Liquid waste management/Sewage disposal

Introduction

Sewage refers to wastewater generated by a community, comprising both solid and liquid excreta, originating from residential areas, street and yard cleaning, as well as industrial sources. It typically manifests as contaminated water with an unpleasant odor. The term “sullage” specifically pertains to wastewater devoid of excreta, such as the effluents from kitchens and bathrooms.

Composition of sewage

Water: 99.9%
Organic and inorganic solids: 0.1%

In terms of microbial content:

One gram of feces may contain approximately 1000 million Escherichia coli (E. coli), 10-100 million fecal streptococci, and 1-10 million spores of Clostridium perfringens.

Sources

Municipalities (houses, businesses, institutions): Originating from sinks, tubs, showers, and toilets.
Agricultural activities: Rainfall and irrigation runoff from farms, carrying fertilizer, pesticides, manure, and soil.
Industry: Industrial wastewater may encompass petroleum products, metals, acids, and other chemicals, as well as nutrients and sediments.

Health aspects of sewage

Creation of nuisance, unsightliness, and unpleasant odors.
Breeding of flies and mosquitoes.
Pollution of soil and water supplies.
Contamination of food.
Increase in the incidence of diseases, especially enteric and helminthic diseases.

Decomposition of organic matters

Aerobic process: In the presence of oxygen, organic matter undergoes breakdown into CO2, water, ammonia, nitrites, nitrates, and sulfates through the action of bacteria, including fungi and protozoa.
Anaerobic process: Anaerobic bacteria decompose organic waste into methane, ammonia, CO2, and H2 in the absence of oxygen.

Modern sewage treatment

The treatment of sewage can be divided into two stages: primary treatment and secondary treatment. In the primary treatment, solids are separated from the sewage through a combination of screening and sedimentation. These separated solids undergo anaerobic digestion, representing the first stage of purification. In the secondary treatment, the effluent resulting from the primary treatment undergoes aerobic oxidation as the second stage of purification.

Primary treatment

1. Screening

Sewage arriving at a treatment facility undergoes initial screening by passing through a metal screen. This screen effectively captures large floating objects such as pieces of wood, rags, masses of garbage, and deceased animals. The removal of these materials is crucial to prevent the clogging of the treatment plant. Typically, the screen comprises vertical or inclined steel bars, usually set approximately 5 cm (2 in) apart. In some plants, the screens are of the fixed type, while in others, they are of the moving type. The screenings are periodically removed either manually or mechanically and disposed of through methods such as trenching or burial.

2. Grit chamber

After screening, the sewage undergoes the next stage in the primary treatment, passing through a long narrow chamber known as the grit chamber or detritus chamber. This chamber, measuring approximately 10 to 20 meters in length, is designed to maintain a constant velocity of about 1 foot per second, with a detention period ranging from 30 seconds to 1 minute. The primary function of the grit chamber is to facilitate the settlement of heavier solids, such as sand and gravel, while allowing organic matter to pass through.

The collected grit at the bottom of the chamber is periodically or continuously removed and disposed of through methods like plain dumping or trenching.

3. Primary sedimentation

After passing through the grit chamber, the sewage is directed to a large tank known as the primary sedimentation tank. This tank is expansive, holding approximately 1/4 to the dry weather flow. Various designs exist for primary sedimentation tanks, with the rectangular tank being the most common. The sewage is allowed to flow very slowly across the tank at a velocity of 1-2 feet per minute, spending about 6-8 hours in the tank.

During this prolonged period of relatively still conditions in the tank, a significant amount of purification occurs primarily through the sedimentation of suspended matter. Nearly 50-70% of the solids settle down under the influence of gravity, resulting in a reduction of approximately 30 to 40 per cent in the number of coliform organisms. The organic matter that settles down is termed sludge and is removed by mechanically operated devices without disrupting the tank’s operation.

Simultaneously, a minor amount of biological action takes place wherein microorganisms present in the sewage attack complex organic solids, breaking them down into simpler soluble substances and ammonia. A fraction of fat and grease rises to the surface, forming scum, which is removed periodically. When the sewage contains organic trade wastes, chemicals such as lime, aluminum sulfate, and ferrous sulfate are added. The addition of one of these chemicals facilitates the rapid precipitation of animal protein material.

Secondary treatment

The effluent from the primary sedimentation tank still contains a proportion of organic matter in solution or a colloidal state and numerous living organisms. It has a high demand for oxygen and can cause pollution of soil or water. It undergoes further treatment, aerobic oxidation, by one of the following methods:

a. Trickling filter method:

The trickling filter or percolating filter is a bed of crushed stones or cinker, 1 to 2 meters (4-8 ft.) deep and 2 to 30 meters (6-100 ft.) in diameter, depending on the size of the population. The effluent from the primary sedimentation tank is sprinkled uniformly on the surface of the bed by a revolving device. This device consists of hollow pipes, each with a row of holes. The pipes keep rotating, sprinkling the effluent in a thin film on the surface of the filter. Over the surface and down through the filter, a complex biological growth occurs, including algae, fungi, protozoa, and bacteria of many kinds, known as the “zoogleal layer.” As the effluent percolates through the filter bed, it gets oxidized by the bacterial flora in the zoogleal layer. The term “filter” is a misnomer, as the action of the filter is purely biological, not one of filtration. The trickling filters are efficient in purifying sewage, as they do not need rest pauses due to the free circulation of wind through the beds supplying the oxygen needed by the zoogleal flora. The biological growth or zoogleal layer undergoes a continuous cycle of growth, death, and sloughing off, forming a material called “humus.” The oxidized sewage is then directed into the secondary sedimentation tanks or humus tanks.

b. Activated sludge process:

The activated sludge process is the modern method of purifying sewage, replacing the trickling filter. The “heart” of the activated sludge process is the aeration tank. The effluent from the primary sedimentation tank is mixed with sludge drawn from the final settling tank (also known as activated sludge or return sludge; this sludge is a rich culture of aerobic bacteria). The proportion of activated sludge to the incoming effluent is around 20 to 30%. The mixture undergoes aeration in the aeration chamber for about 6 to 8 hours, accomplished either by mechanical agitation or by forcing compressed air continuously from the bottom of the aeration tank (diffuse aeration). During aeration, the organic matter in the sewage oxidizes into carbon dioxide, nitrates, and water with the help of the aerobic bacteria in the activated sludge. Typhoid and cholera organisms are destroyed, and coliforms are significantly reduced. Activated sludge plants occupy less space and are suitable for larger cities, while the trickling filter method may be preferred for smaller towns due to cost-effectiveness and ease of operation.

Secondary Sedimentation:

The oxidized sewage from the trickling filter or aeration chamber is led into the secondary sedimentation tank, where it is detained for 2-3 hours. The sludge that collects in the secondary sedimentation tank is called ‘aerated sludge’ or activated sludge, as it is fully aerated. It differs from the sludge in the primary sedimentation tank in that it is practically inoffensive and is rich in bacteria, nitrogen, and phosphates. It is a valuable manure, if dehydrated. Part of the activated sludge is pumped back into the “aeration tanks” in the activated sludge process, and the rest is pumped into the sludge digestion tanks for treatment and disposal.

Sludge Digestion

One of the greatest problems associated with sewage treatment is the treatment and disposal of the resulting sludge. One million gallons of sewage produces 15-20 tons of sludge. The sludge is a thick, black mass containing 95% water, and it has a revolting odor. There are several methods of sludge disposal:

a. Digestion: Modern sewage treatment plants employ the digestion of sludge as the method of treatment. If sludge is incubated under favorable conditions of temperature and pH, it undergoes anaerobic auto-digestion, in which complex solids are broken down into water, carbon dioxide, methane, and ammonia. The volume of sludge is considerably reduced. Complete sludge digestion takes 3-4 weeks or longer. The residue is an inoffensive, sticky, and tarry mud that dries readily and forms excellent manure. Sludge digestion is carried out in special tanks known as “sludge digestion tanks.” Methane gas, a by-product of sludge digestion, can be used for heating and lighting purposes.

b. Sea disposal: Coastal towns and cities can dispose of sludge by pumping it into the sea.

c. Land: Sludge can be disposed of by composting with town refuse.

Disposal of effluent

(a) Disposal by dilution: Disposal into water courses such as rivers and streams is called ‘disposal by dilution.’ The effluent is diluted in the body of water, and the impurities are oxidized by the dissolved oxygen in water. The diluting capacity of the river or the receiving body of water and its dissolved oxygen contents are important considerations before discharging the effluent into a river or any body of water. Since people use river water for drinking purposes, the effluent must be rendered free from pathogenic organisms by adequate chlorination. The Royal Commission in England in its Fifth Report (1908) recommended that effluent from a sewage treatment plant should not have more than 30 mg/litre of suspended solids, and the 5-day BOD of the effluent, including the suspended matter, should not exceed 20 mg/litre. These standards have been the backbone of subsequent work on the purity of sewage effluent. In recent years, industry has released hundreds of new chemicals into the sewerage system. Some of these chemicals are not removed by biological treatment, and consequently, the effluent may contain substances toxic to humans or substances that can harm fish, damage agriculture, or interfere with the normal functioning of a stream. In many places in the UK, effluent standards have been raised from the original Royal Commission values of 30 mg per liter of suspended solids and 20 mg per liter of BOD to 10 mg per liter of each. The World Health Organization is addressing this problem and fostering research in “tertiary” methods of treatment or “polishing” the effluent further.

(b) Disposal on land: If suitable land is available, the effluent can be used for irrigation purposes (e.g., the Okhla Sewage Treatment Plant in Delhi).

Other methods of sewage disposal.

Sea outfall
River outfall
Land treatment
Oxidation ponds
Oxidation ditches.

a. Sea outfall: Sea coast towns and cities may dispose of their sewage by discharging it into the sea. For instance, nearly two-thirds of untreated sewage from Greater Mumbai is discharged every day into the Arabian Sea. Purification takes place through dilution in the large body of seawater, and the solids gradually oxidize. The drawback of this method is that offensive solid matter may be washed back to the shore, creating a public nuisance. To prevent this, the sewage outfall is designed to discharge the sewage into deep water at multiple points.

b. River outfall: Raw sewage should never be discharged into rivers. Current practice involves purifying sewage before discharge into rivers. The extent of purification required depends on the dilution the river provides for aeration and self-purification.

c. Land treatment (Sewage farming): If sufficient and suitable land (porous soil) is available, sewage may be applied to the land after grit removal, screening, and a short period of settlement. This treatment, known as Sewage Farming or Broad Irrigation, is practiced in some Indian towns and cities. Approximately one acre of land is needed to treat the sewage of 100-300 persons. The land is first divided into ridges and furrows. Sewage is intermittently fed into the furrows, and crops are grown on the ridges. Suitable crops are those that do not come in contact with sewage and are likely to be eaten raw, such as fodder grass and potatoes. Fruit trees with fruits high above the ground (e.g., plantain) can also be grown. However, certain crops like sugar cane, coriander, cucumber, tomato, onion, etc., should be avoided. Proper management by a competent agricultural expert is crucial, and during the rainy season, alternate disposal methods may be necessary.

d. Oxidation pond: A cost-effective method of sewage treatment is the oxidation pond, also known as the waste stabilization pond, redox pond, or sewage lagoon. The term “waste stabilization pond” is more appropriate, encompassing both sewage and industrial wastes. Although an older method, oxidation ponds have gained attention from public health engineers recently. An oxidation pond is an open, shallow pool, 1 to 1.5 meters (3-5 ft.) deep, with an inlet and outlet. Essential components for an oxidation pond include algae, certain bacteria feeding on decaying organic matter, and sunlight. Organic matter in the sewage is oxidized by bacteria to simple chemical compounds, such as carbon dioxide, ammonia, and water. Algae, aided by sunlight, utilize carbon dioxide, water, and inorganic minerals for growth, establishing a mutually beneficial biological balance. Oxygen required for oxidation comes from the atmosphere and the algae, which releases oxygen under sunlight. Oxidation ponds function aerobically during sunshine hours and partially anaerobic during the night. Properly maintained oxidation ponds produce an effluent suitable for growing vegetable crops through land irrigation or for discharge into rivers or other water bodies after appropriate treatment. Mosquito nuisance is minimized by controlling weed growth and maintaining a clear water line. Properly maintained oxidation ponds do not produce odor nuisances and have become an established method for small community sewage purification.

e. Oxidation ditches: Other recommended methods include (1) oxidation ditches and (2) aerated lagoons. These methods use mechanical rotors for extended aeration. For the treatment of wastes from a population between 5,000 to 20,000, an oxidation ditch requires one acre, compared to 22 acres for an oxidation pond and 2.5 acres for an aerated lagoon. These low-cost treatment methods are effective for sewage purification.