Principle of Enzymic Hydrolysis Technology

Anaerobic digestion is a series of complex biochemical reactions mediated by consortia of micro-organisms that convert organic compounds to methane and carbon dioxide.  It is a stabilization process achieving odour, pathogen, and mass reduction.  During the process, particulates are solubilised and large polymers are converted to monomers.  Subsequently, the monomers are fermented to simple substances, which in turn get converted into CH₄ and CO₂. 

The Enzymic Hydrolysis Biochemical Pathways diagram (Fig. 1) is an illustration of the digestion process.  Under appropriate condition bacteria would grow and release the enzymes necessary for bio-transformations.  Facultative anaerobes and anaerobes are responsible for the hydrolytic step which is often rate-limiting in processes involving particulate matter.  Optimum fermentation of volatile fatty acids (VFA) occurs at pH 6.3.  Carbon dioxide is a significant consumer of alkalinity which is provided by the ammonia resulting from amino acids degradation.  The acid bacteria have a syntrophic (mutually beneficial) relationship with the acetotrophic methanogens which are good acid scavengers but cannot operate with a VFA/alkalinity ratio > 0.4.  Methanogens tend to have slow growth rates (doubling times of 1.5-4 days).

Pathogen Destruction

The cocktails of fermentative products are toxic to pathogens such as E. coli and their destruction is accelerated by any increase in the reaction temperature as shown below (Fig. 2).

Fig. 2 - The Enzymic Hydrolysis process accelerates E. coli destruction  

Plug Flow Reactors

Proper bioreactor design is essential for good volatile solid destruction and pathogen control.  A multiple reactor design caters for the different growth requirement of microbial consortia and different biochemical conversion reactions since the condition in each reactor can be individually managed, for example to minimise product inhibition.  By linking six completely mixed reactors in series a Plug-flow regime is guaranteed in order to provide the necessary condition for natural selection of micro-organisms and optimum reactions.  A plug flow design also minimises the total reactor volume necessary for the bio-transformations.

Fig. 3 - Sludge digestion with Plug flow Enzymic Hydrolysis pre-treatment typically achieves 50-60%VSR and 99.9% E. coli reduction. 

Enhanced Enzymic Hydrolysis

While conventional thermal pasteurisation techniques can achieve the required pathogen standard, their short treatment times and the high temperature do not allow sufficient biological activity to develop.  High sludge temperatures also require high water temperatures (typically +80?C) and this in turn leads to scaling and potential sludge baking problems.  On the other hand, Enzymic Hydrolysis proved to be an excellent method for generating hydrolytic activities.  By combining the advantage of the original process at 42°C with optimum enzyme activity at 55°C the Enhanced Enzymic Hydrolysis Process (Fig. 4) improves volatile solid reduction (VSR) and biogas yield whilst producing excellent pathogen control capability at the same time. The relatively low operating temperature enables it to work successfully with low-grade waste heat from CHP plants to enable the use of a greater proportion of the biogas for electricity production 

Fig. 4 - Sludge digestion with the Enhanced Enzymic Hydrolysis Process typically achieves 50-60%VSR and 99.9999% E. coli reduction. 

Enzymic Hydrolysis Applications

Enzymic Hydrolysis Technology is a sustainable development because it is based on a biological process using natural micro-organisms and minimum energy.  There is little odour risk because the reactors are totally enclosed. It obviates the need for secondary digesters and eliminates any green house gas emission thereof.  The technology is particularly suitable for the treatment of biodegradable wastes such as sewage sludge, food wastes, agricultural and fermentation residues, etc.  Current applications include:

1. Pathogen reduction (meeting Safe Sludge Matrix; USEPA Class A and Class B Biosolids)
2. Enhance VSR and biogas yield in sludge digestion
3. VFA production (carbon source for biological nutrient removal applications)
4. Plug flow digestion