Achieving stable anaerobic mono-digestion of concentrated waste activated sludge without any pretreatment

Sludge digesters are generally designed using empirical thresholds that were defined several decades ago, typically leading to large digesters displaying low organic loading rates (1-2.5 kgVS.m^-3.d^-1). However, the state of the art has significantly evolved since these rules were set, especially regarding bioprocess modelling and ammonia inhibition. This study demonstrates that digesters can be safely operated at high sludge concentration and total ammonia concentration up to 3.5 gN.L^-1, without any sludge pretreatment. The possibility of operating sludge digesters at organic loading rates of 4 kgVS.m^-3.d^-1 by feeding concentrated sludge was identified through modelling and experimentally confirmed. Based on these results, the present work proposes a new mechanistic digester sizing strategy based on microbial growth and ammonia-related inhibition in lieu of historical empirical methods. Applying such method to sludge digester sizing could lead to very significant volume reduction (25-55%), which would result in reduced process footprint and more competitive building costs.