LOW-COST
WATER SUPPLY AND SANITATION
“Waste-Stabilization Ponds”
Introduction
Sewage from waterbone sanitation
systems must be treated before it is disposed of or reused, because it contains
wastes which pollute the environment. This treatment has two objectives:
·
To
remove organic matter from the sewage which causes pollution;
·
To
remove pathogens (disease-causing organism) which cause serious health risks.
Sewage treatment processes in
developing countries should;
·
Be
effective, low-cost and simple to construct and operate;
·
Need
little imported equipment.
Advantages
and Disadvantages
Waste-stabilization pond systems
are simple to build, reliable, and easy to maintain. Stabilization ponds are a
low-cost process for treating sewage. They require little or no imported
mechanical equipment. Pond systems provide pathogen removal, which is better
than convensional treatment processes. By keeping raw sewage in a series of
shallow ponds for two to three weeks, safe levels of BOD and pathogen removal
can be achieved. The ponds give a consistent standard of treatment and are not
sensitive to sudden load increases.
The only disadvantage is that
waste-stabilization ponds require a substantial amount of land.
Anaerobic
Ponds
Anaerobic ponds are used to
settle out and break down organic matter
They are always placed first in a
series of waste-stabilization ponds. Two
processes take place in anaerobic ponds:
·
Solid
in the incoming sewage settle to the bottom of the pond and form a layer of
sludge.
·
Anaerobic
bacteria (bacteria that require no oxygen) break down the organic matter in the
sludge. This breakdown produces gas (which is lost in the atmosphere) and some
soluble products which pass into other ponds. This process causes a very low
build-up of sludge in the anaerobic pond.
Facultative
Ponds
Facultative ponds are used BOD
and pathogen removal.
They can be the first pond in a
series of treatment ponds or they can receive effluent from an aerobic pond. In
this pond:
·
Some
of the suspended solids settle to the bottom where they are digested
anaerobically. This bottom layer is called the anaerobic layer. Thirty percent
of the BOD reduction of the pond occurs in this layer.
·
There is a layer above the anaerobic layer in
which oxygen is present. Algae growing
in this layer produce the oxygen by photosynthesis . The algae obtain
the nutrients they require to survive from by-products of aerobic bacteria,
which are also present in this layer. Aerobic bacteria require oxygen to
survive and the bacteria get this in turn from algae (as a by-product). The
interdependence between the algae and aerobic bacteria is called symbiosis
(figure 2).
Maturation
Ponds
Maturation ponds are used to
upgrade the effluent from a facultative pond or from another maturation pond.
They should not receive untreated wastewater. Maturation ponds are aerobic for
their whole depth because of their low concentration of organic material and
their high concentration of algae. The ponds are intended mainly for pathogen
removal. The pathogenic organism die off as the wastewater slowly passes
through the maturation pond. The number of ponds required in the stabilization
system depends on the required quality of the final effluent.
Pond
Design Principles
Pre-Design Considerations
There are several factors which
affect the size and layout of a pond system:
1.
The
volume of sewage to be treated
2.
The
strenght of the sewage to be treated
3.
The
desired quantity of the final effluent from the pond system
4.
Climate
Before a pond system can be
designed, values must be established for the first three factors.
The volume of sewage will depend
first on the estimated population of the area to be served at the end of the
design life of the system (which is usually assumed to be between 30 and 50
years), and secondly, the quantity of water each person is expected to use.
This must take into account improvements in the water supply and higher
standards of living, which will mean higher water consumption. It is usually
assumed that 80 percent of domestically used water enters the sewers.
Figure2: The Symbiotic
Relationship between Algae and Bacteria
The “strength” of sewage depends
on its contents of organic materials, and is measured by it biochemical oxygen
demand (BOD). This is the amount of oxygen that would be needed to oxidize the
organic matter in the sewage and is usually measured in milligrams per liter
(mg/l).
To estimate the strength of the
sewage to be treated, the total amount of BOD contributed per person in the
population served in considered. This will vary according to diet and how much
food waste is dispostropical ed of through the sewers, but it most tropical countries it will be roughly 40 g
per person per day. The strength of sewage will be the total BOD per person
divided by the water consumption per person.
However, where the effluent is
used for trickle irrigation, the suspended solids contents will be important.
Suggested values of BOD and FC forr various end uses for the effluent are given
Table 1
Preliminary
Treatment
Before
sewage flows into the firs pond in the series, it may be screened to remove
rags and other large objects that will float and cause problems in the ponds.
The screens can be mechanic ally raked, but manually raked screens have the
advantages that they can not break down and can be fabricated locally. The
screenings should be buried or burned.
Table 1. Suggested Effluent Standars for
Irrigationand Discharge
Method of Reuse
|
BOD (mg/l)
|
Fecal Coliforms Number/100 ml
|
Irrigation of trees, cotton, and other
nonedible crops
|
60
|
50.000
|
Irrigation of citrus, fruit trees, folder
crops, and nuts
|
45
|
10.000
|
Irrigation of deciduous, fruit trees, sugar
cane, cooked vegetables , and sports playing fields
|
35
|
1.000
|
Discharge to a receiving stream
|
25
|
5.000
|
Unrestriched crop irrigation, including
parks and lawns
|
25
|
100
|
In
tropical countires, sewage tends to contain a lot of grit and sand which is
washed into the sewers because people often use sand for cleaning kitchen
utensils. The grit can be removed by passing the sewage along grit removal
channels. If so, the grit should be regularly remove from these channels and
buried. Alternatively, a sump can be constructed below the inlet to the first
pond. The sump store the grit between desludging operations. The sump is easier
to construction and maintain than the grit remove channel, and is therefore
recommended.
Anaerobic
Ponds
Anaerobic
ponds are designed to receive BOD loadings of between 100and 400 gm/m3/d.
The actual loading will depend on climate, with higher loading possible at
higher temperatures. The retention time will usually be in the range of 1 to 2 days
an a rate of BOD removal of up to 80 percent (but normally about 60 percent) is
possible under tropical conditions.
Facultative
Ponds
Facultative
ponds are usually 1 to 1,5 meters deep.
If they are deeper than 1,5 m. the anaerobic layer of the ponds predominates.
If the pond is less than 1 m deep, there will be problems with plants growing
in the pond. Detention time varies from 5 – 30 days.
Maturation
Ponds
The number and size of maturationponds in a
system depend on the bacteriological quality that is required of the effluent.
Construction
The
layout of the various ponds depends on
the quality and topography of the land available. A sample layout is in figure
3.
Embankments
should be simple and cheap to build and, where possible, rely on construction
using homogenous soil with clay or silt
content. If the soils of the available site are not sufficiently impermeable,
the embankments can be constructed with an impermeable clay blanket on the
inside of the embankment.
Where
the permeability of the soil is so high that losses due to infiltration are
likely to exceed local evaporation raes, a lining should be used. This may be
impermeable soil with high clay or silt content or plastic covered by a
protective layer of soil.
At
the water level, the embankments must be protected from erosion for a distance
of 200 mm vertically above and below the water surface. Grouted rocks and
stones are usually the simplest method for doing this, but pre-cast concrete
slabs, if available, can be used.
The
side slopes of the embankments will usually be about 1:3. With some soils ,
they may be steeper, but only if very thorough investigation confirm that this
is feasible . (figure 4)
The
embankment crest should be wide enough for all vehicles to drive to all parts
of the pond system, and should be approximately 0,5 m above pond level.
Pond
inlets to anaerobic and facultative ponds should discharge below survace to
maintain anaerobic conditions in anaerobic ponds and to limit scum formation in
facultative ponds. The inlets should discharge at least 10 m from the edge of
the pond to reduce the possibility of sludge banks building up at the pond
edge.
Interpond
connection should be as simple as possible and should allow for flexible
operation and expansion of the pond system. Anaerobic ponds should be provided
with a means of being completely emptied for desludging.
Ponds
inlets and outlets should be sited where they will keep effluent from traveling
directly across the ponds. They are usually located in diagonally opposite corners.
Operation
and Maintenance
The
maintenance of waste – stabilization ponds is simple because they are a simple
treatment system. A well maintained pond system will yield a good effluent
quality with little labor.
Vegetation
on embankments must be kept short bt cutting or moving because tall vegetation
will shelter the ponds from wind. This will reduce the efficiency of the pond
as wind is required to mix the layers of water
in the pond. Plants in the pond and near the water’s edge should be
removed because they shade the ponds and encourage mosquito breeding. Lining
part of the inner slopes with rocks of stones will help prevent aquatic plants
from growing in shallowa water.
Scum
mats may form on the pond survace because of gas bubbles which carry sludge
particles to the survace to form mats of floating algae. These mats must be
broken down (by water jet or long-handled rake) or removed. Scum removed from
ponds should buried.
Three-Hour
Practical
1.
List
the three basic types of waste-stabilization ponds and describe the processes,
principal functions, and maintenance requirements for each pond type.
Processes
Anaerobic
ponds. Organic
matter is removed by sedimentation and anaerobic digestion. This produces gas
and soluble nutrients and leaves sludge.
Facultative
pands. Organic
matter is removed by sedimentation and aerobic treatment (upper layer) and
anaerobic treatment (lower layer). Algae in the anerobic layer produce oxygen
by photosynthesis and use nutrients and CO2 produced by bacteria. Bacteria use
oxygen from the algae for breakdown of organic matter producing nutrients and
carbon dioxide.
Maturation
ponds. Bacteria
break down wastes. Algae, through photosynthetic processes maintain an aerobic
environment
Principal
Function
Anaerobic
ponds.
Organic matter removal and partial
treatment.
Facultative
pands. Pathogen
destruction and organic matter treatment.
Maturation
ponds.pathogen
and suspended solids removal.
Maintenance
Required
Anaerobic
ponds. Require
desludging when half-full of sludge (every 2 to 5 years); periodic vegetation
and scum control.
Facultative
pands. Require
scum matter break-up or removal. Desludging done very infrequently. Periodic
vegetation control along banks, as required.
Maturation
ponds. Require
periodic vegetation control.
2.
A
pond system consisting of a facultative pond and maturation ponds is required
to treat 100.000 m3/day of domestic sewage which has a BOD of 630
mg/l. The average temperature during the coolest season is 20oC and
the required bacterial quality of the effluent is less than 5.000 fecal
coliforms/100 ml. Influent quality is 5 x 107 fecal coliforms/100
ml.
Determine pond dimensions and
retention times if facultative and maturation ponds are to be used.
Facultative
pond
Area loading :
mid-depth area :
assuming
a depth of 1,5 m :
Maturation Pond
For
three maturation ponds with 3 days retention time
Pound
volume
assuming
pond depth = 1,5 m;
