Activated sludge (AS) contains substantial quantities of hydrolytic enzymes. These are adsorbed to the sludge flocs or embedded in extracellular polymeric substances surrounding microbial cells.
Ultrasonic disruption of the sludge flocs significantly increases the enzyme extractability from the sludge. However, this does not increase the anaerobic digestion process performance.
Increased Methane Production
The improved treatment properties associated with sludge pretreatment can have numerous economic benefits to the Water Industry, including enhanced sludge settleability, dewaterability and anaerobic digestion for biogas production. This ultimately enables the re-use of sludge as a valuable resource, promoting sustainable wastewater management and increasing renewable energy generation.
The results showed that sludge hygienisation significantly increased the methane potential of mixed primary and biosludge, compared to untreated sludge. Moreover, the effect on methane potential was more significant than the effect of individual enzymes on sludge solubilisation and TSS reduction. This was attributed to the ability of the enzymes to disrupt the refractory cell wall, thereby releasing inner cellular materials that are otherwise not accessible to the microorganism consortium during AD.
These cellular materials, including polysaccharides, amino acids and fatty acid chains, are normally converted to methane by the autotrophic methanogens. The fact that the microbial community is able to absorb these nutrients even after effective sludge pretreatment, suggests an ecological strategy for surviving in conditions of nutrient deprivation, similar to the behaviour of other organisms with the ability to survive under extreme stress conditions (e.g. fungi).
The microbial activity (as dehydrogenase activity) at various sampling points was analyzed using the Enzyme Activity Index, EAI. The EAI value of sludge hygienised with cellulase and amylase was found to be comparable to that of the untreated sludge at sample point 6, taken from the aeration tank thickening belt.
Lower Strength Sludge Liquors
Anaerobic digestion of sewage sludge converts organic carbonaceous material into methane and carbon dioxide. The end products are gases, stabilised sludge solids that can be dewatered and disposed of or further processed, and sludge liquor that is often discarded or further treated.
The addition of hydrolytic enzymes can enhance anaerobic sludge digestion by disrupting the microorganism's cell envelope in order to promote biodegradation. The disruption of the EPS matrix and cell wall helps to reduce the resistance that this material poses to direct anaerobic digestion by breaking down refractory compounds such as polysaccharides, glycoproteins, and cellulose.
Enzyme pretreatment can also be used to improve sludge filterability. The enzymes significantly increased the pore width and surface area of the sludge granules, making them more readily accessible to the digestive bacteria. Several previous experiments using aerobic pretreatment, temperature-phased anaerobic digestion (TPAD), and combinations of these methods showed that the use of enzymes could improve sludge dewaterability by increasing methane production.
The improved digestibility of sludges with the assistance of enzymes also reduces the concentration of biochemical oxygen demand in the sludge effluent. This is because the enzymes help to maintain a high level of food for the microorganisms throughout the digestion process, which results in lower BOD and COD levels in the final effluent [20]. The higher the concentration of food available for the biomass, the more the microorganisms will consume and therefore the more the amount of BOD and COD will be removed from the final effluent.
Reduced Disposal Costs
As a consequence of the enhanced bioconversion in anaerobic digestion, significantly lower disposal costs can be achieved, as the produced methane may be used to replace fossil fuels and the digestate can also be applied as a fertiliser. However, the production of methane and sludge liquids from wastewater sludge is primarily dependent on the digester operational conditions and the content of the sludge feedstock.
A wide range of different pretreatment strategies for enhancing the microbial consortium's enzymatic activity have been developed (aerobic pretreatment, temperature-phased anaerobic digestion, enzyme-assisted pretreatment). The latter is based on combining the inherent enzymatic sludge degradation with exogenous enzymes that assist in degrading refractory organic compounds that resist sludge's natural enzymatic attack.
Sonication of sludge has shown to be an effective method for enhancing the microbial consortium's performance during AD. It can cause the loosening or even destruction of refractory EPS structures, thereby making inner cellular compounds accessible to the microorganisms. This improves the solubilization of these organics and thus their upcoming utilization. In addition, sludge dewaterability is enhanced owing to the release of bound water in the pretreated sludge.
To evaluate the impact of sonication on the microbial consortium in an industrial WWTP, a sludge sample was obtained from the thickening belt and subjected to a series of sonication experiments with various conditioning times. The activities of protease, lipase, - and b-amylase and cellulase were assessed, as these hydrolytic enzymes are the most important for anaerobic digestion. The results showed that the sonication treatment induced an optimal level of enzyme extraction from the sludge. The extracts were found to be rich in protease, - and a-amylase but lacking in b-amylase. The activities of these enzymes were significantly increased by the addition of the surfactant Triton X100 to the extraction solution.
Increased Productivity
The enzyme pretreatment significantly improves the dewaterability of WASP, which allows for a significant reduction in the amount of water needed for sludge conditioning and thereby lowers the costs associated with sludge disposal. This is primarily due to the fact that the addition of cellulase and protease significantly increases the amount of soluble protein in the sludge, thus increasing the water holding capacity of the sludge.
The increase in soluble protein also significantly reduces the amount of water required for anaerobic digestion. This is due to the fact that soluble protein can be converted into methane during AD, which has a lower density and hence a lower specific gravity than water.
As a result of the increased soluble protein content, there is also an increase in the rate at which the digested sludge can be pressed. This, in turn, results in a much shorter processing time and lower sludge cake strength.
Moreover, the hydrolytic enzymes in sludge flocs are apparently retained, despite extreme nutrient deprivation. This is likely a microbial ecological strategy to ensure that hydrolases are available when organic carbon substrates become available again and to minimise the waste of enzyme synthesis.
Many studies have investigated methods of enhancing sludge's AD performance through pretreatment. Chemical Oxygen Demand (COD) solubilization is a typical/common parameter used for this purpose, and has been found to correlate directly with enhanced AD performance. However, not all COD solubilization improvements are equally valuable.