Discussion on Papers 5–7

MR HUNTINGTON and MR RUMSEY, Paper 7 Raw sewage is extremely variable in its composition but, provided the discharge of non-domestic sewage to sewers is properly controlled, sewage should contain few contaminants which are in sufficient strength to be troublesome.

For land-based treatment, where the effluent is discharged into a river or stream, biological oxygen demand, suspended solids and ammonia, are usually critical, while for marine treatment, bacteria levels are important. Land-based treatment only reduces bacteria levels by one order of magnitude (Fig. 1) and a beach close to the mouth of a river can be seriously affected by high bacteria levels carried by that river. The combined effect of bacterial die-off during treatment and dilution on receipt in the river generally ranges between only 80 and 160, which is very small compared with dilutions of 5000 plus which are achieved with properly designed marine outfalls.