Performance of sequencing batch reactor for nutrient removal as functions of operating variables

Sequencing batch reactors were originally used for COD and phosphate removal from wastewaters. Recent regulations over nutrient discharges to natural water systems resulted in modifications in sequential batch reactor (SBR) systems to achieve nitrification, denitrification along with COD and phosphate removal. An SBR treatment system consists of a sequencing operation including the steps of fill, react, settle, decant, and idle in the same reactor. 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. Since the same reactor is used for biological oxidation and sedimentation in SBR operations, capital and operating costs are lower than conventional activated sludge processes. Therefore, SBR operations may be more advantageous for treatment of small volume wastewaters in rural areas. In this respect, this thesis is composed of six main parts investigating different number of steps, different hydraulic retention times (HRT), different sludge ages, different carbon sources, different medium compositions and different specific nutrient loading rates (SNLR) on the biological nutrient removal from a synthetic wastewater. In the first part of this study, three different operations with different number of steps were used and their nutrient removal performances were compared. Those operations were a three-step operation consisting of anaerobic/ anoxic/ oxic( An/ Ax/ Ox ) phases; a four- step operation consisting of anaerobic/ oxic/ anoxic/ oxic ( An/ Ox/ Ax/ Ox ) phases and a five-step operation consisting of ( An/ Ax/ Ox/ Ax/ Ox ) phases. Effects of different operations on percent COD, nitrogen (NH4-N, NO3-N)Ill and phosphate removal (PO4-P) performances were investigated. The sludge age was constant at 10 days. The best percent nutrient ( nitrogen and phosphate ) and percent carbon removals were achieved when the five step operation was used. The highest percent COD, NH4-N and PO4-P removals obtained with the five-step operation were 94%, 90 %, and 57 %, respectively. In the second part of this study, nutrient removal from synthetic wastewater by a five-step sequencing batch operation was studied at different hydraulic retention times (HRT). The nutrient removal process consisted of anaerobic (An), anoxic (Ax), oxic (Ox), anoxic (Ax), oxic (Ox) and settling phases. Sludge age was kept constant at 10 days, while hydraulic retention times (HRT) of each phase was varied. Effects of hydraulic retention time of each phase on percent COD, nitrogen (NH4-N, NO3-N) and phosphate (PO4-P) removals were investigated. Hydraulic retention time of each phase was varied at five different levels and the the most suitable retention time resulting in maximum overall percent nutrient removal was determined. The highest observed percent COD, nitrogen (NH4-N and NO3-N) and phosphate (PO4-P) removals were 96%, 87%, 81% and 90%, respectively which were obtained with An/ Ax/ Ox/ Ax/ Ox phase retention times of 2/ 1/ 4.5/ 1.5/ 1.5 h. In the third part of the study, nutrient removal was studied at different sludge ages by using a five-step SBR operation at the pre-determined hydraulic retention times. The nutrient removal process consisted of anaerobic, anoxic I, oxic I, anoxic II, oxic II and settling phases. Hydraulic retention times (HRT) of the aforementioned phases were kept constant at 2/1/4.5/1.5/1.5 h. Settling phase was Vi h for all experiments. Sludge age was varied between 5 to 30 days at six different levels. Effects of sludge age on percent COD, nitrogen (NH4-N, NO3-N) and phosphate (PO4-P) removal were investigated and the optimal sludge age resulting in maximum percent nutrient removal was determined. The highest percent COD (94%), NH4-N (84%) and P04-P (70%) removals were obtained at the sludge age of 10 days, although a sludge age of 15 days resulted in slightly lower values. Sludge ages larger than 15 days resulted in lower percent nutrient removals as compared to those obtained at 10 or 15 days of sludge age. Sludge volume index (SVI) was also minimum (55 mL/g) at sludge ageIV of 10 days. Biomass(MLSS) concentration increased with sludge age resulting in MLSS concentration of 3500 mg/L at sludge age of 30 days. On the basis of these results, a sludge age of 10 days was found to be optimal resulting in maximum percent nutrient removals and minimum sludge volume index. In the fourth part of the thesis, a three-step sequencing batch operation was used for nutrient (COD, NH4-N, PO4-P) removal from synthetic wastewater by using different carbohydrates and organic acids as carbon sources. The operation consisted of anaerobic, anoxic and oxic ( An/ Ax/ Ox ) phases with durations of 2/ 1/ 4.5 h. Different carbohydrates glucose, lactose, sucrose, maltose and a mixture of glucose / maltose (50/50) were used as the sole carbon source in the first phase of experiments. Sludge age was kept constant at 10 days. Glucose was found to be the most suitable carbohydrate source with percent COD, NH4-N and PO4-P removals of 96%, 99% and 94%, respectively. Nutrient removals obtained with sucrose as the sole carbon source were close to those obtained with glucose. A mixture of glucose and organic acids (50/50) such as acetic, citric, propionic and butyric acid were used as carbon source in the second phase of the experiments. Percent COD, NH4-N and PO4-P removals were 93%, 97% and 96%, respectively when a mixture of glucose and acetic acid (50/50) was used. Glucose-citric acid (50/50) combination also resulted in comparable nutrient removals. Changing the ratio of glucose / acetic acid in the mixture did not improve the nutrient removal performance. In the fifth part of this study, nutrient removal from synthetic wastewater was studied as a function of nutrient medium composition using a five-step sequencing batch reactor (SBR). The nutrient removal process was consisted of anaerobic, anoxic, oxic, anoxic, oxic phases with hydraulic retention times (HRT) of 2/ 1/ 4.5/ 1.5/ 1.5 h. Sludge age and settling phase were 10 days and 14 hour, respectively in all experiments. Initial chemical oxygen demand (COD) concentration was constant at 1200 mg/L. COD/N and COD/P ratios in the nutrient medium were considered as independent variables. Box-Wilson statistical experiment design was used to determine the effects of independent variables on percent COD, NH4-N, PO4-P removals. The results were correlated by a response function and the coefficientswere determined by regression analysis. The COD/N/ P ratio resulting in maximum percent COD, NH4-N and PO4-P removals was determined to be 100 / 2 / 0.54 resulting in 95% COD, 94% NH4-N and 99% P04-P removals. In the last part of this study, nutrient removal from a synthetic wastewater by a five-step SBR operation was studied at different specific nutrient loading rates (SNLR ). The operation consisted of anaerobic (An), anoxic (Ax), oxic (Ox), anoxic (An) and oxic (Ox) phases with hydraulic retention times (HRT) of 2 / 1 / 4.5 / 1.5 / 1.5 h, respectively with a 45 minutes settling phase. Initial COD concentration was varied between 600 to 4800 mg/L at eight different levels with constant COD / N / P ratio of 100 / 3.33 / 0.7. Effects of specific nutrient loading rates (SNLR) on percent COD, NH4-N and PO4-P removal were investigated. Percent nutrient removals decreased and effluent nutrient levels increased with increasing nutrient loading rates. The highest percent COD (99%), NH4-N (99%) and PO4-P (97%) removals were obtained with the initial COD concentration of 600 mg/L at COD loading rate of nearly 40 mg COD/g biomass.h. However, the sludge volume index (SVI) decreased with increasing COD loading rate resulting minimum SVI of 46 mL/g at COD loading rate of nearly 86 mg COD/g biomass.h. Biomass concentration increased with increasing specific nutrient loading rate resulting in biomass concentration of 3.84 g/L at COD loading rate of 86 mg COD/g biomass.h. In summary, nutrient removal from a synthetic wastewater was extensively studied and a five-step SBR operation with 10 days sludge age was found to be the most suitable among the others tested. The effects of hydraulic retention times of each step, wastewater composition and the nutrient loading rates on the system performance were investigated and the optimum operational values were determined.