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/m2/d (LLR) and 10 L/m2/d (HLR). Following a start-up period of 70 days, an average of 90% and 71% of NH4(+)-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 NH4(+)-N (p < 0.05) and higher NO3(-)-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 ofwoodchips at the base ofthe biofilters to enhance nitrogen removal.

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Three pilot-scale biofilters consisting of 1 m aerobic woodchip layers and 0.5 m submerged woodchip layers were constructed to treat the separated pig manure anaerobic digestate (SAD). Reductions of 54% dry matter, 80% of chemical oxygen demand (COD), 93% of 5-day biochemical oxygen demand (BOD5), 86% of total nitrogen (TN) and 79% of total phosphorus (TP) were achieved. The study results confirm the occurrence of nitrification in the aerobic woodchip layers and denitrification in the submerged layers. The woodchip biofilters can be used for large-scale SAD treatment before landspreading so as to meet the stringent Nitrates Directive.

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