SEPTIC TANK

L-33

Unit-III

Environmental Engineering – II

Working of Septic tank Septic tank is sludge digestion cum

sedimentation tank with large detention time.

It is suitable for population up to 300.

The septic tank is a buried, water-tight container

usually made of concrete, brick masonry,

fiberglass or polyethylene.

Its job is to hold the wastewater long enough to

allow solids to settle down to the bottom (forming

sludge), while the oil and grease floats to the top

(as scum).

Compartments and a T-shaped outlet prevent

the sludge and scum from leaving the tank and

traveling into the drainfield area.

The liquid wastewater (effluent) then exits the

tank into the drainfield.

Finally, the wastewater percolates into the soil,

naturally removing harmful coliform bacteria,

viruses, and nutrients.

LAYOUT OF

SEPTIC

TANK

TREATMENT

Following reaction occurs:

Organic materials +Nutrients = CH4 +

CO2 +NH3 + H2S + 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 (NH3).

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.

DESIGN OF SEPTIC TANK

A. DETENTION PERIOD METHOD

B. RATIONAL METHOD

A. Detention period method

1. Detention period (t) is assumed in between

12 to 36 hrs (common value = 24 hrs)

2. Assuming rate of sewage flow (lpcd) [90 to

150 lpcd] and knowing population to be

served; total discharge (Q) is determined.

3. Volume is calculated by

V=Q. t

4. Depth is assumed 1 to 2 m.

5. Surface area is calculated by

As = V / depth

6. Assuming L/B ratio in between 2 to 4

length and breadth can be fixed.

B. Rational Method

1. Settling zone is kept min 30 cm deep.

2. Digestion zone is provided as 0.032

m3/capita.

3. Sludge storage zone of 7.30 m3/100 persons

for cleaning interval of 1 year is provided.

4. Surface area of 0.92 m2/10 lpm of flow is

provided.

5. Free board of 0.3 m is provided.

( depth 1 to 2 m, L/B ratio 2 to 4)

Estimated peak discharge

No. of users Peak discharge

(lpm)

100 240

150 360

200 480

300 720

Advantages

1. Ease in construction.

2. No maintenance problem(except periodical

cleaning).

3. Cost of construction is less.

4. Effluent can be discharged without any

trouble.

5. It is suitable for isolated community.

6. Very little attention and skilled attendance is

needed.

Disadvantages 1. If ST is not properly functioning, effluent will

be very foul & dark.

2. Leakage of gases from top cover of ST may

cause bed smells & environmental pollution.

3. Periodical cleaning, removal and disposal of

sludge remains a tedious problem.

4. Unpredictable functioning.

5. Size required is large for larger communities.

6. Effluent contains high BOD.

7. Problem of mosquito nuisance.

8. Problem of ground water contamination

Disposal Of Effluent

Ways Of Disposal Of Effluent Are:-

1. Soak Pit.

2. Dispersion trenches

3. Sub-Surface Irrigation

4. Leaching Cess-Pools.

5. Treated In Trickling Filters

6. Recycling.

Soak Pit Soak Pit does not provide adequate treatment

for raw wastewater and the pit will clog quickly.

A Soak Pit should be used for discharging pre

settled black water or grey water .

Soak pits are appropriate for rural and

suburban settlements.

They depend on soil with a sufficient absorptive

capacity.

Soak pits are used for soils where percolation

rate is below 25 min/cm.

They are not appropriate for areas that are

prone to flooding or have high groundwater

tables.

The total subsurface soil area required for

soak pits and dispersion trenches is given by

q = 130/(√t)

Where,

q = Max rate of effluent application in Lit/d/m2

of leaching surface.

T= Standard percolation rate for the soil in

min/cm

Total trench/soak pit area required can be

calculated by

A = (Q/q)

Where,

Q = total flow = Population x Sewage flow

rate.

Advantages

1. Can be built and repaired with locally

available materials.

2. Small land area required.

3. Power conservative.

4. Can be built and maintained with locally

available materials.

5. Simple technique for all users.

Disadvantages

1. Pre-treatment is required to prevent

clogging, although eventual clogging is

inevitable.

2. Negatively affects soil and groundwater

properties.

Dispersion trenches

Trenches are 0.5 to 1 m deep and 1 m wide.

Maximum length of trench is 30 m.

Spacing between trenches is 2m.

One distribution box is provided for every 3 to 4

trenches

Cleaning of septic tank

Objective questions

1. Septic tank is ________ cum _______

tank.

2. Detention period adopted in septic

tanks is __ to __ hrs.

3. Septic tank can be used for population

up to __ users.

Theory questions

Q1] Explain working of septic tank with

neat sketch.

Q2] write short note on

1. Design of septic tank

2. Septic tank Effluent disposal