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Horizontal settling tank

A horizontal settling tank is used to clean water from sludge and different solids. It is made from stainless or ferrous rolled stock with applied modern anti-corrosion paints and coatings (zinotan).

Horizontal Settling Tank

A horizontal settling tank is one of the types of rectangular tanks. This article represents a rectangular tank equipped with water separating and water collecting devices and, besides, piping to feed the water to be clarified, as well as a device to remove the sediment formed. Horizontal settling tanks are generally applied at stations which treat water from drinking and household industrial water pipelines, with their capacities reaching more than 30000–50000 m3/day with extraction from water of different substances treated with a coagulant.

Any horizontal settling tank comprises:

  • Working part in which the suspension is settled
  • Lower part in which the settled sediment is collected.

Horizontal Settling Tank Design

To design a horizontal settling tank with a particular completeness degree of insoluble impurities extraction from the waste water, the following data are commonly used:

  • First, waste water volume
  • Second, initial concentration of suspended copper (Cu) in the waste water
  • Third, allowed final concentration of chrome (Cr) in the settling-vat water, which is assumed in accordance with the existing sanitary standards or may be stipulated by the process requirements.
  • Fourth, conditional settling velocity U0 of the particles which should be extracted from the water;
  • Fifth, water column height h in the considered laboratory cylinder where the process analysis, i. e. settling from waste water, is performed;
  • Sixth, exponent n showing the influence of suspended particles agglomeration on their settling.

The process of settling in waste waters, as well as the sediment compaction accompanying it, influences both the economy and stability of the sewage treatment facilities, in particular, for biological treatment.

The excess activated sludge volume in aeration tanks increases with the suspended particles carried from the settling tanks. Therewith, the moisture content of the activated sludge exceeds that of the sediment manyfold. Such a process implies increase in the capacities of sludge thickeners, as well as all the downstream facilities, to treat the surplus activated sludge.

In order to increase the efficiency of horizontal settling tanks, provided that the waste water contains no more than 300 mg/l of suspended substances, additional measures are to be taken:

  • First, add chemical agents to waste water, in particular, coagulants which help increasing the impurities settling velocity;
  • Second, add substances which settle well, such as activated sludge, which act as a sorbent and also a biocoagulant.
  • Third, aerate waste waters to flocculate fine insoluble impurities.
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