Page 434 - Environment: The Science Behind the Stories

Raw sewage enters
treatment facility

1 Screens and
grit tank
Solid objects
and grit removed

Oils and greases Solids disposed
float to the top at landfill
Solids sink to
2 Primary the bottom
clarifier
Oils, greases,
and solids
removed
Gases chemically
treated to reduce
odor

3 Aeration basin
Microbes consume
organic matter

Some solids returned
to seed aeration
basin with new microbes
4 Secondary
clarifier
Remaining oils,
greases, and
solids removed Sludge sent to
anaerobic digester

5 Filtering and
disinfection
Water filtered
with coal and
sand, and/or
disinfected with
chlorine or UV
light CHAPTER 15 • Fr E shwat E r s yst E m s and rE sour CE s

Effluent discharged Gas to Biosolids for
into waterways generate cropland
electricity
Figure 15.26 Shown here is a generalized process from a modern, environmentally sensitive wastewater treatment facility. Waste-
water initially passes through screens to remove large debris and into grit tanks to let grit settle 1 . It then enters tanks called primary clarifiers
2 , in which solids settle to the bottom and oils and greases float to the top for removal. Clarified water then proceeds to aeration basins 3
that oxygenate the water to encourage decomposition by aerobic bacteria. Water then passes into secondary clarifier tanks 4 for removal of
further solids and oils. Next, the water may be purified 5 by chemical treatment with chlorine, passage through carbon filters, and/or exposure
to ultraviolet light. The treated water (called effluent) may then be piped into natural water bodies, used for urban irrigation, flowed through a con-
structed wetland, or used to recharge groundwater. In addition, most treatment facilities use anaerobic bacteria to digest sludge removed from 433
the wastewater. Biosolids from digesters may be sent to farm fields as fertilizer, and gas from digestion may be used to generate electric power.

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