Purification of Water

Facilitator: Dr. NAVPREET Assistant Professor, Department of Community Medicine Govt. Medical College & Hospital, Chandigarh. Specific Learning Objectives

• At the end of session, the learner shall be able to: 1. Know the aim of water purification. 2. Describe the purification of water on a large scale. 3. Differentiate between slow sand filter & rapid sand filter 4. Describe various methods of disinfection. 5. Describe the purification of water on a small scale. 6. Enumerate water quality – criteria & standards.

• The aim of water purification is to produce and maintain water that is: hygienically safe, aesthetically attractive and palatable, in an economical manner.

• On a large scale • On a small scale Purification of water on a large scale

• The components of a typical water purification system comprise one or all of the following measures:  Pre‐treatment,  Filtration, and  Disinfection Pre‐treatment

• The sub‐steps included in pre‐treatment are: Storage, Coagulation, Rapid Mixing, Flocculation, and Sedimentation. Storage

• Natural purification: Physical Chemical Biological Filtration

• Slow sand or Biological filter • Rapid sand or Mechanical filter SLOW SAND FILTER

• First used for treatment of water in 1804 in Scotland and subsequently in London. • The elements of slow sand filter are: Supernatant (raw) water A bed of graded sand An under‐drainage system A system of filter control valves.

Supernatant (raw) water

• 1 ‐ 1.5 metre

• Purposes: To overcome the resistance of filter bed To provide waiting period (3 to 12 hours)

• Level of supernatant water is always kept constant. Sand bed

• Most important part of the filter. • 1 ‐ 1.2 meter.

• Effective diameter of sand grains: 0.2‐0.3 mm. • It is supported by layer of graded gravels (30‐ 40 cm) • Rate of filtration: 2‐3 m.g.a.d. (million gallons per acre per day) • Vital layer Vital layer

• Schmutzdecke layer, Zoogleal layer or Biological layer. • ‘Ripening’ of the filter. • ‘Heart’ of slow sand filter.

Under‐drainage system

• Porous or perforated pipes • Purpose: Provide an outlet for filtered water Support the filter medium above Filter box

• From top to bottom:

Supernatant water 1‐1.5 meter

Sand bed 1.2 meter

Gravel support 0.3‐0.4 meter

Filter bottom 0.16 meter Filter control

• To maintain a constant rate of filtration. • ‘Venturi meter’ • ‘Loss of head’ : if > 1.3 meter, it is uneconomical. • Filter cleaning: • Scrapping off the top portion of sand layer • Manually or by mechanical equipment. Advantages of slow sand filter

• Simple to construct and operate. • Cost of construction is cheaper. • Quality of filtered water is high.

RAPID SAND FILTER

• The first rapid sand filter was installed in the USA in 1885. • Two types: Gravity type (Paterson’s filter) Pressure type (Candy’s filter) Steps in purification of water by Rapid Sand Filter • Coagulation • Rapid mixing • Flocculation • Sedimentation • Filtration Alum

Mixing Flocculation Sedimentation Filter chamber chamber tank Filter beds

• Sand is the filtering medium • Effective size of sand particle: 0.4‐0.7 mm. • Depth of sand bed: 1 meter. • Rate of filtration: 200 m.g.a.d. • Gravel supports the sand. • Under drains at the bottom of the filter beds. Filtration

• Formation of slimy layer by ‘alum‐floc’ which are still not removed • “Backwashing” Advantages

• It can deal with raw water directly. • It occupies less space. • Filtration is rapid (40‐50 times that of a slow sand filter) • More flexibility in its operation. BUZZ!

• Slow sand filter vs. Rapid sand filter DISINFECTION

• Chlorination • Ozonation • Membrane processes Chlorination

• Principles • Breakpoint chlorination • Orthotolidine Test Free available

> residual formed Chlororganics & Chloramines partly destroyed ‐‐‐‐‐‐‐‐‐‐ Formation of chloroganics & chloramines residual

Chlorine destroyed by reducing compound Chlorine Free available residual Combined residual Chlorine added ‐‐‐‐‐‐‐‐‐‐ >

Purification of water on a small scale

• Boiling • Chemical disinfection: Bleaching powder Chlorine solution High test hypochlorite Chlorine tablets Iodine Potassium permanganate • Filtration • Ultraviolet irradiation • Multi‐stage reverse osmosis Water Quality‐ Criteria & Standards

• Acceptability aspects Physical parameters Inorganic constituents • Microbiological aspects • Chemical aspects Inorganic constituents Organic constituents • Radiological aspects Microbiological aspects

• Bacteriological indicators Coliform organisms Faecal streptococci Clostridium perfringens • Virological aspects • Biological aspects Protozoa Helminths Surveillance of Drinking Water Quality • Sanitary survey • Sampling • Bacteriological surveillance • Biological examination • Chemical surveillance Bacteriological surveillance

• Presumptive coliform test Multiple tube method Membrane filtration technique • Detection of Faecal Streptococci & Cl. Perfringens • Colony count Take home message

• Purification of water is essential. • Rapid sand filters are preferred but, slow sand filters are also in use. • Chlorination of water is effective and cheap method for disinfection. • Everyone can play his/her role in purification of water e.g. at your home.