Stabilization ponds have been in use in the United States since 1901. Presently, there are approximately 7,500 stabilization ponds being used to treat wastewater and industrial wastes throughout the nation. These ponds (also referred to as lagoons) can be used alone or in combination with other wastetreatment processes. Stabilization ponds treat a wide variety of pollutants; therefore, it is imperative that they are constructed correctly to minimize the risk of soil and groundwater contamination. Unfortunately, contamination occurs periodically and an understanding of groundwater hydrology and soil dynamics are needed to ensure effective remediation. This report will focus on the different types of stabilization ponds,their methods of wastetreatment,their construction and the potential hazards they pose to soil and groundwater if they leak.
There are four general types of wastewater ponds. All use
microorganisms to degrade and detoxify organic and inorganic
constituents; the types of organisms differ among the four
categories. Basic classification involves the description of the
dominant biological reaction that takes place in the pond. The
four principle types are:
Facultative ponds are the most common form of wastewater lagoon. They are usually 1.2 to 2.5 meters in depth, with an aerobic layer which overlies an anaerobic layer that usually contains sludge deposits. The crux of facultative operation is oxygen production by surface reaeration and photosynthetic algae; the algae present one of the most serious problems associated with facultative ponds. These ponds require large land areas to maintain suitable BOD loadings. If structural failure occurs, a large area of an aquifer and soil are at risk of being contaminated.
Anaerobic Ponds have such a heavy organic loading that there
is no aerobic zone. These ponds have average detention times of
20 to 50 days. Two dominant biological reactions are acid
formation and methane fermentation. These ponds are typically
used for the treatment of strong industiral and agricultural
wastes and they tend to produce odorous compounds. Sodium nitrate
and grease crusts are used to combat these odors. These compounds
coupled with the acidic compounds formed through fermentation can
be very damaging to soil and groundwater if the lagoon leaks.
Picture of Anaerobic Pond Treating Strong Agricultural Waste.
In aerated ponds, oxygen is supplied through diffused or mechanical aeration. These ponds are generally 2 to 6 meters in depth with detention times of 3 to 10 days and they are advantageous because they require very little land area.
Aerobic ponds maintain dissolved oxygen throughout. They are typically 30 to 45 cm deep which allows sunlight to penetrate at full depth. Detention time is usually 3 to 5 days. Because the detention time is so short, very little coliform destruction will result. These coliforms pose a hazard to soil and groundwater purity if the lagoon leaks.
Picture of Aerobic Lagoon
A major function of stabilization ponds is the removal of nutrients from wastewater and other waste by-products. Although the effluent which comes from the lagoons has little nutrient content, the lagoon itself serves as a nutrient sink. The high concentration of nutrients contained in the pond pose a risk to soil and groundwater as well. The high concentration of nitrogen and phosphorous are examples of excess nutrients contained in many lagoons. Nitrogen can undergo a number of physical and chemical processes which include ammonification, nitrification, denitrification and settling in organic particulate form. Phosphorus is assimilated into algae cells within the pond. Both nitrogen and phosphorous become potential contaminants to groundwater. Nitrogen is especially risky since nitrates, by-products of nitrogen decomposition, are known to cause "Blue Baby Syndrome" in infants who ingest water which contains excessive nitrate.
Pathogenic organisms such as Shigella, Salmonella,
Escherichia, Leptospira, Vibria and Francisella are contained in
some lagoons. Viruses and protozoa also exist in abundance in
these ponds. Water is not the natural habitat of these organisms
but it serves as a means of transport to new hosts. Although
pathogenic organisms are usually unable to survive or multiply
for long periods in water, studies have shown that viable numbers
of organisms remain present in some lagoons at all times. This
becomes a particular concern for lagoons which are located near
high water tables that merge into nearby streams and reservoirs.
E-Coli is a major contaminant contained in wastewater lagoons.
In order to prevent contamination of groundwater, pond seepage must not be able to occur. Therefore, pond sealing is a primary requirement in the successful design, construction, and maintenance of a lagoon. Pond sealers fall into three categories: 1.) cement liners, 2.) synthetic and rubber liners, and 3.) chemical and natural treatment sealers.
Cement liners can be constructed of asphalt and montmorillonite clay as well as cement. The usual construction of a cement liner involves the preparation of a gravel bed approximately 15 cm deep followed by the application of a clay, cement, or asphalt layer which settles through the gravel and seals the voids. This forms a formidable barrier to pond seepage. The small amount of seepage that can occur is usually .2 -.25 m* day. It is important to note that clay tends to be the best primary liner in this category because it shrinks and swells according to variable temperatures and wet-dry conditions. Cement and asphalt liners can crack under such conditions.
Synthetic liners are the most popular of all liners because seepage is impossible if they remain unpunctured and uncorroded; therefore, it is important for the synthetic liners to be laid cautiously and properly during their installation. Synthetics are available in large sheets, they're chemically inert and economical to install. These liners are particularly useful in ponds which contain toxic wastewaters. When used in conjunction with cement liners, synthetic liners allow no seepage.
Natural liners are formed by the physical clogging of soil pores by settled solids, biological clogging caused by microbial growth at the pond lining, and chemical clogging of soil voids due to ionic exchange. This method of pond sealing is haphazard and unreliable because the dominant mechanism of the three natural sealants depend upon the characteristics of the wastes being treated.
While most liners perform satisfactorily for an average of 15
years, premature failure usually results from 1 or more of the
following factors:
It is important to keep these factors in mind when designing, constructing and maintaining lagoons. Most states set guidelines regarding the factors aforementioned. It is imperative that an owner/operator of a lagoon meet the prescribed specifications of that state; if the owner fails to meet these requirements and the lagoon contaminates it's surroundings, he/she can be fined heavily by the state DEQ and the EPA. Described below are the general guidelines set by the Virginia Department of Environmental Quality for lagoon design construction and maintenance
All surface lagoons must be designed with a liner on all portions of the impoundment. The liner must be intended, constructed, and installed to prevent any flow of wastes out of the lagoon into the subsurface soil or groundwater during the entire life of the lagoon. The liners must be placed upon a foundation capable of providing support to the liner. The foundation must provide resistance to pressure gradients above and below the liner to prevent failure due to settlement, compression or uplift. The operator of the lagoon must install a leachate collection system. The lagoon must be designed and constructed to prevent overtopping resulting from wind, excessive rainfall, malfunction of the level controller, and human error. The operator must demonstrate that the lagoon is located, designed, and operated in a manner which does not allow migration of waste into the groundwater or surface water at anytime. During lagoon construction, the liner must be inspected to ensure tight seams and joints and the absence of tears and punctures. The soil based and admixed liners must also be inspected for imperfections. After construction of the lagoon is complete, a certified engineer must examine the lagoon to ensure that the lagoon will operate according to it's design specifications.
During operation of the lagoon, weekly inspections of the lagoon need to be made. Lagoons should also be checked for structural integrity after every major storm. The lagoon should be removed from service if the contents in the lagoon suddenly drop for no reason. If a leak is responsible for the drop in effluent level, it must be located and plugged immediately. If the leak can not be located, the operator must remove the waste from the lagoon and inspect the liner. If the hole can not be patched and if the leak remains, the lagoon must be decommissioned. When the surface impoundment can no longer be used, the removal of all the waste, liner, and any contaminated soil must be removed from the site. The waste will be treated as either solid or hazardous waste.
When a surface impoundment fails, soil and groundwater contamination is likely to occur. If the lagoon contains animal wastes, the pollutants tend to be nitrates, phosphorous and fecal coliforms. Phosphorous is usually sorbed to soil particles rather quickly; however, nitrate leaches readily and can pose serious threats to human health in some instances (see Potential Contaminants). The transport of fecal coliforms into groundwater is limited by the porosity of the soil matrix; transport is easiest through sand and most difficult through clay. The extent of contamination from animal waste storage ponds depends upon the location of the water table. If the surface impoundment of a lagoon containing chemical or hazardous waste fails, the pollutant can be sorbed to soil particles or dispersed in groundwater; this is dependent upon the porosity of the soil matrix, the depth to groundwater, the partition coefficient of the waste and it's chemical structure. Send comments or suggestions to:
Middlebrooks, E. Municipal Wastewater Stabilization Ponds. Washington, D.C. Office of Water. 1983
Virginia Environmental/Safety Regulations. Section 9 VAC 20-60-840.
Freeze and Cherry. Groundwater. Prentice Hall,Englewood Cliffs, NJ. 1979
Bedient P.H, Rifai H.S., Newell C.J: 1994 Groundwater Contamination: Transport and Remediation. Prentice Hall, New York
Send comments or suggestions to:
Student Authors: Lorence Pope lpope@vt.edu, lpope@vt.edu and Jeff Taylor
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
