Biocides with Controlled Degradation for Environmentally Friendly and Cost-Effective Fecal Sludge Management

SIMPLE SUMMARY: During the storage and/or transportation of fecal sludge (FS), it is treated with biocides, such as quaternary ammonium compounds and biguanide derivatives, to control microbial activity and unpleasant odors. These biocides are very effective, which, however, have drawbacks in the disposal of biocide-treated FS, such as toxicity to activated sludge in biological wastewater treatment plants. In this work, the feasibility of using biocides naturally degradable in alkaline medium for more environmentally friendly management of FS was evaluated. The original strategy was based on alkalinization of the medium due to gradual decomposition of urea in FS. The four selected biocides were shown to effectively control microbial activity and degrade after biocidal function, allowing such biocide-treated FS to be disposed of in wastewater treatment plants in an environmentally sound manner without harming the activated sludge. ABSTRACT: Didecyldimethylammonium chloride (DDAC) and polyhexamethylene guanidine (PHMG) exhibit high antimicrobial activity and are widely used as biocidal agents in chemical toilet additives for the management of fecal sludge (FS). Disposal of such biocide-treated FS to a wastewater treatment plant (WWTP) is a major environmental problem. It is possible to reduce environmental damage through the use of biocidal agents, which easily decompose after performing their main biocidal functions. In this work, it is proposed to use the fact of a gradual increase in pH of FS from the initial 7.5 to 9.0–10.0 due to the decomposition of urea. Six biocidal compounds were selected that are capable of rapidly degrading in an alkaline environment and one that naturally degrades upon prolonged incubation. Four of them: bronopol (30 mg/L), DBNPA (500 mg/L), Sharomix (500 mg/L), and sodium percarbonate (6000 mg/L) have shown promise for environmentally friendly management of FS. In selected dosage, they successfully reduced microbial activity under both aerobic and anaerobic conditions and are cost-effective. After 10 days of incubation, degradation of the biocide occurred as measured by biological oxygen demand (BOD(5)) in biocide-treated FS. Such FS can be discharged to WWTP without severe damage to the activated sludge process, the need for dilution and additional procedures to neutralize toxicity.