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346 Chapter 10 Introduction to Wastewater Systems
daily volume of sludge is thousands of gallons. This makes them bulky and putrescible. To
simplify handling and reuse/disposal, sludge is dewatered and stabilized in varying de-
grees. The following ways and devices are common (see Fig. 10.8):
1. Organic matter in sludge stored in tanks is metabolized and converted into rela-
tively stable residues by bacteria and other saprophytic organisms. Continuing
hosts of living things use the waste matters in the digesting sludge for growth and
energy. Liquefaction and gasification occur. Dissolved oxygen disappears, and the
biomass becomes anaerobic. Sludge-digestion tanks produce concentrated, stabi-
lized, and rapidly dewatering sludges, sludge liquor, and gases of decomposition,
primarily methane and carbon dioxide.
2. Water is removed from sludge (usually digested sludge) run into beds of sand or
other granular materials by evaporation of moisture to the air and percolation of
water into and through the beds. This is accomplished with sludge-drying beds that
produce a spadable sludge cake.
3. Sludge is dewatered by passing it (usually after chemical conditioning) through a
centrifuge, filter press, or a filter medium such as cloth or coiled wires wound
around a drum to which a vacuum is applied. Centrifuges, presses, and vacuum fil-
ters are used for this purpose to produce a sludge cake or paste.
4. Sludge cake or paste is dried by means of heat in flash driers that produce commer-
cially dry sludge granules.
5. The biosolids produced by sludge treatment are applied to land as a soil amend-
ment and fertilizer for agricultural land.
6. Organic matter in partially dewatered or heat-dried sludge is burned as a fuel in in-
cinerators, producing ash.
7. Sludge is thickened in advance of digestion, drying, or dewatering by stirring.
Sludge thickeners produce a more concentrated sludge.
8. Organic matter in thickened sludge is destroyed by wet combustion in sludge re-
torts operating at high pressures and temperatures and producing readily dewa-
tered, mineralized residues.
The marsh gas or methane released during digestion is a combustible gas of high fuel
value and is put to good and varied uses in modern treatment works. After digestion,
wastewater solids are no longer recognizable as such. Their colloidal structure has been
destroyed and they dry rapidly in the air. Heat-dried sludge is stable and essentially sterile.
The final product of incineration is ash. Many of the treatment methods reduce the number
of possibly pathogenic organisms concentrated in the sludge to within the standards for
safe reuse.
Two flow diagrams of wastewater treatment plants are presented in Fig. 10.9. Numerous
other combinations of treatment processes are possible. The plants shown provide so-called
primary and secondary treatment of both wastewater and sludge. Partial treatment of either
or both is often enough. However, more complete treatment may be needed at critical times
of the year; for example, during low summer runoff and high recreational use of receiving
waters. Tertiary treatment, also called water renovation, may be provided to remove residual
nutrients and toxic, foaming, or otherwise objectionable substances.
The works illustrated in Fig. 10.9 will remove from 80% to 95% or more of the sus-
pended solids, putrescible matter, and bacteria. Effluent chlorination can ensure 99% or
higher destruction of bacteria. Partial treatment can achieve removal values between 40%
and 70%.