Influence of Enzyme Additives on the Rheological Properties of Digester Slurry and on Biomethane Yield

Abstract

The use of enzyme additives in anaerobic digestion facilities has increased in recent years. According to the manufacturers, these additives should increase or accelerate the biogas yield and reduce the viscosity of the digester slurry. Such effects were confirmed under laboratory conditions. However, it has not yet been possible to quantify these effects in practice, partly because valid measurements on large-scale plants are expensive and challenging. In this research, a new enzyme product was tested under full-scale conditions. Two digesters were operated at identic process parameters—one digester was treated with an enzyme additive and a second digester was used as reference. A pipe viscometer was designed, constructed and calibrated and the rheological properties of the digester slurry were measured. Non-Newtonian flow behavior was modelled by using the Ostwald–de Baer law. Additionally, the specific biomethane yield of the feedstock was monitored to assess the influence of the enzyme additive on the substrate degradation efficiency. The viscosity measurements revealed a clear effect of the added enzyme product. The consistency factor K was significantly reduced after the enzyme application. There was no observable effect of enzyme application on the substrate degradation efficiency or specific biomethane yield.

Novel Insight on the Impact of Enzymatic Addition on Organic Loading Rate in Anaerobic Digestion

“…It is possible that changes in physical characteristics, such as viscosity might have contributed to the improvements in specific biogas yields and biogas quality as found in other studies. Changes in viscosity in AD has been also linked to the production of extracellular polymeric substances (EPS) and soluble microbial products (SMP), which can also effect mixing, foaming and dewaterability [29]. According to Neyens et al [30], increase dewaterability and the requirement for a lower amount of flocculation polymer indicate a change in sludge surface structure and EPS production.…”

“…Ayol et al [31] reported that this enhanced degradation of the EPS matrix was able to improve sludge solids solubilisation and hence the formation of enzyme-substrate complexes. Muller et al [29] when dosing enzymes at full-scale recorded an influence on rheological parameters with a significantly decreased consistency factor (K) and hence viscosity compared to the control conditions, with possible economic effects on reduced energy demand for mixers and pumps. In this work, the presence of externally added enzymes, in particular in the mode 1/HRT and 2/HRT, have provided additional degradative power.…”

“…Other full-scale studies, showed that the direct addition of hydrolytic enzymes to 30 full-scale plants treating agricultural residues produced up to 35% increase in terms of biogas yield [11]. Whereas Müller et al [12] recorded a decrease in both apparent viscosity and consistency factor in the treated sludge after enzymatic addition, which translated into economic benefits from reduced energy demand for mixers and pumps. Other studies have shown enhanced sludge solubilisation but no improved biogas production [8,13].…”

Anaerobic Digestion in the 21st Century

“…extended to waste products either rich in recalcitrant lignocellulose or containing inhibitory substances such as ammonia (see the studies of Wedwitschka et al [5] and Mahato et al [6] in this Special Issue). The development and evaluation of various pretreatment technologies for lignocellulosic biomass is a hot topic of AD research that several articles in this Special Issue deal with (see the studies of Müller et al [7], Schumacher et al [8], and Monlau et al [9]). The effect of the inoculum on the microbial community structure and performance of the AD process is still an enigma.…”