| TECH-TIP: Tools for Monitoring Organic Carbon and Disinfection by-Products
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TECH-TIP: Tools for Monitoring Organic Carbon and Disinfection by-Products

Currently regulated in the United States by the Stage 2 D/DBP Rule, DBPs can be a challenge for many water treatment plants.  DBPs form when the dissolved organic carbon (DOC) present in the water reacts with a disinfectant, such as free chlorine, to form halogenated organic compounds, such as trihalomethanes (TTHM) and haloacetic acids (HAA).  October’s Tech Tip described the various ways that DBP concentrations can be controlled, both at the treatment plant and in the distribution system.  This article focuses primarily on DOC and DBP monitoring techniques that may commonly be applied by plants that utilize DOC removal strategies as a primary means of limiting DBP formation.  

The removal of DOC is most commonly achieved by coagulation or enhanced coagulation processes, although some plants may also use processes such as enhanced softening, ion exchange, granular activated carbon/powdered activated carbon, and nanofiltration or reverse osmosis membranes for DOC removal.  Plants utilizing these processes typically also strive to balance DOC removal with turbidity control to achieve simultaneous compliance with multiple treatment plant regulations, and it can take some trial and error to identify treatment conditions that are able to achieve the greatest reduction in both DOC and turbidity.  Employing some basic analytical testing for organic carbon and related parameters, at locations throughout the treatment process train, can help plant staff better understand and trend raw water conditions, adjust treatment conditions based on water quality, evaluate the effectiveness of treatment, and potentially predict DBP formation.      

Sampling Locations and Frequency
Key sampling locations for organic carbon and related parameters include raw water/source water, post-coagulation, and post-filtration.  Plants attempting to optimize coagulation conditions through the use of jar testing may also benefit from analyzing organic carbon as part of the jar testing procedure.  Samples for DBP analysis are typically collected from various locations in the distribution system, although samples for DBP formation potential testing may also be collected at various treatment plant locations, such as before and after coagulation and filtration, in order to quantify the impact of the treatment process on DBP formation.  These analytical techniques will be described in greater detail in the following section.

Although the D/DBP Rule requires a minimum of monthly monitoring for total organic carbon (TOC) in each of the plant’s raw water sources, more frequent monitoring of TOC or a surrogate parameter has several benefits.  Data collected more frequently can provide plant staff with a more thorough understanding of trends in organic carbon concentration that may occur through the year.  This can be beneficial, because TOC concentrations frequently vary on a seasonal basis.  Understanding of seasonal trends can help plant staff to anticipate and prepare for expected changes in raw water quality.  Although collection of this type of baseline data can be very useful, plant staff should be aware that it can take several seasons to establish a baseline that is representative of water quality throughout the year.   Continuous analysis of raw water quality with respect to organic carbon can also allow plant staff to respond to raw water quality changes by making nearly immediate changes in treatment conditions.  Plants with multiple raw water sources may also utilize TOC information as a means to select or control source water blending.  Plants that apply a TOC removal process may also benefit from frequent or continuous monitoring downstream of the primary TOC removal process so that the effectiveness of the treatment process may be assessed.       

Analysis Parameters
There are many types of carbon present in water that comprise the amount of total carbon (TC) in a water sample.  TC consists of total inorganic carbon (TIC) and total organic carbon (TOC).  Total inorganic carbon is that which is contributed by inorganic compounds such as bicarbonate and carbonate ions.  TIC is not included in a TOC analysis, and most analytical methods either involve purging the TIC from the sample prior to analysis or subtracting the TIC from the TC concentration to calculate TOC by difference, as follows:

TC – TIC = TOC

Total organic carbon can be further broken down into components such as dissolved organic carbon (DOC), particulate organic carbon (POC), and even assimilable organic carbon (AOC), which is the portion of carbon that may be utilized as a food source for bacteria.  The component of TOC that is typically of most interest to drinking water treatment plants is the DOC, defined as the portion of the organic carbon that can pass through a 0.45 micron filter.  DOC can be an important process control measurement for drinking water treatment plants, because it is the DOC that reacts with disinfectants present in the water to form DBPs.

TOC and DOC can be analyzed in a variety of ways, many of which are described in Standard Methods for the Examination of Water and Wastewater, 21st Edition, Section 5310.  Both benchtop and continuous analyzers are commercially available for TOC analysis, many of which employ the persulfate-UV oxidation method to oxidize the organic carbon to carbon dioxide, which is then measured by an NDIR or conductivity detector.  These analyzers provide very useful data to plants focused on controlling DOC and DBP formation.  However, their cost can be prohibitively expensive for some plants, particularly smaller facilities with more limited resources.  Plants may also send samples to a certified lab for TOC analysis, particularly for reporting purposes.  However, the turnaround time to receive results from an outside lab may not be rapid enough to apply the results for process control purposes.  

One alternative that plants may consider is the use of a surrogate parameter, such as UV254 absorbance, as an indicator of the water’s DOC concentration.  The UV254 absorbance method measures the portion of the organic carbon that absorbs ultraviolet light at a wavelength of 254 nanometers, which is typically associated with the structure of carbon contributed to the water from a natural organic carbon source.  The UV254 absorbance method is described in Standard Methods 5910B.  This method can be run using a handheld or benchtop UV spectrophotometer.  Continuous monitors for UV254 absorbance are also commercially available.  The analytical method consists of filtering a sample (typically through a 0.45 micron filter, similar to that used in a DOC analysis method) and reading the sample on the spectrophotometer versus an organic-free water blank.  Due to the wavelength of the light used, quartz cells are used for this analysis.  

In many cases, UV254 absorbance may be correlated with DOC concentrations.  A correlation may be established by measuring both DOC and UV254 absorbance on a split sample for multiple samples collected over a period of time.  Because the relationship between UV254 absorbance and DOC can vary on a seasonal basis, the correlation should be established and verified periodically at a variety of times throughout the year and for a variety of raw water quality conditions.  It is recommended to apply caution in using UV254 absorbance to predict a specific DOC concentration – it may be more suitable to use UV254 as a general indicator of general trends in DOC concentrations.  Once a correlation is established, UV254 may be measured on a regular, or even continuous, basis to capture raw water quality, adjust treatment process conditions, and monitor the effectiveness of treatment.  For plants without a UV spectrophotometer, colorimetric methods are also available for TOC/DOC analysis that can be run in the lab using colorimeters or spectrophotometers to read the results.  Although these methods take longer to run than a UV254 measurement, they can still provide useful water quality feedback to plant staff within a period of several hours.  With any analytical method, it is important to accurately follow the procedure and be aware of any potential interferences that may exist in the sample.  

Although measuring UV254 absorbance is a useful method employed by many water treatment plants, it can be beneficial to evaluate additional parameters for DBP control.  Some of these parameters include:

  • Bromide – The presence of bromide contributes to the formation of brominated DBPs, such as bromoform.  Bromide has a high molecular weight than chloride, so brominated DBPs can cause additional challenges with DBP compliance.  The presence of bromide also contributes to the formation of bromate in plants that apply ozone.  Plants with known or expected bromide in the raw water may monitor bromide so that its impact can be minimized or controlled.
  • DBPs (TTHM and HAA) - DBPs are commonly measured in teh lab using GC or GC/MS, and many plants send samples to an outside lab for DBP analysis.  Plants that prefer a more immediate means of in-house DBP analysis may benefit from the colorimetric methods, benchtop analyzers, and continuous monitors that are available commercially. 
  • SDS DBP Formation - An SDS (simulated distribution system) DBP formation potential test may be run to assess the impact of treatment conditions on DBP formation.  This test, described in Standard Methods 5710C, is more advanced and may be run as part of a bench or pilot testing analysis.  The method involves dosing samples with chlorine at typical distribution system pH levels and incubating samples for times and at temperatures reflective of typical distribution system conditions.  The objective of this test is to attempt to quantify the concentrations of DBPs that may form in a treated sample under typical distribution system conditions. 

Any treatment processes applied or changes made to control DBP formation should also consider the impact of these treatment processes on turbidity – the Partnership for Safe Water’s key water quality performance indicator.  Whether running jar tests or altering coagulation conditions to increase organics removal, it is important to maintain or even increase turbidity monitoring to ensure that particulate removal is not compromised.  

Setting Goals
A key component of both the Partnership for Safe Water’s treatment plant and distribution system optimization programs is establishing goals and developing action plans that enable utilities to meet the established goals.  While the Partnership sets goals for turbidity (treatment), disinfectant residual, pressure, and main breaks (all three for distribution), establishing goals for other parameters that are critical to plant performance is encouraged.  The development of standard operating procedures (SOPs) and action plans to achieve these goals is also recommended.  

For plants focused on minimizing DBP formation, these goals may be related to organic carbon, in addition to turbidity.  The analytical tools described in this Tech-Tip provide a means for plants to establish baseline performance and make the data-driven decisions that are essential to optimizing plant operations and performance.

References

Standard Methods for the Examination of Water and Wastewater, 21st Edition

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