Water Demand and Supply

In this Section, we will deal with few terms and concepts related to water demand forecasting for a city.

Various types of water demands

• Total Annual Volume (V) in Liters or Million Liters
• Annual Average rate of draft in Liters/day = V/365
• Per Capita Demand (q) = Annual average rate of draft in liters/day/person (lpcd)
• Average rate of draft in liters per day per service

As per IS : 1172-1953 – The minimum domestic consumption of water for a town or a city with full flushing system should be taken as 200 l/h/d.

On an average, a per capita demand of 20 liters/head/day is usually enough for commercial and institutional demand.

Fire Demand

For a city having population P > 50,000

\color{blue}\large{Kiloliters\: of\: water\: required = 100\sqrt{P}}

Kuichling’s Formula :

\color{blue}\large{Q = 3182\sqrt{P}}

Q : Amount of water in liters/minute.
P : Population in thousands.

National Board of Fire Formula :

For P < 2,00,000

\color{blue}\large{Q = 4637\sqrt{P}[1-0.01\sqrt{P}]}

Per Capita Demand :

\color{blue}\large{q = \frac{V}{365\times Design\:Population}}

Factors affecting Per capita demand of Water :

• Size of City
• Climatic Condition
• Type of Gentry and habits of people
• Industrial and Commercial activities
• Quality of water supplies
• Pressure in distribution system
• Development of sewerage facilities
• System of supply
• Cost of water
• Policy of metering and method of charging

Variation in Water Demand :

We generally take Maximum Daily Consumption as 180 % of Average Daily Consumption

So, Maximum daily consumption = 1.8 X q

Maximum Hourly Consumption = 1.5 X average hourly consumption

Now, Maximum hourly consumption of maximum day = peak demand

Goodrich Formula

\color{blue}\large{P =180t^{-0.1}} 

P : Percentage of average draft for time t in days.

If t= 1 : Daily variation

t= 7 : Weekly variation

Coincident draft = Maximum Daily Draft + Fire Draft

We design Sources of supply such as wells etc. for maximum daily consumption or Average Daily Consumption.

Total water demand = Maximum daily demand + Fire demand

Design of elements of water supply :

• Pipe Mains : Maximum Daily Consumption
• Filter and Other Units : Twice the Average Daily
• Pumps : Twice the Average Daily
• Distribution System : Coincident Draft or Maximum Hourly Draft

Water Supply may have a design period of 30 years.

Population Data and Population Growth :

Data is generally collected at the interval of ten years (Decennial Census)

Ideal population growth curve : The curve is S shaped, and it is called logistic curve.

For this curve :

\color{blue}\large{\frac{\mathrm{d} P}{\mathrm{d} t}\propto (P_{S}-P)} 

\color{blue}{\log_{e}\frac{P_{S}-P}{P}\:-\: \log_{e}\frac{P_{S}-P_{0}}{P_{0}}=-KP_{S}t} 

Population Forecast Method

Arithmetic Increase Method

\color{black}\small{\frac{\mathrm{d} P}{\mathrm{d} t}=K}

\color{black}\small{\int_{P_{1}}^{P_{2}}dP=\int_{t_{1}}^{t_{2}}Kdt }

\color{blue}\large{P_{2}-P_{1}=K (t_{2}-t_{1})}

Geometric Increase Method

We assume Percentage growth rate to be constant. Compounding is done at every decade.

\color{blue}\large{P_{n} = P_{0}\left ( 1 + \frac{r}{100} \right )^{n}}

r : assumed growth rate.

Method of varying increment or Incremental increase method.

Growth rate is progressively increasing or decreasing.

\color{blue}{P_{n} = P_{0}+n\bar{x}+\frac{n(n+1)}{2}\bar{y}}

x is mean of increase in population and y is mean of incremental increase.

Other methods :

• Decreasing rate of growth method
• Simple graphical method
• Comparative graphical method
• Zoning or master plan
• Logistic Curve method

Water Quality Control

This section provides all information about standards that we need to keep in mind for quality of supply water regarding physical, chemical as well as biological properties of water.

Quality Control of Municipal and Industrial Water Supply

Characteristics of Water

Physical Characteristics of Water

Turbidity

Large amount of suspended matter such as clay, silt or other finely divided organic material are present in water. It will appear to be muddy, cloudy or turbid in appearance.

Measurement of turbidity of water

Turbidity rod or Turbidimeter : Interference caused by water sample to light is used as principle. It is expressed in mg/l or ppm.

The standard unit is that is produced by 1 mg of finely divided silica in 1 liters of distilled water. The length at which needle disappear in turbid water is measured.

A turbidity of 5 mg/l is easily detectable.

IS permissible limit for turbidity = 5 – 10 units

Turbidimeters :

Jackson’s Turbidimeter

Addition of water is stopped as soon as the image of candle flame cease to be seen. Then we calibrate Height of water column to measure turbidity.

Unit is JTU

Bayle’s Turbidimeter

Modern nephalometers is in use. Range is 0 – 10 mg/l.

Light bulb is seen in galvanised box.

Nephalometers

The light intensity is measured at right angle to incident light. Such an instrument use photometer to measure intensity of light passing through turbid water, after same is scattered at right angles.

Unit is Nephalomeric Turbidity Unit (NTU), also known as Formazin Turbidity Unit (FTU).

It is used as unit since, Formazin polymers are used as reference turbidity standard.

Electric conductivity increases with increase in Total dissolved Solids.

Colour

Water should not be objectionable from aesthetic and psychological point of view. It can be measured by comparing the colour of water samples with other standard glass tubes (Nesler tubes), containing solutions of different colour intensities.

The standard unit of colour : one milligram of platinum cobalt dissolved in 1 Liter of distilled water.

Maximum permissible colour for domestic water = 20 ppm.

Taste and Odour of Water

The dissolved organic material or the inorganic salts, or dissolved gases may impart taste and odour in water.

Gases : H₂S, CH₄, CO₂, O₂ etc.

Mineral : NaCl, Fe, CO₃^2- , SO₄^2- , Phenol, oil etc.

Tastes imparted by dissolved O₂ and CO₂ are generally desirable.

The threshold odour number TON, represents the dilution ratio at which odour is hardly detectable.

The threshold number of water should be 1 and should never exceed 3.

In routine examination, we determine odour in cold water.

Temperature

For potable water, temperature about 10 degree centigrades are highly desirable, while temperature above 25 degrees are considered objectionable.

Specific Conductivity of Water

Measure number of ions by portable ionic water tester, called conductivity sensor.

It measures the amount of electricity conducted by 1 cm of water, expressed in mho/cm.

ISO renamed this unit as Siemens.

\color{blue}{TDS (mg/l) = conductivity(\mu s/cm) \times 0.67}

Chemical Characteristics of Water

Total Dissolved solids and Total suspended solids

Evaporating a sample of water and weighing the dry residue left.

For suspended solid, we do filtration.

Permissible amount : 500 ppm

Rejection value : 2000 ppm

p^H of water

Logarithm of reciprocal of hydrogen ion concentration.

For pure water at 25° C

Product of concentration of [H⁺] and [OH^–] = 10^-14 mol/L

p^H = 7 : Neutral

p^H > 7 : Alkaline – Bicarbonates and Carbonates, p^H < 7 : Acidic – Mineral acid, free carbon dioxide, sulphates of Fe and Al.

Measurement of p^H : Potentiometer

Permissible p^H value : 6.6 – 8.5

Higher acidic value cause tuberculation and corrosion while higher alkaline value cause incrustation and psychological impact on human being.

Hardness of water

It prevents formation of sufficient lather.

We measure it in terms of CaCO₃ equivalent.

Total Hardness in mg/L as CaCO₃

\color{blue}{H = \left [[Ca^{++}]\times \frac{50}{20}  \right ]+\left [[Mg^{++}]\times \frac{50}{12}  \right ]}

Chloride Content

Found due to NaCl, leaching of marine deposits, brine and industrial and domestic waste.

Concentration > 250 mg/L : produce salty taste.

Obtained by titrating with silver nitrate and potassium chromate as indicator.

Nitrogen Content

It is indicator of presence of organic matter.

Forms :

• Free Ammonia
• Albuminoid or organic nitrogen
• Nitrites
• Nitrates

Free Ammonia

First stage of decomposition of organic matter indicated by release of free ammonia.

Its value should not exceed 0.15 mg/L.

Nitrites

• Indicate presence of partly decomposed organic matter.
• Colour developed by sulphonic acid nepthamine.
• It should be nil amount as it is highly dangerous.

Nitrates

• Indicate fully oxidised organic matter and old pollution.
• Colour developed by phenol di sulphonic acid.
• Concentration should not exceed 45 mg/L.
• Too much nitrate cause methemoglobinemia (blue baby syndrome)

Organic Nitrogen

• It should not exceed 0.3 mg/L
• Determined by Kjedahl Nitrogen.
• Adding strong Alkaline solution KMnO₄ to already boiled water sample, ammonia gas is liberated.

They can be determined by colour matching method.

Indicator used for Iron testing is 1,10 Phenanthroline

Microscopial Characteristics

Most bacteria are harmless while some are harmful and are called Pathogens.

Aerobic bacteria requires oxygen while Anaerobic bacteria works in absence of oxygen.

Facultative bacterial works with or without oxygen.

Coliforms

• The Coliforms are the rod shaped non pathogenic bacteria, whose presence or absence indicates fecal pollution.
• They dwell in lower portion of intestine.
• Lactose fermenters
• Non spore forming
• Gram negative
• Detection of E Coli in drinking water is taken as evidence of recent pollution with human feces.

Water is presumed to be safe if no coliform is seen, as coliform live longer than pathogen.

Other Microbes : Algae, Plankton, and Fungi

Purification of Water Supplies

This section covers various methodologies that we use for purification of water supplies both physically and chemically in a systematic order.

Purification of Water Supplies

Method of purification of water

• Screening
• Plain Sedimentation
• Sedimentation aided with Coagulation
• Filtration
• Disinfection
• Aeration
• Softening
• Miscellaneous Treatment
  • Desalination
  • Removal of Iron and Manganese
  • Fluoridation
  • Recarbonation

Screening

Most of big and visible objects, such as trees, branches, fish etc. can be removed by screening.

Screens are generally provided in front of pumps or the intakes.

Coarse Screen are sometimes present in front of fine screen. Coarse screen consist of parallel iron rods placed vertically or at a slight slope, at about 2.5 to 5 cm apart.

The fine screens are usually made of woven wire mesh with opening not more than 6 mm²

Normally fine screen faces clogging.

The coarse screens are also kept inclined at about (45 – 60) degrees to the horizontal. It increase opening area to reduce flow velocity.

Types

• A fixed bar type screen
• Movable bar type or travelling bar type screen : endless belt of fine screen mounted on a chain.

Plain Sedimentation

Particles tend to settle down due to greater specific gravity. They remain in suspension due to turbulence of water.

The basin in which the flow of water is retarded, is called settling or sedimentation tank or clarifier.

The theoretical average time for which the water is detained in the tank, is called the detention period.

Theory of sedimentation factors :

Velocity of flow

It carries particles horizontally. Greater the flow area, the lesser is the velocity, so more easily the particles will settle down.

Viscosity of Water

It varies inversely with the temperature. Warm water is less viscous, so provide less resistance to sedimentation.

The size, shape and specific gravity of particles

More gravity cause more settling.

Stoke’s Law

For d < 0.1 mm

\color{blue}{V_{S} = \frac{g}{18}(G-1)\frac{d^{2}}{\mu}}

V_s : velocity of settlement in m/s

d : diameter of particle in m

G : specific gravity of particle = ρ_g/ρ_w

μ : kinematic viscosity of water in m²/s

In General

\color{blue}{V_{S} =\left [\frac{\frac{4}{3}gd(G-1)}{C_{D}}  \right ]^{\frac{1}{2}}}

C_D : Coefficient of drag : depends on flow regime.

For turbulent flow :

C_D = 0.4

For Laminar flow :

C_D = 24/Re

Re : Reynolds number

\color{blue}\large{Re = \frac{Vd}{\mu}}

For d > 1 mm : Newton’s Formula

\color{blue}{V_{S} = 1.8\sqrt{gd(G-1)}}

The actual velocity of settlement of particle is much less than the theoretical value because

• Non sphericity of particle
• Upward displacement of fluid caused by settling of other particles
• Convectional currents

Sedimentation aided with coagulation

Coagulation

Chemical technique which is directed towards destabilisation of charged colloidal particles.

Flocculation

Slow mixing technique which promotes the agglomeration of destabilised particles.

These coagulants cause formation of flocs, which are gelatinous precipitate.

The fine colloidal particles in water get attracted and absorbed in these flocs, increasing their size.

Most of the colloidal particles in waste water are negatively charged. The stationary charged layer on the surface is surrounded by a bound layer of water.

In the bound layer, called stern layer, ions of opposite charge drawn from the bulk solution, produce a rapid drop in potential, called stern potential.

A more gradual drop, called zeta potential occurs between the shear surface of waste layer and point of electroneutrality in solution.

Clariflocculator

It can remove turbidity upto (10 -20) mg/L. Also it reduces bacteria upto 70 %.

Structure

• Feeding Device : Dry and wet feeding (solution of required concentration)
• Mixing Device : Pump air, basin with baffle walls
• Flocculation Tank
• Sedimentation Tank

Velocity of flow in channel between baffle is controlled to value of 0.15 to 0.45 m/s and the detention period is about 20 to 50 minutes.

Flocculation tank

Best flock will form by violently mixing the mixture of water and coagulant, followed by gentle stirring to permit build up of floc particles.

Structure

• Rectangular tank with paddles
• Speed : 2 – 3 rpm
• Detention : 20 – 60 minutes
• Clear distance : 15 – 30 cm from wall
• Rolling motor prevents flocs from settling

Chemical for Coagulation

Alum

Al_2​(SO₄​)₃​⋅18H₂​O+3Ca(HCO₃​)₂​→3CuSO₄​+2Al(OH)₃​↓+6CO₂​

• It reacts with bicarbonate alkalinities.
• The Aluminium hydroxide is formed as precipitate.
• It imparts permanent hardness in form of calcium sulphate.
• It fixes a limit on maintaining a p^H of (6.5 – 8.3) for good floc.
• Dosage : (5 – 85) mg/L ; Normally 17 mg/L
• It is difficult to dewater the sludge formed and not easy to dispose.
• The only drawback of Alum is the small range of p^H required.

Copperas

Ferrous Sulphate

FeSO₄​⋅7H₂​O+Ca(OH)₂​→CaSO_4​+Fe(OH)₂​)↓+7H₂​O

FeSO₄​⋅7H₂​O+Ca(HCO₃​)_2​→CaSO₄​+Fe(HCO₃​)₂​+7H₂​O

4Fe(OH)_2​+O₂​+2H₂​O→4Fe(OH)₃​↓

• Added in conjugation of lime.
• Ferrous hydroxide further oxidize to ferric hydroxide.
• For uncolored raw water
• Cheaper than Alum.
• Function properly in p^H range of 8.5 and above.
• Does not give good results in coloured raw water.

Chlorinated Copperas

6(FeSO₄​⋅7H₂​O)+3Cl₂​→2Fe₂​SO₃​+2FeCl₃​+42H₂​O

It is effective in pH range of 4 to 7.

Sodium Aluminate

• One and half time costlier than Alum.
• For water which do not have natural desired alkalinity.
• For boiler feed.

Na₂​Al₂​O₄​+Ca(OH)₂​→CaAl₂​O_4​↓+Na₂​CO₃​+CO₂​↑+H₂​O

Desalination of Water

Brackish water is unfit for drinking. Desalination is process of removing the salt.

It is practically used to denote process used for reduction of TDS (mg/l)

It is measured in field with a conductivity meter. The unit used is mili Siemens per m.

Sea water contains on average 3.5 % of salt. Landlocked lakes are more salty as they are more shallow than ocean (more evaporation due to more surface area).

Methods of Desalination of Water

Desalination by evaporation and distillation

• Water obtained is purest form that man can think of to produce at any large scale.
• Use of evaporation (Teflon lining)
• Flash Evaporation : hot water is suddenly cooled under less pressure. Water is vapourised suddenly like a flash and actual boiling point is never reached.
• One form involves spraying hot brine under pressure through nozzle into chambers which is at lower pressure and cooler.

Electro dialysis

The bond between Na⁺ and Cl^– is broken with the help of electricity. They move and stick to opposite electrode.

The segregation is achieved by means of thin plastic like sheets called membrane. They are made of peculiar chemical substances, called ion exchange resins and are very thin, say 1/180^th of Cm.

• Right kind of resin can remove only positive or negative ion but not both.
• Such one stack can remove upto 50 % salinity.
• It is a compact machine
• Cost is small and easy to operate
• Ideal machine for small town or a remote place.
• This process is also effective in demineralization.

Desalination of Water by Reverse Osmosis

Water and salt molecules are separated by forcing the salt solution against a semi permeable membrane barrier, which permits flow of solvent (water) but not salt.

The Osmotic Pressure is directly proportional to Total Dissolved Solid.

\color{blue}\large{\pi = [C]RT}

[C] : concentration of salt

Membrane supported by grids.

It does not work below 60,000 KN/m² and usually operate at about 100,000 KN/m²

Pressure driven process according to membrane pore size :

• Microfiltration : (0.1μm – 10μm) – Algae, Bacteria
• Ultrafiltration : (0.001μm – 0.1μm) – Virus
• Nanofiltration : (1nm – 100nm) – organic compound salt
• Reverse Osmosis : (0.0001μm – 0.001μm) – Dissolved salts, Ca, Mg, Na etc.

Nano Filtration and RO helps in filtration of dissolved salt.

RO can be used for moderate salty water.

Crystallization or Freezing

When salt water freezes, ice formed in beginning is almost free from salt. This ice when melted can give good water.

Solar Distillation

Distillation of water is done using solar power.

Removal of Iron and Manganese from Water

Fe and Mn are generally dissolved in water. When exposed to air, they reduce forms insoluble visible oxidized ferric iron and manganic manganese.

When their content exceed 0.3 mg/l and 0.05 mg/l respectively, they become objectionable.

Cause discolouration of textiles.

Incrustation of water mains due to deposition of oxide.

Unpleasant in taste.

The reduced iron in water promotes growth of autotrophic bacteria in distribution mains pipes. The elimination of iron bacteria is generally difficult and expensive.

Removal Method

When present without combination with organic matter :

Aeration followed by Coagulation, Sedimentation and Filtration. During Aeration, the soluble ferrous and manganese compound may oxidize into insoluble ferric and manganic compounds that can be easily sedimented out.

When present in combination with organic matter :

It becomes difficult to break bond between them. The bond can be removed either by adding lime and thereby increasing the p^H Value of water to about (8.5 – 9) or by adding chlorine and potassium permanganate.

Manganese Zeolite

A natural green sand, coated with manganese dioxide.

It is used to remove soluble iron and manganese. After Zeolite becomes saturated with metal ions, it can be regenerated by back washing with Potassium Permanganate.

Fluoridation

• Adding extra dose of fluorine.
• Optimum value of 1 mg/l.
• Its reduction cause dental caries in children. It aids in formation of permanent teeth in children and treatment of Osteoporosis in old men.

Compound Added :

• Sodium Fluoride (NaF)
• Sodium Silico Fluoride (Na₂SiF₆)
• Hydro Silico Fluoride (H₂SiF₆)
  

Excess Amount (> 1.5 mg/l) cause spotting of teeth, grey to black discolouration of enamel, skin irritation, and deformation of bone.

Defluoridation Methods

• Nalgonda Technique : Rural India
• RO
• Ion Exchange
• Activated Alumina Absorption or Prashanti technology