Control of septage sanitization by limes and lactic acid fermentation

Considering low-cost and effective fecal sludge (FS) treatment alternatives is essential to avoid risk to human health and to ensure safe disposal in landfills and soils. This research assesses optimal pH adjustment of two techniques for sanitizing de-watered FS from a septic sewage-treatment plant. The preliminary analysis evaluated the efficiency of lactic acid fermentation (LAF) by two lactic acid strains: Lactobacillus casei and Lactobacillus acidophilus. The homofermentative strain was chosen to evaluate three supplementary sugars: smashed carrot, sugarcane-derived molasses, and anhydrous dextrose. Lime treatment was examined using two materials, CaO, 105% calcium carbonate equivalent (CCE)_, and Ca(OH)₂, 75% CCE. Four samples were retrieved from a passive thermal drying bed, two characterized by its available nutrient content; all samples were analyzed for the pathogenic profile, and total coliforms (TC) were selected as indicators. For LAF, an inoculating rate of Lactobacillus casei 10 g/100 g sludge was found effective in decreasing the pH below 4.0 after 30 days of fermentation, using 22% w/w dextrose/septage and 20% w/w molasses/septage, where molasses contains 20.7% of soluble sugars. In the case of lime treatment, the pH was fitted by a power-law relationship to the rate of lime applied in a septage with an initial pH lower than 7.0. A Langmuir type equation fitted better the liming of two septages with initial pHs above 7.0. The rate of lime CaO 10% w/w was observed to increase the pH above critical value, 12, after 1 h and 24 h. Analysis confirmed the total elimination of TC in samples with pH < 4.0 and pH > 12, contrasting the respective controls. Rates of CaO considering the initial pH of the FS are recommended in order to reach pH 12. Septage sanitization can be completed using either CaO or lactic acid fermentation with molasses; selecting the ideal method will rely on cost-benefit analysis. Sanitization can be considered as well to improve safety soil nutrient recycling practices.