Journal of Environmental Treatment Techniques

2019, Volume 6, Issue 1, Pages: 113-141

J. Environ. Treat. Tech.

ISSN: 2309-1185

Journal web link: http://www.jett.dormaj.com

A Review on Different Aerobic and Anaerobic

Treatment Methods in Dairy Industry

Wastewater

1

2,3

1

4

5

Amin Goli , Ahmad Shamiri , Susan Khosroyar , Amirreza Talaiekhozani , Reza Sanaye ,

7

*

Kourosh Azizi

1

- Chemical Engineering Department, Islamic Azad University Branch of Quchan, Quchan, Iran

-Chemical & Petroleum Engineering Department, Faculty of Engineering, Technology & Built Environment, UCSI University,

6000 Kuala Lumpur, Malaysia

2

5

3-Process System Engineering Center, Faculty of Engineering, Technology & Built Environment, UCSI University, 56000 Kuala

Lumpur, Malaysia

4

-Department ofCivil Engineering, Jami Institute of Technology, Isfahan, Iran

- Department of Cancer proteomics, Shiraz University of Medical Sciences, Shiraz, Iran

- Department of Medical Nanotechnology, Shiraz University of Medical Sciences, Shiraz, Iran

- Department of Medical Entomology, School of Health, Shiraz University of Medical Sciences, Shiraz, Iran

5

6

7

Received: 02/08/2018

Accepted: 10/02/2019

Published: 30/03/2019

Abstract

Dairy industries have grown tremendously in many regions around the world due to the growth of demand for milk-related

products. Dairy industries release wastewater containing high chemical oxygen demand (COD), biological oxygen demand (BOD),

nutrients, in addition to organic and inorganic substances. Such wastewater, if improperly treated, severely pollutes water resources.

For many years, anaerobic–aerobic processes have been used to remarkable effect in the treatment of dairy industry wastewater.

Previously, a large portion of wastewater treatment was carried out in conventional anaerobic–aerobic treatment units. Nowadays,

high-rate anaerobic–aerobic bioreactors are progressively employed for treating wastewater with high COD content. This paper

reviews dairy wastewater sources, their production, and characteristics. Furthermore, different types of high-rate anaerobic–aerobic

wastewater treatment methods currently available, including aerobic and anaerobic bioreactors over and above hybrid anaerobic–

aerobic bioreactors, are discussed. The strong points and the weaknesses of different individual and combined anaerobic and aerobic

bioreactors are highlighted; they are then compared to point out future areas of investigation for full usage and application of these

methods for wastewater treatment. The comparison demonstrates that using an integrated bioreactor is advantageous in treating

highly polluted industrial wastewater. The combination of aerobic and anaerobic degradation pathways in an individual bioreactor

can enhance overall degradation efficiency. Furthermore, this combination appears as an attractive alternative from the technical,

economic, and environmental perspectives, especially wherever space is a limiting factor.

Keywords: Dairy product, Industrial dairy wastewater, Anaerobic–aerobic treatment, Anaerobic–aerobic bioreactors, Wastewater

treatment

1

vary significantly, depending on the product (16-18). Water

1

Introduction

pollution causes a sharp drop in dissolved oxygen.

Compounds such as albumin, casein, lactose, and milk fat

are highly biodegradable and decompose rapidly, becoming

rancid and septic. Another main water pollutant from the

dairy industry is whey (19-21). The liquid whey and the

casein that remain in milk after fat removal are used to make

cheese. In the milk clotting (coagulation) process, milk is

separated into curd and whey by the action of milk clotting

enzymes such as rennet or any edible acidic compound

Nowadays, many environmental crises threaten human

life (1-4). One of the most dangerous is the waste from the

dairy industry and milk factories (5-10). Any effort to

prevent environmental pollution from the expansion of the

dairy industry and milk factories should take into account the

effluents they produce (9, 11-15). Milk is the raw material of

the dairy industry, regardless of whether the finished dairy

product is fresh milk, powdered milk, or some other

products. The quality and quantity of wastewater produced

Corresponding author: Professor Dr. Kourosh Azizi, Department of Medical Entomology, School of Health, Shiraz University

of Medical Sciences, Shiraz, Iran

113

Journal of Environmental Treatment Techniques

2019, Volume 6, Issue 1, Pages: 113-141

like lemon juice or vinegar. Clots (rennin curds) contain

casein, fats, minerals, and vitamins, while whey contains

lactose, soluble proteins and/or whey proteins, enzymes,

organic acids, water-soluble vitamins and minerals (20, 22-

2

Characteristics of the dairy effluents

Dairy industry wastewater is generally produced

intermittently; therefore, effluents’ flow rates alter

significantly. Wide seasonal variations are also frequent and

related to the volume of milk received for processing, which

is usually high in summer and low in winter (38). In general,

dairy industry produces lots of wastewaters—nearly 0.2–10

L of waste per liter of processed milk (39-41). Given that the

dairy industry generates various products, such as milk,

butter, yoghurt, ice-cream, and different desserts, the

characteristics of dairy industry effluents also change

significantly based on the operation techniques used in this

process (31). Wastewater characteristics in the dairy industry

affected by using of acid and alkaline cleaners and sanitizers,

generally leads to a highly variable pH (29, 42-44). In

literature, information on the dairy wastewater

characteristics of full-scale operations is rare. To the best of

our knowledge, only one study has been published, which

presents wide information on the specific characteristics of

dairy wastewater from several full-scale operations (29).

Figs. 2 and 3 are some indication that a comprehensive

review in this field is next to necessary in order to draw

general conclusions and provide some guided perspectives

for future research. This is despite the fact that a few reviews

involving related topics of aerobic/anaerobic biological

treatment of dairy wastewater have appeared recently (40,

27). From the economic and environmental point of view,

whey disposal highlight itself as a thorny issue (28). The

present rules and regulations executed by environmental

bodies have had a significant positive impact on new

technologies. This has resulted in enhancing treatment of

dairy products. Dairy product treatment methods are

classified into four categories: chemical, physical,

biological, and hybrid methods (which are also called mixed

methods or multistage dairy product treatment systems). It is

worthy of mentioning that by mixed [multistage] treatment

here we mean a combination of aerobic and anaerobic

methods. This classification is presented in Fig. 1. Cleaning

of transport lines, equipment between production cycles,

tank trucks, milk storage tanks, and equipment malfunctions

as well as operational errors generate most of the wastewater

in the dairy industry (29, 30). Wastewater of dairy industry

is often treated utilizing coagulation/flocculation and

sedimentation processes. The main drawbacks of these

methods are their high coagulant cost, high sludge

production, and poor removal of COD. Hence, biological

treatment is typically recommended for treating dairy

wastewater (31).

Anaerobic granular sludge sequencing batch reactor

45). Many researchers have turned their attention to

(

SBR) was used by Schwarzenbeck et al. (32) for the treating

anaerobic treatment methods more than the aerobic methods;

in point of fact, as indicated by the number of citations per

year, interest is still growing (see Figs. 2 and 3) (46, 47).

In general, aerobic and anaerobic processes can be

applied for treating of dairy wastewater in order to obtain a

high level of organic removal efficiency. Nonetheless, these

processes suffer from a number of restrictions which reduce

their effectiveness. Aerobic processes are commonly used

for treating of low strength effluents (biodegradable COD

contents lower than 1000 mg/L) while anaerobic processes

are commonly used for treating of highly polluted effluents

of dairy industry wastewater. They presented COD, total

nitrogen, and total phosphorus removal efficiencies of 90%,

80%, and 67%, respectively, at 8 h of hydraulic retention

time (HRT). Kushwaha et al. (33) employed an SBR in order

to remove COD and nitrogen from dairy wastewater: they

obtained COD and total Kjeldahl nitrogen (TKN) removal

efficiencies of 96.7% and 76.7%, respectively at an HRT of

24 h. Sirianuntapiboon et al. (34) used an SBR biofilm

system for milk industry wastewater treatment. They

presented 97.9% and 79.3% removal efficiencies of COD

and TKN, respectively at an organic loading rate of 680 g

(

biodegradable COD contents greater than 4000 mg/L)(48).

3

biological oxygen demand (BOD

5

)/m d. A study by Omil et

A comparison of aerobic and anaerobic processes are

presented in Table 1. High energy demands of aerobic

treatment systems is the biggest disadvantage of these

processes. Furthermore, dairy wastewater is warm and

highly polluted, providing an ideal condition for anaerobic

treatment. Moreover, no demand for aeration, the minor

level of excess sludge generation, and low area requirement

are additional advantages of anaerobic treatment processes

al. (35) demonstrated that an anaerobic filter reactor (AFR)

coupled with an SBR can achieve a COD removal efficiency

3

over 90% at an organic loading rate of 5–6 kg COD/m d. An

upflow anaerobic sludge blanket (UASB) bioreactor was

applied for the treatment of wastewater from the cheese

production industry. The COD removal efficiency thereof

96% was obtained at an HRT and organic loading rate of 5.3

h and 10.4 g COD/l d, respectively (36). In addition, a

membrane SBR was used to treat dairy industry wastewater.

Thence, high quality effluent results were obtained at 12 h

HRT with BOD, total suspended solids, total nitrogen and

total phosphorus removal at 97-98%, 100%, 96% and 80%,

respectively (37).

A number of recent qualified, comprehensive research

and review papers were published with emphasis on

different topics of the aerobic/anaerobic biological treatment

methods which are discussed in the next sections.

(

in comparison to aerobic processes). The stated positive

points prompted the fast development of biological systems

for treating dairy industry wastewater using conventional

anaerobic–aerobic treatment plants or high-rate bioreactors

were developed to reduce the capital cost of the process.

However, investigation of high-rate anaerobic-aerobic

treatments is limited and not well documented. Therefore,

the main objective of this review is to summarize and discuss

the feasibility of high-rate anaerobic-aerobic treatment

methods for removing organic compounds from wastewater

of dairy industry. Additionally, characteristics and sources

of dairy wastewater are discussed in this review.

114

Journal of Environmental Treatment Techniques

2019, Volume 6, Issue 1, Pages: 113-141

2

1

40

20

00

80

60

40

20

00

800

750

700

650

600

550

500

450

400

350

300

250

8

6

4

2

0

200

1

50

00

1

50

0

20002001200220032004200520062007200820092010201120122013201420152016

Year

Fig. 2: Citations on “aerobic treatment of dairy wastewater” per

year, showing the increasing research interest in this topic. Data

from ISI Web of Knowledge, Thomson Reuters.

Fig. 3: Citations on “anaerobic treatment of dairy wastewater” per

year, showing the increasing research interest in this topic. Data

from ISI Web of Knowledge, Thomson Reuters.

Chemical oxidation

Chemical methods

Physical methods

Ozone action

Screening

Degreasers

Aerobic:

-

Activated sludge process (ASP)

Conventional or percolating filter

Rotating biological contactors (RBC)

Sequencing batch reactors (SBR)

Membrane bioreactor (MBR)

Dairy wastewater

treatment methods

Biological

methods

Anaerobic:

-

Anaerobic digestion AD

Completely stirred tank reactor CSTR

Upflow anaerobic film (UAF)

Upflow anaerobic sludge blanket (UASB)

Membrane anaerobic reactor system (MARS)

Expanded bed and/or fluidized-bed digesters- Fixed-bed digester

Anaerobic contact process

-

RBC and SBR

-

UASB and AFB

Hybrid methods

Aerobic-anaerobic fixed-film bioreactor (FFB)

UBF and MBR

Fig. 1: Classification of dairy wastewater treatment methods.

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Journal of Environmental Treatment Techniques

2019, Volume 6, Issue 1, Pages: 113-141

Table 1: Comparison of aerobic and anaerobic treatment methods (48-51).

Feature

Aerobic

Anaerobic

Organic removal efficiency

Wastewater quality

Organic loading rate

Sludge generation

Nutrient demand

High

Great

Moderate

High

Moderate to poor

High

Low

High

Low

Alkalinity demand

Energy demand

Temperature sensitivity

Odor

Low

High

Low

High for certain industrial waste

Low to moderate

High

Less opportunity for odors

Potential odor problems

Bioenergy and nutrient recovery

Mode of treatment

No

Yes

Total (rely on feedstock characteristics)

Essentially pretreatment

General characteristics of dairy waste effluents from

full-scale operations are summarized in Table 2 (52-60).

High COD contents demonstrate that wastewater of dairy

industry is highly polluted and fluctuates in nature.

Considerable amounts of the organic compounds and

nutrients in dairy wastewater are obtained from milk and

milk products. Nitrogen is mainly derived from milk

proteins in the wastewater of dairy industry. It is presented

in different forms, either as organic nitrogen (i.e., proteins,

were reported to be capable of suppressing the synthesis of

exopeptidases, a collection of enzymes assisting protein

hydrolysis (69). The anaerobic degradation of proteins

mechanism and the impact of ammonia on this process were

studied in detail by researchers (70-74). The main protein in

milk composition and dairy wastewater is casein. Casein

degrades quickly while fed to acclimated anaerobic reactors:

the consequence degradation products of this mechanism are

non-inhibitory (75). Lipids are basically inhibitory

compounds produced during dairy effluent anaerobic

treatment. Existing literature has limited information on the

lipids anaerobic digestibility. Lipids are hydrolyzed to

glycerol and long chain fatty acids (LCFAs) during

anaerobic degradation process, followed by b-oxidation.

This produces acetate and hydrogen (69). Low

bioavailability of lipids makes the mechanism of lipids

biodegradation complicated (76). Glycerol obtained from

lipid hydrolysis was found to be a non-inhibitory component

(75), while LCFAs were presented to inhibit methanogenic

bacteria (77). The inhibitory effect of lipids on anaerobic

mechanisms can be associated with the existence of LCFAs,

which postpone methane production (78). While lipids do

not create crucial issues in aerobic processes, they

sometimes adversely affect the typical processes of single-

phase anaerobic treatment (79, 80). Saturated LCFAs

reported having a lower inhibitory impact than unsaturated

LCFAs. Unsaturated LCFAs actively inhibit methane

generation from acetate and moderately inhibit b-oxidation.

Consequently, it would be better to convert unsaturated

LCFAs to saturated LCFAs in order to avoid lipid inhibition

in anaerobic mechanisms (80). Difficulties experienced with

lipids in anaerobic treatment processes have been presented

in the literature (81-85).

+

- -

2 3

, and NO

urea, and nucleic acids) or as ions (i.e., NH

4

, NO

)

. The common forms of phosphorus are inorganic like

3-

-4

7

O ), though

orthophosphate (PO

4

) and polyphosphate (P

2

there are organic phosphorus forms present, too (61). Other

methods that can be utilized to measure wastewater pollution

level and treatability are concentrations of suspended solids

(

SS) and volatile suspended solids (VSS) (29). SS in

wastewater of dairy industry are derived from coagulated

milk, cheese curd, or flavoring ingredients (62).

Concentrations of selected elements, including sodium (Na),

potassium (K), calcium (Ca), magnesium (Mg), iron (Fe),

cobalt (Co), nickel (Ni), and manganese (Mn), are listed in

Table 3. In particular, high Na concentrations signify the

extensive utilization of alkaline cleaners at dairy industries.

The concentrations of heavy metals including copper (Cu),

nickel (Ni), and zinc (Zn) were reportedly low enough not to

adversely affect the performance of biological treatment (29,

52). Wastewater of dairy industry is simply made of

degradable carbohydrates, mainly lactose; and also less

biodegradable proteins and lipids (63). In cheese factory

wastewater, 97.7% of total COD was formed via compounds

such as lactose, lactate, protein, as well as fat (55). As a

result, dairy wastewater may be considered as a complex

substance (63-65). Lactose is the main carbohydrate in dairy

wastewater and is an easily available substance for anaerobic

bacteria. Lactose anaerobic methanation requires

collaborative biological activity from acidogens, acetogens,

and methanogens (66). Components such as organic acids,

namely, acetate, propionate, iso- and normal- butyrate, iso-

and normal valerate, caproate, lactate, formate, and ethanol

can be produced from lactose anaerobic fermentation (67,

3

Treatment technologies

3

.1 Primary treatment (Screening, Degreasers)

Screen is used in wastewater treatment in order to

eliminate large particles that may cause damage to pumps

and blocking of downstream (86). For purposes of avoiding

further increase in COD content due to solid solubilization,

physical screening of dairy wastewater should occur as fast

as possible (87, 88). Wendorff (89) suggested the utilizing

of a wire screen and grit chamber with a screen orifice size

of 9.5 mm, whereas Hemming (87) suggested the employing

of finer spaced, mechanically brushed, or inclined screens of

68). Kisaalita et al. (67) presented two methods of carbon

flow for the acidogenic fermentation of lactose including

carbon flow from pyruvate to butyrate and lactate, both

taking place in parallel. The existence of high carbohydrate

levels in synthetic dairy wastewater leads to some decrease

in the synthesized proteolytic enzymes content, resulting in

low contents of protein degradation (63). Carbohydrates

40 mesh (about 0.39 mm) for reducing solids.

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Journal of Environmental Treatment Techniques

2019, Volume 6, Issue 1, Pages: 113-141

Table. 3: Concentrations of selected elements in dairy effluents

Effluent type

Creamery

Na

mg/l)

K

(mg/l)

Ca

(mg/l)

Mg

(mg/l)

Fe

(mg/l)

Co

(mg/l)

Ni

(mg/l)

Mn

(mg/l)

Reference

(

170-

35–40

35-40

5-8

2-5

0.05-

0.15

0.5-1.0

0.02-

0.10

)43(

200

Cheese/whey

Cheese/alcohol

Cheese/beverages

Cheese/whey

735a

423a

453a

419a

123–

42.8a

41.2a

8.6a

35.8a

8-160 12–120

47.7a

54.3a

33.6a

52.3a

11.4a

8.3a

16.9a

11.0a

2–97

)29(

)42(

Mixed dairy

0.5-6.7

0

0-0.13

0.03-

0.43

2

324

720–

80

Mean concentrations are reported.

Cheese

530–

950

)57(

9

a

As reported by Droste (86), in order to avoid the settling

of coarse matter in wastewater, precautionary measures

should be carried out before screening. He recommends

ensuring a 1:2 ratio of depth to width of the approach channel

to the screen and water velocity not lower than 0.6 m/s.

Screens ought to be cleaned out using manual or mechanical

methods and screened material disposed of at a landfill site.

After screening, fat, oil, and grease (FOG) compounds must

be removed. Fat adheres to degreasers. Most pond FOG

mass float to the water surface by gravity method and are

manually removed (69).

In case of self-operating and easily-constructed system,

wastewater flows across a series of cells and FOG mass,

generally floating on the surface, is separated by retention

within the cells. Disadvantages comprise of continuous

monitoring and cleaning to avoid FOG buildup, as well as

reduction of removal efficiency at pH higher than 8 (58).

Occasionally, to facilitate this work in aeration ponds, air

flotation and dissolved air flotation (DAF) are used to

transfer fat particles to the surface and prevent mildew and

odor production.

3.2.1 Activated sludge process

As presented by Smith (102), a typical activated sludge

process (ASP) is an ongoing treatment process which uses a

group of microorganisms that are suspended in wastewater

in an aeration tank to absorb, adsorb, and biodegrade organic

pollutants. A simplified diagram of this process is shown in

Fig. 4. A portion of the organic compound is effectively

oxidized to innocuous end products and other inorganic

matters in order to supply energy for sustain growing of

microbial as well as formation of biomass (flocs). Flocs are

maintained in suspension by one of the air blown into the

bottom of the tank or mechanical aeration methods. The

dissolved oxygen content in the aeration tank is crucial and

should be in the range of 3–5 mg/L. The duration of aeration

as well as cell residence time has to be considered for

designing of an aeration tank. The mixture flows to a

sedimentation tank from the aeration tank where the

activated sludge flocs form larger particles that settle as

sludge. The biological aerobic metabolism process produces

large quantities of sludge (0.6 kg dry sludge per kg of BOD5

removed). Certain sludge is recycled to the aeration tank;

however, the remaining must be processed and disposed of

in an environmentally sound manner, which is a major

running cost. There are numerous alterations of ASP; yet in

all cases, the main energy-consuming operation is providing

oxygen during aeration process. With ASPs, issues usually

occurring are bulking (103-105), foam production, iron, and

carbonates precipitation, extreme sludge production, as well

as a decline in efficiency during winter. Various reports

present that ASP has been employed to successfully treat

wastewater of dairy industry(105). Donkin and Russell (106)

reported that reliable COD removal of over 90%, as well as

TN decrement of 65%, could be achieved with milk powder

and butter wastewater respectively. Removing of

phosphorus compounds was less reliable and appeared to be

sensitive to environmental variations.

3

.2 Major aerobic biological treatment methods

Wastewater treatment usually extends from physical

treatment to biological treatment systems. Numerous studies

were conducted on the wastewater biological treatment. An

aerobic biological treatment technique relies on

microorganisms grown in an environment which is rich in

2

oxygen to oxidize organic materials to CO , water, and

cellular compound. Remarkable data on laboratory- and

field-scale aerobic treatments confirmed that aerobic

treatment is reliable and cost-effective for production of

high-quality effluent (101). Start-up typically needs an

adjustment period to allow the expansion of a competitive

microbial community. In this process, ammonia-nitrogen

can be successfully removed in order to avoid disposal

problems. Foaming and poor solid–liquid separation are

common issues of aerobic processes. Many aerobic

biological treatment techniques were developed to treat

dairy production wastewater, such as activated sludge (AS),

conventional or percolating filter, rotating biological

contactor (RBC), sequencing batch reactor (SBR), and

membrane bioreactor (MBR). The pros and cons of these

methods are presented in Table 4.

In 2013, the effect of varying retention time was

investigated in the AS system by Lateef et al. (107). The

removal efficiencies of COD and BOD were 96% within five

days (107). Increased retention time did not notably affect

BOD and COD removal. However, increasing retention

5

time has several advantages. For example, longer retention

time and aeration time result in uniformity in these ponds, in

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Journal of Environmental Treatment Techniques

2019, Volume 6, Issue 1, Pages: 113-141

which the action keeps the immune system from organic

shocks. The second advantage is lower sludge production

due to the digested part of microorganisms in this section.

These two advantages make use of the system in the dairy

industry. On the other hand, the bulking phenomenon due to

the lack of sedimentation creates excessive foam and toxic

materials, meaning that projected and actual efficiencies of

these systems are contingent on operator experience (84).

Table 4: Advantages and drawbacks of using different types of aerobic processes

Advantages Disadvantages

Reactor type



Low effluent quality

Higher sludge production

Energy-consuming operation

Bulking

Foam production

Precipitation of iron and carbonates

Decrease in efficiency during winter



Easy to operate

and install

Odor-free

Light footprint

ASP



High removal efficiency



Can be blocked by precipitated ferric

hydroxide and carbonates

Not appropriate for the treatment of high-

strength wastewaters

Conventional

or percolating

filter

Very efficient in removal of ammonia

Appropriate for small- to medium-sized

communities



Simple and reliable process



Odor problems may occur

Needs permanent skilled technical

operator for operation and maintenance

purposes



High removal efficiency

Low power input needed

Easy to operate



Needs to be protected against sunlight,

wind, and rain (especially against

freezing in cold climates)

Considerable investment, operation and

maintenance costs

Contact media not available at local

market

Continuous electricity supply required

Low maintenance

Less operator attention

Lower operating costs

Well-controlled against organic shocks

Low space requirement

Low sludge production

No risk of channeling

RBC



(

but uses less energy compared to

trickling filters or ASPs in terms of

comparable degradation rates)



Easy to modify cycles

Small footprint

Cost effective

Low flow applications

Wider wastewater strength variations

High removal efficiency

Capable of achieving nitrification,

de-nitrification, and phosphorous

removal



High energy consumption

Difficult to adjust cycle times for small

communities

SBR



Frequent sludge disposal



Wide operation flexibility

Minimal sludge bulking

Minor operation and maintenance

issues



May be operated remotely

High effluent quality

High volumetric load possible

High rate of degradation

Lower sludge production

More compact



Control of membrane fouling

Membrane fouling

Aeration limitations

Stress on sludge in external MBR

Membrane pollution

High cost

MBR

Energy saving

High removal efficiency

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Feed

Effluent

Sedimentation

Clarifier

Sludge return

Excess sludge

Fig. 4: Simplified illustration of activated sludge process for aerobic wastewater treatment system (102).

3

.2.2 Conventional or percolating filter

Aerobic filters like typical trickling or percolating filters

of approximately 2 m because deeper filters improve

anaerobic growth with subsequent odor issues. These having

been saied, filters with synthetic media are able to fully

aerobic up to about 8 m (108). The final wastewater flows to

a sedimentation or clarifying tank to separate sludge and

particles from the carrier medium. In general, organic

loading for dairy wastewater must not be higher than 0.28–

as shown in Fig. 5, are examples of the oldest biological

treatment techniques that are able to produce high-quality

effluents (102). The carrier media (20–100 mm diameter)

may include pumice, rock, gravel, or plastic pieces, which

are populated by a diverse microbial community. A storage

tank wastewater is usually poured over the medium and then

trickles via a medium with 2 m bed. The sticky microbial

film growing on the carrier medium absorbs the organic

elements of the wastewater and absorbs compounds will be

decomposed aerobically in the film. Deposited sludge lyre

needs to be removed periodically. The downward flow as

well as natural convection currents resulting from

temperature differences between the air and added

wastewater contribute to facilitating aerobic conditions. The

decomposition process might be improved by using forced

ventilation, but the air must be deodorized in clarifying tanks

in order to be used in this system. Typical filters with aerobic

microbes growing on rock or gravel are restricted to a depth

3

0.30 kg BOD/m and recirculation is required for this system

(109). Kessler (110) presented a dairy effluent BOD removal

of 92% and since the BOD of the final wastewater was still

high, further treatment of effluent was performed in an

oxidation pond to decrease the BOD content. An essential

issue is that trickling filters can be clogged by deposited

ferric hydroxide and carbonates, with associated decrement

of microbial activity. When overloading occurs with dairy

wastewater, the medium will be clogged with heavy

biological and fat films. Maris et al. (111) presented that

biological filters are not suitable for treating high-strength

wastewaters because filter blocking by organic precipitated

on the filter medium is commonly found.

Distribution

Feed

Aerati

Carrier

Effluent Outlet

Fig. 5: Simplified illustration of aerobic filter for wastewater treatment processes (110)

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2019, Volume 6, Issue 1, Pages: 113-141

3

.2.3 Rotating biological contactors

The rotating biological contactor (RBC) design includes

are carried out sequentially in SBR systems are: fill, react

(aeration), settle (sedimentation/clarification), draw (the

effluent is decanted), and idle. Wastewater is mixed without

aeration process in order to provide metabolism of the

fermentable components while the tank is filled. The

following stage is aeration process that contributes to the

oxidation and biomass formation processes. Afterwards,

sludge is settled and the treated wastewater is separated to

finalize the cycle. SBR depends strongly on the site operator

to regulate the duration of each stage to reflect fluctuations

in effluent composition (113). Owing to their nearly light

footprint, SBRs are effective in locations where land is

limited. It is likewise simple to improve cycles within the

system for nutrient separation if required. In addition, SBRs

are economical if treatment beyond the biological is

necessary, like filtration. They also suggest potential capital

savings by removing the need for clarifiers, are considered a

reasonable choice for low flow applications, and allow for

greater effluent strength alterations. SBRs need a high level

of maintenance because of the timing units and controls.

Based on the downstream processes, it may be required to

equalize the wastewater after it exits the SBR. Eroglu et al.

(53) and Samkutty et al. (114) proposed that SBR should be

a practical primary and secondary treatment choice for

treating dairy plant effluent with COD removals of 91% -

97%. Torrijos et al. (2001) (115) obtained the COD removal

circular discs (Fig. 6) formed by high-density plastic or other

lightweight materials (102). The discs that rotate at 1–3 rpm

are located on a horizontal shaft such that 40%-60% of the

disc surface stick out from the tank, making it possible for

oxygen to transfer from the atmosphere to the exposed films.

The oxidation of organic compounds of the effluent will be

faciliated through developing a biofilm on the disc surface.

The biofilm sludge will be torn off and removed from the

sedimentation tank as soon as it becomes extremely thick.

2

Operation efficiency is according to the g BOD per m of

disc surface per day (102). Rusten et al. (112) presented

COD removal efficiency of 85% with an organic loading rate

3

(

OLR) of 500 g COD/m hour when treating dairy effluent.

The RBC process suggests various superiority over the ASP

for employ in dairy effluent treatment. The major benefits of

RBC process are low power input needed, ease of operation,

and low maintenance. Moreover, pumping, aeration, and

wasting/recycling of solids are not necessary, thus reducing

the required operator attention. In addition, the process of

nitrogen separation is relatively easy, and only inspection

and lubrication are involved in routine maintenance. The

rotating discs mostly act as trickling filters; nevertheless, as

compared to the trickling filter, RBC requires less land and

lower operating costs. Another advantage of this system is

the large amounts of microbial mass that can protect against

organic shocks to the system.

3

efficiency of 97% at a loading rate of 0.50 kg COD/m day

using SBR in treating effluent from small cheese-making

dairies. Meanwhile, Li and Zhang (116) efficiently operated

an SBR at an HRT of 24 hours for treating dairy effluent

with a COD content of 10 g/L and they achieved the removal

efficiencies of 80% in COD, 63% in total solids, 66% in

volatile solids, 75% in Kjeldahl nitrogen, and 38% in TN.

3.2.4 Sequencing batch reactor

A sequencing batch reactor (SBR) is a single-tank fill-

and-draw system (Fig. 7) that aerates, settles, withdraws

effluents, and recycles solids (40, 102). The typical steps that

Disc exposed

to air

Rotating discs

Shaft

Effluent

Feed

Holding tank

Side view

Front view

Fig. 6: Simplified illustration of aerobic effluent treatment processes: rotating biological contactor (112)

121

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Influent

Settle

Fill

React

Air off

Add substrate

DRAW

Air on

Waste

sludge

IDLE

Remove

effluent

Air off

Fig. 7: Simplified illustration of aerobic wastewater treatment processes: sequencing batch reactor (102)

3

.2.5 Membrane bioreactor

Recently, membrane bioreactors (MBRs), particularly

(121) and back flushing (122) in order to decrease membrane

fouling. Similar to most membrane filtration systems,

membrane fouling, as well as fouling control are main issues

for a cost-effective and feasible MBR system (123). In

addition to MLSS, soluble microbial products are considered

major membrane foulants (122). In testing the performance

of MBR for dairy effluent treatment with an HRT of 36

hours and 0.13 kg COD/kg MLSS, a COD removal

efficiency of 95% was observed. Increasing MLSS

concentration in the MBR system did not considerably affect

removal efficiency of BOD5 and COD. The concentrations

of COD and BOD5 used were 400 and 550 mg/L, and

removal efficiencies achieved were 95% and 91%

respectively.

submerged-type membranes (Fig. 8), have been gaining

attention owing to their better treated wastewater quality and

lower sludge generation compared to typical ASPs (40, 117-

119). In an MBR, membranes play a key role in solid/liquid

separation. There are two types of MBRs which are differ

based on the placement of the membrane unit. Membranes

are either submerged in the reactor or placed externally. The

submerged membrane unit has attracted more attention

recently since it is a compact system as well as uses low

energy (117, 118, 120). One drawback of this system is that

control of membrane fouling is more difficult compared to

external membrane system. Different techniques have been

implemented, including the intermittent suction method

Retentive Recycle

(

b)

(

a)

Influent

Permeate

Permeat

Waste sludge

Waste

sludge

Bioreactor

Fig. 8: The configuration of MBR system: (a) submerged MBR and (b) side-stream MBR configuration

122

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3

.2.6 Factors governing aerobic reactor choice

Aerobic granular activated sludge SBR (GAS-SBR) was

treatment with high COD content without prior dilution, as

required by aerobic processes, decreases the required space

as well as related costs. Usually, there are no bad odors if the

process is operated properly (134). High capital cost, long

startup periods, rigorous control of operating conditions, and

higher sensitivity to variable loads and organic shocks, in

addition to toxic compositions are the drawbacks of

anaerobic systems (135). Ammonia nitrogen is accordingly

discharged with the digester effluent and generating oxygen

demand in the receiving water since it is not separated in an

anaerobic system. In order to obtain reasonable discharge

standards, a complementary treatment is also necessary. As

demonstrated in previous works, a remarkable disadvantage

of aerobic treatment plants are the high energy demands.

COD contents of dairy wastewater change considerably;

furthermore, dairy wastewaters are warm and highly

polluted, making them ideal for anaerobic treatment (135).

Furthermore, no aeration is required, the level of sludge

production is low, and the area demand is low.

recently proposed to provide a significant aerobic treatment

rate as well as superior settling (40, 124). Schwarzenbeck et

al. (32) presented 90% COD, 80% TN, and 67% TP removal

efficiencies in a GAS SBR. While mixed culture AS is

normally employed by scientists to treat dairy wastewater,

bio-augmentation (adding external microorganisms with

significant degradation capacity for specific wastewater)

was successful at improving performance (125). Loperena et

al. (126) demonstrated that, whereas commercial and mixed

activated-sludge inocula presented similar rates of COD

removal in batch experiments for treating dairy industrial

wastewater, COD degradation rate was higher for

commercial inocula.

3.3 Major anaerobic biological treatment methods

Various anaerobic biological treatment techniques were

developed to treat dairy products, including anaerobic

digestion (AD), completely stirred tank reactor (CSTR),

UASB, upflow anaerobic filter, anaerobic contact process,

expanded bed and/or fluidized-bed digesters, fixed-bed

digesters, and membrane anaerobic reactor system (MARS).

Biological treatment methods are environmentally friendly

3.3.2 Completely stirred tank reactor

One of the simplest types of anaerobic digester is

completely stirred tank reactor (CSTR) (136, 137). Sahm

(138) stated that the OLR rate ranges from 1 kg to 4 kg

3

-1

for treating polluted air and also do not produce NO

x

, SO

x

,

organic dry matter m day , and the digesters generally have

3

or secondary pollutants. Various factors, like pH,

temperature, and gaseous retention time, have significant

effects on biological processes and should be in optimum

condition for obtaining high efficiency. The advantages and

drawbacks of these methods are laid-out in Table 5.

An anaerobic filter is able to operate by individual-

feeding such as upflow, downflow and horizontal direction

as well as multiple-feeding (40, 91, 127-129). The upflow

anaerobic filter was widely applied for treating of whey; it is

able to operate efficiently for treating of low and highly

polluted dairy effluent at short HRT and high organic

loading rate (130). Operating conditions and anaerobic

filters treatment performances for treating dairy wastewater

are presented in Table 6.

capacities of 500 to 700 m . CSTRs reactors are usually

employed for concentrated effluents, particularly those

whose polluting matter is mostly SS and has COD values

greater than 30,000 mg/L. There is no biomass retention in

this reactor; as a result, the HRT and sludge retention time

(SRT) are not separated, necessitating long retention times

that depend on the growth rate of the slowest-growing

bacteria in the process of digestion. Ross (139) reported that

the HRT of the typical digesters is similar to SRT, which can

vary from 15 to 20 days. This type of digester was employed

by Lebrato et al. (140) for treating effluent of the cheese

factory. Although 90% COD separation was obtained, the

digester could only work at a minimum HRT of 9.0 days,

which was likely because of biomass washout. The effluent,

containing 80% washing water and 20% whey, had a COD

of 17,000 mg/L. This type of reactor is very beneficial for

laboratory scale studies, but it is hardly a feasible choice for

industrial scale treatment because of its HRT limitation.

3.3.1 Anaerobic digestion

A biological process carried out via an active microbial

community without presenting of exogenous electron

acceptors is known as anaerobic digestion (AD). In this

process, up to 95% of the organic load in a waste stream can

be turned into biogas (methane and carbon dioxide), while

the rest is used for cell growth and maintenance (131). In

general, anaerobic processes are rather efficient and cost-

effective for the biological stabilization of dairy effluents

because the high-energy associated with aeration in aerobic

systems is not required (59, 132). AD also produces

methane, a source of heat and power (133). Furthermore,

minor sludge is produced, diminishing problems associated

with sludge removal. AD systems require nutrient such as

nitrogen and phosphorus significantly lower than aerobic

systems (108). Pathogenic organisms are generally

destroyed, and the final sludge has a high soil conditioning

content if the heavy metals level is low. Dairy effluents

3.3.3 Upflow anaerobic sludge blanket

Lettinga et al. (1991) (141) designed a UASB reactor for

commercial applications. A schematic diagram of this

reactor is shown in Figure 9.

UASB reactor was used for treating maize-starch effluents

in South Africa for the first time (142), however the full

potential of UASB reactor was only discovered after a

significant development program by Lettinga in the late

1970s (141, 143). The UASB has only recently been used in

anaerobic treatment. Through UASB, pollutants in

wastewater are degraded by microbes that produce 75%-

4 2

80% CH by volume, 15%-25% CO , and minor amounts of

, H , and other gases.

N

2

123

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Table 5: Advantages and drawbacks of using different forms of anaerobic processes

Advantages Disadvantages

Reactor type



Low energy requirements



High capital cost

Less sludge is generated

Methane production, which can be utilized as a heat or

power source

Less requirements to N and P

Lake of pathogenic organisms

Less space requirements

Cost effective

High removal efficiency

No biomass retention

High removal efficiency

Continuous operation

Reasonable temperature control

Simply adapts to two-phase runs

Reasonable control

Ease of operation

Minor operating (labor) cost

Easy to clean

Long startup periods

Strict control of operating

conditions

Greater sensitivity to variables

loads and organic shocks

Toxic compounds

AD



High energy requirements



Very low conversion per unit

volume

By-passing and channeling

probable with weak agitation

performance

CSTR

High removal efficiencies

No support material is required

Cost-effective

Great decrement in organics

Is able to tolerate high OLRs (up to 10 kg BOD/m /d)



Long start-up period

Sufficient amount of granular

seed sludge

3

UASB

and hydraulic loading rates



Minor sludge generation (and thus, infrequent deluging

required)

Biogas is able to be applied for energy (but generally

needs scrubbing first)



Reactor needs skilled operation



Requires expert design and

construction

Low reduction of pathogens

and nutrients

Effluent and sludge require

further treatment and/or proper

discharge

Risk of clogging, depending on

pre- and primary treatment

Removing and cleaning the

clogged filter media are

difficult



High OLRs

Short HRT

High removal efficiency

Stable against organic and hydraulic shock loading

No electrical energy is needed

Minor sludge production; the sludge is stabilized

Upflow

anaerobic filter



Enhances biomass retention

High removal efficiency

MARS



High retention time

Fixed-bed

digester



Saturated region

Difficult to design accurately



High removal efficiency

Expanded bed

and/or

fluidized-bed

digesters

Fixed-bed

digester



High removal efficiencies

Can control and optimize the biological film thickness

Elimination of bed clogging

Low hydraulic head

Greater surface area

Capital cost is lower



Problems of channeling

Plugging

Gas hold-up

Poor settling properties

High removal efficiencies

Varies temperature ranges

No oxygen requirements

Ethane is a useful end product

Anaerobic

contact process



High retention time

Poor settling properties

124

Journal of Environmental Treatment Techniques

2019, Volume 6, Issue 1, Pages: 113-141

Table 6: Operating conditions and anaerobic filter performances for dairy effluent treatment (40).

Feed type

Wastewater

Packing

media

Temp.

( C)

pH

HRT

(d)

OLR (kg Influent COD CH

COD m³COD (g Remova (m

-1 −1

4

3

yield

Reference

o

_k_g−1

d )

L )

l (%)

CH

4

COD)

Up-flow

Whey

Dairy

Polyethyle 37

ne/clay

7

10-15

1-5

2.2-3.3

1-4

33

90

_

(144)

(97)

Flocor

35

5.7-

7

6.8-

7.2

_

5-20

2-6

72-90.2

0.089-0.28

0.32-0.34

0.32-0.39

0.236-0.26

0.158-0.35

.5

Polypropy 32-34

lene

PVC rings 35

0.83-

12.5

2

0.5-6.5

1.44-6.29

2.6

(95)

>80

97.9-

98.8

Synthetic

dairy

Simulated

whey

3-12

13

(145)

(146)

(147)

Down-

flow

Polyethyle 35

ne

5.9-

7.8

6.9

5

1

66-93.6

Horizontal Synthetic

flow whey

Ceramic

40

1-10.2

85-93.8

-

Methane gas contains high calorific level; thus, by

utilizing this type of reactor, the produced methane is

separated and used as an alternative energy source. This

system, therefore, is feasible and efficient for the waste

contains a high level of BOD. The rather simple design of

the UASB digester (Fig. 9) is according to the premier

settling properties of granular sludge. The granules’ growth

and development are keys to the success of the UASB

reactor. Note that the presence of granules in the UASB

digester eventually contributes to removing the HRT from

the SRT. Therefore, efficient granulation is necessary to

achievie a short HRT without inducing biomass washout.

The effluent is fed from bottom and exits at the top through

an internal baffle system to separate the gas, sludge, and

liquid phases. The granular sludge and biogas are separated

percentages of COD with an organic loading rate of 31 g/L/D

of COD were 90 percent (148). An anaerobic upflow filter

was used in a 2008 study on the treatment of whey. In 2008,

a UASB system was used to evaluate the removal efficiency

of whey. With a concentration of 5000 mg/L and an HRT

variety of 1 to 4 days, the removal efficiency was increased

dramatically from 70% to 90% (6). Performance evaluation

of the dairy wastewater treatment using UASB reactors

under various experimental conditions and at various scales

are summarized in Table 7. As shown in this Table, COD

removal from dairy wastewaters in UASB reactors changes

from 50% to 98%.

3

using this device. A COD loading of 30 kg/m day can be

treated with a COD separation efficiency of 85%–95% at

optimal conditions. The methane level of the produced

biogas ranges from 80% to 90% (v/v). HRTs of as low as 4

hours are practical, with superior settling sludge and SRT of

greater than 100 days. The UASB is highly economical

because it uses less pump energy for the recirculation of

effluent and does not require other expenses. The UASB

system is greatly relies on its granulation process with a

particular organic wastewater in comparison with other

anaerobic technologies. Removal efficiencies of 95%–99%

can be obtained by employing the UASB. The main point of

the UASB system is that it does not need support material

for retaining high-density anaerobic sludge. But, the absence

of carriers makes the availability and maintenance of

biomass that settles easily, either as flocs or as dense

granules (0.5–2.5 mm in size) necessary. In order to separate

biogas and bacterial mass which are returned into the active

lower zone of the reactor on the other side, a three-phase

separator (biogas, liquid, and biomass) is required.

Fig. 9: Schematic diagram of a UASB system (48, 149). (Reprinted

from Biological Wastewater Treatment in Warm Climate Regions,

p. 723, ISBN 9781843390022 with permission from the copyright

holders, IWA).

In 1991, UASB reactor performance was evaluated in

the anaerobic wastewater treatment of cheese. The

125

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Table 7: Results of the UASB reactors performance for the dairy wastewater treatment (150)

Reactor

volume (L)

_

COD IN (mg/L)

COD removal (%)

Reference

1440

700-1200

_

1500-2000

4056

101-541

2038-4728

1250-2250

800-4080

5240

20,314

1000-2000

500-3300

53-91.5

50-93

96-98

90

79.4/85.5

98

(151)

(152)

(6)

3

6

1

6

5

1

4

1

6

1.7

.8

0/2

(153)

(154)

(155)

(156)

(157)

(158)

(159)

(160)

(161)

(162)

69

20,120

4

.32

8,000

20

.6

78.78-87.06

64

78

88.23

95

67

3

3.3.4 Upflow anaerobic filter

yield (YCH4/COD removed) of 0.327 m /kg COD removed

Young and McCarty developed the upflow anaerobic

was achieved.

filter in 1969 (163). Its mechanism is like the aerobic

trickling filter mechanism. The upflow anaerobic filter is

loaded with inert support material, like gravel, rocks, coke,

or plastic media; therefore, the system does not require

biomass removal or sludge recycling. The AFR can be

functioned as a downflow or an upflow filter reactor with an

OLR ranging from 1 kg/m to 15 kg/m day COD and COD

separation efficiencies of 75%–95%. The treatment

temperature is between 20 °C and 358 °C with HRTs ranging

from 0.2 to 3 days. The potential risk of clogging via

undegraded SS, mineral precipitates, or bacterial biomass is

the major disadvantage of the upflow anaerobic filter. Their

usage is also limited to effluents with COD ranging from

3.3.5 Membrane anaerobic reactor system

The digester effluent is filtered via a filtration membrane

in a membrane anaerobic reactor system (MARS). Li and

Corrado (167) investigated the MARS (well mixed digester

with an operation volume of 37,850 L integrated with a

microfiltration membrane process) on cheese whey

including up to 62,000 mg/L of COD. The digester effluent

was filtrated by the membrane and permeate was released.

The retentate, which contained biomass and SS, was

recycled to the digester. The COD separation efficiency was

99.5% at an HRT of 7.5 days. The most important

achievement of the study was that the process control

parameters attained in the pilot plant could adequately be

employed to their industrial-scale plant. The anaerobic

digestion ultrafiltration system (ADUF) similar to

membrane system has successfully been employed in lab-

scale and pilot-scale investigations of dairy wastewaters

(168). The ADUF process does not employ microfiltration,

but rather an ultrafiltration membrane. Therefore, far better

biomass retention efficiency is achievable with the ADUF.

Prieto et al. (169) developed a composite bioactive

membrane for wastewater treatment and used it to produce

3

1000 to 10,000 mg/L (139). Bonastre and Paris (164)

reported 51 anaerobic filter applications, of which 5 were

applied for pilot plants and 3 were used for industrial-scale

dairy effluent treatment. The anaerobic filters were worked

at HRTs ranging from 12 to 48 hours, while COD separation

ranged from 60% to 98%. The organic loading rate changed

3

from 1.7 to 20.0 kg COD/m day.

Separated phase digesters are developed to spatially

remove acid-forming bacteria and acid-consuming bacteria.

Separated phase digesters are beneficial for treating effluents

either with unbalanced ratios of carbon to nitrogen (C:N),

like effluents with high protein concentrations, or effluents

that acidify quickly, like dairy effluents (165). The main

points of these digesters are high OLRs and short HRTs.

Burgess (166) presented two cases where dairy effluents

were treated applying a separated phase industrial-scale

process. One dairy had an effluent with a COD content of

2

and capture hydrogen (H ) which is a source of energy.

3.3.6 Fixed-bed digester

The fixed-bed digester (Fig. 10) includes permanent

porous carrier materials. Through extracellular

polysaccharides, bacteria are able to attach to the surface of

the packing material and still stay in tight contact with the

passing effluent. The wastewater is fed either at the bottom

or at the top to make upflow or downflow arrangements,

(170) employing a downflow fixed-film digester for treating

deproteinized cheese whey with an average COD of 59,000

mg/L. The digester obtained a COD decrement of 90%–95%

50,000 mg/L and a pH value of 4.5. Two phases of digester

were worked at 35°C. The acidogenic reactor was functioned

at an HRT of 24 hours, and the methanogenic reactor was

operated at an HRT of 3.3 days. 50% of the COD was

transformed to organic acids in the acidification tank, while

only 12% of the COD was separated. The OLR for the

3

at an HRT and OLR of 2.0–2.5 days and 12.5 kg COD/m

3

acidification and methane reactors were 50.0 kg COD/m

day respectively. The deproteinized cheese whey had a mean

pH of 2.9 although the digester pH was frequently above 7.0

(171). De Haast et al. (172) employed a bench-scale fixed-

bed digester including an inert polyethylene bacterial carrier

for treating cheese whey. They achieved best results at an

3

day and 9.0 kg COD/m day respectively. A total COD

decrement of 72% was obtained. The methane content in

biogas was 62% and the supplied data showed that a methane

126

Journal of Environmental Treatment Techniques

2019, Volume 6, Issue 1, Pages: 113-141

HRT of 3.5 days, with 85%–87% COD separation. The

wastewater, this specific dairy wastewater seems to be at the

bottom end of the scale in terms of its COD content and

organic load. The dairy effluent was apparently generated by

a dairy with very good product-loss control and a relatively

great level of water use (165).

3

organic loading rate was 3.8 kg COD/m day, and biogas

3

yield amounted to 0.42 m /kg COD added per day. The

biogas contained a methane level of between 55% and 60%,

and 63.7% of the calorific value of the substrate was

conserved in the methane.

Biogas

Effluent

Bed

Trap

Pump

Feed

Fig. 10: A simplified illustration of anaerobic wastewater

treatment processes: fixed-bed digester.

Feed

3

.3.7 Expanded bed and/or fluidized-bed digesters

In comparison with the fixed-bed reactor, anaerobic

Fig. 11: A simplified illustration of anaerobic effluent

treatment processes: fluidized-bed digester.

fluidized bed reactor has superior mass and heat transfer

characteristics. Furthermore, it has a great level of attached

biomass, which is rich in microbial diversity. It becomes

stable quickly after changing operational conditions (40,

3

.3.8 Anaerobic contact process

The anaerobic contact system (Fig. 12) was built in 1955

177). This system is basically an anaerobic ASP that

comprises of a well-mixed anaerobic reactor followed by a

model of biomass separator. The removed biomass is

returned back to the reactor, therefore decreasing the

retention time from the typical 20–30 days to 10 days. Given

that the bacteria are maintained and recycled, this model of

plant would be capable of treating medium-strength effluent

(

173, 174). Operating conditions and evaluation of anaerobic

fluidized bed reactors for treating of dairy industry

wastewater are summarized in Table 8. As shown in Fig. 11,

in fluidized bed digesters, effluents pass upwards via a bed

of suspended media to which bacteria attach (175). The

carrier medium continuously remains in suspension through

strong, efficient liquid phase recirculation. The carrier media

consist of plastic granules, sand particles, glass beads, clay

particles, as well as activated charcoal fragments.

Parameters that contribute to having an efficient fluidization

system for this process are (a) utmost contact between the

liquid and the fine particles carrying the bacteria; (b)

avoiding issues of channeling, plugging, and gas hold-up,

generally occurring in packed beds; and (c) the ability to

control and optimize the thickness of biological film (138).

Toldra et al. (176) applied this process for treating dairy

effluent with a COD of only 200–500 mg/L at an HRT of 8.0

hours and a COD separation of 80%. Note that with the

fundamental changes found between different types of dairy

(

200–20,000 mg/L COD) very effectively at great organic

loading rates (138). The organic loading rate can change

3

between 1 kg/m .day and 6 kg/m .day COD with COD

separation efficiencies of 80%–95%. The treatment

temperature varies between 30°C and 40°C. A main

drawback of this process is the poor settling properties of the

anaerobic biomass from the digester effluent. Dissolved air

(

178) and biogas flotations methods (179) were used as

alternative sludge removal methods in this system. Although

the contact digester is assumed to be obsolete, numerous

dairies throughout the world still use this system (180).

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Table 8: Anaerobic fluidized bed reactors performances at different operating conditions for dairy effluent treatment (40)

128

Journal of Environmental Treatment Techniques

2019, Volume 6, Issue 1, Pages: 113-141

Mixing

Biogas

Degassifier

Feed

Clarifier

Solids recycle

Fig. 12: A simplified illustration of anaerobic effluent treatment processes (Contact digester).

3

.3.9 Factors influencing choice of anaerobic reactor

Compared to the established technologies in the market,

Factoring all of the findings, the maximum loading rates

that may be achieved with soluble wastewater is highest in

UASB, followed by EGSB, fluidized bed reactor, and

anaerobic filter. Accordingly, Rajeshwari et al. (187) held

the same order in terms of the requirements of land area and

the capital cost of the reactors. Moreover, only minimum

maintenance and digester operation are required, provided

that the process is adequately stable against the changes in

conditions of environment and fluctuations in the

wastewater characteristics. The potential utilization of a

reactor determines the susceptibility of the process and,

consequently, a system that operates at loading conditions

near the maximum level has higher sensitivity as compared

to the other systems. Table 9 outlines the recommendations

in choosing a reactor based on the comparison of numerous

types of reactors (187).

industries prefer a better and more dependable technology

that would need minimal land area and capital. In terms of

an AD system, a process capable of running at high loading

rates of organic and hydraulic, fits the technology criteria.

Ideally, this process requires minimal maintenance and

operation. In order to determine an ideal reactor type for a

certain application, a systematic evaluation must be

conducted on several configurations between the reactor and

the wastewater stream. Notably, three factors influence the

organic and hydraulic loading potential of a reactor, namely

(

i) the amount of active biomass per unit volume that can be

retained by a reactor, (ii) contact opportunity between the

retained biomass and the incoming wastewater, and (iii)

diffusion of the substrate within the biomass.

Considering the determinants, the most prominent

option is the granular sludge UASB reactor. Prominently, the

only constraints that the reactor has are its granules’

propensity to float at high loading rates [the granules tend to

shear]. To a smaller extent, these limitations also apply to

attached biomass reactors such as fluidized bed, fixed film,

and rotary biological contactors. The media occupy the

space, causing the attached biomass reactors to have a

relatively lower capacity for biomass retention of the reactor

per unit volume. This capacity is determined by the film

thickness whilst the fluidized bed reactor has the highest rate

because it has a large surface area for the attachment of the

biomass. Furthermore, the fluidized bed and expanded

granular sludge bed (EGSB) systems demonstrate further

contact between the retained biomass and the incoming

wastewater. Nevertheless, the diffusion of the substrate

within the biomass in these configurations is restricted

because of the high upflow velocity.

3.4 Major combined biological treatment methods

Depending on the type of pollution and based on

biological treatment methods, wastewater treatment systems

are categorized into aerobic, anaerobic, and combination

methods. As mentioned above, dairy industrial wastewater

with a high organic concentration always encounters

significant complications. Thus, aerobic and anaerobic

methods cannot sufficiently remove all low- and high-

loading combinations. As a result, combining two methods

or multi-stage methods can compensate for the weaknesses

of each individual method and improve the performance of

processes (188). Recently, combined anaerobic reactors

have been widely used for the treatment of dairy wastewater

and performance evaluation and operating conditions of

theses reactors are listed in Table 10.

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Table 9: Recommended factors to choose anaerobic reactors.

Grade order

Factors

Operational competence

Fixed film < UASB < RBC < fluidized bed

Energy usage

UASB <fixed film < EGSB < fluidized bed < RBC

RBC < fixed film < UASB < EGSB < fluidized bed

Capital cost, land area demand, operation and maintenance

Table 10: Anaerobic integrated reactors performances at different operating conditions for dairy effluent treatment (40).

3

.4.1 Anaerobic RBC and aerobic SBR systems

In an RBC system, microorganisms form a biological

illustrates a diagram of an anaerobic RBC system. Both

anaerobic and aerobic RBC reactors have a similar

configuration with the exception of a covered tank in the

former, which prevents contact with air (193). Notably, the

sole application of anaerobic RBC in the treatment of highly

polluted synthetic wastewater yields a final COD of the RBC

film and attach to an inert support medium that has a

sequential disc configuration. The support medium is

submerged partly or totally; it also gradually rotates around

a horizontal axis in a tank with flowing wastewater. Fig. 13

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2019, Volume 6, Issue 1, Pages: 113-141

effluent that is still deemed as excessively high. The initial

COD concentrations of the synthetic wastewater were

between 3248 and 12,150 mg/L. Despite achieving

satisfactory efficiencies of overall COD removal at an HRT

of 32 h (ranging from 74 % to 82 %), it is necessary to

perform more treatment (194).

3.4.2 UASB and aerobic fluidized bed system

Mobile supports fill the fluidised bed reactors; hence, the

particles covered with biofilm are fluidized through the

liquid recirculation. Consequently, the constraint of

substrate diffusion which is common in the stationary bed

process is eliminated. Fig. 14 depicts a graphic of an aerobic

fluidised bed (AFB) system. Heijnen et al. (200) and Shieh

and Keenan (1986) (201) outlined the advantages of the AFB

reactor: (i) high biomass concentration, (ii) short HRT, (iii)

high organic loading rate, (iv) small external mass transfer

resistance, (v) large surface area for mass transfer, and (vi)

no bed clogging. Contrary to this, Lazarova and Manem

(202) and Saravanane and Murthy (203) stated that the

system’s applicability on a large industrial scale would be

hampered by a number of issues, namely the control of the

biofilm’s thickness, oxygen distribution system, and bed

expansion. In addition, the system also records elevated

energy consumption as it operates on an exceedingly high

ratio of liquid circulation.

Typically, there are three phases in the general mode of

operation for an AFB reactor in the treatment of wastewater:

(1) the discrete solid phase of inert particles with

immobilized microbial cells, (2) the discrete air bubbles, and

(3) the continuous aqueous solution. A research by Tavares

et al. (1995) showed that AFB process with the three phases

resulted in the attainment of high percentage (82%) in the

average COD removal efficiency during a synthetic

wastewater treatment. It is to be noted that, the initial feed

content was 180 mg/L and the process was performed at a

low average HRT of 30 min. With this result, the reactor

showed its potential in treating lowly polluted wastewater

has COD content of 100 mg/L to 200 mg/L (204).

Furthermore, a research by Yu et al. (65) on the treatment of

synthetic textile medium-polluted wastewater with COD

content of 2700 mg/L obtained a total of 75% COD removal

efficiency. This result was achieved by combining UASB

with the AFB reactor at an overall HRT of 14 h. Compared

to the aerobic system, the combination of UASB and AFB

generated 45% lower sludge volume. Nonetheless, the

anaerobic biomass (∼1 g volatile solid [VS]/L) incorporated

into the AFB reactor to improve the removal of COD led to

an increasing level of suspended solids (SS). This is because

the anaerobic biomass deactivates at a fast rate under aerobic

conditions; consequently, the particular activity of aerobes is

diluted by the dead anaerobic cells.

The RBC system has several advantages: (i) short

retention time, (ii) low energy requirements, (iii) low

operating costs, (iv) excellent process control, and (v) ability

to handle an extensive range of flows. On the contrary, the

main disadvantage of the system is its susceptibility to

wastewater characteristics. This causes restricted

operational flexibility to different operating and loading

conditions. Apart from these, frequent maintenance is also

required on its mechanical drive units and shaft bearings.

Furthermore, the fill- and draw-AS systems have an

improved version, the aerobic SBR, which is comprised of

one or more tanks. Each tank has the capability to perform

solid separation and waste stabilization. Kim et al. (195)

explained the advantages of the SBR process: (i) flexibility

in the treatment of variable flows, (ii) providing options for

aerobic or anaerobic conditions in the same tank, (iii)

minimum operator interaction, (iv) efficient oxygen contact

with microorganisms and substrates, (v) good removal

efficiency, and (vi) small floor space. Due to these benefits,

the process has been implemented at an increasing rate in the

industrial (196-198) and municipal (199) wastewater

treatment.

A combination of the anaerobic RBC and aerobic SBR

systems will lead to efficient bioenergy production and

waste treatment system as high methane production rates can

be achieved via anaerobic RBC, and diluted waste can be

treated efficiently via the aerobic SBR. To that end, an

integration of anaerobic RBC and three aerobic SBRs was

adopted in the treatment of screened dairy manure and also

in the treatment of a mixture of cheese whey and dairy

manure. In general, this combination system is able to attain

a significant reduction in COD (a minimum of 98%) and

generate a considerable amount of methane gas.

To that end, minimal cell mass from the UASB reactor

has to be ensured prior to the process in the AFB reactor.

This is to circumvent the anaerobes from having biological

activity with high turbidity that does not have any

contribution. The biological treatment of industrial

wastewater of medium level of pollution may apply the

UASB-AFB system due to its reduced sludge formation,

high pH tolerance, and stable performance in the removal of

COD. The UASB-AFB system is also ideal in economic,

technical, and environment terms, particularly when space is

a constraint.

Fig. 13: Simplified diagram of an anaerobic RBC reactor (149).

(Reprinted from Biological Wastewater Treatment in Warm

Climate Regions, p. 719, ISBN 9781843390022 with permission

from the copyright holders, IWA).

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2019, Volume 6, Issue 1, Pages: 113-141

of recirculation ratio (R/F) and anaerobic/aerobic volume

ratio (Van:Vae). Accordingly, the downflow manner was

applied for the FFBs, and recirculation of the aerobic

effluent was done on the anaerobic FFB. Consequently, the

removal of COD was prominently evident in the anaerobic

FFB. Furthermore, this effect was inflated with an increase

in the contribution of denitrification, namely as the R/F

increased from 1 to 6. Apart from that, a smaller volume of

aerobic FFB compared to the anaerobic FFB also led to an

increase in the fraction of COD removed in the anaerobic

FFB. This is explained by the large recirculation in the

anaerobic FFB feed that favoured denitrification as opposed

to the detriment of the methanogenic process and the

generation of biogas.

3.4.4 Anaerobic upflow bed filter and aerobic membrane

bioreactor system

An anaerobic hybrid reactor, the anaerobic upflow bed

filter (UBF) is a combination of an anaerobic FFB and a

UASB. UASB is installed at the lower part of the UBF

reactor which develops the granular sludge. Conversely,

FFB is found at the upper part of the UBF with the support

of stationary packing material. Notably, the problems of

clogging and biomass washout occur regularly in both

anaerobic FFBs and UASBs; UBF is capable of eliminating

these problems. In aerobic MBRs, membrane filtration is

merged with biodegradation processes and sieving enables

the occurrence of solid-liquid separation. MBR retains solid

Fig. 14: Schematic diagram of an AFB reactor (149). (Modified

from Biological Wastewater Treatment in Warm Climate Regions,

p. 717, ISBN 9781843390022 with permission from the copyright

holders, IWA).

materials,

pathogenic

bacteria,

biomass,

and

macromolecules while simultaneously tolerating smaller

solution species and water to permeate the membrane (207,

3

.4.3 Anaerobic–aerobic fixed film bioreactor system

Compared to suspension culture, the immobilized cells

208). Consequently, the process ensures the production of

high-quality effluent. Dhaouadi and Marrot (209), Muller et

al. (210), and Wang and Wu (211) listed a number of

advantages that MBR has: (i) separation of solid retention

time (SRT) from HRT, (ii) high-quality effluent, (iii)

reduced production of sludge due to the endogenous

respiration during the long SRT, and (iv) low rate of sludge-

loading. Normally, the membrane-retained aqueous and

particulate-based enzymes disappear in the conventional

step of sedimentation clarification. This situation is different

in MBR, and hence, the metabolic rate with this process

would be improved (212).

on the surface of the media, or fixed film, are superior

because they (i) have wider variation in population, (ii) have

higher rate(s) of growth, (iii) demonstrate faster utilization

of the substrate concerning free biomass, and (iv) are less

sensitive to the variations in environment in terms of pH,

temperature, and toxic substances. Bishop (205) stated that

the fixed cells endure physiological modifications because

of the increasing local concentration of enzymes and

nutrients or the extracellular polymeric matrix, which have a

selective effect on toxic or inhibitory substances.

Consequently, these cells exhibit the outlined advantages.

Apart from that, Del Pozo and Diez (206) examined a

combination of two FFBs – anaerobic and aerobic – with

arranged media, linked serially with recirculation for the

treatment of wastewater from a poultry slaughterhouse. The

implementation of FFB in the slaughterhouse wastewater

was done due to the severe problems of flotation in the

suspended biomass systems, which were caused by the

substantial levels of grease and oil. Clogging was avoided by

placing the long corrugated PVC tubes as the support media

vertically. Moreover, the tubes have a rough structure that

attributed to the increase in specific surface and acted as a

protection from stress forces for the biomass attached. As a

result, the overall COD removal efficiency of 92% was

On the contrary, membrane fouling is

a major

disadvantage in the adoption of MBR. Generally, this issue

is addressed by applying cross-flow filtration. A study by

Ahn et al. (213) investigated the treatment of highly polluted

wastewater with COD content range of 6000 mg/L to 14,500

mg/L. The anaerobic UBF-aerobic MBR system was

implemented at a relatively short HRT (24 h) and the results

showed significant removal of COD at 99%. Fig. 16

illustrates a diagram of this system. Apart from that, and

despite the superiority of the system, membrane fouling was

still evident. Compared to a unit MBR that was run under

similar settings, the trans-membrane pressure was

approximately nine times higher. The increased extracellular

polymeric substance and hydrophobicity led to serious

fouling in the system. In addition, the membrane-coupled

ASP is a representative of the MBR system. It merges the

AS system with membranes and performs specifically

3

attained at an organic loading rate of 0.39 kg/m .day. Fig. 15

depicts the illustration of the anaerobic-aerobic FFB.

These having been said, analysis of the fraction of COD

removed by every reactor was done by evaluating the effects

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2019, Volume 6, Issue 1, Pages: 113-141

efficiently in organic matter removal as an advanced

secondary wastewater treatment process.

Fig. 15: Schematic diagram of anaerobic–aerobic FFBs (206). (Reprinted from Water Research, Organic matter removal in combined anaerobic-

aerobic fixed-film bioreactors, 37 (2003) 3561–3568, with permission from the copyright holders, Elsevier).

Fig. 16: Schematic representation of the UBF-aerobic MBR system (213). (Reprinted from Desalination, Simultaneous high-

strength organic and nitrogen removal with combined anaerobic upflow bed filter and aerobic membrane bioreactor, 202 (2007)

114–121, with permission from the copyright holders, Elsevier).

Nevertheless, the system is not effective for the

elimination of other nutrients. Seghezzo et al. (214) claimed

that phosphorus and nitrogen are the main causes of

eutrophication and have detrimental effects on receiving

water. Accordingly, the removal of biological nutrients via

the MBR system has to be enhanced. On a separate note, Bae

et al. (37) proved that the modes of operation do not

influence the stable and high BOD removal of 97% or 98%

for the MBR system. Moreover, membrane separation

resulted in effluent free of SS. Other than these, the high

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BOD:TKN ratio of the influent caused the nitrifying bacteria

not to be cultivated at an adequate rate. Thence, assimilation

or synthesis of new cells devours nitrogen as its nutrient and,

hence, the removal of nitrogen turned out to be fairly high

during the operation, reaching 96%. Furthermore, the

constraint of the biological removal process caused by the

high concentration of phosphorus in the influent resulted in

low removal efficiency. Nonetheless, optimization of the

system enabled the removal efficiency to reach 80%.

restrictions are addressed through the development and

application of new high-rate bioreactors that provide higher

yields of methane for the production of biogas and ensure

better removal of organic matters at shorter HRTs. Special

care has been paid to the integrated anaerobic-aerobic

bioreactors – a combination of anaerobic and aerobic

processes in a single bioreactor – in an effort to minimize the

limitations in terms of odors, minimal sludge production,

and space. Compact integrated bioreactors are anticipated to

treat an extensive range of industrial and municipal

wastewater of high organic pollution. The simple, yet

economical technology generates renewable energy, and has

a remarkable efficiency of treatment. Nonetheless, the

majority of the integrated bioreactors stated in this study are

not implemented on a large scale in the industry. Thus, more

extensive analysis on the performance and capability of

these reactors is essential, particularly on a bigger scale.

Improvements to the system are also fundamental with

recommendations such as utilizing suspended carrier or

packing mediums and installing a biogas capture system. In

general, an anaerobic RBC integrated with aerobic SBR

system can achieve a considerable COD reduction to

produce remarkable amounts of methane gas. High pH

tolerance, reduced sludge formation, and stable COD

removal performance of the UASB–AFB system make this

system beneficial in the biological treatment of industrial

wastewaters of medium-level pollution. Furthermore, the

UASB–AFB configuration emerges as an attractive

alternative from the technical, economical, and

environmental perspectives; especially when space is a

limiting factor. A significant COD removal in the treatment

of highly polluted wastewater with high COD content can be

achieved by integration of an anaerobic UBF and aerobic

MBR systems at a relatively short HRT. Although the

performance of this system is impressive, membrane fouling

is an issue that should be addressed in this process.

Conducting various researches is required for current

biological methods of dairy wastewater treatment in order to

enhance energy production and organic removal efficiency

as well as reduce the operating cost and environmental

impact.

4

Challenges of handling polluted dairy in

future

Demand for dairy products is foreseen to increase in the

future and inevitably to gear up the amount of wastewater

from the industry. Every year, this industry increases their

usage of chemical materials. Serious consequences may be

faced by future generations if no new improved technologies

are developed at a fast rate. As the main user and the

significant generator of wastewater, the dairy industry can

potentially reuse the wastewater. Boilers, cooling systems,

and washing of plants are a few examples for the utilization

of purified wastewater. Additionally, the dairy industry will

enjoy direct benefit from in-house treatment of wastewater

with the prominent reduction in charge of levies for

reception of wastewater. For instance, 70% of the total

savings from AD in the United Kingdom are attributed to the

lower costs for discharge. Apart from that, the dairy industry

will gain indirect benefits from fields that use effluents for

irrigation of pastures. Therefore, efficient management of

wastewater in the dairy industry is essential for these

reasons.

5

Conclusion

For choosing an appropriate wastewater treatment

method, a process assessment in addition to an economic

analysis are required. Effluent composition, concentrations,

volumes produced, susceptibility to treatment, and the

environmental impact are examples of important factors for

selecting an efficient treatment method. The operational

procedures and design must consider the fluctuation in the

quantity and quality of wastewater from the dairy industry.

According to the literature, the biological methods are

revealed as the most economical approach in the organics

removal since they are relatively easier to control.

Nonetheless, the anaerobic methods are also outstanding as

they have low rates of sludge production and have lower

energy requirements. As no particular process of treating

dairy wastewater may comply with the minimum

requirements for discharge of effluent, application of a

combined process that is particularly designed to treat

specific dairy wastewater is necessitated. Over the past

decades, awareness of the anaerobic-aerobic treatments has

been increasing; this is actually attributed to a number of

advantages: (i) low chemical consumption, (ii) low energy

consumption, (iii) low sludge production, (iv) huge potential

for resource recovery, (v) less equipment requirements, and

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