A Modified UCT Method for Enhanced Biological Phosphorus Removal
Sustainable process-product development & green chemistry
Sustainable & Clean Technologies - Ib: Extraction & Remediation (T1-4b)
Keywords: nutrient removal, wastewater, UCT method, step feeding, BNRAS
Nitrogen and phosphorus are limiting nutrients in most freshwater systems and cause eutrophication. Biological Nutrient Removal Activated Sludge (BNRAS) systems remove carbon, nitrogen and phosphorus by biological means with low costs and less waste sludge production. One of the most commonly applied BNRAS methods for urban wastewater treatment relies on the University of Cape Town (UCT) concept. The UCT process was designed to minimize the effect of nitrate to the anaerobic contact zone, which is crucial for maintaining truly anaerobic conditions and thus, allowing biological phosphorus release. Step feeding is an attractive process to eliminate the need for internal recycling and optimize organic carbon utilization for denitrification, and thus it results to energy savings. Multiple stage cascades optimize removal efficiency with minimum reactor volume and provide operational safety. On this basis, the UCT approach could be enhanced by combining multiple stages of anoxic and oxic zones with the step feeding process.
The pilot-scale prototype BNRAS system presented here combines both, the idea of UCT concept and the step denitrification cascade for integrated removal of carbon, nitrogen and phosphorus. The experimental set-up of 44 L operational volume consists of an anaerobic selector and stepwise feeding in subsequent anoxic and oxic tanks. Raw wastewater with influent flow rates ranging between 48 – 168 L/d was fed to the unit at Hydraulic Residence Times (HRT) of 5 – 18 h and specific BOD5 loading rates of 0.08 – 0.82 kg BOD5/ (kg MLVSS ּd). Influent flow rate (QF) is distributed at percentages of 60/25/15, 40/30/30 and 25/40/35 % to the anaerobic selector (AN), 2nd (DN2) and 3rd (DN3) anoxic tank respectively. The overall Sludge Retention Time (SRT, θc) was kept constant at 10 d and temperature at 20˚C.
Theoretical expectations were confirmed since the proposed flow scheme does not permit nitrate introduction to the anaerobic selector, whereas step feeding to anoxic tanks adds essential carbon compounds for the denitrification process and no internal recycling is necessary. The results of two year operation show high removal efficiencies of organic matter of 89% as total COD removal and 95% removal for BOD5, complete nitrification (95% removal of ammonium-nitrogen), 90% removal of Total Kjeldahl Nitrogen (TKN) and total nitrogen removal through denitrification of 73%. Phosphorus removal attains a mean value of 67% but ranges intensively because of fluctuations in the incoming phosphate concentration. All removal efficiencies receive constantly the highest values at the feeding ratio of 60/25/15. Moreover, plant configuration provides operational conditions that suppress filamentous bacteria and favour growth of floc-formers, and thus leads to high sludge settleability characteristics (Sludge Volume Index ≈ 100 ml/g).
The highest removal efficiency and the optimum operation are recorded at HRT of about 9 hrs and influent flow rate of 96 L/d, which is distributed by 60% to the anaerobic selector, by 25% to second anoxic tank and by 15% to the last anoxic tank. Removal efficiency of organic substrate is 94% and 98% for COD and BOD5 respectively, 99% for ammonium nitrogen, 94% for TKN, 83% for total nitrogen and 93% for orthophosphates. Nitrate nitrogen effluent concentration is about 7 mg/L.
See the full pdf manuscript of the abstract.
Presented Wednesday 19, 15:00 to 15:20, in session Sustainable & Clean Technologies - Ib: Extraction-Remediation (T1-4b).
