Waste water treatment Techniques – Physical, chemical and biological methods

Wastewater treatment is the process of removing contaminants from wastewater and household sewage, both runoff (effluents) and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce an environmentally-safe fluid waste stream (or treated effluent) and a solid waste (or treated sludge) suitable for disposal or reuse (usually as farm fertilizer).

Wastewater Treatment Methods

Physical

Sedimentation (Clarification)  
Screening  
Aeration  
Filtration  
Flotation and Skimming  
Degassification  
Equalization

Chemical

Chlorination  
Ozonation  
Neutralization  
Coagulation  
Adsorption  
Ion Exchange

Biological

Aerobic 
Activated Sludge Treatment Methods  
Trickling Filtration  
Oxidation Ponds  
Lagoons  
Aerobic Digestion 
Anaerobic 
Anaerobic Digestion  
Septic Tanks  
Lagoons 

Physical methods include processes where no gross chemical or biological changes are carried out and strictly physical phenomena are used to improve or treat the wastewater.  

Examples would be coarse screening to remove larger entrained objects and sedimentation (or clarification). In the process of sedimentation, physical phenomena relating to the settling of solids by gravity are allowed to operate.  Usually this consists of simply holding a wastewater for a short period of time in a tank under quiescent conditions, allowing the heavier solids to settle, and removing the "clarified" effluent.  Sedimentation for solids separation is a very common process operation and is routinely employed at the beginning and end of wastewater treatment operations.While sedimentation is one of the most common physical treatment processes that is used to achieve treatment, another physical treatment process consists of aeration -- that is, physically adding air, usually to provide oxygen to the wastewater.  Still other physical phenomena used in treatment consists of filtration.  Here wastewater is passed through a filter medium to separate solids.  An example would be the use of sand filters to further remove entrained solids from a treated wastewater.  Certain phenomena will occur during the sedimentation process and can be advantageously used to further improve water quality.  Permitting greases or oils, for example, to float to the surface and skimming or physically removing them from the wastewaters is often carried out as part of the overall treatment process. 

In certain industrial wastewater treatment processes strong or undesirable wastes are sometimes produced over short periods of time.  Since such "slugs" or periodic inputs of such wastes would damage a biological treatment process, these wastes are sometimes held, mixed with other wastewaters, and gradually released, thus eliminating "shocks" to the treatment plant.  This is call equalization.  Another type of "equalization" can be used to even out wide variations in flow rates.  For example, the wet well of a pump station can receive widely varying amounts of wastewater and, in turn, pump the wastes onward at more uniform rates. 

Chemical treatment consists of using some chemical reaction or reactions to improve the water quality.  Probably the most commonly used chemical process is chlorination.  Chlorine, a strong oxidizing chemical, is used to kill bacteria and to slow down the rate of decomposition of the wastewater.  Bacterial kill is achieved when vital biological processes are affected by the chlorine.  Another strong oxidizing agent that has also been used as an oxidizing disinfectant is ozone. 

A chemical process commonly used in many industrial wastewater treatment operations is neutralization.  Neutralization consists of the addition of acid or base to adjust pH levels back to neutrality.  Since lime is a base it is sometimes used in the neutralization of acid wastes. 

Coagulation consists of the addition of a chemical that, through a chemical reaction, forms an insoluble end product that serves to remove substances from the wastewater.  Polyvalent metals are commonly used as coagulating chemicals in wastewater treatment and typical coagulants would include lime (that can also be used in neutralization), certain iron containing compounds (such as ferric chloride or ferric sulfate) and alum (aluminum sulfate). 

Certain processes may actually be physical and chemical in nature.  The use of activated carbon to "adsorb" or remove organics, for example, involves both chemical and physical processes.  Processes such as ion exchange, which involves exchanging certain ions for others, are not used to any great extent in wastewater treatment. 

Biological treatment methods use microorganisms, mostly bacteria, in the biochemical decomposition of wastewaters to stable end products.  More microorganisms, or sludges, are formed and a portion of the waste is converted to carbon dioxide, water and other end products.  Generally, biological treatment methods can be divided into aerobic and anaerobic methods, based on availability of dissolved oxygen. 

The purpose of wastewater treatment is generally to remove from the wastewater enough solids to permit the remainder to be discharged to a receiving water without interfering with its best or proper use.  The solids which are removed are primarily organic but may also include inorganic solids.  Treatment must also be provided for the solids and liquids which are removed as sludge.  Finally, treatment to control odors, to retard biological activity, or destroy pathogenic organisms may also be needed. 

While the devices used in wastewater treatment are numerous and will probably combine physical, chemical and biological methods, they may all be generally grouped under six methods: 

1.     Preliminary Treatment

2.     Primary Treatment

3.     Secondary Treatment

4.     Disinfection

5.     Sludge Treatment

6.     Tertiary Treatment

Degrees of treatment are sometimes indicated by use of the terms primary, secondary and tertiary treatment.  Tertiary treatment, properly, would be any treatment added onto or following secondary treatment. 

 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.