Sewage disposal- Septic tank and STP

SEWAGE DISPOSAL BUILDING SERVICES A. Septic tank B. Sewage treatment plant HIMA KARTHIK. S 1190100870 II YEAR IV SEM SPA VIJAYAWADA

A. SEPTIC TANK Septic tank is a water-tight masonry tank made of brick work, concrete, fibreglass, PVC or plastic, into which blackwater (household sewage) is admitted for treatment.

It’s a satisfactory means of disposing excreta and liquid wastes from individual dwellings, small groups of houses and institutions which have adequate water supplies, but do not have access to a public sewage system.

Concrete septic tank with two chambers

The main design features of a septic tank are as follows: • Capacity: A capacity of 20-30 gallons or 2.5- 5 c.ft. Per person is recommended for household septic tanks. The minimum capacity of a septic tank should be at least 500 gallons.

•Length: The length is usually twice the breadth.

•Depth: The depth of a septic tank is from 1.5 to 2 m (5- 7 ft.).

•Liquid depth: The recommended liquid depth is only 1.2 m (4 ft.).

•Air space: There should be a minimum air space of 30 cm (12 in.) between the level of liquid in the tank and the under surface of the cover.

•Bottom: In some septic tanks, the bottom is sloping towards the inlet end. This facilitates retention of solids.

•Inlet and outlet: There are an inlet and outlet pipe, which are submerged.

•Cover: The septic tank is covered by a concrete slab of suitable thickness and provided with a manhole.

•Retention Period: Septic tanks are designed in this country to allow a retention period of 24 hours.

Too long a retention period will result in undue septicity of the effluent whereas too short a period gives insufficient treatment.

WORKING OF A SEPTIC TANK A septic system has a simple design. It is an underground watertight container (mostly rectangular or round) made of fiberglass, plastic or concrete.

• The tank is connected with two pipes (for inlet and outlet).

The inlet pipe collects the water waste in the septic tank, long enough that the solid and liquid waste is separated from each other.

The outlet pipe also called the drain field, moves out the preprocessed wastewater from the septic tank and spreads it evenly in the soil and watercourses.

• After a while, the wastewater separates in 3 layers.

The top layer is oils and grease and floats above all the waste. This is called scum.

The middle layer is the wastewater along with waste particles.

The bottom layer consists of heavier particles that are heavier than water and form a layer of sludge.

• Inside the tank bacteria from the wastewater breaks down the solid waste.

• These bacteria decompose the solid waste rapidly allowing the liquids to separate and drain away more easily.

OPERATION AND MAINTENANCE • The use of soap water and disinfectants such as phenol should be avoided.

• The contents of the septic tank should be bailed out at least once a year. This operation is called “desludging”. The bailed-out sludge is disposed of by trenching.

• Newly built septic tanks are first filled with water up to the outlet level and then seeded with ripe sludge drawn from another septic tank.

AQUA PRIVY • The aqua privy functions like a septic tank and has been used in different regions in the country. The privy consists of a water-tight chamber filled with water.

• The shape of the tank may be circular or rectangular. A capacity of one cubic metre (35 c.ft.) is recommended for a small family, allowing 6 years or more for cleansing purposes. Aqua privies are designed for public use also.

• Night soil undergoes purification by anaerobic digestion. Since there is evolution of gases, a vent should be provided for the escape of gases into the atmosphere, the vent should be open above the roof of dwellings.

• The digested sludge which accumulates in the tank should be removed at intervals.

B. SEWAGE TREATMENT PLANT Sewage treatment happens in three mainstagespretreatment, primary and secondary Pre-treatment is the preliminary removal of wastewater or sludge constituents, such as oil, grease, and various solids (e.g., sand, fibres and trash).

PRIMARY TREATMENT Primary treatment of wastewater involves sedimentation of solid waste within the water. This is done after filtering out larger contaminants within the water. Wastewater is passed through several tanks and filters that separate water from contaminants.

Screening- Sewage arriving at a disposal work is first passed through a metal screen which intercepts large floating objects such as pieces of wood, rags, masses of garbage and dead animals. Their removal is necessary to prevent clogging of the treatment plant. The screen consists of vertical or inclined steel bars usually set 5 cm (2 in) apart.

GRIT CHAMBER • Sewage is then passed through a long narrow chamber called the grit chamber or detritus chamber.

• This chamber is approximately 10 to 20 metres in length; it is so designed as to maintain a constant velocity of about 1 foot per second, with a detention period of 30 seconds to 1 minute.

• The function of the grit chamber is to allow the settlement of heavier solids such as sand and gravel, while permitting the organic matter to pass through.

Grit chamber

PRIMARY SEDIMENTATION • Sewage is now admitted into a huge tank called the primary sedimentation tank. It is a very large tank, holding from 1 / 4 to 1 / 3 the dry weather flow. Sewage is made to flow very slowly across the tank at a velocity of 1- 2 feet per minute.

• The sewage spends about 6-8 hours in the tank. Nearly 50-70 per cent of the solids settle down under the influence of gravity. A reduction of between 30 to 40 per cent in the number of coliform organisms is obtained.

Primary sedimentation tank

• The organic matter which settles down is called sludge and is removed by mechanically operated devices, without disturbing the operation in the tank.

• When the sewage contains organic trade wastes, it is treated with chemicals such as lime, aluminium sulphate and ferrous sulphate. Addition of one of these chemicals precipitates the animal protein material quickly.

SECONDARY TREATMENT Secondary treatment is a treatment process for wastewater (or sewage) to achieve a certain degree of effluent quality by using a sewage treatment plant with physical phase separation to remove settleable solids and a biological process to remove dissolved and suspended organic compounds.

• The effluent from the primary sedimentation tank still contains a proportion of organic matter in solution or colloidal state, and numerous living organisms.

• It is subjected to further treatment, aerobic oxidation, by one of the following methods trickling filter method and activated sludge process.

TRICKLING FILTER METHOD • The trickling filter or percolating filter is a bed of crushed stones, 1 to 2 m (4- 8 ft.) deep and 2 to 30 m (6-100 ft.) in diameter, dependin upon the size of the population.

• The device consists of hollow pipes each of which have 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 very complex biological growth consisting of algae, fungi, protozoa and bacteria of many kinds occurs. This is 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 action of the filter is thus purely a biological one, and not one of filtration as the name suggests.

The term “filter” is a misnomer. The trickling filters are very efficient in purifying sewage.

•They do not need rest pauses, because wind blows freely through the beds supplying the oxygen needed by the zoogleal flora.

•The biological growth or zoogleal layer lives, grows and dies. The dead matter sloughs off, breaks away and is washed down the filter.

• It is a light green, flocculent material and is called “humus”. The oxidized sewage is now led into the secondary sedimentation tanks or humus tanks

ACTIVATED SLUDGE PROCESS • 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 mixture is subjected to aeration in the aeration chamber for about 6 to 8 hours.

• During the process of aeration, the organic matter of the sewage gets oxidized into carbon dioxide, nitrates and water with the help of the aerobic bacteria in the activated sludge.

SECONDARY SEDIMENTATION The purpose of secondary clarifiers is to separate the incoming biomass from the biological reactors into clarified effluent and thickened sludge. For processes such as trickling filters and rotating biological contactors, the solids are typically settled and wasted, similar to primary sedimentation.

• The sludge that collects in the secondary sedimentation tank is called ‘aerated sludge’ or activated sludge, because 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 pumped into the sludge digestion tanks for treatment and disposal Secondary sedimentation tank

REFERENCES • https://medium.com/waste-disposal-hub/how-septic-tanks-work-and-when-to-emp ty-them-346a4fe4fe6f • https://www.organicawater.com/primary-secondary-tertiary-wastewater-treatmentwork/#:~:text=Primary%20Wastewater%20Treatment,that%20separate%20water%2 0from%20contaminants .

• https://www.arvindenvisol.com/secondary-wastewater-treatment/ • https://aesarabia.com/trickling-filters/

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