ETP treats the waste materials into neutralized molecule or reduces the harmful ingredients. Industrial wastewater treatment covers the mechanisms and processes used to treat waters that have been contaminated in some way by anthropogenic industrial or commercial activities prior to its release into the environment or its re-use.
In textile mills the waste water effluent contains:
- Organic matter
- Inorganic matter
- Dissolved solids
- Suspended solids
- Chemicals & Auxiliaries
- Metal toxicants
They directly or indirectly effect the colour, alkalinity, pH, hardness, BOD, COD values of water.
What are the sources of waste water in Textile mills?
The main sources of waste water from textile process are:
Why treat waste water?
Waste water released from textile processing units is treated in effluent treatment plants due to several reasons as follows:
- Basic aim is Environmental protection
- Legal requirement
- Customers requirement
- Corporate social responsibility
- Societal requirement
- Maintenance of clean water
- Conservation and protection of water for industrial and agricultural uses
Categories of Effluent Treatment Plant Processes
There are three (3) Categories by nature of treatment being used in ETP as follows:
By degree of treatment:
There are 4 main types of treatment in ETP by degree of treatment as follows:
- Preliminary treatment
- Primary treatment
- Secondary treatment
- Tertiary/advanced treatment
Categories of treatment in Degree of treatment does have physical, chemical, biological processes in them.
- Preliminary treatment is mostly physical treatment
- Primary treatment may contain both physical and biological treatment
- Secondary treatment also known as biological treatment
- Tertiary treatment contains both chemical and biological processes
Above mentioned treatment processes further contain below mentioned processes in table
|Preliminary Treatment||Primary Treatment|
|Secondary Treatment||Tertiary Treatment|
|Activated Sludge Process|
Activated carbon filters
Preliminary treatment get rid of overweight solids and materials that can be easily collected from the effluent and can harm or choke the pumps and skimmers of primary treatment clarifiers. These are in-organic materials and insoluble organic pollutants (i.e. huge floating and suspended solid material, gravel, oil & grease) which are inert and cause problems to further chemical and biological treatments. The presence and sequence of preliminary treatment units totally depend on the characteristics of effluent that is to be treated. All preliminary treatment consists of physical separation techniques by controlling flow rate of effluent. The physical separation techniques are as follows:
The influent sewage water is screened to remove all large objects like fabric parts, plastic packets, or any other container etc. According to size of the solid particles to be removed there are further three main types of screens
- Bar Screens
- Course Screens
- Fine Screens
The purpose of a scraper is to remove solids, produce a cleaner effluent and concentrate solids. Concentration of solids removed from the wastewater reduces the volume of sludge for dewatering and/or disposal.
Grit chambers are long narrow tanks that are designed to slow down the flow so that solids particles will settle out of the water. When waste flows into the grit chamber, particles settle down to the bottom of the chamber based on their size, their specific gravity, and the speed of roll in the tank
A skimming tank is a chamber designed so that floating matter rises and remains on surface of the wastewater until removed, while the liquid flows continuously through outlet or partition below the water lines. Removes oils, greases, etc.
In this tank air is used to remove the solid particles and oil and greasy materials.
In Primary treatment, sedimentation process is used for most of the solids that can be settled are physically separated or removed from the wastewater. When some chemicals are used in primary sedimentation tanks, some of the mixture of solids(colloidal) are also eliminated form waste water.
The process of primary treatment is used to reduce the pace of the wastewater necessarily to permit solids to settle and floatable material to surface. That’s why, primary devices may contain settling tanks, clarifiers or sedimentation tanks. Because of variations in design, operation, and application, settling tanks can be divided into following groups:
In this tank as name presents, the solid particles are allowed to settle down based on gravity. This process does take some time. This process is also used to slow down the speed of the waste water flow.
A flocculent (chemical substance) is added in waste water to increase the settling speed. Small particle based on charge combine together and create large particles known as flocs and settle down based on gravity.
The function of equalization tanks is not only to act as holding tanks to help balance flow in wastewater treatment processes, but to also act as an excellent holding zones for other liquids, agents and chemicals added in the wastewater treatment systems. Aeration might be required to maintain the waste water in original phase during holding time.
The most critical stage in most of the industrial waste water treatment processes is the process of Neutralization. It is critical to neutralize the waste water before going to biological process otherwise it could kill the bacteria or plants
Secondary Treatment/Biological Treatment:
Secondary treatment process depends significantly on the aerobic organisms which biochemically degrade the organic materials to inorganic or stable organic solids. It is similar to the recovery zone in the self-purification of a stream.
Secondary treatment removes dissolved and suspended biological matter. This is typically performed by indigenous, water-borne micro-organisms in a managed habitat. This process may require a separation process to remove the micro-organisms from the treated water prior to discharge or tertiary treatment.
Activated Sludge Process:
It is a type of wastewater treatment process for treating wastewater using aeration and a biological floc composed of bacteria and protozoa. The method of Activated-sludge, is a process in which sludge, the stored, bacteria-rich collections of settling tanks and basins, is spread on the incoming waste water and the combination is stirred for numerous hours in the presence of appropriate air supply.
Organic matter from wastewater is removed by using trickling filters. The TF is an aerobic treatment system that utilizes microorganisms attached to a medium to remove organic matter from wastewater.
A bed of solid media on the surface of which bacteria are attached is trickling filer. Wastewater is irrigated on the solid media. It is also known as biological filter to make it clear that its not a mechanical process.
The aerated lagoon (or aerated pool) could be simply effluent treatment system consisting of a pond with artificial aeration to endorse the biological oxidation of wastewaters. The Aeration process increases the efficiency of treatment process, that can reduce energy costs in some cases. Lagoons with aeration system needs less land area and shorter detention times.
Large cement tanks having 3-5 m depth. Effluent coming from Primary Treatment is stored in these tanks for the duration of 2-6 days which is then aerated mechanically. After aeration for 2-6 days, a healthy sludge in flocculent form is made, which performs the oxidation of organic matter. It can removes upto 90% of BOD.
An oxidation pond is a large shallow pond. Stabilization of organic matter is brought out by bacteria. Oxygen is required for this purpose of metabolism and is supplied by algae. The algae utilizes the carbon dioxide released by bacteria for photosynthesis.
For effective treatment:
- Maximum sunlight penetration (for photosynthesis),
- Wind action for mixing
- Neutral Aeration
Tertiary treatment is the final treatment, meant for ‘polishing’ the effluent and removal of pollutants not removed in previous treatment processes. These pollutants may include soluble inorganic compounds such as phosphorous or nitrogen which may support algae growth in receiving waters. Tertiary treatment also removes organic materials that contribute to color, taste, odor, bacteria, viruses, BOD, COD, or other soluble minerals that can interfere with ultimate re-use of the wastewater. Preferred when treated water is need to be reuse or discharge is into the ecosystem.
It produces high quality effluent which can be reuse further for commercial and industrial applications. Treated water can be reuse for the irrigation of a golf course, green way or park, construction work, industrial process, etc. If the water is satisfactorily clean then it can also be used for recharge of groundwater. Sometimes, the treated water is disinfected by chemicals or physically depending on the location of discharge.
The Sand Filters are specifically designed to remove additional Biological Oxygen Demand (BOD) and Suspended Solids (SS) from sewage and Effluent treatment plants. This low cost and low maintenance sand filter tertiary treatment technique is an ideal addition for new and existing (permanent or temporary) process units. Sand filters have the capacity to produce high-quality water without even using chemicals additives. Passing flocculated water through a rapid gravity sand filter strains out the floc and the particles trapped within it, reducing numbers of bacteria and removing most of the solids. Sand of varying grades is used as the medium for the filters
Activated Carbon Filters
It is used in treatments plants to remove micropollutants or difficult contaminants from industrial effluents. Carbon filtering is a method of filtering that utilizes a bed of activated carbon to remove pollutants and impurities, using chemical absorption. Each particle/granule of carbon provides a large surface area/pore structure, allowing contaminants the maximum possible exposure to the active sites within the filter media.
The killing or removal of pathogenic microorganisms or their deactivation is known as water disinfection. Destruction or deactivation of microorganisms results in closure of their reproduction and growth. Physical or chemicals disinfectants are means through which disinfection can be achieved.
- Chlorine dioxide
- Several acids and bases etc.
Ultraviolet light (UV)
- Electronic radiation
- Gamma rays
The exchange of irons between an electrolyte solution, and a complex or between two electrodes is called ion exchange.
The term ion exchange is significantly used in case of purification process, decontamination, or separation of aqueous and other solutions that contain ions with the use of solid or mineral ion exchangers.
Through this process it is possible to regenerate resins. It has low initial cost. Ion exchange method is extensively used to remove hardness, iron and magnesium salts. But is does not removes bacteria effectively.
This process is most commonly used to recover H2SO4 , Cu, Pb, Hg, Cr, Ni, and also the removal of cyanides after recovery of Cr. from waste water Majorly used for recovery of Cr, Ni, Phosphate and H2SO4, Cu, Pb, Hg and removal of cyanides from wastewater after Cr recovery.