Design of Settlement Tanks and the use of Chemicals to Aid Precipitation of Suspended Solids

Traditionally, the design of settlement tanks has been based on practical experience linked to key criteria, which have evolved from many sources. In this paper, the design criteria which affect the performance of both primary and secondary tanks are examined and a procedure, linked to critical velocities and loading rates, is put forward to enable the design requirements to be rationalized. Based on the authors’ considerable practical experience, the proposed procedure can be used with confidence to design tanks which will achieve required performance, particularly in the case of secondary tanks (for separation of both activated and humus sludges). In particular, the importance of a correctly sized inlet stilling chamber, to reduce the feed velocity to around 0.003–0.005 m/s is explained. The use of chemicals to aid removal of suspended solids (and hence BOD) has been practised for many years. Today, there is renewed interest, particularly for phosphate removal and for the treatment of some industrial wastewaters. The composition and nature of sewage affects the potential benefits of coagulant chemicals when added at the treatment works’ inlet. The introduction of organic polymers has significantly improved performance and reduced costs. The major disadvantage of using chemicals relates to the significant increase in quantity of sludge that is produced, which will require both treatment and disposal, increasing operational costs.