Environmental Engineering Reference
In-Depth Information
taBle 32.14
characteristics of solids Produced during Wastewater treatment
solids or sludge
description
Screenings
Screenings include all types of organic and inorganic materials large enough to be removed on bar
racks. The organic content varies, depending on the nature of the system and the season of the year.
This sludge is usually not the subject of digestion processes, because of the large size of particles/
pieces. It is generally disposed in a landfill.
Grit
Grit is usually made up of the heavier inorganic solids that settle with relatively high velocities.
Depending on the conditions, grit may also contain significant amounts of organic matter,
especially fats and grease.
Scum/grease
Scum consists of the floatable materials skimmed from the surface of primary and secondary
settling tanks and from grit chambers and chlorine contact tanks, if so equipped. Scum may
contain grease, vegetable and mineral oils, animal fats, waxes, soaps, food wastes, vegetable and
fruit skins, hair, paper and cotton, cigarette tips, plastic materials, condoms, grit particles, and
similar material. The specific gravity of scum is less than 1.0 (usually around 0.95).
Primary sludge
Sludge from primary tanks is usually gray in color, slimy and in most cases, has an extremely
offensive odor. If sludge is left in storage, it can readily be partially digested.
Sludge from
chemical
precipitation
Sludge from chemical precipitation with metal salts is usually dark in color, though its surface may
be red if it contains much iron. Lime sludge is gray-brown. The odor of chemical sludge may be
unpleasant, but not as offensive as of primary sludge. The hydrate of iron or aluminium in it makes
the chemical sludge gelatinous. If the sludge is left in the tank it undergoes similar decomposition
as primary sludge, only at slower rate.
Activated sludge
Activated sludge generally has a brown flocculent appearance. If the color is dark, the sludge may
be approaching a septic condition. If the color is lighter than usual, there may have been under
aeration with a tendency for the solids to settle slowly. Sludge in good condition has an inoffensive
“earthy” odor. The sludge tends to become septic rapidly and then has an odor of putrefaction
Activated sludge will digest alone or when mixed with primary sludge.
Trickling filter
sludge
Humus sludge from trickling filters is brown in color, flocculent and relatively inoffensive when
fresh. It generally undergoes decomposition more slowly then other undigested sludges. When
trickling filter sludge contains many worms, it may become inoffensive quickly. Trickling filter
sludge digests readily.
32.5.2.1 specific constituents: trace compounds, nitrogen,
Phosphorous, Potassium, and heavy metals
The consideration of these constituents is mainly important when sludge is applied to land. Sludge
characteristics that affect the suitability for land application and for beneficial use include organic
content (usually measured as volatile solids), nutrients, pathogens, metals, and toxic organics.
In most cases when sludge is applied to land, there are sufficient nutrients present for good plant
growth. However, trace elements and heavy metals may limit the sludge application to land. Trace
elements are those inorganic chemical elements that in very small quantities can be essential or
detrimental for plants and animals. Heavy metals are some of these trace elements in the sludge
and mostly are the limiting factor for sludge land application. The typical heavy metal content in
wastewater sludge is presented in Table 32.16.
32.5.2.2 sludge quantities
Data on the sludge quantities produced from various processes and operations are presented in
Table 32.17. Although data in Table 32.8 can be useful as presented, it must be noted that quantity
of sludge can vary widely. The contribution of raw municipal wastewater (in the U.S.) is about
400 L PE -1 /day (Novotny et al. 1989; Hammer 1996) with total solids of less than 0.1%, 100-350
mg/L suspended solids and 110-400 mg/L BOD 5 . These numbers add up to sludge quantity of 54
 
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