Biogas Production Potential from the TORWASH® Filtrates
The F-CUBED project´s pilot tests used three different feedstocks of paper sludge, olive pomace and orange peels which were hydrothermally treated in the TORWASH® reactor and then dewatered and separated into a solid fraction and a liquid filtrate. After treatment, the filtrate still contains organic material that could be used for generating biogas in an anaerobic bioreactor.
The potential for biogas production is measured in an anaerobic biodegradability test for each of the three filtrates. The anaerobic biodegradability gives information regarding the anaerobic treatability of the waste stream. The aim of the biodegradability test is to provide information about the maximum fraction of the COD (chemical oxygen demand) in a particular wastewater that can be converted anaerobically. Anaerobic micro-organisms convert the organics that are present in the filtrate into biogas, mainly consisting of methane. The presence of sufficient nutrients (nitrogen and phosphate) for the needs of the micro-organisms is also checked. The tests were conducted under standardised conditions of a temperature of 35 °C, a neutral pH and sufficient micro- and macronutrients to ensure representative results.
Results show an anaerobic biodegradability of 62% for paper sludge, 57% for olive pomace and 77% for orange peels. In all tests, sufficient nitrogen and phosphorus were available and therefore would not need to be added in a full-scale anaerobic reactor.
The anaerobic biodegradability testing demonstrated that all feedstocks have the potential for biogas production and F-CUBED partner PAQUES detailed the parameters for the preliminary designs of full-scale anaerobic reactors and the associated investment and operational costs based upon the volumes and feed capacities of the residues. Estimated methane production based on the full-scale reactor designs for paper sludge, olive pomace and orange peels are 1,240, 6,540 and 990 Nm3/d (normal cubic metre per day) respectively. The generated biogas can be used as energy source for the TORWASH® process and the downstream processing steps.