Assessment of Indicator Organisms and Long-Term Stability in Advanced Alkaline Systems

Since the routine examination of biosolids for the presence of pathogens is very tedious, difficult and time consuming, indicator microbes are employed to assess the potential presence of pathogens. The objective/benefit of using indicator organisms is that tests for those microbes should be more simple, more routine, inexpensive and sufficiently reliable with respect to precision and accuracy (NRC, 2002). In addition to the development of new, reliable surrogate organisms, stability of alkaline material must be addressed.Over the last 30 years, viruses have typically been inactivated before the helminth eggs at temperatures below 70°C or temperature of pasteurization. In the cases where the temperature is below pasteurization temperature, the inactivation agent has been the ammonia concentration in alkaline systems. Looking at our studies on an advanced alkaline process in which the temperature is held at 55°C for 60 minutes, the helminth egg inactivation required a 55°C where as the virus inactivation occurred at a temperature range of 45 to 50°C. The data observed by our testing on an advanced alkaline process was also documented by EPA scientists in the early 1980s with Ward's and Berg's groups (Berg, 1968; Ward et al, 1976; Ward and Ashley, 1977; Ward and Ashley, 1978; Ward, 1978).Biosolids must be stabilized in order to reduce odors, which have been noted as a major concern with respect to alkaline stabilization. Stabilization is designed to address potential putrefaction processes, odiferous releases and vector attraction concerns. Also, most alkaline processes are open systems (i.e. the reactor is open to atmosphere rather than air-tight) in which temperature is more difficult to control, and factors such as increased pressure or bactericidal action of unionized ammonia are not present to aid in disinfection. The stabilization portion of this research evaluates the long term stability of a closed alkaline system product resulting from new operating conditions established by testing previously conducted and approved by EPA's Pathogen Equivalency Committee. The conditions formerly established as optimum to achieve required pathogen destruction resulted in the ability of the system to operate at a lower temperature of 55°C as opposed to the original temperature of 70°C designated under the 503 regulations.