A recent trend in the treatment and disposal of sewage sludge is land application to agricultural and municipal lands. Many questions have arisen in concern for public heath safety because of this practice. The application of sludge through spraying and other means leads to concern of how potential contaminants may affect the groundwater in the affected regions. Currently sludge is monitored through Section 405 of the Clean Water Act (CWA). The major concerns for groundwater contamination are excess nutrients (Nitrogen and Phosphorus), heavy metals, organic compounds and pathogens. The ceiling concentrations of many contaminants are listed in figure 1. These concentrations are provided by the EPA to monitor the sludge that may be applied to agricultural or municipal land. The allowable concentration levels are determined by the EPA in accordance with human health advisories and with consideration of acceptable amounts available for uptake by agricultural plants planted on the site.
(1) Ceiling Concentrations
Table 1 of Sec 503.13 - Ceiling Concentrations |
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| Pollutant | Ceiling Conc (mg/kg) | Pollutant | Ceiling conc (mg/kg) |
|---|---|---|---|
| arsenic | 75 | cadmium | 85 |
| copper | 4300 | lead | 840 |
| mercury | 57 | molybdenum | 75 |
| nickel | 420 | selenium | 100 |
| zinc | 7500 | ||
Dry weight basis.
(2) Cumulative pollutant loading rates.
Table 2 of Sec 503.13 - Cumulative Pollutant Loading Rates |
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| Pollutant | Cumulative Loading Rate (kg/ha) | Pollutant | Cumulative Loading Rate (kg/ha) |
|---|---|---|---|
| arsenic | 41 | cadmium | 39 |
| copper | 1500 | lead | 300 |
| mercury | 17 | nickel | 420 |
| selenium | 100 | zinc | 2800 |
(3) Pollutant concentrations.
Table 3 of Sec 503.13 - Pollutant Concentrations |
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| Pollutant | Monthly Avg. Conc. (mg/kg) | Pollutant | Monthly Avg. Conc. (mg/kg) |
|---|---|---|---|
| arsenic | 41 | cadmium | 39 |
| copper | 1500 | lead | 300 |
| mercury | 17 | nickel | 420 |
| selenium | 100 | zinc | 2800 |
Dry weight basis.
(4) Annual pollutant loading rates.
Table 4 of Sec 503.13 - Annual Pollutant Loading Rates |
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| Pollutant | Annual pollutant loading rate. (kg/ha-365d) | Pollutant | Annual pollutant loading rate. (kg/ha-365d) |
|---|---|---|---|
| arsenic | 2.0 | cadmium | 1.9 |
| copper | 75 | lead | 15 |
| mercury | 0.85 | nickel | 21 |
| selenium | 5.0 | zinc | 140 |
Sludge is the dry by-product of the treatment of waste water. Waste water is treated to remove pathogens and to break up potentially harmful organic compounds. After the waste water has be treated properly it must be tested in order to see any potential problems that may affect human health adversely.
Once sludge has been tested and deemed suitable for land application it can be applied using (most often) two different methods. In each method the dry sludge needs to be mixed with water to make the sludge easier to apply. First of all, sludge is applied through direct spraying. Usually a large truck with spray and pump capabilities travels over the land and applies the sludge directly to the soil surface. The sludge can then be mixed with the top soil through discing by a tractor. Another method used is injection, where a truck has special soil intrusive adaptations that pump the sludge into the soil about 6 to 10 inches below the surface. The adaptation resembles a soil aerator used to break up compacted soils.
Many potential problems from sludge application to land may occur. The most significant problem affecting groundwater is excess concentrations of nutrients entering the groundwater. Both nitrogen and phosphorus must be present in the sludge to provide nutrients for the plants that grow on the applied land. When excess nutrients are not consumed by the vegetation they may enter the groundwater. However, phosphorus is insoluble and generally adheres to soil particles preventing it from contaminating the groundwater. On the other hand, nitrogen is very soluble and could easily contaminate an aquifer through leaching or from a rise in the water table.
Sludge also contains heavy metals which could potentially pollute the groundwater. Many heavy metals are insoluble and are adsorbed by soil particles but several heavy metals, such as cadmium, are soluble and may pollute the groundwater. Heavy metals tend to accumulate in soils over time, increasing the chances of groundwater contamination. Minute concentration of heavy metals can adversely affect human health.
Pathogen and organic contamination of groundwater are concerns of the land application of sewage sludge. However, these contaminants are not often found in sewage sludge deemed acceptable for land application. Sewage sludge must be tested for pathogen and organic concentrations to insure no damage to human heath. But in some cases, chlorinated organics, in extremely low concentrations, may enter the groundwater and adversely affect human health. Chlorinated hydrocarbons are proven carcinogens in low concentrations.
In order for a site to be considered for sewage sludge application many tests are done to ensure the groundwater will not be contaminated. If the highest seasonal water table is less than 24 inches below the ground surface the land is deemed unfit for sewage sludge application. Also, tests for perculation rates, soil saturation conditions and evapotranspiration rates are considered before a landowner may be permitted to apply sewage sludge. Allowable nitrogen concentrations should be suited for the appropriate vegetation being grown on the land to avoid having excess that may contaminate the groundwater. Monitoring wells should be placed around the site to check concentrations of possible contaminants. Land is not considered for sewage sludge application if it borders within 100 feet of a domestic or drinking water well.
The land application of sewage sludge can improve agricultural production while having little affect on groundwater purity. As long as safe guards are followed and the sites are properly maintained and monitored severe groundwater contamination is not likely. The EPA and DEQ strictly test and monitor sludge application. According to the National Academy of Sciences' National Research Council report on "Use of Reclaimed Water and Sludge in Food Crop Production" (1996), "There have been no reported outbreaks of infectious disease associated with a population's exposure ... to adequately treated and properly distributed reclaimed water or sludge applied to agricultural land." The biological accumulation of harmful contaminants is much more difficult to monitor and no substantial data can link sewage sludge to adverse human health problems. Hopefully, through proper maintenance sewage sludge application to land may provide an useful and environmental friendly way to recycle treated sewage.
Send comments or suggestions to:
Student Authors: Scott Lowe and Juliane Min
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
