Editing Turbidity (section)

==Standards and test methods==
[[File:Turbidimeters.JPG|thumb|[[Turbidimeter]]s used at a water purification plant to measure turbidity (in NTU) of raw water and clear water after filtration.]]

===Drinking water standards===
Governments have set standards on the allowable turbidity in drinking water. In the United States, [[public water system]]s that use conventional or direct filtration methods must not have a turbidity higher than 1.0 NTU at the plant outlet and all samples for turbidity must be less than or equal to 0.3 NTU for at least 95 percent of the samples in any month. Systems that use filtration other than the conventional or direct filtration must follow state limits, which must include turbidity at no time exceeding 5 NTU. Many drinking water utilities strive to achieve levels as low as 0.1 NTU.<ref>{{cite web |title=National Primary Drinking Water Regulations |url=https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations |date=2022-01-26 |publisher=EPA}}</ref> The European turbidity standard is 4 NTU.<ref>{{cite web |title=What are the drinking water standards? |url=http://dwi.defra.gov.uk/consumers/advice-leaflets/standards.pdf |date=June 2017 |publisher=Drinking Water Inspectorate (UK) |location=London |archive-url=https://web.archive.org/web/20201111190539/http://dwi.defra.gov.uk/consumers/advice-leaflets/standards.pdf |archive-date=2020-11-11}}</ref>

===Ambient water standards===
====United States====
The US [[United States Environmental Protection Agency|Environmental Protection Agency]] (EPA) has published water quality criteria for turbidity.<ref>{{cite report |title=Quality Criteria for Water |url=https://www.epa.gov/sites/default/files/2018-10/documents/quality-criteria-water-1986.pdf |date=1986 |publisher=EPA |id=EPA 440/5-86-001. (Commonly known as the "Gold Book.")}}</ref> These criteria are scientific assessments of the effects of turbidity, which are used by states to develop [[Clean Water Act#Water Quality Standards Program|water quality standards]] for water bodies. (States may also publish their own criteria.) Some states have [[Promulgation|promulgated]] water quality standards for turbidity, including:
* [[Louisiana]]: 25, 50 or 150 NTU, or background plus 10 percent, depending on the water body.<ref>Louisiana Department of Environmental Quality. Baton Rouge, Louisiana.[http://epa.gov/waterscience/standards/wqslibrary/la/la_6_wqs.pdf "Surface Water Quality Standards."] ''Louisiana Administrative Code (LAC).'' Title 33, Part IX, Chapter 11. August 6, 2007.</ref>
* [[Vermont]]: 10 NTU or 25 NTU, depending on water body classification.<ref>Vermont Water Resources Board. Montpelier, Vermont. [http://www.nrb.state.vt.us/wrp/publications/wqs.pdf "Vermont Water Quality Standards."] {{webarchive|url=https://web.archive.org/web/20131231093748/http://www.nrb.state.vt.us/wrp/publications/wqs.pdf |date=2013-12-31 }} January 25, 2006.</ref>
* [[Washington (state)|Washington]]: 5 NTU over background (when background is 50 NTU or less), or 10 percent increase when background is over 50 NTU.<ref>Washington Department of Ecology. Olympia, Washington.[http://www.epa.gov/waterscience/standards/wqslibrary/wa/wa_10_chapter173-201a.pdf "Water Quality Standards for Surface Waters of the State of Washington."] ''Washington Administrative Code (WAC).'' Chapter 173-201A. November 18, 1997.</ref>

===Analytical methods===
Published analytical test methods for turbidity include:
* ISO 7027 "Water Quality: Determination of Turbidity"<ref>International Organization for Standardization. Geneva, Switzerland.
[https://www.iso.org/standard/62801.html "ISO 7027-1:2016 Water quality – Determination of turbidity – Part 1: Quantitative Methods."] 2016 and [https://www.iso.org/standard/62801.html "ISO 7027-2:2019 Water quality – Determination of turbidity – Part 2: Semi-quantitative methods for the assessment of transparency of waters."] 2019.</ref>
* US EPA Method No. 180.1, "Turbidity"<ref>EPA. Environmental Monitoring Systems Laboratory. Cincinnati, Ohio. [https://www.epa.gov/cwa-methods/approved-cwa-test-methods-inorganic-non-metals "Method 180.1: Determination of Turbidity by Nephelometry; Revision 2.0."] August 1993.</ref>
* "Standard Methods", No. 2130B.<ref>{{cite book |title=Standard Methods For the Examination of Water and Wastewater |edition=21 |editor-last1=Eaton |editor-first1=Andrew D. |editor-last2=Greenberg |editor-first2=Arnold E. |editor-last3=Rice |editor-first3=Eugene W. |editor-last4=Clesceri |editor-first4=Lenore S. |editor-last5=Franson |editor-first5=Mary Ann H. |year=2005 |publisher=American Public Health Association |isbn=978-0-87553-047-5 |id=Also available on CD-ROM and [http://www.standardmethods.org/ online] by subscription}}</ref>