pH Control Equipment is essential to most commercial scale anaerobic digestion plants to ensure that the fermentation process can continue efficiently. 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. pH correction chemical dosing systems are one of the most critical and are needed to correct pH imbalances when they occur. If a pH imbalance persists and becomes large dangerous sulphur dioxide may be produced, and biogas production can be halted. However, pH dosing systems are available on the market which can operate reliably within biogas digesters and avoid problems occurring.
For very simple small low cost micro digesters, of the types of biogas plant 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”, which is effectively the result of a loss of pH control. Monitoring of pH is needed for larger plants.
The bacteria which provide these anaerobic processes are particularly sensitive to pH and their activity slows and becomes “inhibited” when the reactor substrate pH is out of their accustomed range. A pH value near neutral is the optimum for anaerobic digestion and below 6.8 methanogenic activity is inhibited. An out of balance 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 do not have the space to discuss those here. 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,according to researchers.
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 of wastewater treatment works sludges (Sewage Works Sludges), 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). For these plants pH monitoring provides an early warning that something abnormal is occurring in time for it to be corrected without biogast production time being lost.
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. In this instance a pH monitoring and pH correction chemical dosing equipment package is needed.
pH Control Equipment 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 on AD Process Control
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.