EVALUATION OF THE DUAL DIGESTION SYSTEM .3. CONSIDERATIONS IN THE PROCESS DESIGN OF THE AEROBIC REACTOR

In this paper, design considerations for pure oxygen and air oxygenate d aerobic reactors in dual digestion are outlined based on the steady state heat balance. Accepting that the objectives of the aerobic react or are sludge pretreatment through oxygen limitation and pasteurisatio n, it is demonstrated that 3 parameters are of crucial importance to d esign which centres around minimising the reactor retention time: (1) the oxygen consumption rate of the sludge (OCR(bio)) which fixes the m aximum biological beat generation rate; (2) the oxygen transfer rate ( OTR) of the oxygenation system which should be less than OCR(bio) to e nsure oxygen limitation; and (3) the oxygen transfer efficiency of the oxygenation system (OTE) which determines the vent gas volumetric flo w rate at a given OTR. For a given OTR, if OTE is high (attainable wit h pure oxygen), vent gas heat losses (through water vapour and sensibl e heats) are small and most of the heat generated is lost via hot effl uent sludge thereby allowing pasteurisation temperatures to be attaine d at short retention times; if OTE is low (which usually is the case w ith air), vent gas heat losses are high, and much less heat can be los t via hot effluent sludge thereby forcing longer retention times to at tain pasteurisation temperatures. However, with heat exchange between reactor effluent and influent sludge flows and with increased mechanic al heat input, heat sources are increased without increasing heat loss es in the vent gas and pro rata reductions in retention time are allow ed.