Turbidity measurement is both a nebulous, oft-misunderstood concept and the master link in a chain of events affecting U.S. Environmental Protection Agency (EPA), drinking water compliance.
It can influence or be influenced by - almost every other link in a water treatment process. Read an overview of turbidity’s relationship to drinking water compliance standards and some tips for keeping a water treatment process in balance.
In layman’s terms, turbidity would appear to be a measure of the aesthetics of water. Cloudy or murky water is definitely less appealing than a sparkling, see-through glass of water.
Unfortunately, that supposedly clear glass of water could be hiding a number of harmful pathogens smaller than 10 microns in size.
Beyond aesthetics, turbidity readings give water treatment plant (WTP) operators a higher degree of confidence in the healthy quality of treated water and provide early indication of filter breakthroughs.
Turbidity readings do not literally “count” the number of Giardia cysts or Cryptosporidium oocysts in a process stream, but they can indicate the relative likelihood of potential problems.
Staying below the regulatory compliance limit of 0.15 nephelometric turbidity units (NTUs) provides a high degree of confidence in the integrity of WTP pretreatment and filtration processes.
The first step in any monitoring effort is demonstrating the quality of the data. With respect to turbidity measurements in drinking water, EPA requirements do a good job of ensuring that status.
They regulate both the quality of water being produced and the legitimacy of the testing methods, along with the maintenance, calibration, and verification processes required to sustain water quality readings at a high degree of reliability.
It is important to distinguish between “calibration verification” and “calibration.”
While some aspects of EPA calibration regulations seem overly cautious in light of the sophistication of today’s turbidity instrumentation, they remain in effect to assure the proper performance of many instruments in the field that are still functioning with 1980s and 1990s technology.
With turbidity instrumentation reliability ensured by the EPA calibration guidelines, there are two key milestones of turbidity compliance for treated water.
The 2 Milestones of EPA turbidity compliance are:
Using modular filtration design, with a turbidimeter on each filtration line, is a good way of monitoring independent filtered streams to satisfy the < 0.15 NTU standard.
It also enables WTP personnel to service any noncompliant line without shutting down more of the filtering operation than absolutely necessary.
WTPs using membrane filtration have yet another requirement designed to protect filtered water quality.
Direct membrane integrity testing — mandated once every 24 hours — confirms the membrane filtration system’s ability to filter out potential pathogens.
Methods for monitoring membrane integrity, including air pressure testing and bubble point testing, are covered in Chapter 3 of the EPA’s Membrane Filtration Guidance Manual. This testing is required regardless of whether or not the plant exceeds the < 0.15 NTU limit on turbidity readings.
While being able to check off all EPA compliance requirements is important, turbidity readings also provide WTP operators with insight to perform their jobs more reliably and more efficiently.
Here are several practices that can simplify the process of meeting or exceeding EPA compliance requirements:
Modern nephelometer designs that reduce the surface area needing to be cleaned by 98 percent, can cut overall cleaning time from 10 minutes to just 1 minute or less and may be carried out automatically.
Those same designs can cut verification time in half and reduce operator involvement for calibration from 15 minutes to just 1 minute per quarter.
Regardless of the type of filtration system used, logging real-time turbidity readings and tracking their trends is a good way to forecast when it is time to shut down a filter system, backwash it and bring it back on-line before a catastrophic failure occurs.
This preventive approach can be more efficient than simply setting an alarm for turbidity readings exceeding the 0.15 NTU limit. This is especially true during seasonal extremes, when rising nitrogen levels can generate blooms of diatoms that can plug filter media.
Turbidimeters can also be used to monitor water used in the backwash process to ensure that sufficient cleaning takes place without keeping the system off-line any longer than necessary.
While the EPA establishes the national drinking water standards, state inspectors are responsible for enforcing those federal standards and/or their own more stringent state standards for water quality.
Working with those inspectors can help WTPs understand how to interpret turbidity readings and develop appropriate real-world responses that can keep them off the EPA’s Enforcement and Compliance List.
There are numerous resources available for WTP operators and personnel to address turbidity monitoring, certification and a variety of other water treatment topics.
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