ANAEROBIC DIGESTERS AND BIOGAS
Anaerobic digestion is a biological process in which microorganisms break down biodegradable materials in the absence of oxygen. One of the outcomes of this process is biogas (e.g. methane), which is produced by the fermentation of organic material.
Zeolite is widely used as an “ion exchanger for the removal of ammonium in anaerobic digestion due to the presence of sodium, calcium, and magnesium cations in its crystalline structure” (Montalvo et al., 2012, p. 125). One practical application of this property is in improving the anaerobic process performances in the treatment of wastewater containing high concentrations of nitrogen compounds, including chicken and livestock wastes (Montalvo et al., 2012).
At the same time, zeolite has a great capacity for metal adsorption, a property that is useful for removing toxic materials that can inhibit the microorganisms responsible for anaerobic digestions processes (Montalvo et al., 2012).
Several studies (Kotsopoulos et al., 2008; Duran-Barrantes et al., 2008; Montalvo et al., 2005) have found that zeolite provides successful microbial support in anaerobic digestion processes because of its 1) high capacity for immobilization of microorganisms, 2) capacity for improving the ammonia/ammonium ion equilibrium and 3) ability to reduce ammonia and ammonium ions in solutions (Montalvo et al., 2012, p. 125).
Weiss et al. (2011) examined the colonization of activated clinoptilolite zeolite as carriers for microorganisms involved in biogas processes. Zeolite particle sizes of 1.0 – 2.5 mm were introduced to anaerobic laboratory batch-cultures and bioreactors during biogas production from grass silage (Weiss et al., 2011).
After an incubation period of 5 – 84 days, researchers saw colonization of the zeolite surface. Single strand conformation polymorphism (SSCP) analysis of bacterial fragments confirmed that populations preferred zeolite as an operational environment (Weiss et al., 2011). Also, populations immobilized on zeolite showed pronounced hydrolytic enzyme activity after reincubation in sterilized sludge (Weiss et al., 2011).
Researchers have found that anaerobic processes are efficient in reducing organic matter in pig and cow manure. Also, the methane gas collected during the process has the potential to significantly reduce the cost of waste treatment (Montalvo et al., 2012). An experiment by Milan et al. (2001) examined the effect of natural zeolite concentrations on the anaerobic digestion of swine waste. Zeolite doses in the range of 0.2 – 10 g/l of wastewater were used in batch experiments carried out at temperatures between 27 – 30 degrees Celsius. The anaerobic digestion process was favored by the addition of zeolite doses between 2 – 4 g/l and inhibited at doses beyond 6 g/l (Milan et al., 2001). One explanation for this observation was that large amounts of zeolite can increase the viscosity of the medium, removing available liquids required by microorganisms. However, zeolite aids the anaerobic digestion process when doses are controlled and remain under 6 g/l.
A study by Borja et al. (1993) examined anaerobic digestion of cow manure using a batch reactor with biomass immobilized on natural zeolite. Researchers observed that methane production can decrease considerable because of the accumulation of ammonia nitrogen. However, the use of a zeolite digester supported ion exchange, which resulted in a reduction of ammonia in the system. The methane yield from the zeolite digester was 5 times higher than digesters that did not contain zeolite.