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Dynamic Modelling of Substrate Degradation for Urban Wastewater Treatment by Sequencing Batch Reactor.

Byline: Turgay Dere Recep Ileri Yavuz Demirci and Mehmet Cekim

: This paper presents the dynamic modelling of substrate degradation for urban wastewater treatment by a pilot-scaled sequencing batch reactor including experimental data of a long-term experimental work performed at different operation conditions. During the study pH chemical oxygen demand (COD) total nitrogen (TN) and total phosphorus (TP) were measured to investigate SBR treatment performance. Optimum operation times were determined and kinetic constant (k) was calculated (0.60 h-1) with using experimental results for urban wastewater. The Model Simulation estimates were very good fit with the experimental data under organic loading degradation conditions model simulation predictions well match with the experimental results under disturbed organic loading conditions.

Keywords: Urban wastewater; Sequencing batch reactor (SBR); Operation time (tr); Kinetic constant (k); Experimental and Modelling Simulation.


Sequencing batch reactor (SBR) defined as an advanced activated sludge biological treatment process is usually used for small-scale systems. However there is potential for use in large-scale systems. SBR processes are being able to define as hybrid systems because of including some characteristics of continuously flowing and complete stirred systems. However they have indeed unmatched other characteristics. Respectively fillinging reacting settling pulling and inert periods occur one cycle time. Although conventional activated sludge processes require least two separate tanks they require only ones for aeration and settling. However thet do not require recycling for increasing of treatment performance. [1-6].

SBR is appropriate treatment for organics and nutrient pollution loads of wastewater. SBR technology has been commonly used for municipal wastewater and various industrial wastewater treatments since the invention. However it has been used for landfill leachates treatment. [7 8]. In large volume scaled plants there are two important problems about sludge bulking and low density. They were solved by modified activated sludge system is known as SBR [9]. The various geometric applications of SBR is a significantly for microbial components of the biomass affecting treatment performance [10].

SBR processes are favorable to remove organics and nutrient from wastewater. .Recently these technology were developed for worldwide municipal and industrial wastewater treatment. [2 4 11]. Approximately 1.3% of the wastewater treatment technologies of Germany are Sequencing Batch Reactors (SBRs) [5].

SBR processes are favorable for the wastewater treatment. The wastewater composition for SBR treatment is characterized by a high nutrient content and often changes. SBR has been widely used in wastewater treatment plants [12]. The SBR system is used to treat wastewater having high nitrogen content by nitrificationdenitrification. However it is used for the phosphorous removal. However aerobic SBR systems still have some problems due to the low settle ability of sludge excess sludge production under high organic loading. Less increase is obtained in the removal efficiency due to the limitation of the increasing of biosludge [11].

Superior advantages of sequencing batch reactors than conventional activated sludge processes are flexible operation and the fact that there is no liquid or solids recycling for the treatment of complex wastewater. The high-suspended growth performance of these reactors was obtained according to conventional systems [13-16].

To carry out SBR processes the major requirement is to provide optimal reaction conditions for nonincreasing of filamentous sludge volumes. For that design factors of SBR are important.These factors are cycle time per day phase separation oxygen feeding nitrification denitrification and phosphorous removal. [17].

Two major phases of the operation of them: aerobic and anoxic to achieve total pollutant removal within minimum time. The nitrogen removal efficiency decreases gradually with increasing ammonium-loading rates at constant chemical oxygen demand (COD) loading. Therefore appropriate concentration control of the carbon source can stimulate the simultaneous nitrification denitrification (SND) to optimize biological nutrient removal of SBR [18]. In SBR the substrate removal kinetic of aerobic granules is dependent size of granules. Small size aerobic granules exhibit higher metabolic activity in the substrate removal rates [19].

To treat total organic carbon (TOC) nutrients and suspended solid (SS) of slaughterhouse wastewater sequencing batch reactors having low capital and operational costs are recommended by European Commission [20].

Our main objective is to investigate optimum kinetics for urban wastewater treatment by sequencing batch reactor.

Results and Discussions

Experimental Results

During the experimental studies chemical oxygen demand (COD) soluble total nitrogen (TN) and soluble total phosphorus (TP) were observed to determine the optimum reaction time for the treatment of urban wastewater by sequencing batch reactor.

Fig. 1 shows the effluent concentrations and removal rates of COD TN and TP at five different reaction times. The removal rate of COD ranged from 80% to 85% (Fig. 1a.) while concentrations in effluent kept among 30 - 40 mg/L (Fig. 1b.). Maximum removal rates (85%) were obtained at 2 h (h) reaction time. At 1.5h reaction time the removal rate of COD was 83% [21]. In Mines and Milton's

1998 study relating to domestic wastewater influent COD concentrations varied between 248 mg/L and 291 mg/L and effluent COD concentrations varied between 42 mg/L and 53 mg/L [22]. In Yilmaz et al. 2006 study for domestic wastewater the effluent COD values ranged from 50 to 70 mg/L while influents COD concentrations ranged from 320 to 480 mg/L [23]. The highest COD treatment efficiency ranging between 75% and 94%. Was obtained in filling period of 2 h and reacting period of 3 h. For grey water the influent COD concentrations varied between 296 mg/L and 564.8 mg/L in Jamrah et al.'s 2008 study [24]. Influent COD was 273 - 490 mg/L and removal efficiency was 64 - 89% for domestic wastewater [25]. The COD removal was obtained in first 20 minute of the reacting period [26].

The removal rates of TN and TP ranged from 93% to 95% and 33 - 39% the maximum removal rates obtained at 1.5 h and 2 h respectively In an activated sludge system nitrogen elements besides organic substrate are used for heterotrophic bacteria growth. When NH4N and NOxN coexist the microorganisms preferably use NH4N and then further use NOxN after complete nitrification resulting in TN reduction. This effect is more obvious in the former stages where there is high Mixed Liquor Suspended Solids (MLSS) [27]. The average removal efficiency of SBR was 47% for TN concentration of domestic wastewater was between 43.06 mg/L and 146.67 mg/L because of too low C/N ratio of domestic wastewater. To increase TN removal efficiency of SBR carbon source should be provided at the aerobic phase or stage [25]. Soluble total phosphorus (TP) removal efficiency ranged from approximately 88% at an SRT of 9.3 days to 78% at an SRT of 18.3 days and influent

TP values ranged from 5.7 to 7.2 mg/L; average effluent soluble TP concentrations ranged from 0.6 to 1.3 mg/L in the study of Mines and Milton 1998 [22]. For municipal wastewater characteristics of TN-TP values were minimum 28; maximum 38 mg/L minimum 3.0; maximum 5.5 mg/L TN- TP removal performances of SBR were 61%-61% 81%-95% and 83%- 97% without organic carbon source with acetic acid addition with waste activated sludge alkaline fermentation liquid addition respectively [28]. To remove soluble phosphorus pollution activated sludge process is executed with sequential anaerobic and aerobic phases. A crucial phase is anaerobic phase. In activated sludge predominant species of bacterial population are generally Acinetobacter Pseudomonas and Candidatus accumulibacter phosphates accumulating bacteria. However activated sludge is being able to absorb soluble phosphorus. We discussed an event that in long-term running SBR systems high performance of phosphorus removal would not continue. Because phosphorus saturation of sludge occur soon after running of them. Hence SBR was operated with a steady state [29]. For phosphorus pollution removal of domestic wastewater SBR performance has worsen rapidly during a bad performance stage. Consequently inert releasing phosphorus occured. To treat this trouble the phosphorus content of sludge was decreased by washing of clean water [26]. Observing the removal rates of the parameters

mostly the maximum removal rates were obtained at 1.5 h or 2 h reaction times. Optimum reaction time was chosen 1.5 h to save energy because of the removal rates at 1.5 h was close to that obtained at 2 h reaction time. The total time (filling reaction or reacting settling pulling or discharging and inert times were 0.5 1.5 1.5 and 0.5 h respectively) for one cycle was 4 h. The treatment system was operated at 6 cycles per day. The volume of treated wastewater totalled 36 L/day in sequencing batch reactor.

Comparison of the Theoretical and the Experimental Studies

In Fig. 2 theoretical calculated COD concentrations and removal rates compared with the experimental results. Reaction constant k= 0.60 h-1 was calculated average effluent concentration (Se) was experimentally determined 3710-3 kg/m3 at 1.5 h reaction time and 1.5 h settle time for sequencing batch reactor. Parameters used in sequencing batch reactor modelling and its simulation were given in Table-1.

Table-1: Pre-treatment wastewater's average characteristics.

Simulation results of the SBR show that COD effluent concentrations (Se) decreased non- linearly with increased reaction time (tr). However COD removal rate (E) increased non-linearly when the COD influent concentration (So) was increased. COD effluent concentrations decreased non-linearly with increased reaction time and COD removal rate showed the same trend as reaction time with increased the flow rate. The effluent COD concentrations are close to each other with increased reaction time. COD effluent concentrations decreased non-linearly when the reaction time was increased. Besides COD removal rate increased non-linearly before the passage of 1.5 h with decreased the volume of activated sludge (Va). COD effluent concentrations decreased non-linearly with increased reaction time. However COD removal rate increased when the total volume of a batch reactor (Vb) was increased.

COD effluent concentrations decreased non-linearly with increased the reaction time. COD removal rate and COD effluent concentrations are close to each other with increased the available volume at the beginning of the filling period (Vab) (Fig. 3 a-e)

The simulation of reaction time shows that COD effluent concentrations increased non-linearly when the flow rate was increased. COD removal efficiencies increased with increased the reaction time. COD effluent concentrations increased linearly with increased COD influent concentrations. COD effluent concentrations decreased when the reaction time (tr) was increased. In addition COD removal efficiencies were very similar each other. COD effluent concentration increased a little bit when increased the activated sludge volume in the batch reactor (Va). However removal efficiencies increased when reaction time was increased. COD effluent concentrations decreased non-linearly with increased the total volume of a batch reactor (Vb). However COD removal efficiencies increased non-linearly with increased the reaction time. Small values than 15 L for the total volume of a batch reactor (Vb) for simulation related our lab-scale study was meaningless.

COD effluent concentrations increased non-linearly with increased the available volume at the beginning of the filling period (Vab) of the reactor. However removal efficiencies increased with increased the reaction time (Fig. 4 a-e). COD effluent concentrations decreased non-linearly when the reaction constant was increased (Fig. 5).

Some studies presented that the main reason of decreasing COD decline was high reaction volume and low reaction times of SBR along with increasing pollution load [23 26]. Similar studies presented that reaction times effect directly COD decline until about 2-3 h depend on influent pollution load (flow rate and concentration).On the other hand at the same time the lower flow rate and the lower amount of substrate requires the lower reaction time to have the large substrate degradation. However the negative effectives of influent concentrations could be tolerated with increasing operational times. The other words influent concentrations could indirectly effect for lower effluent concentrations [28 29]. In this study these relations can be approximately identificated by simulations (Fig. 3 and 4).

During this study the urban wastewater characterization under unsteady state conditions was studied through monitoring the change in the kinetic parameters overtime. By substituting the average values of the urban wastewater kinetics such as S0 Q Vb Va Vab Se for 206 mg/L influent COD concentration at the end of the loading period (Sf) was obtained. Conversion of the effluent COD (Se) with different reaction times (tr) and the reaction constant effecting this conversion have been found by both experimental result and theoretical analysis (Eq. (8)). Reaction constant (k) 0.60 h-1 was calculated for obtained experimental results (So = 206 mg/L Q = 12 L/h Se = 35 mg /L tr = 1.5 h Vb = 40 L Va = 34 L Vab = 6 L). k (reaction constant) is changeable and depends on working experimental conditions. Experimental

Wastewater Composition

Pre-treatment wastewater and activated sludge was obtained from Karaman Municipal Wastewater Treatment Plant in Sakarya Turkey. Pre- treatment wastewater's average characteristics were given in Table-2. COD TN and TP were measured according to the Standard Methods [30 31]. Each experiment was repeated three times for SBR influent and effluent depending on reaction time so that we could study variability; the table also lists the average value of each set of three values.

Reactor Configuration and Experimental Setup

Experiments were performed in a lab-scaled SBR with the size of 35 cm length 35 cm width and 45 cm height. The SBR had a working volume of 40 L was operated Aeration was accomplished by air stones from the bottom of the reactor and mechanical mixer was worked at 1200 rpm during the reaction period. Peristaltic pumps performed wastewater (feed and discharge). For every cycle time continuously influent volume of filling and continuously effluent volume of pulling were six liter (6 L). The lab-scaled SBR set up is shown in Fig. 6. Respectively fillinging reacting settling pulling and inert periods occur one cycle time [2 6 9 20 21]. Periods of the cycle time especially the fillinging reacting settling and departments of the reactor volume limit physical operating conditions. The duration of any period is being able to increased or decreased but the others affect the reactor performance negatively. For example minimum settling periods cause to reduce effluent quality and to decrease sludge quantity in the reactor [2].

The performance of a lab-scaled SBR treating urban wastewater was observed for different reaction times were between 0.5 -4 h. Kinetic studies were accomplished by experimental data. Optimum reaction time was obtained (1.5 h) and the reaction constant (k) calculated (0.60 h-1) for urban wastewater. The total time for one cycle was 4 h. The treatment system was operated for 6 cycles per day.

Theoretical Approach

After filling the remaining substrate concentration is a variable function of the volume and the kinetic expression for substrate removal that applies during the filling-period.

Table-2: Parameters used in sequencing batch reactor modelling and its simulation.

The results of this study show that optimum reaction time was 1.5 h and reaction constant is between 0.30 h-1 and 0.60 h-1 for treating urban wastewaters by sequencing batch reactor.

Variation of effluent COD's with reaction time and effective parameters on this variation were researched. tr S0 Q k and Vab parameters were effective but Va and Vb parameters were not more effective according to simulation studies of SBR.

Sequencing batch reactor (SBR) is observed as a useful system and it offers a significant improvement to the treatment of urban wastewater. The chosen theory can be used to define SBR kinetics with optimum operational conditions and efficiency equation developed for this new study to define treatment efficiency of SBR.


The authors would like to thank the financial support of this project (2008-50.02.002) given by research funds of Sakarya University in Sakarya Turkey.


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Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Date:Oct 31, 2014
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