Biological stoichiometric analysis of nutrition and ammonia toxicity in thermophilic anaerobic co-digestion of organic substrates under different organic loading rates

This study used chemostats under different Organic Loading Rates (OLRs) to investigate the co-digestion of kitchen waste, swine wastewater sludge, and fruit and vegetable waste. In kitchen waste, mean Total Chemical Oxygen Demand (TCOD), Oil and Grease (O&G), and moisture content (MC) was 101.5 g/L, 33.0 g/L, and 82.7%, respectively. The TCOD/Total Kjeldahl Nitrogen (TKN) and TCOD/Total Phosphorus (TP) of kitchen waste were 319.5 and 230, respectively. In swine wastewater sludge, TCOD/TKN was 4.56–43.9 and TCOD/TP was 2.02–31.8. Biodegradability tests of fruit and vegetable waste showed that COD removal exceeded 56%, and methane recovery exceeded 80%. Co-digestion of these three organic wastes in chemostats suggests that the system functioned stably up to an OLR of 9.52 g COD/L-d at a Hydraulic Retention Time (HRT) of 5 days. When the OLR increased to 12.54 g COD/L-d, average COD and Volatile Suspended Solids (VSS) removal efficiencies decreased from 90% to 76.5% and from 93% to 76.5%, respectively. The analyzed NH3–N concentration is 28% less than the stoichiometry-predicted concentration. The discrepancy may be due to differences in substrate biodegradabilities, TKN sampling and analysis procedures, and parameters used for stoichiometry calculations.

► The TCOD and TCOD/TKN of kitchen waste were 101.5 g/L and 319.5, respectively.
► Batch study using respirometer showed all COD removal efficiencies exceeded 50%.
► Co-digestion of organic wastes showed system functioned stably up to 9.5 g COD/L-d.
► The analyzed NH3–N was 28% less than stoichiometry-predicted NH3–N concentration.