The disposal of waste sludge is a problem of growing importance in waste water treatment plants. It can represent up to 50 % of operating costs. Anaerobic digestion plays an important role due to its ability to transform organic matter into biogas which is used as a renewable source of energy. The aim of the study was to examine whether the addition of supplements or pre-treatment of concentrated secondary (waste-activated) sludge could improve the production of biogas from it and also reduce the final content of organic matter in the digestate in Domžale-Kamnik wastewater treamtment plant (WWTP). Five experiments were performed in two pilot-scale CSTR bioreactors (one as the experimental and the other as the control reactor) with total volume of 200 L. Digestate from a full-scale anaerobic digester was used as an inoculum. Experiments were conducted for 30 days; duringthat 10 L (three times) and 20 L (two times) of concentrated secondary sludge was added as a substrate. To enhance the biogas production in anaerobic digestion process two different commercial enzymatic/bacterial supplements (MethaPlus® L120, Agrovit and BioCat+™, Citadel) and mechanical desintegration (hydrodinamic cavitation) as a pre-treatment process were applied. During the process pH, alkalinity, Kjeldahl's and ammonium nitrogen, shortchain fatty acids, chemical oxygen demand and organic matter were monitored off-line and volume of biogas on-line. The mechanical pre-treatment at double volumetric loading rate enhanced the biogas production most successfully, up to 10,8 %. Addition of enzymatic supplement at single volumetric loading rate enhanced the biogas production up to 6,3 %. Meanwhile, addition of bacterial supplement at single volumetric loading rate reduce biogas production up to 8 %.Content of organic matter did not decreased during experiments.
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