Removal of organic carbon, nitrogen and phosphorus from wastewater using a novel biofilm reactor
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During this research study, a pumped flow biofilm reactor (PFBR) technology developed at NUI Galway for decentralised wastewater treatment was examined at laboratory scale and field scale. The PFBR technology comprised two reactor tanks (Feed and Discharge Reactors), each containing stationary biofilm media modules, with aeration provided by alternately transferring the wastewater between the two reactor tanks. The aims of this research include: (i) to investigate the characteristics of the PFBR in nutrient removal when treating high strength wastewater; (ii) to examine the emission and generation of nitrous oxide (N2O) in the PFBR when treating wastewater; and (iii) to characterise a field-scale PFBR when treating municipal wastewater arising from a typical Irish town at the NUI Galway Water Research Facility (WRF) in terms of performance, energy consumption, and sludge yield. The laboratory-scale PFBR was operated while treating a high strength synthetic domestic wastewater containing a high phosphorus concentration in a 15-hour treatment cycle and monitored for carbon, nitrogen (N) and phosphorus (P) removal over a study period of 195 days. A number of detailed phase studies were also carried out. On average, the total chemical oxygen demand (CODt), filtered total nitrogen (TNf) and ammonium-nitrogen (NH¬4+-N) removal efficiencies were 97%, 82% and 99%, respectively. Enhanced biological phosphorus removal was observed with an average orthophosphate-phosphorus removal of 43% achieved. Detailed analysis on P release and uptake rates were also carried out and showed that with an extended aeration period, further P could be removed. The generation and emission of N2O in the laboratory-scale PFBR was also investigated while treating a high-strength synthetic domestic wastewater. Carbon and nutrient (N and P) removals during a phase study (when operating at steady-state) were examined and compared with the nutrient removal study. Mass transfer coefficients (K), describing the transfer of soluble N2O from the liquid phase to the atmosphere due to molecular diffusion in quiescent conditions and enhanced diffusion in bulk fluid circulation conditions, were calculated. The overall generation (G) and emission (Q) of N2O in the PFBR while treating synthetic wastewater was then calculated and it was determined that 2.0% of the influent total nitrogen was removed through N2O emission. The field-scale PFBR was operated and examined during two studies (Studies 1 and 2) treating 25-29 m3 municipal wastewater per day at the NUI Galway WRF. In the 100-day Study 1, an average 5-day biochemical oxygen demand (BOD5) removal efficiency of 94% was achieved, while in 34-day Study 2, the BOD5 removal efficiency was up to 95%. Suspended solids (SS) effluent concentrations were 14 and 9 mg SS/l in Studies 1 and 2, respectively. NH4+-N removal efficiencies for Studies 1 and 2 were 80% and 82%, respectively. A number of additional parameters were monitored at the field-scale PFBR, including: energy; flow; dissolved oxygen; oxidation-reduction potential; and pH, as well as operational and maintenance requirements. The PFBR has been shown to offer a viable solution for decentralised wastewater treatment due to ease of operation, simple maintenance, low running costs and low sludge yield.
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