The problem of undrinkable water should be taken more seriously than at the moment
Water pollution of surface water bodies in Sindh is much more than worse than previously thought. The discharge of untreated municipal wastewater of cities, towns, and wastewater of industries and factories. The Manchar Lake receives agricultural return flows, the components of which include salinity, agro-chemicals (pesticides) and fertilizers (nitrates and phosphates), in addition to the untreated domestic and industrial effluents. Due to the significant discharge of saline effluents from MNVD (Main Nara Valley Drain), the water quality of Manchar Lake has deteriorated to a large extent. Therefore, before the clean-up work of Manchar Lake is taken in hand, the saline effluents from MNVD need to be stopped forthwith.
Water pollution, over the years, has become a major issue, in countries with economies in transition. Rivers such as Yellow (China), Ganges (India) and Amu and Syr Darya (Central Asia) are highly polluted rivers.
Everybody knows that since water from surface water bodies is generally unsafe for drinking purposes, unless properly treated, the WHO (World Health Organization) has prescribed guidelines for drinking-water quality. According to the WHO the primary aim of the guidelines is the protection of public health.
All well-designed and well-operated water treatment plants are capable of producing finished waters that is safe and wholesome. If value of any parameter in the finished water is much higher (than the acceptable limit), then, it could be due to a problem in the operation of the plant. High turbidity (greater than 10 NTU — nephelomtric turbidity unit) in treated water could be due to, among others: hydrogenion concentration of incoming water in the coagulation unit is not in accordance with the type of coagulant used; temperature of raw water is low; flash mixing is not being done; flocculator paddles rotate at high speed, or rotate with jerks, causing breaking-up of flocs, inlet of sedimentation tanks is such that, it creates turbulence and disturbance in sedimentation tanks, inadequate water retention time in sedimentation tanks, due to sludge accumulation at the tanks’ bottom, operation of sludge scrappers in sedimentation tanks is jerky, thereby causing resuspension of sludge particles, filtration units are bypassed filtration rates are higher than the designed rates; air scouring and backwashing of filters is not done regularly (in case of rapid sand filters, the filters need to be backwashed once every 24 hours, depending upon the raw water turbidity) and air and washwater pressures are inadequate to clean and wash the filters fully.
When rapid sand filter system is used for water treatment, proper pretreatment of raw water (with coagulant) is an important pre-requisite. In fact, the ability of rapid sand filters, to handle almost any quality of raw water, is mainly due to the proper pretreatment of raw water, by way of coagulation.
The water treatment plant managers must understand that, the plant will never give clear and acceptable finished water if proper and adequate coagulation, consistent with the requirements, laid down by the plant’s design criteria, for the amount of coagulant to be used, is not done even if the rest of the treatment units (flocculation, sedimentation and filtration) are working normally.
Experience has shown that, with the exception of few heavier particles (greater than 1,000 micrometer in size), almost no bacteria and viruses are removed if uncoagulated water is passed through rest of the treatment units. If the finished water has high turbidity (greater than 10 NTU), it will escape disinfection, since microorganisms encase themselves in the turbid particles, where the disinfectant cannot reach.
Proper operation of water treatment plant includes regular monitoring of raw water and treated water quality. Most water impurities are in dissolved form and cannot be seen by naked eye. Proper record of influent and effluent water quality should be maintained. All tests must be conducted in accordance with the procedures laid down in the Standard Methods for the Examination of Water and Wastewater.
Filtration is all essential unit of the water treatment plant. It is not advisable to supply water from water treatment plant without filtration. Filtration is both a physical and a chemical process. The whole depth of filters comes into action (in case of rapid sand filtration system), which removes a wide variety of contaminants including bacteria, viruses, turbidity, colour, oxidized iron and manganese, radioactive particles, chemicals added during pretreatment and to some extent, algae.
Water disinfection is practised for destroying or inactivating disease-producing organisms. It does not imply complete destruction of all living organisms, which is sterilization. Disinfection has played a major role in protecting public health through the reduction of waterborne diseases. Plant managers ensure that residual chlorine of 0.3mg/l is maintained in the distribution system, to prevent water contamination. Chlorine is typically used for disinfection. Chlorinators are used for this purpose, which add chlorine at the required dosage (rate of chlorine application per unit of water flow). Random addition of bleaching powder is not all acceptable method in water treatment.
However, while chlorine is very effective against bacteria and viruses, it is relatively ineffective against pathogenic protozoa, such as crytosporidium and giardia. Chlorine interacts with natural organic matter in water to produce trihalomethanes (chloroform, bromoform, dibromochloromethane, and bromodichloromethane), haloacetic acids and bromate that are carcinogenic. Environmental engineers suggest use of chlorine dioxide, chloramines and ozone as alternative disinfectants. Finished water should first be tested for the presence of trihalomethanes, before changing the disinfectant.
Water is described as “hard” if it is high in dissolved minerals, specifically calcium and magnesium. Hard water is not a health risk, but is a nuisance as it consumes too much soap, cause scale formation on plumbing fixtures and creates problems in washing cloths. Hard waters are less corrosive than soft waters, but they become corrosive when water is hot.
Chloride in excess of 100mg/l imparts a salty taste. Chlorides levels in finished of 250mg/l and above may cause health-related problems. Water containing about 500mg/l of sulphate taste bitter and, at 1,000mg/l would be cathartic. Moderate salinity in raw water (up to 4,000mg/l TDS) can be removed by a properly operated water treatment plant. High water salinity (e.g., seawater salinity of 30,000mg/l TDS) is removed through desalination process (e.g. reverse osmosis, multi-stage flash, and vertical tube evaporation).
Customarily, the parameter, “electrical conductivity,” is used for irrigation water quality. Instead, the parameter “total dissolved solids,” is used, when water quality is analyzed for drinking purposes. Electrical conductivity is, however, a quick measure of its total dissolved concentration.
Blue-green algae, a type of photosynthetic bacteria, called “eyanobacteria” are present in most ecosystems, including lakes, rivers, wetlands and creeks, when conditions are favourable. Occurrence of algal blooms is troublesome. They cause major problems. They cause musty tastes and odour in waters. Some species produce toxins that are known to kill livestock, wildlife and humans. Blue-green algae produce three types of toxins: endotoxins (cause allergies), neurotoxins (cause nerve damage) and hepatotoxins (cause liver damage). To put the toxicity of blue-green algae in perspective, Microcystis, one of the types of blue-green algae, produces a toxin called microcystin LR, which is 200 times more toxic than cyanide. And the blue-green algae are not destroyed by boiling water.
Environmental agencies are of the view that blue-green algal cell concentration of more than 2,000 cells per ml can cause potential toxicity problems for humans in drinking-waters. For control of algal cells (other forms) in streams, it is suggested that an effluent standard for oxidation ponds, that could be expected to have a beneficial effect oil a stream’s oxygen balance, should be not more than 10,000 cells per ml.
Presence of ammonia in water is an indicator of pollution. During water treatment, ammonia reacts with chlorine and thus little (about 30 per cent) chlorine is available for disinfection. It causes taste and odour problems. Ammonia is not of direct importance for health at the levels expected in drinking-water.
Conventional water treatment plants (rapid sand filters), cannot remove most heavy metals and pesticides. A well-operated rapid sand water treatment plant, with an advanced treatment (also called tertiary treatment) unit of granular activated carbon (GAC) can remove the algal cells. A process, that is better suited for algae removal is the dissolved air flotation, in which the coagulated particulate matter, including algae, is floated to the top of a clarification tank. GAC can also remove pesticides in the water.
If it is found that, high levels of turbidity (500 mg/l and above) in raw water is frequently encountered (which would shorten the filter runs), then it is advisable to install “roughing filters,” as the first pretreatment step. Also known as pre-filters, they are used extensively used in rural areas. The filter medium of a roughing filter is composed of relatively coarse material, ranging in size from 25 to 4 millimetres. Filtration rate is typically 0.3 to 1.5 meters per hour and produce effluent with turbidity of levels, suitable for further treatment. Extensive application of roughing filters is found in developing countries. Likewise, if frequent disinfection problems are encountered (chlorinators are out of order, or disinfectants procurement is a problem), then a slow sand filtration system should be added, as a last treatment unit. Operated at the filtration rates of 0.1 to 0.2 meters per hour, in which the layer on the surface of sand, called “schmutzdecke”, (a thin slimy matting of material, largely organic in origin) is also formed, the slow sand filters are highly efficient in removing microorganisms, fine organic and inorganic matter, organic compounds dissolved in water and some commonly-occurring pesticides. Slow sand filters, however, require large area of land. Slow sand filtration system for water treatment in rural areas, is used extensively in developing countries.
The problem of undrinkable water should be taken more seriously than at the moment 
