WASTERWATER TREATMENT

PART I

To meet current and proposed water pollution standards it is often necessary to use newer treatment operations and to improve the efficiency of conventional processes. The treatment of water and wastewater requires a variety of techniques and processes involving unit operations, transfer processes, thermodynamics and reaction kinetics.

The major industrial users of water are the primary manufactures of metals, chemicals, paper, petroleum, and food products. The pollutant levels in wastewater are often characterized by solid content and by biochemical oxygen demand, which is a measure of the - dissolved oxygen used by microorganisms in biological oxidation of organic matter. The total biochemical oxygen demand (BOD) of aqueous industrial wastes is three times the total BOD of wastes entering municipal waste water treatment plants. Over 90% of the industrial BOD is generated by the chemical, paper, food, and petroleum industries.

The primary metals industries together with these four industries contribute 90% of the solids entering industrial wastewater. The total solids entering sewage treatment plants from domestic wastes are less than one-half of the total solids in industrial wastes. It is apparent that heavier industrial use of the available water must be accompanied by greater treatment to ensure that the levels of toxic chemicals do not increase and become harmful.

As our standard of living advances, our demand for water accelerates, we have to meet the needs of an increasing world population by irrigating more of the unproductive areas and fulfilling the demands for an even greater industrial output. These needs require close stewardship of our water resources carried out by chemists to preserve water quality through waste treatment and to ensure adequate quantities through recycle.

The characteristics of wastewater are broadly classified into physical chemical and biological according to the type of measurement test that has to be performed. The analyses range from the very specific quanti­tative tests usually applied for chemicals to the broad group tests applied to biological classes.

The most important physical characteristic of waste-water is its solids content as it affects the esthetics, clarity and color of the water.

The chemical impurities of wastewater can adversely affect the environment in many different ways. Although the nitrogen, phosphorous and dissolved solids can be removed by the addition of chemicals and by certain tertiary methods, they are not easily removed in a conventional plant. Soluble organics can deplete oxygen levels in streams, and give taste and odor to water supplies. Toxic materials can affect food chains as well as public health. Nutrients can cause eutrophication of lakes. Although some chemical tests are specific, many determine broad classifications due to the variety of compounds found in wastewater. If water reuse is to be widely practiced, these materials will have to be removed.

Biological tests on water and wastewater determine whether pathogenic organisms are present by testing for certain indicator organisms. Biological information is needed to measure water quality for such uses as drinking and swimming, and to assess the degree of treatment of the wastewater before its discharge to the environment.

Many operations are used to purify water before discharge to the environment.

The main objectives of conventional wastewater treatment processes are reduction of biochemical oxygen demand, suspended solids and pathogenic organisms. In addition, it may be necessary to remove nutrients, toxic components, nonbiodegradable compounds, and dissolved solids. Since most contaminants are present in low concentrations, the treatment processes must be able to function effectively with dilute streams.

1. What is necessary to do in order to meet modern water pollution standards?

2. What does the current treatment of water and wastewater require?

3. What are the major industrial users of this century?

4. What is BOD?

5. Where is wastewater treated? Do such plants treat all the industrial wastewater?

6. Why do the modern water consumption standards require close stewardship of chemists?

7. What are the possible ways to preserve water quality?

8. What are the most important characteristics of wastewater?

9. How do chemical impurities influence water quality?

7. To understand the information of the following text check up whether you understand these words correctly:

Removal, settling, clarifier, thickener, tank, clarification, thickening, flocculate, coagulate, softening, stabilization, conditioning, dewatering.

8. Match the phrases in A with the corresponding word-combinations in B:

A B

1. limited pretreatment 1. важное обстоятельство

2. mass transfer 2. реактор для обработки отстоя

3. important consideration 3. массопередача

4. sludge reactor 4. отстойники

5. settling tanks 5. ограниченная предварительная обработка

10.Read the text and arrange the items of the plan according to the information given in the second part.

1. The purpose of biological waste treatment.

2. Mass transfer in wastewater treatment systems.

3. The characteristics of industrial wastes.

4. Treatment process for sludge.

5. Sedimentation process in wastewater treatment plant.