Managing, Operating Multisubstrate Biogas Facilities
Anaerobic digestion means different things to different people. To the biologist, it is a genetically diverse ecosystem. To the civil and mechanical engineer, it is an engineered environment. To the chemical engineer it is a heterogeneous autocatalytic sequence parallel reaction system. Is it any wonder then that more than 40 percent of biogas plant failures occur after start up and are due to monitoring and control errors?
Not only must the biogas manager possess the normal skill set associated with managing a business, but he or she must indeed also possess at least some of the skills of engineers and biologists. This begins with a thorough understanding of the fundamentals of biological processes, because this is what drives most of the decisions that he or she needs to make on a daily basis.
Substrate inventory and recipe management: For a biogas plant processing multiple substrates, knowledge of the biology will dictate the recipe, or mixture of substrates. The biogas plant manager needs to know what feedstock can be processed and in what ratio, what contaminants in the feedstock will inhibit the organisms, and what mass load and frequency to feed the digesters. Once these factors are known, managing the inventory of feedstock becomes common sense.
Process monitoring: All biogas plants require measurement of certain process parameters to enable effective process control. Faced with parameters like pH, total solids, volatile solids, chemical oxygen demand, volatile fatty acids, alkalinity, ammonia and biogas composition, the biogas plant manager not only needs to know what these parameters mean, but also how they relate to and are affected by the biological processes occurring. Over and above these parameters, there are a host of other parameters that can be used by the biogas plant manager to diagnose digester health. For example, the presence of light metal cations in high enough concentrations will inhibit the microorganisms. The biogas plant manager needs to understand the biological processes and know what to look for if the routine monitoring data is insufficient to diagnose digester health.
Process control: A thorough working knowledge of biological processes will inform the biogas plant manager of the temperature the process should be operated at, the impact changes in temperature has on the process, how mixing impacts the efficiency of the process, what causes foaming and how to deal with it if it occurs, what solids concentration range the digesters should be operated at and what happens if this range is exceeded, which other parameters should be measured and how they are used to control the process, and what biogas flow and composition say about the stability of the process and what to do when these trend in the wrong direction.
In diagnosing digester upsets, which inevitably occur, the cause may be due to a single factor, or multiple factors. Additionally, the biogas plant manager has, or should have, a number of process indicators at his or her disposal that allow diagnosis of the upset and to take corrective action. It is only an understanding of how biological systems work that allows the biogas plant manager to avoid upset conditions or to correctly diagnose and correct upset conditions.
Author: Anthony Leske
Engineering Manager, Himark BioGas