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Environmental consideration of dairy wastewater treatment using hybrid sequencing batch reactor

Mahmoud Nasr,* Ahmed Elreedy, Amr Abdel-Kader, Walid Elbarki and

Medhat Moustafa

Department of Sanitary EngineeringAlexandria University

Alexandria 21544, Egypt

Key Words: Attached growth, dairy wastewater, hybrid system, organic loading rate, suspended biomass

*Corresponding authorEmail: mahmmoudsaid@gmail.com

ABSTRACT

INTRODUCTION

In this study, biological treatment of dairy wastewater industry was investigated. Two units, con-ventional sequencing batch reactor (CSBR) and hybrid sequencing batch reactor (HSBR) were

-1 -1operated, with the same source at organic loading rates (OLRs) of 1.75, 2.33, and 3.50 g COD L d . The seed sludge was collected from a treatment plant situated in an Egyptian Starch, Yeast and Detergents Company. The operating sequence of both CSBR and HSBR systems consisted of five steps: fill (2 h), react (aerobic: 19 h and anoxic: 1 h), settle (1 h), draw (0.5 h) and idle (0.5 h). In

2 -3the hybrid system, plastic balls with specific surface area of 900 m m and porosity of 94% were randomly equipped at the bottom of the reactor. Results showed that, in the two reactors, the chemical oxygen demand (COD) and total Kjeldahl nitrogen (TKN) removal efficiencies decreased with increasing OLRs. However, better performance was observed in the hybrid system. In the

-1 -1HSBR, at increasing the OLR from 1.75 to 3.50 g COD L d , COD and TKN removal efficiencies decreased from 98 to 92% and from 92 to 83%, respectively. Similarly, in the CSBR, removal of COD decreased from 88 to 76% and removal of TKN decreased from 71 to 54%, at the same range of OLR. Moreover, the hybrid system obtained higher specific substrate utilization rates (0.096-0.13

-1 -1h ) and biomass growth yield coefficient (0.25-0.67 g VSS g COD), as compared to the CSBR. Good performance of the HSBR was explained by the wide variation of microbial community dominated in the system, including suspended heterotrophs and attached nitrifiers. The conventional treatment systems could be upgraded to handle higher organic loads by adding packed media in the biological units. .

In Egypt, wastewater treatment is mainly con-sidered a vital and strategic objective issue [1]. In this context, the government has an ongoing investment programme aiming at protecting the country's finite water resources [2]. Moreover, increasing of urbaniza-tion and industrial development has caused huge in-creases in the consumption of water resources as well as deterioration in their quality. However, lack of fi-nancial support is a great obstacle towards constructing new wastewater treatment plants. Consequently, it is worthwhile upgrading the existing treatment units to handle the rapid increase in flow as well as organic loads. Biological treatment process of wastewater is usu-

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ally carried out by microorganisms that can be classi-fied as being either suspended or attached growth [3]. Moreover, the wastewater treatment efficiency is directly related to the total amount of bacteria inocu-lated in the bioreactors [4]. In the recent year, there has been an increasing interest in combining the two different processes (suspended and biofilm growth) as a hybrid system by adding carrier media into the biological reactor [5]. Adding biomass carriers in the biological units is a promising method to obtain higher sludge concentration than in the conventional treat-ment process, but without a significant increase of the sludge load to the settling phase [6]. Various studies have been conducted on hybrid systems, concerning the carrier media as being fixed [7] or freely moving inside the reactor volume [8]. In the attached systems,

Sustain. Environ. Res., 24(6), 449-456 (2014)

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the biofilm is naturally stacked on the carrier media, resulting in retention of slow-growing microorganisms, such as nitrifiers [9]. By maintaining both suspended heterotrophs and attached nitrifiers in the same reactor, the consumption of dissolved organic substrates would be prompted in the suspension, avoiding competition between nitrifiers and heterothophs in the biofilm [10]. Dairy wastewater contains high concentrations of chemical oxygen demand (COD), biological oxygen demand (BOD) and nutrients (N and P) [11]. Such wastewater should be strictly treated prior to its dis-charge into receiving water body to prevent contami-nation of water resources and environmental deteriora-tion. Previous studies investigated the treatment of dairy wastewaters using anaerobic biological methods including up-flow anaerobic sludge blanket reactor and anaerobic filters [12]. However, anaerobic digestion needs post-treatment to remove remaining contaminants including organic matter and nitrogen species [13]. Among biological treatment processes, sequencing batch reactor (SBR) has been recognized as a promising technology for treating dairy waste-water [14]. SBR is a highly flexible system used for the biological treatment of industrial wastewaters allowing efficient COD and BOD removal [15]. In this system the conventional biological reactor and the settling tank are combined together, where equal-ization, aeration, and clarification can be achieved in the same unit [16]. Researches on dairy wastewater treatment using SBR can be found in previous studies [17-19]. In this study, two different systems were fed from dairy wastewater source; conventional SBR (CSBR) and hybrid SBR (HSBR). The media used in the hybrid system was plastic balls with specific surface area of

2 -3900 m m and porosity of 94%. Performance of the two systems was investigated at organic loading rate

-1 -1(OLR) range: 1.75-3.50 g COD L d . Additionally, kinetic parameters were estimated.

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MATERIALS AND METHODS

1. Substrate and Inoculum

The wastewater was collected weekly from a real dairy wastewater industry situated in Borg El-Arab city, Alexandria. The substrate was stored into an

3equalization tank (35 m ) in order to secure constant concentrations during the day. The substrate was diluted with tape water and necessary nutrients for microorganism growth were added. The main compo-sition of the influent wastewater is listed in Table 1. The calculated BOD/TKN/TP ratio was 90/2/1. Moreover, BOD/COD ratio was about 0.77, meaning that most of the organic compounds in the feed are biodegradable. The seed sludge was collected in 10 L container

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Table 1. Characteristics of synthetic wastewater used in the experiments

-1COD (g L )

-1BOD (g L )

Nitrogen-1(mg L )

Soluble

Particulate

Total

Soluble

Particulate

Total

NH -N4

NO -N3

TKN

aRange

3.2-7.0

0.8-3.0

4.0-10.0

2.3-5.2

0.4-1.8

2.7-7.0

40-60

0.1-0.2

70-110

50-70

70-500

4-7

34-35

bFeed

5.6

1.4

7.0

4.5

0.9

5.4

80

0.1

120

60

260

7.4

25-32a bDairy wastewater industry composition; Feed composition

-1TP (mg L )-1O&G (mg L )

pH

Temp. (°C)

from a treatment plant situated in the Egyptian Starch, Yeast and Detergents Company, Alexandria. The collected sludge was settled for thickening before inoculating in the reactors. The sludge characteristics

-1were 3 g volatile suspended solids (VSS) L , VSS/total -1suspended solid (TSS): 0.78, 7 g COD L and pH: 7.4.

The HSBR was continuously fed for about one month before starting the experiments to enrich the ball carriers with attached biofilms.

2. Experimental Set-up

Lab-scale apparatus was installed in the laboratory of Sanitary Engineering Department - Faculty of Engineering at Alexandria University. The pilot plant consisted of two units (CSBR and HSBR) operated with the same feed wastewater. The squared reactors (45 cm in length and 35 cm in depth, with a total working volume of 45 L) were made of Plexiglas material. Diffused aeration system was fixed at the units' bottom, and the air bubbles were supplied from an air compressor (DARI-100/280DEC-2 HP-1.5 kW-220 V, 50 Hz). Dissolved oxygen concentrations were

-1maintained at 3.5-4.5 mg L . For the HSBR system, polyvinyl chloride plastic balls (400 balls), with a total

2surface area of 2.21 m were randomly placed at the bottom of the reactor. Each ball has the following properties: size: 2.5 cm in diameter, specific surface

2 -3area 900 m m , and porosity 94%. The experimental procedure was operated at three

-1different OLRs with influent 7 g COD L . During the first run (0-45 d), the feeding flow was equal to 11.25

-1L d , which is corresponding to an OLR of 1.75 g -1 -1COD L d . With the aim of investigating the system

performance at higher loading rates, the flow was

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Nasr et al., Sustain. Environ. Res., 24(6), 449-456 (2014)

Parameter

451

-1further increased to 15 and 22.5 L d , achieving OLR -1 -1of 2.33 and 3.50 g COD L d , respectively. The pilot

plant was investigated for carbon and nitrogen removal at the three operating OLRs.

3. SBR Operation

The operating sequence of both CSBR and HSBR systems consisted of five steps: fill, react, settle, draw and idle. During the fill period, feed wastewater was added to the reactor within 2 h. The fill process was fully aerated and allowed the wastewater level to rise from 50-55% of capacity (at the end of the idle period) to 100%. After that, aeration was continued again for 19 h. The chosen aeration time was based on the optimal operating mode observed by Neczaj et al. [14]. After aeration ceases, the system was kept under anoxic condition for 1 h. Then, bio-sludge was allowed to separate from the liquid under quiescent conditions (settle period: 1 h). After clarifying the reactors, the supernatant (35% V) was withdrawn within 0.5 h. In the idle step (0.5 h), excess biomass was wasted from the bottom of the reactors at values around 10-15% of V. After that, fresh wastewater was filled into the reactor and the above operating program was repeated again. Samples were withdrawn from the reactors at the beginning and at the end of each cycle for analysis. The BOD, SS, and total Kjeldahl nitrogen (TKN) were determined by using the Standard Methods [20]. COD was determined by the dichromate method using spectrophotometer (DR2010 Colorimeter Hach, USA). In order to measure the biofilm concentration on the plastic balls, 12 carriers were gathered from the HSBR (4 samples per run), dried in an oven for 24 h at 105 °C and then weighted (W ). The carriers were 1

washed, dried another night at 105 °C and then weighted again (W ). The total amount of the attached 2

biomass was obtained by subtracting W from W . 2 1

Mixed liquor volatile suspended solids (MLVSS) in suspension were determined by calculating the weight loss on ignition at 550 °C for 30 min.

4. Kinetic Studies

Substrate degradation rate is expressed in Eq. 1 according to the first order equation. The net biomass growth rate (Eq. 2) was calculated from the biomass growth rate and the biomass decay rate. The results of kinetic studies can be used for predicting the treatment performance of full-scale reactors at the same opera-tional conditions. The presented model described substrate consumption and cellular growth for both purely-suspended and hybrid systems, assuming: (i) the transformation processes occurring in both sus-pended biomass and biofilm were not considered, (ii) the removal of readily and slowly biodegradable organic substances was explained by the first order

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constant (k), (iii) aerobic and anoxic cell yields were expressed by the stoichiometric yield coefficient, and (iv) the effects of diffusive resistance on substrate re-moval and the growths competing for a rate-limiting substrate (Monod's equation) were not included. However, a wide range of biofilm models has been conducted and are currently available in a guideline for biofilm model [21]. .

(1)

(2)

Where dS/dt is substrate utilization rate in mg -1 -1COD L h , dX /dt is net biomass growth rate in mg V

-1 -1VSS L h , k is first order constant of substrate degra--1dation in h , Y is ratio between the biomass produced

-1to the substrate mass consumed in g VSS g COD, and -1k is endogenous or decay coefficient in h . d .

RESULTS AND DISCUSSION

1. Effect of OLR on COD Removal

Figure 1 showed the effluent COD concentrations at operational OLR ranged from 1.75 to 3.50 g COD

-1 -1 -1 -1L d . In the first run at OLR of 1.75 g COD L d , the systems reached the steady state after 12 d. Effluent

-1COD was 810 ± 8 and 167 ± 5 mg L for CSBR and HSBR, respectively. After that OLR was stepwise in-

-1 -1creased to 2.33 g COD L d at day 48 of operation. Gradually, the effluent COD in the CSBR and HSBR

-1increased to 1153 ± 24 and 290 ± 8 mg L , respec-tively. From day 90 on, similar trends were observed

-1at maximum applied loading rate of 3.50 g COD L -1d , where effluent COD increased again to 1674 ±

-150 and 577 ± 7 mg L , respectively. It should be highlighted that, in the CSBR, the effluent COD exceeded the Egyptian law 93/62 for discharge to

-1sewer system (< 1100 mg L , as modified by Decree 44/2000). The relationship between COD loading rate and removal efficiency is shown in Fig. 2 with the percent-age of COD removed decreasing linearly with in-creased OLRs over the range applied in this study. In the CSBR, during the first operating OLR (1.75 g

-1 -1COD L d ), COD removal efficiency was 88% of -1influent COD 7000 mg L , while in the HSBR system,

COD efficiency was 98%. Further increase of OLR values, the HSBR always achieved higher performance as compared to the conventional reactor due to the presence of attached biomass [10]. Chen et al. [22] stated that, when a hybrid system is applied, a readily biodegradable organic substance can be effectively removed. Similar findings were observed by Wang et al. [4] who found that more than 80% feed COD could

-1 -1be removed up to OLR of about 3.5 g COD L d in a

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Nasr et al., Sustain. Environ. Res., 24(6), 449-456 (2014)

452

Fig. 1. Applied OLRs and corresponding effluent COD values of CSBR and HSBR during the operation time.

hybrid system. Artiga et al. [10] achieved COD remov-al efficiency of 95% at increasing the OLR stepwise up

-1 -1to 4.5 g COD L d . Consequently, the existing CSBR can be upgraded to attain better performance at higher organic loads by adding carrier media inside the reac-tors. It is noted that at much higher OLRs of 6.9-24.5

-1 -1g COD-soluble L d , COD removal efficiency could only be achieved in the range of 55 to 76% using carrier materials [23]. The substrate balance model developed by Borja et al. [24] defines the COD balance of the reactor based on: (i) the reactor is operated under steady state conditions, (ii) the particulate SS in the influent are readily biodegradable, and (iii) the VSS in the effluent corresponds to the generated biomass [25]. The COD mass balance of the reactors (Table 2) was computed considering influent and effluent by-products as equiv-alent COD. The calculated mass balance, in the CSBR (72-81%) was lower than 100% due to neglecting the fraction of COD consumed for cell maintenance and growth. However, due to the attachment of biofilm on the media, biomass in the hybrid system was increased, leading to more COD storage. Thus, lower values of COD balance (53-65%) were obtained in HSBR. Sludge volume index (SVI) was used to quantify the settling characteristics of the MLSS. In this study,

-1SVI was found in the range of 90-140 mL g show-ing the good sludge settling characteristics.

2. Effect of OLR on TKN Removal

As shown in Fig. 2, the TKN conversion decreased from 71 to 54% in CSBR and from 92 to 83% in HSBR, at increasing OLRs from 1.75 to 3.50 g COD

-1 -1L d . The hybrid system was able to achieve high TKN removal throughout the whole experimental campaign, even during the highest OLR. As presented in Fig. 3, nitrifying bacteria convert ammonia to nitrate in the aeration phase [26], resulting in decreasing

-1ammonia levels to 14, 20 and 25 mg L in CSBR, and -14, 6 and 12 mg L in HSBR at OLRs of 1.75, 2.33 and

-1 -13.50 g COD L d , respectively. Those results corre-sponded to the specific nitrification rates in the hybrid

-1 -1system of 0.65-0.86 mg NH -N g VSS h , which were 4

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Fig. 2. Effect of OLRs on COD removal, TKN removal, specific nitrification rate, substrate degradation first order (k), and mass yield (Y) of CSBR and HSBR. .

Nasr et al., Sustain. Environ. Res., 24(6), 449-456 (2014)

-1 -1Table 2. COD balance of CSBR and HSBR at OLRs: 1.75, 2.33 and 3.50 g COD L d

% COD-soluble-eff.a% VSS-eff.

a% Excess sludgeb% COD balance

1.75

8

3

61

72

2.33

10

7

64

81

3.50

15

9

51

75

1.75

< 1

2

51

53

2.33

1

3

61

65

3.50

1

7

49

57

CSBR-1 -1OLR (g COD L d )

HSBR-1 -1OLR (g COD L d )

a -1 bBased on 1.42 g COD g VSS; COD balance (%) = (COD-soluble-eff. + VSS-eff. + Excess sludge)/COD-in

slightly higher than those obtained in the CSBR of -1 -10.62-0.82 mg NH -N g VSS h (Fig. 2). Simultane-4

ously, denitrification process occurred in the anoxic phase, achieving the specific denitrification rates of

-1 -10.19-0.26 and 0.31-0.32 mg NO -N g VSS h for 3

CSBR and HSBR, respectively. The efficient nitrogen removal in the hybrid system can be explained by: (i) the “seeding” effect of nitrifiers from the biofilm to the mixed liquor, due to biofilm detachment from the carriers and/or collisions of the carriers with the walls "wall effect" [9], and (ii) the increase of sludge retention time (SRT) in the hybrid system leads to a favourable environment for nitrifying bacteria [10]. In HSBR, due to the entrap-ment of biomass on the plastic media, higher biomass

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Nasr et al., Sustain. Environ. Res., 24(6), 449-456 (2014)

moval efficiency of 59% obtained by Sirianuntapiboon et al. [28] using membrane coupled SBR at 1.34 g

-1 -1BOD L d , demonstrating that the good use of pack-ing media.

3. Model Verification and Kinetic Parameters Estimation

By plotting -ln(S/S ) versus t, a straight line 0

passing through the origin is obtained, whose slope will be k. Moreover, a plot of (X -X )/(X .t) versus V,0 V,t V

(S -S )/(X .t) leads to a straight line; whose intercept 0 t V

and slope will be Y and k respectively. The values thus d

obtained (data not shown) indicate that k values were -10.09, 0.075 and 0.06 h at OLRs of 1.75, 2.33 and 3.50

-1 -1g COD L d , respectively. Trends of k demonstrated that COD removal efficiency decreased with increasing the OLRs (Fig. 2). However, HSBR showed higher k

-1values (0.13, 0.12 and 0.10 h ) for the corresponding OLRs, respectively. Those results indicate that, CSBR requires longer hydraulic retention time to achieve the same organic matter oxidation of the HSBR under the same operating conditions. Consequently, smaller size and compacted units of HSBR are enough for the treat-ment of dairy wastewater as compared to the conven-tional reactor. Biomass Y depends on concentration of biomass in the bioreactor and amount of excess sludge wasted [29]. The packed material increased the opportunity for higher biomass concentration in the system, due to biofilm detachment from the carriers, leading to higher Y values. However, for both systems, Y values in-creased with OLRs (Fig. 2), suggesting that the higher COD loads increased the biomass growth as well as the excess sludge. In CSBR, at increasing OLR from

-1 -11.75 to 3.50 g COD L d , biomass in the system in--1creases from 3700 to 4340 mg L , with elevating of

-1excess sludge concentration from 5608 to 6250 mg L , causing an enhancement in Y values from 0.14 to 0.40

-1g VSS g COD. Same trends were observed in the HSBR system, where Y increased from 0.25 to 0.67 g

-1VSS g COD at the same OLR values. Similarly, Leyva-Díaz et al. [9] found that the amount of hetero-trophic biomass yield in a hybrid membrane bioreactor

-1was 0.53 g VSS g COD. Moreover, Wang et al. [4] observed that the total biomass concentration in hybrid

-1reactors increased from 4300 to 5750 mg L , with X /X at 53-64% at increasing OLR from 0.76 to attached total

-1 -13.04 g COD L d . If the substrate consumption rate is fast, the cell decay of microorganisms is increased (i.e., k increased d

with Y values) [9]. Consequently, k raised from d-10.0018 to 0.0082 h in CSBR and from 0.003 to 0.013

-1h in HSBR, at increasing OLR from 1.75 to 3.50 g -1 -1COD L d . Similar findings were also observed by

Kaewsuk et al. [30]. Although the results offered a good description of

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the reactors performance, in terms of COD removal and biomass production, the observed yield (Y ) obs

showed different behavior. In a system containing a certain amount of biomass, part of the carbon source is mainly used for storage and maintenance. Therefore, the Y (Eq. 3) is a function of the maximum growth obs

yield (Y ), SRT and k as described by Van Loos-max d

drecht and Henze [31]:

Even if Y was significantly higher in the HSBR for all OLR values, the Y values were approximately obs

similar for both reactors (CSBR: 0.16 and HSBR: 0.20 -1g VSS g COD). This may be attributed to the lower

growth rate of certain types of microorganisms with different cell yields in the hybrid system, mainly caused by the attached growth. Apparently, this case is different than processes involving pure suspended or attached biomass. Muller et al. [32] stated that theo-retically the anoxic cell yield coefficient is about 83% of its aerobic value. The presence of two zones within the packed media (an aerated outer zone and an anoxic deeper zone) would lower cell yield. The simulated results fit through the actual data with a correlation coefficient value above 0.90 (Fig. 4), indicating that the kinetic coefficients were reliable with this experimental work.

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(3)

CONCLUSIONS

ACKNOWLEDGEMENT

REFERENCES

Dairy wastewater treatment was investigated using CSBR and HSBR. Both reactors showed a similar trend in the correlations between the kinetic parame-ters; Y and k are positively increased with OLRs and d

negatively correlated with k. However, the hybrid system showed better kinetic performance, so the re-moval percentages of COD were higher (92-98%) compared to CSBR (76-88%). Similarly, the efficiency of TKN removal decreased with increased OLRs, and the hybrid system was superior. It can be concluded that the conventional treatment systems could be upgraded to handle higher organic loads by adding packed media to the biological units. .

This work was supported by the Sanitary Engi-neering Department, Faculty of Engineering, Alexan-dria University, Egypt. .

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Discussions of this paper may appear in the discus-sion section of a future issue. All discussions shouldbe submitted to the Editor-in-Chief within six monthsof publication. .

Manuscript Received: Revision Received:

and Accepted:

January 19, 2014April 9, 2014 May 1, 2014

456 Nasr et al., Sustain. Environ. Res., 24(6), 449-456 (2014)

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