Solubilization of primary sewage sludge by freeze-dried Lactobacillus brevis

Primary sewage sludge solubilization by Lactobacillus brevis was studied in a 15-L lab-scale anaerobic reactor. L. brevis was inoculated in a low-cost broth medium to optimize strain growth and activity and enhance the medium's usability and economic feasibility over commercial MRS broth. L. brevis was freeze-dried, powdered, and inoculated in the treatment groups, with un-inoculated controls maintained under similar conditions. The effect of the powdered components on primary sewage sludge solubility was examined using an unwashed powdered form (p-type) and a powdered form washed twice with distilled water (w-type). Both inoculants showed maximum solubilization rates of 36.9% and 20.7%, respectively, which were higher than that observed in controls (6.1%). Thus, L. brevis can solubilize sludge without the addition of other soluble substrates. Total volatile fatty acid concentrations (mg COD L−1) were 7990 (AVE: 6800) and 5574 (AVE: 4293) mg L−1 in p-type and w-type reactors, respectively, which were higher than that of controls, i.e., 3333 (AVE: 2393) mg L−1. Real-time polymerase chain reaction showed that L. brevis cell count in anaerobic reactors decreased gradually with time (initial: 107 cells mL−1; day 19: 105 cells mL−1). Nevertheless, a solubilization rate >35% was maintained following a single inoculation of freeze-dried L. brevis. •Non-washed powdered Lactobacillus maximized TVFAs concentrations in primary sewage sludge.•The population changes with powdered and washed Lactobacillus were similar using rt-PCR.•Use of non-washed Lactobacillus may be useful in the sludge solubilization in the field.