Aeration of anaerobic pig slurry for ammonia oxidation

Anaerobic ponds (AP) are common practice at Australian piggeries for the treatment of pig waste, with effluent discharge from ponds often reused on-farm (APL, 2010; Tucker et al. 2010; Buchanan et al. 2013). The present study was conducted as part of a larger project looking to boost pig effluent as an alternative sustainable resource and improve the pork industry’s environmental status. This will be achieved through the conversion of greenhouse gas emissions to a renewable energy source via the growth of microalgae in integrated piggery wastewater treatment systems (WWTS) (Buchanan et al. 2013). Effluent from AP is rich in ammonia (NH₃), pathogenic organisms, and high-suspended solid (SS) loads. High levels in pig effluent, if untreated, could inhibit algal growth or pose a potential risk to pig health when reused as shed flushing material, which are concerns for reuse (Buchanan et al. 2013). Aerobic treatment for the oxidation of NH₃ to nitrate (NO₃) is a potential method to alleviate the adverse effects of NH₃ on algal growth and to reduce the concentrations of SS and pathogens. The objective of this preliminary experiment was to determine, at laboratory scale, NH₃ oxidation within an aerobic reactor fed AP effluent at an aeration level of 10% saturation (0.7 mg O₂/L) and a 5-day theoretical hydraulic retention time (THRT).

Anaerobic pig slurry (ANPS) collected from an AP at a local South Australian piggery, was pumped intermittently through a bench top aerobic reactor over a 25–30 day period to achieve the desired 5-day THRT. The ANPS was aerated by blowing air intermittently through the reactor to maintain a dissolved oxygen level of 0.7 mg O₂ /L. Influent and effluent samples were collected and analysed at 4–5 day and 2–3 day intervals, respectively. Table 1 summarises the results of a series of chemical analyses performed on inlet and outlet slurry post aerobic treatment, using standard wastewater analysis methods (APHA 1995) to assess the oxidation potential (nitrification) of the system under these conditions.

Table 1.  Mean nutrient levels in anaerobic pig slurry before and after aerobic treatment at 10% saturation and a 5-day THRT. Values are mean ± SD

The mean inlet ammonium (NH₄-N) concentration at the start of the experiment was 1.5 ± 0.7 g/L (mean ± SD) and that of suspended solids was 0.8 ± 0.1 g/L (Table 1). Post-aerobic treatment showed mean NH₄-N and SS levels had decreased by 28.8% and 52.2% respectively. This, in conjunction with increased NO₂-N and NO₃-N from zero in the inlet effluent to 0.2 ± 0.1 g/L and 0.1 ± 0.0 g/L detected in the outlet effluent, demonstrated that NH₃oxidation had occurred. Reducing NH₄-N to its non-toxic form NO₃-N can lead to lower disease potentials associated with NH₃ exposure and improved water quality. Both are vital and beneficial for reuse on-farm.

Findings from this preliminary experiment suggest aeration of ANPS to be a positive candidate for the treatment of piggery waste. Ammonia oxidation did occur, however the conversion of NH₄-N to NO₃-N was relatively low. Further research will assess the oxidation capability of the integrated system under different conditions of DO and THRT to best identify optimal operating conditions to achieve maximum nitrification.