water purification - large scale

WATER PURIFICATION ON A LARGE SCALE

Dr. S. A. Rizwan, M.D., Assistant ProfessorDepartment of Community MedicineVMCHRI, Madurai

LEARNING OBJECTIVES• Define the aim of water purification methods in public

health• List out the various methods for water purification in large

scale• Describe the steps of each method• Describe the advantages and disadvantages of each

method

WATER POLLUTION• Natural • Man made: Urbanization and industrialization• Sources of pollution• Sewage • Industrial and trade waste• Agricultural pollutants• Physical pollutants, viz heat, radioactive substances

• Indicators of pollution• Total suspended solids, biochemical oxygen demand (BOD) at 20

deg. C, concentration of chlorides, nitrogen and phosphorus

WATER RELATED DISEASES• Biological (Water-borne diseases)

• Caused by infective agent • Viral, Bacterial, Protozoal, Helminthic, Leptospiral

• Caused by aquatic host• Snail, Cyclops

• Chemical• Dental• Cyanosis in infants• Cardiovascular • Inadequate use of water• Insect breeding

WATER POLLUTION LAW• Water (Prevention and Control of Pollution) Act , 1974• Central and State Water Boards and Joint Water Boards

endowed with wide powers for controlling pollution

WATER PURIFICATION• Large scale

1. Storage2. Filtration

• Slow sand filter• Rapid sand filter

3. Disinfection• Chlorination• Ozonation• Other agents• Membrane processes

WATER PURIFICATION• Small scale

1. Household purification• Boiling• Chemical disinfection: Bleaching powder, Chlorine solution , High test

hypochlorite (HTH), Chlorine tablets, Iodine, Potassium permanganate• Household filtration

2. Disinfection of well• By adding bleaching powder• Double pot method

1. STORAGE

1. STORAGE • In natural or artificial reservoirs

• Effects of storage:• Physical: gravity – 90% suspended impurities settle down in one

day• Chemical: oxidizing action• Biological: only 10% bacteria remains at the end of 1 week

• Optimum period of storage: 2 weeks

2. FILTRATION

2. FILTRATION• Water passed through porous media

1. Slow sand filter: biological2. Rapid sand filter: mechanical

2A. SLOW SAND FILTER

SLOW SAND (BIOLOGICAL) FILTERS

• Used first in 19th century in Scotland

• Elements of slow sand filter1. Filter Box

a) Supernatant waterb) Sand bedc) Under drainage system

2. Filter control valves

SLOW SAND (BIOLOGICAL) FILTERSGeneral plan and layout

SLOW SAND (BIOLOGICAL) FILTERSParts in general

SLOW SAND (BIOLOGICAL) FILTERSCross-sectional view of the filter bed

ELEMENTS OF SLOW SAND FILTER

• Supernatant water • Sand bed• Under drainage system• Filter control valves

ELEMENTS OF SLOW SAND FILTER

• Supernatant water

• Depth: 1 to 1.5 m• Promotes downward flow of water through the sand

bed• Waiting time of 3-12 hours for raw water to undergo

partial purification by sedimentation and oxidation

ELEMENTS OF SLOW SAND FILTER

• Sand bed

• Depth, 1 m (sand of diameter 0.2-0.3 mm), 0.3m (gravel with 0.2 - 1 cm diameter)• Sedimentation• The supernatant water acts as a settling reservoir. Settle-able

particles sink to the sand surface. • Mechanical straining• Particles too big to pass through the gap between the sand

grains are retained

ELEMENTS OF SLOW SAND FILTER

• Vital/ Biological/ Zoogleal/ Schumtzdecke layer

• Slimy, gelatinous layer over sand bed containing threadlike algae, bacteria and diatoms• ‘Heart’ of the slow sand filter• Ripening of filter: Formation of vital layer• Suspended particles are retained by adhesion to the biological

layer• Removes organic matter, holds back bacteria and oxidizes

ammoniacal nitrogen in to nitrates

SCHMUTZDECKE

ELEMENTS OF SLOW SAND FILTER

•Under drainage system

• Depth: 0.15 m• At the bottom of filter bed • Porous pipes: Outlet for filtered water as well as support to the

filter media above• Rate of filtration 0.1-0.4 m3/hr/m3

ELEMENTS OF SLOW SAND FILTER

• Filter control valves• To regulate the flow of water in and out

• Filter cleaning• Increased bed resistance -> Necessary to open the regulating

valves fully -> Scrapping top portion of sand bed up to 2 cm depth -> Time for cleaning the filter

• After 3-4 years new filter bed is constructed

ADVANTAGES OF SLOW SAND FILTER

1. Simple to construct and operate 2. Construction is cheaper than rapid sand filters 3. Physical, chemical and bacteriological quality of

filtered water is very high (99.9 to 99.99 per cent and E. Coli by 99 to 99.9 per cent)

2B. RAPID SAND FILTER

RAPID SAND FILTER• First in1885 in USA• Gravity type (Open)/ Paterson’s• Pressure type (Closed)/ Candy’s

RAPID SAND FILTER

OVERVIEW OF RAPID SAND FILTER

STEPS OF RAPID SAND FILTER1. Coagulation• Addition of Alum (5-40 mg/litre)

2. Rapid mixing• Mixing chamber• Violent mixing of alum (minutes)

STEPS OF RAPID SAND FILTER3. Flocculation

• Flocculation chamber• Slow stirring of water by paddles (30 minutes)• Flocculent ppt. of Aluminium Hydroxide entangles all particulate,

suspended matter along with bacteria

4. Sedimentation• Sedimentation chamber• Flocculent ppt. settle down (removal is done from time to time)• Clear water above goes for filtration

STEPS OF RAPID SAND FILTER5. Filtration• Filter bed• “Effective size” of the sand particles is 0.4-0.7 mm• Graded gravel, 30 to 40 cm • Depth of the water on the top of the sand bed is 1.0 to 1.5 m • Rate of filtration is 5-15 m3/m2/hr• Remaining alum floc forms a slimy layer over sand bed, it holds back

bacteria, oxidize organic matter• Back washing: by air bubbles or water when floc layer becomes very thick,

takes about 15 min

RAPID SAND FILTERCross-sectional view

ADVANTAGES OF RAPID SAND FILTER

1. Rapid sand filter can deal with raw water directly. No preliminary storage is needed

2. The filter beds occupy less space 3. Filtration is rapid, 40-50 times that of a slow sand filter 4. The washing of the filter is easy 5. There is more flexibility in operation

DIFFERENCES BETWEEN SLOW AND RAPID FILTERS

Properties Rapid sand filter Slow sand filter

Area Little space Large areaRate of filtration(m.g.a.d)

200 2-3

Sand size (diameter)

0.4-0.7 mm 0.2-0.3 mm

Pretreatment Coagulation & sedimentation

Sedimentation

Filter cleaning Backwashing ScrapingOperation More skilled Less skilledRemoval of colour Good BetterRemoval of bacteria

98-99% 99.9%-99.99%

3. DISINFECTION

3. DISINFECTION• Criteria for satisfactory disinfectant• Not influenced from properties of water within short

time• Should not be toxic and colour imparting or leave the

water impotable• Available, cheap, easy to use• Residual concentration to deal with recontamination• Detectable by rapid, simple techniques in small

concentration

ACTION OF CHLORINATION• Kills pathogenic bacteria (no effect on spores and viruses)•Oxidize iron, manganese and hydrogen sulphide• Reduces taste and odours• Controls algae•Maintains residual disinfection

MOA OF CHLORINATION•H2O+Cl2 (at pH 7) HCl + HOCl (main disinfectant)

•HOCl (at pH > 8.5) H+ + OCl- (minor action)

•NH3 + Cl2 NH2Cl/ NHCl2/ NCl3+ H2O (Mono, Di, Tri Chloramines)

PRINCIPLES OF CHLORINATION1. Water should be clear, free from turbidity 2. Chlorine demand: Chlorine needed to destroy bacteria, to oxidize

organic matter and to neutralize the ammonia in water3. Free residual chlorine for a contact period of 1 hour is essential4. Breakpoint: Point when chlorine demand of water is met and free

residual chlorine appears5. Breakpoint chlorination: Chlorination beyond the breakpoint . The

principle of break point chlorination is to add sufficient chlorine so that 0.5 mg/L free residual chlorine is present in the water after one hour of contact time

6. Dose of Chlorine = Chlorine demand + Free residual chlorine7. Minimum recommended concentration of free chlorine is 0.5 mg/L for

1hr

METHODS OF CHLORINATION• Chlorine gas (Paterson's chloronome)• Chloramine• Perchloron or high test hypochlorite (HTH)

SUPER CHLORINATION•Method of choice for highly polluted waters•High dose of chlorine is added• After 20 minutes of contact, dechlorination is done with sodium sulphate/ sodium thiosulphate to reduce the taste of excess chlorine

TESTS TO MEASURE RESIDUAL CHLORINE

Orthotolidine Test• Yellow colour• In 10 seconds - free chlorine• In 15 min - both free and combined chlorine

Orthotolidine Arsenite (OTA) Test • Yellow colour • Tests both free and combined chlorine separately• Yellow colour due to nitrites, iron, manganese are

overcome

OTHER DISINFECTION METHODS•Ozone•Used in Europe and Canada• Strong oxidizing agent• Strong virucidal•No residual effect• Should be used with chlorination

OTHER DISINFECTION METHODS•UV Rays• Used in UK• Water should be clear• No residual effect• Expensive

• Chloramine• Chlorine + Ammonia - Chloramine• Less effective than chlorine

MEMBRANE PROCESSES

• High-pressure processes• Lower-pressure processes

MEMBRANE PROCESSES•High-pressure processes• Reverse osmosis

• Rejects monovalent ions and organics of molecular weight >50 daltons• Pore sizes <0.002 μm• Desalination of brackish water and seawater

• Nanofiltration• Allow monovalent ions such as sodium or potassium to pass but reject a

high proportion of divalent ions such as calcium and magnesium• Pore sizes are typically 0.001-0.01 μm• Effective for the removal of colour-forming organic compounds

MEMBRANE PROCESSES• Low-pressure processes•Ultrafiltration• Reject organic molecules of molecular weight above about

800 daltons • Pore sizes 0.002 - 0.03 μm

•Microfiltration• Pore sizes 0.01-12 μm• capable of sieving out particles greater than 0.05 μm• used for water treatment in combination with coagulation

REVIEW

REVIEW 1

• All are true for Rapid Sand Filter except (All India)

a) No preliminary storage of water is requiredb) Operation requires skilled workersc) Frequent washing is not requiredd) Can be gravity or pressure type

REVIEW 2

• All are true for Rapid Sand Filter except (All India)

a) No preliminary storage of water is requiredb) Operation requires skilled workersc) Frequent washing is not requiredd) Can be gravity or pressure type

REVIEW 3

• Disinfection action of chlorine in water is due to (All India)

a) Hydrogen chlorideb) Hypochlorous acidc) Hypochlorite ionsd) Hydrogen ions

REVIEW 4

• Which of the following have residual germicidal effect in water disinfection? (AIIMS)

a) Chlorine onlyb) Chlorine and ozone gasc) Chlorine and UV radiationd) Chlorine, ozone gas and UV radiation

REVIEW 5

• What is used to find the dose of bleaching powder required for disinfection of water? (AIIMS)

a) Chloroscopeb) Chloronomec) Horrock’s apparatusd) Winchester Quart Bottle

REVIEW 6

• Minimum recommended of dose of free residual chlorine for routine chlorination? (AIIMS)

a) 0.5 ppm for 1 hrb) 0.5 ppm for 30 minc) 1.0 ppm for 1 hrd) 1.0 ppm for 30 min

REVIEW 7

• True statement regarding chlorination is (DPG)

a) Orthotoulidine test measures combined chlorine separatelyb) Chlorine acts best when pH is 7c) It kills bacteria, viruses and sporesd) Hypochlorite ions are mainly responsible for disinfection

REVIEW 8

• Slow sand filter is differentiated from rapid sand filter by (PGI)

a) Bacteria removed more effectivelyb) Skilled person is neededc) Cost of construction is cheaperd) Sand particle are smaller sizee) Longer duration is needed

REVIEW 9

• Orthotoulidine test is done for

a) Free chlorineb) Combined chlorinec) Fluorined) Iodine

REVIEW 10

• Schmutzdecke refers to

a) Suspended matter in drinking waterb) Algae in drinking waterc) Alum flocculate in surface of sand filterd) Algae, plankton, diatoms, bacteria on surface of sand filter

THANK YOUEmail your queries to sarizwan1986@outlook.com