Use of Peracetic Acid in Operational Sewage Sludge Disposal to Pasture

Abstract

Intestinal pathogens and parasites are a problem associated with the disposal of sewage sludge to pasture. Those of major importance are ova of the beef tapeworm (Taenia saginata) and Salmonella bacteria. Techniques for routine monitoring of pathogens and intestinal parasite levels in sludge together with design of simple methods for their removal have intensified, because disposal of sewage sludge to pasture is coming under increasing international surveillance. The performance during field trials of a proprietary peracetic acid compound (containing 36 to 40% w/w peracetic acid), hereafter referred to as PAA (100%), is described. These trials under operational conditions showed it to be a suitable bacterial and ovicidal agent for the disinfection of raw, digested and activated sludges. Concentrations ranging from 250 to 1000 mg PAA/l have achieved up to 99% inhibition of hatching and up to 100% destruction in viability of tapeworm embryos suspended in raw and digested sludges. Similarly these concentrations resulted in 5 log reductions in salmonellae seeded in raw sludge. A dose of 250 mg PAA/1 reduced salmonellae levels from 4600 organism/100cm3 to <30/100cm3 during gravity thickening of surplus activated sludge. Results from 46 tanker loads of consolidated surplus activated sludge revealed that salmonellae levels were reduced from 2400 organisms/100 cm3 to <30/100 cm3 at doses of 500, 400 and 300 mg PAA/1. At a lower dose of 150 mg PAA/1 levels were reduced to within a range of <30 to 430 organisms/100 cm3. At optimum disinfection concentrations for the destruction of both bacteria and parasites, PAA was rapidly utilised resulting in safe, readily biodegradable, non-toxic residuals. The technical and practical benefits, principals of disinfecting sludge and ecological safeguards are highlighted in this paper. Use is made of case histories to describe practical operating techniques.