BIOCONSULTING and Engineering Service, S.L. (BIOCONS)

Enzyme Enhanced Sludge Digestion


The core process in the treatment of municipal sludge is the digestion, i.e. anaerobic fermentation. The efficiency rate of this process, which takes some 20 to 30 days, is however, unsatisfactory and impairs its cost-efficiency.

The reasons for this mediocre digestion efficiency lie, in the first instance, in the composition and properties of the sludge. Due to the high hydrolysis stability of the microbial cell wall, and the undiluted constituents of the cellulose fibres, hydrolysis can only proceeds very slowly - as a result of which only mediocre digestion rates are achieved, despite the long digestion period.

Enzymes are bio-catalysts, which when added in small quantities, can substantially influence the direction and speed of bio-chemical reactions. Biocatalytic hydrolysis can operate up to 100 times quicker and considerably more selectively than acid or alkaline catalytic reactions.

Practical trials, in which the selected enzymes were continually fed to the raw sludge, led to substantially improved digestion, up to 12 to 18% higher biogas production and correspondingly less residual digested sludge.

In addition to the optimised performance, the enzymatically achieved increase in degradation and the higher production of digestion gases also led to enhanced cost efficiency of the sludge treatment, with the following key features:

  • Less digested sludge - TR (residual sludge) leads to a lower demand for conditioning agents for sludge dewatering (lime or polymer flocculation agents) and to lower sludge disposal costs (for landfills, agricultural and thermal disposal),
  • Quicker and more efficiently dewatered sludge permits lower energy consumption and a longer operations lifespan of the dewatering facility, and ultimately
  • Higher sludge gas quantities lead to a better energy balance of the digestion process.

This provides for possible annual cost savings of between 1.0 to 1.5 € per resident/year.

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