Treatment of water

This PPT is based on techniques of Treatment of water from hardness, arsenic , fluoride

1. Treatment of water:

2. REMOVAL OF HARDNESS:

3. What is hard and soft water: Water which readily gives a lather with  'soapy'  soap  (not detergents)  is described as SOFT water . Note:  Detergents usually give a good lather with any water . Some of these dissolved substances make the water HARD . ' Hard water ' means the water  does not readily give a good lather  with soap and so wastes soap as well as  causing a 'scum'!  though the 'hardness'  does not affect soap less detergents .

4. Hard water come in two varieties Temporary hardness: Temporary  hardness  is a type of  water hardness caused  by the presence of dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate). When dissolved, these minerals yield calcium and magnesium cations (Ca 2+ , Mg 2+ ) and carbonate and bicarbonate anions (CO 3 2− , HCO 3 − ). Temporary hard water can be softened by boiling the water. Permanent hardness: Permanently hard water cannot be softened by boiling. Permanent hardness is caused by very soluble  magnesium sulfate  (from salt deposits underground) and slightly soluble  calcium sulfate  (from gypsum deposits). In order to  soften  water you must remove the  calcium ions  ( Ca 2+ )  or  magnesium ions  ( Mg 2+ ) ions from it by one means or another

5. Temporary hardness Removal: Chemical Process of Boiling Hard Water We can boil water to remove temporary hardness. Temporary hardness in water can be easily removed by boiling. On boiling, calcium/magnesium bicarbonate decomposes to give NOTE : calcium/magnesium carbonate, which is insoluble in water. Therefore, it precipitates out.

6. Permanent hardness Removal: Adding Washing Soda: Calcium and magnesium ions present in hard water react with sodium carbonate to produce insoluble carbonates. The water now contains soluble and harmless sodium salts.    

7. Calgon Process: Calgon is a trade name of a complex salt, sodium hexametaphosphate (NaPO 3 ) 6 . It is used for softening hard water. Calgon ionizes to give a complex anion: The addition of Calgon to hard water causes the calcium and magnesium ions of hard water to displace sodium ions from the anion of Calgon.

8. Using Ion Exchange Resins: Giant organic molecules having acidic or basic groups are known as Ion-exchange resins. Acid resins contain the acid group ( - COOH ). Acid resins exchange their H +  ions with other cations such as Ca 2+ , Mg 2+ , etc., present in hard water. Acid resins are, therefore known as base-exchange resins.

9. Basic resins exchange their OH - ions with the other anions such as HCO 3 - , Cl - , SO 4 2- , present in hard water. Basic resins, therefore, are also known as acid exchange resins.

10. Fig: 11.5 - Ion-exchange process for water softening In the ion exchange process, hard water is passed through two tanks 'A' and 'B'. Tank- A contains acid resin and tank- B is filled with basic resin . All the cations present in hard water (except H + ) are removed by the acid resin present in Tank- A , and the basic resin present in Tank- B removes all the anions (except OH - ) present in hard water. Water obtained after passage through both the tanks is free from all the cations and anions that make it hard. The water obtained after passing through the ion-exchangers is called deionised water or demineralised water. This is as good as distilled water. The water becomes soft after this process.

11. REMOVAL OF ARSENIC:

12. Introduction: Arsenic (As) is known to be a very toxic element and a carcinogen to human. Even a trace amount of arsenic can be harmful to human health. The World Health Organizations (WHOs) current provisional guideline for arsenic in drinking water is 10 ppb. In India, states like Uttar Pradesh, Bihar, Jharkhand, West Bengal, Assam, Manipur, mainly in Ganga- Meghna -Brahmaputra (GMB) plain covering an area of about 569749 sq km with a population of over 500 million have reported serious illnesses due to presence of arsenic. The arsenic removal from drinking water by physicochemical process provides process for decontamination of water with respect to arsenic. BARC developed know how of ultra filtration (UF) based membrane technology for water decontamination with respect to microbiological contamination at both  domestic  and  community  scale is available for transfer separately. The present technology is a novel Ultra filtration (UF) membrane assisted physicochemical process for removal of arsenic from ground/surface water to make the water safe for drinking. 

13. Application: � Removal of arsenic from ground/surface water to provide safe drinking water free from primary contaminant like arsenic as well as secondary contaminants like iron and microorganisms. � Technology can be adopted at both domestic and community level 

14. Process: UF membrane assisted physicochemical process/device is capable of removing arsenic contamination from ground/surface water for drinking purposes from a feed concentration of 500 ppb or more to less than 10 ppb (which is the desirable limit set by BIS). The entire process involves two steps: 1) Absorption of arsenic species on the in situ generated absorbent by simple addition of two reagents. 2) Filtration of arsenic containing sludge using UF membrane device based on the technology developed by BARC. The two reagents required for the first step are to be prepared using the procedure given in the technology transfer document. The details of the device required for the second step is available in the form of technology with BARC and can be taken separately. These devices are also available with several licensees of BARC in the form of commercial products.

15. Second Method: Arsenic removal from water requires special adsorption media. Granular ferric oxide, titanium and hybrid media that contain iron-impregnated resin are all highly effective, but there are differences in media life. Before choosing a treatment technology, homeowners should ask water treatment providers to estimate the number of days that media can remove arsenic based on their water usage and water test results. The media are either contained in tanks for whole-house treatment or in cartridges for point-of-use (POU) treatment. Whole-house treatment is intended to treat all water for the house. The POU treatment system is installed at one location, such as a kitchen faucet, that provides water for drinking and cooking.

16. Installation: A typical whole-house adsorption system installed by a water treatment professional is shown in Figure 1. The system consists of a flow control module, an incoming water pressure gauge, an untreated water sampling port, a tank containing two cubic feet or more (depending on the size of the household and water test results) of adsorption media with backwash control valves, a shut-off valve and a sampling port for treated water. The system should be thoroughly backwashed before being placed into service. The system requires backwashing every 28 days or after treatment of every 8,000 gallons of water. Most systems have an automatic backwash option based on the volume of water treated or the time since the last backwash. This periodic backwash helps to “fluff” the bed to eliminate channelling and to remove sediment and minerals that increase the pressure and reduce water flow.

17. REMOVAL OF FLUORIDE:

18. Introduction: Fluoride is a normal constituent of natural water samples. Its concentration, though, varies significantly depending on the water source. Although both geological and manmade sources contribute to the occurrence of fluoride in water, the major contribution comes from geological resources. Except in isolated cases, surface waters seldom have fluoride levels exceeding 0.3 mg/l . Examples are streams flowing over granite rich in fluoride minerals and rivers that receive untreated fluoride-rich industrial wastewater. There are several fluoride bearing minerals in the earth's crust. They occur in sedimentary (limestone and sandstone) and igneous (granite) rocks. Weathering of these minerals along with volcanic and fumaroles processes lead to higher fluoride levels in groundwater. Dissolution of these barely soluble minerals depends on the water composition and the time of contact between the source minerals and the water.

19. Guidelines And Standards: Taking health effects into consideration, the World Health Organization (1996) has set a guideline value of 1.5 mg/1 as the maximum permissible level of fluoride in drinking waters. However, it is important to consider climatic conditions, volume of water intake, diet and other factors in setting national standards for fluoride. As the fluoride intake determines health effects, standards are bound to be different for countries with temperate climates and for tropical countries, where significantly more water is consumed.

20. Ways to fight against fluoride: Chemical Additive method: These methods involve the addition of soluble chemicals to the water. Fluoride is removed either by precipitation, co-precipitation, or adsorption onto the formed precipitate. Chemicals include lime used alone or with magnesium or aluminium salts along with coagulant aids. Treatment with lime and magnesium makes the water unsuitable for drinking because of the high pH after treatment. The use of alum and a small amount of lime has been extensively studied for defluoridation of drinking water. The method is popularly known as the Nalgonda technique (RENDWM, 1993), named after the town in India where it was first used at water works level. It involves adding lime (5% of alum), bleaching powder (optional) and alum (Al2(SO4)3.18H2O) in sequence to the water, followed by coagulation, sedimentation and filtration. A much larger dose of alum is required for fluoride removal (150 mg/mg F-), compared with the doses used in routine water treatment.

21. As hydrolysis of alum to aluminium hydroxide releases H+ ions, lime is added to maintain the neutral pH in the treated water. Excess lime is used to hasten sludge settling.

22. The Nalgonda technique has been successfully used at both individual and community levels in India and other developing countries like China and Tanzania. Domestic defluoridation units are designed for the treatment of 40 litres of water.

23. Contact Process: Contact Precipitation: Contact precipitation is a recently reported technique in which fluoride is removed from water through the addition of calcium and phosphate compounds. The presence of a saturated bone charcoal medium acts as a catalyst for the precipitation of fluoride either as CaF2, and/or fluorapatite (Fig. 22.3). Tests at community level in Tanzania have shown promising results of high efficiency. Reliability, good water quality and low cost are reported advantages of this method (Chilton, et al., 1999).

24. Bone char as Defluorinating material: Bone char consists of ground animal bones that have been charred to remove all organic matter. Major components of bone charcoal are calcium phosphate, calcium carbonate and activated carbon. The fluoride removal mechanism involves the replacement of carbonate of bone char by fluoride ion. The method of preparation of bone charcoal is crucial for its fluoride uptake capacity and the treated water quality. Poor quality bone char can impart bad taste and odour to water. Exhausted bone char is regenerated using caustic soda. Since acid dissolves bone char, extreme care has to taken for neutralising caustic soda. Presence of arsenic in water interferes with fluoride removal.

25. Calcined Clay: Freshly fired brick pieces are used in Sri Lanka for the removal of fluoride in domestic defluoridation units The brick bed in the unit is layered on the top with charred coconut shells and pebbles. Water is passed through the unit in an upflow mode. The performance of domestic units has been evaluated in rural areas of Sri Lanka ( Priyanta & Padamsiri 1997). It is reported that efficiency depends on the quality of the freshly burnt bricks. The unit could be used for 25-40 days, when withdrawal of defluoridated water per day was around 8 litres and raw water fluoride concentration was 5 mg/l.

26. Activated Alumina as a defluorinating material: Activated alumina or calcined alumina, is aluminium oxide, Al2O3. It is prepared by low temperature dehydration (300-600°C) of aluminium hydroxides. Activated alumina has been used for defluoridation of drinking water since 1934, just after excess fluoride in water was identified as the cause of fluorosis . The fluoride uptake capacity of activated alumina depends on the specific grade of activated alumina, the particle size and the water chemistry (pH, alkalinity and fluoride concentrations). In large community plants the pH of the raw water is brought down to 5.5 before defluoridation, as this pH has been found to be optimum and it eliminates bicarbonate interference.

27. Activated alumina has been the method of choice for defluoridation of drinking water in developed countries. Generally it is implemented on a large scale in point of source community plants. A few point of use defluoridation units have been developed which can be directly attached to the tap. During recent years this technology is gaining wide attention even in developing countries. Domestic defluoridation units have been developed in India using indigenously manufactured activated alumina, which is commercially available in bulk quantities .