3 Alternative Nutrient-Removal Techniques

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.”

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Taxonomy

• Aquaculture
• Treatment
• Wetlands