INTRODUCTION

 

Methods of waste disposal date from ancient times, and sanitary sewers have been found in the ruins of the prehistoric cities of Crete and the ancient Assyrian cities. Storm-water sewers built by the Romans are still in service today. Although the primary function of these was drainage, the Roman practice of dumping refuse in the streets caused significant quantities of organic matter to be carried along with the rainwater runoff.

The issue of sewage disposal assumed increasing importance in the early 1970s as a result of the general concern expressed in the United States and worldwide about the wider problem of pollution of the human environment, the contamination of the atmosphere, rivers, lakes, oceans, and groundwater by domestic, municipal, agricultural, and industrial waste.

Domestic waste discharge in Persia has run about 200 liters per person daily, governed by such things as climate, habit of the people and the cost of the water. This waste water supply, if discharge is untreated, turns rivers and streams into open sewers with no possibility of reuse of the water for domestic or recreational purposes.

 



Sewage Definitions:

Is the content of a sewer or drain; refused liquids or matter carried off by sewers.

Domestic sewage consists of human wastes, paper, and vegetable matter. This type of waste is organic because it consists of compounds of carbon. Sewage can consist of 99.9% water and 0.1% solids.

 

Sewerage

Is a system of pipes used to collect and carry sewage, which is the wastewater, discharged from domestic premises.

 

Sewage treatment


It means removing impurities so that the remaining waste water can be safely returned to the river or sea and become part of the natural water cycle again. A sewage treatment plant separates solids from liquids by physical processes and purifies the liquid by biological processes.

 


History of the Sewage:

At the beginning of the 20th century, a few cities and industries began to recognize that the discharge of sewage directly into the streams caused health problems, and this led to the construction of sewage-treatment facilities. At about the same time, the septic tank was introduced as a means of treating domestic sewage from individual households both in suburban and rural areas. Because of the abundance of diluting water and the presence of sizable social and economic problems during the first half of the 20th century, few municipalities and industries provided wastewater treatment.

During the 1950s and 1960s, the U.S. government encouraged the prevention of pollution by providing funds for the construction of municipal waste-treatment plants, water-pollution research, and technical training and assistance. New processes were developed to treat sewage, analyze wastewater, and evaluate the effects of pollution on the environment. In spite of these efforts, however, expanding population and industrial and economic growth caused the pollution and health difficulties to increase.

In response to the need to make a coordinated effort to protect the environment, the National Environmental Policy Act (NEPA) was signed into law on January 1, 1970. In December of that year, a new independent body, the Environmental Protection Agency (EPA) was created to bring under one roof all of the pollution-control programs related to air, water, and solid wastes. In 1972 the Water Pollution Control Act Amendments expanded the role of the federal government in water pollution control and significantly increased federal funding for construction of waste-treatment works. Congress has also created regulatory mechanisms and established uniform effluent standards.


Nature of Sewage:

The origin, composition, and quantity of waste are related to existing life patterns. When waste matter enters water, the resulting product is called sewage or wastewater.

Sewage contains mineral, animal and vegetable matter in suspension and large numbers of bacteria. It contains paper, food, grease and almost anything that can be carried by water flowing in a sewer. Fresh sewage has a musty odor, which is not unlikable, but decomposition sets in after a few hours, with the resultant production of foul odors. Such as you have undoubtedly noticed emanating from septic tank effluent.

The concentration of suspended and dissolved matter is actually only about 1/10 of 1%, and about 50% of this is mineral matter that is ineffective. Generalizing, the constitution of sewage is:

                  Nitrogen compounds

                  Carbohydrates

                  Fats and soap

                  Mineral matter

                  Living organisms in the form of plants and animals Mineral

                  Matter Greases

 

SEWAGE TREATMENT:

Plants and animals play an important part in sewage treatment processes by transforming the organic matter to simpler compounds. Sewage is their food supply. Most of them are very small, single celled organisms, but some can be seen with the naked eye .The plants consist of algae and fungi, which are the simplest forms of plants without roots or leaves. Algae contain chlorophyll, which is green coloring matter, and chlorophyll makes carbohydrates from carbon dioxide by photosynthesis. Fungi do not contain chlorophyll and depend upon ready-made food.

Besides domestic sewage there is industrial waste. Many industrial wastes are also organic in composition and can be treated by microorganisms in the same way as domestic sewage. This type of treatment is called biological treatment and the strength of the sewage is measured in terms of B.O.D. or biochemical oxygen demand. This is a measure of the amount of oxygen used by the microorganisms in breaking down the sewage into stable compounds.

Thus in a town the sewerage system will collect the sewage from domestic, commercial, and industrial premises and carry it to the nearest river or to the sea. The dilutions available in the receiving water have traditionally determined the extent of treatment necessary.

In sewage treatment the bacteria play the most important part. They are the single celled organisms of the vegetable kingdom, and are found in air, soil, water and the intestinal tract of man and animals. They multiply by splitting in two and growth is retarded or stimulated by dissolved oxygen, pH, food supply and disinfecting agents.

In sewage treatment we classify bacteria by their oxygen requirement into three groups, the aerobic group, which require dissolved oxygen for their existence; the anaerobic group, which exist in the absence of dissolved oxygen and obtain their oxygen supply from nitrites, sulfates and other organic compounds; the facultative group, which are either aerobic or anaerobic, usually depending upon which predominates.

 

 

 

 

 

 

 

SEWAGE TREATMENT PROCESSES:

1.     Preliminary treatment:


Solids like wood, paper, rags and plastic are removed by screens, washed, dried and taken away for safe disposal at a licensed waste tip. Grit and sand, which would damage pumps, are also removed and disposed of in a similar way.

 


2. Primary treatment:

The remaining solids are separated from the liquid by passing the sewage through large settlement tanks, where most of the solid material sinks to the bottom. About 70% of solids settle out at this stage and are referred to as sludge. The sludge is used on farms after further treatment called sludge treatment.


 


3. Secondary treatment:

A biological process that relies on naturally occurring microorganisms acting to break down organic material and purify the liquid. In a simple sewage treatment process, microorganisms are encouraged to grow on stones over which the sewage is trickled. The microorganisms, which need oxygen to thrive, feed on the bacteria in the sewage and purify the water. These treatment units are called percolating filters.


This process can be speeded up by blowing air into tanks of sewage where the microorganisms float freely and feed on the bacteria. These treatment units are called aeration tanks.

 

 


Following either form of secondary treatment, the wastewater is settled in tanks to separate the biological sludge from the purified wastewater.

 

 

 

 

 

4. Tertiary treatment:

Sometimes, extra treatment is needed to give the wastewater a final polish. This is known as tertiary treatment. It can reduce the organic matter content still further if this is necessary. Various methods may be used, including sand filters, reed beds or grass plots. Disinfections, using ultra violet light to kill bacteria, is another method, and is being used at a number of coastal sewage treatment schemes. Several forms of tertiary treatment are available for example:

1.     Passing the effluent over grass plots

2.     Retaining the effluent in lagoons

3.     Filtering through sand beds

4.     Using a bed of pebbles or wedge wire in the final settling tanks.

5. Nutrient removal:

Consists of reducing the phosphorus and nitrogen in the sewage so as to prevent plant growth in the receiving waters.

In addition to reducing the BOD and suspended solids consideration must nowadays be given to reducing the amount of phosphorus and nitrogen in effluents. These elements act as nutrients or fertilizers and when the effluent is discharged into a lake the phosphorous can cause algae to grow. Thus it is necessary to control the amount of phosphorus discharged with the effluent. This is done by adding aluminum or ferric salts to the treatment process. This has the effect of causing the phosphorus to settle out of the effluent and into the sludge.

Where the effluent is discharged into the sea it may be necessary to reduce the nitrate content by a process of denitrification, which will reduce the nitrate content to 10 mg/l.

 

Summary of Sewage Treatment Processes:

The three general phases of treatment are primary, secondary, and tertiary. During primary treatment, a large percentage of the suspended solids and inorganic material is removed from the sewage. The focus of secondary treatment is reducing organic material by accelerating natural biological processes. Tertiary treatment is necessary when the water will be reused; 99 percent of solids are removed and various chemical processes are used to ensure the water is as free from impurity as possible.

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