Current time-temperature relationships for thermal inactivation of Ascaris eggs at mesophilic temperatures are too conservative and may hamper development of simple, but effective sanitation

Abstract

Ascaris eggs are commonly used as indicators for pathogen inactivation during the treatment of fecal sludge and wastewater due to their highly resistant lipid membrane and ability to survive in the environment for long periods of time. Current guidelines suggest that thermal treatment alone cannot inactivate Ascaris eggs at temperatures below 45 °C, although some evidence in the literature suggests this to be incorrect. Here, we performed a controlled experiment to test the effect of mesophilic temperatures on Ascaris inactivation. We exposed Ascaris suum eggs to a temperature gradient between 34°C and 45 °C under anaerobic and aerobic conditions to observe the required exposure times for a 3-log reduction. Indeed, we found that temperatures lower than 45 °C did inactivate these eggs, and the required exposure times were up to two orders of magnitude shorter than suggested by current guidelines. Results from the anaerobic exposures were used to develop a time-temperature relationship that is appropriate for Ascaris inactivation at mesophilic temperatures. Data from the literature demonstrated that our relationship is conservative, with faster inactivation occurring under environmental conditions when Ascaris eggs were suspended in fecal sludge or manure. A specific aerobic relationship was not developed, but we demonstrated that aerobic conditions cause faster inactivation than anaerobic conditions. Therefore, the anaerobic relationship provides a conservative guideline for both conditions. We demonstrate that relatively low temperatures can considerably impact Ascaris viability and suggest that mesophilic temperatures can be used in waste treatment processes to inactivate pathogens. The development of safe, low-input, mesophilic treatment processes is particularly valuable for ensuring universal access to safe sanitation and excreta management.

Pathogen Reduction Potential in Anaerobic Digestion of Organic Fraction of Municipal Solid Waste and Food Waste

“…They observed inactivation in AD at temperatures between 37 and 45 • C, while at lower temperatures Ascaris stayed active. The time required for a 3-log reduction in Ascaris' viability decreased from 57.5 to 2.12 days as the temperature increased [23]. Ligocka and Paluszak [30] who compared composting and fermentation under mesophilic conditions have also confirmed that the fastest inactivation during AD is taking place at 55 • C. A decrease in the percentage of invasive eggs amounted to almost 8% per hour [30], which is similar to a decrease obtained in present research (6.7-7.1%/h).…”

“…Papajova et al [29] stated that exposing A. suum eggs to the thermophilic conditions is the best way of elimination, due to inactivation of the enzymatic system of the egg. Harrof et al [23] investigated the time-temperature relationship for the inactivation of Ascaris eggs under mesophilic temperatures in aerobic and anaerobic treatment. They observed inactivation in AD at temperatures between 37 and 45 • C, while at lower temperatures Ascaris stayed active.…”

“…has been widely studied in laboratory conditions [16,17], attention must be paid when applying the results as well as time-temperature relationship analyzed in laboratory to large-scale systems [23]. Here, exposure times required for inactivation of pathogens may be affected by uneven heating, fluctuating temperatures or shielding properties of solids [23,24]. Thus, the transfer of the laboratory results to the full scale systems requires further investigation and is an interesting area of research.…”

Resource recovery technologies as microbial risk barriers: towards safe use of excreta in agriculture based on hazard analysis and critical control point

“…Even if a thermophilic temperature is not achieved, a small change in temperature in the mesophilic range facilitates the inactivation of Ascaris eggs. 95 Pathogen inactivation in a composting chamber primarily relies on the heat generated by aerobic biological reactions. Thus, maintaining optimum conditions for biological reactions is important for sanitization.…”

“…Ascaris eggs are not inactivated at temperatures below 36 °C without uncharged ammonium. 95,121 At a thermophilic temperature, the estimated storage time was significantly shortened to <1 d for bacteria and Ascaris eggs and 30 d for murine norovirus 1 (Fig. 4 and Table S5 †).…”