July-August 2000

The topics in this newsletter are considered to be timely and of interest. Comments and suggestions are invited. The use of trade names in this newsletter is not an endorsement of any company or product by the Maryland Cooperative Extension, University of Maryland, College Park.
David S. Ross, Extension Agricultural Engineer

Making Sense of Oxygen Demand Measurements of Organic Matter in Wastewater
-Part 2 of a Series-

In the previous issue of TOPICS, the importance of knowing the organic matter content of a waste stream was explained and two common measures of organic matter content in wastewater, biochemical oxygen demand (BOD) and chemical oxygen demand (COD), were introduced. In this installment, the procedures used to measure the BOD and COD of a wastewater are briefly described. (Standardized methods for carrying out both tests are available1.)

The BOD test. In the BOD test, special glass bottles are filled completely with the wastewater being tested, stoppered to exclude air, and incubated at 20o C. Microorganisms that are present in the wastewater break down the available organic matter, and in doing so, use up (or "breathe") oxygen. The change in the wastewater's dissolved oxygen concentration caused by the microorganisms is the key to the BOD test. The concentration of dissolved oxygen in the wastewater is measured at the beginning and at the end of the test. These measurements are usually reported in terms of mg/L (milligrams of dissolved oxygen per liter of wastewater). The difference between the two dissolved oxygen concentrations is the BOD of the wastewater, which is also usually reported as mg/L (milligrams of oxygen demand per liter of wastewater). Most often, BOD tests are conducted over a five-day period and the resulting measured BOD value is called the five-day BOD (BOD5).

Sometimes wastewaters do not contain significant levels of microorganisms or certain nutrients and may have to be supplemented with these additives. In other cases, the levels of organic matter are too high and the wastewater samples have to be diluted. If microorganisms are added or the wastewater sample is diluted, the measured BOD value must be corrected appropriately.

In addition to the organic matter, any ammonia present in a waste stream may also be oxidized by nitrifying bacteria in a process called nitrification. Nitrification also demands oxygen, which is referred to as nitrogenous BOD (NBOD). A general equation for the overall nitrification process is shown below.

ammonia + oxygen + carbon dioxide + nitrifying bacteria
nitrate + water + new cells + energy

Nitrifying bacteria grow slowly, more slowly than the microorganisms that oxidize organic matter, and it normally takes from 6 to 10 days before they start to consume oxygen. However, as shown in the figure below, if a significant number of nitrifying bacteria are present in the wastewater, they might exert sufficient oxygen demand to introduce error even into the measurement of organic matter using the BOD5 test. In these cases, the wastewater sample being tested should be pretreated with an agent that suppresses nitrifying bacteria, and the results of the BOD test should be reported as CBOD (carbonaceous BOD).


The COD test. In the COD test, the oxygen demand of organic matter in wastewater is measured by allowing it to react with a strong chemical oxidizing agent (potassium dichromate) in an acidic solution, instead of with microorganisms plus oxygen. The test is carried out at high temperatures and in the presence of a catalyst (silver sulfate). The amount of dichromate remaining at the end of the test is measured and subtracted from the amount of dichromate originally added to the wastewater. The equivalent amount of oxygen required to oxidize the organic matter to carbon dioxide is equal to the COD of the wastewater and is determined from the amount of dichromate consumed in the COD test. The COD test is popular because it takes much less time (about 3 hr) than the BOD test, which requires several days. However, the COD test involves the use of toxic chemicals, which must be properly disposed of. In addition, many compounds that cannot be oxidized by microorganisms react with the chemical oxidizing agent. Therefore, for some wastewaters, the amount of organic matter measured using the COD test will be higher than that measured using the BOD test. Examples of BOD and COD values reported2 for several different types of agricultural wastewaters are listed below.

Wastewater type

BOD5 (mg/L)

COD (mg/L)

Tomato processing

 

450-1,600

 

650-2,300

Corn processing

1,600-4,700

3,400-10,100

Cherry processing

660-1,900

1,200-3,800

Poultry plant processing

150-2,400

2-3,200

Milk plant processing

940-4,790

1,240-7,800

Impact of Oxygen Demand Values on the Design of Land Application Systems. When wastewater is being applied to land, microorganisms in the soil carry out the same reactions that occur in the BOD test, and in the process, they consume organic matter in the wastewater and use up oxygen in the soil. If the rate of oxygen demand being applied to land as wastewater exceeds the rate of oxygen transfer to the soil from the atmosphere, then oxygen will become depleted in the top layer of soil due to microbial activity. The application of wastewater to soil at a high rate can cause ponding. High rates of application can also cause an excessive build up of soil microorganisms, which can form a slimy layer that can increase the amount of ponding that occurs. When soils are flooded, not only is crop production impeded, but oxygen transfer to the soil is severely reduced. However, even if the soil is depleted of oxygen, removal of the applied oxygen demand (organic matter) does not stop!

This is possible because some microorganisms present in the soil can substitute nitrate (NO3-) for oxygen in the break down of organic matter, after oxygen has been depleted. The oxidation of organic matter using nitrate is called denitrification. In this process (shown below), nitrate is primarily converted to nitrogen gas (N2), which escapes to the atmosphere.

Organic matter + NO3- + denitrifying microorganisms
carbon dioxide + N2 + water + new cells + energy

After nitrate has been depleted, some soil microorganisms can also break down the applied organic matter using sulfate (SO4-2) in place of nitrate or oxygen. This process is called sulfate-reduction. In sulfate-reduction, no oxygen is consumed, even though the oxygen demand of the wastewater is reduced. Sulfate reduction produces a highly odorous compound, hydrogen sulfide. The production of hydrogen sulfide through sulfate-reduction is one way in which offensive odors can develop when land application systems deplete soil of oxygen. Other microbial reactions occur when sulfate is not present and oxygen and nitrate have been depleted from soil, and these reactions also lead to the decomposition of organic matter and can create odors.

In some instances it may be beneficial to operate a land application treatment system so that oxygen is periodically depleted. This is the case when maximum removal of nitrogen from the applied wastewater is desired. Removal of nitrogen as N2 gas is accomplished through the microbiological process of denitrification described above. Of course, denitrification can only occur if sufficient BOD is available. Odor problems in these systems can be reduced by providing adequate drying times, which allow the soil to become oxygenated, in between wastewater applications. The drying periods also promote nitrification, the conversion of ammonia to nitrate, which is an aerobic process. Organic matter loading rates of 200 to 450 lb of BOD5 per acre per day (224-500 kg/ha/d) have been successfully used in land treatment systems for food processing wastewater by providing adequate drying times in between applications. Of course, for any given site, the selection of a loading rate must take into account a variety of factors such as the permeability of the soil and the crop irrigation needs, and must fall within the limits defined by the disposal permit.

Jennifer G. Becker, Extension Engineer

University of Maryland Conferences

 

The following conferences are scheduled for October 25-27, 2000, in Maryland. Call now for registration availability.

9th Annual Better Composting School
October 25-27, 2000; Hanover, Maryland. Registration information -- call 301-405-1198.

Short Course on Bioremediation of Contaminated Soils and Groundwater
October 25-27, 2000; University of Maryland, College Park, Maryland

This informative course is designed for environmental scientists and engineers, consultants, extension specialists, the public, regulatory personnel, and other interested parties who are involved in making decisions about cleanup of contaminated soils and groundwater. Bioremediation is recognized as a cost-effective and environmentally more desirable alternative to traditional methods for cleaning up many environmental pollutants. This program will address the scientific principles underlying bioremediation, the latest innovative bioremediation technologies, and regulatory aspects of bioremediation. Short course participants will acquire "hands-on" experience using computer modeling software to enhance design and evaluation of bioremediation. Participants will also have an opportunity to put their new knowledge about bioremediation to use in a case study problem-solving exercise.

Instruction and discussions on bioremediation issues will be led by University of Maryland faculty from the Departments of Biological Resources Engineering, Civil and Environmental Engineering, and Natural Resource Sciences and Landscape Architecture. In addition, guest speakers from regulatory agencies and engineering practice have been invited.

The course will be held on the University of Maryland's College Park campus. The cost of the course is $300 and includes continental breakfasts, coffee breaks, lunches, parking, and course materials. Maryland Cooperative Extension personnel will receive a 20% discount. Course participants may receive a certificate for 2.0 continuing education units for an additional $10 processing fee.

Registration and other additional information may be found on-line at http://www.contedu.umd.edu. A brochure that contains additional information and registration form may also be obtained by contacting Ken Carter (Office of Continuing and Extended Education) at kjcarter@deans.umd.edu or (301) 405-6296. Questions concerning course content should be directed to Jennifer Becker (Department of Biological Resources Engineering) at jgbecker@wam.umd.edu or (301) 405-1179.

Jennifer G. Becker, Extension Engineer

NRAES Regional Conference Announcements

 

NRAES (Natural Resource, Agriculture, and Engineering Service) is sponsoring the following two conferences. Conference brochures will be available in October. For further information, contact NRAES, Cooperative Extension, 152 Riley-Robb Hall, Ithaca, New York 14853-5701. Phone (607) 255-7654, Fax (607) 254-8770, E-mail: nraes@cornell.edu, Web site: www.nraes.org.

Biotechnology: Progress or Problem?
A Conference for Developing Community Leaders
January 17-19, 2001; Binghamton, New York

Biotechnology refers to a wide-ranging group of issues, including genetically modified foods, using animals to grow human transplant organs, cloning, genomics, and more. Advances in biotechnology have raised serious ethical, moral, legal, and religious issues for some. For others, biotechnology represents a tremendous opportunity to improve the human condition. This conference is designed for leaders who want to accept the challenge of wrestling with when to say yes and when to exercise caution in accepting, promoting, or implementing new biotechnology developments.

Milking Systems and Parlors
Planning and Managing for Quality Milk and Profitability
A Conference for Producers and Their Advisors
January 30 and 31 and February 1, 2001; Harrisburg, Pennsylvania

This educational conference will focus on milking systems, milking parlors, milking management, and milk handling. Quality milk, milk production per man-hour, and profitability can be optimized on dairy farms through careful management and capital investments. New developments and research have occurred concerning milk parlors, milking systems, data collection and use, cow comfort, and cow handling to improve milk production.

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1American Public Health Association, Standard Methods for the Examination of Water and Wastewater, 19th ed., Washington, D.C., 1995.

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2Loehr, R. 1984. Pollution Control for Agriculture, Second Edition, Academic Press, Inc., Orlando, FL.

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