Environmental Engineering Reference
In-Depth Information
Pond surface
Photosynthesis
(Algae producing oxygen)
Aerobic decomposition
(Bacteria producing CO
2
)
In
Solids
Anaerobic digestion
(settled solids)
Pond bottom
figure 7.1 Stabilization pond processes.
7.2.1.1 Ponds Based on Location and Types of Wastes They Receive
7.2.1.1.1 raw Sewage Stabilization Pond
The raw sewage stabilization pond
is the most common type of pond
(see Figure 7.1). With the exception of screening and shredding, this type
of pond receives no prior treatment. Generally, raw sewage stabilization
ponds are designed to provide a minimum of 45 days of detention time
and to receive no more than 30 lb of BOD
5
per day per acre. The quality
of the discharge is dependent on the time of the year. Summer months
produce high BOD
5
removal but excellent suspended solids removals.
The pond consists of an influent structure, pond berm or walls,
and an effluent structure designed to permit selection of the best qual-
ity effluent. Normal operating depth of the pond is 3 to 5 ft. The process
occurring in the pond involves bacteria decomposing the organics in the
wastewater (aerobically and anaerobically) and algae using the products
of the bacterial action to produce oxygen (photosynthesis). Because this
type of pond is the most commonly used in wastewater treatment, the
process that occurs within the pond is described in greater detail in the
following text.
When wastewater enters the stabilization pond, several processes
begin to occur. These include settling, aerobic decomposition, anaerobic
decomposition, and photosynthesis (see Figure 7.1). Solids in the waste-
water will settle to the bottom of the pond. In addition to the solids in the
wastewater entering the pond, solids that are produced by the biological
activity will also settle to the bottom. Eventually, this will reduce the
detention time and the performance of the pond. When this occurs (nor-
mally in 20 to 30 years), the pond will have to be replaced or cleaned.
