Types of Ponds by Location
In this section we will answer the following questions:
Ponds can be classified based upon their location in the wastewater treatment process and on what type of waste they receive.
Whether wastewater is being treated in a pond or in another type of treatment facility, it follows the same general path. First, the water passes through a series of pretreatment processes including screening and shredding the sewage. Next, the wastewater receives primary treatment which allows some of the solid matter to settle out. From primary treatment, the wastewater moves to secondary treatment where biological processes convert the remaining organic matter into a form which is easier to remove from the wastewater. Treatment may stop after secondary treatment or may continue with tertiary treatment, which reduces the nutrient content of wastewater to prevent algae blooms in the body of water into which the effluent will be released.
The raw sewage stabilization pond, which we explored in depth in the last section, is a primary treatment pond. After water has been treated in a raw sewage stabilization pond or in some other type of primary treatment facility, the water can move on to an oxidation pond, which is a type of secondary treatment. Finally, a polishing pond is a type of tertiary treatment.
These three types of ponds can be used in a series, as shown
in the picture above. Alternatively, they may be used in conjunction
with primary, secondary, and tertiary treatment in a wastewater treatment
plant. The wastewater may receive primary treatment in the treatment
plant then receive secondary treatment in an oxidation pond. Or
the wastewater may receive primary and secondary treatment in a treatment
plant and tertiary treatment in a polishing pond.
Raw Sewage Stabilization Pond
The raw sewage stabilization pond is the most common type of pond. It is a primary treatment facility which receives wastewater which has had no prior treatment (except screening or shredding.)
Like any other primary treatment facility, the purpose of the raw sewage stabilization pond is to settle out most of the solids in the water. In addition, aerobic, facultative, and anaerobic decomposition of organic matter begins in this pond. Oxygen is provided by diffusion from the surface of the pond and from photosynthesis by the algae in the pond. All of these processes occur over the minimum 45 day detention time during which the water stays in the stabilization pond.
As shown in the previous section, the stabilization pond consists of an influent structure, berms or walls surrounding the pond, and an effluent structure designed to permit selection of the best quality effluent. The normal operating depth of the pond is 3 to 5 feet.
The raw sewage stabilization pond is designed to receive no more than 50 pounds of BOD5 per day per acre. The biochemical oxygen demand, or BOD5, is the amount of organic matter which can be biologically oxidized in 5 days at 20°C in the dark. This is a way of measuring how much organic matter is in the water.
The quality of the water discharged from a stabilization
pond will depend on the time of year. During the summer, the
pond removes most of the BOD5 but not very much of the suspended
solids. In contrast, during winter months, the pond will have
poor BOD5 removal but excellent suspended solids removal.
In either case, the water is usually transferred from the raw
sewage stabilization pond to some type of secondary treatment facility.
Water from the stabilization pond or from primary settling tanks of a treatment plant flows into the oxidation pond. In this pond, additional settling of solids and biological treatment of organic matter in the water occurs. Some of the fecal coliform in the water is also removed.
The oxidation pond is very similar in design to the stabilization pond.
We have already dealt with polishing ponds to some extend in the last section. These ponds, also known as finishing ponds, receive water flowing from the oxidation pond or from some other secondary treatment systems. Here, additional BOD5, solids, fecal coliform, and some nutrients are removed from the water.
Polishing ponds have a much shorter detention time than stabilization ponds since they rely entirely on biological processes and no settling occurs here. Water remains in polishing ponds for only 1 to 3 days. A greater detention time may result in an increased concentration of suspended solids in the effluent.
In addition, polishing ponds are typically deeper than the other types of ponds, usually operating at a depth of 5 to 10 feet.
Types of Ponds by Processes
In this section we will answer the following question:
Aerobic Ponds and Aerated Ponds
Ponds can also be classified based on the type of processes occurring within the pond. The types refer to the three types of respiration which we treated in more depth in the lesson on oxygen.
An aerobic pond is a pond in which oxygen is present throughout the pond. All biological activity in the pond is aerobic decomposition. This type of pond is not widely used because, without some type of aeration, the lower portions of a pond tend to lack oxygen and to host anaerobic bacteria, as shown in the picture below. This is why the mud on the bottom of a pond smells like sulphur when it is stirred up. A pond would have to be very shallow in order to host only aerobic decomposition without any aeration.
One way of maintaining aerobic processes throughout the pond is to add oxygen to the water using mechanical or diffused air systems. Ponds which add oxygen to the water in this way are known as aerated ponds. Aerated ponds allow the depth of the pond and/or the acceptable loading levels to be increased. The mechanical or diffused aeration systems can be used to supplement natural oxygen production or to replace it.
The typical pond shown above, with an aerobic zone near the surface and an anaerobic zone near the bottom, is a facultative pond. Like a facultative bacteria, a facultative pond can carry out both aerobic and anaerobic processes based on the presence or absence of oxygen. This is the most common type of pond.
Oxygen is present in the upper portions of the pond, so aerobic processes occur here. there is no oxygen present in the lower levels of the pond, so the processes here are anaerobic or anoxic (lacking sufficient oxygen.)
The facultative pond must have a balance between photosynthesis and aerobic decomposition as shown in the picture below.
Facultative Stabilization Pond
Oxygen is added to the water in two ways. The wind and the surface area prompt oxygen to diffuse into the water from the air. Algae also produce oxygen during photosynthesis when the sun is present.
The oxygen is then used up by bacteria in the aerobic portion of the pond. These bacteria use oxygen to break down organic matter suspended in the water. In turn, the bacteria produce the carbon dioxide which the algae use in photosynthesis.
Some of the solids settle to the bottom of the pond. These solids are broken down by anaerobic bacteria which produce methane or hydrogen sulfide.
The final type of pond is the anaerobic pond. No oxygen is present in this type of pond, so all biological activity within an anaerobic pond is anaerobic decomposition. Wastewater is not usually treated in anaerobic ponds, but these ponds are used to treat high strength industrial wastes.