FAQ: Process biology and start-up
5 questions with answers. Click a question to expand the answer; each has a permanent link.
In winter, is heat from the CHP engine enough to sustain the process?
In a typical agricultural plant running at full power, CHP heat is generally sufficient for the process, but the margin may be small if part of the heat goes to buildings, a dryer or hygienisation. At low engine load or when it is shut down, a backup source is needed, most often a boiler.
Sources: IEA Bioenergy Task 37 — Biogas; Rozporządzenie (UE) nr 142/2011 (wymagania dla produktów ubocznych pochodzenia zwierzęcego) — EUR-Lex
Discussed in the article: Biogas plant heat balance in winter: how much heat the process uses and where to get it
Is insulation of the digester tank more important than pipework insulation?
The tank has the largest surface area, so overall it loses the most, but short, uninsulated sections of pipework, heat exchangers and pumps can cause large local losses and a risk of freezing. Both groups are worth assessing.
Sources: IEA Bioenergy Task 37 — Biogas; Rozporządzenie (UE) nr 142/2011 (wymagania dla produktów ubocznych pochodzenia zwierzęcego) — EUR-Lex
Discussed in the article: Biogas plant heat balance in winter: how much heat the process uses and where to get it
What is a starter for a biogas plant?
A biogas plant starter is a material or set of actions that allow a stable anaerobic microflora to develop in a new or restarted digester.
Most often it is active inoculum, that is, material from an already operating fermentation process, e.g. digestate from a working biogas plant. It supplies hydrolytic and acid-forming bacteria and methanogenic archaea, as well as alkalinity and minerals. A good starter shortens start-up and reduces the risk of acidification, because the new tank does not start from “empty” biology. A starter can be combined with other materials, e.g. slurry, but its quality and its match to the target substrates and temperature matter more than the amount alone. The start-up plan should be prepared by a process engineer for the specific installation.
Sources: IEA Bioenergy Task 37 — Energy from Biogas; Basic Information about Anaerobic Digestion — US EPA
What is inoculum?
Inoculum is biologically active material containing the microorganisms needed for anaerobic digestion, added to a process or test to start the breakdown of the substrate.
It can come from an operating biogas plant (digestate), from the digesters of wastewater treatment plants, from UASB reactors, and as a supplement from slurry or rumen fluid. In a full-scale installation it is used to start up the digester or to rebuild the process after a failure. In the laboratory it is the basis of methane potential tests (BMP), where its activity and dose affect the result. The quality of inoculum is determined by methanogenic activity, the absence of inhibitors, and temperature and substrate matching to the target process.
Sources: IEA Bioenergy Task 37 — Energy from Biogas; Angelidaki et al. (2009), Defining the biomethane potential (BMP) of solid organic wastes and energy crops — Water Science and Technology
Why can pH alone warn too late?
pH alone can warn too late because the contents of the digester have buffering capacity and for a long time mask the build-up of acids.
The carbonate and ammonium buffer system neutralises successive portions of volatile fatty acids (VFA), so the VFA concentration may already be rising while the pH still looks normal. Only after the alkalinity is exhausted does the pH begin to fall quickly, and by then the process is already badly disturbed and recovery takes a long time. For this reason, besides pH, one tracks VFA, alkalinity, the FOS/TAC ratio (the ratio of acids to alkalinity), biogas composition and its production relative to the load. Trends in these indicators are best, not single readings.
Sources: Chen, Cheng, Creamer (2008), Inhibition of anaerobic digestion process: a review — Bioresource Technology; IEA Bioenergy Task 37 — Energy from Biogas