Anaerobic treatment of solid manure residues

Abstract

Spent bedding, which is a mix of bedding stubble, faeces, urine, spilt water and animal feed, is a major by-product of livestock farming in Australia and the United States. In Australia, such residues are stockpiled for passive composting prior to land spreading to harness the nutrient value. Key concerns are uncontrolled greenhouse gas emissions, odour, losses of nutrients, and the potential for ground and surface water contamination. A sustainable management alternative is anaerobic digestion (AD), whereby organic matter in the spent bedding is converted into useable methane energy and nutrients are mobilized for subsequent recovery. However, AD technology is not currently available in Australia at a sufficiently low cost and low level of complexity for on-farm adoption. In addition, the AD characteristics of solid manure residues are not well understood, posing significant uncertainty for AD design and optimization. In response, this thesis examines a leachbed AD approach, which is particularly suitable for high solids wastes such as spend bedding, and can be cost-effective and simple to construct and operate. A pilot scale (200 L) leachbed system was constructed and operated. Spent bedding was collected from Australian piggeries and wetted with water leachate in the enclosed leachbed via two distinct operating modes: trickling and flood-and-drain. The water leachate was heated in both configurations, aimed at a 37 oC operating temperature. Results showed comparable methane recovery for both the trickling and flood-and-drain modes at 50% of the biochemical methane potential (B0 = 195 - 218 L CH4 kg VSfed-1) in both cases. This indicated that AD performance was insensitive to the mode of leachate flow. However, the flood-and-drain leachbed did mobilise more particulates into the leachate than the trickling leachbed, which could cause a materials-handling problem with pumping of leachate on-farm. Inoculation with solid digestate from a previous leachbed (inoculum-to-substrate ratio of 0.22 on a VS basis) hastened the AD start-up, but methane recovery remained at 50 % of the Bo. Post-digestion testing indicated that methane recovery was limited by insufficient indigenous inoculum and/or by chemical inhibition. To clarify the effects of operating conditions on AD performance at high solids content, a series of batch tests were performed at smaller laboratory scale. These tested the viability of indigenous inoculum in spent bedding (from the manure component), together with the effects of operating temperature and solids concentration. Results showed that spent bedding actually did have sufficient indigenous inoculum to recover the full Bo of the spent bedding. However, to operate at higher solid loading (≥ 10 % TS content) and higher temperatures (55 oC), methanogenic activity could be boosted by inoculation with leachate and/or solid digestate from a former leachbed batch. The use of leachate as an inoculum would preserve leachbed volume for fresh waste and easier handling. Chemical analysis of digestate revealed that leachbeds could have high concentrations of the chemical inhibitors ammonia and humic acid (HA). In the pilot trials ammonia concentration was below the inhibitory threshold for AD, so the focus of further study shifted to HA. Inhibition testing showed that hydrolysis was more prone to inhibition than methanogenesis at HA concentrations of 0 to 20 g L-1. Also carbohydrate hydrolysis was more susceptible to HA inhibition than protein hydrolysis. The results suggested that at HA concentration below 5 g L-1, hydrolysis inhibition was due to inactivation of hydrolytic enzymes by HA. However, beyond HA concentration of 5 g L-1, AD seemed to be affected by more complex mechanisms. Inhibition appeared to be reversible, but recovery rate depended on hydrolytic activity. Increased HA inhibition resilience may result from higher microbial activity or microbial concentration, independent of differences in microbial community composition. Overall, the study provided insight and certainty on leachbed performance factors for spent bedding. Likely future applications would include decentralized installations, with leachbeds operating in tandem with existing covered anaerobic ponds for enhanced methane recovery and water reuse.