Abbreviation for “Formazine Nephelometric Unit”. Calibration unit in compliance with standard ISO 7027 for turbidity measurement using the scattered light method. Based on a formazine solution. Calibration (turbidity)
|Typical turbidity values for water||FNU|
|Limit in Central Europe||1|
|Targeted by most waterworks||0.1|
|Guideline WTO (ideal)||< 5 (< 1)|
|Raw water*||< 0.5|
|After flocculation and filtration*||< 0.2|
Subject to the source of Raw water its turbidity can and does very greatly, for instance river water extracted for use in a water treatment plant can easily reach 200FNU and greater.