Zahn-Wellen Is a Test You Should Know

• By Shawn Blenis, Ray Noletto
• Aug 11, 2008

Pre-testing aqueous wastewater streams to determine biodegradability doesn't have to cost an arm and a leg, much less drag on for months. Zahn-Wellen Simulation Testing Protocols, widely used in Europe, can help industrial and municipal wastewater treatment managers in North America achieve a sound environmental management strategy while simultaneously meeting stringent discharge limits and controlling costs.

The Zahn-Wellen test is a 14- to 21-day static test that can be used to determine the inherent biodegradability and toxicity of water-soluble chemicals in a stream under aerobic conditions. The test, named after scientists R. Zahn and H. Wellen who co-authored a paper in the German chemicals publication Chemiker-Zeitung [Issue 98, p. 228-232], can serve as an efficient alternative to a full-scale treatability study.

The U.S. Environmental Protection Agency and state regulators are playing hard ball when it comes to wastewater effluent discharge. These agencies are particularly concerned with watersheds that may be impaired by one or more pollutants. The best example of this tightening regulation is EPA's Total Maximum Daily Load (TMDL) program, which was established to ensure that water quality standards of common watersheds are met and maintained.

In these cases, EPA sets a finite limit on the total amount of pollutants that can be present downstream, regardless of how many facilities are discharging upstream. This means that facilities downstream will be limited to lower discharge limits as a result of upstream discharging.

New wastewater streams must be properly tested before they are discharged into highly regulated and controlled treatment facilities, because if they're not, treatment plant operations and effectiveness may be at risk. This could lead to even larger problems if the treatment plant, which may serve an entire municipality, has to shut down. If permitted conditions are exceeded, the violating company may be subject to heavy fines, and the watershed area could be severely compromised.

When manufacturers introduce a new product line, they typically evaluate the resulting aqueous waste streams on paper first and then subject them to physical testing. Whereas a full-scale treatability test can be prohibitively expensive and time-consuming, the Zahn-Wellen test is a suitable alternative that can save both time and money. It can be used to determine the inherent biodegradability and toxicity of water-soluble chemicals in a stream under aerobic conditions. If the wastewater treatment facility has nitrifying bacteria present, the effect on nitrification also may be evaluated.

The goal of the test is to measure the biological elimination of dissolved organic carbon (DOC) and nitrogen (N) over time. While adaptable to any wastewater treatment plant, Zahn-Wellen Simulation Test Protocols are most suited for facilities with a potential waste stream with high DOC content. Examples include chemical, petrochemical, pharmaceutical, leather tanning and carpet manufacturers or any operation that utilizes solvents and/or toxic chemicals.

The Zahn-Wellen test has three possible outcomes: The first is that the stream is highly biodegradable and readily treatable. The second is that the stream is moderately biodegradable and further testing may be required before scale up. Finally, the stream may be considered inhibitory or toxic and cannot be discharged to a treatment facility as is.

In order to ensure a successful and timely test, samples must be diluted to approximately 200 milligrams per liter DOC and seeded with bacteria from the final treatment facility. Over the course of the 14-day test, operators will measure the pH and temperature. Operators measure DOC removal at the outset and again at 3, 7, and 14 days. A single sample is run in four separate reactor stages: • Reactor 1 contains the waste of interest with essential nutrients such as nitrogen (N) and phosphorous (P) plus an easily biodegradable co-substrate. The co-substrates are used to evaluate the viability of the bacteria in the suspension. Possible results range from total removal of DOC to none at all. Depending on the results, some pretreatment of the waste may be necessary prior to introducing it to the treatment plant.

• Reactor 2 is the reference. This is basically Reactor 1 without the waste of interest and tests the biological activity of the bacterial seed. In this reactor, the results should show at least 90 percent DOC elimination of the co-substrate. A finding less than this level indicates poor activity by the bacterial seed.

• Reactor 3 contains neither aqueous waste nor co-substrate, just nutrients and bacteria. This control reactor is used to prove that the bacteria were sufficiently cleaned of any impurities before the start of the test.

• Reactor 4 is known as the "Strip Test." There is waste, but no bacteria or co-substrate. Copper sulfate is added to kill any bacteria introduced to the reactor. This is used to determine if the organic material is removed by biodegradation or is "stripped" (volatilized).

Daily monitoring helps maintain the optimal temperature and pH for bacterial growth.

The Zahn-Wellen test provides data that indicates the removal capabilities of DOC and N, and whether or not a stream will readily biodegrade, is inhibitory, or is toxic –within weeks. This can save an enormous amount of time, effort, and money that would go into the planning and execution of a full-scale study. In extreme cases, full-scale treatability tests still may be necessary, but for the majority of streams, this is not the case.

Zahn-Wellen is not without its limitations. Samples with low DOC concentrations (below 100 mg/liter) are difficult to test accurately. And waste stream samples with high salt content (greater than 2 percent) or extreme pH levels are problematic, as neutralization may increase the total dissolved solids in excess of 2 percent. In these cases, the Zahn-Wellen test should be used to evaluate toxicity only.

But for most facility managers looking to cut costs without compromising operations, the Zahn-Wellen Test Protocols are a simple, cost-effective and accurate tool for measuring the biodegradability of a potential wastewater effluent stream. A typical Zahn-Wellen test runs in the neighborhood of $2,000. Compare that to the $32,000 price tag associated with a full-scale study, which includes an additional $12,000 of analytical testing for a three-month-long, full scale study.

Interpreting the results of a Zahn-Wellen test and adjusting the treatment program accordingly takes experience, not just with the test itself, but with bringing multiple wastewater streams on-line in a real life plant environment. Manufacturers should consider enlisting the help of an expert consultant with a proven track record in the industry and experience using the test protocols within their own facilities. That said, manufacturing facilities in North America would do well to follow the lead of their European counterparts by exploiting the potential economic and time-saving gains that the Zahn-Wellen protocols can bring.