Treatment of municipal wastewater produces sewage sludge, a dilute suspension of solids in water. Sewage sludge contains significant amounts of essential plant nutrients and organic matter that can benefit crop production. Application of sewage sludge to farmland has been a common practice in Pennsylvania for many years and allows this material to be recycled, rather than incinerated or disposed of in landfills. In addition to its beneficial aspects, however, sewage sludge also contains trace elements, organic chemicals, and pathogens that could harm humans or the environment if they are not properly treated and managed. Under current Pennsylvania regulations, sewage sludges must be treated further before being applied to farmland, to stabilize organic matter and greatly reduce pathogens. In addition, the concentrations of nine trace elements and one synthetic organic chemical (polychlorinated biphenyl, PCB) must be below specified limits. Sewage sludges that meet these quality criteria are known as biosolids. Additional information regarding sewage sludge treatment, use, and regulation in Pennsylvania can be found in the Extension Land Application of Sewage Sludge in Pennsylvania fact sheet series, including What is sewage sludge and what can be done with it?, Use of Biosolids in Crop Production, and A Plain English Tour of the Regulations.
Although the nine regulated trace elements are not the only ones present in sewage sludge or land-applied biosolids, they (and PCBs) were identified through an environmental risk assessment process as possible sources of risk. Therefore, the amounts of these pollutants present in biosolids provide an indication of overall quality and can be used to assess how biosolids will affect the soil. To facilitate this type of assessment, Penn State researchers conducted a comprehensive survey of biosolids that were land applied in Pennsylvania from 1978 to 1997. The results of that survey are summarized in this fact sheet.
The objectives of the survey were to:
- construct a database containing analytical records of biosolids that were produced from 1978 to 1997, including information about the treatment plants that produced them
- determine if changes in biosolids quality occurred from 1978 to 1997
- compare trace element concentrations to current regulatory standards
- calculate expected changes in soil trace element concentrations from repeated biosolids application
Note that although the term biosolids is used throughout this fact sheet, some analytical records contained in the early years of this database represent material that would not have qualified as biosolids under current standards and treatment technologies.
Two approaches were used to gather biosolids analytical records. First, all records from the biosolids testing program at Penn State's Agricultural Analytical Services Laboratory were collected into an initial database. Second, a list of all individual and general biosolids land application permit holders was obtained from the Department of Environmental Protection (D.E.P.). The list was used to identify wastewater treatment plants that were not part of the initial database. Analytical records for the biosolids produced at these plants were obtained from the D.E.P. regional office permit files. The final database consists of 7,746 analytical records of biosolids produced at 177 wastewater treatment plants from 1978 to 1997.
Figure 1. Change in the distribution of cadmium concentrations in Pennsylvania biosolids from 1978 to 1997.
The database records are not evenly distributed among the survey years. In the early years covered by the survey (the late 1970s), relatively few biosolids were being analyzed for potentially hazardous constituents, and only about 50 records were available for each year. As land application increased during the 1980s and as biosolids testing requirements and protocols became more firmly established, the number of analytical records increased steadily. From 1989 to 1997, the survey included between 500 and 700 analyses per year.
Figure 2. Change in the distribution of chromium concentrations in Pennsylvania biosolids from 1978 to 1997.
Although the database contains analytical results for numerous biosolids parameters, this fact sheet summarizes the data obtained for PCBs and the following ten trace elements: arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc. Not all records contain results for each of these substances. In particular, very few analyses were conducted for mercury prior to 1984, or for arsenic, selenium, and molybdenum before 1993.
Figure 3. Change in the distribution of copper concentrations in Pennsylvania biosolids from 1978 to 1997.
Percentile values have been used to indicate the reported distribution and range of values for each trace element. A percentile value is the concentration of a given trace element below which the specified percentage of all reported concentrations fall. For example, the 75th percentile value for cadmium in 1978 was 22 mg/kg, or ppm (Figure 1).¹ This means that 75% of all cadmium records for 1978 were less than or equal to 22 mg/kg, and 25% were greater than 22 mg/kg. The 50th percentile (the concentration above and below which equal numbers of records fall) is also known as the median.
¹ Concentrations are given in milligrams per kilogram (mg/kg), the equivalent of parts per million (ppm).
Changes in biosolids trace element concentrations from 1978 to 1997
Pennsylvania's regulations for land-applied biosolids require testing for arsenic, cadmium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc. Although chromium is no longer regulated, almost all land-applied biosolids continue to be analyzed for this element. Figures 1 through 7 show how the concentration distributions of these trace elements in biosolids have changed with time. Median concentrations of cadmium, chromium, copper, lead, nickel, and zinc each declined from 1978 to 1997, as did mercury from 1984 to 1997. As the trend lines in these figures indicate, most of these elements decreased by 50% or more. The decrease was less substantial only for copper and zinc. Perhaps even more significant is the decreasing range of concentrations of these metals among treatment plants during this time period. This was most apparent with cadmium, chromium, lead, and nickel, where the 90th percentile concentrations decreased by 70 to 90%, and the 25th to 75th percentile ranges decreased by at least 50%. With copper and zinc, the decreases in concentration ranges were smaller.
Figure 4. Change in the distribution of lead concentrations in Pennsylvania biosolids from 1978 to 1997.
The decreases in these trace metals over the past 20 years represent a major improvement in biosolids quality, much of it due to pretreatment programs requiring industries to remove trace elements and organic chemicals from their wastewater before discharging it to municipal sewerage systems. Many municipal water systems also treat drinking water to make it less corrosive, so fewer metals are leached from water pipes and plumbing fixtures.
Figure 5. Change in the distribution of nickel concentrations in Pennsylvania biosolids from 1978 to 1997.
Before 1993, when arsenic, molybdenum, and selenium were added to the list of regulated trace elements, relatively few biosolids were analyzed for these substances. From 1993 to 1997, no significant change occurred in the medians or the concentration ranges of these elements. For this reason, graphs depicting yearly arsenic, molybdenum, and selenium data are not shown here. Information on the concentrations of these elements is contained in Table 1.
Table 1. Median and 95th percentile concentrations of trace elements in 1996-97 Pennsylvania biosolids compared to regulatory standards for land-applied biosolids.
|Trace element||PA median biosolids (mg/kg)||PA 95th percentile biosolids (mg/kg)||Regulatory pollutant concentration limit (mg/kg)||Regulatory ceiling concentration limit (mg/kg)|
|¹ Chromium is not regulated under current Pennsylvania and federal rules. Regulatory limits given in this table for chromium are from the original regulations proposed by the U.S. E.P.A., but are not part of the final rule.|
In addition, little change has occurred in median concentrations and concentration ranges of chromium, lead, nickel, and zinc since 1993. This may indicate that these trace elements have reached their lowest levels possible with existing technologies and treatment programs. Biosolids concentrations of some of these elements may reflect actual concentrations in drinking water and in human waste. If this is the case, further significant declines are not likely to occur without additional incentives and investment. For some trace elements, further declines simply may not be possible. For example, the range of arsenic concentrations in biosolids is comparable to the arsenic concentrations typically found in Pennsylvania's surface soils.
Current biosolids quality
Pennsylvania's regulations contain two concentration standards: the ceiling concentration limit and the pollutant concentration limit. The ceiling concentration limit is the higher of the two, and if the concentration of any of the regulated trace elements exceeds it, the biosolid may not be land applied. If the concentrations of all regulated trace elements fall below the pollutant concentration (the lower of the two limits), and if the biosolid meets certain other quality criteria, it may be land applied with very few limitations. These limits and restrictions are explained more fully in the Land Application of Sewage Sludge in Pennsylvania: A Plain English Tour of the Regulations fact sheet.
Figure 6. Change in the distribution of zinc concentrations in Pennsylvania biosolids from 1978 to 1997.
Table 1 focuses on recent biosolids quality (1996 and 1997), and compares trace element concentration ranges to the regulatory concentration limits. For each of the regulated trace elements, the 95th percentile concentration is lower than the pollutant limit (the more stringent of the two limits), and in most cases it is substantially lower. This means that over 95% of all land-applied Pennsylvania biosolids have trace element concentrations well below the most stringent regulatory standard. Copper is the trace element whose median and 95th percentile concentrations come closest to the regulatory limit.
Figure 7. Change in the distribution of mercury concentrations in Pennsylvania biosolids from 1984 to 1997.
The survey database contains 1,918 PCB analyses from 1994 to 1997. Of these, 91% were reported to be lower than the analytical detection limit, and 81% of the reported detection limits were less than 1 mg/kg. This means that PCB concentrations were too small to be measured by the analytical method used. Clearly, the median concentration of PCBs in Pennsylvania biosolids is less than 1 mg/kg and well below the 4 mg/kg regulatory pollutant concentration limit. Because PCBs could be detected in nearly none of the biosolids, it is impossible to determine percentile values for them, as was done for the trace elements.
Implications for Pollutant Loading of Pennsylvania Soils
Under Pennsylvania's regulations, long-term application of biosolids to a given soil is limited by the total amount of trace elements in the soil after each application. This is known as the cumulative loading limit. Table 2 shows the cumulative loading limits for eight trace elements and how many tons of median Pennsylvania biosolids it would take to reach each of those limits. Copper is the trace element that would first reach its cumulative loading limit after 1,270 tons/acre (on a dry-weight basis) of biosolids had been applied. A typical biosolids application rate is about 4.5 tons/acre on a dry-weight basis, and in practice this amount normally would not be applied to the same field every year. Nevertheless, using this as an annual rate, it would take 282 years to reach the copper limit with median biosolids. Over this length of time, it is reasonable to assume that several different biosolids would be applied, and the overall effect would be similar to adding the median biosolids. Thus the median provides a good estimate of the long-term buildup of trace elements in soil.
Table 2. Amount of Pennsylvania median biosolids required to reach the cumulative loading limit for each regulated trace element. Cumulative loading calculations were conducted using the background soil concentrations in Table 3 as the starting point.
|Trace element||Cumulative pollutant loading limit (lb/acre)||Trace element concentrations in median PA biosolids (lb/ton)||Amount of median PA biosolids required to reach limit (tons/acre)|
The cumulative loading limit is intended to prevent soil concentrations of these trace elements from reaching levels that would constitute a risk to human health or the environment.
Table 3 contains the results of several calculations to assess the effects of adding trace elements or biosolids to soil. The first data column in Table 3 shows the typical background concentrations for surface soils in Pennsylvania. The second column of data shows what the soil concentrations would be if each of the trace elements were added in its pure form to reach the cumulative loading limit. The risk assessments used to develop Pennsylvania's regulations determined that these concentrations would be protective of human health and the environment.
Table 3. Effects of long-term biosolids application on the soil concentrations of regulated trace elements.
|Trace element||Soil concentration - Typical background for PA surface soils (mg/kg)||Soil concentration - If trace elements were added in their pure form to reach the cumulative loading limit¹ (mg/kg)||Soil concentration - That would result from the addition of 1,270 tons/acre of median PA biosolids¹ (mg/kg)|
|¹ These concentrations were calculated using the assumption that one acre of 6-inch-deep soil weighs 1,000 tons.|
When biosolids are mixed into soil, however, trace elements are not added in their pure form. Instead, they are added as a minor part of a much larger volume of biosolids. Consequently, their buildup in the soil is diluted by the biosolids in which they are contained. The third data column in Table 3 shows the soil concentrations that would result from the addition of enough median concentration biosolids to reach the cumulative limit for copper (1,270 tons/acre, see Table 2). The calculations used to determine the values in this column take into account the diluting effect of the biosolids itself. They also make the following assumptions:
- the biosolids are 50% organic matter
- all of the organic matter will decompose
- the biosolids are applied at an annual rate of 4.5 tons/acre
- the biosolids are uniformly mixed in the upper 6 inches of soil
- all trace elements added in the biosolids remain in the soil (no loss or removal occurs by leaching, runoff, erosion, volatilization, or plant uptake)
The calculations in Table 3 show that the addition of biosolids causes trace elements to build up in soil at different rates. When median biosolids are used, copper concentrations come closest to the cumulative loading limit concentration (given in the second data column), followed by zinc and lead, each of which come to about 50% of their cumulative limit concentrations. At the other extreme, soil levels of arsenic are changed very little because arsenic's concentration in the median biosolids is very close to the background soil concentration. Soil buildup of other trace elements ranges from about 10 to 30% of the cumulative limit concentration. Again, it is reasonable to assume that the aggregate effect of adding different biosolids over 282 years of applications would be similar to that of adding the median biosolids. It is also clear that risks from soil buildup of metals can be minimized by using biosolids with low trace element concentrations.
- The quality of Pennsylvania biosolids, as measured by trace element concentrations, has improved substantially since 1978.
- Over 95% of Pennsylvania biosolids have trace element concentrations well below Pennsylvania's most stringent regulatory limits.
- For almost all Pennsylvania biosolids, long-term application will be limited by the buildup of copper in soil. When the limit for copper is reached, other trace elements will have reached only a small fraction (one-tenth to one-half) of their respective cumulative loading limits.
- The composition of individual biosolids will vary from the median values used in these assessments. Therefore, it is important to continually monitor the cumulative loading of all regulated trace elements whenever biosolids are applied to soil.
Prepared by Richard Stehouwer, assistant professor of agronomy.