Coagulation and Disinfection Manual

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This manual is part of a series of guides devised by the Oxfam Public Health Engineering Team to help provide a reliable water supply for populations affected by conflict or natural disaster. The equipment is designed to be used with any or all of the following Oxfam water equipment: Water Pumping equipment, Water Storage equipment, Water Filtration equipment, Water Distribution equipment, Hand-dug Well equipment, and Water Testing Kit. All are designed using available, easily transported equipment which is simple, rapidly assembled, and fully self-contained, to provide an adequate, safe water supply at moderate cost. The principles used in these packages may often be useful in long-term development projects.
Transcript
    1 Oxfam Water Supply Scheme for Emergencies This equipment is part of several packages devised by the Oxfam Public Health Engineering Team to help provide a reliable water supply for populations affected by conflict or natural disaster. The equipment is designed to be used with any or all of the following Oxfam water equipment: Water Pumping equipment, Water Storage equipment, Water Filtration equipment, Water Distribution equipment, Hand-dug Well equipment, and Water Testing Kit. All are designed using available, easily transported equipment which is simple, rapidly assembled, and fully self-contained, to provide an adequate, safe water supply at moderate cost. The principles used in these packages may often be useful in long-term development projects. The Oxfam equipment packages which consist of Oxfam tanks (steel sheets, rubber liners), diesel water pumps, 3” PVC pipes etc, have been used successfully in the last two decades in often harsh environments, ranging from tropical to temperate climatic areas. Although this equipment is designed for emergencies, if installed and protected adequately it can give many years of useful service, though some up-grading works will be necessary to prolong its life. This equipment can be dismantled and re-used elsewhere. However, these Oxfam equipment packages, while being simple to erect over a period of days, yet durable enough to last several years, do not lend themselves to very rapid deployment in a few hours. Increasingly, the nature of work which Oxfam has been required to undertake, has required equipment that can rapidly deployed, then dismantled and moved to other locations and this has led to the development of the so called “rapid response kits” since the mid 1990s. This type of equipment is seen as a necessary complement to the srcinal Oxfam equipment and is best used to provide a start up package in the absence of a detailed assessment and where affected populations are likely to be highly mobile. The relatively higher equipment costs and lack of suitability for anything other than short term water supply, means that the deployment of the “rapid response kits” should be used only where appropriate. Read this manual through before starting installation. Contents SECTION A - EQUIPMENT USE AND SYSTEM DESIGN 2 Coagulation 2 Disinfection 4 SECTION B – LIST OF KITS AND INSTRUCTIONS FOR USE 6 Coagulation and disinfection kits 6 Coagulation 7 Sedimentation - upflow clarifier 8 Disinfection 13 SECTION C - OPERATIONAL INFORMATION 15 Coagulation and flocculation 15 Chlorination 19 SECTION D – DETAILS OF KITS AND SUPPLIERS 22 Full list of kit contents 22 List of suppliers 25    2  SECTION A - EQUIPMENT USE AND SYSTEM DESIGN Wherever possible, water supplies in emergency conditions should be obtained from underground sources, by exploitation of springs, tubewells, or dug wells; as treatment requirements are minimal, because water from these sources is usually low in physical and microbiological contamination. However, in an emergency, such a source may not be available or it may take a long time to develop and the use of surface water from streams, rivers, lakes, or ponds may become necessary. Usually these surface sources are polluted with both physical and microbiological contamination. The use of chemicals to assist in treatment of water is often necessary where water needs to be provided from these contaminated surface water sources, particularly in the early stages of an emergency. While the use of the roughing and slow sand filtration (see Oxfam filtration equipment manual), should be considered for post emergency situations as these treatment methods are more sustainable and appropriate, the relatively speed and efficiency of using chemical treatment methods justifies their use during the early stages of an emergency response. The principles of using chemicals for water treatment apply to both household level (small scale) and treatment plant level (bulk centralised production), but the equipment and methodologies discussed in this manual apply primarily to bulk water treatment. SPHERE recommends maintaining a chlorine residual of 0.2-0.5 mg per litre and turbidity below 5. Where water is not disinfected, there should be no more than 10 faecal coliforms/100ml at point of water delivery. The recommended figure of 15 litres / person / day is used for water supply and this figure is based upon; water requirements for food preparation and consumption, which require higher quality water, as well as water needed for clothes washing and bathing. Where nearby sources of water such as streams and rivers are available and the safe use of these for washing clothes and bathing can be managed, it may be appropriate and necessary to initially size the treatment system on a figure of 10l / person / day. This would provide the water required for food preparation and drinking, i.e. a minimum of 5 litres/person/day and additional water to allow for subsequent increased demand, perhaps due to population expansion. Oxfam uses two basic types/stages of treatment process for treatment of physically and microbiologically contaminated (surface) water: 1.  Water (surface) with high physical contamination, i.e. suspended solids (which often has high microbiological contamination too), needs to be treated using plain sedimentation or a combination of coagulation and flocculation followed by (assisted) sedimentation. Thus the primary role of this stage of treatment is to reduce physical contamination – though it does also have a limited ability to reduce microbiological contamination. 2.  Water (surface) with low physical contamination but with high microbiological contamination can be treated by disinfection only. Thus the primary function of disinfection is to eliminate microbiological contamination – there is little scope for efficient disinfection with chlorine where there are high levels of physical contamination (<5NTU). Note: physical contamination is due to suspended solids - approximate estimates of which are made by measuring turbidity (NTU). Coagulation Coagulation with aluminium sulphate Where high levels of suspended solids exist in the water, reduction of these is necessary in order to be able to disinfect effectively with chlorine and for aesthetic (looks/taste) reasons. Plain sedimentation of solids suspended in water is often slow, but is readily assisted by addition of a coagulant, which causes the solid particles to aggregate (stick) together and so to form larger masses, which settle more rapidly. While effective intake design and plain sedimentation can remove larger particles, colloidal (very fine) matter and organic material such as algae, is often difficult to remove without use of a coagulant. The coagulant most commonly used by Oxfam is aluminum sulphate powder (Oxfam code FAS), which though not a very strong coagulant, does have the advantage that it can be air freighted easily and is quite commonly available in different parts of the world. However, it does have quite a narrow pH range, operating best between pH 6.5    3and 7.5 and outside these limits its efficiency goes down and hence more has to be used to compensate. This occurs as the solubility of aluminium precipitate increases dramatically outside this range, which means that where pH is too high or too low, a floc precipitate will be unable to form easily.  As the addition of (acidic) aluminium sulphate to water lowers the pH (by reacting with its natural alkalinity), there is a risk that water pH may fall outside the optimum range. Where water has insufficient alkalinity or buffering capacity, additional alkali must be provided, usually by the addition of Quick lime, as this will raise the pH of the water. As a guide, around 7 – 14kg of lime added to 95m 3 of water will provide an appropriate level of pH adjustment, though clearly the actual amount should be determined as part of the jar tests. Coagulants such as ferric chloride and ferric sulphate can be ordered and these operate in a wider pH range, but are more hazardous, making them more difficult to transport by air and they are less commonly available. Coagulant aids can also be used where water is particularly difficult to treat, even by coagulation and Oxfam is investigating the use of these as a start up option in acute emergencies to increase the effectiveness of aluminum sulphate. Jar tests should be performed to determine the correct dose of coagulant to use. This will probably be in the range between 25 - 150g/m 3  for aluminum sulphate, but will depend upon the raw water to be treated. Details of how to undertake a  jar test are given in Section C. There are three main stages in using a coagulant and these can be achieved in a variety of ways, choice being dependent upon equipment being available and local circumstances; 1. Dosing of coagulant 2. Floc formation - flocculation 3. Sedimentation Dosing of aluminium sulphate There are several options that Oxfam uses for addition of aluminium sulphate (and some other coagulants) to water; 1. By suction side dosing, using the suction side dosing kit (Oxfam code FASD). The coagulant is sucked into the water stream by the pump and undergoes rapid mixing in the pump chamber. 2. By use of a barrel erected at edge of, or in the tank to drip into inlet or outlet flow. Either a 200 litre-oil drum could be used or the equipment in the constant head dosing kit (Oxfam code FCCD). 3. By use of a precise chemical dosing pump (Oxfam code FDO), which is powered simply by a small hydraulic head (minimum of 1m). Though these have been thoroughly tested and a specification prepared for them (see section D), they are not stocked and this manual does not deal with them in any further detail. Flocculation Once the coagulant has been added (dosed) to the water supply, the right conditions need to be created to enhance the process of floc formation. Typically after a period of rapid mixing/injection into the water stream (as achieved with suction side dosing where water is churned through the pump chamber), the water/coagulant mix should be gently stirred to permit the smaller flocs to come together. Care must be taken not to have the flocs broken up by too strong mixing. Oxfam uses two basic methods for achieving this; 1. The use of a coiled pipe floculator, especially in conjunction with suction side dosing, is much more efficient way of achieving good flocculation and has been recently introduced to Oxfam. 2. Attaching a 2/3m length of hose onto a coagulant/flocculent tank inlet and fixing this along the circumference of the tank to create a circular stirring motion within the tank during the time in which water is being pumped/fed into the tank. This method is the traditional practice but it is less efficient than the coiled pipe floculator. Sedimentation (coagulant assisted) Once the coagulant has been introduced into the water and flocs are starting to form, a period of time is required for these to settle out of the water and form a sediment at the bottom of the tank, enabling clean water to be removed from the clear water above this. The use of specially designed sedimentation tanks complete with special inlet, outlet arrangements and other features, does increase the efficiency of sedimentation and allows a much greater level of process control. However Oxfam or onion tanks can be used to provide a very basic sedimentation tank which will achieve    4the separation of most of the flocs from the treated water. Good dispersion of the aluminium sulphate throughout the water to be treated should be ensured before it is introduced to the tanks. Agitation of water in the settlement tanks must be minimised. Aluminium carry over into the water supply should be measured by checking the presence of aluminium with a comparator (available in Oxfam kit, code FMT) at the tank outlet. Oxfam uses two basic methods of achieving sedimentation: 1. Simple sedimentation in batches, either in Oxfam tanks or Onion tanks. This is the simplest way of achieving this as it requires only basic equipment (i.e. tanks) though it provides little process control, with the risk of both suspended solids and coagulant carry over into the water supply. Once pumping of water into the tank has been completed, water will typically have to sediment between 2 and 6 hours (actual time determined by a jar test) and thus water production rates can be calculated accordingly. 2.  Upflow clarification, which is an advanced sedimentation process, usually run on a continuous basis, which allows a considerably greater level of process control. The Upflow clarifier kit has been developed very recently to be able to achieve this and it warrants a further description because it is so new. The upflow clarifier  .   Oxfam has developed a completely new piece of equipment for use with water treated by coagulants and this can be built inside an Oxfam T11 tank. The evolution and testing of the Upflow clarifier kit (Oxfam code FUC), which though incorporating treatment technology used in permanent water treatment plants, has a number of unique design features to enable it to be engineered to fit into a “rapid response package”. The upflow clarifier has been designed to fit into a complete water treatment system package, which combined with a pump, aluminium sulphate and chlorine dosing systems, offers a fairly rapid and very robust continuos treatment process capable of dealing with high levels of turbidity. The system will take between half and one day to set up, including time taken to erect the T11 tank and to reach stable operating conditions. The system has been tested and can produce between 7-9m 3 /hr, with turbidity reduction from NTU500 to under NTU10, but actual production and performance is dependent upon raw water quality. As it is essentially a sedimentation system, it requires considerably less cleaning and maintenance than pressure filtration systems, which often take less time to set up, but soon loose this benefit with complex cleaning regimes that result when highly turbid water (much above 50 – 100 NTU) is being treated. The various stages in the Upflow clarifier package are:  Dosing of aluminium sulphate, either by having a Tee on the suction hose of the pump, or from the outlet of a raw water/sedimentation tank or hydraulic dosing pump, to dose the coagulant into the water to ensure that it undergoes rapid mixing.  Flocculation by passing through 2 parallel coils of 3” layflat hose wrapped around the Oxfam T11 tank, each 30m long, which act as flocculators by gently stirring the water as it passes through the coils of the pipe.  Sedimentation in the T11 tank in which the upflow clarifier is built, where some flocs should already be forming.  Final filtration through a polishing fabric filter installed at the top of the tank. Water low in suspended solids will flow out from the top of the clarifier and through the tank outlet.  Chlorination at the outlet, where chlorine is added by the constant head chlorine dosing device Disinfection Chlorine is the chemical most widely used for disinfection of treating drinking because of its ease of use, ability to measure its effectiveness, availability and cheapness. Under the right conditions chlorine will kill all viruses and bacteria, but some species of protozoa and helminthes are resistant to chlorine. WHO recommends adequate protection of the source as the most effective way of dealing with these more resistant helminthes and protozoa by preventing faceal contamination entering the water. Protozoa and helminthes are difficult to detect directly, but where these are thought be a risk, it may be necessary to resort to use of Membrane filters to strain out these organisms (the smallest of these are Giardia cysts at 7-10microns, while Cryptosporidium oocysts are 4-6 microns). However though these are able to produce high quality water, they will not provide much water quantity for a low capital investment and thus the
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