The document discusses septic tanks and their role in on-site wastewater treatment systems. It provides details on what a septic tank is, how it works, and its components. Mathematical models are presented to analyze the anaerobic digestion process that occurs in septic tanks. Linear and non-linear equations are fitted to experimental data to model the relationship between inputs, state variables, and outputs of the anaerobic digestion system. Computer programs can also be developed using these models to analyze septic tank performance over time based on given inputs and parameters.

1. by,
Vikas Kumar Verma
Enrol.No.-11519016.
M.Tech. (1stSem.)
Environmental Engg.

2. What is a Septic Tank?
A Septic Tank is a below ground watertight box
(concrete, plastic or fiberglass), often about 9 x
5 x 6 feet. It separates the liquids and solids,
provides digestion of some organics (mainly by
bacteria which live without oxygen) and storage.
It discharges partially filtered and clarified
effluent to the drainfield for final treatment.

3. Who has a Septic Tank System?
If a building (with plumbing facilities) is not
on a city sewer system, then in most cases it
has a septic tank system (also known as an
On-site Wastewater Disposal System, or
Subsurface Wastewater Disposal System) to
dispose of the wastewater.

4. What is an On-site Wastewater
System?
An On-site Wastewater Disposal System collects,
treats and disposes of all the wastewater produced in
a building.
A conventional system includes a Septic Tank, a
Distribution Device and a Drainfield

5. What is a Drainfield?
 The Drainfield (also known as the
nitrification field) is the area which contains
the pipes (and/or other approved materials).
It receives the effluent (partially treated
cloudy liquid) from the septic tank for
distribution, treatment ( mainly by bacteria
that need oxygen) and absorption into the
soil.
 Approved Drainfields are made from many
materials including pipe and gravel, plastic
chambers, concrete blocks, polystyrene
aggregate and other piping systems.

6. What does an On-site
Wastewater Disposal System do?
Avoids the spread of disease by preventing
human sewage from contaminating the
ground surface, well water supplies and
streams.

7. Typical Septic System
House evapotranspiration
well
septic tank
ba sem ent tre nche s
effluent
ba ffle
soil ab so rptio n
tre atme nt
gro und wa ter
strea ms, lake s

8. Diagram of septic tank

9. Schematic of conventional septic tank
Inspection opening Access opening near side wall at Inspection opening 150
150 mm diameter least 600 mm diameter mm diameter
Inlet At least 25 mm
20% of Outlet
Liquid level Liquid depth Water line
At least 75 mm
Scum
20% of Liquid depth
(150 mm, minimum) 40% of
40% of
Scum clear space Liquid depth
Liquid depth
Liquid depth
(75 mm, minimum)
Clear space Scum clear space
(300 mm, minimum)
Sludge
Sludge
First compartment 2/3 length second compartment 1/3 length
Total length equals two to three times width

10. View Cross Section of Tank

12. Element of Septic Tank

13. SEPTIC TANK
 It is a primary horizontal continuous flow
type of sedimentation tank having extra
provision to digestion of settled sludge
 Properties.
 Detention time- 12 to 36 hr
 It removes 60% to 70% of dissolve matters.
 Cleaning period-6 month to 3 years

14. Tank Sizing
 Generally prescribed for individual homes
based on home size
 Criteria: Hydraulic detention time plus
solids storage
 1 to 2 days detention of design flow
 Add solids storage volume equal to 1/3 – 1/2 of
the above hydraulic detention

15. Tank
Materials
Reinforced concrete
Fiberglass-reinforced plastic
(FRP)
Polyethylene/Poly-propylene

16. Tank Functions
 Solids removal by settling & floatation
 60-80% solids removal
 Anaerobic digestion
 Storage of solids

17. Factors that Influence Anaerobic
Digestion
 pH
 Teperature
 Chemicals
 Highly variable flow patterns
 Pharmaceuticals and personal care products
(PPCPs)
 Process wastewaters
 Lack of tank maintenance

18. What Happens in the Tank
IN OUT
SCUM
SLUDGE

19. Anaerobic Digestion
ORGANIC GASES + HUMUS
MATTER
CO2
CH4
H2S
NH3

20. Biological Activity in the Septic
Tank
 Anaerobic (without Oxygen)
 Incomplete
 Cheap and easy
 Reliable
 Gases produced are odoriferous
 Not all solids in tank are biodegradable
 Need to be stored until removed during
pumpout

21. Average Removal of BOD,
TSS, and Grease in Septic Tank
Average Raw Average % Removal
Parameter Sewage Septic
Influent Tank Effluent
BOD (mg/L) 308 122 60
TSS (mg/L) 316 72 77
Grease 102 21 79
(mg/L)

22. Computer program for septic tank
#include<stdio.h>
#include<conio.h>
#include<math.h>
Void main ()
{
Clrscr ();
Double population, per capita rate, detention time, quantity
of sewage, rate of deposition, period of cleaning, volume of
sludge, capacity, depth, surface area, width, length
Print f (“enter population”);
Scan f (“%f”, & “ population”);
Print f (“enter per capita rate”,);
Scan f (“%f”, & “per capita rate”);
Quantity of sewage=0.8*per capita rate*population

23. Print f (“enter rate of deposition”);
Scan f (“%f”, & “ rate of deposition”);
Print f (“enter period of cleaning”);
Scan f (“%f”, & “period of cleaning”);
Volume of sludge= (rate of deposition*period of
cleaning)/1000
Capacity=quantity of sewage+ volume of sludge
Print f (“enter depth”);
Scan f (“%f”, & “depth”);
Surface area= volume of sludge/depth
Width=power (surface area/3), 0.5);
Print f (“width=%d”, b);
Length=3*width
Print f (“length=%d”, l);
getch ();

24. OUTPUT
 Enter population= 120
 Enter per capita rate= 150
 Enter rate of deposition= 30
 Enter depth of tank= 1.5m
 Width= 2.1m
 Length= 6m

25. Role of the septic tank
Anaerobic fermentation of solids
Reduce the load of pathogens in the effluent
Hold the effluent for 2-3 days for improved safety
Retain solid material to prevent blockage of
further disposal system

26.  Advantage
1. It can be easily Disadvantage
constructed. 1. Its size should be
2. No maintenance very large to serve
problem. many people.
3. It excellently 2. Smell problem
remove BOD. 3. It needs periodic
4.Very less amount cleaning.
of solids are
produced.
5. Low cost

27. Some things which can be done to prolong
the life of a Septic System:
 Pump out the accumulated solids from the
Septic Tank regularly (check it every 3 years,
and typically pump it every 5-7 years).
 Don’t overload it (minimize water usage).
 Don’t put things which don’t readily
decompose into a Septic System (grease,
cooking oils, sanitary napkins, chemicals,
etc).
 Don’t drive on it, or build over it.

28. Failures of a Septic System
1.Failure by surfacing
Failing Septic Systems often
show up as a wet or mushy area
above the drainfield (often
because the drainfield can no
longer absorb the effluent).

29. Ways to spot a failing septic
system
 Failing septic
systems can often
be spotted from
the road.
 Nutrients from
effluent often
cause the “grass
to grow greener”.

30. Anaerobic digestion process is widely used for the treatment
of biodegradable organic waste.
Anaerobic digester depends upon different empirical rules.
Mathematical models are very helpful in the better
understanding anaerobic digester . By analyzing theses
models we can also apply computer technique on anaerobic
digesters.

31. The anaerobic digester is conceived as an input-output
model with organic waste as input and carbon dioxide and
methane production per unit weight of substrate as output.
The output is a function of the following state variables-
s. No. Name Symbol
1. pH pH
2. Alkalinity Ca
3. Volatile acid conc. Cva
4. Total nitrogen conc. CN
5. Temperature T

32. For the purpose of analysis these state variables are denoted as
X1, X2, X3,X4 and X5 and the state vector by X.
The input u is in the form of volatile solid (vs) loading done
once everyday and expressed as gm per litre of digester is
denoted by C.
The output vector consists of the following variables-
s. No. Name Symbol
1. Unit carbon qCO2
dioxide discharge
2. Unit methane qCH4
discharge
And for the purpose of analysis they will be denoted by Y1
and Y2; and output vector by Y.

33. The system is described by the following differential equations:
dx/dt = f1(x,u)……. (1)
in which X is (x1,x2,x3,x4,x5 )T. knowing the initial conditions and
the input u(t), equation (1) can be solved by using an appropriate
numerical method. Thus the state variables are expressed as a
function of time and input.
The output is a function of state variables and the input variables,
which can be written as-
Y=f2(X,u)….. (2)
In which Y is (y1, y2)T
In digester input is known & parameters are also known then at
any stage we can know what would be the output.

34. In the present investigation two types of equations, linear and
exponential, are fitted to the data. For mathematical convenience u
is denoted as x6.
1. Linear equation
A typical output y is related to the input and state variables by
the following linear equation-
Y=a1x1+a2x2+a3x3+a4x4+a5x5+a6x6 ……. (3)
In which a1’s are unknown constants to be determined in such
manner that there is a good agreement with the data. The error
ϵi in the ith data van be written as-
ϵi= yi-( a1x1+a2x2+a3x3+a4x4+a5x5+a6x6) ….. (4)
in which yi is the ith observed output.

35. Squaring ϵi (all the errors become positive) and summing one
gets:
E=Σ(yi- Σaj xji)2 (5)
For minimizing E, we have dE/daj=0; for j= 1, 2,3…….6
using equation (5) and (6) one gets
2 (Σyi- Σaj xji) xk =0…. (7)
Simplifying equation (7); one gets
Σaj Σxji xki= Σyi xki ; for k= 1, 2, 3…….6 …. (8)
Equation (8) is a system of linear simultaneous equation in aj
which can be solved using standard procedure.

36. 2. Non linear Equation
The following state output equation is proposed
Y=k.x1a1. x2a2. x3a3. x4a4. x5a5.
In which k is constant. x6a6 …(9)
Taking logarithms one gets
Log y= log k+ a1 log x1+ a2 log x2 + a3 log x3 + a4 log x4 + a5
log x5 + a6 log x6 … (10)
It is a linear formulation in log x and log y . by changing the
variables and using an equation similar to eq. (8) the constant aj
could be evaluated.

37. Conclusion
Septic tank is widely used in waste water treatment. This method has
several significant advantages like very less by-products low energy,
low space requirement and highly efficient.
In septic tank, the treatment process is anaerobic present and due to
modeling of anaerobic system and the analysis of anaerobic
treatment has become simple.
By model analysis we can also write the several computer programs
for given input and parameter at different time.