Prompted by potential benefits, such as improved digester performance, increased process reliability, higher digester gas volumes and gas quality, expanding wastewater treatment plants are evaluating the feasibility of implementing phased anaerobic digestion. Regional wastewater treatment plants in the USA and recently also in Europe have completed years of successful operation of two-phased anaerobic digestion facilities generating higher biogas yields and through cogeneration renewable electrical energy.
Inhalt
1.Introduction
2.Two-phase anaerobic digestion
3.Biogas production, cleaning and energy conversion
4.Conclusion
5. References:
1.Introduction
Prompted by potential benefits, such as improved digester performance, increased process reliability, higher digester gas volumes and gas quality, expanding wastewater treatment plants are evaluating the feasibility of implementing multiphase anaerobic digestion. Regional wastewater treatment plants in the USA such as Inland Empire Utility Agency in California have completed many years of successful operation of two-phased anaerobic digestion facilities generating gas for 50 MW of renewable electric energy.
2.Two-phase anaerobic digestion
Two-phase anaerobic digestion is a process that makes use of the different growth conditions of the anaerobic organisms, the acetogenic and the methanogenic bacteria. The organisms are separated into an acid phase reactor and a methane phase digester. In the acid phase reactor, the solids are hydrolyzed and converted into volatile fatty acids. Due to the conditions present in this digester (lower pH, higher temperatures), the solubility of complex organic matter, typically proteins, fats, amino, acids and polysaccharides is increased. As a result of this improved hydrolysis, the volatile solids reduction is increased and the amount of solids resulting from digestion is reduced.
The benefits identified for two-phase digestion include:
- Increased digester capacity.
- Increased process stability.
- Increased volatile solids destruction.
- Increased gas yields (methane production).
- Improved quality of methane phase digester gas
- Decreased digester foaming episodes
The disadvantages of two-phase digestion are:
- Increased hydrolysis and destruction of proteins in waste activated sludge releases ammonia to the recycle stream.
- Poor quality of acid phase gas (to be treated and flared off).
- Higher capital costs
By separating the organisms in two vessels according to the two phased digestion process, the concentration of active organisms is increased and therefore the necessary volume is reduced.
The higher volatile solids destruction in two-phase digestion process results in a higher gas yield, which can be 33 percent higher compared to conventional digestion. In addition, the lower sludge production in two-phase digestion can reduce disposal costs by 31 percent.
For the digester plant design a process modelling was performed to determine plant capacity, projected sludge production, and maximum available nitrification and denitrification capacity under the intended operating condition, and to establish the design bases for the multiphase digestion process. A sensitivity analysis was performed for gas costs, sludge disposal costs, volatile solids destruction and construction costs. The highest sensitivity on payback resulted in this case from varying construction costs and the maximum achievable volatile solids destruction.
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- Dipl.Ing. Christian Tasser (Autor:in), 2006, Two phased anaerobic digestion in waste treatment plants and renewable energy facilities, München, GRIN Verlag, https://www.grin.com/document/75014
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