Determination of TOC (0-1000 ppm) and TNb (0-100 ppm) in Municipal Wastewater According to EN 1484, USEPA 415.1 and 9060A

• TOC is a valuable sum parameter in municipal wastewater (WW) monitoring. It can be used as a more cost-effective and non-toxic surrogate for COD analysis for process control which also avoids production of toxic waste. TOC is also suitable for effluent control and is the required parameter for reporting in some countries.
• TNb determination in the influent for process control and in the effluent for monitoring according to the EU-Directive can be conducted parallel to TOC measurement (according to NEN-ISO 20236 and ASTM D8083).
• Reliable, quick and direct detection of organic carbon content without interference of other parameters is possible.
• WWTPs that treat high amount of industrial discharges can monitor the influent for high organic load and react accordingly.
• Hach^® QP1680 is a lab TOC analyzer based on catalytic high temperature combustion technology. Its unique maintenance concept results in easy handling, that can be completed in a few quick steps.
• TOC monitoring in the effluent requires standardized determination in some regions.

Method

There are mainly two methods available to measure the TOC.

NPOC

• Use this method if no volatile organics are present. When measuring with the QP1680 standard settings of the NPOC method, 650 μL of 3N hydrochloric acid (HCl) are injected into the sample vial. Then oxygen is bubbled through the sample for 600 s. This purges all IC from the sample. Afterwards the sample is injected into the oven and the TOC is measured.
• Ensure that the sample is acidified to pH < 2. Alkalinity and pH of the sample may require additional sample preparation prior to the measurement.
• Ensure that all IC is sufficiently purged out of the sample by testing an aliquot of the sample spiked with IC.
• NPOC is equal to TOC, when VOC and POC are not present.

TOC = TC – IC Difference Method

• Use this method if volatile organics are present or if TOC ≥ IC. Avoid this method if IC > TOC. For this method both, IC and TC are measured. The TOC is then calculated from the two individual measurements.
• When QP1680 measures IC with standard settings, 100 μL HCl (3N) are added to 500 μL of sample and the IC is purged out of the sample by bubbling oxygen through the sample. Ensure that all IC is purged out of the sample by testing an aliquot of the sample spiked with IC.
• The accuracy of the method is the sum of the accuracies of the two individual measurements. This means, that it is usually less accurate than the NPOC method. That is why the difference method should be avoided if IC > TOC.

The use of 3N HCl acid, 1N H₃ PO₄ or 1.5N H₂ SO₄ is applicable for those two methods to determine TOC.

TN_b

• TNb analysis is based on the sum of inorganic and organic bound nitrogen which is measured under the high temperature combustion. The analysis can be carried out parallely when measuring TC or NPOC.
• Generally speaking it is advisable to choose the calibration range so that most measurements will be towards the middle of the range.
• Typical range TOC: 0–400 mg/L
• Typical range TN: 0–100 mg/L

Analyzer

QP1680 Lab TOC (High Temperature Combustion)

Setup QP1680

• 1000 μL syringe with a 0.8 mm ID needle
• Autosampler attached
• Sample glass vials

Necessary Lab Equipment

• Water pre-treatment system for ultrapure water UPW (water purity min. 18.2 MΩ•cm, max. 1–2 ppb TOC)
• Glassware: volumetric flasks with stoppers (appropriate size)
• Aluminium foil to cover sample vials

Calibration

Before calibrating the QP1680, make sure the blank value is below 50,000 AE and doesn’t show a significant drift. To ensure that, run at
least 10 blank measurements.

Calibration will be done with the calibration wizard.

TIC Stock Standard Solution 1000 mg/L C.

Add 7.04 g of sodium hydrogen carbonate of a 99.5% mass fraction NaHCO₃ into ultra pure water. Add enough water to make the solution exactly 1 L. Mix well until all NaHCO₃ is dissolved in water.

Calibration Procedure

1. Open the single stock calibration wizard.
2. Choose the method to be calibrated e.g. NPOC for the calibration of NPOC. For the calibration of the difference method (TOC = TC – IC), both TC and IC must be calibrated.
3. TC is best calibrated using a KHP standard solution. IC must be calibrated using a carbonate standard solution.
4. Fill in the following parameters:
   • Concentration of stock solution (e.g. 500 mg/L)
   • Fill in the vial position of the stock solution
   • Select the method to be used
5. Fill in the number of calibration points (recommended 10-points).
6. Fill in how many replicates for every point should be run (recommended at least 3).
7. Blank measurement is not necessary for calibration but it is necessary to condition the analyzer beforehand.

Recommendations

• It is best practice to add a stir bar to the sample vial to ensure good homogenization during the measurement. This is particularly important when particles are present in the sample.
• Particles larger than 800μm may clog the needle and should be removed before analysis.
• Minimize the contamination of the system by running an appropriate number of blank water samples prior to calibrations and samples. The blank value should be lower than 50,000 AU and stable.
• It is necessary to run a quality control standard on a regular basis (at least daily) to check the validity of the calibration. Use the “bracketing” function to set up automatic quality control measurements.

Maintenance

• Refresh the UPW in the UPW bottle and the humidifier daily.
• Every week refresh the acid from the acid bottle (Use 3N HCl).
• After refreshing the reagents always run the system method “Fill tubing sampler with liquids”.
• On a daily basis: before analyzing samples, check the injection port for leakages by putting a droplet of water onto the furnace port. If you see bubbles, clean the injection port and test again. If there is still gas coming out of the furnace port, replace the furnace port. Chapter 17.14 in the manual explains the maintenance of the Injection port.
• Carefully clean the sample needle if salt or other solids build up on the outside.
• Clean the IC and waste port if salt or other solids build up on them.
• The method of standard addition (spiking) and/or dilution of the sample can be used to understand matrix effects.
• If the quality control measurement reveals too low recovery, it is necessary to recalibrate the instrument. Eventually it will be necessary to replace the furnace and/or catalyst completely.

Features and Benefits

As stated above, the QP1680 can handle particulates up to 800 μm. The analyzer can also handle dirty samples better than some competitive products. This is because the sample does not travel more than a couple cm past the injection needle on the sampler arm. Therefore, there is never the possibility of the sample contaminating the injection syringe or internal valves. Competitive TOC analyzers are susceptible to dirty samples, from wastewater or industrial waters (i.e. influents, oil refining, etc), clogging internal injection valves that would require cleaning.

The QP1680 also offers an injection needle washing station. This port sprays ultra-pure water onto the outside of the needle to rinse off possible contaminates from previous samples, reducing issues from sample carry over contamination.

Compliance

TOC/NPOC:

ASTM D7573, EN 1484, EPA 415.1, EPA 9060A, ISO 8245, SM 5310B, NEN-ISO 20236

TN_b:

ASTM D8083, EN 12260, ISO11905-2, NEN-ISO 20236