CE 333: Environmental Engineering II

Fall 2023 Semester

Level-3, Term II

Lecture-3: Pour Flush Latrines-1

Md.Yasin
Assistant Professor
Department of Civil Engineering
Ahsanullah University of Science and Technology (AUST)

Pour-Flush Sanitation Technologies

 • Further improvement over “pit latrines”
• An important precondition for such latrines is availability of some water (about
2 to 5 liters per use) for cleansing and flushing (i.e., tube well based water
supply would be required)
• The most vital part is the “water seal” or “other seal” incorporated in the
latrine pan/ slab, which eliminate odour and insects problems

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Pour-Flush Latrine
 Major Components:
(1) Superstructure
(2) Latrine Pan with “water/other
seal”
(3) “Leach Pit” (single or twin)

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Water Seal
• Key component: After each use, about 2 to 5 L of water is poured in to
 flush the waste. Some of the clean flush water remains in the trap thus
maintaining the “water seal”.
• The “depth of the water seal” within the trap unit is very important. A seal
depth of 15 to 25 mm should be
optimum.

Two Types:

(1) Gooseneck type

(2) U-bend type

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Types of Pour Flush Latrines

 There are two basic types of pour flush latrines

1. Direct-Pit Pour Flush Latrine (pit directly below latrine)

2. Offset-Pit Pour Flush Latrine (Single Pit)

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Types
Pour ofLatrine
Flush Pour (pit
Flush Latrines Vent pipe not shown
directly below latrine)

Can use either: Gooseneck/U-Trap

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Types of Pour Flush

Latrines

(2) Offset-Pit Pour Flush Latrine

(Single Pit)

U-bend type or P-Trap used

Md.Yasin, Asst. Professor, Department of Civil Engineering
Vent pipe not shown7

Types of Pour Flush Latrines

(3) Offset Pit Pour Flush Latrine (Alternating Twin Pit)
 Components:

• Superstructure
• Latrine pan with “water
seal”
• Two “leach pits”
• A Y-junction (diversion box)

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Types of
Pour Flush Latrines
(3) Offset Pit Pour Flush Latrine (Alternating Twin Pit)
 • Pits are used alternately; only one pit
used at a time
• When one pit becomes full, flow of
waste is diverted (at the Y-junction) to
the second pit
• Contents of first pit decomposes to
safe,
pathogen free humus within 18 to 24
months
• Contents of the first pit may then be dug
out and the pit becomes ready for re-use.

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Twin Offset Pit Pour Flush Latrine

 Different Pit Arrangements

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Functions of Pour-flush Latrines

• After each use, the latrine is manually flushed with 2-5 L

 of water; total water entering into the pit
usually varies
from 5 to 20 lpcd.
• “Water seal” provides barrier against odour and
insect.
• Pit has to provide:
a) “sufficient volume” for solids storage; and
b) “sufficient area” for infiltration of wastewater into
soil (thus, long-term infiltration capacity of soil is very
important)

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Functions of Pour-flush Latrines (Contd.)

 Users of pour flush technology must understand that
• The “water/other seal” must be maintained at all times in order to effectively
control odour and insects.
• Flow must be diverted to the second pit when the first pit is full (for twin-pit
latrines).
• When the pit is full, it must be kept covered (with 0.5 m soil) for at least one and
a half years for risk free manual emptying.

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Suitability of Pour-Flush Latrines

 1. Pour-flush latrines could be used in both rural and urban areas, where tube
well-based water supply is available.
2. Single pits may be appropriate in urban (slum/low-income communities) areas,
only if they can be de-sludged mechanically. Twin-pits are recommended if the
pits are to be de-sludged manually. However, it is often difficult to construct
twin pits due to space constraint in slum areas.
3. In rural areas mechanical desludging is neither reliable nor affordable.
Consequently, pits should be designed for manual desludging. Hence,
“alternating twin-pit latrines” are preferred.

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Advantages of Pour Flush Latrines

 1. A properly designed pour-flush latrine is a “hygienic latrine” (eliminate odour,
insect problems)
2. Not expensive, easy to construct (especially single- pit latrines)
3. Require low volumes of water for flushing (2-5 L/flush) (tube-well based
water supply sufficient)
4. Can be upgraded to: (i) a septic tank system, or (ii) small bore sewerage
system
5. Can be located, if desired, inside the house (off-set pit latrines).
6. Potential for resource recovery using the digested sludge as soil conditioner

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Disadvantages/ challenges of Pour-Flush Latrines

 1. Flushing water must be available throughout the year.
2. Maintenance of “water seal” is difficult with low-volume manual flushing;
3. Maintenance of Y-junction (twin pit latrines) to be ensured.
4. Desludging difficulties for single pit latrines
5. Risk of groundwater pollution (to be covered in more detail)

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Most Significant Challenge in Pour-Flush Latrines

 1. Maintenance of Water/Other Seal; considerable research underway for
development of suitable “seal”
2. Desludging of pits, for single pit toilets
3. Management of “fecal sludge” generated in pit latrines/septic tanks
(collection, transportation, treatment and disposal)

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Most Suitable Pour-Flush Latrine

 If space is available, then a twin off-set pit pour-flush latrine is the most
suitable option, because it provides a long-term solution and largely
eliminates the problems associated with fecal sludge management.

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Emptying of Pits: Mechanical Emptying

 Fig.: Schematic of VacuTug pit
emptying device developed by
Mirpur Agricultural Workshop
and Training School (MAWTS)

A couple of organizations
have been utilizing VacuTug
at some areas of Dhaka city

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Vacu tug
Md.Yasin, Asst. Professor, Department of Civil Engineering
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Human-Powered Emptying and Transport

Md.Yasin, Asst. Professor, Department of Civil Engineering
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Groundwater Pollution from Pit and Pour-flush Latrines Md.Yasin, Asst. Professor,
 Department of Civil Engineering 21

Groundwater Pollution: Thumb Rule for Pollution Prevention

 Md.Yasin, Asst.

Professor, Department of Civil Engineering

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Groundwater Pollution: High Water Table Areas

 Reducing risk of groundwater
pollution in local aquifer used for
water supply

• Seal the pit bottom of the pit with clay, lean concrete or plastic sheet and to put a
500 mm annulus of fine sand (< 1.0 mm) between pit lining and soil.

(Objective: increasing travel time of infiltrating liquid to groundwater).

Md.Yasin, Asst. Professor, Department of Civil Engineering
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Sand Envelopes around Pits: Can Serve Different Purposes

 Sand envelope around pit:

(a) In “high water table” areas 500 mm

annulus of “fine sand” (< 1.0 mm)
between
pit lining and soil, to retard leaching of
liquid

(b) For “compacted clayey soil” 300 mm

annulus of “coarse sand” surrounding
pit
lining and soil, to facilitate leaching of
liquid

Md.Yasin, Asst. Professor, Department of Civil Engineering

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 Groundwater Pollution: Other Common Reasons

1. Use of cow-dung during well construction (as a substitute for bentonite)

2. Leakage through annular space between well pipe and soil in faulty
platforms
3. Leakage at the joint of PVC/ uPVC strainer pipe and GI well pipe

Md.Yasin, Asst. Professor, Department of Civil Engineering

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Pit/ Pour-flush Latrines for Areas of Different Hydro-geologic Conditions

There are areas where modifications of usual design of different types of pit
latrines is needed to make them appropriate.

Such areas include:

(a) Dry Areas
(b) High Groundwater Table Areas, and
(c) Wet or Flood-prone Areas

Md.Yasin, Asst. Professor, Department of Civil Engineering

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(a) Dry Areas

1. In dry areas (depth of WT 7 m or more), a

pit
or pour-flush latrine (depending on
availability
of water) can be used. Here chance of
groundwater pollution is minimum
2. Single-ring or Multiple-ring pits can be used
3. Availability of water throughout the year is a
problem in some dry areas.
Md.Yasin, Asst. Professor, Department of Civil Engineering
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(b) High Water Table Areas

1. In high WT areas (e.g., char areas), where deep excavations are not possible,
raised pits are commonly used; part of the pit remains below ground level, while
part remains above ground
2. The portion above ground must be made impervious above and immediately
below the ground (with at least 1.5m earth covering around the pit portion
above ground), in order to avoid ingression of surface water or exfiltration of pit
liquid
3. Pit liquid infiltration is achieved through the lower part of the pit below ground
level
4. Two types: Step latrine and Mound latrine
 Md.Yasin, Asst. Professor, Department of Civil Engineering
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(b) High Water Table Areas

Md.Yasin, Asst. Professor, Department of Civil Engineering

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(c) Wet and Flood-prone Areas

1. A major challenge
2. Where surface or groundwater needs to be protected from pollution, pits can be enveloped
with 500 mm fine sand

Md.Yasin, Asst. Professor, Department of Civil Engineering

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(c) Wet and Flood-prone Areas

• One method of sanitation in swamps is to lower concrete rings below the

bottom of the water body; has the disadvantage of filling up of the pit too
 soon.

Md.Yasin, Asst. Professor, Department of Civil Engineering

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