How to Sample Biosolids

Sample before you apply this product of wastewater treatment to croplands.

May 14, 2017

Biosolid, a product of wastewater treatment, is often applied to croplands in order to amend the soil with important plant nutrients such as nitrogen and phosphorus.

Why sample?

Proper sampling of biosolids is an essential step towards analysis of nutrients, metals, organic compounds, and pathogens. In fact, under EPA Rule 503, biosolids must be sampled before sale, use, disposal, or quality checks.

Who samples?

Biosolids are usually sampled by an owner or operator of a biosolids storage facility or publicly owned wastewater treatment works (POTW).

When to sample?

Under EPA Rule 503, biosolids must be sampled before sale, use, disposal, or quality checks. Especially in the case of pathogen testing, analysis is best done as close as possible to the time of use/sale/disposal/etc, so that the analysis reflects the true conditions of the biosolids in storage.

How often to sample, & how many samples?

When sampling at a POTW, the frequency of sampling, the number of grab samples collected, and the number of composites created from the samples depends on the millions of gallons per day (MGD) treated by the POTW (see table below based on Table 2.4-1 in EPA, 1999). Note that the size of the biosolid unit does not affect the total number of samples required.

Design Flow (MGD) Sampling Frequency # Grab Samples # Composited Samples
< 1 once / year 27 1
1 - 10 once / quarter 42 4
10 - 100 every 60 days 48 6
> 100 every month 84 12

Where to sample?

Samples should be collected “downstream” in the POTW design flow to ensure sufficient mixing, and thus reliable results. If sampling a liquid form, try to collect from a pipeline. Once the biosolid is in storage (drying beds, lagoon, piles, etc.), sampling ought to represent any variations in biosolid qualities, for which a method is outlined in the following section. For example, if sampling a lagoon, samples should include material from the bottom, middle, and top of the liquid column. If sampling a drying bed, consider how the material at the surface is drier and more exposed, which could affect certain analyses. In this case, adequate mixing should be performed before sampling.

Collecting a representative sampled

A simple method is suggested by the EPA for collecting grab samples that account for any variations in biosolid qualities.

  1. Draw an outline of the storage unit on paper.

  2. Divide the outlined storage unit into 100 square units.

  3. Assign sequentially the numbers 0-99 to each square.

  4. Use a random number generator or random number table to select locations for sampling. The total number of locations will equal the number of samples required based on design flow of the POTW (see Table 1).

  5. Sample at the center of the squares as it overlays the storage unit.


To composite a sample for analysis requires mixing the individual grab samples together. For compositing liquids, semi-liquids, or semi-solids, mix grab samples together in some container appropriate to the sampling plan. For solids, use the cone and quarter method. Spread out each grab sample into a ring, with one sample spread out on top of another. Then, with a shovel, work around the ring, shoveling the layers of solids into the center, forming a cone-shaped pile, with each consecutive shovelful being placed at the top of the cone. Then flatten out the pile, divide it into quarters, and discard the solids from two quarters opposite each other. The remaining two quarters can be used for a composite sample and, if necessary, a replicate.

Materials for Sampling

Materials should be carefully selected to avoid potential contamination of samples. Nonreactive materials are essential. Teflon and glass materials are ideal, though stainless steel or brass materials can be used, too. No galvanized metals should be used. The use of wide-mouthed containers is recommended. A list of sampling tools is recommended below.

Consistency of Medium Suggested Sampling Tools
liquid coliwasa
muddy/sticky trier
granulated/powdery thief
thick & hard solid shovel, auger, scoop

Storage, shipping, & safety

Various protocols ought to be followed to ensure proper storage of samples as they are sent to the lab for analysis. These protocols are specific to the analysis and can be referenced in the EPA Biosolids Management Handbook Section 2. In general, however, samples are to be kept refrigerated and/or acidified, and samples should be received by the lab within 24 hours of sampling. Often, a chain-of-custody form is to be kept with the samples, being signed by all carriers/receivers of the samples.

Safety in handling biosolids is necessary as to prevent exposure to pathogens. Gloves and waterproof clothing ought to be worn during sampling. Know, too, whether the sampling location, such as a tank or pipe, is under pressure from gases generated from microbial activity in the biosolid.


Environmental Protection Agency (1999) Biosolids Management Handbook, Section 2. Denver, CO: U.S. EPA Region 8.

Environmental Protection Agency (1994) A Plain English Guide to the EPA Part 503 Biosolids Rule, Chapter 6. Washington, D.C: U.S. EPA.