ALT E R N AT IV E WAT E R SU P P LY TE C H N O L O G IE S CONTENTS Introduction The important alternative water supply technologies include: Shallow Shrouded Tube-well (SST) and Very Shallow Shrouded Tube-well (VSST) Deep Tube-well Dug Well Pond Sand Filters Household Filters Infiltration Gallery Rainwater Harvesting INTRODUCTION The type of hand-pump technology suitable for a particular area depends on the groundwater level, water quality and hydrogeological conditions. There are some areas like the costal belt in the southern part of Bangladesh, where the conventional shallow and deep tubewlls technologies are not successful due to the high salinity. Alternative water supply options are needed for those areas. The important alternative water supply technologies include: Shallow shrouded Tube-well (SST) Very shallow shrouded Tube-well (VSST) Pond sand filters Household filters Infiltration gallery Solar desalination Rainwater Harvesting
Deep Tube-well In Bangladesh two types of deep tube-wells as shown in Figure are constructed, manually operated small diameter tube-well similar to shallow tube-wells and large diameter power operated tube-wells called production well. Some areas of the coastal region of Bangladesh is very suitable for construction of deep tubewell. Department of Public Health Engineering has sunk a total of 81,384 deep tube-well mainly in the coastal area to provide safe water to 8.2 million people (DPHE,2000). The identification of areas having suitable deep aquifers and a clear understanding about the mechanism of recharge of these aquifers are needed to develop deep tube-well based water supply systems in Bangladesh. Dug Well Dug well is the oldest method of groundwater withdrawal for water supplies. The mechanism of producing water of low arsenic and other dissolved minerals concentration by dug wells are not fully known. Dug wells are widely used in many countries of the world for domestic water supply.
The flow in a dug wells is actuated by lowering of water table in the well due to withdrawal of water. Usually no special equipment or skill is required for the construction of dug wells. For construction by manual digging, the wells should be at least Community dug wells should be deeper to provide larger surface area for the entry of water to meet higher water demand. Private dug wells are less that 10m deep but dug wells for communal use are usually 20-30 meters deep.1.2 meters in diameter. Pond Sand Filters A prospective option for development of surface water based water supply system is the construction of community type Slow Sand Filters (SSFs) commonly known as Pond Sand Filters (PSFs). Slow sand filters are installed near or on the bank of a pond, which does not dry up in the dry season. The water from the pond is pumped by a manually operated hand tube-well to feed the filter bed, which is raised from the ground, and the treated water is collected through tap(s). The PSF is a low-cost technology with very high efficiency in turbidity and bacterial removal. It has received preference as an alternative water supply system for medium size settlements in arsenic affected areas.
Although PSF has very high bacterial removal efficiency, it may not remove 100% of the pathogens from heavily contaminated surface water. Fig.: Pond Sand Filter for Treatment of Surface Water
The major limitations of PSF Operation and maintenance are difficult; Not suitable for heavily contaminated ponds; People complained of foul taste in pond water and many resorted to using it for cooking only; Conflicts with fish culture; It is difficult to find an appropriate/reserve pond for installation of PSF; Many ponds dry up in the dry season in some parts of the country; Secondary contamination takes place due to lack of proper maintenance. Household Filters Surface water containing impurities can be clarified by a pitcher filter unit or a small sand filter at the household level. It is an old method of water purification, in rural areas of Bangladesh. These processes of water treatment at household level have been phased out with the introduction of tube-wells for village water supply. Pitcher filters are constructed by stacking a number pitchers (Kalshis), one above the other, containing different filter media. Raw water is poured in the top Kalshi and filtered water is collected from the bottom one. In this process, water is mainly clarified by the mechanical straining and adsorption depending of the type of filter media used. Small household filters can be constructed by stacking about 300-450 mm thick well graded sand on a 150-225 mm thick coarse aggregate in a cylindrical container. The container is filled with water and the filtered water is collected from the bottom. Full effectiveness of the filtration process is obtained if the media remain in water all the time. The pitcher and other small household filters cannot completely remove micro-organisms if these are present in large numbers in raw water.
The important characteristics of household filters are: Suitable for surface water treatment; Remove turbidity, color and microorganisms; Complete removal of pathogenic microorganisms is not guaranteed; Not suitable for high-turbid water; Difficulty in cleaning and keeping the system operational. Infiltration Gallery Infiltration Galleries (IG) or wells can be constructed near perennial rivers or ponds to collect infiltrated surface waters for all domestic purposes. Since the water infiltrate through a layer of soil/sand, it is significantly free from suspended impurities including microorganisms usually present in surface water. Again, surface water being the main source of water in the gallery/well, it is free from arsenic. If the soil is impermeable, well graded sand may be placed in between the gallery and surface water source for rapid flow of water. Fig.: An Infiltration Gallery by the Side of a Surface Water Source
Rainwater Harvesting Bangladesh is a tropical country and receives heavy rainfall during the rainy season. In the coastal districts, particularly in the offshore islands of Bangladesh, rainwater harvesting for drinking purposes is a common practice in a limited scale for long time (Chowdhury et al, 1987). In some areas of the coastal region with high salinity problem, about 36 percent households have been found to practice rainwater harvesting in the rainy season for drinking purpose (Hussain and Ziauddin, 1989). In the present context, rainwater harvesting is being seriously considered as an alternative option for water supply in Bangladesh in the arsenic affected areas. Advantages and disadvantages of rainwater collection system Advantages The quality of rainwater is comparatively good. The system is independent and therefore suitable for scattered settlements. Local materials and craftsmanship can be used in construction of rainwater system. No energy costs are incurred in running the system. Ease in maintenance by the owner/user Disadvantages The initial cost may prevent a family from installing a rainwater harvesting system. The water availability is limited by the rainfall intensity and available roof area. Mineral-free rainwater has a flat taste, which may not be liked by many. Mineral-free water may cause nutrition deficiencies in people who are on mineral deficient diets. The poorer segment of the population may not have a roof suitable for rainwater harvesting.
WAA T E R T R E A T M E N T Preliminary Treatment Preliminary treatment is any physical, chemical or mechanical process used on water before it undergoes the main treatment process. The purpose of preliminary treatment processes is to remove any materials which will interfere with further treatment. Pretreatment may include screening, pre-sedimentation, chemical addition, flow measurement, and aeration. Preliminary Treatment / Screens The screens are used to remove rocks, sticks, leaves, and other debris. Very small screens can be used to screen out algae in the water. All objects are removed by physical size separation Screens on the outside of intakes are often cleaned by flushing water from the treatment plant backwards There are two primary types of screens - bar screens and wire-mesh screens. A bar screen is used to remove large debris. The spaces between the bars are two to four inches wide. A wire-mesh screen is used to remove smaller debris. The gaps are about half an inch wide. Water must be flowing slowly in order to pass through a wire-mesh screen - velocities should be no greater than 3.5 inches per second. Preliminary Treatment / Pre-sedimentation - Aeration Preliminary Treatment / Monitoring Flow Measurement: to adjust chemical feed rates, calculate detention times, and monitor the amount of water being treated. It is also monitored for a variety of characteristics including ph, turbidity, total alkalinity, temperature, and coliform bacteria. The ph and total alkalinity of the water will influence the amount of alkali to be added and can also influence the flocculation conditions The level of turbidity will influence the amount of polymer (coagulant) added to the water. Temperature is also measured since cold water does not floc as well as warm water and requires the addition of more polymer
Primary Sedimentation Sedimentation is a treatment process in which the velocity of the water is lowered below the suspension velocity and the suspended particles settle out of the water due to gravity. The process is also known as settling or clarification Settled solids are removed as sludge, and floating solids are removed as scum The efficiency or performance of the process is controlled by: detention time, temperature, tank design, and condition of the equipment. Notes Fig.: Simple Sedimentation Tank sedimentation may not be necessary in low turbidity water of less than 10 NTU In this case, coagulation and flocculation are used to produce pinpoint (very small) floc which is removed from the water in the filters Primary Sedimentation / Location in the Treatment Process The most common form of sedimentation follows coagulation and flocculation and precedes filtration. This type of sedimentation requires chemical addition (in the coagulation/flocculation step) and removes the resulting floc from the water. sedimentation following coagulation/flocculation is meant to remove most of the suspended particles in the water before the water reaches the filters, Sedimentation at this stage in the treatment process should remove 90% of the suspended particles from the water, including bacteria.
The purpose of sedimentation here is to decrease the concentration of suspended particles in the water, reducing the load on the filters. Sedimentation can also occur as part of the pretreatment process, where it is known as presedimentation. Types of sedimentation basins Rectangular basins: have a variety of advantages - predictability, cost-effectiveness, and low maintenance. In addition, rectangular basins are the least likely to short-circuit, especially if the length is at least twice the width. A disadvantage of rectangular basins is the large amount of land area required. Double-deck rectangular basins: This type of basin conserves land area - has higher operation and maintenance costs. Square or circular sedimentation basins with horizontal flow are known as clarifiers. This type of basin is likely to have short-circuiting problems. Solids-contact clarifiers also known as up flow solids-contact clarifiers or up flow sludgeblanket clarifiers combine coagulation, flocculation, and sedimentation within a single basin. found in packaged plants and in cold climates where sedimentation must occur indoors Sedimentation and flotation zones All sedimentation basins have four zones - the inlet zone, the settling zone, the sludge zone, and the outlet zone. In a clarifier, water typically enters the basin from the center rather than from one end and flows out to outlets located around the edges of the basin. But the four zones can still be found within the clarifier Filtration After separating most floc, the water is filtered as the final step to remove remaining suspended particles and unsettled floc 1. Rapid sand filters Use relatively coarse sand and other granular media to remove particles and impurities that have been trapped in a floc through the use of flocculation chemicals-typically salts of aluminium or iron. Water and flocs flows through the filter medium under gravity or under pumped pressure Water moves vertically through sand which often has a layer of activated carbon or anthracite coal (a hard, compact variety of mineral coal). The top layer removes organic compounds Most particles pass through surface layers but are trapped in pore spaces or adhere to sand particles
To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called back flushing or backwashing) compressed air may be blown up through the bottom of the filter to break up the compacted filter media to aid the backwashing process Fig.: Rapid sand filters Rapid sand filters / Advantages and disadvantages Advantages Much higher flow rate than a slow sand filter; Requires relatively small land area Less sensitive to changes in raw water quality, e.g. turbidity requires less quantity of sand Disadvantages Requires greater maintenance than a slow sand filter. For this reason, it is not usually classed as an "appropriate technology,". Generally ineffective against taste and odour problems. Produces large volumes of sludge for disposal. Requires on-going investment in costly flocculation reagents. treatment of raw water with chemicals is essential skilled supervision is essential cost of maintenance is more it cannot remove bacteria
Slow sand filters Slow "artificial" filtration (a variation of bank filtration) to the ground, Water purification plant The filters are carefully constructed using graded layers of sand with the coarsest sand, along with some gravel, at the bottom and finest sand at the top. Drains at the base convey treated water away for disinfection effective slow sand filter may remain in service for many weeks or even months produces water with a very low available nutrient level and low disinfectant levels Slow sand filters are not backwashed; they are maintained by having the top layer of sand scraped off A 'large-scale' form of slow sand filter is the process of bank filtration in a riverbank. Advantages Fig.: Slow sand filters require little or no mechanical power, chemicals or replaceable parts, require minimal operator training and only periodic maintenance, Often an appropriate technology for poor and isolated areas. simple design
Disadvantages Due to the low filtration rate, slow sand filters require extensive land area for a large municipal system. Many municipal systems in grown cities installed rapid sand filters, due to increased demand for drinking water. Membrane filtration Is a treatment process based on the physical separation of compounds from the water phase with the use of a semi-permeable membrane. Most of the membranes used are synthetic membranes made of organic polymers. Can be divided into two categories based on the pore sizes of the membrane: micro- and ultra filtration (MF and UF) remove colloidal substances and microorganisms Nano-filtration and reverse osmosis (NF and RO) remove colloidal substances and microorganisms but also dissolved substances like micro pollutants and ions Micro- and ultra filtration remove substances from the water phase by a sieve mechanism Microfiltration removes bacteria and the larger viruses (to a size of 0.05 μm). Ultra filtration also removes bacteria, but because of the smaller pore size all the larger viruses are removed The removal of suspended solids of MF and UF is at least 99%. Membrane filtration The removal of microorganisms is referred to in log units. A removal of one log unit corresponds to a 90% removal. The removal of 4 log units corresponds to a 99.99% removal. molecular weight cut-off (MWCO) can also be used as an indication of the ability of membranes to reject compounds MWCO is the molecular weight of spherical molecules which are 90% rejected by the membrane s pores. The unit of MWCO is Dalton (1 Dalton is the mass of one hydrogen atom = 1.66x10-27 kg) The MWCO for MF/UF is in the range of 10,000 to 500,000 Dalton (10 to 500 kd). Aeration Aeration is the process of bringing water and air into close contact.
Aeration is the process to remove dissolved gases, such as carbon dioxide, hydrogen sulfide, and to oxidize dissolved metals such as iron. It can also be used to remove volatile organic chemicals (VOC). It happens by: Exposing drops or thin sheets of water to the air or Introducing small bubbles of air and letting them rise through the water. Aeration accomplishes the desired results by: Sweeping or scrubbing action caused by the turbulence of water and air mixing together Oxidizing certain metals and gases Aeration Efficiency CHEMICAL SUBSTANCES AFFECTED BY AERATION The constituents that are commonly affected by aeration are: Volatile organic chemicals, such as benzene, found in gasoline, or trichloroethylene, dichloroethylene, and perchloroethylene, examples of solvents are used in dry cleaning or industrial processes. Carbon dioxide Hydrogen sulfide (rotten-egg odor) Methane (flammable) Iron (will stain clothes and fixtures) Manganese (black stains) Various chemicals causing taste and odor CHEMICAL SUBSTANCES AFFECTED BY AERATION Surface waters have a low CO 2 content (0 to 2 mg/l). Deep lake or reservoir can have high CO 2 content due to the respiration of microscopic animals and lack of abundant plant growth at the lake bottom. Aerators remove CO 2 by the physical scrubbing or sweeping action caused by turbulence. aeration can reduce the CO 2 content to 4.5 mg CO 2 /l Concentration of CO 2 in groundwater is usually higher than in surface water. Water from a deep well normally contains less than 50 mg/l, but a shallow well can have a much higher level, up to 50 to 300 mg/l.
CARBON DIOXIDE REMOVAL The most appropriate treatment for carbon dioxide may be aeration, addition of an alkali, or a combination of the two CO 2 gas dissolves easily in water, resulting in carbonic acid: H 2 O + CO 2 <===> H 2 CO 3 CO 2 is neutralized through the addition of an alkali (basic, ionic salt), such as lime (Ca(OH) 2 ) or soda ash (Na 2 CO 3 ). Lime reacts with carbon dioxide, removing the carbon dioxide from the water as shown below: CO 2 + Ca(OH) 2 <===> CaCO 3 + H 2 O CO 2 above 5 to 15 mg/l in raw water can cause three operating problems: It increases the acidity of the water, making it corrosive by forming a weak acid, H 2 CO 3. It tends to keep iron in solution, thus making iron removal more difficult. It reacts with lime added to soften water, causing an increase in the amount of lime needed for the softening reaction. Taste & Odor Removal Aeration is effective in removing tastes and odors that are caused by volatile materials Volatile materials (e.g Methane and hydrogen sulfide) have low boiling point and will vaporize very easily. Many taste and odor problems in surface water could be caused by oils and by-products that algae produce. Since oils are much less volatile than gases, aeration is only partially effective. Dissolved Oxygen Oxygen is injected into water through aeration to remove the flat taste. The amount of oxygen that the water can hold is dependent on the temp. The colder the water, the more oxygen the water can hold. Water that contains excessive amounts of oxygen can become very corrosive. Excessive oxygen can cause air binding of filters.
Types of Aerators Aerators fall into two general categories. Introduce air into the water or water into the air. The water-to-air method is designed to produce small drops of water that fall through the air The air-to-water method creates small bubbles of air that are injected into the water stream. All aerators are designed to create a greater amount of contact between the air and water to enhance the transfer of the gases. WATER INTO AIR Cascade Aerators Consists of a series of steps that the water flows over. Aeration is accomplished in the splash zones. The aeration action is similar to a flowing stream. Splash areas are created by placing blocks across the incline. Cascade aerators used to oxidize iron and to partially reduce dissolved gases. the oldest and most common type of aerators. Cone Aerators Are used primarily to oxidize iron and manganese prior to filtration. The water pumped to the top of the cones and then allowed to cascade down through the aerator.
Slat and Coke Aerators Similar to the cascade and cone types. They usually consist of three-to-five stacked trays, which have spaced wooden slats in them. The trays are filled with fist-sized pieces of coke, rock, ceramic balls, limestone, or other materials. The primary purpose of the materials is provide additional surface contact area between the air and water.