Night-soil was anaerobically digested in small experimental tank with three different methods; gas recirculation stirring, mechanical stirring and simple static digestion. The digestion progressed only upon gas stirring. Concerning gas stirring from the reason of construction of the apparatus desulfation and removal of water from the gas were carried out. During this procedure ammonia was also removed.
Consequently, it became evident that the removal of ammonia was intimately relating to the process of the digestion and facilitating the process of digestion. The effect of gas stirring on digestion was different between night-soil obtained during summertime and those obtained during the season of relativly low temperature. The ammonia gas produced by night-soil obtained during the season of low temperature was more readily removed in the present experiment, indicating the influence of the mode of existence of ammonia in night-soil on the process of digestion.
Results of experiments are summarized as follows.
1) Gas stirring alone facilitated the process of digestion among three treatments of night-soil, gas stirring, mechnical stirring, simple static treatment at anaerobic condition.
2) When ammonia is readily removed by gas stirring, marked decreased was found in BOD₅, volatile acids alkalinity, albuminoid nitrogen, and total alkalinity, indicating the facilitation of digestion.
3) Night-soil obtained in March and April were rich in ammonia which was readily removed by gas stirring, whereas night-soil obtained in July and September contained a large amount of ammonia which was not easily evaporated by gas stirring. The form of existence of ammonia therefore appears to be diffrent according to the season.