Some would describe sludge as a potpourri of muck. Those who work with the material know that sludge is the solid matter that settles at the bottom of septic tanks or wastewater plant sedimentation tanks. Sludge must be processed by methods which will digest the bacteria or it can be pumped out for land disposal, incineration or composting.

Before sludge is applied to land or used in compost, it must meet the U.S. Environmental Protection A-gency's (EPA) risk-based standards. While no standards are perfect, the heavy metal standards are the result of 25 years of research with the U.S. Department of Agriculture. The confirmation, however, rests in the experiences of hundreds of municipalities who compost their sludge and use the final compost product (For a list of other materials which are composted, see "Versatility" chart on page 70).

Expose Yourself There are several ways in which a person can be exposed to compost pollutants (see table on page 68). Researchers examined the pathways to pollutants and developed hypothetical "highly exposed individuals" (people, plants, animals and even microorganisms) for each pathway. In this hypothetical world, the highly exposed individual don't change their exposure habits and receives the same amount of exposure every day for 70 years. Also, pollutant concentrations and releases near the individual do not change with time.

Recent studies show that as time passes, heavy metals and other soil pollutants are less available to plants or animals, or they become less "bio-available." If a pollutant is not bio-available, plants or animals can't chemically digest the pollutant.

Long-term field studies of sludge have shown that the bio-availability of metals is highest during its first year of use. Thereafter, the metals are increasingly bound and are not available to plants. For example, some forms of cadmium cause public health concerns. Since cadmium isn't bio-available in compost, it can't injure even the exposed home gardeners who spend their life growing foods on compost amended soils.

Studies which fed metal-laden sludge and compost to animals also report a lack of bio-availability. In fact, sheep which were fed from pastures with a high copper content reportedly suffered from copper deficiency. Also, mice which ate lettuce from high-cadmium soil reportedly experienced a cadmium reduction in their kidneys. Compost, which can be used for a long period of time with little risk, also is an effective way to tie up metals in toxic soils.

To further explain bioavailability consider the following examples: Be-fore, during and after World War II, Japanese families on farms ate large amounts of rice that was high in cadmium. As a result, the families suffered from Fanconi syndrome, a disease linked to cadmium build-up in the kidneys. However, in New Zealand, oyster fishers and their families consumed high amounts of cadmium-enriched oysters. Veg-etable gardens in Shipham, United Kingdom, and Stolberg, Germany, also had high cadmium levels from mining wastes. With high-cadmium diets like the Japanese, these residents did not accumulate cadmium in their kidneys and had no adverse health effects. The difference is bio-availability: compost can make metals less bio-available.

Nonetheless, in the EPA standards, bio-availability is linear. The standards use a very conservative assumption that bioavailability of pollutants stays at its beginning rate and does not lessen.

Estimating The Unknowns The "risk reference dose" is a benchmark to measure the relative toxicity of a pollutant. For example, how can someone determine if it's hazardous for sheep to eat 10 mg of copper a day as they graze on compost mixed with soil? Most of the an-swers can be found in the risk reference dose, a series of studies that measure how given doses of toxic pollutants affect certain animals. Risk reference doses were used to estimate the lowest amount of a pollutant that the highly exposed individual in each pathway can safely tolerate.

However, most of the doses used for the Environmental Protection A-gency's standards were based on studies which fed or injected the animal or organism with pure chemical doses of the pollutants. This procedure over-estimates the risk because when pollutants are in compost, soil or food, bioavailability is reduced.

The variables in composting research include: soil type, compost quality, type of plants being grown and the health of nearby residents. To account for these variables, the Environmental Protection Agency routinely adds "uncertainty factors," or an admission that no research can answer all of these questions.

The EPA's ruling added uncertainty factors of 10 to 10,000 to the risk reference doses, depending on the EPA's confidence in existing data. For each pollutant that was more thoroughly researched than another, a factor of 10 has been multiplied in.

For example, if compost with zinc is applied to land that grows food, zinc will be in the food. If the highly exposed individual ate the food containing 5,000 mg of zinc every day of his/her life and was not affected by it, an uncertainty factor of 10 means that the acceptable level of zinc for that pathway is 500 mg per day. An uncertainty factor of 10,000 would put acceptable zinc levels at 0.5 mg per day.

Humans are assumed to be 10 times more sensitive than the animals tested for the reference dose. This assumption, which may not necessarily be true, adds to the safety factors of setting pollutant limits.

The 99th Percentile In addition to the highly exposed individual, EPA also took an aggregate approach, which considered the entire population's exposure to pollutants.

In 1988, the EPA surveyed sewage sludge treatment plants to determine the levels of pollutants in their sludge. Based on the survey, compost pollutants must be at or below 99 percent of the highest levels found in the nation's sewage sludge.

This standard is being used because of the excellent track record that sludge and sludge compost re-portedly have in enhancing the soil without endangering the public or the environment. As an added safety factor, the Environmental Protection Agency chose the lower of the 99th percentile limit and the limit derived from the highly exposed individual.

The U.S. Court of Appeals, D.C. Circuit, ruled that Environ-mental Protection Agency cannot use non-risk arguments like the 99th percentile in a risk-based rule. If risk assessment limits replaced the 99th percentile limits, chromium and se-lenium limits would rise an estimated 2.5 times.

The Appeals Court also found that the "highly exposed individual (HEI)" method for setting selenium limits was unrealistic. For selenium, the HEI is a child who ingests land-applied sludge/compost for five years. EPA must justify the method used to set the selenium limit or change the limit to account for sites with little public contact. As a result, all heavy metals limits which use the same exposure pathway (selenium, ar-senic, cadmium, lead and mercury) may need to be re-examined.

In addition, the court found that EPA lacked sufficient scientific basis for the set chromium limit. This may lead the EPA to remove chromium from its regulated metals list. While the rule is remanded, it remains in effect. EPA either can further justify the rule to the court's satisfaction, or it can change it. EPA expects to re-spond by late this year.

Compost Applications The EPA's regulations intend to protect the public and their environment. According to researchers Cha-ney and Ryan, compost which meets EPA standards, "can safely continue for an indefinite period without risk to agriculture or the environment. There is little evidence that compost ... will comprise risk to highly exposed individuals even at very high cumulative applications."

It's critical to understand the long-term effects of composting, considering its wide range of applications: agriculture, horticulture, and landscaping as well as rehabilitation ma-terial for wetlands, salt damaged roadsides and other disturbed land. It also can decrease erosion, improve plant yield and strength; suppress plant diseases and decrease the need for chemical fertilizers. In addition, manufacturers are using materials that would otherwise be landfilled to compost into a recycled product. In fact, composting methods can be used to neutralize hazardous wastes. For example, the U-nited States Army used composting to neutralize explosives and leachate from munitions. Compost also can be used to rehabilitate old mines, to treat petroleum wastes and to lessen fertilizer run-off from farms.

Meeting safety standards is the key to a flourishing composting in-dustry.