Complete biological nutrient removal (BNR) in a single tank, sequencing batch reactor (SBR) process, is demonstrated here at full-scale on a typical domestic wastewater. The unique feature of the UniFed process is the introduction of the influent into the settled sludge blanket during the settling and decant periods of the SBR operation. This achieves suitable conditions for denitrification and anaerobic phosphate release which is critical to successful biological phosphorus removal. It also achieves a “selector” effect, which helps in generating a compact, well settling biomass in the reactor.
The results of this demonstration show that it is possible to achieve well over 90% removal of COD, nitrogen and phosphorus in such a process. Effluent quality achieved over a six-month operating period directly after commissioning was: 29 mg/l COD, 0.5 mg/l NH4-N, 1.5 mg/l NOx-N and 1.5 mg/l PO4-P (50%-iles of daily samples). During an 8-day, intensive sampling period, the effluent BOD5 was <2 mg/l in all samples and the total phosphorus averaged 0.17 mg/l in the effluent. Detailed sampling and analysis during one cycle and at various depths clearly showed the deliberate stratification achieved in the tank during the settling and decant period, allowing biochemical reactions to occur during this normally “non-productive” period.
The simplicity and flexibility of the UniFed system allows it to be used in numerous applications, particularly for industrial situations where a high degree of uncertainty of the wastewater composition during the design stage or where changing requirements based on changes on the production side are present. The single tank operation without any recycle also reduces the capital costs for a full BNR system compared to the comparatively complex continuous flow processes.