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Tropical Environmental Health
Sanitation 2
Elevated Toilet:
For use in areas with high water table!
We have examined methods for disposal of
faecalmatter: Now consider what happens with the effluent
• In any system using water, the effluent must be disposed
• Effluent consists of sullage water (i.e. laundry, kitchen, cleaning.
• Toilet water containing faeces, urine and other effluent.
• Real engineering principles apply; however in many developing countries this is left to providence….
The Septic TankSchematic of conventional septic tank
Inspection opening150 mm diameter
Inlet
At least 75 mm
At least 25 mm
Access opening near side wall at least 600 mm diameter
Inspection opening 150 mm diameter
Liquid level20% of
Liquid depth Water line
40% ofLiquid depthScum clear space
(75 mm, minimum)
Scum clear space(300 mm, minimum)
20% of Liquid depth(150 mm, minimum)
Scum
Clear space
Sludge Sludge
Outlet
40% ofLiquid depth
Liq
uid
dept
h
First compartment 2/3 length second compartment 2/3 length
Total length equals two to three times width
Liq
uid
dept
h (D
)
0.2D 0.4D
InletOutlet
Scum
Sludge Sludge
Liquid
Solids Solids
Manhole
Two compartment septic tank
Compartmentbaffle
Role of the septic tank
Anaerobic fermentation of solidsReduce the load of pathogens in the effluentHold the effluent for 2-3 days for improved
safetyRetain solid material to prevent blockage of
further disposal system
The soak-away
Absorption field treatment of septic tank effluent
septic tank
Absorption trench system
Distribution box
Absorption trench
1/2 to 1 meter
backfill
Gravel or crushed rocks
Drainage tile(100 mm diameter)
1/2 to1 1/2 meter
150 mm
The field requires periodic maintenance, diversion of the flow at distribution box and repacking of the rock fill, removal of plant roots etc.
Absorption field treatment of septic tank effluent
septic tank
Absorption trench system
Distribution box
Connection to a sewage system:what are the alternatives?
• Conventional sewage connection…expensive• Small bore sewage system: less expensive• use road-side drains, and hope for the best….• Unfortunately this is the common outcome
Comparison between the sewage systems
Storm sewer
combined sewer
sanitary sewer
Small bore is near surface, and runs full
Small bore
Conventional
Small bore is PVC pipe, conventional is made from concrete
Conventional
Small bore
Conventional requires heavy machinery and regular pumping stations to
maintain flow
small bore installation is simpler, and as the pipes are light and fit in small
trenches
Sewer lines can rise or fall, the system operates under slight pressure
There is a BIG –but! Small bore systems cannot handle solids. So all connections
MUST be to septic tanks
House connection
Interceptortank
Small bare sewer
Where do we go from here:
• The effluent must be disposed of in a sanitary manner
• The system should be inexpensive and easy to manage
• Tropical areas do have long hours of sunlight, why not exploit this.
• We can by using oxidation ponds…
Lay out of an oxidation pond systemStabilization pond layout and details
Aerobic pond
Aerobic pond
Aerobic pond
Interpond connection
Facultative pond
Inlet
OutletAnaerobic
pond
Plan layout(not to scale)
5,400 4,000 Varies
Water line
300300
Precast concrete
slabs 3
1
Var
ies Water line
1,80
01,2 0
0
100 mm ofSelected fill
Precast concrete slabs for
Protection against wave erosion
Detail of a typical embankment
Newcells
Algae
O2 CO2
nutrients
Light
Organicmatter
Newcells
Bacteria
Three processes of stabilization: anaerobic
Anaerobic Facultative Maturation
Suspended &Soluble solidsSettleable solids
Sludge layer
The second process is facultative
Anaerobic Facultative Maturation
Suspended &Soluble solidsSettleable solids
Sludge layer
In the maturation pond, pathogens are reduced: the water an be released to a river
Anaerobic Facultative Maturation
Organic matter
Pathogenic organisms