Scientists and engineers have developed methods to treat wastewater and make it reusable or safe for release to the environment. The processes the biological, physical and chemical pollutants from the water.
Wastewater can be first treated close in its primary storage area like
septic tanks and aerobic treatment systems. The water can be collected
there and then transported to various water treatment plants through water
There are basically six different methods through which wastewater is cleaned and turned into reusable water. These methods used various techniques to clean water and their results vary according to the quality of water.
- Preliminary Treatment
Preliminary treatment of water is done to protect the pumping equipment from getting jammed by all the slime and dirt in the water. This method removes and reduces the large floating particles, inorganic solids and pollutants like oil and grease from the water. This is the first step of wastewater treatment and facilitates further decontamination of the water.
In this stage, solid waste particles are removed from the water with the help of devices like Screens, grinders, cutters, grit chambers and pre-aeration tanks. All of these devices are very carefully designed and applied on wastewater from different resources.
- Primary Treatment
In the second method, all the heavy solids are from the wastewater through the process of sedimentation. Sedimentation is the process in which waste water goes through big tanks called primary sedimentation tanks. All the sludge in the water gets can settled on the bottom of this tank and all greasy substance like oil rises to the surface. The oil on surfaces is skimmed off and sludge is removed and sent to be processed separately, through special scrapers fitted inside the tanks. The purpose of this treatment is to decrease the speed of wastewater to allow the solids to settle down and light particles to surface. The water treated through this process is like the water gone through stream.
- Secondary Treatment
This kind of treatment depends on aerobic organisms which biochemically rot organic solids to inorganic particles. In this treatment, water is passed through four different types of devices. At first, it goes through trickling filters to secondary settling tanks. Then, the water is accumulated in final settling tanks to remove additional sludge. The treated water then goes through intermittent sand filters. The water is then, finally filled in a stabilization pond where it goes through the process of chlorination. Even though, this process is performed at every step of wastewater purification, but it becomes much more important in the later stages.
Chlorination helps in disinfecting the water, prevents wastewater decomposition and controls the odour. Even though chlorination is most commonly used methods of wastewater disinfection, there are some other methods used also which can be used for this purpose, for example, the ozone method. This method is much safer for fish and other living forms. It also doesn’t add any peculiar smell or taste to the water like chlorine does.
- Tertiary Wastewater Treatment
Tertiary treatment has been added to wastewater treatment process since 1970's. Tertiary treatment involves additional treatment rendered to wastewater after the secondary treatment. Many times, this only involves extra sand filters to remove increased suspended solids. Sometimes, tertiary treatment may also include removing plant nutrients, nitrogen and phosphorous from the water to improve its purity.
Tertiary treatment has come into practice because of improvement in wastewater treatment units and awareness about environmental effects of the purified wastewater. Tertiary wastewater treatment is also sometimes known as the advanced water treatment. This is the highest degree of water treatment and may include chemical and physical methods to produce high quality of water.
Even though sludge treatment is not directly connected to the process of procurement of fresh water to consumers through wastewater treatment, it is an important step. Sludge is solid waste removed from the wastewater in treatment units. It is essential to treat this sludge to some to prepare it for final disposal. The main objective of sludge treatment is to remove all the water from it and decompose the organic solids into mineral solids or at least in more stable organic ones. Sludge treatment process can be done by the following methods: This is accomplished by a combination of two or more of the following methods:
1. Conditioning with chemicals
3. Digestion with or without heat
5. Heat drying
8. Drying on sand bed -- open or covered
9. Vacuum filtration and
10. Wet oxidation
After purification, treated wastewater becomes almost as pure as potable
water and is ready to be reused for various purposes. Even though, most
countries don’t provide this water for drinking purposes, but it has become
a major source for irrigation and commercial purposes. Treated wastewater
has also helped cities in restoring the depleting underground water levels.
Human feces contain on the order of 10 billion bacteria per gram. This is part of Nature’s design, of course, as the microorganisms play a symbiotic part in the digestive system. Once outside the body, feces pose a disease risk to others, especially if they get in the water supply. Outbreaks of the disease cholera are attributed to human fecal waste in drinking water and routine disinfection of municipal water supplies is driven by the fear of disease.
Industrial disinfection and sterilization commonly is divided into food and non-food applications. Food applications require stricter microbial and bacterial control and according to EPA sterilization in those applications should achieve at least 99.999% efficiency in harmful microorganism removal. For comparison, non-food applications require 99.9% removal. ‘Food applications’ requirements are also applied in other segments of services and industry such as health care, pharmaceuticals, biomedical applications, artificial transplants and other similar ones.
Dissolved oxygen is an important part of water quality. Fish and other aquatic organisms do better when there is enough oxygen in the water. Runoff with a high concentration of organic matter can lower the amount of dissolved oxygen. The parameter biochemical oxygen demand (BOD) load is often measured. If it is too high, that is a problem.
Ion exchange systems can be evaluated using bench scale columns and experimental procedures common in water treatment evaluation. Column tests are similar to those conducted for adsorption evaluation. During development the engineer obtains enough data to develop a breakthrough curve, tracking volume treated to the effluent quality, and establishing a service time relation to the breakthrough point. Process information obtained from pilot testing including
- Regeneration conditions and expected resin recovery
- Column hydraulic loading and effects on service time
- Service time
- Alternative exchange media
Scaleup from column tests is common. Upset conditions must be evaluated to determine whether the data developed is effective through the wide range of influent conditions encountered.
Ion exchange systems are tailored to specific waste characteristics using available resins, in process schemes selected to optimize treatment effectiveness and regeneration costs. Special resins are constantly being developed for specific inorganic or organic component removal, their use dependent on the specific resin characteristics, and precise application can be obtained from the suppliers developing the technology. The resins discussed in this section, whose chemical characteristics are outlined in the appendix, are those commonly employed for water treatment. They are applied as sequential unit processes selected to optimize overall process efficiency and minimize regenerating costs, based on the following guidelines:
(1) Strong acid resins are highly functional over a wide pH range, with low leakage and high exchange rates; but are less economical and efficient to regenerate than weak acid resins.
(2) Weak acid resins are limited to cation removal over a pH range from 7 to 14 and exhibit high leakage. They can be economically and efficiently regenerated, using a weak or strong acid; including that regenerated from second stage strong acid resin.
(3) Strong base resins are highly functional over a wide pH range and high exchange rates, but are less economical and efficient to regenerate than weak base resins.
(4) Weak base resins are limited to anion removal below a pH of 6, but can be economically regenerated at a high efficiency utilizing a weak or strong base; including that regenerated from the second stage strong base resin.
Economical ion exchange application requires optimum use of bed capacity and effective regeneration.
Ozone is employed for treatment of drinking water as well as treatment of water. It was for decades used as an oxidizer for manufacturing and, since the 1970s, also for water purification. Today there are dozens of ozone treatment systems on the market. Ozone is one of the most powerful oxidants available. Occurring in nature at the higher layers of the atmosphere, ozone (O3) is an allotrope of oxygen. Because ozone is unstable and highly reactive, treatment systems produce ozone at the point of use.
Ozone action removes:
- dyes and pigments;
- algae and microorganisms;
- detergents and surfactants;
- nitrites and sulfites