Report No DWI0740

Feb 1986

Injection of raw sewage sludge beneath the soil surface is a more hygienic and aesthetic means of applying sludge to land than surface application.

Since this type of application equates to burying the sludge, raw sludge can be utilised on grassland in this manner and the 6 month 'no grazing' recommendation can be eased. Because of considerable difficulties in obtaining realistic samples in field conditions studies of the survival of salmonellae and faecal indicator bacteria have been made in soil profiles held under controlled conditions of temperature and moisture.

When raw sewage sludge containing relatively high populations of salmonellae, ca 102 per ml, was buried in soils under grass at 9-15 ºC the populations were reduced in 1-4 weeks to <1 cell per ml sludge added. Presumptive coliform bacteria, of initial populations of about 107 per ml, were reduced by some 3-4 orders of magnitude over 8 weeks, and faecal streptococci, initially about 105 per ml, declined by some 2-3 orders of magnitude. There was no significant influence of soil type, whether of clay, loam or sandy texture, or of temperature respecting the values 9, 12 and 15 ºC, on population decline. There was a moderate background population of presumptive coliforms in the untreated soil samples, but few streptococci or salmonellae.

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