EFFECT OF DISSOLVED-OXYGEN ON SLUDGE ACIDIFICATION DURING THE SSDML-PROCESS

There is a distinct possibility that the simultaneous sludge digestion and metal leaching (SSDML) process will encounter oxygen limitation, at least for some duration of the process. This is because of the high oxygen requirement, for sulfur oxidation as well as for sludge solids degradation, combined with the low solubility of oxygen. This can alt er the system kinetics and downgrade the process performance. The pres ent work focuses on the effect of dissolved oxygen concentration and o xygen transfer rates on the process of sludge acidification. SSDML exp eriments were carried out in 20 L capacity laboratory reactors as well as a 4000 L capacity pilot plant. It was found that the duration of o xygen limited kinetics is directly proportional to the concentration o f biodegradable sludge volatile solids and inversely proportional to t he surface area of sulfur and initial concentration of less acidophili c bacteria in the sludge. During this period, the specific growth rate s and sulfate production rates of the sulfur oxidizing bacteria are re duced to a fraction of their actual values due to the limitation in ox ygen. Based on this information, empirical relations were developed an d these were used to simulate oxygen limited SSDML process sequences. The oxygen uptake rate (OUR) and the volumetric oxygen transfer coeffi cient (k(L)a) values were calculated for 12 runs carried out in the pi lot plant. Increase in k(L)a obtained by increasing the aeration rates were marginal at higher aeration rates. Also, very high OUR values ob served in the complete absence of sulfur oxidizing bacterial activity indicate aerobic digestion to be the predominant oxygen consuming step .