Anaerobic hydrolysis during digestion of complex substrates

Complex waste(water) such as, raw sewage, dairy wastewater, slaughterhouse wastewater, fish processing wastewater, primary sludge and the organic fraction of municipal solid waste have been proven to be degradable under anaerobic conditions. However, during the digestion process the conversion of the complex organic molecules into mono- and dimer components, also called the hydrolysis, is often the rate-limiting step. For design and optimization of the anaerobic conversion of complex waste(water) a good knowledge of the hydrolysis kinetics is therefore essential. The scope of this thesis was therefore to clarify the hydrolysis kinetics during the anaerobic digestion of complex waste(water), with emphasis on the hydrolysis of particles, dissolved macromolecules and lipids in coherence with the process conditions during the digestion. The mechanisms of the hydrolysis were elucidated by lab experiments and simulations with mechanistic hydrolysis models. For the hydrolysis of particulate substrates the results presented in this thesis revealed that, at constant pH and digestion temperature, the amount of surface available for the hydrolysis is the most important parameter for the hydrolysis rate and all other parameters are of minor importance.

With respect to dissolved polymers, such as gelatine and dissolved starch, the results indicate that the mechanism of the enzymatic hydrolysis in batch experiments can be described as a random polymerisation process. Moreover, the hydrolysis rate of dissolved components is linearly related to the sludge concentration in the batch experiment. The hydrolysis of neutral lipids under acidogenic conditions is slower as compared to the hydrolysis under methanogenic conditions. Based on the results presented in this thesis it was hypothesised that this is due to positive effect of the methane production on maintaining the lipid-water interface and subsequent higher volumetric hydrolysis rate.

In practice the hydrolysis rate is most commonly described by an empirical first order relation, in which the hydrolysis rate is linearly related to the amount of biodegradable substrate that is available (Eastman and Ferguson, 1981).

The identification of the essential parameters of the hydrolysis mechanisms in this thesis made it possible to evaluate the first order approach and designate the limitations of the relation. The evaluation revealed that the hydrolysis only proceeds according to first order kinetics if no changes in the rate limiting step or the biodegradability occur during the degradation of a substrate. Moreover, the first order hydrolysis constant seems system and substrate specific and the use of literature values for the hydrolysis constant is therefore not advised.

For assessment of a hydrolysis constant in a lab experiment the following guidelines were presented: (1) For waste(water) containing mainly protein and carbohydrates, first order kinetics can be established under acidic and methanogenic conditions in batch or completely stirred tank reactor (CSTR) system. (2) For waste(water) that contains high concentrations of lipids the assessment of the hydrolysis constant for neutral lipids under acid conditions is impossible due to coagulation of the lipid. Under methanogenic conditions the hydrolysis constant can be assessed in a 'multiple flask' batch system. However as (gas) mixing can differ between a laboratory batch and a full-scale CSTR-system, the subsequent effect on the lipid-water interface might cause a difference in the prevailing k _h value of the two systems.