no. 0.547

Biosolids Recycling

by K.A. Barbarick and D.G. Westfall1 (1/07)
  • Biosolids supply N, other plant nutrients and organic matter to soils.
  • Necessary steps in a biosolids application program include:
    1. File a Letter of Intent with the Colorado Department of Public Health and Environment (CDPHE).
    2. Meet all CDPHE requirements regarding trace elements and pathogens.
    3. Apply biosolids at "agronomic rates."
    4. Develop a soil-management program that includes periodic soil and plant sampling and analyses.

Households, businesses, and industries produce wastewater. Biosolids are by-products of the wastewater-treatment process. Farmers, land-reclamation specialists, landscapers and home Gardeners have used these primarily organic materials for over seven decades in the United States.

The Colorado Department of Public Health and Environment (CDPHE) and the U.S. Environmental Protection Agency (EPA) encourage and regulate recycling of biosolids on crop- or rangeland, since they contain plant nutrients and organic matter that can improve production as well as soil conditions.

The CDPHE (1993) defines biosolids as:

"The accumulated residual product resulting from a domestic wastewater treatment works. Biosolids does not include grit or screenings from a wastewater treatment works, grease, commercial or industrial sludges, or domestic or industrial septage."

The EPA announced final national standards (40 CFR Part 503) for beneficial use of biosolids on February 19, 1993. The CDPHE adopted their regulations (1993) in accordance with EPA standards on November 2, 1993. All biosolids recycling and disposal programs must comply with state and federal regulations to protect the environment and public health.

To apply biosolids to land, you must first submit a Letter of Intent to CDPHE to develop a land-application program. The CDPHE will either issue or deny a Notice of Authorization for the plan detailed in a Letter of Intent.

A key aspect of EPA and CDPHE regulations requires application of biosolids at an agronomic rate. The CDPHE (1993) defines agronomic rate as:

"The rate at which biosolids are applied to land such that the amount of nitrogen required by the food crop, feed crop, fiber crop, cover crop or vegetation grown on the land is supplied over a defined growth period, and such that the amount of nitrogen in the biosolids which passes below the root zone of the crop or vegetation grown to groundwater is minimized."

Biosolids Properties

Biosolids can exhibit a wide array of physical and chemical traits. Depending on the extent of dewatering or drying, the solids content of biosolids can range from less than 5 percent to more than 90 percent. Chemical characteristics of biosolids vary between treatment plants and, to a limited extent, within the same plant over time.

Table 1 lists the chemical composition of three Colorado biosolids as representative examples. Because biosolids may contain trace elements, they are classified by their trace-element content due to their potential impact on public health and the environment.

Table 1: Chemical properties (dry weight basis) of three biosolids from Colorado.
Parameter, units Littleton/Englewood1 Fort Collins2 Metro Denver3
EC4, dS/m 11.6 5.0 12.7
Organic N, % 2.88 4.22 6.31
NH4-N, % 0.47 0.40 1.35
NO3-N, % 0.01 0.01 0.01
Phosphorus (P), % 2.52 1.60 2.32
Potassium (K), % 0.283 0.194 0.200
Arsenic (As), mg/kg 4 3 3
Cadmium (Cd), mg/kg 6 5 10
Chromium (Cr), mg/kg 98 40 80
Copper (Cu), mg/kg 558 553 500
Mercury (Hg), mg/kg 0.8 6.2 3.0
Lead (Pb), mg/kg 45 117 138
Molybdenum (Mo), mg/kg 26 16 31
Nickel (Ni), mg/kg 85 19 41
Selenium (Se), mg/kg 13 14 4
Zinc (Zn), mg/kg 942 776 915
1Applied to experimental plots near Bennett, Colorado, in August 1993.
2Applied to experimental plots on the Meadow Springs Ranch near Fort Collins, Colorado, in August 1991.
3MetrogroTM cake chemical analysis, 1993.
4EC is a measure of the soluble salt concentration.

New, aggressive pretreatment programs at the source of generation have dramatically reduced metal concentration in biosolids over the last two decades, minimizing the possibility of environmental damage.

The EPA and CDPHE established guidelines regarding the quantity of trace elements in biosolids and the amount that ultimately can be added to soils growing plants. The CDPHE (1993) classifies biosolids into Grade 1 and Grade 2, depending on metal concentrations (Table 2). The agencies base metal limits on extensive research regarding the effects of biosolids metals on various pathways of exposure, including plant toxicities and adverse effects on animal and human health.

Biosolids are treated to eliminate pathogens (disease-causing organisms) that may reside in wastewater. EPA and CDPHE (1993) require domestic wastewater treatment plants to reduce pathogens and diminish the attraction of Insects and animals before biosolids are applied.

In Colorado, anaerobic (without air) and aerobic (with air) digestion are the most common methods used to treat and stabilize biosolids. Digestion destroys pathogens through heat and attack by beneficial microorganisms (e.g., anaerobic bacteria); it also reduces odors. Municipalities use composting, heat drying and other techniques to further reduce pathogens and stabilize the material.

Applicators may apply Grade 1 biosolids at agronomic rates without restrictions regarding trace metal loading limits. However, CDPHE and EPA limit the annual and cumulative application of Grade 2 biosolids according to the annual and lifetime (cumulative) loading limits in Table 3.

Table 2: Maximum trace element concentrations allowed by CDH (1993) for Grade 1 and Grade 2 biosolids.
Metal Grade 1 Grade 2
  -- mg/kg (dry weight basis)--
As 41 75
Cd 39 85
Cr 1200 3000
Cu 1500 4300
Pb 300 840
Hg 17 57
Mo Not finalized 75
Ni 420 420
Se 36 100
Zn 2800 7500
Table 3: Annual and cumulative pollutant loading limits (CDPHE, 1993).
Metal Annual limit Cumulative limit1
As 1.8 37
Cd 1.7 35
Cr 139 2680
Cu 67 1340
Pb 14 268
Hg 0.75 15
Mo 0.80 16
Ni 19 375
Se 4.5 89
1kg/ha = 1.12 * lbs/Ac

Fertilizer and Soil Amendment Value

Biosolids contain significant amounts of N, P and K (Table 1). They also can provide plant micronutrients such as Zn. Many soils in Colorado exhibit low levels of available Zn and biosolids help alleviate the deficiency of this essential element.

The nature of nutrients in biosolids is different than those found in commercial fertilizers. Stabilization of biosolids during waste treatment produces organic N forms that are not available to plants until they are decomposed by soil microorganisms. When added to soils, microorganisms break down biosolids and release 10 to 50 percent of the organic N as available N (ammonium, NH4+) in the first year following application. Soil microorganisms rapidly convert the NH4+ to nitrate (NO3-). Plants quickly absorb NO3-. It also is mobile in soils, irrespective of whether it originates from commercial N fertilizer or biosolids. The mobility of NO3- increases the potential for groundwater contamination. In essence, biosolids are slow-release N fertilizers that contain low concentrations of plant nutrients.

Frequently, biosolids promote physical changes in soil that are more significant than the plant nutrients they supply. Most Colorado soils contain less than 1.5 percent organic matter. Biosolids can serve as a source of organic material that improves soil tilth, water-holding capacity, structure development and stability, and air and water transport, and can ultimately decrease soil erosion potential.

Cropland Application

Colorado State University has applied Littleton and Englewood biosolids to summer-fallowed dryland winter wheat near Bennett, Colorado, for 13 years. Continuous application of 3 dry tons/acre of biosolids, when compared to 50 or 60 pounds of N/acre as commercial N fertilizer, produced comparable or better wheat yields, higher protein contents, and larger economic return (Barbarick et al., 1991; Barbarick et al., 1992; Ippolito et al., 1992, 1993, 1994).

The greatest challenge in using biosolids for beneficial reuse on crop- and rangeland is to prevent NO3- leaching to groundwater. As biosolids' nutrient value may vary depending on the form (i.e., liquid, dewatered or dried), determining the correct agronomic rate remains a challenge.

However, if the agronomic rate is applied under non-irrigated (dryland) cropping in our semi-arid environment, where water table depths generally are over 100 feet deep, the potential for groundwater contamination is negligible. Under irrigated conditions, if agronomic rates of biosolids based on site specific soil-test and crop-management information are applied, groundwater contamination with NO3- should not occur. Annual monitoring of residual soil NO3-N levels will help guard against groundwater pollution.

Rangeland Application

Recent interest in the use of biosolids on rangeland has developed. Harris-Pierce et al. (1993) found that surface application, without subsequent incorporation of 2 dry tons/acre of biosolids from the Fort Collins wastewater-treatment facility, increased plant canopy cover of rangeland in the first and second seasons following application. Five dry tons/acre increased plant biomass production compared to untreated control plots in just the first season. The interaction of climate and biosolids application did produce some shifts in plant species distribution.

Since biosolids application to rangeland generally involves surface application without incorporation, the effect of biosolids addition on runoff-water quality is a concern.

In a simulated rainfall study (water was applied through a sprinkler system) on 8 and 15 percent slopes, Harris-Pierce et al. (1993) found that under a severe runoff event (4 inches rainfall/hour), an estimated rate of 1.5 dry tons/acre biosolids application would not create any potential NO3-N or trace element pollution problems. If off-site transport is possible, it may be minimized by providing untreated buffer strips around areas of runoff catchment and along edges of drainage ways.

Public Acceptance

Land application of biosolids for beneficial use poses no health or environmental threat if CDPHE and EPA guidelines for trace elements and pathogens are followed and proper soil management is practiced.

Biosolids application also will result in beneficial economic return to agricultural producers and improve soil quality. Municipal pretreatment programs significantly reduce the metal content in biosolids. Proper treatment (i.e., anaerobic digestion) of sewage eliminates most pathogens. Further treatment such as drying, composting and lime stabilization further reduce pathogens.

Some public resistance to biosolids application will probably always exist. Applicators must face issues about odors, fears about health and environmental risks, and act appropriately to ensure odors are minimized and biosolids are handled and applied safely and aesthetically.

Public education should continue to develop trust between all parties. Society can benefit if we can safely recycle the plant nutrients and organic material that constitute biosolids.

Steps for a Successful Land-Application Program

  1. Letter of Intent for CDPHE.
  2. Know the nutrient and trace-metal composition of the biosolids.
  3. Prior to application and for each application thereafter, applicators must sample and analyze the soil from the land application site for pH, NH4-N, NO3-N, total P, conductivity (measure of soil salinity), organic matter and available P. Applicators must also collect and analyze soils for As, Cd, Cr, Cu, Pb, Hg, Mo, Ni, Se and Zn once every five years after application.
  4. Include a soil fertility evaluation in the soil analyses, especially for N, so that the applicator can ascertain the agronomic rate of the biosolids. Over the long term, using soil test data will help prevent an imbalance of plant nutrients.
  5. Judiciously apply biosolids to sloping land to avoid surface water contamination.
  6. Contact CDPHE or EPA to ensure that you meet all necessary requirements for biosolids use. Only CDPHE can approve a program of recycling of biosolids through land application.


  • Barbarick, K.A., R.N. Lerch, D.G. Westfall, R.H. Follett, J. Ippolito, and R. Jepson. 1991. Application of anaerobically digested sewage sludge to dryland winter wheat. Colorado Agricultural Experiment Station, TR91-5.
  • Barbarick, K.A., R.N. Lerch, J.M. Utschig, D.G. Westfall, R.H. Follett, J. Ippolito, R. Jepson, and T.M. McBride. 1992. Eight years of application of sewage sludge to dryland winter wheat. Colorado Agricultural Experiment Station, TB92-1.
  • Colorado Department of Health. 1993. Biosolids Regulation 4.9.0.
  • Harris-Pierce, R.L., E.F. Redente, and K.A. Barbarick. 1993. The effect of sewage sludge application on native rangeland soils and vegetation: Fort Collins-Meadows Springs Ranch. Colorado Agricultural Experiment Station, TR93-6.
  • Ippolito, J., K.A. Barbarick, D.G. Westfall, R.H. Follett, and R. Jepson. 1992. Application of anaerobically digested sewage sludge to dryland winter wheat. Colorado Agricultural Experiment Station, TR92-5.
  • Ippolito, J., K.A. Barbarick, D.G. Westfall, R.H. Follett, and R. Jepson. 1993. Application of anaerobically digested sewage sludge to dryland winter wheat. Colorado Agricultural Experiment Station, TR93-5.
  • Ippolito, J., K.A. Barbarick, D.G. Westfall, and R. Jepson. 1994. Application of anaerobically digested sewage biosolids to dryland winter wheat. Colorado Agricultural Experiment Station, TR94-6.
  • Logan, T.J., and R.L. Chaney. 1983. Utilization of wastewater and sludges on land - metals. pp. 235-323. In A.L. Page (ed.) Proc. of the 1983 Workshop on Utilization of Municipal Wastewater and Sludge on Land. Univ. of California-Riverside.

1K.A. Barbarick, Colorado State University professor, and D.G. Westfall, professor; soil and crop sciences. 7/94. Reviewed 1/07.

Colorado State University, U.S. Department of Agriculture, and Colorado counties cooperating. Extension programs are available to all without discrimination. No endorsement of products mentioned is intended nor is criticism implied of products not mentioned.

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Updated Wednesday, January 08, 2014

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