Waste Water Treatment
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As urbanization intensified and cities grew, domestic sewage and industrial waste were eventually discharged into drainage ditches and sewers, and the entire contents emptied into the nearest watercourse. In major cities, this discharge is enough to destroy even a large body of water.
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Sanitary engineering technology for treating wastewater to reduce its impact on watercourses, pioneered in the United States and England, eventually became economically, socially, and politically feasible. Some of the main pollutants are listed below:
Organic Matter: Many organic matters are synthetic and not biodegradable. They include pesticides, detergents and petroleum hydrocarbons. Certain organic matters which are biodegradable are a source of food for aquatic micro-organisms that use dissolved oxygen to convert the organic matter into energy for growth. Microorganisms combine this matter with oxygen from the water to yield the energy they need to thrive and multiply; unfortunately, this oxygen is also needed by fish and other organisms in the river. Heavy organic pollution can lead to "dead zones" where no fish can be found; sudden releases of heavy organic loads can lead to dramatic "fishkills".
Suspended solids: Solids in water may come from many activities like dredging, construction and soil erosion. The increased turbidity in water actually reduces the light penetration and affects the photosynthesis process of the eco system. These solid particles can clog rivers or channels as they settle under gravity.
Pathogenic bacteria and other disease causing organisms - These are most relevant where the receiving water is used for drinking, or where people would otherwise be in close contact with it; and
Nutrients Nutrients like nitrates and phosphates in water encourage plant growth, especially algae and may come from domestic, industrial and or agricultural waste. These nutrients can lead to high concentrations of unwanted algae, which can themselves become heavy loads of biodegradable organic load Treatment processes may also neutralize or removing industrial wastes and toxic chemicals. This type of treatment should ideally take place at the industrial plant itself, before discharge of their effluent in municipal sewers or water courses.
Wastewater Treatment
Wastewater treatment plants receive inflows from domestic, commercial, industrial, agricultural users and also from storm water runoff and infiltration. Industrial waste mainly comprising of heavy metals, radioactive materials can be treated with proper mechanisms in place. The main purpose of waste water treatment is to control pollution of a receiving water body like river, lake, sea or designated pond. The most common waste water constituents include organic matters measured by BOD or COD, suspended solids, nutrients, bacteria, and toxic chemicals.
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The method and degree of water treatment are important considerations for environmental engineers. Generally speaking, the characteristics of raw water determine the treatment method. Most public water systems are relied on for drinking water as well as for industrial consumption and fire fighting, so that human consumption, the highest use of the water, defines the degree of treatment. Thus, we focus on treatment techniques that produce potable water.
Effective waste water treatment includes:
- Identifying the characteristics of waste water inflow
- Set treatment objectives
- Integrate treatment operations
- Assess the system in view of green engineering, life cycle thinking and sustainability.
Wastewater treatment is closely related to the standards and/or expectations set for the effluent quality. Wastewater treatment processes are designed to achieve improvements in the quality of the wastewater. Widely used terminology refers to three levels of wastewater treatment:
- Pre Treatment
- Primary Treatment
- Secondary Treatment
- Tertiary (or advanced) Treatment.
Pre Treatment
Coagulation and Flocculation
Naturally occurring silt particles suspended in water are difficult to remove because they are very small, often colloidal in size, and possess negative charges, and are thus prevented from coming together to form large particles that could more readily be settled out. The removal of these particles by settling requires first that their charges be neutralized and second that the particles be encouraged to collide with each other. The charge neutralization is called coagulation, and the building of larger flocs from smaller particles is called flocculation.
Settling
When the flocs have been formed they must be separated from the water. This is invariably done in gravity settling tanks that allow the heavier-than-water particles to settle to the bottom. Settling tanks are designed to approximate uniform flow and to minimize turbulence. Hence, the two critical elements of a settling tank are the entrance and exit configurations.
Filtration
The movement of water into the ground and through soil particles and the cleansing action the particles have on contaminants in the water. Soil particles help filter the ground water, and through the years environmental engineers have learned to apply this natural process in water treatment and supply systems, and have developed what we now know as the rapid sand filter. The actual process of separating impurities from carrying liquid by rapid sand filtration involves two processes: filtration and backwashing.
Disinfection
After filtration, the finished water is often disinfected with chlorine. Disinfection kills the remaining microorganisms in the water, some of which may be pathogenic. Chlorine from bottles or drums is fed in correct proportions to the water to obtain a desired level of chlorine in the finished water. When chlorine comes in contact with organic matter, including microorganisms, it oxidizes this material and is in turn itself reduced.
Primary (Mechanical) Treatment
Primary Treatment is designed to remove gross, suspended and floating solids from raw sewage. It includes screening to trap solid objects and sedimentation by gravity to remove suspended solids. This level is sometimes referred to as "mechanical treatment", although chemicals are often used to accelerate the sedimentation process. Primary treatment can reduce the BOD (Biochemical Oxygen Demand) of the incoming wastewater by 20-30% and the total suspended solids by some 50-60%. Primary treatment is usually the first stage of wastewater treatment. Many advanced wastewater treatment plants in industrialized countries have started with primary treatment, and have then added other treatment stages as wastewater load has grown, as the need for treatment has increased, and as resources have become available.
Secondary (Biological) Treatment
Secondary treatment removes the dissolved organic matter that escapes primary treatment. This is achieved by microbes consuming the organic matter as food, and converting it to carbon dioxide, water, and energy for their own growth and reproduction. The biological process is then followed by additional settling tanks ("secondary sedimentation", see photo) to remove more of the suspended solids. About 85% of the suspended solids and BOD can be removed by a well running plant with secondary treatment. Secondary treatment technologies include the basic activated sludge process, the variants of pond and constructed wetland systems, trickling filters and other forms of treatment which use biological activity to break down organic matter.
Tertiary Treatment
Tertiary treatment is simply additional treatment beyond secondary! Tertiary treatment can remove more than 99 percent of all the impurities from sewage, producing an effluent of almost drinking-water quality. The related technology can be very expensive, requiring a high level of technical know-how and well trained treatment plant operators, a steady energy supply, and chemicals and specific equipment which may not be readily available. An example of a typical tertiary treatment process is the modification of a conventional secondary treatment plant to remove additional phosphorus and nitrogen.
Disinfection, typically with chlorine, can be the final step before discharge of the effluent. However, some environmental authorities are concerned that chlorine residuals in the effluent can be a problem in their own right, and have moved away from this process. Disinfection is frequently built into treatment plant design, but not effectively practiced, because of the high cost of chlorine, or the reduced effectiveness of ultraviolet radiation where the water is not sufficiently clear or free of particles.
Conclusion
Water treatment is often necessary if surface water supplies, and sometimes groundwater supplies, are to be available for human use. Because the vast majority of cities use one water distribution system for households, industries, and fire control, large quantities of water often must be made available to satisfy the highest use, which is usually drinking water. However, does it make sense to produce drinkable water and then use it to irrigate lawns? Growing demand for water has prompted serious consideration of dual water supplies: one high-quality supply for drinking and other personal use, and one of lower quality, perhaps reclaimed from wastewater, for urban irrigation, fire fighting, and similar applications.