Thank you for your question on the difference between the final effluent standard produced by a package sewage treatment plant and the stream standard.
The difference between the 2 is as follows:
1. Effluent Standard or Final Effluent Standard is the standard of effluent that a given wastewater treatment plant (WWTP) needs to produce based on a set maximum inlet load in terms of Flow, BOD, SS (Suspended Solids) & Ammonia (NH4), Nitrate (NO3), Nitrite (NO2), Total Nitrogen (TN), Orthophosphate (OP) and Total Phosphate (TP) if specified. The outlet effluent parameters are set relative to the influent parameters. N.B. Industrial effluent may have other parameters listed. Package sewage treatment is concerned with the treatment of normal domestic sewage or equivalent.
2. The Stream standard is the standard of the water in a particular water course to which a sewage treatment plant is discharging into. If a stream standard exists the Effluent standard for a particular WWTP is set by working back from the Stream standard. For example if a water course is used for drinking water, bathing or as a fishery, the stream standard and thus the effluent standard discharging into that stream will be set relatively high and will probably include low BOD/SS and NH4/TN/TP concentrations.
However, if a watercourse can provide significant dilution/biodegradation then the stream standard can be set lower, thus resulting in a lower effluent standard.
The stream standard is often set relative to the 'assimilative capacity' of a watercourse. This means how much dilution/biodegradation it can provide before pollution such as Oxygen stripping or eutrophication results.
Thank you for your question.