Treatment of separated piggery anaerobic digestate liquid using woodchip biofilters

The Irish nitrates directive restricts the land area suitable for landspreading of pig manure, so anaerobic digestion warrants consideration. In this study, six identical Lodgepole pine woodchip biofilters were set up to treat the separated liquid fraction of digestate after anaerobic digestion of pig manure. Two hydraulic loading rates were examined: 5 L/ m ²/d (LLR) and 10 L/ m ²/d (HLR). Following a start-up period of 70 days, an average of 90% and 71% of NH⁺₄-N was removed at LLR and HLR, respectively. LLR resulted in higher total nitrogen removals than HLR (p<0.05). Efficient nitrification occurred at LLR as indicated by the lower NH⁺₄ -N (p<0.05) and higher NO^-₃ -N (p<0.05) concentrations in the biofilter effluent. A batch experiment testing the capacity of saturated woodchips in removing total oxidized nitrogen (TON) from the effluent of the woodchip biofilters showed that TON was reduced by 323 mg/L from 663 mg/L in 360 h, indicating that the aerobic woodchip biofilters should incorporate a saturated layer of woodchips at the base of the biofilters to enhance nitrogen removal.