Tertiary treatment of municipal sewage via slow sand filtration

Abstract

This study is a field evaluation of slow sand filtration as a tertiary treatment process at pilot-scale. It was found out that slow sand filtration is, indeed, very effective in removing contaminants from secondary effluents. In particular, the bacterial removal levels were exceptional to an extent that the filtrate would easily qualify for unrestricted irrigation. In view of the experimental rules, 0.16 m/hr is suggested as a suitable hydraulic loading for the design of similar systems in Saudi Arabia. At this hydraulic loading the observed average removal of BOD, SS, turbidity and total coliform bacteria were 86, 69, 88, and over 99%, respectively, and the length of the filtration run was about 20 days. It was confirmed that most of the purification is occurring at the top layers of the filter such that even a sand bed depth of 35 cm yielded significant levels of contaminants removal. It was also observed that the presence of algae in the filter influent is a very important condition as it adversely affects the filter performance.