A sustainable alternative for COD test

^{Copyright header image: VMM}

The chemical oxygen demand (COD) is one of the oldest parameters used to determine water quality. Since the 1940s, this has been measured to see how much organic material is present in water. A significant part of organic pollutants (leaves, food residue, dead organisms, wastewater discharges, etc.) are broken down by microorganisms using oxygen. This causes the oxygen concentration to drop, putting fish and other aquatic life at risk. The COD indicates how much oxygen will be consumed if the wastewater is oxidised by this chemical process.

Prompted by REACH

The commonly used method to measure the COD has also been the same for decades and is based on an extensive test in which significant amounts of reagents are used. However, many of these reagents – like potassium dichromate, a chromium (VI) compound – are toxic and can even be carcinogenic. Furthermore, the reagents are only used once and are then disposed of. So the current method is everything but sustainable. Partly as a result of the stringent European REACH legislation, which strictly regulates the use of substances like potassium dichromate, both users and the industry have been searching for sustainable alternatives for quite some time. One of these users is the Flanders Environmental Agency (VMM). “We focus strongly on green chemistry, so we try and limit the use of hazardous substances as much as possible,” says Johan Annys of the VMM. “We do the same when analysing waste and surface water.”

Three years ago, VITO therefore joined forces with the German companies Merck and Hach (two major manufacturers of test kits). The aim: developing, testing and validating a sustainable alternative to the existing COD test, in particular for water with a chlorine concentration above 1,000 mg/l. “In the partnership with Merck and Hach, we played the role of reference laboratory,” says Christine Vanhoof of VITO. “We divided the assignment into two parts: one test for what is called the low COD measuring range and one for the high COD measuring range, which respectively have low and high chloride concentrations. This mainly involved reducing the amounts of reagents and eliminating the interference with substances such as chloride. This interference was a problem especially for water samples with high chloride levels.”

Small and practical test kit

For the low COD measuring range, the Merck and Hach labs managed to develop a sustainable test kit that required up to ten times less of the reagents than before. The kit is also much smaller, and therefore much more practical. “The traditional method – which we call the macro-method – is based on large equipment in labs,” says Vanhoof. “The new kit consists of tubes of only 20 millilitres.” The disruptive chloride interference could also be eliminated by adding very small amounts of mercury.

Both Merck and Hach developed a test kit for measuring low COD levels (less than 70 mg O2/l) in waters with high chloride concentrations, but in the end the accredited laboratories only considered the Hach kit due to its user friendliness. Vanhoof: “This test kit also yielded the best time savings and was therefore the most efficient one.” The new test kit was then extensively tested by VITO and the VMM, during which a crystal-clear method and manual were also prepared.

The partners then started on the development of the alternative test for measuring high COD levels (above 70 mg O2/l) in waters with high chloride concentrations. The newly developed kit also appeared to serve as a good basis here, so this test kit was also fully ready and validated a year later. “At that point the bulk of the work was done. The only thing we are still doing within this project is finalising the methodology.”

Another key advantage of the new COD test is that the reagents used are collected, so they no longer need to be processed as waste, creating an (almost) closed cycle. “We collect the used reagents at our lab in Düsseldorf,” says Carsten Schulz, Product Manager at Hach. “We are currently able to recycle and reuse 75 percent of the substances.”

Another advantage compared to the existing macro-method is that the new test method is largely automated. “Companies like Merck and Hach have developed robots to perform analyses,” says Kristof Tirez of VITO. “This was highly labour-intensive when using the existing macro-method.”

Proactively saving costs

The new COD test method for samples with high chloride levels has been introduced this year. The VMM will use it to check the quality of wastewater and surface water. “We’re actually already doing that,” says Johan Annys. “The test kits for samples with low chloride levels have already been available much longer.”

Tirez: “This parameter determines the Flemish discharge charges, which often run to thousands of euros for companies. So it is important that clear and reliable analysis methods are agreed with the accredited laboratories.”

To help the Flemish government as much as possible, but also Flemish companies, VITO has recorded the method for new test kits (in both the low and the high COD measuring ranges) in a compendium. And it doesn’t stop there: VITO also acts a point of contact, which is a role it performs as an independent expert on environmental testing. “Both governments and companies are welcome here if they have any questions or comments about the new tests,” says Vanhoof. “We want to keep supporting them, also now that the development of the tests is finished.”

At Hach they can look back with satisfaction on the partnership with VITO and the VMM, which was carried out based on the design thinking approach. “We collaborated on the basis of an iterative process: we created a prototype, after which VITO and the VMM tested it and provided us with feedback, after which we improved our prototype. This allowed us to create a workflow in which all the problems were resolved by us and the tests were validated by VITO,” says Schulz.