Anaerobic digestion of pre-treated slaughterhouse waste

Anaerobic digestion of pre-treated slaughterhouse waste

Low-rate covered anaerobic lagoons (CALs) offer the Australian red meat processing (RMP) industry an attractive wastewater treatment option with the added benefit of capturing methane-rich biogas that can be combusted to offset onsite fossil fuel consumption. Whilst high-strength, high-fat wastewater generated by the RMP industry provides excellent potential for biogas production, it also presents operational problems and can reduce the performance of anaerobic digestion (AD) systems. Fats, oils and greases, and other solids present in the wastewater are responsible for pipe blockages, degradation of lagoon covers, inhibition of mass transfer of nutrients, and sludge flotation and washout.

This thesis presents an investigation of pre-treatment on AD of high-fat waste cattle slaughterhouse using dissolved air flotation (DAF) sludge as a standard substrate. The first phase of work evaluated four pre-treatment options using biomethane potential (BMP) tests. The pre-treatment methods assessed were thermobaric, chemical, thermochemical and bovine bile as a novel bio-surfactant. Phase 2 examined thermobaric pre-treatment in continuous digestion.

Under batch digestion, thermobaric pre-treatment demonstrated the greatest improvement in the digestion process. Thermobaric pre-treatment was also the most practical for implementation at slaughterhouses, with potential for heat-exchange to reduce pre-treatment cost. Soluble chemical oxygen demand was enhanced from 16.3% in the control to 20.84% (thermobaric), 40.82% (chemical), and 50.7% (thermochemical). Pre-treatment altered volatile fatty acid concentration by -64% (thermobaric), 127% (chemical) and 228% (thermochemical). Lag phase was reduced by 20% in the thermochemical group, and 100% in the thermobaric group. Specific methane production (SMP) was enhanced by 3.28% (chemical), 8.32% (thermobaric), and 8.49% (thermochemical) as a result of pre-treatment.

Bovine bile was dosed at arbitrary concentrations from 0.2-6 g/L. At 0.6 g bile/L, methane yield increased by 7.08%. Doses above 2 g bile/L produced negative impacts on SMP, kinetics and digestion profile. At 6 g/L bile produced a 6% decrease in specific methane production and up to 79% additional inhibitory duration, delayed time of peak methane production 74%, and slowed total digestion time 65%. Reaction kinetics declined linearly with respect to bile addition, reaching half the control value at 6 g/L bile concentration. Subsequent anaerobic toxicity assays using bile in the range of 1-6 g/L revealed the inhibitory nature of bile at higher doses. Economic feasibility assessment showed that, when compared to the current use of bile as a sale product to pharmaceutical companies, the addition of 0.2 g bile/L to existing slaughterhouse waste streams could increase the value of bile to 220% of its current sale value.

Based on the batch BMP results, thermobaric-treated substrate was used for continuous digestion experiments. Thermobaric-treated DAF sludge combined with abattoir wastewater was fed to lab-scale continuous stirred tank reactors (CSTR) for 49 days. While pre-treatment under batch digestion improved methane yield and inhibition, methane yield was decreased by 12.1%, pH was consistently lower, and H2S concentration was 56% higher on average in continuous digestion mode. Under the conditions of this investigation, the benefits measured under batch digestion were not reproduced under continuous digestion. This highlights the value of continuous digestion experiments in evaluating substrates for industrial application.