Water and Wastewater: Filters

Introduction

Windhoek, Namibia, a city in an arid region, suffered chronic freshwater scarcity. In the late 1960s, due to prolonged droughts, the only alternative for survival was to use treated wastewater for drinking purposes. In 1968, it became the first city in the world to directly supplement its drinking-water supplies with treated wastewater.1'1 Windhoek has approximately 2,00,000 residents, with yearly increase of the population of about 5%. Since 1973, an epidemiological study of Windhoek residents has shown no adverse health impacts associated with drinking reclaimed wastewater. It provides 30% of the demand.

In 2002, this treatment plant was rebuilt, and the wastewater effluent is now the primary source for drinking water. This plant treats 24,000 m3/day and has preozonation, flocculation, and filtration units comprising a rapid sand filter, a biological activated carbon filter, and two granular activated carbon (GACs) filters in a series, followed by ultrafiltration (UF).12i This is a classic example of significance of filtration systems in water and wastewater treatment.

Water treatment commonly refers to the actions taken to purify water for domestic and industrial purposes. Generally, processes used for water treatment are physicochemical. Wastewater or sewage treatment processes are mainly biological. Most of the treatment plants purifying surface waters might have two stage processes—filtration and disinfection. For the wastewaters, normally, two more processes are added—pre- and postclarification. Water quality as far as suspended solids are concerned is achieved by filtration. Most of the filters are composed of granular media such as sand, anthracite, soil, etc. These filters have the ability to produce high-quality water when perfectly operated. Clarification and filtration processes are complimentary to each other.

Traditionally, settling of solids was the main process of clarifying water. Filtration was also practiced in ancient times. The earliest filters seem to be very simple—infiltration wells excavated near the banks or lakeshores. Wells on the seashore usually were dug tapping sweet water aquifers flowing from the land to the sea. The technology is modernized with addition of sophisticated UF or reverse osmosis for better-quality water. Filters can be classified based on the following: 1) filtration rate—slow, rapid, and pressured; 2) direction of flow—downflow, upflow, and biflow; 3) filtration medium—sand, soil, and synthetic media; 4) layers of media—single, dual, and multimedia; 5) input water quality—water, sewage, industrial effluents, etc.

Water or wastewater treatment is the process of removing undesirable chemical or biological contaminants. These can be gaseous or solid constituents. Concentration of gaseous contaminants is dependent on factors like solubility, temperature, partial pressure, etc. They can be removed physically by diffusion or temperature increment Size of solid contaminants may range from millimeters to angstroms—less than a nanometer. These contaminants are different types of solids that can be categorized based on their sizes or physicochemical properties. Origin of these solids can be nonbiological or biological. They can be divided into two—depending on their chemical composition—organic and inorganic, as shown in Figure 1. Suspended or dissolved solids having different sizes are shown in Figure 2. Solids larger than suspended solids are termed as floating solids like plastic, paper, cloth, etc. They can be easily removed by putting a screen across the path of water flow, but smaller particles need finer sieves.

Treatment of Water and Wastewater

Treatment includes physical, chemical, and biological processes to remove physical, chemical, and biological contaminants. Its objective is to produce an environmentally safe fluid waste stream (or treated effluent) and solid waste (or treated sludge, http://en.wikipedia.org/wiki/Sludge) suitable for disposal or

Classification of solids in water

FIGURE 1 Classification of solids in water.

Different sizes of solid contaminants in water

FIGURE 2 Different sizes of solid contaminants in water.

reuse (usually as farm fertilizer). Using advanced technology, it is now possible to reuse sewage effluent for drinking water. Singapore has advanced to convert sewage into drinking water on mass scale.

In general, a conventional water treatment plant usually consists of physical treatment (screening, sedimentation, flotation, and/or filtration) and chemical treatment (pH adjustment, coagulation-flocculation process, oxidation- reduction process, adsorption process).131 The degree of the complexity of the treatment plant also depends on the quality of raw water and treated water requirement. In industrial processing, water is used in numerous applications requiring likewise different qualities of water. Examples of different uses are cooling water, water for rinsing and chemical production, boiler feed water, purified water, and water for injection. The growth in population, the increasing costs of treatment and distribution, contamination of fresh water sources, and the sophistication of end users somehow force the development of better water treatment technology.14,51

 
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