Amanda Scott2 and Katrin Schuhen1
Total Organic Carbon (TOC) is an aggregate measurement used by water treatment plants to indicate the amount of organic contamination from natural and non-natural sources. However, it is sometimes desirable to understand what organic compounds make up a TOC value. The M9 SEC DOC Detector was developed to operate as part of an HPLC Size Exclusion Chromatography (SEC) system to separate and quantify both aromatic and nonaromatic organic fractions as a function of molecular weight. Obtaining a complete picture or ‘footprint’ of organic size fractions present in source water, process waters, and finished water can help operators make smarter treatment decisions to optimize processes for time savings, cost savings, and ensure discharge quality. The combined HPLC SEC UVA + DOC system discussed in this article provides a better understanding of all organics present as opposed to only those that have a chromophore or fluorophore. The resulting information can be correlated to size fractions of specific contaminants or functional groups. The aim of this article is to discuss advantages and limitations of the classical TOC method in comparison with new enhanced techniques like Total Organic Carbon Size Exclusion Chromatography (TOC-SEC).
To determine organic contamination levels, wastewater and drinking water treatment facilities are moving toward using the sum parameter total organic carbon (TOC) as opposed to traditional methods of biochemical oxygen demand (BOD) and chemical oxygen demand (COD). In contrast to these traditional methods, TOC analysis is fast and has the advantages of better accuracy, lower sample volume requirements and the ability for complete automation through the use of online instrumentation. Additional advantages of TOC include low waste production and reduced interferences compared to COD and BOD. Hazardous waste is a significant disadvantage of COD, where highly toxic metals (Hg, Ag, Cr) are used and released in sulphuric acid results.
Additional qualitative and quantitative analysis of wastewater is often performed using chromatography coupled with mass spectrometry methods such as LC-MS, LC/MS/MS, GC-MS and GC-MS/MS to specifically detect certain regulated and non-regulated contaminants of concern .
Due to low concentrations of organic contaminants in water (ng/L), an enrichment (i.e., pre-concentration) technique before these actual analyses is needed. Additionally, significant methods development is necessary for these mass spectrometry techniques for optimal quantification of each individual organic compound. Since the number of chemicals in the environment is continuously increasing and real-time monitoring is intended, composite parameters are becoming increasingly important. Nevertheless, analyzing wastewater is challenging due to its variable composition and complexity: ranging from biodegradable natural organic matter (NOM) to anthropogenic organic pollutants. Even though NOM itself is not toxic, it affects mobility and toxicity of organic contaminants [2, 3, 4]. Performing TOC analysis allows for a quantification of the sum total of the organic species in the water; however, a more complete picture of the organic composition of the sample can be useful and can be determined by performing separation of the organics by molecular size. Size exclusion chromatography (SEC) was applied to gather information about quality an treatability of organic matter. Therefore, a modified TOC detector for characterization of NOM was introduced .
All TOC analysers oxidize organic material to CO2 using either wet chemical oxidation or high temperature combustion. The resulting CO2 is then measured to determine a TOC concentration. Before oxidizing organic carbon, the inorganic carbon (IC), atmospheric CO2, carbonate, and bicarbonate in the sample, is removed. This removal is accomplished through lowering the pH by adding acid and then either purging with a carrier gas or diffusion through a CO2 permeable membrane. If the former method is used, then the resulting TOC value is referred to as non-purgeable organic carbon (NPOC). If the latter method is used, typically the resulting total carbon (TC) is measured as along with any remaining IC, and TOC is obtained through subtraction (TOC = TC – IC). The TOC measurement and DOC detection referred to in this article is called UV-Persulfate oxidation whereby organic carbon is converted to CO2 via UV light and the addition of a persulfate oxidizing reagent. The resulting TC and IC are determined using the Sievers Membrane Conductometric Detection (MCD). MCD involves the separation of the CO2 produced through the oxidation of organic matter through a gas-permeable CO2-selective membrane, which prevents interference from other ionic species in the sample.
Modified TOC with HPLC SEC
The applied SEC coupled TOC system combines a size selective separation system with two detectors, a UV detector and a modified TOC analyser operated as a dissolved organic carbon (DOC) detector. The system provides not only TOC, but also size information and UV activity. The separation is driven by differential permeation of molecular sizes when a solution flows through a column with porous packing . Larger molecules elute off the column earlier than smaller molecules, which are held back due to diffusion into the packing (fig. 1). With this system, the molecular weight distribution of NOM can be estimated .
Other mechanisms like hydrophobic interactions, ion exchange, ion-exclusion, and intramolecular electrostatic repulsive interactions, might be relevant in the separation process [7, 8]. As humic substances comprise a large majority of NOM entering a treatment plant, the application of TOC-SEC to NOM and specifically humic substances has been investigated in past .
The TOC-SEC system provides a method for size fractionation of organics based on molecular weight. This application is cost-effective, easy to use, and simple to integrate. Coupling HPLC SEC UVA and SEC DOC detector leads to a total representation of all the dissolved organic compounds, not only those that are aromatic or have a UV or fluorescent signal. Figure 2 shows an example system in the laboratory.
TOC-SEC provides a multi-detector combination system and insights in NOM molecular fractions [4, 6, 7, 9]. The set-up is described in detail in table 1.
Results and Discussion
As an example, a drinking water treatment facility in the United States sources its water from a newly built reservoir. The reservoir is filled from creek water that is downstream of a wastewater treatment plant. The creek provides ample residence time for contaminants originating in the wastewater to degrade therefore making the drinking water facility a pseudo indirect reuse plant. The water is treated with chemical coagulation and flocculation followed by membranes and disinfected with chloramines.
While the overall TOC was similar from the wastewater treatment plant (WWTP) effluent to water treatment plant (WTP) influent as shown in table 2, concentrations of each size fraction changed dramatically due to environmental degradation as shown in figure 3. The WTP effluent had less overall TOC and consequentially lower concentrations in each size fraction. The presence and concentration of organics in different size fractions varies from WWTP effluent to WTP influent to WTP effluent. TOC monitoring and DOC speciation can be used to optimize treatment processes, prove reliability, and meet target contaminant removal. Monitoring bulk TOC in addition to characterizing DOC allows facilities to ensure efficient operations that protect public health and the environment.
Since efficiency of organic matter removal from wastewater is one of the major challenges in water treatment processes, characterization of the remaining organic matter in wastewater effluent by size can indicate treatment quality without having to know the concentration of every single compound. The TOC-SEC system is a useful tool to describe the nature and changes of source water as well as effluent characteristics for discharge. TOC-SEC can also help operators establish optimal treatment settings when bringing up a new treatment system or troubleshooting when a treatment system is down.
The authors acknowledge technical and research support from the GE Analytical Instruments, Boulder/USA.
The author wish to express their sincere appreciation to Dondra Biller, Sydney Jannetta, Nicollette Laroco and Beate Stahl for helpful comments and laboratory co-work.
1 Institute for Environmental Sciences, University of Koblenz, Landau, Germany
2 GE’s Analytical Instruments, Boulder, USA
Jun.-Prof. Dr. Katrin Schuhen
University of Koblenz
Institute for Environmental Sciences
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