Secondary Treatment - Wastewater Operators - page 260

Note: The CWA also states that ponds and trickling filters will be
included in the definition of secondary treatment even if they do not
meet the effluent quality requirements continuously.
Most secondary treatment processes decompose solids aerobically,
producing carbon dioxide, stable solids, and more organisms. Because
solids are produced, all of the biological processes must include some
form of solids removal (e.g., settling tank, filter). Secondary treatment
processes can be separated into two large categories: fixed-film systems
and suspended growth systems.
fixed-film systems are processes that use a biological growth (bio-
mass or slime) attached to some form of media. Wastewater passes over
or around the media and the slime. When the wastewater and slime are
in contact, the organisms remove and oxidize the organic solids. The
media may be stone, redwood, synthetic materials, or any other sub-
stance that is durable (capable of withstanding weather conditions for
many years), provides a large area for slime growth while providing open
space for ventilation, and is not toxic to the organisms in the biomass.
Fixed-film devices include trickling filters and rotating biological con-
tactors. Suspended growth systems are processes that use a biological
growth mixed with the wastewater. Typical suspended growth systems
consist of various modifications of the activated sludge process.
7.2 TreaTMenT Ponds
Wastewater treatment can be accomplished using ponds . Ponds
are relatively easy to build and manage, they accommodate large fluc-
tuations in flow, and they can also provide treatment that approaches
the effectiveness of conventional systems (producing a highly purified
effluent) at a much lower cost. It is the cost advantage that drives many
managers to decide on the pond option. The actual degree of treatment
provided depends on the type and number of ponds used. Ponds can
be used as the sole type of treatment or they can be used in conjunc-
tion with other forms of wastewater treatment; that is, other treatment
processes can be followed by a pond or a pond can be followed by other
treatment processes.
7.2.1 Types of Ponds
Ponds can be classified (named) based on their location in the
system, by the type of wastes they receive, and by the main biological
process occurring in the pond. First, we will take a look at the types
of ponds according to their location and the type wastes they receive:
raw sewage stabilization ponds , oxidation ponds , and polishing ponds .
Then, we will look at ponds classified by the type of processes occurring
within the pond: aerobic ponds , anaerobic ponds , facultative ponds ,
and aerated ponds .