Preliminary Treatment

At most plants preliminary treatment is used to protect pumping equipment and facilitate subsequent treatment processes.  Preliminary devices are designed to remove or cut up the larger suspended and floating solids, to remove the heavy inorganic solids, and to remove excessive amounts of oils or greases. 

To effect the objectives of preliminary treatment, the following devices are commonly used: 

1.       Screens -- rack, bar or fine

2.       Comminuting devices -- grinders, cutters, shredders

3.       Grit chambers

4.      Pre-aeration tanks

    In addition to the above, chlorination may be used in preliminary treatment.  Since chlorination may be used at all stages in treatment, it is considered to be a method by itself. Preliminary treatment devices require careful design and operation. 
 Primary Treatment

    In this treatment, most of the settleable solids are separated or removed from the wastewater by the physical process of sedimentation.  When certain chemicals are used with primary sedimentation tanks, some of the colloidal solids are also removed.  Biological activity of the wastewater in primary treatment is of negligible importance. 

The purpose of primary treatment is to reduce the velocity of the wastewater sufficiently to permit solids to settle and floatable material to surface.  Therefore, primary devices may consist of settling tanks, clarifiers or sedimentation tanks.  Because of variations in design, operation, and application, settling tanks can be divided into four general groups: 

1.       Septic tanks

2.       Two story tanks -- Imhoff and several proprietary or patented units

3.       Plain sedimentation tank with mechanical sludge removal

4.       Upward flow clarifiers with mechanical sludge removal

When chemicals are used, other auxiliary units are employed.  These are: 

1.       Chemical feed units

2.       Mixing devices

3.       Flocculators

    The results obtained by primary treatment, together with anaerobic sludge digestion as described later, are such that they can be compared with the zone of degradation in stream self-purification.  The use of chlorine with primary treatment is discussed under the section on Preliminary Treatment. 

 Secondary Treatment

    Secondary treatment depends primarily upon aerobic organisms which biochemically decompose the organic solids to inorganic or stable organic solids.  It is comparable to the zone of recovery in the self-purification of a stream. 

The devices used in secondary treatment may be divided into four groups: 

1.       Trickling filters with secondary settling tanks

2.       Activated sludge and modifications with final settling tanks

3.       Intermittent sand filters

4.       Stabilization ponds

The use of chlorine with secondary treatment is discussed under the section on Secondary Treatment 
 Chlorination

This is a method of treatment which has been employed for many purposes in all stages in wastewater treatment, and even prior to preliminary treatment.  It involves the application of chlorine to the wastewater for the following purposes: 

1.       Disinfection or destruction of pathogenic organisms

2.       Prevention of wastewater decomposition --

 (a) odor control, and 
(b) protection of plant structures 

1.       Aid in plant operation --

(a) sedimentation, 
(b) trickling filters, 
(c) activated sludge bulking 

1.       Reduction or delay of biochemical oxygen demand (BOD)

    While chlorination has been commonly used over the years, especially for disinfection, other methods to achieve disinfection as well as to achieve similar treatment ends are also used.  Among the most common is the use of ozone.  In view of the toxicity of chlorine and chlorinated compounds for fish as well as other living forms, ozonation may be more commonly used in the future.  This process will be more fully discussed in the section on disinfection. 

 Sludge Treatment

    The solids removed from wastewater in both primary and secondary treatment units, together with the water removed with them, constitute wastewater sludge.  It is generally necessary to subject sludge to some treatment to prepare or condition it for ultimate disposal.  Such treatment has two objectives -- the removal of part or all of the water in the sludge to reduce its volume, and the decomposition of the putrescible organic solids to mineral solids or to relatively stable organic solids.  This is accomplished by a combination of two or more of the following methods: 

1.       Thickening

2.       Digestion with or without heat

3.       Drying on sand bed -- open or covered

4.       Conditioning with chemicals

5.       Elutriation

6.       Vacuum filtration

7.       Heat drying

8.       Incineration

9.       Wet oxidation

10.     Centrifuging

Package UnitsThe term "package units" is used in the field to describe equipment which has been put on the market by a number of manufacturers that is intended to provide wastewater treatment by the use of prefabricated or modular units.  Package units can also refer to a complete installation, including both mechanisms and prefabricated containers.  This term is also applied to installations where only the mechanisms are purchased and the containers constructed by the purchaser in accordance with plans and specifications prepared by the manufacturer. 

Though specific limitations have not been established, individual package units have, in general, been small installations serving a limited population. 

Package units have been adapted to practically all the treatment devices, either singly or in various combinations that have been mentioned. 
 
 

Tertiary and Advanced Wastewater Treatment

    The terms "primary" and "secondary" treatment have been used to generally describe a degree of treatment;  for example, settling and biological wastewater treatment.  Since the early 1970's "tertiary" treatment has come into use to describe additional treatment following secondary treatment.  Quite often this merely indicates the use of intermittent sand filters for increased removal of suspended solids from the wastewater.  In other cases, tertiary treatment has been used to describe processes which remove plant nutrients, primarily nitrogen and phosphorous, from wastewater. 

Improvement and upgrading of wastewater treatment units as well as the need to minimize environmental effects has led to the increased use of tertiary treatment. 

A term that is also sometimes used to indicate treatment of a wastewater by methods other than primary or biological (secondary) treatment is advanced treatment.  This degree of treatment is usually achieved by chemical (for example coagulation) methods as well as physical methods (flocculation, settling and activated carbon adsorption) to produce a high quality effluent water.