Lesson 18:

Stabilization Ponds



In this lesson we will answer the following questions:
  • What are the different types of ponds used in wastewater treatment?
  • What are the process control calucations needed for pond operation?

Reading Assignment

Along with the online lesson, read Chapter 9: Wastewater Stabilization Ponds in your textbook Operation of Wastewater Treatment Plants Volume I .


Introduction to Sewage Ponds


Ponds are probably one of nature's most economical ways of treating sewage and producing a highly purified effluent (end product.)  The degree of treatment provided by ponds depends upon the type and number of ponds used.  Ponds can be used as the sole type of water treatment or can be used in conjunction with other forms of wastewater treatment.  

The beginning of this lesson will be concerned with raw sewage stabilization ponds in particular and with the requirements which must be met when building sewage treatment ponds in general.  The second half of the lesson will consider other types of ponds used in wastewater treatment.  



Advantages and Disadvantages

Ponds have many advantages and disadvantages compared to treatment in plants. Both have to deal with aeration of the water being treated, but in ponds, oxygen is transferred directly into the water across the surface area without the need for any equipment. A plant, in contrast, must install an aerator to add oxygen to the water.

The natural method of aeration used by a sewage pond takes much longer than an aerator does to add oxygen to the water. As a result, ponds treat sewage much more slowly than package plants do.  The minimum detention time of a pond is 45 days .  In contrast, a package plant has a two to four hour detention time.  And, since ponds must hold the wastewater much longer than package plants do, the ponds must also have a much larger area to retain the sewage.  

If the time and area are available, sewage ponds are very economical facilities to maintain.  Package plants require frequent monitoring for various parameters such as ammonia and B.O.D.  In contrast, ponds require only one visit per day to monitor pH and D.O.  




Sewage ponds are very simple to construct.  A bulldozer is used to remove soil from the ground and create a basin in which water can collect.  However, the pond and surrounding area must be planned in such a way that the human and natural environments surrounding the pond are not damaged.    

The first requirement of a sewage pond is that it must be surrounded by a berm (a mound or wall of earth) or an embankment (a raised structure to hold back the water, such as that shown in the photograph at the beginning of this lesson).  The berm or embankment prevents storm water from running into the pond.  Without a berm, a heavy storm could cause the sewage pond to overflow and send untreated sewage out into the surrounding area.  

The soil in which a pond is built must be impermeable.  This will prevent the sewage from being absorbed into the ground and from leaking pollutants into the area.

A pond must be completely fenced to keep unwanted visitors out.  In addition, the area around the fence must be mowed to keep out vermin which could dig holes into the sides of the pond.  Tree growth must be restricted near the pond since roots could enter the pond and provide a way for sewage to escape if the trees died.  

Sewage ponds must be encircled by a windbreak, which usually consists of a row of pine trees.  The windbreak will prevent the pond's odors from disturbing the nearby residents and will also make the area aesthetically pleasing.  

The depth of the pond is another important factor.  The pond must be greater than two feet deep at all parts to exclude plant growth.  Plants growing at the edge of a pond will create areas of still water in which mosquitoes will lay their eggs.  But at depths of over six feet, anaerobic conditions occur, so regulations stipulate that the depth of a sewage pond can be no more than 5 feet.  



Health of the Surrounding Environment

The final requirement which must be met when constructing sewage ponds is to be sensitive to any streams or rivers into which the effluent from the pond will be released.  This entails knowing the classification of the stream, whether the stream contains any endangered species, and whether there are any existing contaminants in the stream.  

One way of protecting the surrounding environment is by adding a finishing pond.  A finishing pond , also known as a polishing pond , is like a finishing school - it prepares the water to go out into the world.  The finishing pond is installed between the sewage pond and the stream as shown below.  

One of the largest problems when water is released directly from a sewage pond into a stream is algae .  Sewage ponds are perfect environments for these one-celled plants.  Food is readily available, as is moisture and sunlight, so algae grow quickly and become quite numerous.  

When water from a sewage pond, rich in algae, is released directly into a stream the stream can be harmed.  The large quantities of algae use up the water's oxygen at night or during an algal bloom (when the algae reproduce very quickly).  Without the oxygen they need to survive, the fish in the stream die.  

A finishing pond can eliminate this problem.  Finishing ponds are usually stocked with fish, such as carp, which eat the algae in the water.  Finishing ponds also allow the quality of the effluent to be monitored before it is released into the stream.  As a result, streams being fed by finishing ponds tend to be healthier than those fed directly from sewage ponds. 



Types of Ponds by Location


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 BOD 5 per day per acre.  The biochemical oxygen demand , or BOD 5 , 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 BOD 5 but not very much of the suspended solids.  In contrast, during winter months, the pond will have poor BOD 5 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.  



Oxidation Pond

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.  

Polishing 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 BOD 5 , 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

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.  

Facultative Pond

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.  

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.

Anaerobic Pond

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.



Pond Process Control Calculations

Wastewater operators and treatment managers use various process control operations to determine the state of operation (status) of the pond. These operations include determining pond area, pond volume, flow rate, hydraulic detention, hydraulic loading, population loading, and organic loading.



Pond Area in Acres



Pond Volume in Acre Feet



Flow Rate in Acre-feet/Day



Hydraulic Detention Time, Days


Normally in the range of 30-120 days


Hydraulic Loading, inches/day



Organic Loading





When compared with other wastewater treatment systems involving biological treatment, a pond treatment system is the simplest to operate and maintain. Pond operation and maintenance activities include collecting and testing samples for dissolved oxygen and pH, removing weeds and other debris (scum) from the pond, mowing the berms, repairing erosion, and removing burrowing animals. Dissolved oxygen and pH levels in the pond will vary throughout the day. Normal operation will result in very high dissolved oxygen and pH levels due to the natural processes occurring.

The three types of ponds based on location within the treatment process are raw sewage stabilization pond (primary treatment), oxidation pond (secondary treatment), and polishing or finishing pond (tertiary treatment). The four types of ponds based on type of respiration are aerobic pond (oxygen present throughout the pond; aerobic respiration), aerated pond (oxygen artificially introduced; aerobic respiration), facultative pond (oxygen present in upper portions of the pond; aerobic and anaerobic respiration), and anaerobic pond (no oxygen present; anaerobic respiration; used to treat high strength industrial wastes).




Work the following crossword puzzle that comes from definitions in your textbook. You may either print the puzzle out, complete it and mail or fax back to the instructor or you may send an email with the correct answers numbered accordingly.





Answer the questions in the Lesson 18 quiz .  When you have gotten all the answers correct, print the page and either mail or fax it to the instructor.  You may also take the quiz online and submit your grade directly into the database for grading purposes.