Effect of sulfur concentration on sludge acidification during the ssdml process

Abstract

Heavy metals present in sewage sludges can be leached out, among other processes, by a process using a consortium of sulfur oxidizing bacteria. The metal leaching could be combined with aerobic sludge digestion to result in the Simultaneous Sludge Digestion and Metal Leaching (SSDML) process. A detailed knowledge of the effect of sulfur concentration on the pH variations of the sludge during the process is essential to a complete understanding of the process kinetics. To achieve this objective, SSDML experiments were conducted in Erlenmeyer flasks and 201 reactors and the results were analysed. The sludge pH was seen to rise initially, which was attributed to a lag in the activity of sulfur oxidizing bacteria combined with the production of unidentified material, which are basic in their effect on the sludge pH. The lag period was found to depend on (i) the surface area of sulfur available for bacterial attachment and (ii) the number of bacteria present in the sludge capable of initiating bacterial colonies on these surfaces. The rate of change of sludge pH during the lag period was found to depend on the sludge pH itself and the two were related using a second degree polynomial. The specific growth and product formation rates of the sulfur oxidizing bacteria were found to vary linearly with the surface area of sulfur available when the bacterial metabolism was sulfur limited and mathematical relations were developed for the same. The rate constants were evaluated by calculating the instantaneous growth and sulfate formation rates and comparing the resultant change in the sludge pH with experimentally observed values. These equations were used to simulate the sulfate production and pH variations in the sludge for various conditions. The simulation results matched the experimental results very well, confirming the validity of the relations developed.