Septic Tank and Soak Pit

Septic Tank And Soak Pit

Why Do We need A Sewer System

Each time we flush the toilet or wash so that down the sink’s drain,we create sewage (also known in polite society as wastewater).

One question that many people might ask is, “Why not simply dump this waste-water onto the ground outside the house, or into a nearby stream?” There are three main things about wastewater that make it so that  you don’t want to release into the environment

Domestic Sewage Treatment Septic Tank And Soak Pit
Domestic Sewage Treatment Septic Tank And Soak Pit

Septic Tank

In rural areas where houses are spaced so far apart that a sewer system would be too expensive to install, people install their own, private sewage treatment plants. These are called septic tanks. A septic tank is a key component of the septic system or sewer system.

In North America approximately 25% of the population  relies on septic tanks; this can include suburbs and small towns as well as rural areas

Septic tanks are generally installed by isolated communities like schools, hospitals & institutions. Septic tank may be defined as primary sedimentation tank. Having capacity to remove 60 to 70% dissolved matter.

Construction Details

Tank is designed to prevent directs currents between the tank inlet and outlet. T-pipes or baffles at inlet & outlet with submerged ends to stop currents and retaining the scum. Manhole covers are provided in top slab to permit inspection & maintenance. Top of tank is covered with RCC slab. Sometimes stairs steps are also provided. Provide vent pipe to escape foul gases. Side sump-well with portable pump. Two Compartment Septic Tank.


Septic Tank Collection System

Design Considerations

  • CAPACITY OF SEPTIC TANK:-capable of storing sewage flow during detention period &extra volume of sludge for .5 to 3 yrs.
  • DETENTION PERIOD:- 12 to 36 hrs (adopted 24hrs)
  • LENGTH TO WIDTH RATIO:- Length is 2 to3 times of width.


Sewage enter through the Inlet. There are three layers

  1. Anything that floats rises to the top and forms a layer known as the scum layer.
  2. Anything heavier than water sinks to form the sludge layer.
  3. In the middle is a fairly clear water layer. This body of water contains bacteria and chemicals like nitrogen and phosphorous that act as fertilizers, but it is largely free of solids.
  4. Organic matter is digested by bacteria. Digestion of settled sludge is carried out by ANAROBIC DECOMPOSITION also called sludge digestion.
  5. Due to decomposition foul gases are produced thus mass of sludge is decreased. BOD of water also decreased .


“Prevention is better than cure” Regular septic tank cleaning is the only way to prevent septic tank systems from clogging and well. Pumping is only way of cleaning. Frequency of cleaning depends on volume of  solids, amount of indigestible solids and the ambient temperature.

Potential Problems

  • Oils and grease are difficult to degrade can cause the inlet drains to block.
  • Flushing non-biodegradable items (cigarette butts & cotton buds) clog a septic tank.
  • Chemicals (pesticides, herbicides)and  inorganic materials(paints or solvents)damage the working of a septic tank.
  • Roots from trees  may clog and rupture them.
  • Excessive water entering the system will overload it and cause it to fail.
  • Septic tanks by themselves are ineffective at removing nitrogen compounds that can potentially cause algal blooms in receiving waters
  • All toilet paper are not suitable to dispose in septic tank(some toilet paper was specifically marked as suitable for use in septic systems and some was not).

Environmental Issues

Organic materials +Nutrients       =       CH4 + CO2 +NH3 +Biomass

Sulfates, under the anaerobic conditions of septic tanks, are reduced to hydrogen sulfide (H2S), a pungent and toxic gas. Nitrates and organic nitrogen compounds are reduced to ammonia.  Because of the anaerobic conditions, fermentation processes take place, which ultimately generate carbon dioxide and methane. Methane, a potent greenhouse gas. Waste water also contain phosphate and phosphate can be the limiting nutrient for plant growth in many ecosystems.


  • Ease in construction.
  • No maintenance problem(except periodical cleaning)
  • Excellently functioning ST can considerably reduce suspended solids &BOD from sewage.
  • Due to digestion reduction in volume is about 60% &reduction in weight is about 30%.
  • It is suitable for isolated community.


  • If ST is not properly functioning, effluent will be very foul &dark.
  • Leakage of gases from top cover of ST may caused  bed smells &environmental pollution.
  • Periodical cleaning, removal and disposal of sludge remains a tedious problem.

Disposal Of Effluent

Ways Of Disposal Of Effluent Are:-

  • Soak Pit.
  • Sub-Surface Irrigation
  • Leaching Cess-Pools.
  • Treated In Trickling Filters
  • Recycling.

Soak Pit Tank

Read About soak pit here




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