STUDY ON A MODIFIED BIOFILM SBR FOR PHOSPHORUS AND NITROGEN REMOVAL

Abstract

This thesis records and discusses the results of a laboratory study on biological removal of phosphorus and nitrogen from a synthetic wastewater. Issues of particular interest included system start-up characteristics, microbiology and reactor operating conditions. A modified Biofilm SBR system was developed and operated over a period of 330 days in order to test different operating conditions.

The study consisted of three periods. Period I was aimed at studying the start-up characteristics of the system, while Period II focused on investigating the system's phosphorus and nitrogen removal efficiencies, and internal biochemical reactions occurring within a reacting cycle. The effects of changing operating conditions, such as SRT, on the system performance were investigated in Period III. An additional batch test was conducted during Period III to further confirm the pertinent rate constants obtained from the above three periods.

With the influent concentrations of 15.38 to 46.6 lmg/l (loading rates of 0.022 to 0.066 kg/m3-d) for PO43- -P, 75.91 to 120.9 mg/l (0.107 to 0.171 kg/m 3-d) for NH4 +-N and 373 to 436 mg/l (1.123 to 1.220 kg/m3-d) for COD, removal efficiencies of greater than 90.4% for TP, 90.0% for TN and 94% for COD were achieved. Ammonia-N(NH4 +-N) was completely removed from the final effluent. Other parameters such as TP, NO3- -N, NO- 2- -N, COD, TSS and pH satisfied the effluent discharge standards. Results of the study demonstrated the system was well designed for phosphorus, nitrogen and organic removals.

The removal efficiency of the system was found to be influenced by the COD/P and COD/N ratios and SRT conditions. The reactions within the system could be identified by changes in the oxidation-reduction potential (ORP), pH, and dissolved oxygen (DO) levels. Therefore, these three parameters could be used for system operation control purposes.

Three stages in the development of a polyP biomass, namely primary acclimation, rapid shift and balanced growth stages, were identified in this study. This finding provided a new means for evaluating biomass development and phosphorus removal capability in biological phosphorus removal systems.

The results of the study also found that the changes in the MLVSS/MLSS ratios were directly related to the enrichment of the biomass with polyP organisms and enhancement of the phosphorus uptake. Based on this finding, a simple methodology for monitoring the biomass's development in terms of polyP organism accumulation was established.

Based on the experimental results obtained in this study, a stoichiometric biochemical model was developed to illustrate the relationship between phosphorus release and changes in the MLVSS/MLSS ratio. The model established the quantitative relationship between MLVSS and MLSS concentrations and phosphorus release and organic uptake. The equation obtained suggested that the anaerobic metabolic conversions in this system would probably be associated with the TCA cycle.