Design of Primary Sedimentation Tank

A primary sedimentation tank is a simple settling tank, normally rectangular or circular (cylindrical) in shape. This unit is provided in the domestic wastewater treatment system mainly to remove a large porting on suspended material, which is primarily inorganic in nature. However a small fraction of biodegradable settleable organic matter also gets removed with inorganic fraction. The conventional primary sedimentation tank normally removes about 60 – 70% of suspended solids and 20 – 30% of associated BOD (organic materials).

Provision of channels (PST) with common walls in between in case of rectangular tanks will save space and will be economical for large treatment plants for simplicity of sludge removal. It is also advised to provide two or more small tanks instead of one large tank due to ease of maintenance without disturbing treatment process.

Design Criteria A proper design of wastewater treatment units requires degree of treatment required in the region where PST is to be installed, experience and knowledge of electrical and mechanical devises being used in the treatment used. It standards and shapes vary slightly from different professionals and from different regions. Following design criteria may adopted as an initial understanding of design process.

Problem Design a primary sedimentation tank for a town having a population of 500,000. Assume suitable data where required.