Biological nitrogen removal in a modified anoxic/oxic process for piggery wastewater treatment

Piggery wastewater is characterized by its high ammonium concentration and low COD/nitrogen ratio. The indiscriminate discharge of untreated piggery wastewater from scattered household pig farms has posed a potential risk of surface water pollution in rural areas of southern China. In this study, an anoxic/oxic (A/O) process with elastic fillers tightly packed in each tank was investigated for piggery wastewater treatment. Two anoxic tanks and two aerobic tanks were used in series and the hydraulic retention time of each was 24h. Dissolved oxygen concentration in the aerobic tanks was intentionally controlled at below 2mg L-1 for the implementation of nitritation. After sludge acclimation, three recycle ratios from 100 to 300% were tested. More than 90% of organic matter was removed from raw piggery wastewater despite recycle ratio values. However, the NH4+-N concentrations in the effluent slightly increased from 30 to 65mg L-1 as the recycle ratio increased. Higher recycle ratio benefited the removal of total nitrogen (TN) and dramatically increased the nitrite accumulation rate in the aerobic tanks. The tightly packed elastic fillers successfully prevented the loss of sludge and increased the biomass in reactors. Moreover, the formation of biofilms on the surface of the elastic fillers developed simultaneous nitritation and denitrification in the aerobic tanks, which counted for approximately 30% of the total removed nitrogen in the proposed system. The employment of elastic fillers in A/O process effectively improved TN removal at a relatively smaller recycle ratio, and consequently reduced the running cost of recirculation for nitrogen removal.