Leachate after aerobic stabilization of municipal solid waste supplemented by waste glycerine from saponification to improve biogas production during co-digestion

Highlights: • Up to 13% waste glycerine (WG) content in the feedstock ensured effective co-digestion. • Methane content in biogas lower after co-digestion with 13% WG than with 6% WG. • Leachate_OFMSW with a low N–NH₄:TKN ratio (0.1) caused rapid ammonium consumption. • The COD:TKN ratio affected characteristics of post-processing wastewater and biogas. • WG from saponification is an excellent co-substrate for leachate_OFMSW. Co-digestion of leachate from aerobic stabilization of OFMSW (leachate_OFMSW) with waste glycerine (WG) from saponification is a promising approach for municipal waste and wastewater treatment. The effect of feedstock composition on co-digestion efficiency was examined. Two feedstocks with different contents of WG (6%, 13%) were used, which affected the COD:TKN ratios in those feedstocks (60, 100). Leachate_OFMSW provided nitrogen and the micro- and macroelements. In both feedstocks, the N–NH₄:TKN ratio was low (0.08), indicating that the dominant form of nitrogen was organic nitrogen (N_org). Co-digestion was carried out in upflow anaerobic-sludge blanket reactors (37 ± 2 °C; OLR, 4 kg COD m⁻³ d⁻¹). This process ensured highly effective COD removal (over 97%). At a feedstock COD:TKN ratio of 60, specific biogas production (SBP) was ca. 0.22 m³ kg⁻¹ COD, and methane content, 61.6%. Increasing the COD:TKN ratio to 100 significantly increased SBP to ca. 0.36 m³ kg⁻¹ COD, but reduced biogas quality (54.2% CH₄). Although N_org had to be ammonified to ammonium because of the low N–NH₄:TKN ratio in the feedstocks, the ammonium concentration in the post-processing wastewater remained low. Moreover, the low ammonium concentrations meant that the pH (8.8) did not have a toxic effect on anaerobic co-digestion (free ammonia concentrations were merely ca. 28 and 5.5 mg L⁻¹, respectively). However, at higher feedstock COD:TKN ratios than those tested, ammonium could be exhausted, which would impede methane production. These results indicate that WG is an effective co-substrate for leachate_OFMSW if an appropriate amount of WG is added to the feedstock.