March-April 2002

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

Two Articles in This Issue

Personal Health - Mold Identification
Where Is the Nitrogen in a Septic System?

Personal Health - Mold Identification
Gary K. Felton

Exposure to mold is common both inside and outside the home, but some people are more sensitive to mold than others, especially those with allergies and asthma. Mold exposure may cause cold-like symptoms, watery eyes, sore throat, wheezing and dizziness, and trigger asthma attacks.

Because some mold spores are very small and can easily be breathed deeply into the lungs, it is a health risk to live in houses with high mold levels. Exposure to high spore levels can cause the development of an allergy to mold. If there is a musty smell, then there is a mold problem that must be addressed.

Is a simple "smell test" sufficient? If the smell test is positive, remediation is necessary. The environment that supports the mold should be eliminated first. If, after a thorough remediation of a mold problem, people are still being affected, then more specific testing may be required. It should be done by an industrial hygienist or other trained professional.

Molds can only be positively identified with a microscope. Bluish-green to green molds are usually Penicillium or Aspergillus. Black to brown-black molds can be Aspergillus niger, Alternaria alternata, Cladosporium herbarium, Cladosporium sphaerospermum, or Stachybotrys chartarum (a highly toxic mold). Reddish or pink molds are usually species of Fusarium.

Typically, identification of the specific type of mold is not recommended initially because it is expensive and requires a specialist. However, one of the most common questions is, "How do I test for mold?" If a client insists on testing, the American Industrial Hygiene Association has a website that lists consultants and labs by state. The URL is: http://www.aiha.org/.

Inexpensive Testing with Results in 24 Hours

One company claims to have the necessary protocol to identify mold problems (not a specific mold!) rapidly and inexpensively. The following is their information.

There are over 100,000 different types of mold but only 50 or so are problematic. This company uses an inexpensive tape test to give information and guidance on what to do next. It is an easy way to determine if the mold is toxic and will require more intensive laboratory study and remediation cost.

The web site, http://themoldsource.com/starter.html (The Mold Source), is a source of mold information. For testing, the Mold Testing Lab at http://www.moldtestinglab.com/ is available. The Mold Testing Lab has experts in toxic mold detection and they have developed a rapid, relatively inexpensive (at $40), and effective method to quickly determine if toxic mold exists in your home or office. With the results from the Mold Testing Lab, you will be able to determine if the expensive professional testing is necessary. This initial testing is easy and convenient. The Mold Testing Lab provides a full report and an analysis, via email, within 24 hours of receiving your samples.

Air Testing

Before air monitoring is required, there must be evidence from visual inspection that ventilation systems or building components may be contaminated. The purpose of such air monitoring is to assess the extent of contamination throughout the building. If air monitoring is conducted, personnel conducting the sampling must be trained on proper air sampling methods for microbial contamination. The laboratory must supply specific culture media for the detection of Stachybotrys, such as malt extract agar (MEA), corn meal agar (CMA), cellulose agar, etc. For a qualified professional in your area, consult testing@themoldsource.com.

More Information

There is a wide variety of information on mold available at the following web sites:

http://www.ext.nodak.edu/extpubs/ageng/structu/ae1179w.htm

http://nyc.gov/html/doh/html/epi/moldrpt1.html

http://www.epa.gov/iaq/pubs/moldresources.html

http://www.epa.gov/iaq/molds/toc.html

Where Is the Nitrogen in a Septic System?
Dr. Gary K. Felton

To many people, a septic system is truly a black box that is out of sight and out of mind. Because nutrient discharge from septic systems is one of many nutrient concerns to people involved with water quality, understanding the concentration and fate of nitrogen in domestic septic systems allows intelligent discussion of septic systems and the impact of alternative systems.

Incoming

The nitrogen content of wastewater is not constant from place to place or from time to time. To add to the confusion, measurement techniques have inherent inaccuracies, and inappropriate sample handling can skew results of nitrogen analysis tremendously. However, some "typical" values that are national averages from a couple different sources are presented below. These are some typical concentrations for domestic sewage (mg/L). Note that these values are for sewer systems, not septic systems. There has been much more work done on sewerage than on septage.

These are the values you can expect to be sent to the septic tank. Probably, the medium and strong columns are more representative of single-family septage, both because there is no industrial water to dilute concentrations and because the water conservation message has, over the last 25 years, resulted in modest decreases in total per capita flows which should lead to greater concentrations in the sewage.

Septage

As mentioned earlier, there are separated solids in septage. It should be noted that septage values have coefficients of variation (CV) of 100% to 150%. This suggests that using a “typical” value for septage is meaningless in many cases. The settled portion contains typical values of:

Treatment begins as soon as domestic wastewater enters the septic tank. Heavier solids settle out to form a sludge (septage) layer at the bottom of the tank, while oils, grease, and other light materials float to the top to form a scum layer. Microorganisms solubilize the suspended and settled solids and often can convert soluble organic compounds to stable forms. From the moment you flush a toilet, two competing actions (physical and microbial solubilization and separation) take place. Hence some nitrogen sources are mixed and some are stratified.

Septic tanks are primarily anaerobic (oxygen-free) and as anaerobic bacteria attack organic nitrogen forms, they release NH₄⁺, which is quickly converted to NH₃, some of which is released as gas.

Nitrogen is present in domestic septage in both dissolved and solid forms. However, there are essentially no nitrites and nitrates in incoming domestic septage. Instead, nitrogen is added to septic systems as ammonia and organic nitrogen. The organic nitrogen consists of urea (CO(NH₂)₂) as well as amines (CH₃NH₂), skatole (C₉H₉N), and diamines (NH₂ (CH₂)4NH₂, NH₂ (CH₂)5NH₂). One source (Mitchell, 1972) also noted that there is approximately 0.025% NH₄Cl (25 mg/L). This is probably the result of residual chlorine in drinking water reacting with amines.

In case of oxygen

If O₂ is present, aerobic bacteria will also release NH₄⁺ from organic nitrogen, and if enough O₂ is present, they will convert the NH₄⁺ to NO₂^- and then rapidly to NO₃^-. Septic inflow is usually somewhat aerated, but the high BOD (biological oxygen demand) quickly utilizes any oxygen, so standard septic systems do not have much aerobic activity. Many bacteria in wastewater are not strictly anaerobic or aerobic and can function in both environments. These bacteria can convert the NO₃^- produced under aerobic conditions to N₂ gas. All microbial processes result in the production of more bacterial cells, which contain approximately 14% nitrogen (dry weight basis), primarily in the form of proteins.

In the trenches

Typical wastewater treatment plants have an aerobic stage that converts ammonia to nitrite and subsequently to nitrate. This step, called nitrification, is not a part of the typical septic system, but would occur if aeration were added as a stage in the multi-compartment tanks. Hence, the typical anaerobic septic tank receives ammonia that is not nitrified, and that ammonia exits in the effluent.

Treatment of septic tank effluent occurs 1) as it flows over the porous medium in the trench, 2) as it infiltrates into the soil (primarily through the sidewalls of the trenches), and 3) as it percolates through soil. The porous media in the trench is anaerobic and is both a physical filter and a biological growth media for facultative anaerobes. At the porous media-soil interface, a biomat forms over time. The microorganisms in the biomat metabolize organic matter in the effluent.

Sand systems

An "intermittent sand filter" is a shallow bed of sand (24"-30") that has both a distribution system and an underdrain system. The intermittent dosing is treated by physical, chemical, and biological transformations. Solids are removed by physical straining and by filtering through local biomats that develop where effluent is introduced. Ammonia from the septic tank effluent is oxidized to NO₃^- (nitrification) under aerobic conditions by nitrifying bacteria including nitrosomonas and nitrobactor. If the effluent flow is properly controlled, an anaerobic zone near the underdrains exists and denitrification occurs. This can account for removal of as much as 45% of the total nitrogen load!

The mound system is a modification of the intermittent sand filter. It still must have an intermittent dosing. The underdrain is replaced by putting the mound directly on the soil with the assumption that the original soil is sufficiently porous to perform the draining function. Hence, it is essential that mound construction not compact the base of the mound to a degree that effluent hits this layer and runs out the sides of the mound. Tilling the soil prior to construction would not be a bad thing.

If the permeability of the base of the mound is correct, then an anaerobic layer develops at the sand-soil interface. Because this layer was probably sod, it is high in organic matter, providing the carbon source necessary for denitrification. This was not part of the original design concept but a nice side effect.

Hence, a well-performing sand mound system removes ammonia, and the microbes also attack organic nitrogen and remove it. Under good operating conditions, you can expect that effluent from a sand mound or an intermittent sand filter would have no organic nitrogen, less that 0.5 mg/L ammonia and approximately 25 mg/L nitrate nitrogen. There are more construction concerns that cause a mound to be anything but well performing. Not all sand mounds result in relatively clean percolation water.

References:

McGhee, T.J. 1991. Water Supply and Sewerage. McGraw-Hill, Inc., New York, NY. Chapters 8, 18, 25.

Mitchell, R. 1972. Water Pollution Microbiology. Wiley Interscience, Div of John Wiley & Sons, Inc. New York, NY.

Metcalf and Eddy Inc. 1991. Wastewater Engineering: Treatment, Disposal, Reuse. 3^rd Ed. Revised by G. Tchobanoglous and F.L. Burton. McGraw-Hill, Inc. New York, NY.

Simpson, T.W. 1991. Characterization of septage quality in Virginia. Project Report to Va. Dept. of Health for Proj. 230-11-110-003-8483881. Dept. of Crop and Soil Environ. Sci., VPI & SU, Blacksburg, VA.

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^aThe first row comes from McGhee (1991) and the second row comes from Metcalf and Eddy (1991).

^bThe first row is from Metcalf and Eddy (1991) and the second row is from Simpson (1991).