Secondary Treatment - Wastewater Operators - page 262

Bacteria and other microorganisms use the organic matter as a food
source. They use oxygen (aerobic decomposition), organic matter, and
nutrients to produce carbon dioxide, water, and stable solids, which may
settle out, as well as more organisms. The carbon dioxide is an essential
component of the photosynthesis process occurring near the surface of
the pond. Organisms also use the solids that settle out as food material;
however, the oxygen levels at the bottom of the pond are extremely low
so the process used is anaerobic decomposition. The organisms use the
organic matter to produce gases (e.g., hydrogen sulfide, methane) that
are dissolved in the water, stable solids, and more organisms. Near the
surface of the pond, a population of green algae develops that can use
the carbon dioxide produced by the bacterial population, nutrients, and
sunlight to produce more algae and oxygen, which is dissolved into the
water. The dissolved oxygen is then used by organisms in the aerobic
decomposition process.
When compared with other wastewater treatment systems involv-
ing biological treatment, a stabilization pond treatment system is the
simplest to operate and maintain. Operation and maintenance activities
include collecting and testing samples for dissolved oxygen (DO) and
pH, removing weeds and other debris (scum) from the pond, mowing the
berms, repairing erosion, and removing burrowing animals.
Note: When operating properly, the stabilization pond will exhibit a wide
variation in both DO and pH due, in part, to photosynthesis occurring in
the system. Also, natural processes occurring in the system will cause the
levels of dissolved oxygen and pH in the pond to vary throughout the day.
7.2.1.1.2 oxidation Pond
An oxidation pond , which is normally designed using the same cri-
teria as the stabilization pond, receives flows that have passed through
a stabilization pond or primary settling tank. This type of pond pro-
vides biological treatment, additional settling, and some reduction in
the number of fecal coliform present.
7.2.1.1.3 Polishing Pond
A polishing pond , which uses the same equipment as a stabiliza-
tion pond, receives flow from an oxidation pond or from other secondary
treatment systems. Polishing ponds remove additional BOD 5 , solids, fecal
coliform, and some nutrients. They are designed to provide 1 to 3 days
of detention time and normally operate at a depth of 5 to 10 ft. Excessive
detention time or too shallow a depth will result in alga growth, which
increases influent suspended solids concentrations.
7.2.1.2 Ponds Based on the Type of Processes Occurring Within
Ponds may also be classified based on the type of processes occur-
ring within the pond, including aerobic, anaerobic, facultative, and aer-
ated processes.