How to Transition from conventional MAD to THP digestion; experiences from Basingstoke
Merry, J.1 and Fountain, P.2, 1Stantec, UK, 2Thames Water, UK
(free)
A common reason for installing a thermal hydrolysis process (THP) is to increase the loading on existing “conventional” digesters, thereby delaying the need to build additional digestion capacity. During commissioning, the change to a hydrolysed feed and ramping up to a higher solids loading can cause significant digestion process instability, particularly at intermediate ammonia concentrations
(around 2000 mgNH4-N/l).
In conventional digesters, the ammonia concentration is normally below 1500 mgNH4-N/l and most methane production will be achieved by acetoclastic methanogens which thrive under “low ammonia” conditions. In THP digesters, however, the ammonia concentration can be as high as 3000 mgNH4-N/l and most methane production is achieved by hydrogenotrophic methanogens which are better able to tolerate “high ammonia” conditions.
The transition to operation at high digester ammonia concentrations must allow sufficient time for the population of hydrogenotrophic methanogens to grow so as to offset the fall in the population of acetoclastic methanogens. If transition is too fast, failure of the digestion process could occur which could have costly implications for reseeding and programme delays.
This paper focuses on experience gained at Basingstoke during the recent transition to THP digestion, and includes lessons learnt and tips for how to mitigate process risks during this transition period.
