Improved Efficiency of Anaerobic Digestion and the Production of Biogas
In the last few years there has been a quantitative increase in the number of plants treating organic material from urban waste streams using Anaerobic digestion (AD). The process is being rapidly incorporated as the preferred option for treating organic material, not only from urban waste treatment plants, but also from industries related to agriculture.
There is no doubt that Anaerobic digestion is now an important weapon in the armoury being employed against the effects of global warming. If organic material is treated before it reaches landfill or as a mechanism to reduce the noxious effects of agricultural waste, it can be safely treated and the products, including Biogas, can be collected and reused instead of being wasted.
The process of Anaerobic digestion is well understood and the technology is mature, however, many plants do not fully optimise production of Biogas, as one of the components that can inhibit Biogas production is the prevalence of ammonia is the influent. Ammonia, if not properly treated, is habitually concentrated during the Anaerobic digestion process, and can seriously inhibit or indeed arrest the production of Biogas. One technology that has been successfully employed in the removal of ammonia and optimising the efficiency of Anaerobic digestion is its removal using thermal applications.
Two forms of ammonia are present in wastewater and become concentrated during the Anaerobic digestion process: ammonia gas (NH3) and the ammonium ion (NH4+). Both forms can cause inhibition in an AD system, although primary inhibition occurs as a result of the presence of NH3. Figures vary, but as ammonium ion concentration increases in an anaerobic digester reactor, typically above 1000 ppm, performance in terms of Biogas production, drops off. Full inhibition of AD occurs at around 5000 ppm. In order to combat this effect, thermal ammonia removal has proven to be highly effective on an industrial scale; to the point where recovery and reuse of the ammonia removed from Anaerobic digestion process water is becoming attractive both economically and environmentally.
The future now lies in improving efficiency and enhancing the possibility of ammonia recovery so that a useful commodity is not needlessly wasted. Ammonia is valuable not only from the point of view of its intrinsic value as a fertilizer and a myriad of other products, but also as an efficient vehicle for the safe storage and transport of hydrogen.
If waste heat from renewable energy processes can be used to drive the system, the ammonia recovered could be used in many industrial processes including the generation of energy either by ‘cracking’ the ammonia to produce hydrogen or by using ammonia directly as a fuel.