Assessment of the Effectiveness of Tropical Lagoon Treatment for the Converting of Domestic Septage into a Value-Added Product
Worldwide, more than two billion people rely on onsite sanitation technologies for excreta treatment and/or disposal. The proper management of domestic septage remains a challenge in most cities in developing countries. A field experiment was conducted in Kinshasa/DR Congo from June 2013 to September 2015 to explore the effectiveness of lagoon treatment for the disinfection and the stabilization of domestic septage. Four lagoons were designed by combining two design factors: Total Solids content (< 5% versus 8% - 12%) and the protection from rainfall (Yes versus No). Each lagoon was a truncated pyramid with a usable volume of 28 m3 and 3m depth. The monitoring of pathogens inactivation was conducted from September 2014 over 12,4 months using coliforms bacteria and Ascaris lumbricoides eggs in sentinel chambers as indicator organisms, respectively with the multiple tubes technique and the Tulane Method. Local climate conditions and abiotic characteristics of the stored septage were monitored monthly. Four-log reduction of fecal coliforms organisms were observed within 10 months of septage storage in the two types of lagoons. At the end of this study, 3-log and less than 2-log reductions of Ascaris eggs were observed, respectively in the lagoons with high and low solids content. The inactivation curve of Ascaris eggs was characterized by a lag phase with a slight inactivation followed by an active inactivation phase. The inactivation rate constants for the active phase were 0.071 and 0.034 day-1, respectively in the lagoons with high and low solids content. Multiple linear regression model showed that the storage time, pH, and TS were significantly associated with percent Ascaris eggs inactivation. Trace elements concentrations in the final product were below threshold ceiling values for their reuse in agriculture. Total volatile solids concentration reduction in the lagoons with high solids content was below the stabilization limit set at 38% by USEPA. In conclusion, lagoon storage of domestic septage with at least 8% TS for 12 to 15 months is an effective option for the disinfection of domestic septage under the conditions of this study. The resulting biosolids constitute a value-added product which can be safely reused in agriculture.