---
kind: "section"
citation: "40 C.F.R. § 503.31"
title: "40"
number: "503.31"
heading: "Special definitions."
url: "https://uscodex.org/cfr/40/503.31"
---

# §503.31. Special definitions.

- (a) **Aerobic digestion—** is the biochemical decomposition of organic matter in sewage sludge into carbon dioxide and water by microorganisms in the presence of air.
- (b) **Anaerobic digestion—** is the biochemical decomposition of organic matter in sewage sludge into methane gas and carbon dioxide by microorganisms in the absence of air.
- (c) **Density of microorganisms—** is the number of microorganisms per unit mass of total solids (dry weight) in the sewage sludge.
- (d) **Land with a high potential for public exposure—** is land that the public uses frequently. This includes, but is not limited to, a public contact site and a reclamation site located in a populated area (e.g, a construction site located in a city).
- (e) **Land with a low potential for public exposure—** is land that the public uses infrequently. This includes, but is not limited to, agricultural land, forest, and a reclamation site located in an unpopulated area (e.g., a strip mine located in a rural area).
- (f) **Pathogenic organisms—** are disease-causing organisms. These include, but are not limited to, certain bacteria, protozoa, viruses, and viable helminth ova.
- (g) pH means the logarithm of the reciprocal of the hydrogen ion concentration measured at 25 °Centigrade or measured at another temperature and then converted to an equivalent value at 25 °Centigrade.
- (h) **Specific oxygen uptake rate (SOUR)—** is the mass of oxygen consumed per unit time per unit mass of total solids (dry weight basis) in the sewage sludge.
- (i) **Total solids—** are the materials in sewage sludge that remain as residue when the sewage sludge is dried at 103 to 105 degrees Celsius.
- (j) **Unstabilized solids—** are organic materials in sewage sludge that have not been treated in either an aerobic or anaerobic treatment process.
- (k) **Vector attraction—** is the characteristic of sewage sludge that attracts rodents, flies, mosquitos, or other organisms capable of transporting infectious agents.
- (l) **Volatile solids—** is the amount of the total solids in sewage sludge lost when the sewage sludge is combusted at 550 degrees Celsius in the presence of excess air.

## Notes

### Amendments

[58 FR 9387, Feb. 19, 1993, as amended at 64 FR 42571, Aug. 4, 1999]

### Authority

Authority: Sections 405 (d) and (e) of the Clean Water Act, as amended by Pub. L. 95-217, sec. 54(d), 91 Stat. 1591 (33 U.S.C. 1345 (d) and (e)); and Pub. L. 100-4, title IV, sec. 406 (a), (b), 101 Stat., 71, 72 (33 U.S.C. 1251 et seq. ).

### Source

Source: 58 FR 9387, Feb. 19, 1993, unless otherwise noted.

### Amendments

[58 FR 9387, Feb. 19, 1993, as amended at 64 FR 42571, Aug. 4, 1999]