The use of hydrodynamic cavitation for waste-to-energy approach to enhance methane production from waste activated sludge.

Anaerobic digestion in wastewater treatment plants converts its unwanted end product - waste activated sludge into biogas. Even if the process is well established, pre-treatment of the sludge can further improve its efficiency. In this study, four treatment regimes for increasing methane production through prior sludge disintegration were investigated using lab-scale cavitation generator and real sludge samples. Three different cavitating (attached cavitation regime, developed cloud shedding cavitation regime and cavitation in a wake regime) and one non-cavitating regime at elevated static pressure were studied in detail for their effectiveness on physical and chemical properties of sludge samples. Volume-weighted mean diameter D[4,3] of sludge's particles decreased by up to 92%, specific surface area increased by up to 611%, while viscosity (at a shear rate of 3.0 s-1) increased by up to 39% in the non-cavitating and decreased by up to 24% in all three cavitating regimes. Chemical changes were more pronounced in cavitating regimes, where released soluble chemical oxygen demand (sCOD) and increase of dissolved organic matter (DOM) compounds by up to 175% and 122% were achieved, respectively. Methane production increased in all four cases, with the highest increase of 70% corresponding to 312 mL CH4 g-1 COD. However, this treatment was not particularly efficient in terms of energy consumption. The best energy balance was found for the regime with a biochemical methane potencial increase of 43%.

Self-purification processes of Lake Cerknica as a combination of wetland and SBR reactor.

Lake Cerknica is a periodically intermittent lake which may extend its surface up to 26 km(2) and reach 80 km(3) in volume. Lakes tend to age over time. Lake Cerknica does not possess properties of a real lake or those of usual wetlands thus making all of its physical, chemical, and biological processes unique. The feature with the greatest impact on plant development and animal life is the alternating nature of the lake where water from the lake is drained through the lake's bottom dries and refloods. Lake Cerknica was compared with a plant-based water treatment system and a sequential reactor with an approximately 6-month filling and emptying cycle. Lake Cerknica's basic processes of nutrient purification are the deposition of suspended nutrients on the lake bottom, integration of nutrients in plants, and partial denitrification.