Acclimatization of anaerobic sludge for UASB reactor start-up

Abstract

The Upflow Anaerobic Sludge Bed (UASB) reactor represents a high rate anaerobic wastewater treatment system. The majority of the active biomass in the reactor is present in the form of sludge granules which possess excellent settling properties. If no acclimatized (granular) sludge is available, the first start-up is performed using unadapted seed material. The subject of this study is the start-up of UASB-reactors using digested sewage sludge as seed material and volatile fatty acids (M) as substrate. A relation was demonstrated between the dry suspended solids content of digested sewage sludge and its settleability and methanogenic activity. The analysis of coenzyme F420 proved a reliable method to assess the potential methanogenic activity of anaerobic sludge. Several environmental conditions were found to influence the length of the gas production lag phase of digested sewage sludge following the supply of a VFA-feed, e.g. the biomass concentration and the mixing intensity. Exceptionally high sludge growth yields were determined with acetate as single substrate. The rate of UASB-reactor start-up has been examined in relation to the type and quantity of seed sludge and the composition and strength of the wastewater. Two types of sludge wash-out are distinguished, i.e. sludge bed expansion wash-out and sludge bed erosion wash-out. The latter type represents a key feature of the UASB-reactor, i.e. the selection pressure exerted on the sludge particles, which is essential in the development of granular sludge from the digested sewage sludge seed. Indications were obtained that the use of different types of digested sewage sludge leads to the development of different types of granular sludge, as a result of differences in the average biomass retention time during the early stages of start-up. The role of Methanothrix and Methanosarcina is discussed. The development of bulking anaerobic sludge is reinforced by seeding with a dilute digested sewage sludge type (≤40 kgDSS.m -3 ). This will prolong the start-up time. Guidelines are provided with respect to the choice of a seed sludge depending upon the strength and composition of the wastewater.