Sanitisation of sewage sludge in a solar heated reactor: inactivation of total coliforms and Escherichia coli

ABSTRACT The application of sewage sludge in agriculture has become a common practice in many countries, which is attractive in many aspects, especially for the input of organic matter and nutrients in the soil. Despite the benefits of sludge use in agriculture and for the environment , this practice needs to be conducted considering safety aspects regarding sanitary conditions. In this context, the aim of this study was to evaluate the efficiency of a thermal disinfection process of sewage sludge using solar energy. The sludge was heated through a heat exchanger built with copper pipes and installed inside the reactor, in which water heated in flat plate solar collectors circulates. Sixteen experimental tests were performed under different solar irradiation conditions. The thermal inactivation of Escherichia coli in the reactor was evaluated using the first order kinetic model in non-isothermal conditions. The process proved effective in those tests with an average solar irradiation period above 500 W.h.m-2, and the reduction of E. coli was between 4.2 and 7.1 units log10; and between 4.8 and 7.4 units log10 of total coliforms. When tests were conducted in days with lower levels of solar irradiation, the efficiency of sanitisation was compromised, due to the low temperatures reached by the sludge.