2.3 Construct denitrification barriers in priority areas where septic system upgrades or conversions to central sewer are unlikely

Chapter 2. Septic Systems
Science Solutions
Activities to Reduce Nutrient Inputs

Key Message: Denitrification barriers installed along the edge of priority water bodies can intercept and reduce nitrates from septic systems carried by groundwater.

Importance

Septic systems can contribute nutrients, including nitrogen, to nearby water bodies. In Sarasota County, nitrogen—particularly in the form of nitrate—is the nutrient of greatest concern. In areas where septic systems are underperforming or failing, and upgrades or conversions are unlikely to occur in the near term, constructing a denitrification barrier between the polluted groundwater plume and a priority water body can intercept and reduce nitrates before they enter surface waters.

Overview

Denitrification barriers typically consist of a trench filled with carbon-rich material like sawdust or woodchips that intercepts nitrate-contaminated groundwater (Figure 2.3.1). Denitrifying bacteria in the trench use the carbon source to convert nitrate in the groundwater to nitrogen gas, which is released into the atmosphere. This process can be a cost-effective solution in locations where septic systems cannot be feasibly upgraded to enhanced systems or connected to central sewer service.

diagram of septic system
diagram of a septic tank with denitrification barrier

Figure 2.3.1 Septic system with additional nitrogen treatment in the flow pathway from tank to leaching field (top) and in the flow pathway from groundwater to receiving waterbody (bottom). Source: EPA

A 600-foot network of denitrification barriers constructed at The Bay Park in Sarasota serves as a proof of concept. The system removes nitrates from more than 15 million gallons of stormwater annually that flow through the 53-acre park before entering the bayou and Sarasota Bay. Research into performance of denitrification barriers is also underway in Lakewood Ranch.

denitrification trench at the Bay

Installation of a denitrification barrier at The Bay Park in Sarasota. Source: The Bay Park Conservancy

Approach

Constructing a denitrification barrier adjacent to a water body that receives groundwater from septic system areas costs approximately $100 per linear foot. The cost-effectiveness and return on investment (ROI) of this intervention depends on site-specific factors such as the length of the barrier, the contributing area, and the number of upstream septic systems it serves.

To illustrate the variability in ROI, two areas along the Siesta Waterway in South Venice were evaluated. This waterway flows into Alligator Creek and is bordered by neighborhoods served by septic systems (Figure 2.3.2).

satellite image of Siesta waterway

Figure 2.3.2. Siesta Waterway flows into Alligator Creek in South Venice, and the surrounding neighborhood is served by septic systems. Denitrification barriers could be installed along the northeast and southwest banks to intercept and treat nutrients in groundwater flowing from septic fields. Arrows indicate groundwater and surface water flow direction. Black dots indicate sample stations with measurements of total nitrogen (TN) in mg/L. Stations in red indicate nitrogen levels exceeding state water quality standards. Source: Water quality data from Sarasota County Government.

The northeast side would require 4,685 linear feet of denitrification barrier at an estimated cost of $468,500. This barrier would serve an estimated 292 septic systems. Using a 60% nitrate removal efficiency and annual nitrate nitrogen loads of 6.9 and 13.8 pounds per system (representing 50% and 0% natural denitrification, respectively), the cost per pound of nitrogen removed would be approximately $36.65 and $17.83, respectively.

The southwest side would require 4,748 linear feet of barrier at an estimated cost of $474,800, serving about 669 septic systems. Using the same assumptions, the cost per pound of nitrogen removed would be approximately $14.15 and $7.08, respectively.

As an interim nitrogen-reduction strategy in areas where septic system upgrades are unlikely, Sarasota County could consider denitrification barriers between polluted groundwater plumes and priority waterbodies.

Resources

  • Sarasota County Department of Health
  • Sarasota County Government Stormwater Utility

Status

Denitrification trench at The Bay Park, Sarasota is installed and functioning
Research into denitrification trenches at Lakewood Ranch is ongoing

Performance Measure

Optimized return on investments to maximize nitrogen reduction from septic systems

Experts or Leads

Damann Anderson, Stephen Suau, Dave Tomasko, Pio Lombardo

Cost Estimate

$100,000–$1,000,000

Related Activities

Chapter 1.2, Chapter 2.1, Chapter 2.2, Chapter 2.4, Chapter 2.5

 

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Other Septic System Activities

2.1 Quantify annual nutrient loads from septic systems

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