NIST SRM 2781 — domestic sludge from Denver as a reference material for metals: certified values include Pb 200.8 mg/kg, Cd 12.78 mg/kg and Hg 3.68 mg/kg
On 2 April 2026 the US NIST described reference material SRM 2781 Domestic Sludge: sludge from a domestic wastewater treatment plant in Denver, dried, ground and homogenised. Laboratories use it for quality control of heavy metal determinations in sludge used as fertiliser, to check results against levels set by the EPA.
- Domestic sludge can contain antibiotics, pesticides, oils and heavy metals. Once sterilised it may be used as a soil fertiliser, and high concentrations of metals such as lead and arsenic can contaminate soil and the food chain. The NIST news item mentions freeze-drying, but according to the certificate the USGS dried about 182 kg of partially dehydrated sewage cake from Metropolitan Denver Sewage Disposal District No. 1 at ambient temperature in a forced-air chamber, ground it to pass a 74 µm sieve, blended it for 24 h and radiation-sterilised it. A unit of SRM 2781 is a bottle with 40 g of dried, pulverised sludge.
- The certificate (issue of 28 May 2025) gives certified mass fractions on a dry-mass basis: As 7.81 ± 0.67, Cd 12.78 ± 0.63, Cr 202 ± 14, Cu 627.8 ± 18.4, Pb 200.8 ± 4.2, Hg 3.68 ± 0.14, Mo 46.6 ± 5.4, Ni 80.2 ± 1.8, Se 16.0 ± 1.5, Ag 97.6 ± 6.5 and Zn 1273 ± 68 mg/kg, and nitrogen 4.78 ± 0.16%.
- The minimum sample mass is 100 mg (dry mass). For non-volatile elements samples are vacuum-dried at room temperature for 24 h or oven-dried for 2 h at 110 °C; volatile elements (As, Hg, Se) and organic constituents are determined on the material as received, with a moisture correction from a separate sample (mass loss at certification 4.7–6.6%).
- The certified values are valid until 31 May 2050 if the material is stored properly — in its original bottle below 30 °C; non-certified values for other inorganic and organic constituents are given in Appendix A of the certificate. The original SRM 2781 certificate dates from 22 June 1995.
Source: NIST — Muck Around and Find Out: NIST’s Domestic Sludge SRM (2.04.2026)