Utilizing Zeolite with Anaerobic Digesters and Biogas
Zeolite successfully aids in anaerobic digestion because of its capacity to immobilize microorganisms and remove ammonium. It is useful in removing methane from animal waste, therefore reducing the cost of waste treatment.
Anaerobic Digestion '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. 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.
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 digestion processes.
Several studies have found that zeolite provides successful microbial support in anaerobic digestion processes because of its:
- High capacity for immobilization of microorganisms
- Capacity for improving the ammonia/ammonium ion equilibrium
- Ability to reduce ammonia and ammonium ions in solutions
Zeolite as a Carrier in Biogas Processes
Weiss 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.
After an incubation period of 5 - 84 days, researchers saw the colonization of the zeolite surface. Single-strand conformation polymorphism (SSCP) analysis of bacterial fragments confirmed that populations preferred zeolite as an operational environment. Also, populations immobilized on zeolite showed pronounced hydrolytic enzyme activity after reincubation in sterilized sludge.
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. An experiment by Milan 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. 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 examined the anaerobic digestion of cow manure using a batch reactor with biomass immobilized on natural zeolite. Researchers observed that methane production can decrease considerably 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.