Enzyme Attack

While progressive consulting companies are already using alternative wastewater treatment methods such as constructed wetlands, Professor Ray Legge, Ph.D. is taking these treatment methods one step further. Legge, a faculty member of the Department of Chemical Engineering at the University of Waterloo, is studying both constructed wetlands and immobilized enzyme technology. These methods overcome the drawbacks of traditional treatments for wastewater and provide new low-energy options.

Enzyme technology

Legge’s enzyme research focuses on removing phenol, one of the most significant industrial pollutants. Phenol is involved in the production of everything from pulp and paper, wood, steel, and metals to resins, plastics and petroleum. Conventional biological treatments sometimes can only treat a narrow range of contaminant concentrations. They also produce large amount of sludge and involve delays due to the need for biological acclimatization. Enzyme technology will address some of these negative issues and may also solve special waste problems. Says Legge: “The approach would be ‘designer,’ related to the phenolic waste of concern.”

In one study, Legge’s team investigated the removal of aqueous phenol using immobilized enzymes in both bench-scale (1.2 L) and pilot-scale (60 L), three-phase fluidized bed reactors. The enzyme used was a fungal tyrosinase immobilized in a system of chitosan and alginate. The immobilization matrix consisted of a chitosan matrix cross-linked with glutaraldehyde with an aliginate-filled pore space.

“This support matrix showed superior mechanical properties,” says Legge. “Along with retaining the unique adsorptive characteristics of the chitosan, it allowed reuse of the enzyme in repeated batch applications.” In the bench-scale studies, more than 92% of the phenol could be removed from the feed water using an immobilized enzyme volume of 18.5% and a residence time of the liquid phase of 150 minutes. Subsequent runs showed a lower percentage of phenol removal, but Legge says, “It is likely that we can get improved performance on repeated runs with a higher loading of the immobilized enzyme.”

Legge’s team is seeking private sector partners for commercialization and is conducting cost comparisons with other technologies.

Constructed wetlands

Wetlands are characterized by low capital, operating, energy and maintenance costs. Additionally, they can be used in a wide range of applications, including treating acid mine drainage, agricultural water (field run-off and livestock operations), storm water run-off, domestic wastewater and industrial wastewater. Legge’s work to date at Waterloo has focused primarily on treating metal-containing wastewater (iron and chromium) and storm water run-off. Iron can accompany organic pollutants; chromium can be found in tanning factory effluent; storm water and agricultural run-off may contain pathogens such as E. coli and Cryptosporidium.

The mystery of exactly how a natural system such as a wetland purifies water isn’t easy to unravel. “The problem with wetlands is that we don’t understand the fundamentals too well,” says Legge. “In terms of constructed wetlands, the climate must be conducive and it must be simple to operate.” Dr. Legge’s current studies will yield information on the relative importance of various removal mechanisms and the ecological influences on their performance.

Researchers are using indoor systems where factors such as temperature, precipitation, and light levels can be controlled. According to Legge, “A field-scale system is preferable to a pilot-scale system, since it avoids difficulties commonly encountered with design scale-up. A robust design is specified in order to allow for future research efforts to be conducted using the same experimental system.”

To do these experiments, the team designed and built “core units,” or “mesocosms” — the smallest units within a subsurface constructed wetland. In one study, wastewater containing nutrients and sodium bromide tracers was passed through mesocosm columns containing pea-gravel medium. Planted, unplanted and abiotic (all biological organisms removed) models were compared.

Every 3.5 days, the mesocosms were fed a synthetic wastewater mixture of molasses, urea, sodium phosphate, magnesium sulphate, potassium carbonate and yeast extract, and once a month, an injection of 1% activated sludge from the Waterloo Regional Wastewater Treatment Plant was added. Legge and his colleagues have so far found that re-aeration did not significantly affect the rates of removal for abiotic contaminants such as metals. The presence of plants did not seem to affect the rates of removal of pathogens or contaminants either. However, they found that higher bioactivity (presence of bacteria, microbes, etc.) had a significant effect on the removal of pathogens.

More study of the plant aspect is in the works.

Treena Hein, B.Sc. is a science writer based in Pembroke, Ontario.