Types of Ponds

Ponds can be classified (named) based on their location in the system, by the type wastes they receive, and by the main biological process occurring in the pond. This section takes a look at the types of ponds classified according to their location and the type wastes they receive: raw sewage stabilization ponds (see Figure 14.1), oxidation ponds, and polishing ponds. In the following section, ponds are classified by the type of processes occurring within the pond: aerobic ponds, anaerobic ponds, facultative ponds, and aerated ponds.

Pond surface

Photosynthesis (Algae producing oxygen)

Aerobic decomposition (Bacteria producing CO2)

Solids

Anaerobic digestion (settled solids)

Pond bottom

Pond bottom

Figure 14.1 Stabilization pond processes.

14.2.1.1 Ponds Based on Location and Types of Wastes They Receive

14.2.1.1.1 Raw Sewage Stabilization Pond

The raw sewage stabilization pond is the most common type of pond. 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' detention time and to receive no more than 30 pounds of BODs per day per acre. The quality of the discharge is dependent on the time of the year. Summer months produce high BOD5 removal but excellent suspended solids removals.

The pond consists of an influent structure, pond berm, or walls and an effluent structure designed to allow selection of the best quality effluent. Normal operating depth of the pond is 3 to 5 feet. 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.

When wastewater enters the stabilization pond, several processes begin to occur. These include settling, aerobic decomposition, anaerobic decomposition, and photosynthesis (refer to Figure 19.3). Solids in the wastewater will settle to the bottom of the pond. In addition to the solids in the wastewater entering the pond, solids 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 (20 to 30 years is normal), the pond will have to be replaced or cleaned.

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, stable solids (which may settle out), and 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), which are dissolved in the water; stable solids; and more organisms. Near the surface of the pond, a population of green algae will develop which 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 involving 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 to the photosynthesis occurring in the system. Normal operation, however, will result in very high DO and pH levels.

14.2.1.1.2 Oxidation Pond

An oxidation pond, which is normally designed using the same criteria as the stabilization pond, receives flows that have passed through a stabilization pond or primary settling tank. This type of pond provides biological treatment, additional settling, and some reduction in the number of fecal coliform present.

14.2.1.1.3 Polishing Pond

A polishing pond, which uses the same equipment as a stabilization pond, receives flow from an oxidation pond or from other secondary treatment systems. Polishing ponds remove additional BOD5, solids, and fecal coliform and some nutrients. They are designed to provide 1 to 3 days' detention time and normally operate at a depth of 5 to 10 feet. Excessive detention time or too shallow a depth will result in algae growth, which increases influent suspended solids concentrations.

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