Alternatives to lower the carbon demand of biological nutrient removal processes
Jabari Barjesteh, Pouria
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There are ongoing researches to develop alternatives to reduce the carbon demand of biological nutrient removal (BNR) processes. In this study the feasibility of three alternatives were investigated: 1- Simultaneous denitritation and phosphorous removal in integrated fixed-film activated sludge (IFAS) systems, 2- Hydrolysis of particulate COD (pCOD) under extended anaerobic condition, and 3- Hydrolysis/fermentation of activated sludge in sludge holding tank (SHT). The simultaneous denitritation and phosphorous removal process (performed by denitrifying phosphorous accumulating organisms (DPAO)) uses carbon simultaneously for phosphorous and nitrogen removal and avoids the conventional pathway of nitrogen removal; thus, it could reduce the carbon demand of BNR process. The use of nitrite in this process would raise the operational concern of free nitrous acid (FNA) inhibition on biomass activity. Therefore, the feasibility of long term DPAO’s activity with nitrite and the FNA inhibition was investigated in an integrated fixed film activated sludge (IFAS) system. It was found that the long term simultaneous denitritation and phosphorous removal in both suspended and attached biomass of IFAS system is feasible and the risk of FNA inhibition on BNR process is moderated by the contribution of attached biomass in the IFAS system. The potential of hydrolysis of pCOD under extended anaerobic condition to produce additional carbon for enhanced biological phosphorous removal (EBPR) was investigated. Results showed that the potential of this alternative to enhance EBPR is limited in practice due to low rate of anaerobic hydrolysis. The currently used kinetics of anaerobic hydrolysis by commercial simulators could overestimate the EBPR efficiency. The potential of hydrolysis/fermentation of activated sludge in SHT to produce additional carbon for BNR process was investigated. Results showed that SHT could enhance BNR efficiency if the activated sludge contains relatively high biodegradable pCOD (plants with low SRT and no primary sedimentation). The results of BioWin simulation indicated that the simulator could overestimate the potential benefit of SHT by failing to simulate the effect of starvation condition in SHT on EBPR. In operation of SHT, a relatively low retention time with high sludge load could increase its potential to enhance the BNR efficiency.