A sequencing batch reactor (SBR) was used for the treatment of abattoir wastewater to produce effluent with desirable nitrogen and phosphorus levels for irrigation. The SBR cycle consisted of an anaerobic phase with wastewater feeding, a relatively short aerobic period (allowing full ammonium oxidation), a second anoxic period with feeding, followed by settling and decanting. This design of operation allowed biological nitrification and denitrification via nitrite, and therefore with reduced demand for aeration and COD for nitrogen removal. The design also allowed ammonium, rather than oxidized nitrogen, being the primary nitrogen species in the effluent. Biological phosphorus removal was also achieved, with an effluent level desirable for irrigation. A high-level of nitrite accumulation (40 mg N/L) in the reactor caused inhibition to the biological P uptake. This problem was solved through process optimization. The cycle time of the SBR was reduced, with the wastewater load per cycle also reduced, while the daily hydraulic loading maintained. This modification proved to be an effective method to ensure reliable N and P removal. N2O accumulation was measured in two experiments simulating the anoxic phase of the SBR and using nitrite and nitrate respectively as electron donors. The estimated N2O emissions for both experiments were very low.
Keywords: abattoir wastewater, biological phosphorus removal, irrigation, nitrite, N2O, sequencing batch reactor