ABSTRACT
Primary treatment reduces the organic loading on downstream treatment processes by removing a large amount of settleable, suspended, and floatable materials.
Primary treatment reduces the velocity of the wastewater through a clarifier to approximately 1 to 2 ft/min so that settling and floatation can take place. This slowing of the flow enhances the removal of the suspended solids in the wastewater.
Primary settling tanks remove the floated grease and scum, remove the settled sludge solids, and collect them for pumped transfer to disposal or further treatment.
Clarifiers used may be rectangular or circular. In rectangular clarifiers, the wastewater flows from one end to the other and the settled sludge is moved to a hopper at one end, either by flights set on parallel chains, or by a single bottom scraper set on a travelling bridge. Floating material such as grease and oil are collected by a surface skimmer.
In circular tanks, the wastewater usually enters at the middle and flows outward. Settled sludge is pushed to a hopper that is in the middle of the tank bottom, and floating material is removed by a surface skimmer.
Factors affecting primary clarifier performance include the following:
– Rate of flow through the clarifer.
– Wastewater characteristics (strength; temperature; amount and type of industrial wastes; and the density, size, and shapes of particles).
– 26Performance of pretreatment processes.
– Nature and amount of any wastes recycled to the primary clarifier.
Key factors in primary clarifier operation include the following concepts:
– Retention time, hours = ( volume, gals ) ( 24 hrs/day ) flow, gallons per day https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203734216/4406a4e1-8626-47b2-b2d2-ea441a87fc3d/content/eq1.tif"/>
– Surface loading rate, gal/day sq/ft = flow, gallons per day surface area, sq/ft https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203734216/4406a4e1-8626-47b2-b2d2-ea441a87fc3d/content/eq2.tif"/>
– Solid loading rate, lbs/day sq/ft = solid into clarifier, lbs/day surface area, sq/ft https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203734216/4406a4e1-8626-47b2-b2d2-ea441a87fc3d/content/eq3.tif"/>
– Weir overflow rate, gal/day lineal ft = flow, gallons per day weir length, lineal ft https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203734216/4406a4e1-8626-47b2-b2d2-ea441a87fc3d/content/eq4.tif"/>
