Thursday, 17 July 2014

Wastewater Treatment Ponds

Lesson 5:
Wastewater Treatment Ponds

In this lesson we will answer the following questions:
  • What are wastewater treatment ponds?
  • What are the types of bacteria at work in most ponds?
  • How is oxygen provided to the wastewater?
  • What is the algae cycle and why is it important?
  • What role does temperature play in wastewater treatment ponds?

Reading Assignment
Read the online lesson and also Chapter 9 in textbook, Operation of Wastewater Treatment Plants Vol. 1.

Wastewater treatment using ponds can be an economical way of treatment which produces effluent that is highly purified. The number and the type of ponds used are the determining factors as to the degree of treatment that is provided.
Another name for wastewater treatment ponds is waste stabilization ponds . For this lesson we use waste stabilization ponds because these ponds help to stabilize the wastewater before it is passed on to receiving water. They can also be referred to as oxidation ponds or sewage lagoons .
The waste stabilization pond is a biological treatment process , where bacteria use organic matter in the wastewater as food. The three types of bacteria at work in most ponds are the aerobic, anaerobic, and the facultative bacteria.
Because of unpleasant conditions associated with the anaerobic decomposition, plant operators must make sure that there is enough dissolved oxygen (D.O.) in the pond to make sure that it will be the aerobic and facultative bacteria that will be predominant, rather than having anaerobic decomposition take place.

How oxygen is supplied in the ponds
One way that oxygen is supplied to the wastewater is by algae . The algae produce the oxygen needed by the bacteria and the bacteria in turn produce carbon dioxide and other things that are needed by the algae.
It is important to remember that the algae will thrive when there is sunlight present. They cannot grow at nighttime, or during extended periods of cloudy weather. The water in the ponds has to be clear and not too deep. If the sunlight can reach the bottom, then the algae will grow throughout the pond depth. The following drawing shows the Algae Cycle)

If the water is not clear or if the sunlight is cutoff the algae will not grow. If this happens, the D.O. level in the pond will decrease and you can expect an increase in anaerobic activity at the bottom of the pond. The thickness of the anaerobic layer at the bottom will probably increase.
In ponds that are over three feet deep , you can expect that there will be a zone where the concentration of dissolved oxygen will vary from time to time. You can also expect facultative bacteria living in this zone because only they can adjust to the aerobic and anaerobic conditions existing.

How weather and seasons of the year affect the ponds
Another thing to be aware of are the things that you can expect to happen in the pond during the winter. In winter, shorter periods of daylight, cloudy weather, and snow or ice can prevent the sunlight from reaching the wastewater. Small amounts of oxygen will be produced by the algae and there will be no aeration from wind action because of the ice cover. Colder temperatures will slow down the activity of the bacteria . By this, the pond is continuously discharging and the effluent will be worse. But in the spring when the pond warms up, there will be an increase in bacterial action on the material that was deposited during the winter. The result will be odors from anaerobic conditions. Another reason for the pond odors in the spring is spring turnover. As the pond warms, the bottom layer of the wastewater starts to mix throughout the pond. When this happens, it brings with it smelly gases and some solids. Spring turnover odors usually last about 7 days.
Although there is a greater die-off of bacteria in ponds than in other treatment methods because of the longer detention time in ponds, pond effluent may still require disinfection to preserve receiving stream quality. It is the bacteria that do the treatment in the ponds. The kind of bacterial action that will take place is determined by the amount of dissolved oxygen available in the pond.

Types of Wastewater Treatment Ponds
In this section we will look at 7 different types of wastewater treatment ponds (waste stabilization ponds) and what process occurs in each.

Aerobic Pond
In the aerobic pond oxygen is present throughout the pond and all biological activity is aerobic decomposition. Aerobic ponds are a maximum of two feet deep, so that the sunlight can reach throughout the entire depth of the pond, which will let the algae grow throughout. The oxygen they give off allows aerobic process microorganisms to live. Aerobic ponds are not used in colder climates because they will completely freeze in the winter.

Anaerobic Pond
Anaerobic ponds are normally used to treat high strength concentrated industrial waste and no oxygen is present in the pond. All the biological activity is anaerobic decomposition. These ponds are 8 to 12 feet deep and are anaerobic throughout. Scum forms on the top of the most anaerobic ponds. This scum stops air from mixing with the wastewater. Because there is no dissolved oxygen in the pond the anaerobic bacteria will be a work. The gases that is produced by the anaerobic bacterial action causes odor problems and these types of ponds are not used very often.

Facultative Pond
Facultative ponds are used the most to treat municipal wastewater. The ponds are usually 4 to 6 feet deep and the sludge at the bottom is anaerobic, while the 1 to 2 feet of the top of the pond is aerobic. In the middle, the amount of dissolved oxygen varies and either aerobic or anaerobic decomposition will take place, depending on how much dissolved oxygen is available.
Facultative Stabilization Pond

Oxidation Pond
This type of pond is designed to receive flows that have passed through a stabilization pond or primary settling tanks. These ponds provide biological treatment and additional settling and some reduction in the number of fecal coliform present. They are normally designed by the same specifications as the stabilization pond.

Raw Sewage Stabilization Pond
This type of pond is the designed to receive wastewater that has had no prior treatment except for screening and shredding and is the most common of them all. These ponds are designed to provide a minimum of 45 days detention time and to receive no more than 50 pounds of BOD 5 per day per acre. The normal operating depth is 3-5 feet. In this time of pond the time of the year determines the quality of the discharge. The summer months produce high BOD 5 removals but low suspended solids removal and in the winter months there is poor BOD 5 removal but excellent suspended solids removals. The process that occur in this type of pond are, settling, aerobic, facultative, anaerobic decomposition and photosynthesis to produce required oxygen.

Polishing Pond
This type of pond is designed to receive flow from the oxidation pond and from other secondary treatment systems. This process removes additional BOD 5, solids and fecal coliform and other nutrient removal. These types of ponds provide only a 1 to 3 day detention time and normally operate at a depth of 5-10 feet. If the detention time excessive, then there will be an increase in the effluent suspended solids concentration.

Aerated Pond
In these ponds, oxygen is provided by using mechanical or a diffused air system (artificial aeration). The artificial aeration also keeps the wastewater in the pond mixed, keeping organics and bacteria in contact. The aeration allows the pond to have shorter detention times and heavier loading.

Wastewater treatment ponds are designed for different types waste and may or may not contain oxygen. Not all ponds are suited for all regions or climates. Dissolved oxygen levels must be maintained. Algae must be present to provide oxygen. Not all ponds have the same detention time and must be maintained for odor control.

Answer the following questions and email your answers to
  1. Why do we consider wastewater treatment ponds an economical treatment method?
  2. What is the most common type of pond used?
  3. What is provided in the oxidation pond how is it designed?
  4. What is removed in a polishing pond?
  5. How is oxygen supplied in an aerated pond?
  6. Oxygen is present in what type/types of ponds and which ponds have no oxygen?
  7. What type of pond has no oxygen in the lower level?
  8. How many pounds per day per acre can a raw sewage stabilization pond receive?
  9. What months have poor BOD5?
  10. What type of pond receives industrial wastes?

Answer the questiosn for the Lesson 5 Quiz. When you have completed the quiz, print it and fax or email it to your instructor. You may also take the quiz online and directly submit it into the database for a grade.

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