Design of conventional type floculator-based water treatment plant for Pingla & Panskura block in West Bengal

Abstract

The aim of water treatment is to produce and maintain water that is hygienically safe, aesthetically pleasing, and palatable in an economical manner. Though the treatment of water would achieve the desired quality, the evaluation of its quality should not be confined to the end of the treatment facilities but should be extended to the point of consumer use. Water to be supplied for public use must be potable i.e., satisfactory for drinking purposes from the standpoint of its chemical, physical and biological characteristics. Drinking water should, preferably, be obtained from a source free from pollution. The raw water normally available from surface water sources is, however, not directly suitable for drinking purposes. The objective of water treatment is to produce safe and potable drinking water. Some of the common treatment processes used in the past include Plain sedimentation, Slow Sand filtration, Rapid Sand filtration with Coagulation-flocculation units as essential pretreatment units. Pressure filters and Diatomaceous filters have been used though very rarely. Roughing filters are used, under certain circumstances, as pretreatment units for the conventional filters. The treatment processes may need pretreatment like pre-chlorination and aeration prior to conventional treatment. The pretreatment processes comprising of Coagulation and Flocculation have been discussed under the main title of Rapid Sand filters in para 5.4 of this chapter. Detailed discussion on all such aspects as well as recommended unit operations, is given in the Manual on Water Supply and Treatment (1999 Edition) Ministry of Urban Development. Water is a precious commodity. Most of the earth water is sea water. About 2.5% of the water is fresh water that does not contain significant levels of dissolved minerals or salt and two third of that is frozen in ice caps and glaciers. In total only 0.01% of the total water of the planet is accessible for consumption. Clean drinking water is a basic human need. Unfortunately, more than one in six people still lack reliable access to this precious resource in developing world. India accounts for 2.45% of land area and 4% of water resources of the world but represents 16% of the world population. With the present population growth-rate (1.9 per cent per year), the population is expected to cross the 1.5 billion marks by 2050. The Planning Commission, Government of India has estimated the water demand increase from 710 BCM (Billion Cubic Meters) in 2010 to almost 1180 BCM in 2050 with domestic and industrial water consumption expected to increase almost 2.5 times. The trend of urbanization in India is exerting stress on civic authorities to provide basic requirement such as safe drinking water, sanitation, and infrastructure. The rapid growth of population has exerted the portable water demand, which requires exploration of raw water sources, developing treatment and distribution systems. The raw water quality available in India varies significantly, resulting in modifications to the conventional water treatment scheme consisting of aeration, chemical coagulation, flocculation, sedimentation, filtration, and disinfection. The backwash water and sludge generation from water treatment plants are of environment concern in terms of disposal. Therefore, optimization of chemical dosing and filter runs carries importance to reduce the rejects from the water treatment plants. Also, there is a need to study the water treatment plants for their operational status and to explore the best feasible mechanism to ensure proper drinking water production with least possible rejects and its management. With this backdrop, the Central Pollution Control Board (CPCB), studied water treatment plants located across the country, for prevailing raw water quality, water treatment technologies, operational practices, chemical consumption and rejects management. The method of treatment to be employed depends on the nature of raw water constituents and the desired standards of water quality as per IS: 10500 (2012). The traditional surface water sources are streams, rivers, tributaries, manmade and natural lakes, and canals. Traditional ground water sources are dug well and bore wells. The seawater desalination and reuse of treated sewage (indirect potable use) are considered as prospective sources. The impurities in raw water can be classified as suspended solids, colloidal solids, and dissolved solids which may be harmful to humans. Impurities also can be classified as inorganic, organic, and microbial contaminants. For drinking water treatment to remove suspended or precipitated solids and contaminants, unit processes viz. coagulation, flocculation, clarification, filtration, and disinfection are a must. Recently, biological filtration (after conventional filtration) is also included to remove organics and Disinfection by Products (DBP). The degree of treatment shall be directly related to the quality of raw water including emergent contaminants and DBPs. As mentioned earlier, Coagulation, Flocculation, Clarification/ precipitation, Filtration and Disinfection are the main unit operations and processes classified as Conventional Treatment. Normally these processes and units are employed to treat surface waters and ground water influenced by surface water. The surface water sources in India have predominantly inorganic suspended solids. However, in the lean flow season due to pollution by sewage, industrial wastewater, farm discharges organic suspended and colloidal impurities also occur in the raw water. Coloured water (due to Natural decay of vegetation) also contributes to organic load. Microbial contaminants viz., algae is also present in many polluted water in lean flow season. Jar tests with different coagulants and coagulant aid give a representative analysis of settleable floc formation. Maximum organic loading in terms of BOD and COD should be restricted to 5 mg/L and 10 mg/L respectively.