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370 Industrial Wastewater Treatment, Recycling, and Reuse
Secondary Tertiary
Primary treatment treatment
treatment
Removal of: Removal of:
Removal of:
Organic pollutants Micro pollutants,
Suspended solids,
and solids dissolved solids
oil & grease
separation Disinfection
Stages of wastewater treatment
Figure 9.1 Stages of wastewater treatment.
Primary Treatment: Primary treatment generally comprises the removal of
inert solids by using screens, a grit removal system, and clarifiers; oil is
removed by means of a corrugated plate interceptor, American Petroleum
Institute separator, dissolved air flotation, or induced gas flotation.
Secondary Treatment: Secondary treatment is the heart of the wastewater
treatment plant and comprises the removal of dissolved organic pollutants.
Organic removal can be achieved by biodegradation or chemical oxidation.
Generally, biodegradation is preferred over chemical oxidation mainly due
to its economical advantage. Biodegradation may be aerobic or anaerobic.
This chapter will mainly focus on the aerobic biological treatment method.
Tertiary Treatment: Depending on the end use of treated water, tertiary
treatment can have one or more of the following process systems:
1. Removal of suspended solids to less than 10 mg/L: Use of filters such as
pressure sand filter, dual media filter, multimedia filters, or ultra-filtration
(UF) membranes.
2. Removal of any particular hard-to-degrade contaminant: Treatment methods
can vary from simple chemical oxidation to advanced oxidation
processes (mixture of different oxidants).
3. Removal of bacteria, viruses: Disinfection by means of chlorination or UV
disinfection.
4. Removal of dissolved solids: Reverse osmosis.
9.2 SECONDARY WASTEWATER TREATMENT
Generally, biological treatment methods are used as a secondary stage for
removal of biodegradable organic matter, as well as removal of nutrients
and suspended solids. Biodegradable organics, principally composed of
proteins, carbohydrates, and fats, if discharged untreated in to the environ-
ment, can lead to the depletion of oxygen and the development of septic