HARAMAYA UNIVERSITY

HARAMAYA INSTITUTE OF TECHNOLOGY

School of Water Resource and Environmental
Engineering
Department of Water Supply and Environmental
Engineering
Course:Sanitation,G5-WSEE

Prepared by Inst.Boki.T
 Definition of sanitation
What do we mean by "sanitation"?
 With respect to
Safe collection,
defining of sanitation storage,
most professionals treatment and
disposal/re-use/r
would agree that ecycling of human
excreta Management
"sanitation" as a whole Collection and
/re-use/
management
is a “big idea” which of industrial recycling of
waste & HW solid wastes
covers:
Sanitation
Treatment
and Drainage and
disposal/re- disposal/re-
use/recycling use/recycling of
of sewage Drainage of grey water
effluents storm
water
• Sanitation is the means of collecting and disposing of
excreta and community liquid waste in a hygienic way so
as not to endanger the health of individuals or the
community as a whole (WHO 1992).

• The World Health Organization states that: Sanitation

generally refers to the provision of facilities and services for
the safe disposal of human waste
Review of sanitation options
• The challenge is to decide what aspects are the most
important, In other words, what aspect of the problem is
going to be dealt with as a priority.

• Countries or communities may try different solution to

solve sanitation problems.

• The key issue here is that each community, region or

country needs to workout what is the most sensible and cost
effective way of thinking about sanitation in the short and
long term and then act accordingly.
In general review of sanitation option can answer the
following questions
• Why we focus on sanitation?......
• How does sanitation affect the environment?.......
• What is the size of the problem?......
• What Conditions to be fulfilled by sanitation
system?.....
Why we focus on sanitation?
• Progress in sanitation and improved hygiene has
greatly improved health, but many people still have no
adequate means of disposing of their waste.
• This is a growing nuisance for heavily populated areas,
carrying the risk of infectious disease, particularly to
vulnerable groups such as the very young, the elderly.
• Poorly controlled waste also means of daily exposure
to an unpleasant environment.
• The build up of fecal contamination in rivers and other
waters is not only a human risk, But also other species
are affected, like ecological balance of the
environment.
cont’d….
The discharge of untreated wastewater and excreta
into the environment affects human health by
several routes:
• By polluting drinking water
• Entry into the food chain, for example via fruits,
vegetables or fish and shellfish
• Bathing, recreational and other contact with
contaminated waters
• By providing breeding sites for flies and insects
that spread diseases
How does sanitation affect the environment?
• In regions where a large proportion of the population is not
served with adequate sanitation:
sewage flows directly into streams, rivers, lakes and
wetlands, affecting coastal and marine ecosystems, fouling
the environment and exposing millions of children to disease
• Particularly in the context of urbanization: domestic
wastewater, sewage and solid waste improperly discharged
results to providing breeding grounds for communicable
disease vectors and contributing to air, water and soil
pollution
The results of poor waste management also contribute
to a loss of valuable biodiversity.
What is the size of the problem?
• In 2004, only 59% of the world population had
access to any type of improved sanitation facility;
In other words, 4 out of 10 people around the world
have no access to improved sanitation and improved
water supply

• They are obliged to defecate in the open or use

unsanitary facilities, with a serious risk of exposure
to sanitation-related diseases.
Open defecation
• Where there are no latrines people resort to
defecation in the open area.
• This may be in special places for defecation generally
accepted by the community, such as defecation fields,
rubbish and manure heaps, or under trees,
• result of the open defecation encourages flies, which
spread faeces-related diseases and;
• Surface water run-off from places where people have
defecated results in water pollution.
• In view of the health hazards created and the
degradation of the environmental safety
Open defecation cont’d…
 Sanitation coverage in Ethiopia

Source : WHO / UNICEF

What conditions must be fulfilled by a sanitation system?
A sanitation system must:
Protect and promote health
• it should keep disease-carrying waste and insects away from
people.
Protect the environment
• avoid air, soil, water pollution, return nutrients/resources to the
soil, and conserve water and energy.
Be simple
• the system must be operational with locally available resources
(human and material).
• Where technical skills are limited, simple technologies should
be favored.
Cont’d….
Be affordable
• total costs (including capital, operational,
maintenance costs) must be within the users’ ability
to pay.
Be culturally acceptable
• it should be adapted to local customs, beliefs and
desires.
Work for everyone
• it should address the health needs of children,
adults, men, and women.
Sanitation and public health
• The importance of the isolation of waste lies in an
effort to prevent diseases which can be transmitted
through human waste, which affect both developed
countries as well as developing countries to differing
degrees.
• It is estimated that up to 5 million people die each
year from preventable water-borne diseases, as a
result of inadequate sanitation and hygiene
practices.
• The effects of sanitation has impacted the society
throughout history.
Cont’d……
Relevant disease which affect the public health
include:
• Waterborne diseases, which can contaminate
drinking water
• Diseases transmitted by the fecal-oral route
• Hookworm, where eggs can survive in the soil
Due to this Many countries are struggling with
diseases due to unhealthy living conditions and
unfiltered water all in the name of poor sanitation.
then Proper sanitation is a necessity for a healthy
life and produces an enhanced feeling of wellbeing.
What diseases are associated with poor sanitation?
• Human wastes have been implicated in the transmission
of many infectious diseases including cholera, typhoid,
infectious hepatitis, polio, cryptosporidiosis, and
ascariasis.
• Poor sanitation results:
– many infections means that the ideal opportunity to
spread disease causing organisms
– plenty of waste and excreta for the flies to breed on,
and
– unsafe water to drink, wash with or swim in
Cont’d….
• Trachoma is closely linked to poor sanitation and is one
of the best examples of an infection readily preventable
through basic hygiene.

• Infectious agents are not the only health concerns

associated with wastewater and excreta.

• But also Heavy metals, toxic organic and inorganic

substances also can pose serious threats to human health
and the environment
– particularly when industrial wastes are added to the
waste stream.
How does sanitation prevent disease?
• For a sanitation system to provide the greatest health
protection to the individual, the community, and society
at large; it must

i. Isolate the user from their own excreta

ii. Prevent nuisance organisms (e.g. flies) from
contacting the excreta and subsequently transmitting
disease to humans; and
iii. Inactivate the pathogens before they enter the
environment or prevent the excreta from entering
the environment
Social and economic determinants of sanitation
I. Social aspects
• Consideration should be given to the institutions of a
political, economic and social nature that are operating at
the national and/or local level, such as government, the
civil service, religious institutions, schools and colleges,
and the family, and to the forms of leadership and
authority that are generally accepted by the majority of the
people.
• It is also important to consider the various roles and
patterns of behavior of individuals and social groups
• to determine who is traditionally responsible for such
areas as water supplies, environmental hygiene, family
health and children's defecation habits, etc.
What is the economic determinant of sanitation?
The health impact of inadequate sanitation leads to
a number of financial and economic costs
including:
• direct medical costs associated with treating
sanitation related illnesses
• lost income through reduced or lost productivity
and the government costs of providing health
services
Cont’d…
Additionally, sanitation also leads to time and
effort losses due to:
• distant or inadequate sanitation facilities,
• lower product quality resulting from poor
water quality
• reduced income from tourism (due to high
risk of contamination and disease) and clean
up costs.
• contribute to economic growth.
Types of sanitation (controlling mechanism of
human waste)
For practical purposes sanitation can be divided in
to on-site and off-site technologies.
• On-site systems (e.g. latrines), store and/or treat
excreta at the point of generation.
• Off-site systems (e.g. sewerage) excreta is
transported to another location for treatment,
disposal or use.
 Some on-site systems, particularly in densely
populated regions or with permanent structures,
will have off-site treatment components as well.
The concept of on-site sanitation
• On-site sanitation can be substantially cheaper
and easier to promote than sewerage networks
particularly in rural and peri-urban areas, because
space availability and population density are not
constraining factors on its adoption
• On-site sanitation can be classified into two main
categories:
◦ ‘wet’ which require water for flushing
◦ ‘dry’ which do not require any water for flushing
On-site sanitation technical options
Various sanitation systems are introduced based on their:
• suitability for particular situations
• the constraints on their use
• their advantage and disadvantages
 Each community must choose the most feasible and convenient
option to provide necessary health protection.
 Selecting the most appropriate option requires a thorough analysis of
all factors including:
 cost
 cultural acceptability
 simplicity of design and construction
 operation and maintenance
 local availability of materials and skills
types of latrines under on-site sanitation are :-
• Types of pits
 Shallow pit
 Simple pit latrine
 Ventilated improve pit latrine (VIP)
• Septic tank
• Imhoff tank

• Aqua privy
• Shallow pit (Arborloo)
◦ People working on farms may dig a small hole each time
they defecate and then cover the faeces with soil.
◦ Pits about 300mm deep may be used for several weeks.
◦ Excavated soil is heaped beside the pit and some is put
over the faeces after each use.
◦ In shallow pits:
• Decomposition is rapid because of the large bacterial
population in the topsoil,
• Flies breed in large numbers and Hookworm larvae
spread around the holes
◦ Hookworm larvae can migrate upwards from excrete
buried less than 1m deep, to penetrate the soles of the
Shallow pit
 Shallow pit
Advantages Disadvantages
• No cost • Considerable fly
• Benefit to farmers as nuisance
fertilizer • Spread of hookworm
larvae
Simple Pit Latrine
1. Effective volume of pit
• (up to 0.5 m from hole)
• 2. Defecation hole
3. Slab
4. Cover
5. Superstructure
6. Roof
7. Slab seating
8. Drainage channel
9. Water table (pit 1.5m above)
When the pit is 50 cm from the
surface:
• move superstructure and slab to
a neighbouring place and
fill the pit with soil.
• Do not dig this place again for at
least two years.
Simple Pit Latrine
The principle of operation
• Wastes such as excrete, anal cleaning materials,
sullage and refuse are deposited in a hole in the
ground.
• The liquids percolate into the surrounding soil and
the organic material decomposes producing:
– gases such as carbon dioxide and methane, which are
liberated to the atmosphere or disperse into the
surrounding soil;
– liquids, which percolate into the surrounding soil;
– a decomposed and consolidated residue.
 Cont’d…
 are widely used in most developing countries.

 The health benefits and convenience depend upon the

quality of the design, construction and maintenance.

 pits should be designed to last as long as possible.

 a design life of 15-20 years is perfectly reasonable.

Cont’d…
 Simple pit latrine
Advantages Disadvantages
• Relatively cheap • Considerable nuisance
• Can be constructed by because of flies, insects
the user (particularly in (mosquitoes if pit is wet)
rural areas) • Bad smell
• Does not need water to
function
• Easy to maintain

Simple pit latrine

High potential for groundwater pollution
Ventilated improved pit latrines (VIP)
• The principle is to cancel or to reduce harmful
side-effects like (smells and flies) related to
traditional latrines by providing a vent pipe higher
than the superstructure.
• Smells are evacuated through the vent pipe.
• Venting the pit dries the waste which assists
natural decomposition and destruction of potential
pathogens,
ultimately rendering a safe humus-like waste
product.
 Eliminating the primary
Vector (flies) greatly reduces
possibility of transmission
Of potential public
health risks.

VIP single-pit latrine

 VIP double-pit latrine

User interface

Receiving unit(in
this case Under
ground)
 Cont’d…..
Advantages Disadvantages
• Relatively cheap • Darkness is indispensable
• Can be constructed by the user within the superstructure
• Does not need water to to fight off flies
function • Only functions properly
• Easy to maintain
when conveniently
• No smell and no flies
oriented towards the wind
• It can take different
• No surrounding obstacle
sorts of cleansing materials (solid
and liquid) (trees and buildings) should be
higher than the vent pipe
 Basic Design Principles of VIP latrine

• The vent pipe should have an internal diameter of at least

110mm to a maximum of 150mm and reach more than
300mm above the highest point of the toilet superstructure.
• It can be made out of PVC, bricks, pet bottles or iron pipes.
• The mesh size of the fly screen must be large enough to
prevent clogging with dust and allow air to circulate freely
• Pits designed to last 25 to 30 years are not uncommon and
design life of 15 to 20 years is perfectly reasonable.
Cont’d…….
• The depth of the pit is at least 2m, but usually more
than 3m.
• The depth is usually limited by the groundwater table
or rocky underground.
• Because of the static properties, a round pit with a
diameter of more than 1.5m guarantees a stable
construction and avoids collapse.
• A horizontal distance of 30m between the pit and a
water source is recommended to limit exposure to
contamination.
• In densely populated areas with many pit latrines, the
risk of a groundwater contamination is even higher.
 Pit sizing for latrines

To size pits or tanks, it is important to determine:

• the rate at which sludge (including faeces, urine
and anal cleansing material) will accumulate, and
• the rate at which effluent will infiltrate in the
surrounding ground.
• The top 0.5m of a pit should not be filled to allow
for safe backfilling and to prevent splashing,
unpleasant sights and increased odour and fly
nuisance.
 Pit sizing for latrines
• The approximate volume(m3 )size of the pit can be calculated
as a function of the following equation:
V=
Where
V: Pit volume (m3)
N: Number of users
R: Sludge accumulation rate (liters/cap/year) is listed in the table
below
D: Pit life (years)_ Design period/life
Af: Infiltration area (m2); (water depth = Af/pit circumference)
W: Amount of water used for flushing (liters/cap/day)
I: Infiltration rates (liters/m2/day
Recommended infiltration capacities
Waste deposited and conditions Sludge accumulation rate,
S(liters/capita/year)
Wastes retained in water where 40
degradable anal cleaning materials are
used
Waste retained in water where non- 60
degradable anal cleaning materials are
used
Waste retained in dry conditions where 60
degradable anal cleaning materials are
used
Waste retained in dry conditions where 90
non-degradable anal cleaning materials
are used
• The term ‘wastes retained in water’ when
applied to a pit latrine, means that wastes are in
a section of the pit below the water-table.
• This method assumes that liquid waste is
absorbed by the surrounding soil.
• If liquid remains in the pit, it will fill far more
rapidly.
• It should also be noted that soil pores become
clogged with time, thus reducing or even
stopping infiltration.
 Principle and operation
• Waste drops into the pit
• organic material decomposes
• excess liquids percolate into the surrounding soil.
• Natural airflow through the top-structure and moving
across the top of the vent pipe removes smells and
vents gases from the pit to the atmosphere.
• A darkened interior is maintained to attract insects
towards the light at the top of the vent pipe.
• A separate hand washing facility is required
 EXAMPLES

1. A family of six intends to construct a pit latrine

to last 20 years. The family uses water for anal
cleaning and intends to use the toilet as a
bathing area. The ground is mainly fine sand
with a water table 3 m below the surface:
Design pit latrine!
Solution:
Given: pop. Number = 6, N = 20
Take R value from table as 40liter/c/year
 EXAMPLES

1. Volume (V) = (N x Dx R)/1000

=20years x 6 x (40/1000m3/year) = 4.8m3

2. Sludge depth

If the pit is to be circular, assume an inside diameter of

1.3 m, the sludge depth will be:

= = 3.65m
3. The infiltration capacity of a fine sandy soil is
about 331iter/m² per day (see from table )
Assuming the volume of water entering the pit
each day is 200liter then the infiltration area
required will be:
Infiltration area = 6.1 m²
Water depth = = = 1.49 m
Assuming a soil seal depth of 0.5 m, the total
depth required for the pit is:
3.65 + 1.49 + 0.5 = 5.6 m
 Septic tank
• A septic tank is an underground watertight settling chamber
which is raw sewage is delivered through a pipe from
plumbing fixtures inside a house or other building.
• The sewage is partially treated in the tank by separation
of solids to form sludge and scum.
• Effluent from the tank infiltrates into the ground through
drains or a soak pit.
• The system works well where the soil is permeable and not
liable to flooding or waterlogging
• The sludge is removed at appropriate intervals to ensure
that it does not occupy too great a proportion of the tank
capacity.
Septic tank
 Design features of septic tank

As septic tank is a settling digestion tank, its rational design is

based on the following three functions. It is expected to
perform:
i. Sedimentation to remove the maximum possible amounts of
suspended solids from the sewage
ii. Digestion of settled solid resulting in a much reduced
volume of dense, digested sludge, and
iii. Storage of sludge and scum accumulating in between
successive cleanings thereby preventing their escape
• Hence, the tank should be capable of storing the sewage
flow during the detention period and additional volume of
sludge for 6 months to 3 years depending on the periodicity
of cleaning
 Detention period
• A detention period of 24 to 48 hours based on the
average daily flow of sewage can be used.
Inlet and outlet baffles
• The baffles or tees should extend of about 20cm
above the top sewage line.
• Inlet should penetrate by about 30cm below the
top sewage line and outlet should penetrate of
about 40% of the depth of the sewage. (to prevent
direct currents b/n inlet and outlet)
• The outlet invert level should be kept 5 to 7.5cm
below the inlet invert level
 Length to width ratio
Septic tanks are usually rectangular with their length at
about 2 to 3 times the width (L = 2B to 3B)
• The width should not be less than 90cm
• Depth is ranges from 1.5 to 1.8m
Sludge withdrawal
• The rate of accumulation of sludge has been
recommended as 30 l/c/year (0.082 l/c/day)
• Sludge is withdrawn from the septic tank either half
year or yearly.
• For small domestic tanks, de-sludging may be done at
least once in 6 months to 3 years.
 Operation principle of septic tank
• Human wastes from the toilet are flushed into a
septic tank.
• The liquid is retained in the septic tank for at
least 24 hours.
• In the septic tank solids settle out to the bottom
where they undergo biological digestion.
• The liquids pass out of the tank and into a
subsoil drainage system (soak-away).
• Digested sludge gradually builds up in the tank
and requires eventual removal by tanker
 Steps to design septic tank

1st step: to calculate liquid retention volume

• If the septic tank accepts sullage as well as toilet waste
and If the water supply per person is known, the sewage
flow may be taken as 70-80% of the water supply
• If only WCs are connected to the septic tank, the sewage
flow is estimated from an assumption about the number
of times each user is likely to flush the WC.
Then, the minimum volume of liquid in 24hrs is:
V1 = P x q liters
Where V1 = required volume for 24 hours' liquid retention;
P = number of people served by the tank;
q = sewage flow per person (l/c/d).
 2nd step: volume of sludge and scum
V2 = P x N x F x R
Where V2 = the required sludge and scum storage capacity
in liters;
N = the number of years between desludging (often 2-5yr)
F = a factor which relates the sludge digestion rate to
temperature and the desludging interval
R = the rate of sludge and scum accumulation
may be taken as 25 liters/person/year for tanks
receiving WC waste only, and 40 liters per person per
year for tanks receiving WC waste and sullage.
Therefore, the total capacity of the tank is:
V = V 1 + V2
• Table of the sizing factor F
 Example:
1. Design a septic tank suitable for a household
with up to eight occupants in a low-density
housing area in which the houses have full
plumbing, all household wastes go to the septic
tank and the nominal water supply is 200liters
per person per day. Water is used for anal
cleaning and the ambient temperature is not
less than 25°C for most of the year. Effluent
flows to drainage field in porous silty clay.
Solution:
Step1
Volume of liquid entering the tank each day
V1 = P x q P = 8
Rate of water supply 200l/c/d, all sullage go to tank
Assume 80% of water supply released as ww.
q = 0.8 x Q = 0.8 x 200 = 160l/c/d
Therefore, V1 = 8 * 160l/d = 1280litres
 Step 2

The volume of sludge and scum is given by

V2 = P x N x F x R but, P = 8 ….given
Assume N (desludging interval) is 3 years;
from Table 3, F= 1.0 versus the given temperature;
as all wastes go to septic tank R = 401iters/c/yr

Therefore: V2 = 8 x 3 x 1.0 x 40 = 960 liters

Total tank volume = V1 + V2 = 1280 + 960 = 2240

liters = 2.24 m³
Step 3 tank dimension
Assume liquid depth = 1.5m
If tank width is W meter
Assume two compartment, the length of first
compartment = 2W and length of the second = W
Volume of the tank(V) = H*L*W, but L= 2W
2.24m3 = 1.5m*(2W+W)*W= 4.5W2
W =0.7m, so the length of the first compartment =
2*0.7m = 1.4m and length of the second becomes 0.7m
Depth of tank from floor to soffit of cover slab
= liquid depth + freeboard
depth = 1.5 + 0.3 = 1.8m
Step 5: effluent disposal:

Determine the size of drainage field required in porous

silty clay
the sewage flow is 1280 l per day.
From Table 2, the infiltration rate for soil is 20 l per m² per
day.
wall area required == 64 m²
Assume depth of the trench (from the bottom of the pipe to
the bottom of the trench) is 0.6 m,
length of trench required is = = 54 m
This allows for infiltration on both sides of the trench.
If the plot is large enough, the drainage field should consist
of two trenches, each 27 m long, connected in parallel.
2. Design a septic tank for a household having five
occupants in a medium density housing area in
which the houses have full plumbing. Only WC
wastes go to the septic tank, and paper is used for
anal cleaning. The ambient temperature is more
than 10°C throughout the year. Also determine the
size of soak pit required in porous silty clay to
dispose of the effluent from the septic tank.
Solution:
1st -Daily volume of liquid
V1 = P x q
If the WC has a 10-litre cistern and each person flushes it four times a
day, then the sewage flow:
q = 4 x 10 = 40l/c/d and
V1 = 5 x 40 = 200 liters.
2nd -Volume for sludge and scum
V2 = P x N x F x R
Assume N is 3 years; from Table 4, F = 1.0; as only WC wastes go to
septic tank R = 25liter/c/year.
V2 = 5 x 3 x 1.0 x 25 = 375 liters
Vtotal = V1 + V2 = 200+375 = 5751iters = 0.575 m³
Cont’d…
As this is less than the minimum recommended volume of 1.0 m³, the
dimensions for the minimum volume should be calculated.
Assume liquid depth = 1.5 m.
Assume tank width is Wm.
design two compartments:
length of first = 2W and length of second = W
Volume of tank (V) = 1.5 x (2W + W) x W = 4.5 W²
4.5 W² = 1.0 m³,
then, W = 0.47 m
As this is less than the recommended minimum width of 0.6 m, take W
= 0.6 m.
Length of first compartment (2W) = 1.2 m
Length of second compartment (W) = 0.6 m
Cont’d….
Depth of tank from floor to soffit of cover slab
= 1.5 m (liquid depth) + 0.3 m (freeboard) = 1.8 m
The tank volume (excluding freeboard) is: (1.2 + 0.6) x 0.6 x
1.5 = 1.62 m³
5th : size of soakpit
The sewage flow is 200l/d… from previous estimation.
From Table , the infiltration rate for sewage through porous silty
clay is 20liters per m² per day.
Therefore, the wall area required == 10 m²
Assume pit diameter as 1.5m, and then the depth required from
the bottom of the pipe from the septic tank to the bottom of the
pit is:
Pit depth = = 2.12 m
Imhoff Tank
• The technology was developed in the Emscher District of
Germany and patented in 1906 by Dr. Imhoff.
• An Imhoff tank, which works similar to a communal septic tank, is
in fact a two storied tank, so they are sometimes also known as
Two-story Digestion tank.
• An Imhoff tank will remove 30 to 40 % of BOD and 60 to 65% of
solids.
• It is a compact and efficient system for pre-treatment of
municipal wastewater.
• The pre-treated wastewater from the Imhoff tank requires a
secondary treatment.
e.g. leach field, soak pits, horizontal flow, vertical flow or
free-surface constructed wetlands.
• The Imhoff tank may be either rectangular or
circular and is divided into three compartments:
1. The upper section or sedimentation compartment
2. The lower section or digestion compartment and
3. The gas vent or scum section
• The settling of solids occurs in an upper chamber
and digestion of the solids in the lower chamber.
• The two chambers are separated by a sloping
partition that contains narrow slots through which
the solids passes into the lower chamber.
Operation principle of Imhoff
• The digested sludge from the bottom of the hoppers is
removed periodically (after 30 to 45 days, depending
upon the temperature of sludge) through the cast-iron de-
sludging pipes provided in each compartment.
• Gas vent or Scum chamber is provided above the
digestion chamber and alongside the sedimentation
chamber to take care of the gases escaping to the surface.
• In order to prevent the particles of the sludge or scum
from entering into the sedimentation chamber from the
digestion chamber, the scum and sludge must be
maintained at least 45cm above and below the slot
respectively. (neutral zone)
Solids settle out in the upper sedimentation chamber
and gradually flow into the lower digestion
chamber.
• In the digestion chamber, solids accumulate and
slowly digest.
• Digestion chamber is divided into a number of
interconnected compartments.
• The bottom of each digestion compartment is
made up in the form of an inverted cone or hopper
with side sloping 1:1, so as to concentrate the
sludge at the bottom of the hopper.
Fig of Imhoff Tank
Design Considerations of Imhoff Tank
Sedimentation chamber
Detention period: 2 to 4 hours (usually 2 hours)
• Flowing velocity should not be more than 0.3m/minute
• Surface loading should not exceed 30,000 liters/m2 of
plan area per day
• Length of tank should preferably not exceed 30m or so.
• Length to the width ratio varies between 3 to 5.
• Depth of the chamber should be kept shallow as far as
possible. Practically a total depth of 9 to 11m has been
found to be satisfactory. Depth of SC 3 to 3.5m.
• The free board should be provided may be about 45cm.
 Digestion chamber
• It is divided into a number of interconnected compartments.
• The bottom of each digestion compartment is made up in the
form of an inverted cone or hopper with side sloping 1:1, so as to
concentrate the sludge at the bottom of the hopper.
• The digested sludge from the bottom of the hoppers is
removed periodically (after 30 to 45 days, depending upon the
temperature of sludge)
• through the cast-iron de- slugging pipes provided in each
compartment.
• The chamber is generally designed for a minimum capacity of
57 liters per capita
• In warmer climates the capacity may be reduced to about 35
to 40 liters per capita, where shorter periods between the sludge
withdrawals are possible
 Gas vent or Scum chamber
• It is provided above the digestion chamber and
alongside the sedimentation chamber to take care of
the gases escaping to the surface.
• The surface area of the scum chamber should be
about 25 to 30% of the area of horizontal projection
of the digestion chamber.
• The width of vent should be 60cm or more
• The scum and sludge must be maintained at least
45cm above and below the slot respectively. to
prevent the particles of the sludge or scum from
entering into the sedimentation chamber from the
digestion chamber
Example
• Design an imhoff tank to treat the sewage from a small town
with 30,000 population. The rate of sewage may be assumed as
150 l/c/d. Make suitable assumptions, wherever needed.
Solution
Design of Sedimentation Chamber
The sewage discharge per day
= 30,000 ∗ 150 = 4.5 M liters/day = 4500m3/day
Assuming a detention period of sewage in the sedimentation
chamber as 2 hours, the volume of sewage entering in 2 hours,
i.e. the capacity of the sedimentation chamber
= 150 ∗ 30,000 ∗2/24liters = 375,000 liters = 375m3
Assume an effective depth of 3m (effective depth
includes part of the bottom sloping walls of the
chamber) and length to width ratio 4
The area of the tank becomes
A = V/depth = = 125m2
Width of the tank becomes
A =L*W but L= 4W, then A= 4w2 , W = =5.5m
Length of the chamber becomes 22.36m ≈ 23 m.
This length is satisfactory and with width of 5.5m
Check for velocity
Length of tank = Velocity * Detention time
15m = Velocity in m/min * (2 * 60 min.)
Velocity = = 0.1916 < 0.3 it is safe!

Check for Surface Loading

Surface loading = = L/m2/day
= < 30,000 L/m2/day . . . It is safe
Hence, the dimensions chosen can be accepted.
 Pour-flush latrines or toilets

A pour-flush latrine is composed • The pit can be dug far

of a pan with a water-seal from the superstructure
installed in a superstructure. and connected with a pipe
• The water-seal is connected to or covered drains.
a pit by a pipe. • The pan is laid down on
• The water-seal flushes out the floor on top of the
excreta with just enough water-seal for urine
water: reception, under minimal
to drain off the solids and
hygienic conditions.
to restore the water level in the
water-seal
Pour-flush latrines or toilets
 Pour-flush latrines or toilets
Advantages Disadvantages
• Relatively cheap
• A source of water is
• No smells, no flies and
mosquitoes
needed
• Agreeable to use • The use of solid
• Can be improved by a cleansing material is not
connection to a sewer network advisable (except paper)
at the right moment
• Skilled labor (mason) is
• Low water consumption 2 – 3
liters of water for each flush required
• Latrine can be in house
Design Principles of Twin-Pit Pour-Flush Toilets
• The pour-flush squatting pan consist of a steep
bottom slope and a water seal trap.
• From there, faeces and flushing water is directed
to one of two leach pits.
• An inspection chamber containing a Y junction is
normally built between the pits and the pan so that
the excrete can be channeled into either pit.
• Each pit is designed to last for about 3 years
before it gets filled.
Pour-flush Toilet (Twin-pit Model)
Cont’d……
• The water seal at the bottom of the pour-flush toilet or pan
should have a slope of 25 to 30°.
• Water seals should be made out of plastic or ceramic to
prevent clogs and to make cleaning easier (concrete may clog
more easily if it is rough or textured).
• The optimal depth of the water seal is approximately 2cm to
minimize the water required to flush the excreta.
• The trap should be approximately 7cm in diameter
• The pits are constructed in brick line (much like a
honeycomb to facilitate the liquid to flow out) or with
perforated concrete tubes (e.g. prefabricated).
• A wooden or concrete slab to prevent people falling into
them
Pits location
• To remain accessible, pits should be constructed in
open
ground. But if no space is available, they can also be
constructed below the toilet.

• Pits should not be situated in drainage lines or the

paths of
storm water drains to prevent cross-contamination.
• The pits should not be located in depressions where
water is likely to collect.
• The pits should be constructed apart 1m from any
structural foundation as leachate can negatively
impact structural supports
 pits are to be located are prerequisites.
• If hydro-geological conditions are not known,
minimal distance of 30m from water sources
should always be maintained!
• No risk in alluvial soils (silt mixed with fine sand)
exists and
where the pit bottom is at least 2m above the
maximum groundwater level.
• Virus and bacteria can travel hundreds of meters in
saturated conditions.
• There is a risk for groundwater pollution where
Water table is high Crack and fissures in bedrock
allow short-cutted flow
Principle of operation pour flush toilet
• Excreta deposited on the pan are flushed by a low volume
of hand poured water: about 2-3 liters of water are
required .
• The excreta flushed into the leach pit are biodegraded
under both anaerobic and aerobic conditions.
• The water, together with the liquid and soluble products of
biodegradation, pass through the leach pit wall into the
surrounding soil and are thereby disposed of
• if the soil has no sufficient long-term infiltrative capacity
the liquid effluent can be removed by shallow gradient
small bore sewers.
• The solid products of biodegradation accumulate in the
leach pit, which in time fills up will desludged.
Example
An offset pour-flush double-pit latrine is to be constructed
for a family of six who use water for anal cleaning. The
groundwater table is within 0.5 m of the surface during
the rainy season and the soil is a sandy loam. Desludging
period for twin pit pour flush toilet is 2years
Solution:
1. Sludge volume
V= N x D x R
Use sludge accumulation rate as 40 l/year.
Sludge volume = 6 x 2year x (40/1000year) = 0.42
m³
Sludge depth
If pit is rectangular with 1.2 m wide and 1.2 m long,
sludge depth = = 0.33 m
An offset pour-flush toilet uses about 2liter of water per
flush. Assuming 20 flushes per day the total liquid
inflow will be:
2 x 20 = 40 liters
If 6liter of urine enter the pit each day, the total daily
inflow of liquid will be 46liter.
The infiltration rate for sandy loam is about 25 l/m²/day
(see from table ); infiltration area required = = 1.84 m²
Cont’d….
The perimeter length of each pit is 1.2 x 4 = 4.8 m,
liquid depth = = 0.38 m

Total depth of pit = slugde depth + water depth +

depth to bottom of inlet pipe (take 0.2m) (minimum
value is 0.1m)

Total depth = 0.33m + 0.38m + 0.2m = 0.91m

Aqua-privy
• has a watertight tank immediately under the latrine floor.
• It is simply a latrine constructed directly above a septic
tank.
• Excreta drop directly into the tank through a pipe.
• The bottom of the pipe is submerged in the liquid in the
tank, forming a water seal to prevent escape of flies,
mosquitos and smell.
• The tank functions like a septic tank.
• Effluent usually infiltrates into the ground through a
soak pit.
• Accumulated solids (sludge) must be removed regularly
Aqua-privy
 Aqua-privy
Advantages Disadvantages
• Water must be available nearby
• Does not need piped • More expensive than VIP or
pour-flush latrine
water on site
• Fly, mosquito and smell nuisance
• Less expensive than a if seal is lost ‘coz no
water’because insufficient water
septic tank is added
• Regular desludging required
and sludge needs careful handling
• Permeable soil required to
dispose of effluent