One- and two-stage anaerobic co-digestion of cucumber waste and sewage sludge.

The demand for uniformly sized and shaped produce that are aesthetically pleasing results in significant food waste throughout the world. Cucumber waste is a major agricultural waste product in a number of countries, especially areas with high pickle production. Opportunity exists for wastewater treatment plants containing anaerobic digesters to utilize cucumber agricultural and industrial waste for biogas production. The biomethane potential of cucumber waste as a substrate for co-digestion with sewage sludge was assessed. The impact of long-term co-digestion of cucumber was then evaluated using mesophilic continuously stirred tank reactors (CSTRs), in both single- and two-stage anaerobic co-digestion with sewage sludge. Ground cucumber waste was added to sewage sludge at 8% of the volume (4.5-4.6% of the organic load) and CSTRs were maintained for five hydraulic retention times (HRTs). One-stage co-digestion of cucumber waste produced comparable gas levels as CSTRs without cucumbers (averaging 219 and 221 m3/kgVS/h, respectively) after two HRTs. The two-stage cucumber co-digestion CSTR averaged 64% higher specific gas than the control and single-stage digester, although the volumetric gas produced was lower (averaging 152 m3/kgVS/h) likely due to gas loss in the first stage resulting in a lower organic load rate. After four HRTs, relative methanogen content showed dramatic differences in levels of hydrogenotrophic methanogens for the two-stage digester, while the one-stage digester containing cucumber waste showed minor differences relative to the control. Cucumber waste co-digestion with sewage sludge is effective although numerous conditions could be utilized to optimize gas production.