SECONDARY TREATMENT OF WATER BY FILTERATION

FILTRATION

The most important stage in water treatment is filtration. In filtration the water is passed through he thick layer of sand. Through filtration following effects are seen:

• Chemical properties of water are changed.
• Suspended and colloidal impurities in finely divided state are removed. 
• The number of bacteria are also reduced.

The filtration is based on four actions:

1) Mechanical Staining

The suspended particles that can not pass through the pores of sand are arrested and removed through the  action of mechanical straining.

2) Sedimentation

The spaces in the sand particles act as the small sedimentation tanks. The impurities are arrested in voids of sand and these adhere to the particles of sand due to 2 reasons:

• due to physical attraction between two particles of matter
• due to presence of gelatinous film developed on sand grains by previous bacteria.

3) Biological Metabolism

The growth and life processes in living cells is called as biological metabolism. The action of filter is based on biological metabolism process. When bacteria are held in voids of sand, a zoological jelly is formed around. The film consist of large colonies of bacteria. Bacteria feed on organic impurities in water. They convert impurities to harmless compounds by biochemical reactions.

4) Electrolytic Changes

The action of filter can also be explained by ionic theory. It describes that when two opposite charges come close in contact to each other the charges are neutralized and new chemical substances are made. It is seen that some of the sand grain filters are charged with the electricity of some polarity. When particles of suspended and dissolved matter containing electricity of opposite polarity come in contact with sand grains, they neutralize each other and results in alteration of chemical properties of water. 

FILTER SAND

The sand used in filter is free from clay, loam, vegetable matter and organic impurities. It should be uniform in nature and size. The classification of filter sand depends upon effective size and uniformity coefficient. The effective size of sand depicts the size of sieve in mm through which 10 % of sample by weight is passed. The uniformity coefficient is the ratio of sieve size in mm through which 60 % of the sample of sand by weight will pass to the effective size of sand.

CLASSIFICATION OF FILTERS

The filters are classified into two categories:

• Slow Sand Filters
• Rapid Sand Filter

Rapid sand filters are further categorized into two sub classes:

• Gravity type rapid filters
• Pressure type rapid filters

SLOW SAND FILTERS

The purpose of slow sand filters is to pass the water slowly through the layer of sand that is placed above the base material and thus purification process is aimed to improve the biological, chemical and physical properties of water. These are best suitable for rural areas in developing countries. because of its simple and maintenance procedures. It pure water at low cost. 

ESSENTIAL PARTS 

A slow sand filter consist of following parts:

• Enclosure tank
• Under drainage system
• Base material
• Filter media of sand
• Appurtenances

The rate of filtration from slow sand filters varies from 100 to 200 liters / hour/ meter square of filter area. 

EFFICIENCY OF SLOW SAND FILTERS

• They remove about 98 to 99 % of bacterial load from raw water.
• They may remove 20 to 25 % color of raw water.
• They can remove the turbidity to an extent of 50 ppm. 

RAPID SAND FILTERS

The main disadvantage of slow sand filters is that it needs considerable space for its installation. This makes it uneconomical for the places where the land values are high. This led the engineers the need to increase the rate of filtration that could be increased by two ways:

• by increasing the size of sand so that the friction of water passing through the filter media is minimized.
• by allowing the water to pass under pressure though the filter media

The first one is achieved by gravity type rapid sand filter and second is achieved by pressure rapid filters. 

The parts of rapid sand filters are same as the slow sand filters.

Read More

COAGULATION AS PRIMARY TREATMENT OF WATER

Coagulation is the process used to make bigger sized particles by adding certain chemicals called as coagulants. These coagulants attach and react with the impurities in water and convert them into settled sizes. 

GENERAL COAGULANTS

Some coagulants that are used for coagulation are as follows:

• Aluminium sulphate
•  Chlorinated copperas
• Ferrous sulphate and lime
• Magnesium Carbonate
• Polyelectrolytes
• Sodium aluminate

1) Aluminium Sulphate

It is also known as filter alum or alum. Its chemical formula is Al₂ (SO4)₃, 18 H₂O. Alum is an effective coagulant and its use in water treatment is universal. Alum in water treatment is supplied and used in the form of flakes, solid lumps and in solution form.

The benefits from alum are as follows:

• It reduce the taste and odor.
• It reduces the turbidity of water.
• It is cheap.
• It is simple in working and not require skilled supervision.
• It produce crystal clear water.
• The floc formed by this coagulant is better.
• The floc formed is tough and can not be broken easily.

Normally bicabonate alklanity is present in water. The chemical reaction involved in this is:

Al₂ (SO4)₃. 18 H₂O + 3Ca(HCO₃)₂   ----->       2Al (OH)₃ + 3CaSO₄ + 18 H₂O + 6CO₂

_{The aluminium hydrooxide formed is not soluble in water. It acts as floc. Some of permanent hardness is caused due to calcium sulphate and carbon dioxide cause hardness.}

Read More

PRE TREATMENT OF WATER

If the raw water has good quality than it can be directly goes in secondary treatment processes of flocculation/ coagulation and sedimentation. Prior to the secondary treatment some steps are followed. They are as follows:

• Screening
• Storage
• Chemical pre-treatment

SCREENING

Coarse screens with inclined bars of 25 mm in diameter and 100 mm spacing avoid the large floating materials from entering the treatment plant. Raking is done with the inclination of bars. The velocities set for screens is 0.5 m/s that may be automatically or manually raked down. If there is no facilitation of storage than fine screens are fitted after the coarse screens. If here is storage than fine screens are placed at the outlet of storage tanks. Fine screens have the openings with 6 mm diameter or square. Exclusive forms of screens are generally used and they are normally automatically cleaned. 

The screens can be of circular drum type or travelling belt type as in vertical escalator. Screens poses the head loss that accounts for hydraulic calculations. Another type of screening is micro screening with mesh opening of 20 micrometer to 40 micrometer. Such screens are used to treat the uncontaminated waters and moderately colored waters. 

STORAGE

Storage is required for municipal water supply systems to meet variable water demand, fire protection, and for emergency needs. The reservoirs used are of three types:

• Surface Reservoir
• Standpipes
• Elevated Tanks

Surface Reservoirs

Surface reservoirs are located at the location where sufficient water pressure is provided. They are normally covered to prevent the contamination. 

Standpipes

These are tall cylindrical tanks. Its upper portion is used for storage and its lower portion supports the structure. The standpipes with the height of 15 m are not economical but above this height the storage tanks become the choice. Water demands of residential areas changes over day. 

Elevated Tanks

In recent years elevated tanks have becoming less famous due to their high cost and due to availability of invariable speed pumps and controls that make it possible to adjust pumping rates with varying demand. The location, type of storage and storage size must be determined. This depend on population and purpose of storage.

Read More

INTRODUCTION OF WATER TREATMENT PROCESSES

Modern technologies have greatly reduced the spread of waterborne diseases such as cholera and typhoid fever. These diseases are not much a concern now for public health as they were before. The main key for this advancement is the recognition that human wastes are the source of contamination of public water and it could be eliminated by effective water treatment strategies and waste disposal systems. 

In 1802 the filtration of drinking water was practiced in Paisly, Scotland. It was used by water vendors in London, England in 1828. In U.S the filtration of drinking water was first used in 1872 by the city of Poughkeepsie, New York. In this century the technology to make water safe for drinking has becoming widespread in Europe and North America. 

There are four classes of water treatment:

Class A  

In this class no treatment is required for some borehole water Occasional upland water

Class B

In this class borehole water or occasional water is used in public water supply. Disinfection is practiced to maintain the purity along the water pipelines. Chlorination with chlorine has been very debated. The alternative of chlorine has been searching out. Sometimes in this class aeration is used to remove the hydrogen sulphide odors and taste and to increase the level of oxygen in water. 

Class C

It is known as standard water treatment and it is used for lowland rivers and reservoirs.

Class D

Class D is special water treatment. It is used when the source is downstream of urban developments or when high quality water is required by the industries. Th other processes include: membrane technology, iron and manganese removal, chemical oxidation and carbon adsorption. 

Water treatment is designed to provide the standard quality water at taps. Four considerations are used in this:

• Source Selection
• Water Quality Protection
• Treatment method to be used
• prevention of re contamination

Precautions used to avoid groundwater and surface water pollution are:

• Prohibition of discharge of sanitary and storm sewers close to water reservoir
• installing fences to avoid pollution from recreational use of water
• restriction on application of fertilizers and pesticides in areas that drain to reservoirs

Screening, coagulation, flocculation, filtration and disinfection are used to surface water treatment.

These plants have the task to remove:

• Particulate substances such as sand and clay, organic matter, bacteria and algae
• dissolved substances causing color and hardness
• Pathogenic bacteria and viruses

Read More

TREATMENT OF AIR PARTICLE EMISSION BY BAGHOUSE COLLECTORS AND ELECTROPRECIPITATORS

BAGHOUSE COLLECTORS

Baghouse or fabric collectors are same like vacuum cleaner on large scale. Through these collectors dry particles are removed from dry and low temperature gas stream (0 to 275 degree centigrade). Cloth sock of about 15 cm in diameter or up to 10 m long is suspended in the chamber. The air is forced to pass through the sock and is discharged by the fabric. The fabric may be woven that is more common. The other fabric materials used are cotton, synthetics, fiberglass. Each material has its own adaptance to the gas, particle temperature and physical and chemical characteristics. 

The cloth from which bag or sock are made may consist of holes that exceed 100 µm. If these are correctly performed then greater than 99 % efficiency could be achieved for particles of diameter above than 1 µm. For smaller particles collection the filter cake is used on the cloth as filtration medium. As the filter cake thickens, the pressure loss and power cost increases. If the porous filter cake thickens too much, then the pressure loss increases that cake may collapse into more compact masses. If these pores are filled by liquid then same problems occurs. So, it is said that these collectors are used for dry particles collection and special care must be taken to avoid excessive condensation from the gas stream.

The filter cake is removed from small bag by simply shaking the bag so that the cake falls by itself. For large industrial collectors bag is cleaned by passing the ring jel of air alongside the bag and after some moments the flow is reversed. Some particles reentrainment occurs while cleaning. These particles are again removed in second cycle of cleaning. To avoid the need of bag shaking, to maintain the thickness of filter cake, and to avoid excessive pressure loss the volume flow rate by collectors is maintained at 0.5 to 2 m³ per meter square of cloth. 

The pressure drop across baghouse range from 5 to 40 cm of water for shaking periods ranged between 4 to 5 times per hour to once in many hours. A typical life of bag is 2 to 3 years. 

Fiber mat particle collectors are performed at low pressure drop and are disposable. They can be washed and reused many times. They are used in air conditioning system and hot air domestic systems.  

ELECTROSTATIC PRECIPITATORS

The voltage difference is maintained at as high level as possible. Electrons are released at the electrode in the cornea discharge and attach to particles and charge the particles. The charged particles or the molecules having same polarity as electrodes move toward ground surface due to electrostatic forces. 

Electrostatic Precipitators

Migrating ions liquid or solid and particles in gas stream and thus giving the particles charge that result in particle motion towards collector plates. When particles attach to plates they stick there and form an insulating blanket.   

For precipitator design gas and particle resistivity are important parameters. Resistivity varies with temperature and chemical composition. Precipitator efficiency is as high as 99 % for particles above than 2 µm at pressure loss of 5 cm of water or less. 

  

Read More

AIR PARTICLE EMISSION BY SCRUBBERS

Scrubbers or wet collectors are designed to increase the particle size by using water or slurry droplets because it is easy to collect the larger particles. 

Wet Scrubber

There are various types of scrubbers but two types are discussed here:

• Conventional Scrubbers
• Venturi Scrubbers

There are several modes of particle collection in scrubbers. In the upper part of scrubber the falling water droplets collide and collect the particles from the upward moving dity gas. In packed section special shapes are used to increase the area of contact between liquid and aerosols. The plugging is a problem faced in packed section despite the specific shapes used. Below the packed section there exists a flooded perforated disc. that supports many centimeters of water and allowed the contact between liquid and water bubbles containing particles. The liquid passes through perforations and fall in another falling drop collection section. 

The particles collection is not achieved in all the droplets and particles collision due to surface tension of droplets and particles wettability properties. Chemicals are added to reduce the surface tension of droplets and to improve their abilities to absorb the gases and particles. 

Venturi Scrubber

The liquid containing particles are moved to the bottom of tower and shifted to settling basin or filtering device for the removal of particles. The water is recirculated with or without treatment to result zero discharge system. 

The demistor the exit point of scrubber is a particles collector. It is designed to remove the drops of liquid from gas stream. 

The size of water drops is critical in determining the performance of scrubber. If the water drops are large than particles then drag forces displace the particles out of the path of falling drops and collision number decreases. 

The pressure loss from conventional scrubbers is 15 to 40 cm of water. Collector efficiency increase with the pressure loss and it may be as high as 95 %.

Scrubbers are designed to operate at high temperature and avoid corrosion. Operating cost is high for pressure loss scrubbers but the capital cost is comparatively low from other collectors. 

Read More

USE OF GRAVITATIONAL SETTLING CHAMBERS AND CYCLONES COLLECTORS FOR AIR EMISSION CONTROL

The air particles emission must control the particles emissions ranging in size from 1 µm to more than 100 µm in diameter. Collectors are designed according to the physics of collecting mechanisms.

GRAVITATIONAL SETTLING CHAMBERS

These are simple chambers and expensive. In this gravitational forces dominate vertical particle motion. These are simple expansions in duct where horizontal velocity of particles is reduced to give time to settle the particles by gravity. On particle viscous force and gravitational forces are equal but in opposite direction and the particle is expected to fall at terminal velocity. The horizontal component of viscous force is negligible because the particle moves with the velocity of gas stream.

The expressions used for the efficiency of the collector are:

η_g= 1- exp {-µ_tL/µH}

η_g= 1- exp {-gd2p _PpL/18µuH}

exp is used to represent exponential

η_{g = efficiency of removal as a fraction}

_{L=Length of collector in meter}

_{H= Depth of collector in meter}

_{u= horizontal velocity of gas and particles through collector}

d_{p=diameter of particle}

_{Pp= density of particle}

_{The performance of the collector varied from design prediction due to turbulence and variation in flow in collector.}

CYCLONES/ INTERNAL COLLECTORS

These collectors depends upon centrifugal forces to separate the heavier particles from lighter gas molecules. Particles laden gases enter in cyclone at top and spiral downward along casing in solid body rotation at entrance velocity u. Particles move outside of spiral. The only exit for particles is upward from central pipe. 

The magnitude of centrifugal force is 

F_c= m_p u²_T/r

UT= tragential velocity of particle

r= radius of curvature of particle trajectory

The pressure drop through conventional cyclone is 5 to 15 cm of 

water by high efficiency cyclone is 10 to 30 cm. of water.

Gravitational settling chambers and simple inertial separators have 

no moving parts. They may be fabricated by metals that can 

withstand the high temperature and resist corrosion. They are 

effective for solid and liquid particles. 

Read More