Microwave and Ultrasound Based Methods in Sludge Treatment: A Review

The amount of waste activated sludge (WAS) is increasing annually, and since it presents potential environmental and health-related risks, an appropriate treatment and stabilization process is needed. It has been shown in numerous studies in the past few decades that amongst the advanced treatment methods of sludge, microwave and ultrasound-based processes offer promising and effective alternatives. The main advantage of these physical methods is that they are energy-efficient, easy to implement and can be combined with other types of treatment procedures without major difficulties. In this review article we would like to present the recent scientific results of the microwave, ultrasound and combined (microwave-alkaline, microwave-H2O2, ultrasound-alkaline and ultrasound-H2O2) treatment of wastewater sludge, in terms of different process-efficiency indicators. Although the obtained results somewhat vary between the different scientific papers, it can be undoubtedly stated that both MW and US—either individually or in combination with chemical treatments—can enhance several aspects of sludge processing, like increasing the SCOD/TCOD rate, disintegration degree (DD), or the anaerobic digestibility (AD), but the extent of these increments clearly depends on the treatment conditions or parameters.

The effect of free nitrous acid pretreatment on the anaerobic digestibility of thickened waste activated sludge

Sludge pretreatment technologies as an avenue to improve solids handling in a WWTP has gained attention and significant research efforts are being directed towards studying several available techniques. The use of FNA as a chemical pretreatment for the AD has shown the potential to enhance the hydrolysis stage by releasing the internal organic matter of TWAS via its biocidal action. The effect of FNA on improving the biodegradability of TWAS was investigated in this thesis. The effect of the FNA on the TWAS characteristics and the methane production in batch tests was first studied. The optimum FNA dose was determined from the batch tests based on both solubilization and methane yields and then tested in a semi-continuous flow system. As the semi-continuous flow system failed when the optimum FNA dose obtained from the batch study was used, another set of semi-continuous flow experiments were conducted using different FNA doses

The effect of free nitrous acid pretreatment on the anaerobic digestibility of thickened waste activated sludge

Sludge pretreatment technologies as an avenue to improve solids handling in a WWTP has gained attention and significant research efforts are being directed towards studying several available techniques. The use of FNA as a chemical pretreatment for the AD has shown the potential to enhance the hydrolysis stage by releasing the internal organic matter of TWAS via its biocidal action. The effect of FNA on improving the biodegradability of TWAS was investigated in this thesis. The effect of the FNA on the TWAS characteristics and the methane production in batch tests was first studied. The optimum FNA dose was determined from the batch tests based on both solubilization and methane yields and then tested in a semi-continuous flow system. As the semi-continuous flow system failed when the optimum FNA dose obtained from the batch study was used, another set of semi-continuous flow experiments were conducted using different FNA doses

Effective Pre-Treatments for Enhancement of Biodegradation of Agricultural Lignocellulosic Wastes in Anaerobic Digestion – A Review

Agricultural crop residues like stems, straws and leaves are valuable resources for biofuel production, especially methane, due to anaerobic digestion. Biogas from agricultural lignocellulosic wastes is capable of attaining sustainable energy yields without environmental pollution. Farmers in many developing countries burn these wastes throughout their fields, imposing environmental hazard due to emission of greenhouse gases. The main problem in this field is the recalcitrance of the agricultural lignocellulose waste that limits its enzymatic degradation and hydrolysis efficiency and consequently decreases biogas production. Therefore, efficient pre-treatments prior to anaerobic digestion are essential. Various pre-treatment methods are used for increasing the anaerobic digestibility of lignocellulose biomass, such as physical (mechanical, thermal, etc.), chemical, biological and combined pre-treatments. This paper reviews different pre-treatments used in anaerobic digestion for the agricultural lignocellulosic wastes and explains the advantages and disadvantages of each. The most frequently used pre-treatments for main agricultural wastes in process of biogas production are also introduced.