Enhanced Microbial Digestion is the process of adding specific cultures of microorganisms to WSP in quantities sufficient to increase the average rate of hydrolysis, resulting in reduction of accumulated organic sludge. Many species of bacteria are capable of producing exoenzymes that hydrolyse wastewater sludge, but they grow at a slow rate so don’t often occur in high numbers in WSP. Moreover, different exoenzymes hydrolyse specific organic function groups, so different exoenzymes are needed to digest the different (e.g. starch, cellulose, protein and fat) components of WSP sludge. Therefore, Enhanced Microbial Digestion adds sufficient quantities of a diverse range of exoenzyme producing bacteria, which together can significantly increase the rate of hydrolysis compared to the natural rate of hydrolysis in the WSP.


One version of Enhanced Microbial Digestion is the addition to WSP of mixed culture bacteria that are capable of high rate and broad spectrum enzyme activity. This method requires that the added bacteria are capable of surviving in the WSP environment and capable of producing the quantity and quality of needed exoenzymes to significantly increase the existing rate of hydrolysis in the WSP. Over time, the added bacteria that are producing the exoenzymes will die-out due to their slow reproduction rate. Therefore, repeat additions of bacterial cultures will be needed to maintain the increased hydrolysis rate.

A second version of Enhanced Microbial Digestion is the addition of commercially prepared enzymes to the sludge in WSP. With this option, the enzyme product must contain all the digestive functions needed to hydrolyse the WSP sludge. As enzymes are proteins, and protein digestion must occur for Enhanced Microbial Digestion to be effective, enzyme products will have a limited time of usefulness in the WSP. Frequent repeat doses will be required to continue treatment. Both versions of Enhanced Microbial Digestion are available in New Zealand.


Hydrolysis is considered the slowest ‘rate-determining’ step in Enhanced Microbial Digestion. The product of hydrolysis is low molecular weight soluble organic material which is readily consumed by any bacteria that are present in the wastewater system. Thus, in theory, the use of Enhanced Microbial Digestion should not cause an increased discharge of soluble organic material in the final effluent. However, especially in systems with extremely short retention times, care must be used to ensure that effluent BOD / COD concentrations do not increase while using Enhanced Microbial Digestion.


All of the following are important when evaluating the possible use of Enhanced Microbial Digestion to reduce accumulated sludge in the WSP

  • Sludge solids concentration and volume. Any use of Enhanced Microbial Digestion will require knowing the wet sludge volume and dry solids concentrations in the WSP prior to application. Accurately measuring the sludge volume and solids concentration during the course of application is essential to ensuring measurable treatment success. As Enhanced Microbial Digestion progresses and organic solids are consumed in the upper more biologically active sludges, the lower compacted solids will begin to hydrate and increase in volume until equilibrium in solids concentration is reached throughout the sludge column. This can be monitored as a progress indicator during the first phases of treatment. The initial reduction in solids concentrations can equate to the removal of significant quantities of material while the overall sludge volume may not have reduced by much.
  • Safety. Whether using an enzyme product or mixed bacterial cultures or a combination of both, the safety of applicators and compliance with New Zealand and local laws with respect to biosecurity are essential. All users of Enhanced Microbial Digestion should ensure that the supplier provides the proper Safety Data Sheets, and where applicable, proof of legal importation of the products into New Zealand.
  • Effluent Quality. In WSP with heavy sludge accumulation or where effluent compliance is borderline, caution must be exercised to ensure good effluent quality. In such situations, a gradual stepwise initiation of the treatment program with ongoing monitoring is essential.
  • Cost Effectiveness. The cost / benefit of standard sludge removal vs Enhanced Microbial Digestion should be compared. Enhanced Microbial Digestion, when properly implemented and measured, has the potential to significantly reduce the cost of sludge removal in WSP.

The rate of reduction of accumulated sludge will depend on many factors including the age of the sludge, history of chemical addition to the sludge, presence or absence of aeration and mixing, climate, and influent loading. For example, Enhanced Microbial Digestion will theoretically digest organic sludge, but will not digest inorganic sludge (grit). Therefore, the amount of sludge reduction that can be achieved with an old, digested pond sludge which has a high inert solids fraction is less than a younger sludge with a lower inert fraction.

The most important considerations for the WSP manager are to know the general rate of sludge build up over time, the starting sludge volume and solids concentrations, intermediate and final sludge inventory, maintaining or improving the quality of the final effluent discharge, and a cost comparison (e.g. Enhanced Microbial Digestion compared with sludge removal, dewatering and disposal).