Water Supply Engineering

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This section covers the importance and types of water, the historical development of water supply schemes, the objectives and components of a water supply system with its schematic layout, the financing of such systems, the sources of water, the quantity and quality required, drinking water quality standards and the objectives of treatment.

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Objectives and Components • The objectives of a public water supply system are to provide water that is safe and wholesome, adequate in quantity, available at adequate pressure, reliable and continuous, conveniently accessible, and affordable.

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Safety comes first: a supply that is plentiful but contaminated is worse than useless, because it spreads disease efficiently. • The components in order, which form the schematic of every scheme: source → intake → raw water transmission main → treatment plant → clear water reservoir → pumping (if needed) → transmission main → service reservoir → distribution system → service connections. • In a Nepali hill gravity scheme the arrangement is characteristically source (usually a spring) → intake → sedimentation tank → reservoir tank → transmission line with break pressure tanks → distribution line → tapstands, with no pumping and often only simple treatment. • Financing: capital cost is met from government grant, donor assistance, loans and community contribution in cash, kind or labour; recurrent cost should be met from tariff revenue.

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The principle that decides the success of a scheme is that the tariff must at least cover operation and maintenance, or the scheme will fail when the first pump or valve does — which is why so many rural schemes collapse a few years after construction.

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Tariff structures are flat rate, uniform volumetric, or increasing block, the last being preferred because it cross-subsidises a basic lifeline quantity for poor households while discouraging waste.

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Sources of Water Source Character Advantages Limitations Rainwater Collected from roofs or catchments Soft, low in dissolved solids, available at point of use Seasonal, needs large storage, first flush contaminated by roof debris Spring Groundwater emerging naturally at the surface Generally good quality, no pumping needed, the mainstay of Nepali hill schemes Yield varies seasonally and may fail in the dry season; vulnerable to contamination at the eye Well and tube well Groundwater abstracted by pumping Reliable, generally free of pathogens, needs little treatment Requires pumping; may carry iron, manganese, arsenic or fluoride; over-abstraction lowers the table River Surface water Large and dependable yield Turbid and polluted; always requires full treatment; flow varies greatly between seasons Lake and reservoir Impounded surface water Storage evens out seasonal variation; sedimentation occurs naturally Algal growth, taste and odour, stratification, evaporation loss • Selection criteria for a source: adequate and dependable yield in the driest month of the driest year on record; acceptable quality requiring the least treatment; sufficient elevation to give gravity flow if possible; proximity to the demand centre; freedom from present and likely future pollution; secure legal right to the water; and acceptable cost of development.

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Yield must be assessed at the dry-season minimum, not the annual average — this is the point most often missed and the commonest cause of scheme failure in Nepal. • A spring should be protected at the source by an intake chamber built into the eye of the spring, with a cut-off drain above to divert surface runoff, fencing to exclude animals, and no latrine or waste disposal within the protection zone. • Nepal's water resource is abundant in the aggregate but very unevenly distributed in time and space: about 80 per cent of the flow arrives in the four monsoon months, while demand is greatest in the dry season, and many hill settlements sit far above their nearest reliable source.

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Quantity of Water • Design period is the number of years for which the works are designed to be adequate, commonly 15 to 30 years.

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It is chosen by balancing the economy of scale of building large now against the cost of capital tied up idle, the useful life of the components — pipes 30 years or more, pumps 10 to 15 — the ease of future extension, and the reliability of the population forecast.

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Components that are difficult or disruptive to extend, such as transmission mains and intakes, are given a longer design period than those easily added to, such as pumps and treatment units. • Population forecasting methods: arithmetic increase, which assumes a constant absolute increment and suits large, old, saturated towns; geometric increase, which assumes a constant percentage growth and suits young, rapidly growing towns but over-predicts if extended too far; incremental increase, which allows for the change in the increment and is generally the most satisfactory; decreasing rate of growth; graphical extension and comparison with similar towns; and the logistic curve, which represents the full S-shaped growth to a saturation value. • Per capita demand covers domestic, commercial and industrial, public or civic, and losses.

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Nepali design practice commonly adopts about 45 litres per person per day for a rural scheme with public tapstands, and 65 to 100 litres per person per day where private connections are provided; urban design figures are higher.

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Losses and wastage are allowed for explicitly rather than hidden in the per capita figure. • Variation in demand, which supplies many numerical questions: maximum daily demand is commonly taken as 1.8 times the average daily demand, and peak hourly demand as 1.5 times the maximum daily — that is 2.7 times the average.

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The design flows follow from this: the source and intake are sized for the maximum daily demand; the treatment plant and transmission main for the maximum daily demand; and the distribution system for the peak hourly demand, or for the maximum daily plus fire demand, whichever is greater.

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Knowing which component is designed for which flow is examined directly. • Fire demand is an addition to the maximum daily demand in urban systems, estimated by empirical formulae such as Kuichling's; in small rural schemes it is normally not provided for.

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Quality and Standards • Water quality parameters are grouped as physical (turbidity, colour, taste, odour, temperature, total solids), chemical (pH, hardness, alkalinity, chlorides, sulphates, iron, manganese, fluoride, arsenic, nitrate, dissolved oxygen, toxic metals) and biological (total coliform, E. coli or thermotolerant coliform, other pathogens). • The essential bacteriological principle:

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E. coli is used as an indicator organism rather than testing for pathogens directly, because it is present in very large numbers in faecal matter, is easily and cheaply detected, survives in water a little longer than most pathogens, and is not normally present otherwise.

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Its presence therefore demonstrates faecal contamination and hence the possibility of pathogens.

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The standard for drinking water is zero E. coli in 100 ml, and this requirement is absolute. • Water-related diseases fall into four groups that should be distinguished: waterborne, transmitted by drinking contaminated water — cholera, typhoid, dysentery, hepatitis A, giardiasis; water-washed, caused by insufficient water for hygiene — trachoma, scabies, skin and eye infections; water-based, involving an aquatic intermediate host — schistosomiasis, guinea worm; and water-related insect vector, where the insect breeds in or near water — malaria, dengue, filariasis.

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The point examined is that increasing the quantity of water available reduces water-washed disease even if the quality is unchanged, so quantity and quality both matter. • Chemical parameters of particular concern in Nepal: arsenic in Terai groundwater, where the national standard is 50 μg/L against the WHO guideline of 10 μg/L and long-term exposure causes arsenicosis and cancer; iron and manganese, which cause staining and taste; ammonia in Kathmandu Valley groundwater; and fluoride, deficiency of which causes dental caries and excess of which causes fluorosis. • Nepal's drinking water standard is the National Drinking Water Quality Standard, 2062 (2005) with its Directives, administered under the Water Resources Act and the relevant regulations; the WHO Guidelines for Drinking-water Quality are the international reference.

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A guideline is advisory and a standard is enforceable — a distinction examined directly.

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Standards are periodically revised, so current values must be verified. • Objectives of treatment: to remove suspended and colloidal matter so the water is clear; to remove or inactivate pathogenic organisms so it is safe; to remove objectionable colour, taste and odour so it is palatable; to remove or reduce dissolved constituents that are harmful or troublesome, such as hardness, iron, manganese, arsenic and excess fluoride; and to render the water non-corrosive and non-scaling so that it does not damage the distribution system.

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This section covers aquatic ecology, water pollution and its effects, water quality standards, water quality assessment and management, and strategies for water pollution control.

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Aquatic Ecology • An aquatic ecosystem comprises producers (algae and aquatic plants), consumers (zooplankton, invertebrates, fish) and decomposers (bacteria and fungi), together with the physical and chemical environment. • Dissolved oxygen is the master variable of a natural water.

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Its solubility decreases as temperature rises and as salinity rises, and it increases with pressure.

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Saturation at 20 °C is about 9.1 mg/L, and most fish require at least 4-5 mg/L, with cold-water species such as trout needing more than warm-water species such as carp.

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Oxygen enters the water by reaeration from the atmosphere and by photosynthesis, and is removed by respiration and by the decomposition of organic matter. • Stratification of a lake or reservoir is important for water supply.

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In summer a warm, less dense epilimnion overlies a cold, dense hypolimnion, separated by the thermocline, across which mixing is very limited.

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The hypolimnion therefore becomes depleted of oxygen, and under anaerobic conditions iron and manganese are reduced to their soluble forms and hydrogen sulphide is produced — which is why an intake must be able to draw from more than one level.

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In autumn the surface cools, the density difference disappears and the lake overturns, mixing the accumulated material through the whole depth and often causing a sudden deterioration in raw water quality.

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Multi-level intakes exist precisely for this reason, and the point is examined directly. • Self-purification of a stream is the natural recovery of water quality downstream of a discharge, through dilution, sedimentation, biological oxidation, reaeration and the die-off of pathogens.

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The oxygen sag curve describes the resulting dissolved oxygen profile: oxygen falls below the discharge as deoxygenation by microbial respiration exceeds reaeration, reaches a minimum at the critical point where the two rates are equal, and then recovers.

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The Streeter-Phelps equation models this, and its two coefficients are the deoxygenation rate constant and the reaeration rate constant.

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Self-purification is faster in a shallow, turbulent, cool stream, because reaeration is more rapid, and slower in a deep, sluggish, warm one. • Zones below a discharge, in order: the zone of degradation, the zone of active decomposition, the zone of recovery and the zone of clear water.

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Water Pollution and Its Effects Pollutant class Sources Effects Oxygen-demanding organic matter Sewage, food processing, pulp and paper, distillery Depletes dissolved oxygen, killing fish and producing septic conditions and odour Pathogens Sewage, animal waste, open defecation Waterborne disease — cholera, typhoid, dysentery, hepatitis Nutrients (N and P) Sewage, detergents, fertilizer runoff Eutrophication, algal blooms, oxygen depletion on decay; nitrate in drinking water causes methaemoglobinaemia Suspended solids Erosion, construction, mining, runoff Turbidity, reduced light penetration, smothering of habitat, shielding of pathogens from disinfection Toxic metals and organics Industry, mining, pesticides, tanneries Acute and chronic toxicity, bioaccumulation and biomagnification Acids and alkalis Industry, mine drainage Disruption of pH, mobilisation of metals, corrosion Thermal discharge Power stations, industrial cooling Reduces oxygen solubility while increasing biological oxygen demand — a double penalty Oil and grease Spills, workshops, runoff Films preventing reaeration, coating of organisms, taste and odour • Point sources discharge at a discrete identifiable location and are relatively easy to regulate; non-point or diffuse sources — agricultural runoff, urban drainage, atmospheric deposition — are spread out and are much harder to control, which is why they now dominate the remaining pollution problem in many countries. • Eutrophication is the enrichment of a water body with nutrients.

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Phosphorus is usually the limiting nutrient in fresh water and nitrogen in coastal waters, which is why phosphate was removed from detergents.

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The sequence is nutrient enrichment → algal bloom → death and decay of the algae → oxygen consumption by decomposers → fish kill, and the effect is worst at night and just after a bloom collapses.

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Assessment and Indices • The principal indices, whose definitions and relations are examined: • BOD — the oxygen consumed by micro-organisms in oxidising the biodegradable organic matter, conventionally measured over five days at 20 °C (BOD₅).

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Five days is used because it captures roughly 68-70 per cent of the ultimate BOD and because the test was standardised on the travel time of British rivers to the sea.

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The reaction is first order, BODt = L(1 − 10−kt), where L is the ultimate BOD. • COD — the oxygen required to oxidise all chemically oxidisable matter, measured in about three hours with dichromate.

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COD is always greater than BOD, because it includes matter micro-organisms cannot degrade.

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The ratio indicates biodegradability: above about 0.5 the effluent responds well to biological treatment, and below about 0.3 it does not. • The advantage of COD is speed — three hours against five days — which is why it is used for process control, while BOD remains the parameter in most discharge standards because it reflects the actual oxygen demand on the receiving water. • Other parameters:

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TOC, dissolved oxygen, total and volatile suspended solids, nitrogen forms, phosphorus, and the coliform count. • Sampling: a grab sample represents one instant and is required for dissolved oxygen, pH, temperature, chlorine residual and bacteriological analysis, since these change on storage; a composite sample, made up in proportion to flow over a period, gives the average condition and is used for BOD, COD and solids.

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Which parameters demand a grab sample is a standard question. • Water quality index methods combine several parameters into a single number for communication to the public and for comparing water bodies over time; their weakness is that aggregation can conceal a single parameter that is grossly unacceptable. • Biological monitoring using benthic macro-invertebrates — the Nepalese Biotic Score being the index developed for Nepal's rivers — has the advantage that the organisms integrate conditions over weeks or months and therefore reveal intermittent pollution that spot chemical sampling would miss.

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Standards and Control Strategies • Effluent standards limit the quality of a discharge at the point it leaves the premises; stream or ambient standards specify the quality to be maintained in the receiving water according to its designated use — drinking supply, bathing, fisheries, irrigation or industrial cooling.

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Effluent standards are easy to monitor and enforce but ignore the assimilative capacity of the receiving water; stream standards relate directly to protection but are harder to attribute and enforce.

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Most systems use effluent standards as the primary enforceable instrument. • The control strategy hierarchy, which should be given in order: • 1.

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Prevention and source reduction — cleaner production, process change, substitution of materials. • 2.

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Reuse and recycling within the process. • 3.

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Treatment to the required standard before discharge. • 4.

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Controlled disposal, using the assimilative capacity of the receiving water where that is acceptable. • Additional instruments: regulatory — standards, consents, licensing and enforcement; economic — effluent charges, pollution taxes, tradable permits, subsidies for treatment; and voluntary — cleaner production agreements, certification and public disclosure of discharge data, which has proved surprisingly effective where enforcement capacity is weak. • Catchment or river basin management treats the whole catchment as the management unit, since a discharge standard applied to each source individually may still allow the cumulative load to exceed the river's capacity. • In Nepal the framework is the Environment Protection Act, 2076 and Rules, 2077, with generic and industry-specific effluent standards and tolerance limits for various uses of water; the Water Resources Act, 2049 establishes the priority of uses, with drinking water given first priority.

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The ordering of priority — drinking water first, then irrigation, then other uses — is examined directly.

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Standards are revised periodically and should be verified against current official publication.

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This section covers the site selection, classification, characteristics and design of intakes, the types and purposes of pipe materials, joints, valves and fittings, break pressure tanks, service reservoirs and the determination of their capacity, and the design of branched and looped distribution systems.

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Intakes • An intake is the structure through which raw water is drawn from the source.

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Its functions are to admit water of the best available quality, exclude floating and suspended matter, provide for regulation of the flow, and remain stable and accessible under all conditions of the source. • Site selection criteria: water of the best quality available at that source; a site free from pollution, especially upstream of any settlement or discharge; stable banks and bed, not on the outside of a bend where scour occurs nor in a zone of deposition; adequate depth of water at the lowest stage; protection from floods, ice and debris; sound foundation conditions; and convenient, year-round access for operation and maintenance.

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An intake on a river should be located upstream of the town it serves, not downstream — a point that seems obvious but is examined. • Classification is by the source — river, reservoir, lake, canal or spring — and by the arrangement — submerged or exposed, wet or dry intake.

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In a wet intake the tower is filled with water at the same level as the source; in a dry intake the water enters the conduit directly and the tower stays dry, which makes the tower more vulnerable to flotation when empty but allows the valves to be inspected in the dry. • Characteristics of a well-designed intake: sufficient submergence to avoid drawing in air and floating matter; a low entry velocity, commonly 0.1-0.3 m/s, so that fish and debris are not drawn against the screen; ports at more than one level so that the best quality water can be selected as conditions change; coarse and fine screens with provision for cleaning; a sump or grit chamber; gates or valves for isolation; and access for maintenance at all seasons. • In Nepal, spring intakes and small river or stream intakes are by far the commonest.

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A spring intake consists of a protective chamber built into the eye of the spring with a permeable back wall or collection pipes, a sedimentation compartment, an overflow, a washout and a lockable cover, together with a cut-off drain above to divert surface runoff and fencing to exclude animals.

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A stream intake for a hill scheme is typically a side intake with a small weir, a coarse screen and a silt basin.

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Pipe Materials, Joints, Valves and Fittings Material Advantages Limitations Typical use Galvanised iron (GI) Strong, resists impact, easily threaded, withstands high pressure Corrodes, heavy, limited life in aggressive water Exposed sections, river crossings, high-pressure lengths in hill schemes High density polyethylene (HDPE) Light, flexible, corrosion-free, long coils reduce joints, tolerates ground movement Damaged by ultraviolet light if exposed, softens at high temperature, can be damaged mechanically Buried transmission and distribution mains; the standard pipe for rural schemes Unplasticised PVC (uPVC) Cheap, light, smooth bore, corrosion-free Brittle, especially when cold; degraded by sunlight; rigid so needs careful bedding Buried distribution lines Ductile iron (DI) Strong, tough, high pressure capacity, long life Heavy and costly, needs corrosion protection in aggressive soil Large urban mains Steel Very high strength, suits large diameters and long spans Corrodes, requires lining and coating Large transmission mains, bridge crossings Concrete and prestressed concrete Durable, no corrosion of the concrete, large diameters Heavy, difficult to repair, subject to sulphate attack Large gravity mains • Joints: socket and spigot with rubber ring, which accommodates movement and is quick to make; flanged, which is rigid, demountable and used for pumps, valves and above-ground work; screwed or threaded for GI; welded for steel; solvent-cemented for uPVC; and fusion-welded or compression-fitted for HDPE.

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A flexible joint is used where settlement or thermal movement is expected and a rigid joint where the connection must be demountable. • Valves and their purposes, which are examined as a set: • Sluice or gate valve — isolation; fully open or fully closed, never used for throttling, since partial opening causes vibration and erosion of the seat. • Globe or control valve — regulation of flow, at the cost of a high head loss. • Non-return (check or reflux) valve — permits flow in one direction only; fitted on a pump delivery to prevent reverse flow when the pump stops. • Air valve — fitted at every summit, to release accumulated air, which would otherwise form an air lock and reduce the carrying capacity, and to admit air when the line is drained so that it does not collapse. • Washout or scour valve — fitted at every low point, to drain the line and flush out accumulated sediment. • Pressure-reducing valve — lowers the downstream pressure to a set value. • Float valve — controls the inflow to a tank at a set level. • The pairing to remember is air valves at the high points and washouts at the low points, and it is asked in almost every paper. • Fittings include bends, tees, reducers, couplings, unions, plugs and caps, saddles for service connections, and thrust blocks at bends, tees and dead ends, which resist the unbalanced hydraulic force that would otherwise pull the joints apart.

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Break Pressure Tanks and Service Reservoirs • A break pressure tank is a small tank, open to the atmosphere, placed in a gravity main to reduce the pressure in the pipe below it to zero at that point, so that the static head on the downstream length is limited to the fall below the tank. • Its purposes: to prevent the pressure exceeding the rating of the pipe, so that a lighter and cheaper class can be used; to protect fittings and tapstands from excessive pressure; and to provide a convenient point for flow control, air release and inspection.

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It is placed wherever the static head would otherwise exceed the permissible pressure for the pipe class, and on a long steep hill scheme several may be required in series.

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Break pressure tanks are the characteristic feature of Nepali gravity schemes and are examined directly. • A clear water reservoir at the treatment plant stores treated water to even out the difference between the steady output of the plant and the varying demand on the transmission main, and to give contact time for disinfection. • A service reservoir is located within or near the distribution area.

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Its functions are to balance the hourly variation between the steady inflow and the varying demand; to provide reserve for a breakdown or interruption of supply; to provide reserve for firefighting; to maintain adequate and reasonably constant pressure in the distribution system; and to allow the source, treatment and pumping works to be designed for the maximum daily rather than the peak hourly demand, which is a very substantial economy. • Capacity determination — the standard examination calculation: total capacity = balancing storage + breakdown reserve + fire reserve. • Balancing storage is found from the mass curve, by plotting cumulative demand and cumulative supply against time and taking the sum of the greatest surplus and the greatest deficit; in the absence of hourly data it is commonly taken as one-third to one-half of the maximum daily demand. • Breakdown reserve is usually a quarter to a half of the maximum daily demand, according to the reliability of the supply.

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Fire reserve is calculated from the fire flow and the duration required, and is normally omitted in small rural schemes. • An elevated reservoir is used where the ground is flat and the required pressure cannot otherwise be obtained; a ground-level or underground reservoir is used where high ground is available, which is almost always the case in Nepal's hills and is much cheaper.

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Branched and Looped Distribution Systems Feature Branched (tree, dead-end) system Looped (grid, ring) system Layout Mains branching progressively, each point fed by one route Interconnected loops, each point fed by more than one route Reliability A break cuts off everything beyond it Supply maintained from the other direction during a break or repair Water quality Dead ends where water stagnates, losing chlorine residual and causing taste and odour Circulation throughout, so little stagnation Pressure Falls progressively along the branch; poor at the extremities More uniform, since flow reaches a point by several paths Analysis Simple; flows determined directly by summing demands Requires iterative network analysis, as by Hardy Cross Cost Lower; less pipe and fewer valves Higher Suitability Small towns, villages, irregular development along a road or ridge Towns with a regular street pattern and where reliability matters • The decisive contrast, which is what the examination asks for: the branched system is cheaper and simpler to design but unreliable and prone to stagnation at its dead ends; the looped system costs more and requires network analysis but gives continuity of supply during repairs and avoids stagnation.

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Dead ends should be provided with a washout so that they can be flushed. • Other layouts: the radial system, in which the area is divided into zones each fed from a central reservoir, giving high pressure and quick service; and the ring or circular system, in which a main surrounds the area and branches feed inward. • Design requirements for a distribution system: adequate pressure at every point — commonly a minimum residual of about 10 m of head at the consumer, and often more where multi-storey buildings are served; velocities normally between about 0.6 and 2-3 m/s, the lower limit to prevent sedimentation and the upper to limit head loss and surge; a minimum pipe diameter for maintenance and fire flow; valves placed so that any section can be isolated without shutting down a large area; air valves at summits and washouts at low points; and the system laid at adequate depth with the water main above and at horizontal separation from any sewer, so that leakage from the sewer cannot contaminate the supply.

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The rule that the water main must be laid above the sewer, and never in the same trench, is a certainty in the examination.

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This section covers the objectives of water treatment and the removal of impurities through screening, plain sedimentation, sedimentation with coagulation, filtration, disinfection, softening and miscellaneous treatments, the principles of granular media filtration including slow sand, rapid gravity and roughing filters with backwashing and air scouring, sludge treatment, the calculation of effective size, uniformity coefficient and head loss, and advanced processes.

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The Treatment Train • The conventional sequence is: screening → aeration (where required) → coagulation → flocculation → sedimentation → filtration → disinfection → storage.

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Each unit prepares the water for the next, and the order cannot be altered — sedimentation before filtration, and filtration before disinfection, because turbidity shields organisms from the disinfectant. • Which impurity is removed where: screens remove floating and large suspended matter; aeration removes dissolved gases and oxidises iron and manganese; plain sedimentation removes settleable solids; coagulation and flocculation convert colloids into settleable flocs; sedimentation then removes them; filtration removes the remaining fine and colloidal matter and much of the bacteria; and disinfection destroys the pathogens that remain. • Aeration by cascade, spray, tray or diffused air serves to add oxygen, drive off carbon dioxide, hydrogen sulphide and volatile taste and odour compounds, and oxidise iron and manganese to insoluble forms that can then be settled and filtered.

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Screening and Sedimentation • Coarse screens (bar racks) with openings of 50-100 mm remove large debris; fine screens of 5-20 mm remove smaller material.

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Screens are cleaned manually or mechanically, and the approach velocity is kept around 0.6-1.0 m/s to prevent both deposition and excessive head loss. • Plain sedimentation removes discrete particles under gravity.

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The governing quantity is the surface overflow rate, SOR = Q/As, which has the units of velocity and equals the settling velocity of the smallest particle that is completely removed. • The fundamental and most examined result of ideal settling theory is that the removal of discrete particles depends only on the surface area of the tank and not on its depth, because a particle entering at the surface must settle the full depth in the time it takes to traverse the tank, and both depth terms cancel.

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Depth matters in practice for sludge storage, for avoiding scour and for hydraulic stability, but not for the theoretical efficiency.

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This conclusion is the basis of the tube and plate settler, which multiplies the effective settling area within the same tank volume. • Detention time t = V/Q, commonly 2-4 hours in a plain sedimentation tank and 1.5-3 hours after coagulation.

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Typical surface overflow rates are 15-30 m³/m²·d for plain sedimentation and 20-40 m³/m²·d with coagulation. • Zones of a settling tank: inlet, settling, sludge and outlet.

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The inlet must distribute the flow uniformly across the full section and dissipate its energy, and the outlet weir must draw off uniformly; short-circuiting caused by poor inlet or outlet arrangement, by density currents or by wind is the commonest reason a tank performs below its theoretical capacity. • Types: horizontal flow rectangular, radial flow circular, upflow clarifier, and the solids-contact clarifier, which combines mixing, flocculation and settling in one unit and uses a sludge blanket to improve floc formation.

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Coagulation and Flocculation • Colloidal particles of 1 nm to 1 μm carry a negative surface charge, so they repel one another and remain in suspension indefinitely — the settling time of a 1 μm clay particle is measured in years.

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Coagulation destabilises them and flocculation brings them together. • The distinction must be exact, because it is examined constantly: coagulation is the chemical destabilisation of the colloids by addition of a coagulant, accomplished in a rapid mix of 30-60 seconds at high velocity gradient; flocculation is the slow, gentle stirring for 20-40 minutes at low velocity gradient which brings the destabilised particles into contact so that they aggregate into settleable flocs.

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Mixing that is too vigorous during flocculation breaks the flocs up again, which is why the velocity gradient must be low and is often tapered, decreasing along the flocculator. • Mechanisms of coagulation: double layer compression, charge neutralisation, sweep coagulation by enmeshment in the precipitating hydroxide, and interparticle bridging by polymers. • Coagulants: alum, Al₂(SO₄)₃·18H₂O, is the commonest and cheapest and works best at pH 5.5-8.0; ferric chloride and ferric sulphate work over a wider pH range, 4-11, and give a denser floc; polyaluminium chloride requires a smaller dose and consumes less alkalinity; and lime and soda ash are used as coagulant aids as well as for softening. • Alum consumes alkalinity, so where the natural alkalinity is insufficient the pH falls and coagulation fails; lime or soda ash must then be added.

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This alkalinity requirement is a standard examination point. • Coagulant aids — polyelectrolytes, activated silica, bentonite clay and adjustment of pH — strengthen the floc, increase its density and reduce the coagulant dose. • The jar test is the standard laboratory method for determining the optimum coagulant dose and pH, since the optimum depends on the particular raw water and cannot be calculated theoretically — a point often asked.

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Filtration • Mechanisms of granular media filtration: straining at the surface, sedimentation within the pores, interception, adsorption and, in a slow sand filter, biological action.

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Straining alone accounts for only a small part of the removal in a rapid filter, where depth filtration within the bed dominates. • Effective size and uniformity coefficient — the two definitions that appear in every paper: • Effective size, D₁₀, is the sieve size in millimetres through which 10 per cent of the sand by weight passes. • Uniformity coefficient is D₆₀/D₁₀, the ratio of the size passing 60 per cent to the effective size.

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A uniformity coefficient of 1 would mean perfectly uniform sand, and the lower the value the more uniform the medium.

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Typical values are D₁₀ of 0.2-0.4 mm with a uniformity coefficient of 2-3 for slow sand filters, and D₁₀ of 0.4-0.7 mm with a uniformity coefficient of 1.3-1.7 for rapid gravity filters. • Head loss through a clean bed is given by the Carman-Kozeny or Rose equation, and it increases as the filter clogs; filtration is stopped and the filter cleaned when the head loss reaches the available head — typically 2-3 m in a rapid filter — or when the filtrate quality deteriorates, whichever occurs first. • Negative head develops when the head loss in the upper part of the bed exceeds the depth of water above it, so the pressure there falls below atmospheric; dissolved air comes out of solution and forms bubbles that blind the bed, a condition called air binding.

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Providing sufficient depth of water over the bed prevents it.

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Feature Slow sand filter Rapid gravity filter Filtration rate 0.1-0.2 m/h (2-5 m³/m²·d) 5-15 m/h (120-360 m³/m²·d) Effective size 0.2-0.4 mm 0.4-0.7 mm Feature Slow sand filter Rapid gravity filter Uniformity coefficient 2-3 1.3-1.7 Area required Large, about 50 times that of a rapid filter Small Pre-treatment No coagulation required; plain sedimentation only Coagulation and flocculation essential Cleaning Scraping off the top 20-25 mm of sand, every 1-3 months Backwashing, every 24-48 hours Removal mechanism Biological action in the schmutzdecke plus physical straining Predominantly physical, within the depth of the bed Bacterial removal Very high, 99-99.9 per cent Lower, 90-99 per cent; disinfection essential Operation Simple, little skill, no chemicals Skilled operation, chemicals, power for backwashing Suitability Low turbidity raw water, small communities, rural areas High and variable turbidity, urban supplies • The schmutzdecke, the biological layer that forms at the surface of a slow sand filter after a ripening period of a few days to weeks, is responsible for much of its purification, and this biological action is what a rapid filter lacks.

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When the filter is scraped, only the top 20-25 mm is removed and the schmutzdecke must re-form, so the filtrate is run to waste until it ripens again.

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Slow sand filtration is particularly well suited to Nepali rural schemes because it needs no chemicals, no power and little skill. • Backwashing reverses the flow at 0.4-1.0 m/min to expand the bed by 20-50 per cent, fluidising the grains so that the accumulated material is released and carried away, for 5-10 minutes.

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Air scouring, in which air is blown up through the bed before or with the water, greatly improves cleaning by breaking up mudballs and dislodging material that water alone cannot remove, and it reduces the quantity of wash water needed.

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Wash water use is typically 1-5 per cent of the filtered output, and excessive expansion is wasteful and can carry media out of the filter. • A roughing filter is a coarse-media filter, of gravel graded from coarse to fine in the direction of flow, placed before a slow sand filter to reduce a high turbidity that would otherwise clog it.

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It may be horizontal flow, upflow or downflow, and it acts principally by sedimentation within the pores rather than by straining.

26

Roughing filtration with slow sand filtration — multi-stage filtration — is the standard arrangement for turbid hill streams in Nepal, and it avoids the need for chemical coagulation entirely. • Dual and multi-media filters place coarse light anthracite above fine heavy sand, so that the water meets coarse media first and the whole depth of the bed is used, giving a longer run and greater capacity than a single medium in which the finest grains end up on top after backwashing.

27

Disinfection • Disinfection destroys pathogenic organisms; sterilisation destroys all organisms and is neither necessary nor practicable for a public supply. • Chlorine is the standard disinfectant because it is effective, cheap, easily applied and measured, and — uniquely among the common disinfectants — leaves a residual that protects the water throughout the distribution system.

28

The residual is the decisive advantage and is examined directly. • Chemistry:

29

Cl₂ + H₂O → HOCl + HCl, and HOCl ⇌ H⁺ + OCl⁻.

30

Hypochlorous acid and hypochlorite ion together constitute the free available chlorine, and HOCl is some 80-100 times more effective than OCl⁻.

31

Since the dissociation is governed by pH, with HOCl dominant below pH 7.5 and OCl⁻ above it, chlorination is far more effective in slightly acidic water and much less so in alkaline water.

32

This pH dependence is the most examined fact about chlorination. • Combined chlorine (chloramines) forms when chlorine reacts with ammonia; it is a weaker and slower disinfectant but far more persistent, so it is used to maintain a residual in a long distribution system. • Breakpoint chlorination: as chlorine is added, it is first consumed by reducing agents, then forms chloramines whose concentration rises, then destroys those chloramines so that the measured residual falls to a minimum — the breakpoint — beyond which further addition appears entirely as free residual chlorine.

33

Dosing beyond the breakpoint is necessary to obtain a free residual. • Factors affecting efficiency: contact time, concentration, pH, temperature, turbidity and the nature of the organism.

34

The product of concentration and contact time, the CT value, is the measure used in practice.

35

Turbidity must be removed first, because particles shield organisms from the disinfectant — which is why filtration always precedes disinfection. • A residual of 0.2-0.5 mg/L free chlorine is normally maintained at the far end of the distribution system, with a contact time of at least 30 minutes. • Disinfection by-products: chlorine reacts with natural organic matter to form trihalomethanes and haloacetic acids, some of which are suspected carcinogens.

36

They are minimised by removing the organic precursors before chlorination rather than by reducing the disinfection, since the risk from inadequate disinfection is immediate and far greater than the long-term risk from by-products — a judgement worth stating explicitly. • Alternatives: ozone, the most powerful, which also removes taste, odour and colour but leaves no residual and is costly; ultraviolet radiation, which is effective against bacteria, viruses and, importantly, the chlorine-resistant protozoa Cryptosporidium and Giardia, but also leaves no residual and requires clear water; and, at household level, boiling, solar disinfection (SODIS), chlorine tablets and ceramic or biosand filters.

37

That ozone and UV leave no residual is their common and decisive limitation for a piped system.

38

Softening, Sludge and Advanced Processes • Hardness is caused by calcium and magnesium ions and is expressed as mg/L of CaCO₃.

39

Carbonate (temporary) hardness is associated with bicarbonate and is removed by boiling or by lime; non-carbonate (permanent) hardness is associated with sulphate and chloride and requires soda ash or ion exchange.

40

Water of under 75 mg/L is soft and over 300 mg/L very hard.

41

Hard water wastes soap, forms scale in boilers and pipes, and spoils cooking; very soft water is corrosive. • Softening methods: the lime-soda process, in which lime removes carbonate hardness and soda ash non-carbonate hardness, with excess lime used where magnesium must be removed; the zeolite or ion exchange process, which exchanges calcium and magnesium for sodium and can reduce hardness to almost zero but adds sodium and cannot handle turbid water; and membrane processes.

42

Blending softened with unsoftened water is usual, since water of zero hardness is corrosive. • Sludge treatment: alum sludge from the clarifiers is thickened by gravity, conditioned, dewatered on drying beds, by centrifuge or filter press, and then disposed of by landfill or land application; lagoons are used in small plants.

43

Alum sludge is gelatinous and very difficult to dewater, which is the principal practical difficulty.

44

Discharging clarifier and backwash sludge directly to a watercourse is unacceptable and is a common failing of small plants. • Advanced processes: • Activated carbon — powdered activated carbon dosed before sedimentation for intermittent taste and odour problems, or granular activated carbon in a bed for continuous removal of organic compounds, taste, odour and colour by adsorption. • Ion exchange — used for softening, demineralisation, nitrate removal and arsenic removal, with the resin regenerated by brine or acid and alkali; the spent regenerant is a concentrated waste needing disposal. • Membrane processes, whose order must be known: microfiltration removes suspended solids, protozoa and most bacteria; ultrafiltration adds viruses and macromolecules; nanofiltration removes divalent ions and is therefore used for softening; and reverse osmosis removes monovalent ions, giving desalination.

45

The required pressure and the cost rise in that order, and reverse osmosis requires the highest pressure because it must overcome the osmotic pressure of the solution.

46

All membrane processes suffer from fouling and require pre-treatment, and all produce a concentrate that must be disposed of. • Iron and manganese removal by aeration followed by sedimentation and filtration, or by contact filtration with an oxidising medium.

47

Arsenic removal by oxidation to the pentavalent form followed by coagulation with iron salts, adsorption on activated alumina or iron-based media, or membrane treatment; the trivalent form must be oxidised first, because it is far less readily removed.

48

Fluoride removal by activated alumina, bone char or the Nalgonda technique using alum and lime.

49

Defluoridation and arsenic removal are both of direct relevance in parts of Nepal.

1

This section covers the systems of supply, clear water and service reservoirs and the determination of their capacity, the layout and design of distribution systems including branched and looped networks, network analysis, non-revenue water and the strategies for its management.

2

Systems of Supply • Continuous supply delivers water at all hours.

3

Its advantages are that the mains remain pressurised so that contamination cannot be drawn in through leaks, water is always available, less household storage is needed, and leaks are detected because they are always flowing.

4

It requires more water and better management. • Intermittent supply delivers for a few hours a day.

5

Its apparent advantage is that it saves water and allows a smaller scheme, but the disadvantages are severe and are examined directly: when the mains are depressurised, contaminated groundwater is drawn in through leaks and joints, so the water quality is compromised; consumers must store water, with further risk of contamination; households with storage draw more than their share, so the supply is inequitable; the repeated filling and emptying causes surge and accelerates deterioration of the pipes; and leakage is concealed because the leaks do not flow while the system is empty.

6

Continuous supply is therefore the objective everywhere, and intermittent supply is a symptom of inadequate resource or management rather than a design choice. • Methods of distribution: gravity, where the source is high enough and no pumping is required, which is the ideal and is the usual case in Nepal's hills; direct pumping, in which pumps feed the distribution system directly, requiring variable output and giving no reserve if power fails; and combined pumping with storage, in which pumps deliver at a steady rate into an elevated or ground reservoir which then supplies the network, giving reserve, steady pump operation and constant pressure.

7

The combined system is the commonest and is generally the best, and the reason is that the reservoir absorbs the demand variation so the pump can run steadily at its best efficiency point.

8

Reservoirs and Capacity • A clear water reservoir at the plant provides disinfection contact time and evens out the difference between plant output and transmission demand; a service reservoir in the distribution area balances the hourly variation, provides breakdown and fire reserve, and maintains pressure. • Capacity = balancing storage + breakdown reserve + fire reserve, as set out in 5.3. • The mass curve method: cumulative demand and cumulative supply are plotted against time over the day; the required balancing storage is the sum of the maximum ordinate above the supply line and the maximum ordinate below it.

9

Equivalently, in tabular form, the storage is the difference between the maximum cumulative surplus and the maximum cumulative deficit. • Where hourly demand data are unavailable, balancing storage is commonly taken as one-third of the maximum daily demand for a continuous supply, and the total capacity as one-third to one-half of the maximum daily demand. • Location and elevation: the reservoir should be as close to the centre of demand as possible, to minimise the length of large mains; high enough to deliver the minimum residual pressure at the most unfavourable point, usually the highest and most distant; and not so high that the pressure at the lowest point exceeds the pipe rating.

10

In flat terrain an elevated tank is required; in hilly terrain a ground-level reservoir on high ground serves the same purpose far more cheaply, which is why elevated tanks are rare in Nepal's hills. • Construction: reservoirs are watertight, covered against contamination and light (which would promote algal growth), ventilated with screened vents, provided with inlet, outlet, overflow, washout and bypass, with the inlet and outlet arranged at opposite ends so that the water circulates rather than short-circuits, and with access for inspection and cleaning.

11

Two compartments are provided wherever possible so that one can be cleaned while the other remains in service.

12

Distribution Layout, Design and Analysis • The four layouts: dead-end or branched; gridiron or looped; circular or ring; and radial.

13

Their characteristics were compared in 5.3; the looped system is preferred wherever reliability and water quality matter, and the branched system where cost governs and development is linear. • Design criteria: the minimum residual pressure at the consumer, commonly taken as about 10 m of head for a single-storey supply and more where taller buildings are served; the maximum pressure, limited by the pipe class and by the tendency of high pressure to increase leakage; velocities of about 0.6-3 m/s; and a minimum pipe size for maintenance. • A point worth carrying: leakage increases with pressure, so pressure management — reducing excess pressure at night and in low-lying zones with pressure-reducing valves — is one of the cheapest and most effective means of reducing losses. • Analysis of a branched system is direct: the flow in each pipe is the sum of the demands it serves, and the head loss is computed from Hazen-Williams or Darcy-Weisbach working back from the critical point. • Analysis of a looped network requires iteration, because the flow distribution is not determined by continuity alone.

14

Two conditions must be satisfied: the algebraic sum of flows at each junction is zero, and the algebraic sum of head losses around each closed loop is zero.

15

These are the analogues of Kirchhoff's laws. • The Hardy Cross method assumes a flow distribution satisfying continuity and applies a correction to each loop, ΔQ = −Σhf/(n Σ|hf/Q|), with n = 2 for Darcy-Weisbach and 1.85 for Hazen-Williams, repeating until the loop head losses balance.

16

Modern practice uses computer models such as EPANET, which solve the equations simultaneously and also model water quality and chlorine decay. • Equivalent pipe: a single pipe that would give the same head loss for the same discharge as the system it replaces, used to simplify a network before analysis.

17

Non-Revenue Water • Non-revenue water (NRW) is the difference between the volume of water put into the system and the volume for which revenue is collected.

18

The definition and its components are examined directly. • Its three components: • 1.

19

Real (physical) losses — leakage from mains, service connections and reservoirs, and reservoir overflow.

20

This is water physically lost. • 2.

21

Apparent (commercial) losses — unauthorised consumption or theft, meter under-registration, and errors in meter reading, billing and data handling.

22

The water reaches a consumer but is not paid for. • 3.

23

Unbilled authorised consumption — water used legitimately without charge, such as firefighting, mains flushing and public standposts. • Note that NRW is not the same as leakage: leakage is only the real-loss component, and in many systems the apparent losses are as large.

24

This distinction is the point most often missed. • NRW in Nepali urban systems has commonly been reported in the range of 30-40 per cent or higher, against a well-managed international level of under 20 per cent; figures vary between towns and over time and should be checked against current utility data. • Why it matters: it wastes a scarce resource and the energy and chemicals spent treating it; it deprives the utility of the revenue needed for operation and maintenance, creating a downward spiral in which poor revenue causes poor maintenance which causes more losses; it lowers the pressure and hence the service to consumers; and leaks are a route by which contamination enters a depressurised main. • Management strategies, which should be given in a structured list: • Measurement and district metering — the system cannot be managed until it is measured, so bulk metering and the establishment of district metered areas with permanent flow monitoring come first.

25

Minimum night flow analysis, which measures flow in the small hours when legitimate consumption is least, is the standard method of estimating leakage in a zone. • Active leakage control — systematic survey with acoustic and correlating equipment, rather than waiting for leaks to become visible; most water is lost through small leaks that never surface. • Speed and quality of repair — the loss from a leak is proportional to the time it runs. • Pressure management — reducing excess pressure reduces both the leakage rate from existing leaks and the frequency of new bursts. • Asset management and pipe replacement — renewing the pipes with the worst burst record. • For apparent losses — universal metering, a programme of meter testing and replacement, regularisation of illegal connections, and improvement of the billing system and customer database. • Institutional measures — a dedicated NRW unit, staff training, and an accurate and complete asset inventory.

1

This section covers fluid properties, pressure and head, the hydraulic gradient, positive displacement and centrifugal pumps, pump and system curves, pump components and selection, pressure transient or surge analysis, pumping station and sump design, pumping system design and solar pumping systems.

2

Heads and the Hydraulic Gradient • The heads must be distinguished precisely, since the examination turns on them: • Static suction lift or head — the vertical distance from the water surface to the pump centreline, a lift if the water is below the pump and a head if above. • Static delivery head — from the pump centreline to the delivery water surface. • Total static head — the vertical distance between the two water surfaces, independent of the pipe route. • Total dynamic head (TDH) — total static head plus all friction and minor losses plus the residual velocity head.

3

This is the head the pump must actually generate, and it is the quantity used in selection. • The hydraulic grade line plots p/ρg + z and the total energy line adds the velocity head; across a pump both rise abruptly by the head added, and thereafter the TEL falls steadily because of friction, as set out in Chapter 2. • Pump power: water (hydraulic) power = ρgQH; shaft or brake power = ρgQH/ηpump; and motor input power = ρgQH/(ηpump × ηmotor).

4

Overall efficiency is the product of the individual efficiencies, and typical centrifugal pump efficiencies are 60-85 per cent.

5

Types of Pump Class Types Characteristics Application Centrifugal (rotodynamic) Radial, mixed and axial flow; single and multistage; submersible; vertical turbine Smooth non-pulsating flow; discharge falls as head rises; requires priming; handles suspended matter; simple and cheap The great majority of water supply duties Reciprocating (positive displacement) Piston, plunger, diaphragm Fixed discharge almost independent of head; very high head possible; self-priming; pulsating flow; requires a relief valve Chemical dosing, small high-head duties, sludge Rotary positive displacement Gear, screw, lobe, vane, peristaltic Steady flow, handles viscous liquids, self-priming Sludge and chemical transfer Special Jet, air-lift, hydraulic ram No moving parts in the liquid or none at all; low efficiency Deep wells, aerated water, and the hydram, which needs no external power at all • The essential contrast between a centrifugal and a positive displacement pump is examined directly: the centrifugal pump's discharge depends strongly on the head against which it works and falls to zero at the shut-off head, whereas the positive displacement pump delivers an almost fixed volume per stroke or revolution regardless of head, so its delivery must never be closed against without a relief valve or the pipe or pump will burst. • The hydraulic ram (hydram) deserves note in the Nepali context: it uses the energy of a large flow falling a small distance to lift a small flow a large distance, with no external power and only two moving parts, and it is well suited to hill springs and streams where a small elevated supply is needed. • Pump components: impeller, casing (volute or diffuser), shaft, bearings, stuffing box or mechanical seal, wear rings, suction and delivery nozzles, and the coupled motor.

6

A mechanical seal leaks less and needs less attention than gland packing but costs more and cannot be adjusted in service.

7

Pump and System Curves • The pump characteristic curve plots head against discharge for a given speed and impeller, and it falls as discharge increases; the efficiency and power curves are plotted on the same axes.

8

The best efficiency point (BEP) is the discharge at which efficiency is greatest, and a pump should be selected so that its duty point lies at or close to the BEP.

9

Operating far from the BEP causes recirculation, vibration, noise, bearing and seal wear and shortened life, which is why oversizing a pump is a mistake rather than a safety margin — a point worth stating explicitly. • The system curve plots the head the system requires against discharge:

10

H = static head + kQ², so it starts at the static head at zero flow and rises as a parabola. • The operating (duty) point is the intersection of the pump curve and the system curve, and this is the single most examined idea in the section.

11

It follows that the operating point moves when either curve changes: closing a valve steepens the system curve and moves the point to lower flow and higher head; an increase in static lift shifts the system curve upward; and the roughening of the pipes with age steepens it gradually, so that the delivery of an ageing system falls. • Pumps in series add their heads at the same discharge, and are used where the head is too great for one pump; pumps in parallel add their discharges at the same head, and are used where the flow is too great, or to match a variable demand by running one, two or three units.

12

Note that two identical pumps in parallel do not deliver twice the flow, because the system curve rises with discharge, so the combined duty point occurs at a higher head where each pump delivers less than it would alone — a standard examination trap. • Affinity laws for a given pump:

13

Q ∝ N, H ∝ N² and P ∝ N³, so a small reduction in speed gives a large reduction in power, which is the basis of variable speed drive as an energy-saving measure. • Specific speed Ns = N√Q/H3/4 classifies the impeller: a low specific speed indicates a radial flow impeller for high head and low flow, and a high specific speed an axial flow impeller for low head and high flow. • NPSH and cavitation:

14

NPSH available = (patm − pvapour)/ρg − static suction lift − suction friction loss.

15

NPSH required is a property of the pump, given by the manufacturer and increasing with discharge.

16

Cavitation is avoided by keeping NPSHA greater than NPSHR, with a margin of at least 0.5-1.0 m.

17

Remedies, in order of preference: lower the pump or raise the water level to reduce the suction lift; shorten and enlarge the suction pipe and remove unnecessary fittings; reduce the flow; cool the liquid; or select a pump with a lower NPSH requirement.

18

Note that altitude reduces atmospheric pressure and therefore reduces NPSH available, which matters in Nepal's hills.

19

Surge, Station and Sump Design • Pressure transients (surge or water hammer) arise from pump start-up and, far more dangerously, from sudden pump stoppage on power failure, and from rapid valve operation.

20

The downsurge following a pump trip can reduce the pressure to vapour pressure, causing column separation whose subsequent rejoining produces a far greater pressure rise than the original surge. • Protection: slow-closing valves; a flywheel on the pump set to extend the run-down time; an air vessel or surge tank; surge-anticipating and pressure-relief valves; air valves at summits to admit air and prevent column separation; a bypass around the pump with a non-return valve; and selection of a pipe class with adequate margin.

21

The analysis was set out in Chapter 2. • Pumping station design: locate above the design flood level, or protect it; provide adequate space for installation, operation and removal of the largest unit; arrange for lifting by hoist or crane; provide standby capacity so that the design flow can be delivered with the largest unit out of service; provide adequate ventilation, lighting and drainage; allow for noise and vibration control; ensure secure and reliable power with standby generation where the supply is unreliable; and provide flow, pressure and power metering.

22

The standby requirement — that the duty flow must be maintained with the largest pump out of action — is examined directly. • Sump or wet well design: the volume must be sufficient to limit the number of pump starts per hour, since frequent starting overheats the motor; the usual criterion is a minimum cycle time giving no more than 6-10 starts per hour for small motors and fewer for large, and the required volume for a single pump is V = QpT/4, where T is the minimum cycle time. • Other sump requirements: adequate submergence over the suction bell to prevent vortex formation and air entrainment, which would cause loss of capacity and vibration; a smooth approach flow without sudden changes of direction or velocity, with the pumps not placed one behind another; a hopper-shaped floor sloping to the suction so that solids do not settle; and provision for screening, isolation and cleaning.

23

Vortex formation from inadequate submergence is the commonest hydraulic fault in sump design.

24

Solar Pumping • A solar photovoltaic pumping system comprises a PV array, a controller or inverter, the pump and motor — commonly a submersible with a brushless DC or an AC motor driven through an inverter — and a storage tank, together with cabling, a mounting structure and protection. • The defining design principle is that water is stored rather than electricity: a tank sized for two to three days' demand is far cheaper, longer-lived and less troublesome than a battery bank, and it is also more robust, since a battery fails long before the array does.

25

This point is examined directly. • Sizing proceeds from the daily water requirement and the total dynamic head to the hydraulic energy needed, which is divided by the wire-to-water efficiency to give the electrical energy, and then by the peak sun hours at the site — commonly 4-5 hours a day in Nepal — to give the array size.

26

The array must be sized for the worst month, not the annual average, and the worst month is usually in the monsoon when cloud reduces insolation, or in winter when the days are short. • Advantages: no fuel cost and no fuel supply problem; very low maintenance with no moving parts in the array; long life of 20-25 years for the modules; no emissions and no noise; suits remote sites with no grid; and the output is naturally greatest in the dry sunny season when water demand is highest. • Limitations: high capital cost; output varies with the weather and is nil at night; requires storage; theft of panels is a real problem at remote unattended sites; and the array must be kept clean and unshaded, since shading of even a part of a module disproportionately reduces the output of the whole string. • Solar pumping has become an important technology in Nepal for rural water supply and small-scale irrigation at sites without reliable grid power, and its economics compare increasingly favourably with diesel pumping once fuel transport to remote hill sites is taken into account.