Nutrient removal performance of a five-step sequencing batch reactor as a function of wastewater composition
Introduction
Sequencing batch reactors (SBRs) were originally used for COD and phosphate removal from wastewaters [1], [2], [3], [4], [5], [6], [7], [8], [9]. Recent nutrient discharge regulations resulted in modifications in SBR systems to achieve nitrification and denitrification along with COD and phosphate removal. SBR treatment system consists of a sequencing operation including the steps of fill, react, settle, decant, and idle [10]. When biological nutrient removal is desired, the steps in the react cycle are adjusted to provide anaerobic, anoxic and aerobic phases in certain number and sequence. The anaerobic phase is usually used as a selector for organisms capable of excess phosphate uptake which also results in COD removal from the wastewater and polyhydroxy butyrate (PHB) synthesis. The anoxic phase is mainly for denitrification of nitrate ions present in the wastewater or produced during the oxic phase with simultaneous removal of carbonaceous compounds. The oxic phase is the major step for oxidation of carbonaceous compounds, nitrification and excess phosphate uptake. When these steps are used in a cyclic operation, removal of carbonaceous, nitrogenous and phosphate compounds are achieved.
Numerous studies have been reported in the literature on nutrient removal [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Umble and Ketchum [12] have used a SBR for biological treatment of a municipal wastewater. A 12-h total cycle time resulted in BOD5, TSS, and NH3-N removals of 98, 90 and 89%, respectively.
Chang and Hao [13] have investigated nutrient removal to identify process variables affecting performance of an SBR. In general, the system achieved removal efficiencies of 91, 98, and 98%, respectively, for COD, total nitrogen and phosphate removals at a solids retention time of 10 days, with a total cycle time of 6 h.
Demuynck et al. [19] have determined that a sequence of short aerobic/anoxic phases was better than the usual sequence of aerobic phase followed by anoxic phase. Addition of supplementary COD was found to be necessary during the anoxic phase to obtain complete nitrogen removal. An optimization algorithm was used to minimize the cycle time and up to 50% savings on extra COD supply and 30% savings on aeration time were achieved by optimization.
Andreottola et al. [20] have developed an algorithm for optimization of the cycle length and phase distribution in order to minimize effluent nitrogen concentration. Optimization results were carried out with 3.3 h of anoxic and 4.2 h of anaerobic phase durations. Effluent concentrations of nitrate, nitrite and ammonia were 2.9, 0.04 and 0.06 mg l−1, respectively, after optimization.
Chang et al. [21] carried out experimental studies in a small-scale SBR system to define important parameters affecting process performance. Varying HRT's with BOD concentrations of 100–200 mg l−1 yielded maximum removal efficiencies for N and P within 1-3-2 h of anaerobic-aerobic-anoxic phases, respectively. Final N and P concentrations of less than 2 mg l−1 were obtained.
Zuniga and Martinez [22] have investigated the possibility of combined phosphorus and nitrogen removal in a biofilm SBR using an operation strategy with four reaction phases: Anaerobic/Aerobic/Anoxic/Aerobic. The system was operated successfully with COD, phosphate and ammonia nitrogen removals of 89±1, 75±15, and 87±10%, respectively.
Sang-Ill et al. [23] used fermented swine waste instead of acetate for supplementation of bench-scale SBRs to improve nutrient removal and found almost no difference in performance of the reactors supplemented with either acetate or fermented swine waste both yielding nitrogen and phosphate removals of 90 and 89%, respectively.
In all of the aforementioned studies a three or four step operation was used for nutrient removal. Nutrient removal performance of a five-step SBR operation has not been investigated systematically as a function of wastewater composition. The five-step operation has significant advantages over three and four steps, since this operation provides additional anoxic and oxic phases resulting in removal of excess nitrogen and phosphate. Short and repeated steps of anoxic and oxic phases enable the organisms to perform better in denitrification, nitrification, COD and excess phosphate removal. Due to the aforementioned advantages of the five-step SBR operation, it was the major objective of this study to investigate systematically the effect of wastewater composition on the nutrient removal performance of a five-step SBR. The total cycle consisted of anaerobic–anoxic–oxic–anoxic and oxic (An–Ax–Ox–Ax–Ox) phases with HRT's of 2/1/4.5/1.5/1.5 h, which were determined to be optimal in previous studies. Sludge age (SRT) was constant at 10 days. COD/N and COD/P ratios in the nutrient medium were varied while initial COD was kept constant at 1200 mg COD l−1. COD, NH4-N and PO4-P removal performances were determined as a function of the COD/N and COD/P ratios. The optimal ratios were also determined.
Section snippets
Experimental set-up
A schematic diagram of the experimental set up is depicted in Fig. 1. A fermenter (Bioflo IIC, New Brunswick) with 5 l working volume was used as the SBR. The fermenter was microprocessor controlled for aeration, agitation, temperature, pH, and dissolved oxygen (DO). Aeration was provided using an air pump and a sparger. Agitation speed was varied between 25 and 300 rpm. The pH and DO of the nutrient medium were continuously monitored by relevant probes.
Wastewater composition
Synthetic wastewater used throughout the
Results and discussion
The results of the Box–Wilson statistical design experiments are presented in Table 2 along with the removal efficiencies for COD, NH4-N and PO4-P predicted from the response functions. Table 3 summarizes the coefficients of response functions for each dependent variable which were determined by the regression analysis. The constants presented in Table 3 were used to predict the removal efficiencies presented in Table 2. Nutrient removal efficiencies predicted from the response functions were
Conclusions
One of the most important factors affecting the performance of biological nutrient removal processes is the composition or the ratio of COD/N/P in the wastewater. In order to determine the affects of nutrient composition on nutrient removal efficiency in an SBR, a Box–Wilson statistical experimental design was used by considering COD/NH4-N (X1) and COD/PO4-P (X2) ratios as independent variables while initial COD was constant at 1200 mg l−1. The objective functions were COD, NH4-N and PO4-P
Acknowledgements
This study was supported by the research funds of Uludag University, Bursa, Turkey.
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