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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
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