As we all know check-ups at the doctor are key to understanding the health of our digestive system. Also, what food we consume and the rate at which we eat greatly affects how our digestive system performs. Interestingly, the same principals apply to Anaerobic Digesters, as they do to us humans.
Celtic Anglian Water operates the largest municipal sludge treatment facility in Ireland at Ringsend in Dublin. This consists of four large anaerobic digesters which are preceded by a CAMBI-hydrolysis treatment plant.
Anaerobic Digestion is a fairly robust process when it is in equilibrium with a consistent, stable feed. However, maintaining such a balance can sometimes be challenging in wastewater treatment plants, where highly variable loads are the order of the day.
Organic overloading leading to pH depression in anaerobic digesters is one of the most common process challenges facing operators. This occurs as the growth rates of acid producing bacteria populations are significantly higher than those of methane producing bacteria. In an event when digester organic loading is suddenly increased the acid forming bacteria thrives on the increasing substrate and production of volatile acids increases, depending on digester alkalinity. This eventually begins to depress the pH of the digester.
So why is this an issue? Well the methane producing bacteria like a consistent, and usually relatively high pH. With a high level of sensitivity even small changes in organic loading rates can lead to pH changes which greatly inhibit the methane bacteria, thus creating a process upset which can prove very difficult to recover from.
Therefore, understanding the digester’s health through the careful monitoring of a number of trends is key to understanding when organic loading rates reach limits that result in digester stress. Daily monitoring of digester pH, temperature and biogas composition is a necessity for high rate digesters. Monitoring of volatile solids destruction, gas conversion rates, volatile acid composition and alkalinity should also be conducted regularly. Ultimately, a digester is like the human gut and it likes the same feed at the same time every day. Unfortunately reality will often dictate that this is not possible, but just make sure to know your limits!
Pictured Clionadh Williams doing monthly measurements on the gamma probes at the Ringsend Hydrolysis plant