Septic tanks are buried,
watertight receptacles designed and constructed to receive and partially treat
wastewater. The tank separates solids from the liquid, provides limited digestion
of organic matter, stores solids, and allows the clarified liquid to discharge
for further treatment and disposal. Settleable solids and partially decomposed
sludge accumulate at the bottom of the tank, while scum rises to the top of the
tank’s liquid level. The partially clarified liquid is allowed to flow through
an outlet opening position below the floating scum layer. The clarified liquid
will be disposed of to the absorption field for further treatment and disposal. That impure water also
shifted to soakage pit for the additional treatment.
Septic tanks serve following functions:
- ü Removal of solids. As sewage enters the septic tank, its rate of flow is reduced so that the larger solids sink to the bottom and soaps, grease, and smaller solids rise to the surface. These solids are retained in the tank, and the clarified effluent with suspended and dissolved solids is discharged.
- ü Bacterial Action.
The solids and the liquids in the tank are partially decomposed by bacteria and
other natural processes. These bacteria are called anaerobic because they
thrive in the absence of free oxygen. This decomposition of sewage under
anaerobic conditions is termed “septic,” hence the name of the system (and the
cause of the odor).
- ü Sludge and scum
storage. Sludge is the accumulation of solids at the bottom of the tank, while
scum is a partially submerged mat of floating solids that may form at or near
the surface. Space must exist in the tank to store these residues during the
intervals between pumping. Otherwise, the sludge and scum will eventually be
scoured from the tank and will clog the leach field and receiving soil.
Septic System Dos and Don’ts
Dos
· Check with the local regulatory agency or inspector/pumper if you have a garbage disposal unit to make sure that your septic system can handle this additional waste.
· Check with your local health department before using additives. Commercial septic tank additives do not eliminate the need for periodic pumping and can be harmful to the system.
· Use water efficiently to avoid overloading the septic system. Be sure to repair leaky faucets or toilets. Use high-efficiency fixtures.
· Use commercial bathroom cleaners and laundry detergents in moderation. Many people prefer to clean their toilets, sinks, showers, and tubs with a mild detergent or baking soda.
· Check with your local regulatory agency or inspector/pumper before allowing water softener backwash to enter your septic tank.
· Keep records of repairs, pumping's, inspections, permits issued, and other system maintenance activities.
· Learn the location of your septic system. Keep a sketch of it with your maintenance record for service visits. The Tank should be constructed at the correct place & required size.
· Have your septic system inspected and pumped as necessary by a licensed inspector/contractor.
· Plant only grass over and near your septic system. Roots from nearby trees or shrubs might clog and damage the drainfield.
· Construct the septic tank without disturbing the groundwater.
· Manhole should provide to access the tank easily.
Don’ts
•
Your
septic system is not a trash can. Don’t put dental floss, feminine hygiene
products, condoms, diapers, cotton swabs, cigarette butts, coffee grounds, cat
litter, paper towels, latex paint, pesticides, or other hazardous chemicals
into your system.
•
Don’t
use caustic drain openers for a clogged drain. Instead, use boiling water or a
drain snake to open clogs.
•
Don’t
drive or park vehicles on any part of your septic system. Doing so can compact
the soil in your drainfield or damage the pipes, tank, or other septic system
components.
•
The
other source of water should not enter in the septic tank.
•
The
wastewater should not run in the natural ground to secure the environment.
Problem Signs in Septic Tanks
The septic tank will need
to be checked if there are signs that it is not working properly.
Some signs that a septic
tank is not working properly are:
Ø The
sewage in the toilet or the liquid waste from other fixtures flows away very
slowly
Ø Liquid
waste overflows from the disconnector trap
Ø Wet
areas are seen at the top of the septic tank
Ø There
is a strong unpleasant smell near the septic tank
Ø The
grass around the tank is very green and growing well
Soak Pit
A pit dug under the natural ground, constructed in a circular shape with dry bricks or stone with the mortar joint free, where water can easily soak into the ground is known as soak pit. The top portion was closed by RCC Slab & accessible. It should not less than 900mm Dia, 1500mm Depth. At the same time, it should not below 2m from the groundwater.
Types of septic tank
|
Concrete septic tank (onsite build or
precast) |
|
|
Advantage |
Disadvantage |
|
Heavy when compare to plastic septic
tanks which helps the tank not to float during ground wet conditions |
Can
be cracked which results the spreading of bad odor. |
|
It
won’t rust |
Expensive |
|
Strengthen over time or withstand for
many decades |
Installation
process need skilled labors |
|
100
% water tight and pump out process is easy |
Sometimes
collapse when heavy forces is applied |
|
Steel septic tanks (Rarely use or not in use nowadays) |
|
|
Advantage |
Disadvantage |
|
Due to its heavy weight, floating won’t
happen even when the ground water is high. |
Can
easily rust and it result a maximum life span of 2 decades |
|
Fiber glass
septic tank |
|
|
Advantage |
Disadvantage |
|
Strong
enough than plastic septic tanks |
Can
float to the ground surface when the ground water is high as because of its
weight |
|
Economical than concrete septic tank |
|
|
It
won’t rust |
|
|
Water tight and highly durable |
|
|
Plastic
septic tanks |
|
|
Advantage |
Disadvantage |
|
Installation
cost is lower when compare to concrete septic tanks |
Because
of less weight can float to the ground surface when the ground is wet |
|
Resistance to cracking |
Can’t
park heavy vehicles as it can change the shape of septic tank and which
result in the explosion of tank due to high pressure |
|
It
won’t rust |
100
% water tight only when it is not damaged ( change of getting damage is more) |
Figure 1: Typical one chamber brick septic tank
Figure 2: Typical two chamber concrete septic tank
IMPORTANT POINTS
1. Septic tank discharging into either a subsurface disposal field or one or more seepage pits shall be required for the approval of drainage and sanitation plans for the places where public sewers are not available.
2.
Such disposal method shall
be designed by a licensed professional in accordance with the requirement of
the provisions of this Code and regulations of the concerned authorities.
3.
The design of such system
shall be on the basis of location with respect to wells or other sources of
water, soil permeability, ground water elevation, area available and maximum
occupancy of the building.
4.
Sullage water shall not be
discharged into the septic tank.
5.
Effluent from septic tank(s) shall not discharge into open
water courses.
6. The minimum distance for various components of the disposal system shall be in accordance with Table below
Table 1: Location of Components of Sewage Disposal System
|
System component |
Distance (m) |
||||
|
Building Foundation |
Well |
Stream |
Seepage Pit |
Dry Well |
|
|
Septic tank |
1.5 |
8 |
- |
1.5 |
- |
|
Disposal field |
3 |
15 |
7.5 |
6 |
6 |
|
Seepage pit |
4.5 |
15 |
15 |
6 |
6 |
|
Dry well |
3 |
15 |
- |
6 |
- |
7. The flow into a septic tank may be calculated on the basis of plumbing fixtures discharging soil wastes simultaneously into it. The capacity of septic tank for residential buildings shall be determined according to the formula (1). For other occupancies a reduction factor shall be used as shown in table below.
The volume of a septic tank may be computed using the following equation:
𝑉 = 𝑃𝑄𝑡 + 𝑃𝑆𝑦 …………………..(1)
Where,
𝑉 =
Volume of the septic tank (litre)
𝑃 =
Number of persons served
𝑄 =
Flow, litre/capita/day (lpcd)
(The flow may be computed
considering waste water flow 60% to 70% of the water consumption or on the
basis of the plumbing fixtures
discharging simultaneously into the septic tank. In absence of these data
the
waste water flow for the Occupancy groups A, C and D may be considered 120 lpcd
for cities, 50 lpcd for district town and 20 lpcd
for thanas and rural areas. For other Occupancy groups a waste water flow of 10
lpcd may be considered.)
𝑡 =
Liquid retention time, day (minimum 1 day)
𝑆 =
Volume required for sludge and scum (0.04 m3/capita/year)
𝑦 =
Desludging frequency, year (Minimum 1 year).
If computed volume is less than
2000 litres then minimum volume of the septic tank shall be 2000 litre.
Table 2: Reduction factors for various occupancies
|
Type of development |
|
Factor |
|
(i) Church: Hall-
|
Seating capacity Residential
Staff |
0.01 1.00 |
|
(ii) School: |
Primary
& Secondary School-student and staff population per session |
0.25 |
|
(iii) School: |
Institution of Higher Learning-Total enrolment and
staff |
0.25 |
|
(iv)
Community Centre: |
Staff/worker Resident Kindergarten-student and staff
population per session Maximum gathering |
0.25 1.00 0.08 0.01 |
|
(v)
Factory: |
One shift per 24 hours Two/three sifts per 24 hours |
0.50 1.00 |
|
(vi) Market: |
Hawker
/employee |
0.25 |
|
(vii) Hawker/Cooked Food center: |
No. of stall |
1.75 |
|
(viii)
Cinema: |
Hall-seating capacity Staff |
0.03 0.50 |
|
(ix) Bar & Snack Bar,
Coffee-House, Restaurant: |
Seating capacity |
0.20 |
|
(x)
Hotel: |
No. of beds Staff/worker |
0.70 0.50 |
|
(xi) Office: |
No. of staff |
0.50 |
8. The septic tank shall have a
minimum liquid capacity of 2000 liters, minimum width 1 m and minimum liquid
depth 1 m. The minimum length of a septic tank shall be at least thrice its
width. It is recommended that the maximum length of a septic tank shall be not
more than 4 times its width.
9.
The maximum size of a septic
tank shall be limited to the number of users not exceeding 300 persons for
residential buildings.
10. The volume required for digested
sludge and scum may be computed on the basis of 0.04 m3/capita/year. There
shall be a clearance between top of the liquid level and bottom of the tank
cover slab which shall be at least 300 mm.
11.
The liquid retention time of
a septic tank shall be at least 1 day.
12.
The desludging frequency of
a septic tank shall be at least 6 months interval and maximum once a year.
13. It is recommended to
use a two chamber septic tank when the capacity of a septic tank exceeds 3000
liters. The inlet compartment of a two chamber septic tank shall have a
capacity not less than two-third of its total capacity (Figure 2).
14. The septic tank shall be constructed of corrosion resistant
material and be of permanent water tight construction. The manhole cover and
the roof of the tank shall be designed for at least 7 kPa live load. The inlet
compartment shall be provided with a manhole. Outlet compartment shall also be
provided with a manhole.
References:
1.
BNBC-2020
2.
www.des.nh.gov
3.
Environmental Health Practitioner Manual: A resource manual for Environmental
Health Practitioners working with
Aboriginal and Torres Strait Islander Communities
4. EPA United States Environmental Protection Agency.