The operator should know at all-time what’s in the digester. The main concern for an operator should be the contaminants and the organic loading rate of each substrates that goes in the digester

In general

  • During filling and emptying, pay attention to digester pressure fluctuations and ensure good accessibility to the operating equipment. (over and under pressure problem)
  • Avoid ignition sources
  • Operator must be physically on site for each material transfer procedures and during all transfer duration
  • Always reduce feeding if a problem is suspected
  • For each new incoming truck of substrates, operator should know the origin/quality of the substrates. Operator must judge if it is adequate to introduce this substrates in the system being careful not to introduce sand, grit, contaminants or over feed.


  • Fill the operation log
  • In the control room, at the control interface, check whether malfunction lights are illuminated.
  • Check if the heating system is working and the temperature is constant
  • For all inlets and outlets, assure that the liquid manure/substrate flow is maintained according to the process regulations.
  • Check the fill levels in the digester and storage.
  • Make sure that there is no unusual scum or foam in tank
  • Make sure that all pumps and mixer work correctly
  • Walk around the system to see any visual, sounds or odors problems 

Feed input

To avoid problems you should accept only substrates with limited contaminants like plastic, wood branches, rocks, grit and others none biodegradable matters. The substrates origin and tonnage should be kept in the operation log.

To evaluate the feeding regime of your digester you should work with the organic loading rate (OLR). OLR is amount of volatile solid (kg) you can feed per day per volume (m³) (kg VS/m³*day).

Example, for a CSTR digester OLR should be between 1 and 3.5.

If you know the mass of substrates each day you may calculate the HRT by dividing the operational volume by the daily volume of substrates.

If you don’t know your substrate you should perform a laboratory test for at least, %TS and %VS.


Temperature is an important aspect of anaerobic digestion. Regarding the operating temperature (psychrophilic, mesophilic or thermophilic) you should avoid any drastic change in temperature.


The pH is a good indicator of stability in the process. The system pH should be in neutral range 6.5 to 7.5. The change of operation method or substrates can result in imbalance and accumulation of VFA (lower pH). To reduce the risk of acidification ensure frequent FOS/TAC ratio


The ratio FOS/TAC is the ratio of volatile organic acids to alkaline buffer capacity and measure the risk of acidification of a biogas plant.

FOS/TAC ratios chart

FOS/TAC ratios Background Measure
> 0.6 Highly excessive biomass input Stop adding biomass
0.5 – 0.6 Excessive biomass input Add less biomass
0.4 – 0.5 Plant is heavily loaded Monitor the plant more closely
0.3 – 0.4 Biogas production at a maximum Keep biomass input constant
0.2 – 0.3 Biomass input is too low Slowly increase the biomass input
< 0.2 Biomass input is far too low Rapidly increase the biomass input

How to do the FOS/TAC ratio

  • Take 20 mL sample and add 180 mL of demineralized water
  • Put under agitation and connect a pH-meter
  • Titrate with H2SO4 0,05 mol/L (0,1 N)
  • Note the volume of H2SO4 when your pH reach 5 (this volume is called: V1)
  • Note the volume of H2SO4 needed to pass pH 5 to pH 4.4 (this volume is called : V2)


FOS = (V2 * 1.66 -0.15) * 500

TAC = V1 * 250