Improving the economic performance of anaerobic digestion by integrating lactic acid recovery into two-stage food waste digestion

The global production of food waste (FW) is of significant economic and environmental concern, having been estimated to produce 8% of globally produced anthropogenic greenhouse gas emissions and result in the loss of nearly USD1 trillion each year. Consequently, the correct disposal and recovery of value from FWs is a global challenge and responsibility. Anaerobic digestion (AD) is a capable technology which can recycle FW to produce renewable energy and recover nutrients. However high capital, operational, and management costs and low value of biogas and digestate lead to questionable economic benefit. As a result, the AD technology heavily relies on subsidies and policy incentives for feasibility. Integration of lactic acid (LA) production technologies into AD could convert the low-value process into a high-value LA-AD biorefinery, reducing reliance on government support. This study aims to address the above by exploring the integration of LA production to AD within the FW context. This involved detailed investigations into the production of LA from FWs, including within the commercial FW context, and integration of FW fermentation for LA into existing AD facilities. Accordingly, following optimisation of LA fermentation, and exploring the feasibility of recovering LA along with its impact on downstream AD, multiple integration scenarios were proposed detailing the potential economic benefits from integrating LA production into FW AD. The assessment of LA production within the FW context was first explored. For this, LA production within a pre-fermenter at a commercial two-stage FW AD facility was monitored, exploring the impact of environmental conditions, feedstock composition, and operational procedures on LA production performance and stability. Results showed standard operation of the pre-fermenter, favoured the formation of LA leading to LA being the dominant organic acid produced from fermentation. Furthermore, standard operation of the AD facility led to the selective dominance of Lactobacillus, a bacterium commonly associated with LA production. While LA production fluctuated over the monitoring period, the LA concentration was surprisingly stable, especially considering the variation in process variables (pH, temperature, retention time, feed rate, and feed composition). Even so, it was outlined that there would be significant opportunity to improve LA production performance, and consequently, economic performance by targeted process optimisation and control. Optimisation of LA fermentation showed the commercially adapted inoculum was capable of high LA yields and selectivities. In addition, the results showed optimal conditions promoted the growth of Lactobacillus, while alternative flanking microorganisms were inhibited. Moreover, optimisation effectively eliminated the conversion of LA to butyrate, allowing the sustained accumulation of LA. Further study of the commercial inoculum showed LA production could be effectively enhanced by supplementing FW with a simple carbohydrate (sucrose) and implementing partial digestate recirculation. While digestate enhanced LA production, it also increased microbial diversity which promoted the production of alternate organic acids. However, the effects of digestate could be effectively controlled through sucrose addition, which promoted the growth of Lactobacillus and inhibited the growth of the flanking community. Following optimisation of fermentation, the feasibility of recovering LA from complex fermentation media and its impact on downstream AD performance was explored. While real commercial broth reduced LA uptake, compared to pure LA solutions, LA was effectively recovered from highly complex fermentation media. Moreover, LA recovery only led to a minor reduction in methane production following the AD of the solid and liquid extraction residues. In this respect, LA production could outweigh the loss in methane production in terms of relative value, indicating the LA-AD biorefinery concept could be commercially attractive. A technoeconomic assessment indeed showed the benefit of integrating LA production into two-stage FW AD, yielding a highly profitable scenario. Furthermore, while integration scenarios were most profitable, Greenfield LA-AD biorefinery scenarios showed significantly higher profitability estimates compared to sole FW AD. Finally, the insight achieved into different aspects of the LA-AD biorefinery led to a series of recommendations for future research in the context of the FW biorefinery concept