SETTLEMENT TANKS

Description

A settlement tank is used to separate organic and inorganic solids from liquid. The solid material, which is denser than the liquid, falls under the influence of gravity to the base of the tank, which is often sloping or conical, from where it is pumped away as a sludge. Some solid materials may not settle easily, or may take too long to settle under the influence of gravity alone. Coagulants and flocculants, such as polyelectrolytes, ferric sulphate or lime, may be added to enhance the agglomeration of particles, and speed up settlement.

Clarifiers, which are used to produce clarified liquor, are usually large circular tanks. However, some incorporate internal parallel plates or lamellae to enhance settlement and speed up the removal of suspended solids and associated BOD. Parallel plates can be constructed from solid metal sheeting, wedge wire or even from woven metal strips. The latter are flexible and this promotes self cleaning. Clarifiers can be constructed from a range of materials including those resistant to aggressive chemicals.

Sludge thickeners are usually cylindrical. A picket fence is used to assist settlement and allow degassing and the removal of thickened sludge may be assisted by scrapers.

Application

Typical applications for settlement tanks exist in the food, mineral and chemical industries and in water and sewage treatment. Settlement tanks can remove sand, gravel, ceramics and organic solid waste. Certain dissolved metals can be settled by converting them to their hydroxide form.

The rate at which a settlement tank can settle solids or sludge is restricted by the upward flow of liquid through a clarifier _ the higher the throughput, the higher the proportion of solids that fail to settle. On the other hand, if the flow is too low, biological activity in the sludge can produce gas and cause the sludge to float.

Sizing

A typical clarifier treating about 40 m3/hour of liquid effluent would require a lift pump of about 3 kW, a sludge transfer pump of about 1 kW, chemical dosing pumps of about 03 kW, a control panel of about 03 kW, and a scraper drive motor of about 025 kW. Clarifiers can produce a sludge of 1-4% solids.

A typical motor size for the picket fence in a sludge thickener would be 1-2 kW. Suitable upward flow velocities are typically 09 m/h to 14 m/h. Sludge thickeners can produce a sludge of up to 75% solids.