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The Effect of Aerobic and Anaerobic Digestion on Eggs of Ascaris Lumbricoides Var. Suum in Night-Soil

W. L. Reyes^*, C. W. Krusé AND M. St.C. Batson

Aerobic and anaerobic batch digestions of night-soil seeded with eggs of pig ascaris (Ascaris var. suum) as a test organism were investigated at various temperatures to determine whether the eggs are preserved, cultured, or destroyed.

At low temperatures, both aerobic and anaerobic digestions tend to preserve Ascaris eggs which subsequently develop normally when removed to a more favorable environment. In the 25° to 35°C temperature range, both systems result in egg destruction attributable to a factor or factors other than heat killing. Oxygen starvation during the period of egg development may be a lethal factor in anaerobic digestion. In the aerobic systems, embryonation of and spontaneous hatching within the digester contribute to the destruction of the eggs.

In neither system is the destruction of eggs complete at the end point of night-soil stabilization, unless temperatures are held at or above 38°C for the anaerobic and 45°C for the aerobic digestion. At these temperatures, night-soil is well-digested and drainable in about 30 days of anaerobic and 20 days of aerobic digestion.

A pasteurization curve was prepared for the time-temperature necessary for complete destruction of fully embryonated eggs as measured by the ability of inducing in vitro hatching. Simple heating of raw night-soil at 55°C for 20 minutes should provide public health safety. Offensive odors and poor dewatering characteristics of the undigested material present, however, some practical objections. The esthetic and economic advantages of one method over the other, whether digestion is anaerobically, aerobically, or by raw night-soil pasteurization, will require a complete engineering analysis before the method of choice is selected.

^* Institute of Hygiene, University of the Philippines, Manila.

Ministry of Health, Labor and Housing, British Guiana.