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).

Evaluation of the anaerobic co-digestion of sewage sludge and tomato waste at mesophilic temperature.

Sewage sludge is a hazardous waste, which must be managed adequately. Mesophilic anaerobic digestion is a widely employed treatment for sewage sludge involving several disadvantages such as low methane yield, poor biodegradability, and nutrient imbalance. Tomato waste was proposed as an easily biodegradable co-substrate to increase the viability of the process in a centralized system. The mixture proportion of sewage sludge and tomato waste evaluated was 95:5 (wet weight), respectively. The stability was maintained within correct parameters in an organic loading rate from 0.4 to 2.2 kg total volatile solids (VS)/m(3) day. Moreover, the methane yield coefficient was 159 l/kg VS (0 °C, 1 atm), and the studied mixture showed a high anaerobic biodegradability of 95 % (in VS). Although the ammonia concentration increased until 1,864 ± 23 mg/l, no inhibition phenomenon was determined in the stability variables, methane yield, or kinetics parameters studied.