3 Alternative Nutrient-Removal Techniques

With a little help from water-quality managers, Mother Nature can do the job of remediating impaired or at-risk watersheds, while making them more resilient to nutrient loading.

The U.S. EPA’s “watershed approach” to pollution management takes (some of) the pressure off wastewater treatment plants (WWTPs) — though that is not the specific intent — in favor of a more holistic approach to nutrient management. Instead of focusing strictly on WWTP discharge limits, this approach considers the role of nonpoint sources, such as agricultural runoff, and the impact that nutrient loads have on the overall health of the watershed. The EPA acknowledges that temporal and spatial elements — when and where nutrients originate — are difficult to resolve. Therefore it might benefit water-quality managers (and satisfy regulators) to make the watershed itself more resilient to nutrients it receives.

Researchers Kurt Stephenson and Leonard Shabman, with partial funding and support from the EPA, presented a report during last year’s Southern Agricultural Economics Association Annual Meetings that describes three effective methods of in-water “nutrient assimilation.” The following methods can be used to remediate impaired waters back to health, and they may also be counted as load reductions in calculating a watershed’s total maximum daily load (TMDL) — the numeric limit on the amount of nitrogen and phosphorus that a watershed can handle.

Managed Wetlands

Wetlands are efficient absorbers of nutrients, especially for nitrogen — important in that nitrogen is more expensive for WWTPs to remove than phosphorous. Stephenson and Shabman cite a study in the Midwest that showed nutrient assimilation wetlands to remove 274 pounds of nitrogen and 24 pounds of phosphorus per acre. The wetlands work by holding water in place and collecting nutrient-rich sediment, which is then absorbed into the roots of its plants. Managed wetlands optimize nutrient uptake by actively managing the volume and timing of the water flow, as well as the type of vegetation that works best. Landowners can be incentivized to construct wetlands through payments by dischargers purchasing nutrient credits, including WWTPs — possibly a less expensive solution to meeting nutrient effluent requirements than plant upgrades.

Shellfish Aquaculture Enhancement

Oysters have also proven their value as natural nutrient consumers. Technically, it’s phytoplankton that consumes the nutrients, and the oysters feed on the phytoplankton. The nutrients passed onto the oysters eventually become part of their shells — a process called biosequestration. Boston reintroduced oysters to the Charles River to help remediate the formerly embattled waterway from a “D” to a “B+” (as graded by the EPA).

The shellfish (mussels or clams can also be used) are harvested by aquaculture — spawned in hatcheries and reared in upwellers. When introduced to an impaired waterway, they are strategically placed and managed so as not to disrupt wild oyster populations.

The practice has recently been suggested for remediation of the Potomac River and the Chesapeake Bay. Researchers from the Virginia Commonwealth University (VCU) conducted a test study in the Chesapeake, finding that eight large-scale oyster farms (1 million oysters each) could remove a ton of nitrogen.  Though it isn’t feasible that oysters alone could remediate the Bay, the researchers concluded that oyster aquaculture outperforms many other nonpoint source controls, namely agricultural best management practices (BMPs), and has the additional benefits of “enhancing public awareness of water quality issues, shifting attitudes toward stewardship, and stimulating local economies.”

Aquatic Plant Biomass Harvests

There are many ways to describe a biomass of aquatic plants set up for nutrient removal — managed aquatic plant systems (MAPS), floating wetlands, floating plant treatment systems, artificial floating meadows are but a few. They also vary in types of vegetation, which includes multiple species of microalgae, macro algae (seaweed), and aquatic plants (e.g. water hyacinths). Finally, there are locational, or style, differences at play; they can be planted within the impaired waterway, in an outgrowth area where water can be diverted and returned, or on an engineered plot floating on the surface.

A number of research studies have examined the efficacy of MAPS, and the outlook is positive. One study showed a large-scale MAPS operation in Florida to remove 1,300 kilogram/hectare (km/ha) of nitrogen and 330 kg/ha of phosphorus utilizing algal turf scrubbers — flat surfaces covered with an engineered geomembrane to harvest algae. Another study indicated that seaweed grown for food production removed “thousands of metric tons of nitrogen.”

One of the latest, high-profile forays into MAPS is a pilot project using of red seaweed (Gracilaria) to remove nutrients in New York’s Long Island Sound, which is benefiting from mussel aquaculture as well.

All-Of-The-Above Approach

Much like President Obama’s “all of the above” platform for energy production, resolving the problem of excessive nutrient loading may require a multipronged approach. Nutrient assimilation should be part of the conversation — especially as TMDLs get ever more stringent. Lower nutrient effluent limits mean dramatically higher costs for WWTPs, according to the National Association of Clean Water Agencies (NACWA). Citing a cost study for enhanced phosphorus removal at a 10-MGD WWTP discharging into the Chesapeake Bay, the study noted the following financial impact:

“As effluent target concentrations are reduced, the unit costs per pound of phosphorus removed increases substantially. Costs for enhanced levels of phosphorus removal with chemical addition and filtration for effluent phosphorus of approximately 0.5 mg/L quadruple (~13 $/lb). As effluent limits approach the limits of treatment technology in the range of 0.05 to 0.10 mg/L, additional filtration steps and/or membranes are required. The unit costs per pound of phosphorus removed increases from base nutrient removal levels by an order of magnitude (~25 to 37 $/lb).”

A report published by Environmental Science & Technology detailed similar findings for nitrogen:

“As limits of technology are approached, nitrogen reductions at point sources become increasingly more expensive. Costs of nitrogen removal at WWTPs in the Connecticut River Basin have been estimated to increase from $12 per pound at 8 mg/L total nitrogen discharged to $14 per pound at 5 mg/L total nitrogen discharged to $37 per pound at 3 mg/L total nitrogen discharged. It is not unusual for costs to upgrade individual plants to a higher level of nitrogen reduction to run into the tens and hundreds of millions of dollars.”

Individual WWTPs have varying infrastructure, environmental, and effluent-limit requirements, so treatment costs will certainly vary. The same is true for in-water solutions such as managed wetlands, shellfish aquaculture enhancement, and aquatic plant biomass harvests, though the logistics affecting cost may be different. The point is to share the burden (and perhaps the cost) of nutrient removal. An “all of the above” approach that considers nutrient assimilation alongside point-source treatment, stormwater management, and nonpoint BMPs gives municipalities — and waterways — a fighting chance against excess nutrients and toxic algal blooms.