Water Quality Control Chapter 3 (IV) PDF

Water Quality Control Chapter 3 (IV) Filtration 1 Introduction All of the particles may not be removed by prior unit operation and processes (sedimentation, c...

Water Quality Control Chapter 3 (IV) Filtration 1 Introduction All of the particles may not be removed by prior unit operation and processes (sedimentation, coagulation and flocculation) Remaining fine floc particles, suspended & colloidal particles, color, dissolved minerals and microorganisms & bacteria needs to be removed prior supplying to public. Filtration is the process of passing water through thick layers of porous media which is most of the cases is a layer of sand supported on a bed of gravel. 2 Introduction It is a solid-liquid separation process in which the liquid passes through a porous medium to remove as much fine suspended solids as possible. Applications: In water treatment plants, a polishing step to remove small flocs and other particles that are not removed in settling 3 Introduction  FILTRATION Actual Filtration Back Washing process by which cleaning of filter medium the water is cleaned During filtration; Water containing suspended matter is applied to the top of the filter bed As the water filters through the porous medium, the suspended matter in the fluid is removed by a variety of mechanisms. 4 Theory of Filtration Since the filter medium is permeable only to the fluid, it retains the solid particles and permits only the fluid to pass through which is collected as the filtrate. The volume of filtrate collected per unit time (dV/dt) is termed as the rate of filtration. As the filtration proceeds, solid particle accumulate on the filter medium forming a packed bed of solids, called filter cake.  As the thickness of the cake increases resistance to flow of filtrate increases rate of filtration gradually decreases. If rate is maintained to be constant then pressure difference driving force (-ΔP) will increase. Therefore, a batch filter is operated either at constant pressure or at constant rate. 5 Theory of Filtration Mechanical Straining Sedimentation and adsorption Biological Metabolism Electrolytic changes  Refer to the following sources to learn more about these mechanisms: Cleasby & Logsdon (1999, pp. 8.32–8.39), Viessman et al. (2009, pp. 343–345), and Metcalf & Eddy (2003, pp. 1047–1049). 6 Theory of Filtration Mechanical Straining Simplest action during filtration. Suspended particles having size more than that of filter voids are arrested and removed, when water passes through filter media. Takes place in few centimeters of depth of filter media. Sedimentation The interstices between the sand grains act as miniature of sedimentation tank.  Due to physical forces of attraction between suspended particles and sand grain Due to the presence of gelatinous coating formed on the sand grains by previously deposited colloidal matter and bacteria Continuous voids of filter media acts as ‘tube settler’ i.e. shallow depth sedimentation tank. Colloids, fine suspended particles and bacteria are removed. 7 Theory of Filtration Biological metabolism Organic matters such as algae, plankton also caught by voids between sand grains and these matters are used by bacteria for survival and convert them into harmless compounds from biological metabolism. These harmless compounds formed are deposited at the surface of sand in a form of a layer which contains a zoological jelly in which biological activities are at highest. This film is known as ‘dirty skin’ or ‘Schmutzdecke’. This layer further enhances in adsorbing and straining activities. Bacteria not only break organic impurities into harmless compounds but they destroy each other and make a balanced life in the filter.  8 Theory of Filtration Figure: Scanning electron microscope (SEM) images of schmutzdecke flocs separated from the sand particles of a slow sand filter (SSF) of the Zurich waterworks (Zurich, CH). The schmutzdecke comprises both organic and inorganic materials and a broad diversity of indigenous bacteria (Hammes et al., 2011) https://doi.org/10.1016/B978-0-08-088504-9.00386-X 9 Theory of Filtration  Electrolytic Action: As per ionic theory, when two substances of opposite charges come into contact, the charge is neutralized and in doing so, new chemical substances are formed. Sand particles in filter media also have charges of some polarity which attracts the suspended, colloidal and dissolved matters of opposing polarity in neutralizing and changes the chemical characteristics of water. After a long use, charges in the sand grains get exhausted and it becomes necessary to clean filter for regeneration of charges. 10 Types of Filters Based on the filtration rate and driving force to  overcome frictional resistance encountered by the water flowing through the filter, the filters are classified as: a) Slow Sand Filter (SSF) Gravity Type b) Rapid Sand Filter (RSF) Gravity Type Pressure Type or Pressure Filter (PF) 11 Types of Filters Filter differs with respect to: i. Head required for filtration ii. Rate of filtration iii. Composition of filter media iv. Method and frequency of cleaning  Commonly used filter materials are a. Sand b. Anthracite c. Garnet sand or limenite d. Other locally available material Coconut husk Gravel12 Sand Anthracite Garnet sand Crushed glass Rice Husk Types of Filters 13 SSF SSF consists of fine sand, supported by gravel. This earliest type of gravity filter has a slow rate of filtration (≤ 1/30th of that of RSF or PF). Requires large area of land and quantity of sand, as rate of filtration is low Surface scrapping is done for bed cleaning; requires manpower. Suitable when land, labor, and filter sand are readily available at low cost. 14 SSF Pre-treatment like sedimentation is not required if turbidity of raw water is