활성슬러지 모델 No.3를 이용한 SBCOD의 가수분해 과정과 미생물의 EPS 형성에 관한 연구

Abstract

starch, oxygen uptake rate, activated sludge model, EPS

ABSTRACT

Since the effluent water quality from wastewater treatment plant in Korea has been strengthen by the Ministry of Environment lately, advanced nutrients treatments become important for nutrients like nitrogen and phosphorus removal from wastewater. In biological process, microorganisms require organic substrate as readily biodegradable form for removal of nitrogen and phosphorus. If there are lack of readily biodegrable substrates (RBCOD), external carbon sources such as methanol, organic acid or slowly biodegradable substrate(SBCOD) should be supplied to the biological reactor. Most of SBCODs are removed at primary clarifier or used as organic substrate for RBCOD in wastewater treatment plant without primary clarifier. In case of the latter, proper design and operation condition are required as a variation of influent characteristics. Activated sludge model simulation could help to find the optimum operation conditions for these wastewater process. In this study, characteristics of SBCOD hydrolysis by activated sludge have been examined through batch tests under various conditions and the results are analyzed using activated sludge model No.3(ASM3). A storage product(XSTO) which is a variable of ASM3 has been especially focused on for interpretation with bound extracellular polymeric substances(B-EPS) of activated sludge. This study results would suggest the real SBCOD degradation mechanism and help operation of wastewater treatment system without primary clarifier in the future.

Starch has been used for SBCOD model substrate. Activated sludge used in all batch tests are well acclimated to the starch at lab-scale sequencing batch reactor. Oxygen uptake rate(OUR) measurements and batch tests under differentiating biomass concentration and electron acceptor conditionsfor are performed for investigating substrate characteristics and features of SBCOD hydrolysis. The results of the experiments are compared with variation of carbohydrate and protein contents in B-EPS and XSTO in ASM3 simulation. Soluble starch stock is injected twice(0hour, 24hours) during OUR measurements for 48hours. The results shows that substrate utilization occurs more in second injection period than first injection period. Comparison batch tests performed for investigating hydrolysis characteristics of each period based on OUR measurement results. In both the first and second period, hydrolysis rate increased as biomass concentration increased. Electron acceptor conditions affected hydrolysis rate only in the first period. Hydrolysis rate under aerobic condition was the fastest of all the rates under the other electron conditions, anoxic and anaerobic. The rates under anoxic and anaerobic conditions seemed to be similar. The hydrolysis was independent from the electron acceptors during the second period, which shows that already synthesized enzyme activity is not affected by electron acceptors. Another batch tests for ASM3 simulation and B-EPS measurements were performed under similar conditions. ASM3 simulation results showed that a variable of storage product had changed in proportion to variation of carbohydrate contents in B-EPS. Hydrolysis rate of soluble starch depended on sludge characteristics. The more amount of carbohydrate contents in B-EPS increased hydrolysis rate of the substrate. Protein contents also positively affected the hydrolysis rate but less than that of carbohydrate contents. Mechanism proposed by O. Karahan et al.(2005) seems to be appropriate for starch hydrolysis. According to the mechanism, starch is first adsorbed on the biomass, then goes through extracellular hydrolysis in sludge floc. , Once the soluble starch is adsorbed on the biomass, hydrolysis rapidly occurs since the macromolecules of soluble starch have large enzymatic area. So, most of time taken until all of starch is removed should be considered to be time for adsorption between biomass and the starch. On this point of view, carbohydrate contents in B-EPS affect the adsorption rate and protein contents in B-EPS affect the rate of hydrolysis following adsorption. B-EPS of activated sludge helps the aggregation with substrates and contains hydrolytic enzymes. ASM assume that hydrolysis does not require energy which means that electron acceptor does not affect the hydrolysis. To fit OUR model of ASM to the real OUR data, OUR batch test should be performed when the sufficient amount of B-EPS exists since the hydrolysis should not be affected by the electron acceptor conditions.