Working of a Septic Tank
The solids settle down in the tank to form "sludge", while the lighter solids including grease and fat rise to the surface to form scum. The solids are attacked by the anaerobic bacteria and fungi and are broken down into simpler chemical compounds. This is the first stage of purification called anaerobic digestion. The sludge is much reduced in volume as a result of anaerobic digestion and is rendered stable and inoffensive. A portion of the solids is transferred into liquids and gases (principally methane) which rises to the surface in the form of bubbles. The liquid which passes out of the outlet pipe from time to time is called the "effluent". It contains numerous bacteria, cysts, helminthes ova and organic matter in solution or fine suspension. The effluent is allowed to percolate into the subsoil. There are millions of aerobic bacteria in the upper layers of the soil which attack the organic matter present in the effluent. As a result the organic matter is oxidised into stable end products like nitrates, carbon dioxide and water. This stage of purification is called aerobic oxidation.
Operation and maintenance of Septic Tank
- The use of soap water and disinfectants such as phenol should be avoided as they are injurious to the bacterial flora in the septic tank
- Undue accumulation of sludge reduces capacity of the septic tank and interferes with proper working. Hence, the contents of the septic tank should be removed out at least once a year. This is called as desludging
Aqua privy It is same as septic tank in function. The privy consists of a water-tight chamber filled with water. A short length of a drop pipe from the latrine floor dips into the water. The shape of the tank may be circular or rectangular. The size of the tank depends upon the number of users. Night soil undergoes purification by anaerobic digestion. Since there is evolution of gases, a vent should be provided to open above the roof of dwellings.
- Sulabh Shauchalaya: The "Sulabh Shauchalaya" model, the invention of a Patna-based firm, is a low cost pour-flush, water-seal type of latrine, which is now being used in many parts of India. Excreta undergoes bacterial decomposition and is converted to compost. The method requires very little water. Sulabh International investors build and maintain the system of community latrines.
- Chemical closet: It has very limited use under Indian conditions. The closet consists of a metal tank containing a disinfectant fluid. The active ingredients of the fluid are formaldehyde and quaternary ammonium compounds. In addition, a harmless water dye and a deodorizing substance are usually incorporated. A seat with a cover is placed directly over the tank. Nothing except the toilet paper should be thrown into the chemical closet.
- Shallow and Deep Trench Latrines: These are simple trenches dug with ordinary tools. Depth of trenches varies from 3 and 6 feet respectively and length varying depending on the number of users.
Sewered system / Water carriage system
It implies collecting and transporting human excreta and waste water from residential, commercial and industrial areas, by a network of underground pipes (sewers) to the place of final disposal. It is the best method. There are two types in water carriage system – the combined sewer system and the separate sewer system.
- Combined system: The sewers carry both the sewage and surface water
- Separate system: Surface water is not admitted into sewers. The separate system is the system of choice today.
A water carriage system consists of the following elements.
- Household sanitary fittings (plumbing system of buildings)
- House sewers/drain
- Street sewers or Trunk sewers
- Sewer appurtenances: manholes, traps etc.
- Household sanitary fittings: Every house is expected to be connected to the nearest sewer. The usual household sanitary fittings are (i) water closet, (ii) urinal and (iii) wash basin. Water closets may be broadly divided into two types: Indian squatting type and western type
- House drain: it is usually 4 inches in diameter and is laid in the court yard about 6 inches below the ground level on a bed of cement concrete with sufficient gradient towards the main drain. The house drain empties the sewage into the main sewer or public drain.
- Trunk sewers: also known as public sewer, they are laid on a bed of cement below the ground level, with sufficient gradient to ensure "self-cleansing velocity. The trunk sewers collect sewage from several houses and transport to the place of final disposal.
- Sewer appurtenances: These are (a) manholes and (b) traps installed in the sewerage system. MANHOLES are openings built into the sewerage system at the point of direction change or meeting point of many sewers. TRAPS are of various kinds, these are devices designed to prevent foul gases entering the houses and to remove sand, grit and grease from sewage.
What is sewage?
It is waste water from a community containing solid and liquid excreta derived from houses, street, factories and industries. It resembles dirty water with an unpleasant smell. The term sewage applies to waste water which does not contain human excreta. e.g waste water from kitchens and bathrooms. The amount of sewage depends upon:
- Habits of people
- Time of the day
Environmental problems in the absence of proper sewage disposal are:
- Nuisance, unsightliness and unpleasant odours.
- Breeding of flies and mosquitoes
- Pollution of soil and water supplies
- Contamination of food
- Increased incidence of diseases.
Composition of sewage: Sewage contains 99.9 per cent water and 0.01 per cent organic & inorganic solids. The offensive odour of sewage is due to decomposed organic matter.
Aim of Sewage Purification:
- To stabilize the organic matter
- To produce effluent free from pathogens
- Decomposition of organic matter
Decomposition of organic matter: takes place by two processes – aerobic and anaerobic.
- Aerobic Process: The organic matter is broken into simpler compounds in presence of oxygen.
- Anaerobic Process: the sewage is highly concentrated and contains plenty of solids. The products of decomposition are methane, ammonia, CO2 and H.
Modern Sewage Treatment:
Modern sewage treatment plants are based on biological principles of sewage purification, where the purification is brought about by the action of anaerobic and aerobic bacteria.
It is divided into two stages; primary treatment and secondary treatment.
- Primary treatment: the solids are separated from the sewage by screening and sedimentation and subjected to anaerobic digestion which is the first stage in purification;
- Secondary treatment: the effluent is subjected to aerobic oxidation which is the second stage in purification.
Other methods of sewage disposal
- Sea Outfall
- River Outfall
- Land Treatment
- Oxidation Ponds
- Oxidation Ditches