Sewage Parameters 2: Suspended Solids (SS)
Sewage Parameters 2: Suspended Solids (SS)
The next topic in our Sewage Treatment Parameters Education Series is Suspended Solids (SS), which are usually stated in conjunction with Biochemical Oxygen Demand (BOD). The normal European discharge standard for SS is 35 mg/l and is usually quoted in short as ’25/35′ effluent (BOD of 25 mg/l & SS of 35 mg/l) Therefore, most package sewage treatment plants will be designed to achieve at least ’25/35′ for a given populations Suspended solids are the solids visible in influent/effluent sewage which do not settle out easily over time. A more correct definition is; ‘A measurement of the solids in suspension in sewage, influents or effluents normally expressed in mg/l or parts per million (ppm).
Suspended Solids are normally reduced by both physical means such as settlement and also by biological treatment. A typical SS concentration in normal domestic effluent is 300 mg/l, which then needs to be reduce to 35 mg/l or better. To obtain very low Suspended Solids concentrations extra treatment such as with a Saran Filter or Package Final Effluent Polishing Filter (FEPF) might be necessary. A certain amount of BOD is attached to SS, so reducing SS will also lead to a consequent decrease in BOD. An effluent with a low SS will look clear to the eye, while as the SS concentration begins to rise the effluent will become increasingly more cloudy.
Next up will be Population Equivalent or PE.
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