A design for intrinsic control of anaerobic digestion

The article reports on a novel layout which achieves intrinsic control of anaerobic digestion of a wide range of feeds on a continuous basis. The feed is automatically controlled to match the digestion rate. The design consists of a shallow tank concentric with a gas holder resting on stoppers on the tank bottom. The height of the digesting fluid is maintained using the plug flow principle. The feed material is dropped into the annular space which is thereafter covered with a black plastic sheet to enable anoxic conditions to prevail. The bioliquid hydrolyses the material, with the help of prefermenting bacteria, into volatile acids. A steady migration of these volatile acids into the anaerobic digester is maintained by the concentration gradient, which is constantly replenished by the action of a syntrophic community of bacteria, producing methane. Further, the feasibility of the concept is reported on a system with a 45 m³ concrete tank and a 35 m³ steel gas holder by using a mix of domestic wastes and producing gas with 70% methane content reliably for hundred days continuously. A novel simulator is developed to predict the performance of the design at various parameters of operation. It is shown that the performance data for digesters analogous to the design reported here are interchangeable. Based on this, the average productivity of gas for various loadings is predicted within ±10% of the actual yields for the analogous digesters. Because of the nature of the design, digestion of a wide range of feeds are permitted: without the problems faced in conventional layouts. This tremendous potential is realised by maintaining the integrity of the ecosystem of anaerobic digestion through the medium of the bioliquid which pervades the annular space as well as the confines of the anaerobic digester. By utilising the design in practice at normal loadings, its potential for reliable and economic digestion at a fraction of the cost of deep digesters could be realised if the predictions are confirmed at higher loadings by further work. This work has been carried out in the Tata Electric Companies partly in the program on renewable resources.