Effects of intermittent-cycle extended-aeration treatment on the fate of nutrients, metals and bacterial indicators in pig slurry

A farm-scale activated sludge treatment plant for separated pig slurry was operated with four different aeration cycles during a year, with the objective of optimising the process and producing an effluent with low nutrient and pathogen concentrations. The high nitrogen-removals observed (over 95%) appeared to be related to the denitrification achieved through the intermittent operation of the aerator. It was necessary to supply, on average, about 10 kWh/day (i.e. 10 kWh/pig) in order to obtain a treated effluent with mean concentrations of 31•6 mg N-NH4/l and 77•6 mg NO3−N/l. The removal of phosphorus was more variable. Percentage removals ranged from 20 to 90%. Concentrations between 25 and 60 mg PO4-P/l were obtained in the treated effluent. From the experimental data and available observations it could not be concluded whether the removal mechanism for P was predominantly chemical or biological. Mean percentage removals of metals were about 90% for Cu and 85% for Zn. Although the TS value of raw slurry indicated a large dilution of slurry (dilution factor of about 4), the initial bacterial concentrations were relatively high. About 2–3 logarithmic units reductions were observed, but concentrations of the order of 106 faecal coliforms and 107 faecal streptococci were still found in the final effluent.