Effect of Electromagnetic Field on Low Dissolved Oxygen Wastewater Treatment

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 3, Pages: 1101-1106

J. Environ. Treat. Tech.

ISSN: 2309-1185

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

Effect of Electromagnetic Field on Low Dissolved

Oxygen Wastewater Treatment

Nulhazwany Abdul Malik, Khalida Muda, Nur Syamimi Zaidi, Mohamad Darwish*, Ahmad

Hanis Omar@Omri

School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), 81310, Johor Bahru, Malaysia

Received: 13/03/2020

Accepted: 07/07/2020

Published: 20/09/2020

Abstract

Activated sludge (AS) system is a biological treatment process that is widely applied in municipal wastewater treatment. Concentration

of the dissolved oxygen (DO) is one of the important parameters that may influence the performance of AS system. Thus, certain levels of

DO in AS systems should be maintained to achieve high efficiency of pollutants removal. However, the energy consumption of aeration stage

represents approximately 50% of total demand of AS system. Therefore, reducing aeration energy would improve the feasibility of AS

process. This study investigated the enhancement of AS process under low DO condition using electromagnetic field (EMF). The AS was

exposed to EMF at intensity of 3 mT with DO concentrations of 1 and 2 mg/L for 24 hours. The impact of EMF on the biomass concentration,

settling velocity, sludge volume index and pollutants removal were thoroughly investigated. The results indicated significant improvement

in the physical properties of AS exposed to the EMF, which resulted with high accumulation of biomass concentration. The settling velocity

and sludge volume index value of the biomass at the end of the experiment were 95 m/h and 72.6 mL/g, respectively. The reactor exposed to

EMF under 2 mg/L of DO showed the highest removal efficiency of chemical oxygen demand (80%) ammonia (97%), nitrite (99%), and

total nitrogen (84%). Additionally, it was proved that EMF could enhance the settleability of the AS in the treatment system.

Keywords: Electromagnetic field, Activated sludge, Biomass, Nitrate, Ammonia nitrogen

Introduction¹

Generally, most of the mentioned treatment processes have shown

significant improvements under specific magnetic applications (4

Activated sludge process is one of the aerobic biological

treatment methods, which is employed as a core process in more

than 90% of the municipal wastewater treatment plants (12). Yet,

due to the fluctuation of wastewater quality and flow,

conventional activated sludge shows a variety of drawbacks,

including sludge expansion, loose flocs structure as well as

biomass deficiency (13). In addition, the energy consumed by the

aeration process represents approximately 50% of the total energy

usage of the activated sludge process, which can be dramatically

reduced by decreasing the operating dissolved oxygen (DO)

concentration (14). However, according to Holenda, Domokos

The rapid development of humans' communities leads to huge

increase in wastewater generation (1). Population explosion and

expansion of urban areas raise the adverse impacts on water

resources, especially in regions where natural resources are

restricted. This phenomenon reflects the significant growth in

default volumes of wastewater, which makes it an urgent

imperative to develop effective and affordable technologies for

wastewater treatment (2).

Recently, a tremendous increase in the applications of

electromagnetic fields (EMFs) appeared in different domains

including therapeutic and diagnostic medicine, environmental

managements, and industrial procedures (3). Magnetic

technology is a physical treatment technique that was introduced

to avoid the consumption of chemicals such as polyphosphates or

corrosive substances, which are expensive and can be harmful to

the environment and most importantly, human health. This

technology has been implemented in various ways through the

application of either permanent magnets or high-gradient

magnetic separation (HGMS) in combination with magnetic

seeding or magnetic adsorption (4). To date, magnetic field was

applied for the removal of heavy metals (5), organic compounds

(15), DO levels in the aerobic reactors have significant influence

on the behaviour and activity of the heterotrophic and autotrophic

microorganisms that live in the AS system.

Limitation of DO concentration often results in poor

flocculated sludge and more turbid effluents (16). On the other

hand, an excessively high DO, which requires a high air flow rate,

leads to a high energy consumption, and may deteriorate the

sludge quality as well. Moreover, high DO concentration in the

internally recirculated water reduces the efficiency of

denitrification process. To the best of our knowledge, the

enhancement of municipal wastewater treatment using EMF

(6, 7), nutrients consisting of nitrogen and phosphorus

compounds (8, 9) and turbidity and suspended solids (10, 11).

Corresponding author: Mohamad Darwish, School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM),

1310, Johor Bahru, Malaysia. Email: sjmohamad@utm.my.

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2020, Volume 8, Issue 3, Pages: 1101-1106

under the condition of low DO concentration was never tested

before. The present study was therefore aimed at investigating the

feasibility of applying an EMF in maintaining high effluent

quality in municipal wastewater treatment under low

concentration of DO. Specifically, objectives of this study are: (1)

to identify the effect of EMF on the physical properties of

municipal wastewater (i.e. mixed liquor suspended solids

(MLSS), mixed liquor volatile suspended solids (MLVSS),

settling velocity and sludge volume index (SVI)); and (2) to

analyse the impact of EMF on the removal of chemical oxygen

demand (COD), ammonia, nitrite, nitrate and total nitrogen (TN).

Materials and methods

.1 Municipal wastewater characteristics

Figure 1: Schematic diagram of the experimental setup (1: DC Power

supply, 2: Crocodile clip, 3: Column, 4: Stone diffuser, 5: Copper coil, 6:

Retort stand, 7: Tube, 8: Aquarium pump)

Raw samples of municipal wastewater, as well as sludge

biomass, were collected twice a week from a sewage treatment

plant and stored under 4°C. Table shows the main

characteristics of municipal wastewater used in this study.

.3 Analytical methods

The concentrations of MLSS and MLVSS were measured

based on Methods No. 2540D and 2540E, respectively (APHA,

005). The settling velocity was determined by recording the

average time taken for the individual sludge to settle at a certain

height in a glass column filled with tap water (17). In this study,

a 16 cm glass column was used to test the settling velocity. Total

nitrogen, COD, nitrite, and nitrate concentrations were measured

using HACH (DR 6,000) spectrophotometric standard methods,

while ammonia nitrogen was analysed using Nessler method.

Table 1: Characteristics of municipal wastewater

Parameter

Temperature (°C)

Values

22-27

8.1

COD (mg/L)

Ammonia (mg/L)

Total Nitrogen

103-431

18.25-41.5

24-74

(mg/L)

.2 Experimental setup

Figure 1 shows a schematic diagram of a sequential batch

Results and discussion

.1 Effect of EMF on wastewater physical properties

reactor (SBR) used in this study. The SBR consists mainly of

three identical glass columns (Column A, B and C) with a total

volume of 1,300 mL for each column. A copper coil was attached

to Column B and C (Figure 1), which supplied the EMF with an

intensity of 3 mT, while Column A was not exposed to EMF and

kept as a control. Each column was supported with a stone air

diffuser located at the bottom of the column, to provide the system

with the required aeration. The concentrations of DO in Column

A, B and C were maintained at 3, 2 and 1 mg/L, respectively. The

level of DO concentration was monitored using YSI dissolved

oxygen meter (USA). The aeration intensity was continuously

monitored to ensure that the level of DO concentration was within

the specified range. During the start-up period, 200 mL of

activated sludge and 800 mL of raw municipal wastewater were

added into each column, to compose a working volume of 1,000

mL with a total biomass concentration of 2,000 mg/L. The cycle

time of the SBR was 24 hours, consisting of 2 min for feeding

reaction, 23.4 h for aerobic reaction, 30 min for settling and 2 min

for discharge.

The physical properties observed in this study included

biomass concentration, settling velocity and SVI.

.1.1 Biomass Concentration

The profile of biomass concentration (MLSS and MLVSS) is

given in Figure 2(A) and (B). Generally, it can be seen that the

MLSS in Column B was higher than in Column A and C. The

average of MLSS for Column B was 2425±33.2 mg/L, while in

Column A and B the average was 1,720±28.3 and 1,625±7.2

mg/L, respectively. At steady state, MLVSS for Columns A, B

and

C were 368.6±5.8, 438.6±5.8 and 238.6±5.8 mg/L,

respectively. Throughout the experiment, both MLSS and

MLVSS in Column B showed stable reading. Meanwhile, in

Columns A and C, sudden drops were observed in the middle of

the experiment. The high MLSS and MLVSS in Column B was

probably due to the effect of EMF applied to Column B.

Principally, magnetic field could influence the microbial

community composition and metabolisms, which could further

affect the biomass characteristics (18-20).

According to

For the physical characteristic’s determination, the samples

were collected once a week at the middle part of the glass column,

and were used to analyse MLSS, MLVSS, settling velocity and

SVI. With regards to the performance analysis, the samples were

collected twice a week and centrifuged for 5 minutes at 6,000

rpm. The supernatant was used to measure the removal

performance of COD, ammonia, nitrite, nitrate and total nitrogen.

Zieliński, Cydzik-Kwiatkowska (21), this higher concentration in

biomass resulted from the improved absorption and coagulation

in the sludge particles due to the effect of magnetic field. In

Column A (no EMF exposure), the molecules moved in random

manner, while in Column B and C, the exposure to magnetic field

allowed the molecules to align easily according to their positive

and negative charges. Consequently, the molecules were arranged

orderly, thus able to induce coagulation (22).

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2020, Volume 8, Issue 3, Pages: 1101-1106

Figure 3: Settling velocity of activated sludge

Sludge volume index is used to indicate the settling ability of

sludge. Generally, dense and compact flocs have high settling

velocity with low SVI, which shows good settling properties (4).

Accordingly, magnetically exposed activated sludge should have

high settling velocity and low SVI, unlike unexposed sludge (26,

7). Figure 4 shows the results of settling SVI in the three

columns. Fundamentally, the increase in settling velocity would

lower the SVI value. It was observed that SVI values for Column

B (72.6±1 mL/g) was much lower than for Columns A and C

(139.7±2 and 122.3±1 mL/g, respectively). Probably, this could

be referred to the effect of magnetic field that might had increased

the collision among the particles, which lead to the formation of

Figure 2: Profile of biomass concentration (A) MLSS (B) MLVSS in all

columns

larger flocs. Since Column

B displayed higher MLSS

concentration, higher chances of collision occurred, which

encouraged the particles to settle rapidly.

Despite the condition of low DO concentration, Column B

was able to retain high biomass concentration compared to

Column A and C. As more sludge coagulated to each other,

higher settling ability was maintained in Column B. In addition,

less sludge washout was observed resulting in higher biomass

concentration in the Column. The increase in sludge biomass

under the effect of magnetic field was also observed by

Łebkowska, Rutkowska-Narożniak (7) and Zaidi, Sohaili (4).

However, Column C showed the lowest MLSS and MLVSS

throughout the experiment. This may be due to the condition of

low oxygen concentration where the mixing of biomass was not

sufficient. As a result, less biomass was exposed to the magnetic

field, thus no effect of magnetic field was detected.

Figure 4: Profile of SVI for all columns

.1.2 Settling Velocity and Sludge Volume Index (SVI)

Figure 3 shows the results of settling velocity obtained from

all columns. On the average, settling velocities of the

magnetically exposed activated sludge in Column B and C were

.2 Removal Performance

.2.1 Chemical Oxygen Demand (COD)

The profile of COD removal performance is given in Figure

. It was found that the efficiency of COD removal was similar in

5±0.7 and 67±1.2 m/h, respectively. Compared to unexposed

activated sludge in Column A (53.4 ± 3.8 m/h), these values were

greater. As stated by Sears, Alleman (23), changes in settling

velocities were likely associated with an increase in the settled

sludge density. Here, the difference in settling velocity between

magnetically exposed and unexposed activated sludge might have

influenced the size and shape of the flocs, possibly due to the

applied force of magnetic field (24, 25). In fact, the application of

EMF had improved the sedimentation of sludge, owing to the

paramagnetic characteristics of iron existed in wastewater (20).

Basically, this effect varies with different iron concentrations,

magnetic field strength, magnet types as well as Column

operation conditions.

all columns, with a slightly higher percentage in Column B. Most

of previous studies that applied magnetic field intensity within the

range of 5-460 mT illustrated that magnetic field is an

intensifying factor for organic substrate degradation (6, 9, 28). In

the current study, the EMF intensity was as low as 3 mT, which

explains the observed improvement in COD removal. Principally,

magnetic field could stimulate the organism biodegradation

ability of aerobic bacteria in the activated sludge, probably due to

the increase in the concentration of extracellular enzymes

distributed on the bacterial surface when exposed to magnetic

field (6, 20).

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2020, Volume 8, Issue 3, Pages: 1101-1106

biomass loss in the effluent could have occurred during the

experiment. The losses in biomass could have reduced the

presence of aerobic microorganisms responsible in degrading the

ammonia, and this clarified the inefficient removal of ammonia

occurred in Column C.

Figure 5: Profile of COD removal performance

At the initial stage of the experiment, COD removal tended to

fluctuate due to inconsistent COD influent concentrations that

were fed into the systems. On the 25th day of the experiment,

COD removal in Column C witnessed a severe drop with only

.5% of COD was removed. This could be due to the sludge that

has been dominated by the proliferation of filamentous

microorganisms. At low DO levels, filamentous microorganisms

have a greater tolerance compared to floc-forming bacteria.

Consequently, filamentous microorganisms could actively

proliferate as their relative biomass increases, thus limiting the

growth and activity of aerobic microorganisms to biodegrade the

COD (29). Apparently, applying EMF with low intensity was not

sufficient to enhance the removal of COD under low DO levels.

After 35 days, the concentration of COD effluent was almost the

same in the control and magnetically exposed columns. However,

the COD removal in Column B was always higher than Column

A and C, reaching up to 90% of COD removed. Furthermore, the

average COD concentration throughout the experimental period

for Column A, B and C were 46.4 ± 13, 35.4 ± 9 and 43.7 ± 13

mg/L, respectively.

Figure 6: Profile of ammonia removal efficiency (A) and concentration

(B) in all columns

.2.3 Nitrate, nitrite and total nitrogen

The variation of nitrite, nitrate and total nitrogen in the

columns are presented in Figure 7, 8 and 9, respectively. Based

on Figure 7, the nitrite removal for Column B was high compared

to Columns A and C. Moreover, Column B was able to be

.2.2 Ammonia nitrogen

achieved 100% removal starting from the 45 day until the end of

The results of the removal performance of ammonia nitrogen

the experiment. On average, the nitrite removal for Column A and

Column C were 93.8 ± 9 % and 65.5 ± 5% respectively. A

consistent low effluent concentration was indicated in Column A,

while severe fluctuations were observed in Columns B and C. In

general, the average concentration of effluent nitrite for Column

A throughout the experimental period was 0.5±0.7 mg/L while

Column C was 3.5±0.6 mg/L.

were given in Figure 6. In general, Column B showed higher

removal performance compared to Column A and C. During early

stage of the experiment, higher removal was observed in Column

C, indicating that the system was not in stable condition yet.

Starting from day 25, Column B showed higher removal

performance compared to Column C until the end of experiment.

On average, Column B exhibited the highest ammonia removal

Theoretically, nitrite is produced through the oxidation of

ammonia by Nitrosomonas and being removed further by

Nitorbacter into nitrate (30). In the current study, the low effluent

concentration achieved by Column B could be explained by the

theories stated by Tomska and Wolny (9), which stated that

magnetic field has a potential to enhance the growth and activity

of nitrifying bacteria, thus allowing it to undergo the effective

nitrification process. Conversely, the efficiency of nitrite removal

in Column C was much lower at the end of the experiment. This

was most likely because of the low EMF intensity applied, which

did not significantly enhance the activity of nitrifying bacteria in

the low DO condition. Also, low DO concentration reduced the

mixing intensity, which caused the activated sludge to remain

statically at the bottom of the glass column. Eventually, this

situation resulted in restriction of the movement of sludge

(

99±1%) followed by Column A (98±1%), while the lowest

removal was in Column C (83±9%). Although the Columns were

aggravated by low DO condition, Column B was able to maintain

higher removal of ammonia. This finding agrees with the

phenomenon that the magnetic field has an effect in enhancing the

biological activity of ammonium oxidizing bacteria (20), which

improved the degradation activity of influent ammonia

concentration, resulting in high removal percentage in Column B.

Figure 6(B) shows the concentration of ammonia in the

influent and effluent of the three columns. At steady state, the

average effluent concentration of Columns A, B and C were

.6±0.4, 0.4±0.3 and 5.3±0.8 mg/L, respectively. Besides, rapid

fluctuations in effluent concentrations were observed in all

columns. The obtained results suggested that unstable amount of

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2020, Volume 8, Issue 3, Pages: 1101-1106

perpendicularly towards the magnetic field lines. Consequently,

less sludge particles were exposed to the magnetic field, hence,

resulted in higher effluent concentration and lower removal

percentage.

of the raw wastewater used in the study. As shown in Figure 9, it

can be seen that the average removal of total nitrogen for Column

A, B and C was 41.8 ± 7%, 83.8 ± 1% and 63 ± 1.7%,

respectively. The higher treatment performance indicated by

Column B could be owing to the present of the Nitrosomonas and

Nitrobacter bacteria, which became more active under the effect

of the EMF and exhibited strong nitrogen oxidizing activity (8,

0). Moreover, the steady state of total nitrogen concentration was

achieved after 40 days for all columns.

Figure 8: Influent-effluent nitrate concentration in Column A, Column B

and Column C

Figure 7: Profile of the nitrite removal performance (A) and

concentration (B) in all columns

Essentially, oxidation of nitrite into nitrate requires sufficient

source of oxygen supply. However, the current experiment was

set in a condition of low oxygen concentration, in order to study

the influence of magnetic field to overcome such condition. As a

result, this condition affected the oxidation process, causing

obvious variations in nitrite removal efficiency in Column C.

Apart from that, the inconsistent concentration of nitrite in the

influent contributed to the obtained outcome.