Biogas plant feeding stability: why the daily total of feed is not enough

The daily total of feed is not enough to assess digester stability, because the response of the biology is also determined by how the doses are spread over time and by the composition and dry matter of the material; the EPA AgSTAR guide says explicitly that maintaining a stable ecosystem of microorganisms requires a regular schedule for adding waste and minimal variability of its physical and chemical characteristics. The daily average "looks good", and yet an acid pulse can occur in the digester. Below we show why and what follows from it for running the installation.

What the EPA guide says

The AgSTAR operator guide (November 2020), in the chapter on steady feeding (4.1), stresses three things. First, waste should be added on a regular schedule at the design volume, in order to maintain the design retention time. Second, variability of dry matter (TS) and volatile solids (VS) must be limited, so that the organic loading rate (OLR) is constant. Third, in co-digestion, different wastes should be added simultaneously, in constant proportions and preferably mixed before being fed to the digester. The guide notes that inconsistent feeding, resulting for example from lack of storage space, delivery dates, types of substrates and the rate at which they break down, can cause large fluctuations in biogas production; it illustrates this with data from a farm where, after substrates were added to manure under inconsistent feeding, biogas production was clearly higher but also much more variable than in the period without substrates.

Installations usually operate in continuous mode (constant flow over 24 hours) or, more often, semi-continuous mode: waste is added one or several times a day at equal intervals.

Why spreading over time matters

Consider a hypothetical example (the numbers are only illustrative). The digester has 3000 m³, and the daily load is 6 t VS, that is 2 kg VS/(m³·d). When the dose is given in six portions of 1 t VS each, each increases the instantaneous VS content by about 0.33 kg/m³. If the whole dose arrives at once, the jump is about 2 kg/m³, that is six times more, although the daily total is identical. Easily degradable material (for example production waste with sugars) undergoes acidogenesis quickly, while methanogens grow more slowly, so the momentary excess of acids is not consumed immediately. The EPA guide explains this mechanism: an OLR spike allows acid-forming bacteria to outpace the growth rate of methanogens, which lowers the pH; in an extreme case foaming and the death of methanogens occur, and the digester has to be emptied and restarted. We also describe the mechanism in the article Digester acidification: diagnosis and response.

Similarly, a seasonal change in the composition of the material can be invisible in the delivery weight alone. Silage from a different cut or a wetter batch of slurry has a different dry matter and a different VS content, so the same number of tonnes carries a different load. Current TS and VS help to understand the change, and we write about logistics in Silage storage at a biogas plant.

What warning signs the EPA gives

• A drop in CH₄ content in the biogas is the first indicator; it is best measured with a continuous analyser.
• A fall in alkalinity points to an upset earlier than pH, which changes more slowly; a rise in VFA and in the VFA-to-alkalinity ratio is also an indicator.
• pH in the effluent (usually 7.0–8.0 in a properly working digester) is a lagging indicator.
• A rise in the CO₂ share in the biogas, even a gradual one, means a loss of process stability.
• A fall in biogas production indicates that design parameters are not being maintained or that a toxic substance has entered the digester.

Titration indicators are covered in the article FOS/TAC: how to measure and interpret, and the acid profile in Volatile fatty acids (VFA) at a biogas plant.

How to keep records

For co-digestion the guide recommends detailed documentation: recording each waste source, the date and time of delivery and the volume, a pH measurement, and ideally retaining a representative sample of each delivery for possible toxicity, physical and chemical analyses. For waste from outside sources, toxicity tests are carried out regularly; a source of inhibitors in food industry waste can be cleaning and disinfecting agents, for example chlorine compounds and ammonia. In practice the following are useful:

1. a record of the times of each dose (not only the daily total),
2. TS and VS of each batch or change in composition,
3. linking a delivery to a batch in the digester record,
4. daily checks of flows and loads and verification of pump operation (for example by measuring the emptying of the feed tank during the dosing cycle).

Good data make it possible to match fluctuations in production to a specific event in feeding. Indicators for everyday use are described in the article Biogas plant KPIs, and the substrate mix in Co-digestion: planning the substrate mix. New materials are additionally assessed in terms of storage and homogenisation, and the feeding plan is adapted to the technology and experience of the given installation.