Greener Water Filters
Another way to remove pollutants from stormwater runoff is to divert the water into troughs that contain filters made from fiber modified to absorb particular contaminants before the water enters a stream, pond, or other water body. Available at low cost and biodegradable, residue fibers from wood processing would seem the perfect, sustainable choice for stormwater filters.
Unfortunately, when it comes to some pollutants, wood alone is not enough. To work effectively on the phosphates that stream down the storm drains of our urbanized environments, wood fibers need to be modified. In many cases, the suggested modifications use toxic chemicals and multistep processes that increase costs and produce new waste streams. Some options also interfere with biodegradability. To treat phosphates, it seems you have to turn a sustainable solution into yet another problem.
Tom Eberhardt, research scientist with the SRS Utilization of Southern Forest Resources unit in Pineville, LA, is working with university and industry scientists to solve this sustainability conundrum. “Our strategy is to test environmentally benign treatments that increase the capacity of wood fiber filters to trap phosphates, but do not interfere with biodegradability,” says Eberhardt.
In one experiment, Eberhardt and fellow researchers pretreated aspen fibers with iron salts, which have been shown to remove phosphates from water when used on other types of filters. The problem—wood fibers have a limited capacity to retain iron. To provide additional sites to retain iron, the fiber was treated with water containing a nontoxic cellulose derivative. After the fibers were dried, they were soaked in an iron salt solution and then tested for the capacity to remove phosphates from water. In preliminary experiments, the treated fibers were able to remove phosphates from test solutions, while untreated fiber samples showed no removal capacity. “The results are very encouraging,” says Eberhardt. “This process may provide an easy, cost-effective, and sustainable way to improve the ability of wood fiber filters to clean stormwater.”