Water pollution and Control

WATER POLLUTION

Water pollution occurs when undesirable and unwanted substances are added to the water bodies that can cause physical, chemical and biological changes in water quality.

The major sources of water pollutants are as given below:

The major pollutants are:

1. Pathogens: Bacteria and other micro-organisms come under this category. They get into water bodies through domestic sewage and animal excreta. Bacteria like Escherichia coli which are found in human excreta are known vectors of gastrointestinal diseases.

2. Organic wastes: They are generally bio-degradable wastes that pollute the water bodies by mixing with them during their runoff phase. An excess amount of organic matter in water bodies suck up the dissolved oxygen in water thus inhibiting aquatic life in these water bodies.

3. Chemical pollutants: Heavy metals like mercury, cadmium and nickel come under this category. These chemicals when present above the allowable limit can result in damage of the central nervous system, kidneys and liver.

Sources of water pollution

Point source

When contamination originates from a single source, it’s called point source pollution. Examples include wastewater (also called effluent) discharged legally or illegally by a manufacturer, oil refinery, or wastewater treatment facility, as well as contamination from leaking septic systems, chemical and oil spills, and illegal dumping. 

Nonpoint source

Nonpoint source pollution is contamination derived from diffuse sources. These may include agricultural, surface runoffs or stormwater runoff or debris is blown into waterways from the land. Nonpoint source pollution is the leading cause of water pollution as per the studies.

Causes of Water Pollution

The key causatives of water pollution in India are:

·        Industries

·        Urbanization

·        Social and Religious Practices

·        Agriculture

 Effects of Water Pollution

The effect of water pollution depends upon the type of pollutants and its concentration. Also, the location of water bodies is an important factor to determine the levels of pollution.

• Water bodies in the vicinity of urban areas are extremely polluted. This is the result of dumping garbage and toxic chemicals by industrial and commercial establishments.
• Water pollution drastically affects aquatic life. It affects their metabolism, behaviour, causes illness and eventual death. Chemicals such as this travel up the food chain before entering the human body.
• The effect of water pollution can have a huge impact on the food chain. It disrupts the food-chain. Cadmium, Mercury and lead are some toxic substances, these pollutants upon entering the food chain through animals(fish when consumed by animals, humans) can continue to disrupt at higher levels. Eg: Minamata disease caused by severe mercury poisoning.
• Humans are affected by pollution and can contract diseases such as hepatitis through faecal matter in water sources. Poor drinking water treatment and unfit water can always cause an outbreak of infectious diseases such as cholera etc.
• The ecosystem can be critically affected, modified and destructured because of water pollution.

* Biochemical oxygen demand is the amount of oxygen required by bacteria to decompose organic matter in a certain volume of a sample of water. Clean water will have a BOD of below 5 ppm whereas a highly polluted water the source will have a BOD of more than 17 ppm.

Eutrophication

The excessive growth (or bloom) of algae and plankton in a water body due to enrichment of nutrients like Nitrogen and Phosphorus is known as Eutrophication. Eutrophication is considered to be a serious environmental concern since it often results in the deterioration of water quality and the depletion of dissolved oxygen in water bodies. Eutrophic waters can eventually become “dead zones” those are incapable of supporting life.

Causes of Eutrophication

The availability of nutrients such as nitrogen and phosphorus limits the growth of the plant life in an ecosystem. When water bodies are enriched with these nutrients, the growth of algae, and phytoplankton.

The enrichment of water bodies with the nutrients include:

• Fertilizers
• Untreated sewage
• Detergents containing phosphorus
• Industrial discharge of waste.

Among these sources, the primary contributors to eutrophication include agriculture and industrial wastes.

Process of Eutrophication

The excessive growth of algae in eutrophic waters is accompanied by the generation of a large biomass of dead algae. These dead algae sink to the bottom of the water body where they are broken down by bacteria, which consume oxygen in the process.

The overconsumption of oxygen leads to hypoxic conditions (conditions in which the availability of oxygen is low) in the water. The hypoxic conditions at the lower levels of the water body lead to the suffocation and eventual death of larger life forms such as fish.

Effects of Eutrophication

• The algal blooms limit the amount of dissolved oxygen required for respiration by other animal and plant species in the water leading to oxygen depletion.
• The growth of phytoplankton also causes reduced light penetration into the lower depths of the water. This can bring about aquatic dead zones, loss of aquatic life and it also lessens biodiversity.
• Extensive deterioration of water quality and decline in the availability of clean drinking water is resulted.
• The toxic compounds that get accumulated due to excess plant growth and anaerobic conditions can also make their way up the food chain, contributing to various negative health impacts such as cancers. 
• It can also cause death in humans and animals even at the least concentration when ingested in drinking water.
• High nitrogen concentration in drinking water is associated with the ability of inhibiting blood circulation in infants, a condition known as blue baby syndrome.

WASTE WATER TREATMENT (OR) SEWAGE WATER TREATMENT

Wastewater treatment is any process, operation or combination of process and operations that make the wastewater less dangerous and repulsive to man. It is a combination of physical, chemical and biological process. There are three types of operation and process:

(a)  Physical unit operation: It is an operation in which the application of physical forces predominates.

Eg: screening, flocculation, sedimentation and floatation.

(b) Chemical unit process: It is a process in which removal of contaminants are brought about by chemical activity.

Eg: Chemical precipitation, adsorption, ion exchange, electrodialysis.

(c)  Biological unit process: It is a process in which removal of contaminants is brought about by biological activity.

Eg: Activated sludge process, trickling filtration.

Conventional wastewater treatment process:

It consists of a combination of physical, chemical and biological process and operations to remove solids and organic matter. General terms used to describe different degree of waste water treatment are preliminary, primary, secondary and tertiary treatment.

PRIMARY TREATMENT

Sedimentation:    It is a physical water treatment process used to settle out suspended solids in water under influence of gravity. It is after used as a primary stage in modern waste water treatment plant, reducing the content of suspended solids as well as the pollutants embedded in the suspended solids.

         

It is an operation by which the suspended particles in liquid settle by gravity to the bottom of the tank, from which they may be removed as sludge. After some time, lighter solids and oil floats on the surface of liquid and can be separated as sludge.

Coagulation and Flocculation: It is a chemical water treatment techniques typically applied prior to sedimentation and filtration to enhance the ability of a treatment process to remove particles.

Coagulation is a process where water are treated with reagents to form coagulates. Coagulants with charge opposite to that of suspended solids are added to the mass to trap particles thus forming a mass charge enough to settle or be trapped in the filter.

The substances that are used as coagulants. The most common ones are: Alum (Al₂(SO₄)₃.H₂O), ferrous sulphate, ferric chloride.

Flocculation:  It is gentle stirring or agitation to encourage the particles thus formed to agglomerate into masses large enough to settle and be filtered from the solution.

Equalization:

Flow equalization is a method used to overcome the operational problems and flow rate variations to improve the processes.

Its objective is by providing storage to hold water when it is arriving too rapidly and to supply when it is arriving less rapidly.

The main function of equalization is to act as buffer. To collect the incoming raw effluent that comes at widely fluctuating rates and maintain at steady (average) flow rate.

Neutralisation:

· Industrial wastes often contain acidic or alkaline components which require neutralisation before discharge or treatment.

• For waters entering biological treatment processes the “p^H” should be maintained between “6.5 - 9” for optimum growth of the micro-organisms.
• Acidic waste are commonly neutralised with alkaline streams like ammonia, caustic soda / soda ash.
• Lime is the most widely used alkaline material for neutralisation of acid wastes.
• Alkaline wastes require treatment with acidic streams like Sulphuric acid or Hydrochloric acid.

Ion Exchange :

 H₂O  -->  H⁺ + OH^-

 NaCl -->  Na⁺  +  Cl^-

Ion exchange is a treatment method where one or more undesirable ionic contaminants are removed from water by exchange with another or less objectionable ionic substance.

Both the contaminants of the exchanged substance must be dissolved and have the same type of electric charge (+ve or –ve). The use of ion-exchange aims to reduce “Calcium and Magnesium” content in water softening.

Adsorption

Adsorption is a waste water purification technique for removing a wide range of compounds from industrial waste water. Adsorption works on the principle of adhesion. The process of adhesion involves separation of a substance from one phase accompanied by its accumulation or concentration at the surface of another.

 

The process can take place in any of the following system:

·        Liquid-Gas,

·        Liquid-Liquid,

·        Solid-Liquid,

·        Solid-Gas.

• The adsorbing phase is the ‘adsorbent’ and the material concentrated or adsorbed at the surface of adsorbing phase is the ‘adsorbate’.
• Activated carbon was the first widely used adsorbent.
• Activated carbon is an excellent adsorbent because it has a strong affinity for binding organic substances even at low concentration.
• It is considered to be a refining treatment and as such is applied at the end of the most common treatment systems especially after a biological treatment.

 

Electro-dialysis

Electro-dialysis water treatment process is one of the water treatment method that employ reverse osmosis membranes which are semi-permeable to ions based on their charge, under the influence of electric field to reduce the ionic content of water.

Electro-dialysis involves passing water between two plates with opposite electrical charges. The +vely charged ions  in the water are attracted to the plate with negative charge  (cathode) while the –vely charged ions are attracted to the plate with the positive (+) charge. Both the types of ions can be removed from the plates and discarded.

Chemical Precipitation

Chemical precipitation is the most common method for removing dissolved metal from waste water solution containing toxic metals. To convert the dissolved metals into solid particle, a precipitation reagent is added to the mixture.

Filtration can then be used to remove the particles from the mixture.

Eg:    (a)    Softening of water with milk or lime (removed by “Ca” or “Mg”)

(b)   Removal of phosphorous from waste water using Iron Chloride.

(c)    Removal of heavy metals as Hydroxides.

SECONDARY TREATMENT

Aerobic Treatment

Waste water treatment process is governed by aerobic organisms which need oxygen for the breaking process. Aerobic waste water treatment tanks are constantly supplied with oxygen. It has been done by circulating air through the tanks. For effective functioning of aerobic organisms sufficient amounts of oxygen should be present in the aerobic tanks at all times. Therefore, aeration is properly maintained throughout aerobic treatment.

 Anaerobic Treatment

         

Anaerobic waste water treatment is a biological treatment process where organisms, especially bacteria, break down organic material in the waste water in oxygen absent environment. Anaerobic digestion is well known anaerobic waste water treatment process. The degradation of organic material is done anaerobically. For the effective anaerobic digestion of organic material, the entry of air into anaerobic tanks is prevented. 

During anaerobic digestion, Methane and Carbon dioxide are produced. Methane is a biogas. Hence, anaerobic digestion process can be used to produce biogas which can be utilized as electricity.

                                                                      

Biological Treatment

Activated Sludge

Microorganisms such as bacteria and protozoa can use the small particles and dissolved organic matter, not removed in primary treatment, as food. Secondary or biological treatment is performed in a tank containing a “soup” of starved microbes called activated sludge. Like us, these microbes require air to live (they are aerobic organisms) and thus air is pumped into the tank. Microorganisms in this aeration tank use the dissolved and particulate organic matter as food, producing more microorganisms which can be collected and separated from the water in the next step.

It then remains to separate out the microorganisms (activated sludge) so that just clean water is left. This is done in a secondary clarifier which operates in the same manner. Some of the solids collected in the secondary clarifier (return activated sludge) are sent back to the aeration tank to treat more wastewater and the excess (waste activated sludge) is pumped to another location in the pant for further treatment. The clean water that flows out the top of the clarifier is sent along for disinfection.

Trickling filters

Trickling filters are conventional aerobic biological wastewater treatment units, such as active sludge systems or rotating biological contactors which are used to remove organic matter from wastewater. The TF is an aerobic treatment system that utilizes microorganisms attached to a medium to remove organic matter from wastewater. This type of system is common to a number of technologies such as rotating biological contactors and packed bed reactors (Biotowers).

TFs enable organic material in the wastewater to be adsorbed by a population of microorganisms (aerobic, anaerobic, and facultative bacteria; fungi; algae; and protozoa) attached to the medium as a biological film or slime layer (approximately 0.1 to 0.2 mm thick). As the wastewater flows over the medium, microorganisms already in the water gradually attach themselves to the rock, slag, or plastic surface and form a film. The organic material is then degraded by the aerobic microorganisms in the outer part of the slime layer.

As the layer thickens through microbial growth, oxygen cannot penetrate the medium face, and anaerobic organisms develop. As the biological film continues to grow, the microorganisms near the surface lose their ability to cling to the medium, and a portion of the slime layer falls off the filter. This process is known as sloughing. The sloughed solids are picked up by the underdrain system and transported to a clarifier for removal from the wastewater.

Adequate airflow is important to ensure sufficient treatment performance and prevent odours. To evenly distribute the water on the filter, a “rotary sprinkler/distributor” is most often used.

The primary factors that must be considered in the design of trickling filters include

·         the type of filter media to be used

·         the spraying system, and

·         the configuration of the under-drain system

TERTIARY TREATMENT

The final step before releasing the treated wastewater to the environment is to kill microorganisms that might be health hazards. This process is called disinfection.

Disinfection

The process of killing or inactivating pathogens or other microorganisms is known as disinfection. This is done by the following ways:

(a)U.V Treatment:   Exposing water to UV rays will kill the germs and microorganisms in water.

(b) Ozonisation: Ozone is passed into water will kill the germs and microorganisms. It produces powerful disinfectant.

(c) By Passing Chlorine: Chlorine gas is passed to the water. It produces hypochlorous acid, which acts as a disinfectant. Liquid Chlorine is a powerful disinfectant.