Photo Credit: Virginia Department
to Sewage Ponds
In this section we will answer the following
Ponds are probably one of nature's
most economical ways of treating sewage and producing a highly
purified effluent (end product.) The degree of treatment
provided by ponds depends upon the type and number of ponds
used. Ponds can be used as the sole type of water treatment
or can be used in conjunction with other forms of wastewater
The beginning of this lesson will be
concerned with raw sewage stabilization ponds in particular
and with the requirements which must be met when building
sewage treatment ponds in general. The second half of
the lesson will consider other types of ponds used in wastewater
Sewage ponds are a way of treating
wastewater on a larger scale than the septic systems which
we considered in the last lesson. The ponds have many
advantages and disadvantages compared to package plants,
small treatment plants often manufactured at a factory, hauled
to the site, and installed as one facility to treat wastewater.
Both ponds and package plants have to deal with aeration of
the water being treated. In the sewage pond, oxygen
is transferred directly into the water across the surface
area without the need for any equipment. A package plant,
in contrast, must install an aerator to add oxygen to the
But the natural method of aeration
used by a sewage pond takes much longer than an aerator does
to add oxygen to the water. As a result, ponds treat
sewage much more slowly than package plants do. The
minimum detention time of a pond is 45 days. In
contrast, a package plant has a two to four hour detention
time. And, since ponds must hold the wastewater much
longer than package plants do, the ponds must also have a
much larger area to retain the sewage.
If the time and area are available, sewage ponds are very
economical facilities to maintain. Package plants require
frequent monitoring for various parameters such as ammonia
and B.O.D. In contrast, ponds require only one visit
per day to monitor pH and D.O.
Sewage ponds are very simple to construct.
A bulldozer is used to remove soil from the ground and
create a basin in which water can collect. However,
the pond and surrounding area must be planned in such a way
that the human and natural environments surrounding the pond
are not damaged.
The first requirement of a sewage pond is that
it must be surrounded by a berm (a mound or wall of
earth) or an embankment (a raised structure to hold
back the water, such as that shown in the photograph at the
beginning of this lesson). The berm or embankment prevents
storm water from running into the pond. Without a berm,
a heavy storm could cause the sewage pond to overflow and
send untreated sewage out into the surrounding area.
The soil in which a pond is built must be impermeable.
This will prevent the sewage from being absorbed into
the ground and from leaking pollutants into the area.
A pond must be completely fenced to keep unwanted
visitors out. In addition, the area around the fence
must be mowed to keep out vermin which could dig holes into
the sides of the pond. Tree growth must be restricted
near the pond since roots could enter the pond and provide
a way for sewage to escape if the trees died.
Sewage ponds must be encircled by a windbreak,
which usually consists of a row of pine trees. The windbreak
will prevent the pond's odors from disturbing the nearby residents
and will also make the area aesthetically pleasing.
The depth of the pond is another important factor.
The pond must be greater than two feet deep at all parts
to exclude plant growth. Plants growing at the edge
of a pond will create areas of still water in which mosquitoes
will lay their eggs. But at depths of over six feet,
anaerobic conditions occur, so regulations stipulate that
the depth of a sewage pond can be no more than 5 feet.
Health of the Surrounding
The final requirement which must be met when constructing
sewage ponds is to be sensitive to any streams or rivers into
which the effluent from the pond will be released. This
entails knowing the classification of the stream, whether
the stream contains any endangered species, and whether there
are any existing contaminants in the stream.
One way of protecting the surrounding environment
is by adding a finishing pond. A finishing pond,
also known as a polishing pond, is like a finishing
school - it prepares the water to go out into the world. The
finishing pond is installed between the sewage pond and the
stream as shown below.
One of the largest problems when water is released
directly from a sewage pond into a stream is algae.
Sewage ponds are perfect environments for these one-celled
plants. Food is readily available, as is moisture and
sunlight, so algae grow quickly and become quite numerous.
When water from a sewage pond, rich in algae,
is released directly into a stream the stream can be harmed.
The large quantities of algae use up the water's oxygen
at night or during an algal bloom (when the algae reproduce
very quickly). Without the oxygen they need to survive,
the fish in the stream die.
A finishing pond can eliminate this problem. Finishing
ponds are usually stocked with fish, such as carp, which eat
the algae in the water. Finishing ponds also allow the
quality of the effluent to be monitored before it is released
into the stream. As a result, streams being fed by finishing
ponds tend to be healthier than those fed directly from sewage
Part 2: Types of Sewage Ponds