The conversion of existing conventional digestion plant to THP pre-treated digestion is challenging. A controlled transition between these two regimes can generate high saving by maximising the sludge throughput and the biogas generation, while reducing digester downtime. The rate of the ramp-up must be carefully considered to reduce the risk of digester failure and avoid the high costs associated with digester reseeding. In the case of Seafield, lab scale studies have been carried out to optimise the digester commissioning phase during the switch from conventional to THP digestion. For that purpose four different ramp up rates have been tested on bench scale chemostats to replicate the full scale proposed ramp up. The daily monitoring of digester stability and performance parameters showed that all the rates tested at lab scale could be applied on site. Following analysis of these results the full scale commissioning followed a much more aggressive ramp up regime than would have otherwise been endorsed with results that closely matched the lab scale results and helped ensure the successful commissioning of the plant.
Keywords: THP, commissioning, lab scale, full scale, ramp up, up scaling, digestion stability, digestion performances
THP is a proven pre-treatment technology used to optimise the anaerobic digestion process in terms of biogas generation, pathogen kill and reduction of sludge volumes through higher VSD and better dewaterability. When a conventional existing plant is converted to a THP plant, the switch from conventional sludge to THP sludge has to be done with care to avoid lower revenue due to poor performance in the digesters or in extreme cases extra costs because of digesters re-seeding.
Two options can be considered before the commissioning. The first consists of emptying the existing digester and seeding it with THP digested sludge, which is a very costly solution, mostly in term of transport. The other option is a controlled switch from conventional feed to THP feed sludge, which requires close monitoring of the digester stability and performance during the ramp up stage. To define the most optimum way of commissioning the full scale digesters, some laboratory scale trials should be carried out.
The aim of this project was to support the full scale commissioning of the THP & digestion plant at Seafield STW. The main objective is to determine the optimum ramp up rate after the switch from conventional feed to THP feed, while keeping stable digestion process and optimum performance. To achieve this, four different loading rates were tested at lab scale. No major difference in performance was observed between the different ramp up schedules tested (+0.2 to +0.7 kgVS/m3 /day) to reach 3kgVS/m3 /day, indicating that any of the ramp up rates tested could potentially be applied at full scale.
A stable period was proven to be beneficial and did allow all of the digesters to reach stable conditions before the second ramp up. Following the laboratory scale trial, the actual digesters were ramped up successfully to the target VS loading of 4.7kgVS/m3 with a rather aggressive loading rate of +0.5kgVS/m3 /day.
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