ELSEVIER Desalination 142 (2001) 287-293

DESALINATION

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Role of intermittent aeration in domestic wastewater treatment by submerged membrane activated sludge system

Halil Hasar a*, Cumali Klnacl b, Ayhan UnRi a, Ubeyde Ipek a

"Firat University, Faculty of Engineering, Department of Environmental Engineering, 23119-Elazt~, Turkey Fax + 90 (424) 218-19-07; email: hhasar@firat.edu.tr

blstanbul Technical University, Faculty of Civil Engineering, Department of Environmental Engineering, Ayazaga-istanbul, Turkey

Received 3 September 2001; accepted 26 September 2001

Abstract

In this study, the treatment of domestic wastewater in a lab-scale submerged membrane activated sludge system (sMBR) was investigated under different aeration intervals. The COD concentration of the system effluent varied generally between 5 and 25 mg/l and the COD removal at the organic loads of 0.6- 0.8 kg COD/m3.d was observed to be above 98%. The total phosphorus'content of the filtrate was decreased to a level that was less than 1 mg/1 under the aerobic conditions in which the aeration was continuously made. A dramatic increase in the total phosphorus content of filtrate was observed under the aerobic + anoxic conditions in which the aeration was made at differential intervals. The filtrate was free of suspended solid (SS) and total coliform bacteria and a percent removal of 100 was achieved in terms of these parameters. The influent turbidity removal was 97-99.8%. Generally, the removal of total Kjeldahl nitrogen (TKN) and ammonium nitrogen varied in the ranges of 87.8-99.1% and in the ranges of 89.4-99.8%, respectively. While the nitrate concentrations in the filtrate increased to 26.8 mg/l under the aerobic conditions, it was determined that this value was decreased to 2.4 mg/l under the aerobic + anoxic conditions.

Keywords: Activated sludge; Submerged membrane; Domestic wastewater; Treatment

1. Introduction

The first studies related to the combinations of membrane and activated sludge have concen- trated on the systems developed to replace secondary

*Corresponding author.

clarifiers traditionally used in activated sludge treatment systems [1]. The membrane allows maintaining a high biomass concentration within the system because the problems of sludge bulking and the variation of organic load will not effect the treatment efficiency. This combination

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288 H.Hasar et al. /Desalination 142 (2002) 287-293

has been applied to treat the domestic wastewater in large scale [2,3]. Yet, the application of the combination has been limited due to recycling pumps and transmembrane pumps which require high energy costs. The submerged membrane activated sludge is superior to a crossflow mem- brane bioreactor in regard to the power consump- tion as far suction pressure in a submerged mem- brane is generally lower than that of a crossflow membrane and power consumption ofrecirculation pumps is absent in a submerged membrane activated sludge. New modules which can be integrated directly into the activated sludge tank have been developed [4-9]. These submerged membranes separate the biomass from treated water. This new technology offers several advantages over con- ventional processes such as reliability, compact- ness, and excellent treated water quality.

The conventional biological processes for wastewater treatment use biomass recycling to increase the microorganism concentration in the aeration tank, and therefore the degradation rate of organic and nitrogen compounds. Membrane separation techniques have been applied for biomass recycling in biotechnology [10].

The aims of this study are to develop a sMBR with high efficiency, to compare the continuous aeration and intermittent aeration in terms of phosphorus and nitrogen removals and to mate- rialize the secondary and tertiary treatment in an efficient single unit.

2. Material and methods

The experimental system consisted o f an activated sludge bioreactor in which a membrane module was submerged, as seen in Fig. 1. The influent was taken from the feed tank to the bioreactor by the use of a peristaltic pump. The bioreactor which was filled with activated sludge had a working volume of 50 I. The inoculation was made with the activated sludge of 10 I from Elazig Domestic Wastewater Treatment Plant. The reactor was first operated as a conventional activated sludge for 15 d, and thereafter, the membrane module was placed into the bioreactor. An air diffuser was placed under the membrane module to prevent or reduce the fouling of mem- brane and to maintain an aerobic environment for the normal growth of activated sludge culture. A

FeedTank I

Mixer

(a)

lllll Membrane .?.:.-..!..-: D~se~

Suction Pump

Ak Pump

Fig. 1. Experimental set up for submerged membrane activated sludge system.

C D

t 2 D

- m

H. Hasar et al. / Desalination 142 (2002) 287-293 289

stirrer was used to ensure complete mixing of the influent and the sludge. The filtrate was drawn from the membrane module through a set of high transmembrane pressures which consisted of a piston and a compressor. A tubular membrane module of 0.03 txm pore size with 0.3 m 2 surface area was converted to a plate module and used in this study.

The hydraulic retention time (HRT) and the sludge retention time (SRT) were varied between 8-24 h and between 30-100 d, respectively. The operation conditions are given in Table 1.

The flux, temperature and transmembrane pressure were monitored by daily measurements. Turbidity was measured with a turbidity meter (Model 965-10, Orbeco analytical System Inc., USA). The parameters such as the chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), ammonium nitrogen (NH4-N), nitrate (NO 3- N), total phosphorus (TP), ortophosphate (Orto-P) and suspended solid (SS) ofinfluent and filtrate, and MLSS and MLVSS in bioreactor were analyzed according to Standard Methods [11].

3. Results and discussion

The influent and filtrate COD values of the sMBR were operated for 100 d (Fig. 2). The aver-

Table 1 Operation conditions of sMBR

Operation Aeration Operat ion Sludge age, period, d period, d d

1 - 1 5 Continuously 1-30 30 *~

17--46 30 min on 30 rain off 30-50 50 o

o 47-64 60 min on

120 min off 50-75 75 65-75 60 rain on

90 rain off

75-100 60 min on 75-100 100 75 min off

10000

1000

E 100 © r.)

10

+ Influent

Filtrate

L i i r r i

10 20 30 40 50 60 70 80 90 100

Operation time, d

Fig. 2. The variations of COD ofinfluent and filtrate during the operation time.

age flux was approximately 5 l/m2.h. The influent COD fluctuated from 140 and 1150 mg/l. However, the filtrate COD was maintained at a low level, generally less than 25 mg/1. Taking the 100 days of the experiment as a whole, the removal per- centage of COD varied in the range of 83.3% and 99.3%. It was shown that the best results were obtained at the SRT of 50 d. The COD removals were between 83.3-97.8% at the organic loads in the range of 0.016 and 0.2 kg COD/m3.d, between 90.5-99.3% at the organic loads in range of 0.2 and 0.4 kg COD/m3.d, above 94% at the organic loads in range of 0.4 and 0.6 kg COD/m 3.d, and above 98% at the organic loads in range of 0.6 and 0.8 kg COD/m3.d (Fig. 3). This clearly

100

98

96 ~ •

94

92

90 . •

88 •

86 •

84

82

80

0

i i i

0.2 0,4 0.6

Organic load, kg COD/m3d

t

0.8

Fig. 3. The removal of COD vs. organic load.

290 H.Hasar et al. / Desalination 142 (2002) 287-293

----or- Influent TP * Filtrate TP • o 20 18

- - 16

~ 14

~ 12

10

8

6

2

o 1 4 7 10 13 17 25 32 39 46 57 64 71 80 90 97

Operation time, d

Fig. 4. The variations of TP and orto-P ofinfluent and filtrate during the operation.

Influent orto-P Filtrate orto-P 20 18 16 © 14 o a

8 ~

4

2

0

indicated that the sMBR had a potential in treating high-strength urban wastewater.

Fig. 4 indicates the variations of total phos- phorus (TP) and orto-phosphate (orto-P) in the influent and filtrate. Although the biomass in sMBR was between 770 and 1900 mg MLSS/1 during the first operation period (between 1 and 15 d), it was observed that total phosphorus was less than 1 mg/l, except on the first day. In this period, the bioreactor was continuously aerated and the dissolved oxygen concentration varied between 1.90 and 4.00 mg/l. During the second operation period (between 17 and 46 d) in which the aeration was made with 30 min intervals, while the filtrate TP concentrations changed between 0.98 and 2.95 mg/1, the filtrate orto-P concen- trations varied between 0.75 and 2.41 mg/1. During the third operation period (between 47 and 64 d) in which the aeration was made in the form of 60 min on and 120 min off, it was shown that the filtrate TP and orto-P concentrations increased dramatically and reached to 6.74 mg/1 and 3.8 mg/l, respectively. During the fourth operation period (between 65 and 75 d) in which the aeration was made in the form of 60 min on

and 90 min off, the filtrate TP and orto-P concen- trations decreased again, and varied between 1.96-3.78 mg/l and between 1.2-2.9 mg/l, res- pectively. During the fifth operation period (bet- ween 76 and 100 d) in which the aeration was made in the form of 60 min on and 75 min off, the filtrate TP and orto-P concentrations varied between 2.06--4.02 mg/l and between 0.9-2.9 mg/l, respec- tively. TP and orto-P concentrations of filtrate showed large variations at the period in which the aeration was made at intervals. The phosphorus compounds were absorbed by aerobic micro- organisms at aerobic conditions, and then the phosphorus released again at anoxic conditions.

Fig. 5 illustrates the evolution of total Kjeldahl nitrogen (TKN) concentration with operation time. When the influent TKN concentration changed from 27.9 to 55.0 mg/l, the filtrate con- centration reduced the low level of 1.2-6.7 mg/l at continuously aeration period. While the influent TKN concentration varied between 36.96 and 77.56 mg/l at the second period, the filtrate con- centration reduced the level of 1.46-11.42 mg/1 at this period. While the filtrate TKN concentra- tion varied between 0.67-5.04 mg/l at the third

H. Hasar et aL / Desalination 142 (2002) 287-293 291

8O

_ 7 0

60 o

~ 50

"~ 40

~ 3o ~ 20

o

~ Influent

Filtrate

- i i r J i

20 40 60 80 100

Operation time, d

Fig. 5. The variations ofTKN ofinfluent and filtrate during the operation time.

period in which the aeration was made in the form of 60 min on and 120 min off and the anoxic conditions were dominant, the filtrate concen- tration varied between 3.25--4.93 mg/1 during the fourth period and between 2.91-5.6 mg/l during the fifth period. The overall removal varied in the range of 80.8% and 99.6%.

Fig. 6 illustrates the evolution of NH4+-N concentration with the operation time. No NH4 +- N measurement was carried out at the first period. While the influent NH4+-N varied in range of 20.1 and 47.1 mg/l during the operation of 100 d, the filtrate concentrations varied between 0.05-1.93 mg/I during the second period, 0.5-3.6 mg/1 during the third period, 2.18-3.44 mg/l during the fourth period and 1.6-3.5 mg/l during the fifth period. The overall removal varied in the range of 89.4-99.9%, indicating that the NH4+-N in the influent had been deeply nitrified in the sMBR. This was mainly due to two reasons. First, as the nitrifying population was completely confined within the bioreactor, these autotrophic nitrifiers having long generation times were forced to proliferate speedily without any loss [ 12]. Secondly, as the sludge production was low in sMBR processes, nitrifiers in the bioreactor faced less menace from those heterotrophic bacteria.

The influent nitrate varied between 0.01 and 3.9 mg/l during the operation time for 100 d (Fig. 7).

50

40

z"

lO

* Influent

o - Filtrate

0 v , i i i i

0 20 40 60 80 100

Operation time, d

Fig. 6. The variations of NH4*-N of influent and filtrate during the operation time.

30

#~ J~ ~ l n f l u e n t

20

0 1- ~ - i i i i - i i - i - 1 7

0 10 20 30 40 50 60 70 80 90 100

Operation time, d

Fig. 7. The variations of NO3--N of influent and filtrate during the operation time.

At the first period (continuously aerobic), the filt- rate concentration increased to 8.66-26.55 mg/1. The nitrate concentration in the filtrate decreased at intermittent aeration conditions for denitrifi- cation. While the filtrate concentration varied between 8.2 and 26.8 mg/l at the first period, the nitrate concentration in the filtrate was decreased to 2.4 mg/1 during the fourth period in which the aeration was made in the form of 60 min on and 90 min off. This means that both nitrification and denitrifieation may successfully be carded out into single unit through intermittent aeration.

As illustrated in Fig. 8, there was no suspended solid (SS) detected in the sMBR effluent during

292 H.Hasar et al. / Desalination 142 (2002) 287-293

Influent SS o Filtrate SS A Influent Turbidity .t Filtrate Turbidity

1000 " A'" ~ A ~ A ~ / x a ~ ' ~ " ~ ~ " ~ ~ 100

} l0 I00

lO e . ,

0.1

1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.01

1 5 9 13 19 29 39 48 57 67 77 90 100

Operation time, d

Fig. 8. The variations of SS and turbidity of influent and filtrate during the operation time.

w. e -~

z

the 100-d experiment, even though a great fluctu- ation was observed in the inf luent SS concentration. The efficiency of the removal of SS remained as high as 100%, which demon- strated better separation effect of the ultrafiltra- tion membrane module in submerged membrane activated sludge system than that of the settling tank in classic activated sludge system [13]. Although a great fluctuation was observed in the influent turbidity, which was between 13-62 NTU, this value was less than 1 NTU in the filtrate (Fig.8).

The coliform removal is an important quality criteria. For example, the Building Services Research and Information Association and the EPA (Florida) set a standard of zero fecal coliforms per 100 ml for water that is to be reused, e.g. for WC flushing [14]. Total coliform bacteria were not detected in the filtrate, so disinfection of the wastewater is not needed. The membrane module used for solids separation should also be able to combine further tertiary treatment steps normally achieved by additional process units such as filtration or disinfection.

The averaged ratio of MLSS/MLVSS in the bioreactor was observed to be 0.76. The MLSS con- centrations varied between 755 and 1770 mg/1 at

the first period, between 1925 and 5765 mg/l at the second period, between 6750 and 8860 mg/1 at the third period, between 8810 and 14650 mg/l at the fourth period and between 7570 and 9400 mg/l at the fitch period. At the fourth period, the bioreactor was inoculated at large scale, and excess sludge was wasted from bioreactor due to rapid membrane fouling.

4. Conclusions

From the technical point of view, reclamation of urban wastewater by sMBR is basically appli- cable as this new technology offers several advan- tages such as reliability, compactness and excellent treated water quality over conventional processes.

It was shown that the sMBR had a potential in treating high-strength urban wastewater. Total phosphorus was less than 1 mg/l at the period in which the bioreactor was continuously aerated. It was shown that the filtrate TP and orto-P con- centrations increased dramatically at the periods of intermittent aeration because the phosphorus released again at anoxic conditions. The nitrifi- cation process carried out successfully. The nitrate concentration in the filtrate decreased to 2.4 mg/1 at the period in which the aeration was made in

H. Hasar et al. / Desalination 142 (2002) 287-293 293

the form of 60 min on and 90 min off, indicating that the nitrate which occurs by ni tr i f icat ion process had been deeply denitrified in the sMBR. Both nitrification and denitrification could suc- cessfully be carried out into single unit through intermittent aeration. It was shown that the filtrate was free o f SS and total coliform bacteria and turbidity was less than 1 NTU. The membrane module should also be able to combine further tert iary t reatment steps normally achieved by additional process units such as filtration, dis- infection and nitrification/denitrification.

5. Acknowledgment

This Study was supported by the Research Foundation o f Firat University under project No. FUNAF-390.

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