Lactic acid from mixed food wastes at a commercial biogas facility: Effect of feedstock and process conditions

Anaerobic digestion facilities can become biorefineries to produce higher-value products together with biogas energy and nutrient-rich digestate. To inform future biorefinery concepts with lactic acid recovery, the current study monitored organic acids in a pre-fermentation stage at a commercial anaerobic digestion facility. The study assessed lactic acid production performance and the impact of mixed food waste feedstocks and process conditions. Feed rate and feedstock composition varied weekly with waste availability. Normal operating conditions of the pre-fermentation stage included warm ambient conditions (24–35 °C), low pH (3.45 ± 0.03), a short hydraulic retention time (1–3.5 days) and stable organic loading rate (12 ± 2 kgVS.m⁻³.day⁻¹). These conditions favoured lactic acid, being dominant at an encouraging average concentration of 21.70 g L⁻¹, notably without any process optimisation or control. Lactobacillus constituted the majority of the microbial community in the pre-fermentation stage (98.1 %–99.1 % relative abundance) with an unknown Lactobacillus species and L. reuteri being the major species present. Grain processing waste and milk paste were positive influencers of LA concentration. The monitoring results, together with a simple economic evaluation, indicated that lactic acid recovery from a commercial food waste anaerobic digestion facility had baseline feasibility. In addition, there would be significant opportunities to increase economic performance by targeted process control and optimisation.