Effect of Dissolved Oxygen on Sludge Acidification during the SSDML-process

Abstract

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 alter the system kinetics and downgrade the process performance. The present work focuses on the effect of dissolved oxygen concentration and oxygen transfer rates on the process of sludge acidification. SSDML experiments 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 oxygen limited kinetics is directly proportional to the concentration of biodegradable sludge volatile solids and inversely proportional to the surface area of sulfur and initial concentration of less acidophilic bacteria in the sludge. During this period, the specific growth rates and sulfate production rates of the sulfur oxidizing bacteria are reduced to a fraction of their actual values due to the limitation in oxygen. Based on this information, empirical relations were developed and these were used to simulate oxygen limited SSDML process sequences. The oxygen uptake rate (OUR) and the volumetric oxygen transfer coefficient (k_La) values were calculated for 12 runs carried out in the pilot plant. Increase in k_La obtained by increasing the aeration rates were marginal at higher aeration rates. Also, very high OUR values observed in the complete absence of sulfur oxidizing bacterial activity indicate aerobic digestion to be the predominant oxygen consuming step.