Lesson 15: Pack Plant Aeration


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Lesson 14:
Ponds


Photo Credit:  Virginia Department of Health



Introduction to Sewage Ponds

In this section we will answer the following questions:

  • What are the advantages and disadvantages of using ponds to treat wastewater?
  • What requirements must be met when constructing a sewage treatment pond?

 

Introduction

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

Sewage ponds are a way of treating wastewater on a larger scale than the septic systems which we considered in the last lesson.  The ponds have many advantages and disadvantages compared to package plants, small treatment plants often manufactured at a factory, hauled to the site, and installed as one facility to treat wastewater.  

Both ponds and package plants have to deal with aeration of the water being treated.  In the sewage pond, oxygen is transferred directly into the water across the surface area without the need for any equipment.  A package plant, in contrast, must install an aerator to add oxygen to the water.

But 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.  

 

Requirements

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. 

Part 2: Types of Sewage Ponds