Reviewed by Zhiqiang Hu
Professor of Civil and Environmental Engineering

Biosolids are useful for land application. Examples of beneficial use include application to agricultural land and reclamation sites (e.g., mining sites).

Although, it contains organic matter and plant nutrients, such as nitrogen (N) and phosphorus (P), biosolids material from municipal waste treatment plants does not have a uniform composition. Waste characteristics and treatment technologies are different for each community. Therefore, it is required to determine the composition of the biosolids to meet nutrient management guidelines.

General characteristics

The biosolids dry matter content (total solids content) for land application ranges from a low of 0.5 percent to more than 50 percent. Fortunately, biosolids material from a specific plant has a narrow range in solids content over time. Each plant is designed to yield certain amounts of biosolids and effluent.

The sewage treatment facility that generates biosolids from wastewater must meet certain sampling requirements. A daily sample determines the percentage of total dry solids. In addition, each facility must sample the biosolids for chemical content. The Missouri Department of Natural Resources (DNR) dictates the sampling schedule by the volume of biosolids generated,

The facility submits the biosolids samples to an approved laboratory. The lab uses standard U.S. Environmental Protection Agency (EPA) and/or Missouri Department of Natural Resources (DNR) guidelines. The lab reports the analysis to the treatment facility. It is this report that is the subject of this guide.

The laboratory report

All reports include certain information, but the format may vary. Table 1 lists the essentials found in a good report, as well as some nice-to-know facts.

Table 1
Information necessary for a complete laboratory report of the analysis of a biosolids sample

General information
  • Name and address of biosolids source
  • Name and address of the analytical laboratory
  • Signature of the analyst or lab supervisor
  • Date of sampling
  • Date of the analysis
  • Reference of methods used in analysis
Analytical information
All parameters, except percent of solids, are measured on a dry matter basis. The preferred units of expression are milligrams constituent per kilogram dry biosolids (milligrams per kilogram).
  • Total solids
  • Arsenic (As)
  • Cadmium (Cd)
  • Chromium (Cr)
  • Copper (Cn)
  • Zinc (Zn)
  • Total Kjeldahl nitrogen (TKN)
  • Total phosphorus (P)
  • Lead (Pb)
  • Mercury (Hg)
  • Molybdenum (Mo)
  • Nickel (Ni)
  • Selenium (Se)

Additional tests required if TKN is more than 50,000 milligrams per kilogram and/or application rate of biosolids is more than 2 dry tons per acre:

  • Ammonium nitrogen (NH4-N)
  • Nitrate nitrogen (NO3-N)
  • Organic nitrogen


  • pH
  • Specific gravity
  • Calcium (Ca) and magnesium (Mg), if lime stabilization is used.
  • Aluminum (Al), if alum is used in the treatment.


The report's measuring unit should be weight of measured material per unit of total dry solids. The lab takes a volume of biosolids, weighs it, dries it and then weighs the dry material. The following is a calculation of percent of total solids (Equation 1):

Equation 1
_ Weight of dry solids _
Weight of initial sample
x 100= percent total dry solids

The lab results are reported in a total dry solids measurement in one of three units — parts per million (ppm), milligrams per kilogram (mg/kg) or percent (Table 2). These same units can also refer to an "as is" or wet weight analysis, so be sure that the lab report says "Dry Weight." Milligrams of a constituent per kilogram dry biosolids (milligrams per kilogram) is the preferred unit to use. PPM and milligrams per kilogram are numerically equal as shown in Equation 2.

Table 2
Example of analytical information found in a laboratory report made on biosolids from a publicly owned treatment works (POTW)

ConstituentConcentration (milligrams per kilogram, dry weight)
Total solids49,000 (4.9 percent)
Arsenic (As)<5
Cadmium (Cd)5
Chromium (Cr)63
Copper (Cu)709
Lead (Pb)147
Mercury (Hg)8
Molybdenum (Mo)14
Nickel (Ni)50
Selenium (Se)4
Zinc (Zn)1,252
Total Kjeldahl Nitrogen45,226
Organic N25,470
Ammonium N19,757
Nitrate N (NO3 – N)28
Total P19,033
Total K3,789
Source: Columbia — Perche Creek Facility annual report, 1994.

Other conversions are necessary, since other units may be used, (Equations 2, 3 and 4):

  • of ppm to milligrams per kilogram:
Equation 2

1 ppm = 1 milligram per kilogram

1 milligram (mg) = 0.001 gram and 1 kilogram (kg) = 1,000 grams

  • of ppm to percent:
Equation 2
ppm x___1___
= percent
  • of percentage to ppm:
Equation 4

percent x 10,000 = ppm

Another useful calculation is conversion of ppm or milligrams per kilogram to pounds per dry ton. A dry ton is 2,000 pounds.

Equation 5
ppm x 0.002 =__pounds___
dry ton

Some laboratories report results on an as-received basis (wet basis). In such cases, the lab should also report the percentage of total solids in the sample. If the report does not specify wet or dry, contact the lab. Percentage moisture or water may be reported by some laboratories,

Percent water + percent solids = 100 percent

Percent solids = 100 percent – percent water

Conversion from the as-received to the dry basis or vice-versa is easy if you know the percent total dry solids in the sample (Equation 6 and 6a):

Equation 6
percent wet basis
percent solids/100
= percent dry basis
Equation 6a
percent dry basis xpercent solids
= percent wet basis

For example, 1 ppm of an element on the as-received (wet) basis in a sample with 5 percent total solids is:

1 ppm ÷__5__
= 20 ppm dry basis

The wet-to-dry conversion and its reverse calculation are important. The biosolids are applied wet, so you must calculate the quantity of wet biosolids per acre.


Table 1 contains the general information found in laboratory reports. There are labs that report in terms of phosphate and potash. In this case, you are not quite sure what is being reported. Likely the lab means P2O5 and K2O — the units used by the fertilizer industry to sell phosphorus and potassium fertilizers.

You may convert P2O5 percentage and K2O percentage to the preferred elemental basis (P percentage and K percentage) as follows:

Percentage P2O5 x 0.44 = percentage P

Percentage K2O x 0.83 = percentage K

Table 2 contains a biosolids analysis. The following list explains some terms and acronyms used either on lab reports or used to modify the analytical results.

Total Kjeldahl Nitrogen (TKN)
This term refers to certain forms of N in the sample. Kjeldahl first developed this standard procedure. TKN usually includes organic nitrogen and ammonium-N (NH4+-N), but not nitrate-N (NO3--N).

Plant Available Nitrogen (PAN)
PAN is an estimate of the fraction of applied nitrogen which is available to plants. The calculated value includes a percentage of the organic nitrogen and ammonium nitrogen, plus the nitrate nitrogen

Equation 7

PAN = fo (organic N) + fa (NH4-N) + NO3-N

The availability factors, fo and fa, are necessary because all of the organic N is not plant available the first year and some of the ammonium nitrogen may volatilize.

Ammonium nitrogen (NH4+-N)
A portion of N in biosolids may be ammonium nitrogen (NH4+-N). When biosolids containing NH4+-N are applied to soil or crop residues, some of the NH4+-N may be lost by volatilization. The amount of volatilization depends upon temperature, pH of the soil and rainfall or snow, and the time period until a tillage or incorporation operation is performed. An availability factor fa (0.7) is used for surface-applied biosolids. If the biosolids are injected there is no loss, and the availability factor equals 1.

Nitrate nitrogen (NO3--N)
The portion of total N in the nitrate (NO3--N) form is soluble and not subject to direct volatilization. All NO3--N in biosolids is considered plant available.

Organic nitrogen
Organic nitrogen is the portion of total N in biosolids not present as NH4+-N or NO3--N. Typically, 20 percent of the organic nitrogen is plant available the year the biosolids are land applied, i.e., fo = 0.2. The nitrogen becomes available through mineralization by soil microbes.


How can the biosolids data in Table 2 relate to the plant needs on an application site? After all, nutrient management plans require a connection to plant needs. This prevents increasing soil nutrients to levels that will adversely affect ground and surface waters.


The first limit to land application of biosolids deals with nitrogen.

DNR rules state, "Annual biosolids application rates shall not exceed agronomic rates for plant nitrogen needs." The rules set up two different cases for documenting N applications.

Case 1

When the biosolids N content does not exceed 50,000 milligrams per kilogram (5 percent) and the biosolids application rate does not exceed 2 dry tons per acre, PAN reporting is not required.

This guideline means that up to 200 pounds of TKN per acre may be applied without further calculations:

2 dry tons per acre x

2,000 pounds
= 200 pounds TKN per acre per year
Case 2

In situations not covered by Case 1, report nitrogen compounds in the PAN calculations. Use MU Extension fertility guidelines to determine PAN loading and crop removal to determine agronomic rates.

For example, assume the biosolids in Table 2 are surface applied at 3 dry tons per acre.

Step 1
Calculate organic N concentration:

Organic N = TKN - NH4-N

Organic N = 45,226 milligrams per kilogram -19,757 milligrams per kilogram

Organic N = 25,469 milligrams per kilogram

Step 2
Calculate PAN:

PAN milligrams per kilogram = fo (organic N) + fa (NH4-N) + NO3-N

PAN milligrams per kilogram = (0.2) (25,469) + 0.7 (1,975) + 28

PAN milligrams per kilogram = 18,952 milligrams per kilogram

PAN pounds per dry ton = 0.002 x 18,952 (Equation 5)

PAN pounds per dry ton = 38 pounds PAN per dry ton at 3 dry tons per acre

Total PAN pounds per acre = 3 tons x 38 pounds per ton = 114 pounds per acre

The next question that arises concerns the agronomic requirement of the crop. Refer to application requirements.

Other nutrients

Currently, N is the key rate to applying biosolids. Application limits depend upon PAN in the biosolids and crop removal. It is helpful to calculate the amount of other nutrients delivered by the biosolids, also.

There is no availability factor used with any elements listed in the lab report, except N. If you apply 3 dry tons per acre of the biosolids represented in Table 2, the quantity of P and K is 114 and 23, respectively:

19,033 milligrams per kilogram x 0.002 x 3 tons per acre = 114 pounds P per acre

3,789 milligrams per kilogram x 0.002 x 3 tons per acre = 23 pounds K per acre


Check the metal pollutants in the biosolids to determine if any limits are exceeded. The calculation is the same as for P and K.

An example is zinc and mercury when 3 dry tons per acre of biosolids are applied (Table 2):

1,252 milligrams Zn per kilogram x 0.002 x 3 dry tons = 7.5 pounds Zn per acre

8 milligrams Hg per kilogram x 0.002 x 3 dry tons = 0.048 pounds Hg per acre

Application rates in gallons

The sample of biosolids represented in Table 2 has only 4.9 percent solids. It is handled as fluid from the treatment facility to the land application site. In this case, the units of measure may be gallons.

You may prefer to calculate the biosolids per 1,000 gallons, so the hauler can more easily determine the rate to apply per acre.

In the following calculations, nutrient refers to TKN, PAN, P, K and metal.

Step 1
Converting the concentration dry basis to wet basis (Equation 6a):

Nutrient milligrams per kilogram dry basis x

percent total solids
= nutrient milligrams per kilogram wet basis

Step 2
Converting a weight of wet biosolids to gallons:

Assume the weight of a gallon of water (8.3 pounds) is the same as a gallon of biosolids (specific gravity of biosolids is assumed to equal 1):

1,000,000 pounds biosolids
8.3 pounds per gallon

= 120,482 gallons biosolids

This means that if a nutrient concentration in wet biosolids is 1 ppm (1 pound per 1,000,000 pounds biosolids), the concentration is 1 pound per 120,482 gallons or 0.0083 pound per 1,000 gallons. An example is:

19,757 milligrams NH4-N per kilogram (Table 2)

19,757 x

x 0.0083 = 8.0 pounds per 1,000 gallons

Application rate in cubic feet

Biosolids with more than 20 percent total dry solids may likely be handled in cubic feet or in cubic yards. It is assumed that the specific gravity is one. Thus, one cubic foot of biosolids weighs 62.4 pounds.

For example, assume the percent solids in Table 2 is 25 percent and that we are dealing with NH4+-N (19,757 milligrams per kilogram dry basis):

Remember milligrams per kilogram is equal to ppm.

mg NH4-N per kilogram wet = 19,757 x

= 4,939 milligrams per kilogram

__4,939 pounds__
1,000,000 pounds

x 62.4 pounds = 0.308 pounds NH4+-N per cubic foot

Or, NH4-N per wet (as applied) ton could be calculated as follows:

19,757 ppm NH4-N x 0.002 = 39.5 pounds NH4+-N per dry ton

39.5 pounds NH4-N per dry ton x 0.75 = 29.6 pounds NH4+-N per wet ton

Original authors: James R. Brown, School of Natural Resources; Dennis Sievers, Department of Agricultural Engineering; Robert Magai, Environmental Specialist, Missouri Department of Natural Resources