Anaerobic co-digestion of food and algal waste resources

Abstract

Anaerobic digestion is a key energy and resource recovery technology. This work investigated potential organic waste resources to co-digest with household food waste (HFW) to stabilise the process and future-proof feedstock availability. This included novel feedstock macroalgae (seaweed) waste (SW). Hydrothermal (autoclave) pretreatment was also investigated to optimise energy recovery from HFW and SW. Preliminary experiments investigated the behaviour of HFW co-digested with either a green waste (GW) or paper waste (PW), using a batch-test laboratory scale and systematic approach with a revised waste mixture preparation method. Following preliminary trials, the co-digestion of HFW/SW was investigated using an air-dried SW mixture. Batch experiments to determine the biomethane potential (BMP) at different ratios of HFW to SW were set up. Co-digesting HFW and SW at ratio 90:10 (d.w.) achieved a BMP similar to HFW alone (252±13 and 251±1 cm3 g-1 VS, respectively), and a peak methane yield for HFW:SW (90:10) at day 12 of 69±3% compared to a peak methane yield for HFW at day 19 of 70±3%. Addition of SW optimised the C/N ratio, increased concentrations of essential micronutrients and produced an overall increase in reaction kinetics. Concentrations of SW ≥25%, associated with high sulphur levels, reduced final methane productivity. Analysis of the macroalgae strains L. digitata, U. lactuca and F. serratus from the SW mixture was carried out to compare mono-digestion and co-digestion with HFW at a 90:10 ratio and the effect of autoclave pretreatment at 136°C. Co-digestion had a positive impact on methane yields for U. lactuca and F. serratus, whilst autoclave pretreatment had no significant impact on the SW strains When results were modelled for a 320 m3 anaerobic digester treating 8m3 feed per day the theoretical energy balance showed that optimal energy production from pretreated HFW at 8.09 GJ/day respectively could be achieved. To verify the suitability of using macroalgae, known to readily uptake polycyclic aromatic hydrocarbons (PAH), toxicity tests were used to determine the impact of phenanthrene sorbed by U. lactuca on the AD process. Despite U. lactuca’s ability to biosorb phenanthrene in under 2 hours, no impact on the AD process was observed. Overall, results of this study demonstrated that co-digestion of HFW and SW, at batch laboratory scale, provide a viable and sustainable waste revalorisation solution. In addition, low temperature autoclave pretreatment increased methane production (p=0.002) from the AD of HFW.