Monofermentation of Nutritional Waste in Biogas Plants : Pilot - Biogas Plant

Abstract

In a joint project of the partners Ing. Friedrich Bauer GmbH (Kemmelbach, Aus- tria), Enersys GmbH (Donaueschingen, Germany) as general planner, the Biogas - Systemtechnik Deutschland (BST - D company) as constructor and the Institutes of Agricultural Engineering as well as Environmental and Animal Hygiene with Vet- erinary Clinic (head: Prof. Dr. R. Böhm) at Hohenheim University a process engi- neering technique was developed and tested for the anaerobic monofermentation of nutritional wastes with high energy concentrations and low contents of structural components. The research project started with systematic investigations in the bio- gas laboratory of the Institute of Agricultural Engineering at Hohenheim University about monofermentation of nutritional waste to obtain information about the fer- mentation behaviour and the performance of the substrate (compare long version, LANDTECHNIK-NET, volume 01/2004). In a second step intensive process engi- neering and hygienic investigations followed at the pilot biogas plant being erected by Biogas Systemtechnik Deutschland GmbH at Donaueschingen. The general planning was done by Enersys GmbH, Donaueschingen. The trials included two phases in a period of in total 192 days. In phase I the fer- mentation process was started and the tube fermenters of the plant got filled at first exclusively with inoculant material and afterwards they were fed with slowly in- creasing amounts of nutritional waste. After 130 days phase I finished by the end of November 2002. During that time in the tube fermenters 0.971 m3 CH4 m-3 RV d -1 , was produced in average, and in the second fermenter stage 0.431 m3 CH4 m-3 RV -1 were generated under a hydraulic loading rate of 3.18 kg odm m-3 RV d -1 and with a theoretical retention time of the fermentation substrate in both fermenter stages of about 118 d. The gas quality from both fermenter stages reached in average 56.3% CH4 and 36.1% CO2 . Due to a defective temperature measurement device in com- bination with malfunction of the gas counter whose signals were basis for the de- termination of the specific input mass of fresh substrate, the horizontal fermenters got overloaded in the beginning of October. Subsequently the concentration of vola- tile fatty acids increased rapidly up to 16500 ppm, whereby especially the concen- tration of propionic acid strongly increased up to 4690 ppm. As a result the pH - value in the horizontal fermenters dropped down to values of 6.4. By re-inoculating with fermented material from the second fermentation stage methane generation was restarted again. In phase II over a duration of 62 days and after having stabi- lized the process conditions there were generated in average 0.95 m3 CH4 m-3 RV d -1 in the tube fermenters and 0.64 m3 CH4 m-3 RV d-1 in the second fermentation stage with 58.7 % CH4 and 36.6 % CO 2 in the mixed gas. The concentration of volatile fatty acids was at a level of in average 5070 ppm of acidic acid and 3680 ppm of propionic acid. It can be followed that the process has been overloaded due to the high contents of volatile fatty acids. The results from the lab scale investigations and from the pilot plant confirmed that a monofermentation of nutritional waste is feasible but that there are several starting points to optimize the process and the strategies as well concerning aspects of process engineering and fermentation biol- ogy.