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DistributionSystem

Manesh P. MallaAsst. Prof.

Civil EngineeringDepartment

Nepal Engineering College

Course ContentLayout Of Distribution Networks,Systems Of Supply,Methods Of Water Distribution,ESR,Underground Service Reservoir,And Storage capacity of balancingreservoir

Introduction…

The purpose of distribution system is to deliver water toconsumer with appropriate quality, quantity and pressure.

Distribution system is used to describe collectively thefacilities used to supply water from its source to the point ofusage.

Requirements of Good DistributionSystem...

Water quality should not get deteriorated in thedistribution pipes.

It should be capable of supplying water at all the intendedplaces with sufficient pressure head.

It should be capable of supplying the requisite amount ofwater during fire fighting.

The layout should be such that no consumer would bewithout water supply, during the repair of any section of thesystem.

All the distribution pipes should be preferably laid onemetre away or above the sewer lines.

It should be fairly water-tight as to keep losses due toleakage to the minimum.

Layouts of Distribution NetworkThe distribution pipes are generally laid below the road

pavements, and as such their layouts generally follow thelayouts of roads.

There are, in general, four different types of pipe networks;any one of which either singly or in combinations, can beused for a particular place.

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They are:

Dead End System

Radial System

Grid Iron System

Ring System

Dead End System...It is suitable for old towns and cities having no definite

pattern of roads.

Advantages Relatively cheap. Determination of discharges and pressure easier due to less number of cut off valves.Pipe laying simple.Disadvantages Due to many dead ends, stagnation of water occurs in pipes. No water if system fails Many scour valves required Less maintaining of pressure in the far areas. Less water for fire fighting

Radial System...

The area is divided into different zones.

The water is pumped into the distribution reservoir kept inthe middle of each zone.

The supply pipes are laid radially ending towards theperiphery.

Advantages:It gives quick service.Calculation of pipe sizes is easy.

Grid Iron System...It is suitable for cities with rectangular layout, where the

water mains and branches are laid in rectangles.

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Advantages

Water is kept in good circulation due to the absence ofdead ends.

In the cases of a breakdown in some section, water isavailable from some other direction.

Disadvantages

Exact calculation of sizes of pipes is not possible due toprovision of valves on all branches.

Ring System...

The supply main is laid all along the peripheral roadsand sub mains branch out from the mains.

This system also follows the grid iron system with theflow pattern similar in character to that of dead endsystem.

So, determination of the size of pipes is easy.

Advantages

Water can be supplied to any point from at least twodirections.

SYSTEMS OF SUPPLY

(a) Continuous systemWater is supplied to the consumers for all 24 hours of a day

Advantages:Water every time, No stagnant water, Adequate quantity forfire fighting, no need of reservoir

Disadvantages:More wastage, if leakage large amount spilled, supplyinterrupted during maintenance

(a) Continuous system(b) Intermittent system

(b) Intermittent systemWater is supplied to the consumers only during fixed hours

Advantages: Useful when pressure or quantity of water is not available Water can be supplied by turn. Repairing work can be done in non-supply hours. Leakage causes less waste

Disadvantages:Inconvenience, domestic tank needed, no water at fire, loss ofwater, greater dia pipe needed, suction effect, more valvesrequired

Methods of water distribution…

For efficient distribution system adequate water pressurerequired at various points.

Depending upon the level of source, topography of the areaand other local conditions the water may be forced intodistribution system by following ways -

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1. Gravity system

2. Pumping system

3. Combined gravity and pumping system

Gravity system…Suitable when source of supply is at sufficient height.

Most reliable and economical distribution system.

The water head available at the consumer is just minimumrequired.

The remaining head is consumed in the frictional andother losses.

Pumping system…Treated water is directly pumped in to the distribution main

with out storing.

Also called pumping without storage system.

High lifts pumps are required.

If power supply fails, complete stoppage of water supply.

This method is not generally used.

Combined gravity and pumping system

Most common system.

Treated water is pumped and stored in an elevateddistribution reservoir.

Then supplies to consumer by action of gravity.

The excess water during low demand periods get stored inreservoir and get supplied during high demand period.

Economical, efficient and reliable system.

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Distribution Reservoirs...

Distribution reservoirs, also called service reservoirs, arethe storage reservoirs, which store the treated water forsupplying water during emergencies (such as during fires,repairs, etc.) and also to help in absorbing the hourlyfluctuations in the normal water demand.

Functions of Distribution Reservoirs to absorb the hourly variations in demand. to maintain constant pressure in the distribution

mains. water stored can be supplied during emergencies.

Location and Height of Distribution Reservoirs should be located as close as possible to the centre

of demand. water level in the reservoir must be at a sufficient

elevation to permit gravity flow at an adequatepressure.

Types of Reservoirs...

Depending upon their elevation w.r.t ground it may beclassified into

1. Surface reservoirs

2. Elevated reservoirs

Surface reservoirs…

These also called ground reservoir.

Mostly circular or rectangular tank.

Under ground reservoirs are preferred especially when thesize is large.

These reservoirs are constructed on high natural groundsand are usually made of stones, bricks, plain or reinforcedcement concrete.

The side walls are designed to take up the pressure of thewater, when the reservoir is full and the earth pressurewhen it is empty.

The position of ground water table is also considered whiledesigning these reservoirs.

The floors of these reservoirs may constructed with R.C.Cslab or square stone blocks resting on columns.

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To obtain water tightness bitumen compounds are used atall construction joints.

At the top of roof about 60cm thick earth layer isdeposited and maintained green lawns to protect thereservoir from cold and heat.

For aeration of water and inspection, ventilation pipes andstairs are provided.

Under Ground Reservoir

TYPES OF TANKS

R.C.C TANKS: R.C.C tanks are very popular because1) They have long life2) Very little maintenance3) decent appearance

G.I. TANKS: G.I. tanks are generally in rectangular orsquare in shape. Now a days G.I. tanks are not preferringbecause1) Life of the tank is short2) Corrosion of metal3) maintenance cost may be more

HDPE TANKS: Now a days HDPE tanks are very popularfor storing less quantity of water and hence useful forresidential purpose. The following are the advantages ofHDPE tanks1) Handling is easy because of light weight2) Cheap in cost3) Maintenance cost is low4) Cleaning of tanks are easy

ESR...Elevated Storage Reservoirs (ESRs) also referred to as

Overhead Tanks are required at distribution areas whichare not governed and controlled by the gravity system ofdistribution.

These are rectangular, circular or elliptical in shape.

If the topography of the town not suitable for under gravity,the elevated tank or reservoir are used.

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They are constructed where combine gravity and pumpingsystem of water distribution is adopted.

These tanks may be steel or RCC.

Now RCC is commonly preferred.

The accessories of ESR are-

Inlet and outlet pipe, overflow pipe discharging into a drain

Float gauge, indicating depth of water.

Automatic device to stop pumping when the tank is full.

A manhole and ladder.

Ventilator for circulation of fresh air.

Storage Capacity of DistributionReservoirs...

The total storage capacity of a distribution reservoir is thesummation of:

Balancing Storage: The quantity of water required to bestored in the reservoir for equalising or balancingfluctuating demand against constant supply is known as thebalancing storage (or equalising or operating storage).

Breakdown Storage: The breakdown storage or oftencalled emergency storage is the storage preserved in orderto tide over the emergencies posed by the failure of pumps,electricity, or any other mechanism driving the pumps.

A value of about 25% of the total storage capacity ofreservoirs, or 1.5 to 2 times of the average hourly supply,may be considered as enough provision for accounting thisstorage.

Fire Storage: The third component of the total reservoirstorage is the fire storage.

This provision takes care of the requirements of water forextinguishing fires.

A provision of 1 to 4 per person per day is sufficient tomeet the requirement.

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When reserve storage is elevated, amount of fire reservemay be determined byR= (F-P) TR= Reserve storage (liters)F= Fire demand, liters/minP= Reserve fire pumping capacity, liters/minT= Duration of the fire in min

The total reservoir storage can finally be worked out byadding all the three storages.

Analytical Method1. Hourly demand ( outflow) and Supply (inflow) for 24 hrs2. Cumulative inflow and outflow3. Cumulative surplus and deficit4. Max of cumulative deficit (MCD), surplus (MCS), total

outflow (TO) and total inflow (TI)5. Capacity of balancing reservoir (CBR:

i. If TI > TO, CBR = MCS + MCD – TI + TO andii. If TI ≤ TO, CBR = MCS + MCD

ExampleA village in Mid-western development region of Nepal has a design yearpopulation of 500. Per capita demand recommended for that particularvillage is 65 lpcd. The demand is to meet by continuous system of supplyfrom spring source with safe yield of 0.5 lps. The consumption pattern in %of a day is as follows. Is balancing reservoir necessary? Calculate itscapacity by analytical if necessary?

Here,Maximum of cumulative surplus (MCS) = 10700 liters,Maximum of cumulative deficit (MCD) = 6900 litersTotal supply (TI) = 43200 liters,Total demand (TO) = 32500 liters

Now, for TI > TO,Capacity of balancing reservoir (V) = MCS+ MCD-TI + TO= 10700 + 6900-43200 + 32500 = 6900 liters

Membrane Filtration and SODIS

1 Manesh P. Malla (nec)

Membrane filtration

Membrane filters are widely used for filtering both drinking water and sewage. For drinking water, membrane filters can remove virtually all particles larger than 0.2 ᵤ m including giardia and cryptosporidium. Membrane filters are an effective form of tertiary treatment when it is desired to reuse the water for industry, for limited domestic purposes, or before discharging the water into a river that is used by towns further downstream. They are widely used in industry, particularly for beverage preparation (including bottled water). However no filtration can remove substances that are actually dissolved in the water such as phosphorus, nitrates and heavy metal ions.

Processes: Reverse Osmosis: Small solute particles to be separated. Molecular weight < 100 Pore size: 2 – 10 A0 Pressure: > 25 atm. Permeation is main transport mechanism Example: Filtration of salt solution Ultrafiltration: Molecular weight of particles: 103 - 105 Pore size: 20 – 1000 A0 Pressure: 6 – 8 atm. Transport Mechanism: Convection (main) + diffusion Example: Filtration of protein, Red blood cells, polymers, etc.

Nanofiltration: Particles to be separated with Molecular weight: 200 – 1000 Pore size: 5 – 20 A0 Pressure: 15 – 25 atm. Particle retention of salts. Example: Filtration of dyes, small molecular weight organics, etc. Microfiltration: Molecular weight > 1 lakh Pore size: more than 1000 A0 Pressure: 2 – 4 atm. Example: Filtration of clay solution, latex, paint, etc.

Solar water disinfection is a type of portable water purification that uses solar energy to make biologically-contaminated (e.g. bacteria, viruses, protozoa and worms) water safe to drink. Water contaminated with non-biological agents such as toxic chemicals or heavy metals require additional steps to make the water safe to drink.

There are three primary subsets of solar water disinfection:

1. Electric. Solar disinfection using the effects of electricity generated by photovoltaic panels (solar PV).

2. Heat. Solar thermal water disinfection. 3. UV. Solar ultraviolet water disinfection.

Membrane Filtration and SODIS

2 Manesh P. Malla (nec)

Solar disinfection using the effects of electricity generated by photovoltaic typically uses an electrical current to deliver electrolytic processes which disinfect water, for example by generating oxidative free radicals which kill pathogens by damaging their chemical structure. A second approach uses stored solar electricity from a battery, and operates at night or at low light levels to power an ultraviolet lamp to perform secondary solar ultraviolet water disinfection.

Solar thermal water disinfection uses heat from the sun to heat water to 70C-100C for a short period of time. A number of approaches exist here. Solar heat collectors can have lenses in front of them, or use reflectors. They may also use varying levels of insulation or glazing. In addition, some solar thermal water disinfection processes are batch-based, while others (through-flow solar thermal disinfection) operate almost continuously while the sun shines. Water heated to temperatures below 100C is generally referred to as Pasteurized water.

High energy ultraviolet radiation from the sun can also be used to kill pathogens in water. The SODIS method uses a combination of UV light and increased temperature (solar thermal) for disinfecting water using only sunlight and plastic PET bottles. SODIS is a free and effective method for decentralized water treatment, usually applied at the household level and is recommended by the World Health Organization as a viable method for household water treatment and safe storage. SODIS is already applied in numerous developing countries. Educational pamphlets on the method are available in many languages,[2] each equivalent to the English-language version.

Principle of SODIS

Exposure to sunlight has been shown to deactivate diarrhea-causing organisms in polluted drinking water. Three effects of solar radiation are believed to contribute to the inactivation of pathogenic organisms:

• UV-A interferes directly with the metabolism and destroys cell structures of bacteria. • UV-A (wavelength 320–400 nm) reacts with oxygen dissolved in the water and produces

highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) that are believed to also damage pathogens.

• Cumulative solar energy (including the infrared radiation component) heats the water. If the water temperatures rises above 50 °C (122 °F), the disinfection process is three times faster.

At a water temperature of about 30 °C (86 °F), a threshold solar irradiance of at least 500 W/m2 (all spectral light) is required for about 5 hours for SODIS to be efficient. This dose contains energy of 555 Wh/m2 in the range of UV-A and violet light, 350–450 nm, corresponding to about 6 hours of mid-latitude (European) midday summer sunshine.

Membrane Filtration and SODIS

3 Manesh P. Malla (nec)

At water temperatures higher than 45 °C (113 °F), synergistic effects of UV radiation and temperature further enhance the disinfection efficiency.