A Semi-Batch (On-line) Method for Biokinetics Determination in an Enhanced Biological Phosphorus Removal System
Multi-scale and/or multi-disciplinary approach to process-product innovation
Integrated Methodologies for Process Development (T3-7)
Keywords: biokinetics, nitrification, denitrification, phosphorus removal, specific rate
Kinetics of nitrification, denitrification and dephosphatation were studied in the aerobic, anoxic and anaerobic stages of a pilot scale Biological Nutrient Removal (BNR) plant treating municipal wastewater. The configuration of the plant’s design is based on the combination of the UCT (University of Cape Town) process and the step feeding in a three-stage denitrification cascade.
In order to study the process kinetics and to obtain actual values for the investigated kinetic parameters semi-batch experiments were performed. For this purpose, continuous feeding of the treating system was interrupted and instantly the pilot plant was turned into a batch mode operation. No sludge was extracted and experiments were conducted in the actual environment of bacteria life cycle. Thereafter, addition of NO3- and NH4+ into the anoxic and aerobic compartments of the treatment plant respectively, took place, whereas adequate initial concentration of a carbon source (municipal wastewater or acetate) was added in the mixed liquor. Aeration was implemented in the anaerobic tank to reach maximum phosphorus release. At this point and after transition to anaerobic conditions acetic acid was added to provide a readily biodegradable carbon source for phosphorus uptake.
The nitrification process was found to follow Monod type kinetics. The maximum specific nitrification rate, qN,max, was determined to vary between 1.44 and 2.95 g NH4+-N/(g VSSaut∙d) for synthetic wastewater addition. The half saturation constant for the nitrification process, Km,N, was estimated graphically at 2.2 – 6.4 g NH4+-N/L. When the substrate was real wastewater, qN,max averaged at 1.14 g NH4+-Nr/(g VSSaut∙d) and Km,N at 1 g NH4+-N/L. In general, nitrification activity is enhanced along the treatment train, whereas the low values of the half saturation constant show that nitrifiers with high affinity to the influent substrate grow in the system.
Denitrification experimental results indicated zero order kinetics. Two specific denitrification rates, an initial high one, qDN,1 and a following low one, qDN,2, until complete substrate consumption were determined in the anoxic tanks. Using synthetic wastewater qDN,1 and qDN,2 were found to obtain mean values of 0.12 and 0.06 g NO3--N/(g VSShet∙d) respectively. In any case, maximum denitrification activity is recorded in the first anoxic tank of the cascade. Since real municipal wastewater lacks of readily biodegradable organics, values of the specific denitrification rates are lower. During denitrification phosphate concentration was fluctuating, whereas phosphorus was instantly released in nitrate absence. Specific phosphate release rates, phosphate profiles and maximum phosphate release were quite similar in all anoxic tanks. Denitrifying phosphate accumulating organisms (DPAOS) are estimated at 25 %.
By testing the anaerobic sludge of the BNR unit, specific anaerobic phosphorus release rate was determined averagely at 0.03 g PO43--P/(g VSShet∙d), whereas specific aerobic phosphorus uptake rate was estimated averagely at 0.05 g PO43--P/(g VSShet∙d). These values are considered low.
The determination of kinetic parameters can be considered as a useful tool for the process design, operation and improvement of wastewater treatment plants. Furthermore, the study of the biological process kinetics contributes to the better understanding and outline of the complicated biological processes that contemporarily take place within the various stages of BNR wastewater treatment plants. On-line biokinetics determination using the proposed semi-batch method results to real time data acquisition and thus, to actual unit monitoring under operating conditions.
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Presented Thursday 20, 14:40 to 15:00, in session Integrated Methodologies for Process Development (T3-7).
