Anaerobic Digester pH Control
To maintain any large scale biological process within a reactor there are a range of parameters which need to be regularly if not continuously monitored and systems will be needed to correct imbalances when they occur.
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For very simple small low cost digesters of the type that are popular in developing countries those that are responsible for running the digesters will take care that a reasonable balance of materials a fed into the reactor to avoid the system producing odour, and what is often described as “turning sour”.
The bacteria which provide these anaerobic processes are particularly sensitive to pH.
PH is not the only cause of inhibition and indeed other inhibitory substances if present above certain concentrations can “knock-out” the digestion process. Most important of these are NH3, H2S, but we cannot cover those here.
A pH value near neutral is the optimum for anaerobic digestion and below 6.8 methanogenic activity is inhibited.
Due to the presence of a high CO2 content in the gases developed during anaerobic processes (30 to 35 percent CO2), quite a high alkalinity concentration is needed within the digester to maintain a pH near neutral.
An alkalinity concentration in the range of 3000 to 5000 mg/l as CaCO3 is often found to be optimum.
It is also possible for the anaerobic digestion process to require acid addition to reduce the pH if the pH rises too far.
Anaerobic Digesters Used for Sewage Sludge Digestion
During sludge digestion sufficient alkalinity is usually present without addition and is produced by the breakdown of protein and amino acids to produce NH3, which combines with CO2 and H2O to form alkalinity as NH4(HCO3).
Anaerobic Digesters Used for Industrial Wastewater Applications
For industrial wastewater applications, especially for waste containing mainly carbohydrates, it is necessary to add alkalinity for pH control.
Anaerobic Digesters Used for Digestion of Animal Wastes, Municipal Solid Waste and Any Seasonally Varying Biomass
A common feature that these digester types all have to contend with is the variability of the feedstock, which will also be reflected in a highly variable incoming alkalinity.
Continuous monitoring of pH within the feed and the digester itself is advisable, and automatic dosing to maintain reactor pH within a pre-set (largely neutral range) may be appropriate.
Regular laboratory analysis of alkalinity will also be necessary to enable the digester operating staff to predict the buffering capacity of the feed materials.
To keep these digester types operating both acid and alkali dosing provision may be needed. Each would to be used on different occasions if the operator is unable to balance the feedstock content sufficiently to control pH merely by ensuring that enough buffering is present within the incoming material such that additional buffering is not needed.
More Information
The process control of biological treatment processes is an extensive subject. Full time courses are available on a regular basis (for example in the UK through the WRc/WRf).
We would recommend that all Anaerobic Digestion Plant operators are trained through attendance at such courses as the water chemistry and biochemistry is complex and and in-depth understanding is necessary when a digester reaction falters, as they will from time to time.
If a full time course is not possible or appropriate then recourse to one of the excellent text books on the subject of water treatment is recommended. Metcalf and Eddy; Wastewater Engineering Treatment and Reuse, McGrawHill, is one such recommended text book. It also contains worked examples of the method of calculation of the amount of alkalinity needed to provide adequate pH buffering in digesters.
More FAQs
See our Amine Gas Treating page here.
See our Main FAQ Page here.
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