chapter 5

CHAPTER 5

WATER TREATMENT PROCESSES

Water treatment process• The extent of treatment required to be given to particular raw

water depends up on :

The characteristics and quality of the available raw water. The quality requirements of the intended use. Treatment of water, hence, includes:

• Removal of color, dissolved gases, and darkness of water,• Removal of objectionable taste and odor,• Destruction of troublesome micro-organisms and their remains,• Elimination of corrosive properties,• Making water safe for drinking and fit for domestic and other

uses.

Objectives of Treatment

The main objective of water treatment process is to:Preventing disease transmission small animals (invertebrates) and their eggs (ova), protozoa and

their cysts, bacteria that may form spores, and viruses. Chlorine is most commonly used to inactivate such pathogens

Make the water acceptable The taste, appearance and suitability for washing clothes shall all

be considered.Protecting the distribution System from corrosion Corrosion of the system can be reduced by raising the pH of the

water or adding chemicals.

Water Treatment methods

The various water purifying methods/techniques for public water supply are:

• Screening• Plain sedimentation Pre-treatment• Sedimentation aided with coagulation processes• Filtration• Disinfection• Aeration• Softening• Miscellaneous treatments; e.g. Fluoridation, The necessity of a few or all of these steps depends solely up

upon the quality of the available raw water.

Factors Affecting Choice of Treatment Methods

• Limitation of capital,• Availability of skilled and unskilled labor,• Availability of equipment, construction material, and water

treatment chemicals,• Local codes, drinking water standards and material

specifications,• Local traditions, customs and cultural standards, • National sanitation and pollution policies, etc

Pretreatment of water

Screening The purposes of screening the raw water are:• Removal of suspended and floating matter, which clogs pipes,

damages pumps etc.• Clarifications by removal of suspended matter to lighten the

load on subsequent treatment processes. Coarse screens are generally termed as trash racks. They are usually kept inclined at about 45o to 60o to the

horizontal so as to facilitate cleaning of the suspended matter. The approach velocity to the screens should not be more than

0.1 - 0.2 m/s to effect settling of larger bodies. After passing the bar screens, the velocity should be increased to 0.3 - 0.5m/s to avoid settling of sediment.

Sedimentationi. Plain Sedimentation Is a form of pretreatment that provides a low velocity of flow. The purpose of which is to settle some solids by gravity pull. The basin in which the flow of water is retarded is known as the

settling tank or sedimentation tank or sedimentation basin. The theoretical average time for which the water is retained in

the tank is called detention period. The settlement of a particle in water is a function of• The velocity of flow;• The viscosity of water; and • The size, shape, and specific gravity of the particle. A plain sedimentation tank under normal conditions may

remove as much as 70% of the suspended solid.

ii. Sedimentation with coagulation (Coagulation, Flocculation & Sedimentation)

All water, especially surface water, contain both dissolved and suspended particles.

Coagulation and flocculation processes are used to separate the suspended solids from the water.

The suspended particles vary considerably in source, composition charge, particle size, shape, and density.

Correct application of coagulation and flocculation processes and selection of the coagulants depend upon understanding the interaction between these factors.

chemical treatment with coagulants (and often pH correction) is essential so that turbidity can be removed in a reasonably short time by sedimentation and rapid filtration.

Terminologies

• Suspension: A suspension is a dispersion of solid particles in a liquid. These particles scatter light, making the aqueous suspensions cloudy or turbid.

• Colloid: Colloidal particles are very small (1 x10-9 to 200x10-9m) because the random motion of the water molecules is enough to prevent the particles settling.• Flock: A flock is a collection of colloidal particles held together.

Flocks generally have little strength and a density close to the density of water.

• Coagulation: is the process of making conditions favourable for colloidal particles to join together to make flocks. It may be the result of biological processes or the addition of chemicals.

• Flocculation : is the growth of coagulated colloidal particles to join together to form flocks that are large enough to settle under gravity.

….Con’t• A coagulation plant contains the following four units: Feeding device Mixing device or mixing basin Flocculation tank or flocculator Settling or sedimentation tankI. Coagulating chemicals (Coagulants) Cost and ease (economy) of acquiring chemicals are the basis of

selection of a particular coagulating chemical. Alum (Hydrated Aluminium Sulphate), Al2 (SO4)3 .18 H2O) is used widely as a primary coagulant. It is available as lumps,

grains or powder, or as a solution that is strongly acidic. For optimum flock formation the pH should generally be

between 6 and 8.

….Con’t Iron Salts:• These operate over a wider pH range than alum. • Iron flocks are heavier than aluminum flocks but may be harder

to remove from filter sand. pH Correction:• Usually it is necessary to raise the pH, and hydrated lime is

usually preferred for this purpose on cost grounds. Coagulant aids:Polyelectrolyte - is used as coagulant aids to make flocks larger, tougher and denser. They have long molecules with charged sites that link small flocks together.

II. Feeding device• The chemical coagulant, i.e. usually alum is first fed (either dry

or in solution form) in to the raw water through the feeding

device.III. Mixing devices• The desired reaction between the coagulant and the water is

irreversible and takes place in a fraction of a second.• So mixing must be very fast, otherwise, undesired products

might form.• There are two main types of mixing mechanisms: Mechanical mixing Hydraulic mixing

Mechanical mixing

• Mechanical mixing uses an impeller rotated by an electric motor.

• Coagulant should be injected at the eye of the impeller for maximum turbulence.

• The loss of head through the mixing is low but the disadvantages are that motor failures can affect the operation of the whole plant, and

Hydraulic mixing• This method has the advantage of simplicity and access of

component parts for repair and maintenance. • The degree of mixing is lower at lower rates so the likely

variation of the flow rate should be taken in to account at the design stage.

• The coagulant should be injected just upstream of the maximum turbulence. The mixing should appear violent.

• The most common types of hydraulic mixers are: Rectangular and V-notch weirs and Hydraulic jump.

IV. Flocculation tank or flocculators

There are two types of flocculation tanks:• Mechanical flocculators Horizontal flow• Hydraulic flocculators Vertical flow

Filtration Is a process by which water from the sedimentation tank is

allowed to pass through beds of granular material such as sands or filters.

Sand, either fine or coarse, is generally used as a filter media.

The selection of the correct effective size (d10) is very important.

…con’t• Too smaller grain size will lead to very frequent clogging of

filters and will give low filtration rates. • Similarly too large size will permit the suspended particles and

bacteria to pass though it without it being removed.• The gravel which may be used below the sand should be hard

durable, free from impurities, properly rounded and should have a density of about 1600 kg/m3.

The sand filters purify water under four different processes:1) Mechanical Straining• When water percolates through the suspended particles which

are unable to pass through the voids, get arrested and removed by the action of mechanical straining.

2) Sedimentation or Adsorption of particles

• The particles of impurities, settle in the voids of the sand due to following two reasons:

1) Due to physical attraction between impurity particles and sand particles.

2) Because of the presence of gelatinous coating formed on the sand particles by previously caught bacteria and colloidal matter,

impurity particles stick to to sand particles and get removed.3) Biological process• During the process of passage of water through void in the

sand particles, the bacteria are caught in the voids, due to which a zoological jelly or film is formed around the sand particles.

4) Electrolytic charge development

• The sand particles of filter media and the particles of suspend ed and dissolved matter, carry electrical charges of opposite nature.

• Because of having opposite charges, they attract each other and neutralize the charges of each other.

Types of Filters • Basically there are two types of filters: SLOW SAND FILTERS and RAPID SAND FILTERS

Reading assignment

• Advantages and disadvantages ofSLOW SAND FILTERS and RAPID SAND FILTERSBACKWASHING

Disinfection

• Disinfection is a process of killing harmful micro-organisms from water and makes it safe for consumption.

Disinfectants are required to:• Be Economical and easily available in bulk• Be able to destroy all harmful organisms from the water and

make it safe • Be capable to immediately attack harmful organisms when

mixed/used in water• Non toxic and free of objectionable odor and taste after

disinfection• Be easy to detect their strength and concentration in water

Chlorination

• Throughout the history of water treatment, chlorination has been synonymous with disinfection.

• Chlorination was the first method used to disinfect water in approximately 1825.

• Chlorine is universally used for water and wastewater treatment. Among the reasons of its being in famous:

• can be stored for long time without any deterioration in its quality and stability

• is very powerful and efficient in killing bacteria• is economically available• initial cost of plant as well as cost of disinfection are relatively low.• its dosage is precise and there is no likely hood of overdose or

underdone.• there is no sludge formation.

Key Conventional Treatment Non-conventional Treatment

Surface Water

Bar Screen

Silt Trapping or Plain Sedimentation

Fine Screen

Pretreatment Aeration Coagulation

Flocculation

Sedimentation

Clarification

Rapid Sand Filtration

Disinfection

Consumer

Slow Sand Filtration

Miscellaneous Treatment