Improving Petrochemical Wastewater Compliance with TOC

The petrochemical industry generates large amounts of wastewater and effluents that must be treated before being reused or released into natural waterways.

It is routine to measure total organic carbon (TOC) and the total nitrogen bound (TN_b) content, as these contaminants cause eutrophication of surface water recourses, endangering groundwater supplies and aquatic life.

The European Industrial Emissions Directive (IED) advises that Best Available Techniques (BAT) must be implemented within the EU for direct wastewater discharges from mineral oil and gas refining.^1,2

The BAT reference document indicates that TOC and TN_b are of growing importance and should be monitored daily. TOC is a preferred parameter to COD, as it does not necessitate using highly toxic compounds, such as mercury and dichromate (Cr VI).

In many cases, COD and TN content are measured using separate methods. This is a time-consuming and labor-intensive process often associated with chromium-VI-contaminated waste formation.

An empirical conversion factor for TOC to COD can be established using correlation studies. A fully automated analytical process can be implemented for TOC/TN_b calculation according to DIN EN 1484 and DIN EN 12260 (or DIN EN ISO 20236 for both parameters), saving time and resources.^3-5

Effluent waters from refining processes are usually challenging samples for TOC/TN_b analyzers, as tubing and valve feeding in the combustion process can result in carry-over.

The direct injection implemented in the multi N/C 2300, employing a septum-free injection port and a wide-bore needle for optimal particle handling, overcomes this issue.

Direct injection guarantees sample transfer without particle losses and prevents blockages, increasing system uptime and minimizing wear and tear on sensitive Teflon parts within the dosing system.

The injection needle is thermally cleaned before injecting the next sample as it remains in the hot furnace inlet zone during analysis, reliably avoiding carry-over.

The multi N/C 2300 provides a robust solution for simultaneous TOC/TN_b analysis of particulate and oily samples.

Samples and Reagents

In the study presented in this article, samples from various process streams and clean-up stages were collected and analyzed, together with a reference standard. 2 M HCl was employed for automatic sample acidification to a pH < 2.

Sample Preparation and Measurement

Before analysis, samples were stored at 4 °C in a refrigerator and transferred into suitable autosampler vials for measurement. Direct mode analysis using an NPOC/TN method was used for wastewater samples.

Samples were automatically acidified using 2 M HCl in the autosampler run and then purged for five minutes to completely remove TIC. For these measurement sequences, an injection volume of 250 µL was implemented.

Samples were catalytically oxidized at 800 °C in an oxygen-rich atmosphere. To complete sample oxidation, a combustion tube filled with platinum catalyst was employed.

The formed nitrogen oxides were detected using a chemiluminescence detector (alternatively, a ChD detector can be used). Focus radiation non-dispersive infrared detection (FR-NDIR) was employed to detect CO₂.

Calibration

The multi N/C analyzer was calibrated for TOC determination between 1 and 500 mg/L using a potassium hydrogen phthalate standard solution. A multi-point calibration was employed to evaluate the results of NPOC measurement.

A calibration was performed from 1 to 50 mg/L for total bound nitrogen using a potassium nitrate and ammonium sulfate (50:50) solution under the standard DIN EN 12260.

Up to three calibration ranges may be linked to each parameter in this method, spanning a total working range of up to three magnitudes. Detection limits and limits of quantification depend on the selected working range and can be determined from the method characteristics.

Figures 1 and 2. Example of NPOC and TN_b calibration with method parameters. Image Credit: Analytik Jena US

Method Parameters

Table 1 outlines the method settings used to determine NPOC and TN_b contents.

Table 1. Method parameters. Source: Analytik Jena US

Results

Table 2 displays the mean values of three replicate injections with relative standard deviations for various real samples (anonymized) and recoveries of nicotinic acid employed as TOC and TN_b reference solution.

The BAT reference document issued under the IED 2010 / 75 / EU² states that the associated average emission levels (BAT-AEL) for direct wastewater discharges from refining processes can be expected to fall within the following ranges:

• 30-125 mg/L COD, corresponding to 7-32 mg/L TOC
• 1-25 mg/L TN_b

Figure 3. Example of a TOC and TN_b measurement curve for sample 3. Image Credit: Analytik Jena US

Table 2. Results. Source: Analytik Jena US

Conclusions

In the study outlined in this article, undiluted wastewater samples from various sampling points in the wastewater treatment process and with different TOC and TN_b concentrations were measured with outstanding accuracy and precision.

Nicotinic acid was employed as an analytical quality assurance standard (AQA), simultaneously checking for TOC and TN_b recoveries. The reference material achieved exceptional recoveries for organically bound nitrogen.

This study demonstrated the outstanding performance of multi N/C analyzers on demanding wastewater matrices based on the optimized combustion process, which includes selectable combustion temperatures up to 950 °C.

Several factors contribute to this performance, including direct injection with a septum-free pneumatic injection head, a wide-bore needle with an inner diameter of 0.7 mm, proper sample homogenization on the autosampler rack and tubing- and valve-free sample transfer into the combustion system.

Carry-over effects are minimized by ensuring the stainless-steel injection needle remains within the oven head at elevated temperatures during peak integration time, completely evaporating TOC components. The microliter injection syringe is also effectively rinsed, reducing carry-over effects.

The AS 60 autosampler and Self Check System provide a high degree of automation, allowing unattended operation and easy TOC/TN_b analyses, even for challenging samples.

The patented VITA flow-management system compensates for flow fluctuations inside the system caused by sample evaporation. This provides TOC calibration stability for up to a year and saves valuable measurement time.

Figure 4. Multi N/C 2300. Image Credit: Analytik Jena US

References and Further Reading

1. EUR-Lex. EN - EUR-Lex. - 32010L0075. [online] Available at: https://eur-lex.europa.eu/eli/dir/2010/75/oj.
2. Establishing best available techniques (BAT) conclusions, under Directive 2010/75/EU of the European Parliament and of the Council on industrial emissions, for the refining of mineral oil and gas, Official Journal of the European Union, L 307/38, 28.10.2014, Commission Implementing Decision of October 9, 2014
3. ISO 8245:1999. Guidelines for the determination of total organic carbon (TOC) and dissolved organic carbon (DOC). Edition 2. [Reviewed 2021]. (2021). Oceanbestpractices.org. [online] https://repository.oceanbestpractices.org/handle/11329/2516.
4. iTeh Standards. (2024). iTeh Standards. [online] Available at: https://standards.iteh.ai/catalog/standards/cen/d44b69ac-9d8f-4e21-8c2c-054c6b6ae0a3/en-12260-2003?srsltid=AfmBOooVtBYRiTjJaE7I_VOIWVaA2Kg13-IX99vpJmhSfNr0QFlUB3Wg [Accessed 8 Nov. 2024].
5. iTeh Standards. (2024). iTeh Standards. [online] Available at: https://standards.iteh.ai/catalog/standards/cen/0359fb8e-97d2-406c-85b4-d7b13767febb/en-iso-20236-2021?srsltid=AfmBOop5YUNj-I2GYi4nm9zVCPt2TG7d5al0sr1n2quNeV6SG0ii992u [Accessed 8 Nov. 2024].

This information has been sourced, reviewed, and adapted from materials provided by Analytik Jena US.

For more information on this source, please visit Analytik Jena US.