Winter 2009

A Quarterly Newsletter of the Instrumentation Testing Association

ITA Enews

Summer 2010

Instrumentation Testing Association  (ITA)

 

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Total Organic Carbon (TOC) Analyzers

Online Total Organic Carbon (TOC) analyzers can be used at wastewater treatment plants to monitor final effluent for regulatory reporting purposes and at various points throughout the treatment process for preventative control.   Online TOC monitoring at the primary effluent stage provides a feed-forward control strategy for managing plant waste loads and preventing plant upsets.

 

Manufacturers utilize different methodologies for such functions as sample transport, sample conditioning, chemical addition, primary measurement, and secondary signal conditioning and amplification.  Automatic calibration and cleaning cycles are options.

Online TOC Analyzer

(Photo Courtesy of Cole Parmer )

Total Organic Carbon (TOC) Analyzer Assessment

Currently, there are several major types of instrument technologies used for online TOC analyzers:

· High Temperature Catalytic/Combustion Oxidation (HTCO)

· Supercritical Water Oxidation (SCWO)

· Patented Two-Stage Advanced Oxidation (TSAO) Process using Hydroxyl Radicals

· Heated Persulfate Ultraviolet (UV) Oxidation

 

 

Total Carbon (TC) of a wastewater sample is typically composed of a mixture of total organic carbon (TOC) and inorganic carbon (IC) or mathematically TC = TOC + IC or TOC = TC — IC.   Using these equations, there are two methods to determine TOC.  One method is to measure TC and IC independently and subtracting the IC from the TC.  The second method uses an acid to carbonate the sample to chemically remove the IC from the sample and leaving only the TOC in the sample for analysis by the instrument.

 

 All online TOC analyzer technologies oxidize the total organic carbon samples to break the sample’s organic carbon down into a carbon dioxide (CO2) gas. The CO2 gas is measured by a Nondispersive Infrared (NDIR) detector.

High Temperature Catalytic/Combustion Oxidation (HTCO)

The High Temperature Catalytic/Combustion Oxidation (HTCO) online TOC analyzer technology first acidifies the sample to a pH of less than 2 which converts carbonates (inorganic carbon) in the sample to carbonic acid (dissolved carbon dioxide or CO2). The sample is then purged (sparged) with a carrier gas to remove the CO2.  The organic carbon remaining in the sample is oxidized to CO2 using a catalyst and a high temperature oven maintained at 900 degrees Celsius.  The continuously flowing carrier gas transfers the CO2 into a nondispersive infrared (NDIR) detector.  The NDIR detector measures the concentration of CO2 which is proportional to the concentration of the total organic carbon in the sample.  The following figure displays a generic online HTCO TOC analyzer.

Patented Two-Stage Advanced Oxidation (TSAO) Process using Hydroxyl Radicals

The Patented Two-Stage Advanced Oxidation (TSAO) process utilizes a technique that passes oxygen through an ozone generator and combines the ozone with sodium hydroxide to form hydroxyl radicals. The hydroxyl radicals are used in the oxidation process to convert organic carbon in the sample to carbonate and oxalate. 

 

First the sample is acidified and purged (sparged) with a carrier gas to remove inorganic carbon in the form of CO2. Then the acidified sample is mixed with a base (sodium hydroxide) and ozone (two-stage oxidation process).  The ozone reacts with the base (hydroxide ions) forming hydroxyl radicals.  The hydroxyl radicals oxidize the sample into carbonate and oxalate.  Acid is again added to the sample.   Next the ozone oxidizes the manganese contained in the acid. The oxidized manganese reacts with the oxalate formed during the base (sodium hydroxide) oxidation to produce CO2.  At the same time, the acid reacts with the carbonate formed during the base oxidation to also produce CO2 gas. An NDIR detector measures the concentration of CO2 which is proportional to the concentration of the TOC in the sample.

Supercritical Water Oxidation (SCWO)

The Supercritical Water Oxidation (SCWO) online TOC analyzer uses a patented technique that exposes heat and pressure to a wastewater sample to bring the sample into a supercritical state (a state in which the sample is neither a liquid or a gas) and is based on wet chemical oxidation technique.  First the sample is acidified and purged (sparged) with a carrier gas to remove inorganic carbon in the form of CO2.  Then the sample is converted to a supercritical state.  The supercritical sample is then oxidized to CO2 gas using persulfate at elevated temperatures and an NDIR detector measures the concentration of CO2 which is proportional to the concentration of the TOC in the sample.

Heated Persulfate Ultraviolet (UV) Oxidation

The Heated Persulfate Ultraviolet (UV) oxidation process utilizes a combination of chemical and ultraviolet oxidation techniques in a low temperature reactor.  First the sample is acidified and purged (sparged) with a carrier gas to remove inorganic carbon in the form of CO2.  Then the acidified sample is mixed with sodium persulfate and pumped into a UV reactor which oxidizes the organic carbon of the sample into CO2.  An NDIR detector measures the concentration of CO2 which is proportional to the concentration of the TOC in the sample.

Generic HTCO Online TOC Analyzer Schematic

(Source: ITA Online Toxics Measurement Workshop Proceedings)