Sewage disposal
Sewage disposal
The used or we can say wasted liquids water from various areas has to be treated before it accumulates and causes nuisance and population to the environment. The various waste water includes water used by home and then released the wasted water, water from the industries or we can say industrial waste like these all the waste water is to be treated for the purpose to avoid epidemic diseases and for good environment. So given below are the few definition to understand the sewage disposal easily and properly.
Definition
(a) Refuse
It is the waste water which is rejected, which includes sewages, sullage, garbage’s etc.
(b) Sewage
It is the liquid waste from the community including discharge from latrines, canals, industries waste.
(c) Sullage
It is the liquid discharges from the home which is mostly a waste and does not smell bad.
(d) Storm water
It is the rain water from roofs, wastes and building.
(e) Garbage’s
It is dry refuse and it includes decayed fruits, grass, leaves, paper pieces, sweeping, vegetables etc.
(f) Sewer
The underground conduits and drains through which sewage is conveyed is known as sewer. The sewers are available in lengths of 600 mm, 750 mm and 900 mm including the internal depth. Glazed earthen ware sewer pipes are cheap. They are resistant to acid and corrosion. They are hard and durable. They are impervious and can be easily cut.
Disposal
The sewage collected is taken to land or water where it does not create mosquitoes breeding centers. This should be in down-stream and should not damage or pollute the drinking water. It should be also taken care the disposed water is not making and problem to the society.
Necessity of sewage treatment
>To maintain public health and hygiene.
>Prevents the possible contamination of water.
>Prevents atmosphere pollution i.e. bad odour.
>Reduces nuisance caused by flies, mosquitoes and insects.
>Encourages aquatic life.
STAGES OF SEWAGE TREATMENT
Primary treatment
1. Screening and skimming chamber-removal of fats, oils and floating matters.
2. Grit chamber –grits such as stone and sand particles are removed.
3. Primary setting tank-removal of organic matter.
Secondary treatment
4. Trickling filter – output from primary setting tank is made in contact with air and due to aerobic condition, fine dissolved matters are established to flocks.
5. The secondary setting-dispersed for irrigation and chlorination (0)
6. Sludge digestion tank- sludge from primary setting tank and secondary setting tank are stabilized which is accompanied by aerobic action through aerobic bacteria.
BOD & COD
(a) BOD (Biological oxygen demand)
>Indicates the amount of oxygen in mg/liter for the complete oxidation of organic materials in sewage.
>BOD is less for mild sewage.
(b) COD (Chemical oxygen demand)
>Amount of carbon in various types of organic matters
>COD test is a test for oxygen consumed from chemical oxidizing agent like potassium di-carbonate in acid medium for oxidation of organic matters.
Aerobic and Anaerobic Decomposition
(a) Aerobic decomposition is caused by aerobic bacteria in presence of oxygen.
e.g., trickling filter
(b) Anaerobic decomposition is caused by anaerobic bacteria in the absence of oxygen.
e.g., Septic tank and sludge tank.
Septic tank
Septic tank is a water tight tank where the sewage undergoes anaerobic decomposition. They are designed only to treat human excreta and urine. The liquid from septic tank should not be allowed in to open drains and it has to be capable of undergoing secondary treatment.
Design aspects
It is designed only to treat common excreta and urine and not the village. Sewer pipes should be at least 100 mm in dimension and radii of the bend pipes should be long with cleaning eyes. The flow direction should not be greater than 45 degrees. They should be constructed in a area open to sky and possibly not closer to the building walls. A detention period of 24 to 40 hours depending on the average flow is allowed. The sedimentation depth should ne minimum 250 to 00 mm. the raw sewage entering should not disturb he settled sludge and surface scum. Outlet is in the form of dip fixed inside the tank. A minimum free board of 500 mm is given.
Sludge Removal
The digested sludge is removed once or twice in water. The scum in the first should not be disturbed much while removing the sludge. Care should be taken that the tank is not over loaded.
Biological action in a septic tank
The sewage is retained in the septic tank for a specified period of time. During the period, the suspended solids settle down and under goes anaerobic action. Thus they digested and reduced in volume.
Seepage pit or Soak Pit
The fulfillment from the septic tank carries a portion of suspended and organic matters. A seepage pit is a pit in to which the effluent from the septic is allowed to seep or seek in to surrounding soil. It can be constructed in any shape with a minimum cross sectional dimensions of 90 cm. the pit is usually lined with stone, bricks or concrete blocks with open joints.
Dispersion Trench
Dispersion trench is the trench in which the sewage pipes are laid with open joints. The aggregates surround’s the pipes and the effluent from the septic tank through the open joints and is absorbed in the surrounding soil. They are not suitable where there are trees or any other vegetation due to presence of roots.
Advantages of septic tank:
>Easy distribution
>Not costly
>Effluent can be treated easily
>Maintenance is easy.
>Digested sludge gets reduced in volume considerably
>Does not require skilled supervision
Disadvantages of septic tank
>Escape of foul gas may pollute the air.
>Removal of digested sludge is laborious.
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