Wastewater is produced by human activities and has a direct impact on the environment into which it is discharged. Production of more and more wastewater by anthropogenic activities is a genuine issue for public health and for the environment.
Wastewater management is a genuine sanitary and environmental challenge for all the players involved in environmental management all over the world. It is important to treat wastewater and encourage its reuse in order to protect public health and water resources.
Wastewater Treatment Processes
Wastewater treatment usually consists of two major steps – primary treatment and secondary treatment along with a process to dispose of solids (sludge) removed during the two steps.
In some areas where receiving waters are more sensitive to pollution or where specific pollutants have not been removed by secondary treatment, a third step called advanced waste treatment (also called tertiary treatment) may be required. Some plants use pre-chlorination for hydrogen sulfide and odor control prior to beginning any treatment processes.
Preliminary treatment is required to remove the coarse solids and other large materials from raw wastewater.
The operations include use of screens and grates for removal of large materials, comminutors for grinding of coarse solids, pre-aeration for odour control. Sometimes pH correction and removal of oil & grease is also done.
Primary Treatment Processes
Primary wastewater treatment, at times, is the first step in the wastewater treatment process or it may be the second step after the preliminary treatment. It involves physical separation of suspended solids (total suspended solids) from the wastewater using primary clarifiers.
The objective of primary treatment is to remove of settle-able organic and inorganic solids by sedimentation and removal of materials that float (scum) by skimming.
Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are removed during primary sedimentation but colloidal and dissolved constituents are not affected. The effluent from primary sedimentation units is referred to as primary effluent.
Sedimentation chambers are the main units involved in primary treatment but various auxiliary processes such as fine screening, flocculation and floatation may also be used.
The second step may be chemical treatment (generally with lime and alum) which is sometimes preceded by flocculation. The purpose is to remove metals by precipitation but it also removes some associated colloidal BOD. The process generates chemical sludge.
Primary treatment involves various physical-chemical processes:
Flocculation – a physico-chemical process for the aggregation of coagulated colloidal and finely divided suspended matter by physical mixing or chemical coagulant aids.
The process involves mixing of wastewater stream with coagulants in a rapid mix tank, which is then passed on to the flocculation basin.
Sedimentation – this process is aimed to remove easy to settle solids. Sedimentation chambers may also include baffles and oil skimmers to remove grease and floatable solids and may include mechanical scrapers for removal of sludge at the bottom of the chamber.
Dissolved Air Floatation- air-bubbles are introduced into the waste water, they attach themselves to the particles, thus causing them to float. This process of diffused air flotation can be used to remove suspended solids and dispersed oil and grease from oily wastewater.
Clarification – in a clarifier, wastewater is allowed to flow slowly and uniformly, permitting the solids to settle down. The clarified water flows from the top of the clarifier over the weir. Solids get collected at the bottom and sludge are periodically removed, dewatered and safely disposed.
Chemical Treatment Processes
Chemical treatment may be used at any stage in the treatment process as and when required (preferably before biological treatment as it removes toxic chemicals which may kill the microbes). Mainly used methods are:
Neutralization – incoming untreated wastewater has a wide range of pH. Neutralization is the process used for adjusting pH to optimize treatment efficiency.
Acids such as sulphuric or hydrochloric may be added to reduce pH or alkalis such as dehydrated lime or sodium hydroxide may be added to raise pH values.
Precipitation – precipitation is carried out in two steps: in the first step, precipitants are mixed with wastewater allowing the formation of insoluble metal precipitants; in the second step, precipitated metals are removed from wastewater through clarification and/or filtration and the resulting sludge are properly treated, recycled or disposed.
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Secondary Treatment Processes
This process involves decomposition of suspended and dissolved organic matter in waste water using microbes.
The mainly used biological treatment processes are activated sludge process or the biological filtration methods. Biological treatment can be aerobic, anaerobic or facultative.
Activated sludge process– a continuous flow, aerobic biological treatment process that involves suspended growth of aerobic micro- organisms to biodegrade organic contaminants.
Influent is introduced in the aeration basin and allowed to mix with the contents. A suspension of aerobic microbes is maintained in the aeration tank.
A series of biochemical reactions in the basin degrade the organics and generate new bio mass. Microorganisms oxidize the matter into carbon dioxide and water using the supplied oxygen.
These organisms agglomerate colloidal and particulate solids. The mixture is passed to a settling tank or a clarifier where micro-organisms are separated from the treated water.
The settled solids are recycled back to the aeration tank to maintain a desired concentration of micro- organisms in the reactor and some of the excess solids are sent to sludge handling facilities.
Biological filters – These filters are biological reactors filled with media which provide a surface that is repeatedly exposed to wastewater and air and on which a microbial layer can grow. The two most common types of biological filters are;
Trickling Filters: in trickling filters treatment is provide by a fixed film of microbes that forms on the surface which adsorbs organic particles and degrades them aerobically. Wastewater is distributed over a bed made of rock or plastic and flows over the media by gravity.
Rotating Biological Contractor (RBC): the setup consists of a series of discs; about 40% of the area is immersed in wastewater. The RBC provides a surface for microbial slime layer.
The alternating immersion and aeration of a given portion of the disc enhances growth of the attached micro-organisms and facilitates oxidation of organic matter in a relatively short time and provides a high degree of treatment.
Tertiary Treatment Processes
Tertiary treatment is the final cleaning process that improves wastewater quality before it is reused, recycled or discharged to the environment.
Tertiary treatment can involve physical-chemical separation techniques such as activated carbon adsorption, flocculation/precipitation, membranes filtration, ion exchange, de-chlorination and reverse osmosis.
Advanced treatment processes which generally constitute of or are part of the tertiary treatment may also sometimes be used in primary or secondary treatment or used in place of secondary treatment.
Some of the common tertiary treatment processes are described below:
Granular Media Filtration- Many processes fall under this category and the common element being the use of mineral particles as the filtration medium. It removes suspended solids mainly by physical filtration. Two common types of these granular media filers are:
Sand filters: the most common type which consists of either a fixed or moving bed of media that traps and removes suspended solids from water passing through media.
Dualor multimedia filtration: consists of two or more media and it operates with the finer, denser media at the bottom and coarser, less dense media at the top.
Common arrangement is granite base at the bottom, sand in the middle and anthracite coal at the top.
Flow pattern of multimedia filters is usually from top to bottom with gravity flow. These filters require periodic back washing to maintain their efficiency.
These processes are most commonly used for supplemental removal of residual suspended solids from the effluents of chemical treatment processes.
Membrane Filtration: In membrane filtration, a solvent is passed through a semipermeable membrane. The membrane’s permeability is determined by the size of the pores in the membrane. Microfiltration, ultrafiltration and Nano-filtration are examples of membrane filtration techniques.
Reverse Osmosis Systems – This is also a membrane separation method that is used to remove several types of large molecules and ions from solutions through application of pressure to the wastewater on one side of a selective membrane.
The result is that the contaminant is retained on the pressurized side of the membrane and the treated waste water is allowed to pass to the other side.
IonExchange– Ion Exchange can be used in wastewater treatment plants to swap one ion for another for the purpose of demineralization. There are basically two types of ion exchange systems, the anion exchange resins and the cation exchange resins.
It can be used for softening, purification, decontamination, recycling, removal of heavy metals from electroplating wastewaters and other industrial processes, polish wastewater before discharging, removal of ammonium ion from wastewaters, salt removal, purify acids and bases for reuse, removal of radioactive contaminants in the nuclear industry, etc.
Activated carbon, Powdered as well as granular activated carbons are used for the purpose of de-chlorination of organic compounds. Organic compounds in waste water are adsorbed on to the surface of the activated carbon. A number of factors affect the effectiveness of the activated carbon. These include pore size, composition and concentration of the contaminant, temperature and pH of the water and the flow rate or contact time of exposure.
Activated carbon can be applied on a broad spectrum of organic pollutants and is typically used to remove contaminants from water such as pesticides, aromatic compounds such as phenol, absorbable organic halogens, non-biodegradable organic compounds, colour compounds and dyes, chlorinated/halogenated organic compounds, toxic compounds, compounds that normally inhibits biological treatments, oil removal in process condensates, halogens, especially chlorine that oxidizes downstream processes and organics that have the potential to foul ion exchange resins or reverse osmosis membranes.
Ultraviolet (UV) Disinfection – This technique is primarily employed as a disinfection process that inactivates waterborne pathogens without use of chemicals. Additionally, UV is also effective for residual TOC removal, destruction of chloramines and ozone.
Treatment of Sludge
The solid material that is removed from wastewater, called sludge, requires proper treatment and disposal and can often be reused. The ultimate disposal of this material is one of the most difficult and expensive problems of wastewater treatment.
The goal of sludge treatment is to destroy harmful organisms and remove water. The end product of the sludge handling process is a relatively dry material known as ‘cake’. It can be applied to agricultural land as a soil conditioner, placed in landfills, or cleanly burned.
At some plants, sludge serves as a fuel to produce energy. For land application, sludge is often kept in a liquid slurry form for ease of handling and for subsurface injection into soils with special equipment.
In summary, wastewater is generated by both industrial and domestic processes. Waste water treatment technologies allow the reuse of wastewater thereby protecting the public health and water resources.