Slow sand filtration as a tertiary treatment for the secondary effluent from sewage treatment plant

A field-pilot scale slow sand filter (SSF) was constructed at Al-Rustamiya Sewage Treatment Plant (STP) in Baghdad city to investigate the removal efficiency in terms of Biochemical Oxygen Demand (BOD5), Chemical oxygen demand (COD), Total Suspended Solids (TSS) and Chloride concentrations for achieving better secondary effluent quality from this treatment plant. The SSF was designed at a 0.2 m/h filtration rate with filter area 1 m2 and total filter depth of 2.3 m. A filter sand media 0.35 mm in size and 1 m depth was supported by 0.2 m layer of gravel of size 5 mm. The secondary effluent from Al-Rustamiya STP was used as the influent to the slow sand filter. The results showed that the removal of BOD5, COD, TSS, and Chloride were 73.84%, 73.01%, 63.71% and 49.80%, respectively after 28 days of the SSF operation. The overall removal efficiency of Al-Rustamiya STP has been improved by this tertiary treatment reaching to 91.15% for BOD, 86.84% for COD, 86.55% for TSS, and 59.50% for chloride which indicated that the final effluent became acceptable to the Iraqi regulations for disposal.