Processes of filtration with coagulation and flocculation or without are employed to separate suspended solids, odor, organics, free chlorine and heavy metal from water. Solids / Liquids separation in water treatment is done by sedimentation, straining, floatation and filtration. Selection of the specific process or combined processes for removal of suspended solids from water depends on the character of the solids, their concentration and the required filtrate clarity and quality. For example, very large and heavy solids can be removed by a simple bar screen or strainer. Fine solids may require both sedimentation and filtration, sometimes aided by chemical treatment.
Suspended solids are important as pollutants and pathogens are carried on the surface of particles. The smaller the particle size, the greater the total surface area per unit mass of particle in grams, and so the higher the pollutant load that is likely to be carried. A less obvious but more damaging impact of TSS, however, is in increasing the turbidity (opacity, or cloudiness) of water. In natural environments high turbidity prevents light from reaching aquatic plants, thereby adversely affecting photosynthesis; this reduces the rate at which plants generate oxygen, cutting the amount of dissolved oxygen available to fish and other animals. Turbidity also increases water temperature, due to light absorption, and may affect the ability of animals to locate food or breed effectively.
In drinking water, the higher the turbidity level, the higher the risk that people may develop gastrointestinaldiseases. This is especially problematic for immune compromised people, because contaminants like viruses or bacteria can become attached to the suspended solids. The suspended solids interfere with water disinfection with chlorine because the particles act as shields for the virus and bacteria. Similarly, suspended solids can protect bacteria from ultraviolet (UV) sterilization of water.L
In water bodies such as lakes, rivers and reservoirs, high turbidity levels can reduce the amount of light reaching lower depths, which can inhibit growth of submerged aquaticplants and consequently affect species which are dependent on them, such as fish and shellfish. High turbidity levels can also affect the ability of fish gills to absorb dissolved oxygen.
Solids/Liquids separation devices are required for producing good water for human, agricultural and industrial use. Some of the manufacturing industries which use solids/liquids separation devices are Steel, Rice Mills, Food industry, Power plants, Pharmaceutical , chemical industries, engineering industries, vegetable oil refineries, cooling towers, fisheries, Textile mills.
Solids/Liquids separation devices commonly used are clarifiers, tube settlers, lamella clarifiers, High Rate Solid contact clarifies, Sand Filters, Pressure Filters, Multi Media Filters, Ultra Media Filters, Anthracite & Sand Filter, Dual Media Filters, Activated carbon Filters, Iron removal Filters, Arsenic removal Filters, Fluoride removal Filters, Micro filters, Cartridge Filters, Bag Filters, Ultrafiltration membranes.
Type Of Particle
Settling time through 1 m of water
Specific area m2.m-3
Settling time for various particles to settle vertically through 1 meter of water at 20oC, under the influence of gravity alone according to Stroke’s law.Courtsey Water Treatment Handbook Degremont ISBN 2.9503984.1.3
There are special filters designed for typical applications. One such typical application is Mill scale filtration in steel rolling mills.
Mill scale, often called scale, is basically Iron Oxide and is the flaky surface of hot rolled steel. It is formed on the outer surfaces of plates, sheets or other profiles during the production process.
In steelproduction, hot milling and casting large amounts of water are used depending on the scale ofthe operation. As steel leaves the casting operations, cooling and cutting with water producesmill scale (iron oxides).This hot water from the mills is transported to Scale pit for the first stage of clarification. Thispartly clarified water is pumped to settling tank (Second stage of clarification) for removal ofsuspended solids as well as oil and grease. In the final stage, special HIGH RATE FILTER for theremoval of suspended solids and oil contents are used.
The Filter is designed as a Deep Bed Filter with a nozzle less system. The under drain design is
the T-beam, constructed of compact corrosion-protected sectional steel beams arranged side by
side on a flat steel ring. Media selection is critical as the steel particles that need to be filtered out
are dense. At the same time they need to be removed during the backwashing process, or otherwise
after sometime the Filter bed gets clogged. This requires specially graded filter media and perfectly
designed internal arrangement. Special Silica sand media with a specific higher bed depth is used.
The process steps in removal of the particles use a combination of Air and Water in a specificsequence to enable removal of the filtered particles. The higher bed depth also enables are larger
hold up of the filtered particles in the bed thus enabling the filter to take up a higher suspended solids load. Proper Water and Air distribution during service and also especially during the backwashing operation is important. This is ensured by having a uniform gap between the bottom laterals. The Laterals are designed in such a manner so that there are uniform distribution points not only for the water but also an Air bed is generated below these laterals by means of a specially designed Air distribution Header and Lateral arrangement. The generation of this Air bed ensures uniform Air distribution which is essential for proper backwashing.