Mixture optimization of anaerobic co-digestion of tomato and cucumber waste

Greenhouse cultivation has significantly increased the production of vegetables and reduced dependence on environmental conditions. In Mediterranean areas, vegetable crops are nowadays one of the most important sources of organic waste generation. Anaerobic digestion is among the methods used to treat this type of biodegradable waste. However, the selection of the organic wastes to be subjected to this microbial treatment is a crucial aspect due to seasonality and simultaneity of the original crops. In this sense, as waste does not have frequently the proper nutrient balance, co-digestion with other substrates generated in the same geographical area is recommended to ensure the correct development of the process. This work studies the mesophilic co-digestion of tomato waste (TW) and cucumber waste (CW), which are common greenhouse wastes that do not contain an adequate ratio among nutrients (C/N/P) and are generated simultaneously. The influence of the percentage of both wastes in the mixture on the overall performance of the process was evaluated. The combination of TW and CW was found to be feasible in terms of stability, biodegradability and methane yield, which reached a value of 292 mLSTP CH4/g VS (STP: standard temperature and pressure, 0°C and 760 mmHg) for the percentage of tomato in the mixture, which is considered optimal at 55-75%. The most suitable organic load rate was determined for a percentage of 65% of TW, reaching a value of 1562 kg(waste) m(-3) month(-1).