Module 9. Pollutions and control

Lesson 16

16.1 Introduction

Water is the essential element that makes life on earth possible. Without water there would be no life. We usually take water for granted. It flows from our taps when they are turned on. Most of us are able to bathe when we want to, swim when we choose and water our gardens. Like good health we ignore water when we have it.

16.2 Water Availability on the Planet

Although 71% of the earth’s surface is covered by water only a tiny fraction of this water is available to us as fresh water. About 97% of the total water available on earth is found in oceans and is too salty for drinking or irrigation. The remaining 3% is fresh water. Of this 2.997% is locked in ice caps or glaciers. Thus only 0.003% of the earth’ total volume of water is easily available to us as soil moisture, groundwater, water vapor and water in lakes, streams, rivers and wetlands.
Surface water: Water that is found in streams, rivers, lakes, wetlands and artificial reservoirs is called surface water.
* Ground water: Water that percolates into the ground and fills the pores in soil and rock is called groundwater.

* Aquifers: Porous water-saturated layers of sand, gravel or bedrock through which ground water flows are called aquifers. Most aquifers are replenished naturally by rainfall that percolates downward through the soil and rock. This process is called natural recharge. If the withdrawal rate of an aquifer exceeds its natural recharge rate, the water table is lowered. Any pollutant that is discharged onto the land above is also pulled into the aquifer and pollutes the groundwater resulting in polluted water in the nearby wells.

16.3 Causes of Water Pollution

There are several classes of common water pollutants. These are

16.3.1 Disease causing pathogens

Disease-causing agents (pathogens) which include bacteria, viruses, protozoa and parasitic worms enter water from domestic sewage and untreated human and animal wastes.

16.3.2 Oxygen depleting wastes

These are organic wastes that can be decomposed by aerobic bacteria. Large populations of bacteria use up the oxygen present in water to degrade these wastes. In the process this degrades water quality. The amount of oxygen required to break down a certain amount of organic matter is called the biological oxygen demand (BOD). The amount of BOD in the water is an indicator of the level of pollution. If too much organic matter is added to the water all the available oxygen is used up. This causes fish and other forms of oxygen dependent aquatic life to die. Thus anaerobic bacteria begin to break down the wastes. Their anaerobic respiration produces chemicals that have a foul odor and an unpleasant taste that is harmful to human health.

16.3.3 Inorganic plant nutrients

These are water soluble nitrates and phosphates that cause excessive growth of algae and other aquatic plants. The excessive growth of algae and aquatic plants due to added nutrients is called eutrophication. They may interfere with the use of the water by clogging water intake pipes, changing the taste and odor of water and cause a buildup of organic matter. As the organic matter decays, oxygen levels decrease and fish and other aquatic species die.

16.3.4 Water soluble inorganic chemicals

These are acids, salts and compounds of toxic metals such as mercury and lead. High levels of these chemicals can make the water unfit to drink, harm fish and other aquatic life, reduce crop yields and accelerate corrosion of equipment that use this water.

16.3.5 Organic chemicals

These which include oil, gasoline, plastics, pesticides, cleaning solvents, detergent and many other chemicals. These are harmful to aquatic life and human health. They get into the water directly from industrial activity either from improper handling of the chemicals in industries and more often from improper and illegal disposal of chemical wastes.

16.3.6 Sediment of suspended matter

These are insoluble particles of soil and other solids that become suspended in water. This occurs when soil is eroded from the land. High levels of soil particles suspended in water, interferes with the penetration of sunlight. This reduces the photosynthetic activity of aquatic plants and algae disrupting the ecological balance of the aquatic bodies. When the velocity of water in streams and rivers decreases the suspended particles settle down at the bottom as sediments. Excessive sediments that settle down destroys feeding and spawning grounds of fish, clogs and fills lakes, artificial reservoirs etc.

16.3.7 Water soluble radioactive isotopes

These can be concentrated in various tissues and organs as they pass through food chains and food webs. Ionizing radiation emitted by such isotopes can cause birth defects, cancer and genetic damage.

16.3.8 Hot water

Hot water let out by power plants and industries that use large volumes of water to cool the plant result in rise in temperature of the local water bodies. Thermal pollution occurs when industry returns the heated water to a water source.

16.3.9 Oil

Oil is washed into surface water in runoff from machines (lubricant) which also pollutes ground water.

16.4 Groundwater Pollution

Groundwater is easy to deplete and pollute, it gets renewed very slowly and hence must be used judiciously. Groundwater flows are slow and not turbulent hence the contaminants are not effectively diluted and dispersed as compared to surface water. Moreover pumping groundwater and treating it is very slow and costly. Hence it is extremely essential to prevent the pollution of groundwater in the first place. Activities at dairy plants have the potential to contaminate both surface waters and groundwater. Water and land pollution can be avoided by appropriate sitting, design, management and control of the dairy plant.

16.5 Sources of Dairy Waste Water

Approximately 65% of dairy factory losses enter waste water discharge streams and these can have a major impact on the environment.

The main sources of dairy processing plant waste water are:

* Raw material (predominantly milk) and product losses from leaking equipment and pipe lines, and spills caused by equipment overflows and malfunctions and by poor handling procedures

* Materials used for cleaning and sanitizing

* By-products such as whey from the manufacture of cheese and casein.

16.6 Components of Dairy Waste Water

The major contaminants in dairy processing waste water are milk solids that contain milk fat, protein, lactose and lactic acid. Other minor constituents include sodium, potassium, calcium and chloride. Organic waste water strength is measured by either BOD or COD. Typical process waste water has a biochemical oxygen demand (BOD5) of about 2,000 mg/L and a dissolved solids concentration of 1,800 mg/L. BOD5 is a measure of the amount of organic matter that is able to be biologically oxidized over a five day period. Whey has a BOD5 concentration of 30,000-40,000 mg/L. Where the whey is not used as a by-product but is discharged as effluent, it will increase the BOD level of waste water and cause treatment and disposal problems. Whole milk has a BOD of 100,000 mg/L. Although the throughput of milk in dairy plants is generally increasing, the technologies available for reducing and recycling wastes means that the volume of water used and waste water generated is significantly less in modern plants. Because of the highly seasonal nature of milk production, during peak periods the volume of waste water generated at dairy plants may be several times greater than during off peak periods. The batch nature of many processes, and intermittent operations such as cleaning and sanitizing, also means a wide daily variation in waste water flows and quality. Options for dairy factory waste water include:

* Treatment to a suitable standard for reuse or recycling

* Discharge to local authority sewers under a trade waste agreement (with pre-treatment as necessary

* Appropriate treatment and land discharge wherever practicable and environmentally beneficial.

Many dairy plants have technologies in place for recovering waste water and/or condensate(from production of milk powder) for reuse in the dairy plant. Reuse and recycling can considerably decrease the volume of main water required to operate the plant and also reduce the cost of both mains water and waste water disposal. Fats, milk solids and minerals can also be recovered from waste water and recycled – either at the dairy plant or offsite. Cleaning chemicals can also be recovered and reused on site. Treatment and discharge to land, the dissolved salts contained in dairy plant waste water can adversely affect soil structure if waste water is used to irrigate land. Waste water can also leach into underlying groundwater and affect its quality. Dairy plants should maximise the recovery, recycling and reuse of acids and alkali to minimise the dissolved salts and sodium levels in the waste water. High salt levels affect the type of vegetation that grows. The volume and organic load of waste water from just one dairy factory during peak season may well exceed the township's domestic waste. This may overload the sewage treatment plant, cause odours and give rise to poor effluent quality.

16.7 Treatment of a Waste Water System

16.7.1 Segregation

Waste streams from the plant should be segregated – for example, whey can be reused to produce whey powder or stock feed. Used up cleaning solutions should be separated from other waste water streams as they can be treated to recover cleaning agents. Highly saline wastewater should also be discharged separately to an evaporation pond where the salts can be recovered and recycled.

16.7.2. Equalisation and pH control

A balance tank or pond is used as equalisation tank to control pH and temperature. pH can be controlled by adding used up acid and alkali cleaners to neutralise each other.

16.7.3. Fat removal

Coarse milk solids should be removed by screening. Fats can constitute up to 50% of the organic load. Its recovery is therefore significant in any treatment process. Dissolved air flotation is a very effective method of separation of fat.

16.7.4. Removal of organic load

Organic load can be reduced by physical methods – such as microfiltration, reverse osmosis and flotation techniques – or by biological treatments – such as activated sludge systems, trickling filters and anaerobic digesters. Lagoons, land irrigation and grass filtration systems can also reduce organic loads but reduction will occur at a slower rate than the previous methods. Best practice management of the waste stream may include removal of product before treatment.

* Activated sludge: A highly effective method for treatment of dairy plant waste water is the oxidation ditch. This is a development of the extended aeration process where aeration, settling and withdrawal of effluent all takes place in the same tank.The oxidation ditch process is characterised by a long retention time and low net sludge yield. This type of treatment lends itself to biological nitrogen removal.

* Trickling filters: The best trickling filters have a free passage of air to prevent the generation of odours but are sensitive to high or low pH which may result in killing the biomass.

* Lagoons: Highest quality wastewater and low odour generation can be achieved in aerated lagoons which use floating aerators to force oxygen input and reassemble activated sludge systems.

16.7.5 Advanced treatment for reuse

Membrane filtration: This process has the potential for acid and alkali recovery and recycling. Best quality wastewater is obtained by pumping effluent through porous media containing millions of tiny pores. The media area is regularly cleaned by high pressurebackwash using water and/or air. The removal of dissolved solids is best achieved by the passage of water through a semi-permeable membrane that restricts the movement of salts. This process for the desalination of wastewater is based on the osmotic pressures on either side of the membrane.

16.7.6 Land irrigation

In some areas, treated wastewater can be either sprayed on the land or used for irrigation.

16.8 Measures to Reduce Water Pollution in Dairy Plant

The following measures have to be taken to control the water pollution in dairy plant.

* Prevent/reduce raw material and product losses

* Reduce water usage (as minimum as possible)

* Treat wastewater to a suitable standard for reuse or recycling

* Design and construct wastewater treatment system and use the waste water for irrigation.

Last modified: Friday, 5 October 2012, 7:23 AM