Determining Appropriate Biosolids Rates
Determining an appropriate application rate, unfortunately, is not a matter of rigorous science. Older restoration projects using biosolids generally used rates in excess of 100 dry tons/acre. There was generally the perception that more biosolids would result in longer-lasting and more effective restoration efforts. While addition of high rates of materials will often have a positive effect, cost concerns may outweigh this. There has been some research to determine appropriate rates at the lower end of the scale. With only a shallow horizon of contaminated soil, application rates of 25 dry t/ac were found to be equivalent to 50 and 75 t/ac in Palmerton, PA, where an application of 25 dry t/ac in combination with fly ash has maintained a stable vegetative cover for eight years. A project in Silesia, Poland found that 100 t/ac was necessary to establish a vegetative cover on slag piles, while work in Bunker Hill, ID indicated that a higher application rate of a drier biosolids was required to achieve even coverage. As use of biosolids to restore metal contaminated sites is a relatively new practice, it is not yet possible to say whether lower rates are as effective as higher rates over the long term. Areas that have received higher rates are showing a self-sustaining cover up to 30 years after biosolids application. Projects using the lower rates range in age from two to eight years.
Factors to be considered in determining an appropriate application rate are depth and levels of contamination. The deeper and more contaminated sites suggest that higher application rates be used. For example, in cases where a plant cover is to be established on mine tailings with Zn concentrations in excess of 10,000 mg/kg and an acidic soil pH, a higher application rate of 100 t/ac or greater would be appropriate. Where tailings are calcareous and have lower metal concentrations, 25-50 t/ac should be sufficient. Brown field sites with highly variable concentrations of contaminants may also be restored successfully with lower rates.
Rates Determined by Depth of Application
To create a "soil horizon" by biosolids or biosolids products, approximately 115 dt/ac of stable material for every inch of soil built up is required. However, over the first year or so, up to 25% of the fresh biosolids mass may be lost to decomposition, depending upon the biosolids stability at time of application. Thus, the applied amount required may be about 150 dt/ac. Since biosolids generally range from 15-30% solids, every inch of soil requires a wet depth of biosolids over 6 inches (assuming 20% solids).
Similarly, one must consider the reduction of volume of compost material in the years following application as the organics decompose (depending upon the characteristics of the compost, greater than 25% of the volume). Thus, a desired depth of compost of 1" should receive an application of 1.3", or about 180 cy per acre. In terms of dry weight loading, this is about 50 dt/ac (at a bulk density of about 20 lbs/cf) per desired final inch of soil compost.
Nitrogen Management
One
major consideration of heavy applications of biosolids is nitrogen management.
An application of 100 dt/ac may contribute up to 10,000 lbs-N/ac (at 5% N);
half of which may be in an available form during the first year. This amount
of available N (initially in an ammonium form) will either volatilize or be
transformed into nitrate. Two associated concerns exist: nitrification is an
acidifying process; and high potential for high rates of nitrate leaching. This
needs to be considered in terms of groundwater impacts. If biosolids for the
project are lime stabilized, the total N concentration will be lowered by dilution,
and the lime added to the biosolids during treatment will maintain soil pH and
also increase N volatilization.
An alternative to high biosolids only applications, which both reduces the N loading and may actually conserve excess N from the biosolids through immobilization, is co-use of a carbon-rich residual. In this case, biosolids can be applied to the soil in combination with sawdust, primary pulp and paper sludge, paper waste or even some types of yard waste (those that include a significant amount of woody debris). This limits the potential for excess N to nitrify and leach from the site. Adding biosolids in combination with a high C material directly to the soil essentially allows it to compost in place. Total cost for this option would be transportation of materials to the site and direct application of the materials. In normal practice, either: (a) alternate layers of biosolids and C-rich material are laid down, then incorporated; or (b) materials are mixed on site prior to application. Because a C-rich residual is often considerably drier than biosolids, it is much easier to work the soil after application.
pH Adjustment
Determining the appropriate application rate for lime is very important in cases of metal contamination. Plants generally require a pH > 5.5 for good growth. In cases of metal contamination, pH > 7.0 will limit the solubility of metals by both increasing the number and strength of binding sites and decreasing the potential for soluble stable species. It is important to add sufficient limestone to raise soil pH in the surface 18" of soil and to keep it well buffered.
Procedure
Soil samples can be collected in 6" increments. pH measurements should be made on the dried and sieved subsamples. There are standard EPA procedures to determine the lime requirement of a soil. An alternative test is as follows: base (such as 1 M KOH) should be added to 10 g subsamples of the soils that have been mixed with 20 ml of water. A good starting point is addition of 1 ml of 0.8 M KOH, which is equivalent to the addition of 8 t/ac of limestone for the 6" portion of the soil profile. The pH of the soil/water slurry should be taken 1 hour after water addition. The base should then be added and the sample put on a side to side shaker for 24 hrs. The pH of the sample after shaking will be comparable to the pH of the field soil after limestone application. If this is sufficient to bring the pH > 8.5, the amount of lime added is sufficient to neutralize that portion of the soil. If the base brings the pH of the sample to >10.5, then the CCE added is more than required. Redo the incubation using a lower rate of KOH addition. Add the adjusted lime requirements for all horizons tested and you will have your lime requirement for the profile. This is a relatively quick procedure to determine the lime requirement for a site where acidity is an obstacle to revegetation. It is appropriate to use this type of procedure, rather than simply consulting the soil test lab or an agricultural extension agent when you are working with heavily disturbed soils or mine tailings.
High Sulfur Sites
In cases where soils are contaminated with high S minerals or where high S is present, it is also necessary to account for the acidity that can be generated when the S oxidizes when determining the appropriate rate of limestone addition. Sulfur is often present in mine tailings when high S ores have been processed. These ores are stable under anaerobic conditions. As the rocks are ground to small particles and exposed to O2, the minerals are no longer stable and the reduced S will oxidize. When S oxidizes, it generates sulfuric acid which has an adverse effect on plant growth.
There are a number of procedures to test for the acid generating potential of these types of soils. The best way to test a soil for this type of acidity is to consult with scientists who work with these types of materials. Douglas Dollhopf (dollhopf@montana.edu) at Montana State University and Lee Daniels (wdaniels@vt.edu) at Virginia Tech can test your soil for a lime requirement that takes into account acid generating potential.
Regulations and Guidelines
40 CFR 503
Contaminants - metals and organics
The national regulations that define appropriate use of biosolids are detailed
in 40 CFR part 503 (http://www.epa.gov/owm). These guidelines define the maximum
metal concentrations that biosolids may have and still be suitable for land
application. The basis for 40 CFR 503 is primarily the agronomic use of biosolids.
The exposure risk assessment used a pathway approach to evaluate any potential
negative impacts as a result of biosolids use and considered soil reclamation
in its analysis as well. It also defines the maximum metal concentrations that
biosolids may have to be considered exceptional quality materials. These materials
may be used without restriction. Currently the vast majority of biosolids produced
in the country have metal concentrations well below the requirements. Organic
contaminants are not regulated under 40 CFR 503 as concentrations of these materials
were well below concentrations that were deemed to pose a potential risk. Radionucleide
concentrations were not regulated in the 503's. EPA is currently surveying the
radionucleide concentration in biosolids and may issue an advisory or site specific
guidelines for these materials. The technical basis for the 503 regulations
is outlined in one of the support documents found on the EPA website.
Pathogens
Part 503 also regulates pathogen reduction requirements that are necessary to
achieve Class A and Class B standards. Class B biosolids have undergone a Process
to Significantly Reduce Pathogens (PSRP). Use of Class B materials has some
restrictions. For example: no vegetable crops may be grown on the soil for 18
months following application; material may not be applied within 10 m of streams
or rivers; public entry in applied areas is restricted immediately following
application. Full details of these restrictions are outlined in the regulations.
Most generators and contractors are familiar with these restrictions and can
make sure that application is in compliance with the regulations. Most biosolids
from larger municipalities that have anaerobic digestion and high N biosolids
generally fall under Class B standards. Class A materials have undergone a Process
to Further Reduce Pathogens (PFRP), such as high temperature digestion, composting
or heat drying. These materials may be used without any restrictions, so long
as they also meet the 40 CFR 503 limits.
State Regulations
The 40 CFR Part 503 regulations are the minimum standards for biosolids application. Each state has the freedom to apply more stringent standards than those outlined in 503. The EPA regional biosolids coordinator will be familiar with any additional regulations. Many additional regulations relate primarily to agricultural use of biosolids. Use of material for restoration purposes (generally a one-time application) may be exempt from these additional regulations.
Permitting Process
Permits are generally required for all biosolids applications. This is a good means to gain public acceptance of a proposed remedy even though obtaining required permits can be a time consuming process. Use of biosolids for reclamation is also a recommended use in the regulations. A provision is made within the regulations for application in excess of agricultural rates for restoration objectives: 503.14(d) "Bulk sewage sludge shall be applied…. at agronomic rates…unless, in the case of a reclamation site, otherwise specified by the permitting authority." Permits may be required on several levels, depending on the particular region of the country. Generally, the permitting process is best left to the experts. If biosolids are obtained through a municipality, generators can often walk the necessary permits through. Another way to obtain appropriate permits is by working with the regional biosolids coordinator.
Public Acceptance
The public has generally accepted the use of biosolids on agricultural lands. This has not always been the case and in some local areas there are still citizens who need to have the benefits of biosolids use demonstrated before public acceptance is achieved. Years of practice in dealing with public acceptance issues have made many biosolids generators public-acceptance professionals. Generally, a successful biosolids project requires a pro-active approach. It is necessary to be very open with local citizens groups about the nature of the restoration project. This includes being straightforward about the materials to be used as well as their origins. Low-keyed informational meetings (as opposed to formal public meetings or hearings) and articles in local papers are very effective means for gaining public acceptance. A large body of educational materials exists that is excellent for use in public meetings. These include videos and pamphlets that describe what biosolids are, the regulations governing their use, and the benefits associated with biosolids use. The generator or contractor providing biosolids for a project may have access to these types of materials. The Northwest Biosolids Management Association (NBMA - contact Leah Taylor 206 684-1145 http://www.nwbiosolids.org) is an excellent source of general educational material and can also provide detailed literature reviews on the environmental effects of biosolids use.
One of the most often heard objections of those near a biosolids use site is to its unique aroma. Odor can be a challenging obstacle to public acceptance. There are two stages of odor from biosolids. The strongest smell happens immediately after application and is caused by volatilization of ammonia and anaerobic decomposition of sulfur compounds. This dissipates after a day or two. Evolution of different sulfur compounds will result in some less intense lingering odor that will depend upon climatic conditions. Dry and hot or cold conditions will reduce odor intensity in a relatively short period of time, while moist, warm conditions prolong odors. Incorporation also reduces odors. Generally in an agricultural community, familiarity with the use of manure will make acceptance of any odors less of a problem. Use of materials in isolated areas eliminates this as an issue.