Enhancement of biogas generation during anaerobic digestion of excess activated sludge (a review)

Summary

In the process of biological wastewater treatment vast amounts of sludge are generated. Among the methods of sludge handling anaerobic digestion holds a prominent place combining both possible biogas production and prevention of secondary environmental pollution. The rate-limiting stage of anaerobic digestion is hydrolysis. Sludge pretreatment is a common way to increase the rate of hydrolysis and enhance biogas production; developing pretreatment options has been an area of noticeable exploratory activity. Methods of chemical pretreatment (acidic, alkaline, electrochemical oxidation, ozonation, Fenton process, persulfate process, treatment with calcium peroxide), biological pretreatment (two-stage thermophilic/mesophilic process) and physical pretreatment (thermal hydrolysis, microwave treatment, electrokinetic disintegration, ultrasonic treatment, high pressure homogenization) are considered. The patented technologies for pretreatment of wastewater sludge and the main manufacturers of the relative equipment are presented. As an alternative to the traditional anaerobic digestion a microbial electrolysis cell is considered where methane generation during electromethanogenesis occurs in the presence of exoelectrogenic bacteria. A number of wastewater sludge pretreatment technologies are currently available for the implementation on an industrial scale. In this regard, an urgent need in the development of a standardized methodology for analyzing the technical and economic indicators of various anaerobic digestion systems is evident in order to select an optimal option for specific conditions.

Key words

surplus active silt , anaerobic fermentation , thermal hydrolysis , pretreatment , acidic treatment , alkaline treatment , electrochemical treatment , Fenton process , persulfate process , microbial electrolysis cell , electrokinetic disintegration , ultrasonic treatment , homogenization , microwave treatment

Study of the eco-economic efficiency of electrochemical pH correction for reducing hardness salts

Summary

In industry, water recycling systems or units have been widely used to cool equipment and other industrial processes. One of the main problems associated with operating these systems is scale formation owing to the presence of hardness salts, such as calcium and magnesium compounds. Traditional addressing water hardness problems involves the use of chemicals such as phosphates and polyphosphates to bind calcium and magnesium ions, preventing their precipitation. Electrochemical pH correction provides for a more environmentally friendly solution. Changing water pH by electrolysis enhances the precipitation of hardness salts as insoluble compounds, that can be further removed from the system. The theoretical foundations and mechanism of water softening by electrochemical treatment are examined. It is noted, however, that direct reduction of hardness salts using electrochemical pH correction alone is not possible. It is expected that electrochemical methods can significantly enhance water softening processes and improve the efficiency. The eco-economic efficiency of the electrochemical pH correction method for the side stream filtration system of an industrial water recycling unit is estimated.

Key words

softening , electrolyzer , electrochemical treatment , water hardness , water recycling facilities , eco-economic efficiency