Reducing the size of a nitrogen removal activated sludge plant by shortening the retention time of inert solids via sludge storage

Spare nitrification capacity is usually needed for a nitrifying activated sludge plant to counter nitrogen shock loads and/or toxicity incidents. The traditional way to provide this capacity is to apply a sludge retention time that is much longer than needed for treating ordinary loads, resulting in over-designed plants. The approach investigated in this paper is to store the spare biomass in a separate sludge storage tank and return it to the main stream process when a shock nitrogen load or an inhibition/toxicity incident occurs. Model-based analysis reveals that, in such a plant, inert solids stay significantly shorter than the active biomass. This implies that one may extend the retention time of the active biomass to the required level without raising the retention time of the inert solids to the same level, giving rise to the possibility of reducing the size of the plant. Savings on tank volume are estimated to be typically around 20%. Experimental studies carried out on a pilot plant confirmed the theoretical results. Furthermore, they revealed that the decay rate of the nitrifiers in the storage tank could be maintained at a low level by intermittently aerating the sludge, so that an even smaller storage tank is required.