Environmental Engineering FOURTH EDITION

218 ENVIRONMENTAI, ENGINEERING

Table 10-2. Design Loadings for Gravity Thickenersn

Sludge

Design loading

(kg solids/m2-h)

Raw primary 5.2

Waste activated 1.2

Raw primary + waste activated 2.4

Trickling filter sludge 1.8

=0.204 kg/m2-h = lb/ft2-h.

Sludge feed ipckened Flotation unit sludge

I 1 II

release

I valve

T.:i:-hAd injection 1 p

Recycle Effluent

saturation tank receiver

Pressurizing

U pump

Figure 10-9. Flotation thickener.

As the air comes out of the solution, tiny bubbles attach themselves to solids and carry
them upward to be scraped off as thickened sludge.

Sludge Dewatering

Unlike sludge thickening, where the treated sludge continues to behave as a liquid,

dewatered sludge will behave like a solid after treatment. Dewatering is seldom used as

an intermediate process unless the sludge is to be incinerated. Most wastewater plants

use dewatering as a final method of volume reduction before ultimate disposal. In the

United States, the usual dewatering techniques are sand beds, vacuum filters, pressure

filters, belt filters, and centrifuges.

Sand beds have been used for a great many years and are still the most cost-

effective means of dewatering when land is available. The beds consist of tile drains

in gravel, covered by about 26 cm (10 in.) of sand. The sludge is poured on the sand to

a depth of 8-12 in.; liquid is removed by seepage through the sand into the tile drains

and evaporation. Although seepage into the sand results in a substantial loss of water,

it only lasts for a few days. The sand pores are quickly clogged, and drainage into the