Faecal sludge treatment: Optimization of Ascaris eggs enumeration and inactivation, and start-up of an anaerobic thermophilic acid-phase digester

Faecal sludge (FS) accumulates in onsite sanitation technologies, which play a prevailing role in low- and middle-income countries. However, FS management is generally lacking. FS is usually directly discharged in the environment despite its high organic load and concentration in pathogens, including parasites such as Ascaris sp.. It is therefore urgent to find an adequate treatment to stabilise and sanitize FS. One possibility, which still needs to be assessed, is the so called temperature-phased anaerobic digestion (TPAD). The main objective of this study was to evaluate the potential of inactivation of Ascaris eggs in the first phase of TPAD, the acid-phase. However, standard methods to enumerate Ascaris eggs – which are microscopic methods involving an incubation - are lacking. The first part of this study focused on the improvement of existing methods to reliably evaluate the treatment performances. Then, inactivation efficiency of Ascaris eggs at 55°C in pure solution and sludge was evaluated. Inactivation efficiency was finally tested after injection of Ascaris eggs in a 1L anaerobic reactor operated at 55°C, which had been started-up to favour the acid-phase. By following the development of the eggs during the incubation time, it was observed that a period of incubation of 14 days is appropriate. Indeed, 14 days allow all eggs to develop to the larval stage while reducing the loss of eggs due to hatching or lysis. It was also found that the presence of FS does not influence the time of incubation. The inactivation experiments have shown that the temperature as a clear impact on the eggs, and that inactivation was accelerated in the presence of substrate and by anaerobic conditions. Indeed, eggs were inactivated within 3 hours when they were heated at 55°C, in 1h30 when they were introduced in digested sludge at 55°C and in less than 45 minutes when they were injected in the acid phase reactor at 55°C. The exploitation of the acid-phase reactor, with synthetic FS imitating the characteristics of FS as a substrate, allows recommending a hydraulic retention time of four days. This enables to run a stable acid-phase process. Standard methods for the enumeration of Ascaris eggs propose an incubation time of three to four weeks. Its reduction to two weeks is a clear advantage as it decreases the time before obtaining the results. Moreover, this study has shown that TPAD is a possible technology to inactivate Ascaris eggs. If the second phase of TPAD permits to stabilise FS, then this technology would be an adequate treatment for FS in low- and middle-income countries.