2.2 Quantify cost and effectiveness of nutrient reduction options for septic systems

Key Message: Understanding and quantifying the life-cycle costs associated with varying nitrate-removal techniques for varying site characteristics will inform local decision-making about existing and future septic system upgrades or conversions.

Importance

Septic systems can contribute nutrients, including nitrogen, to water bodies. In Sarasota County, nitrogen in the form of nitrate is the nutrient of greatest concern. Options to mitigate nitrate include converting septic systems to central wastewater treatment systems, upgrading septic systems with a denitrification system, or installing a denitrification barrier along the water’s edge to intercept and reduce nitrates from septic systems carried by groundwater (Figure 2.2.1). Understanding and quantifying the life-cycle costs of these options is important to assure that investments in nutrient management are cost-effective.

Overview

A recent study directed by the Florida Legislature refined and calibrated a nitrogen fate and transport model for septic systems, and tested nitrogen-reducing technologies (FDOH 2015, Hazen and Sawyer 2015). Two-stage passive biofiltration septic systems were designed and tested at existing Florida homes to assess their capacity to improve nitrogen removal over conventional systems. In the first stage, organic nitrogen from wastewater was converted to ammonia then nitrate by passively aerating it as it moves through unsaturated media. In the second stage, nitrate was denitrified to nitrogen gas. The saturated biofilters in the second stage contained reactive media, such as sawdust and sulfur, to assist with denitrification. When deployed to test sites at existing homes in Florida, most of the experimental septic systems reduced nitrogen by 90–95% prior to their effluent arriving at the drainfield. The study also documented installation, operating, and maintenance costs to allow comparisons of life-cycle costs between field-tested systems.

Approach

To demonstrate the recommended approach, an assessment was conducted of life-cycle costs associated with options to mitigate estimated nitrate loads from septic systems in Sarasota County. This assessment was based on the following assumptions and limitations:

• There are 35,000 septic systems in Sarasota County (see Chapter 2.3).
• Based upon an average annual net load of 8 pounds per septic system, the average annual nitrate load is 483,000 pounds, excluding failures and natural denitrification (see Chapter 2.1).
• Natural denitrification accounts for 0-50% of nitrate reduction for septic systems in Sarasota County, although actual TN reduction may be as high as 90% (Suau, unpublished data).
• Return on investment (ROI) of nitrate mitigation options were calculated as the ratio of the annual pounds of nitrogen reduced and the annualized life-cycle cost. Life-cycle costs were estimated as the sum of capital costs annualized at 5.5% over 20-years and annual operation/maintenance costs.
• The cost per unit to convert septic systems to central sewer is $15,833.33 plus an estimated $2,800 associated with digging/installing a sewer service line as well as draining and decommissioning the septic tank.
• The premium cost of incorporating a gravity denitrification system into a septic system replacement is $10,000 per unit.
• The cost per unit to construct a denitrification barrier adjacent to a water body that receives groundwater from an area served by septic systems is $100/linear foot.

Option 1 – Conversion of OSTDS to Central Wastewater System

Based on the assumptions above, the cost per unit for converting a septic system to central sewer would be $18,633. Bracketing the annual nitrogen load between 6.9 pounds and 13.8 pounds (assuming 50% and 0% natural denitrification, respectively) would result in a ROI between $2,729.40 and $1,364.73 per pound of nitrogen removed, respectively. In addition, the estimated ROI per unit to upgrade central sewer service to AWT level treatment at an assumed 69% reduction for 6.9 and 13.8 pounds of TN is $768.57 and $1,537.14, respectively. Therefore, the total ROI for conversion of septic systems to central sewer is between $3,487.97 to $2,901.87/pound TN reduced.

Option 2 – Incorporation of denitrification system into septic systems when replaced

The estimated premium cost for installing an advanced denitrification system into new or replacement septic systems is $10,000.00. Bracketing the annual influent nitrate nitrogen load between 6.9 and 13.8 pounds (assuming 50% and 0% natural denitrification, respectively) with a 95% removal efficiency from denitrification would result in a ROI per advanced septic system between $1,525.55 and $762.78/pound TN reduced.

Option 3 – Construction of Denitrification Barriers

The cost-effectiveness and ROI of installing denitrification barriers is site specific. Factors include the length of the denitrification barrier, contributing area, and the number of upstream septic systems served. To illustrate the variable ROI of this option, two areas served by septic systems along the north and south sides of the South Venice Siesta Waterway flowing into Alligator Creek were considered.

First, the area northeast of the Siesta Waterway would require 4,685 linear feet of a denitrification barrier at an estimated cost of $468,500. This denitrification barrier would serve an estimated 292 septic systems. Bracketing the annual unit nitrate nitrogen loads of 6.9 and 13.8 pounds (assuming 50% and 0% natural denitrification, respectively) and a 60% removal efficiency would result in an annual ROI for this denitrification system of $36.65 and $17.83/pound TN reduced, respectively.

Second, the area southwest of the Siesta Waterway would require 4,748 linear feet of denitrification barrier at an estimated cost of $474,800. This denitrification barrier would serve an estimated 669 septic systems. Bracketing the annual unit nitrate nitrogen loads of 6.9 and 13.8 pounds (assuming 50% and 0% natural denitrification, respectively) and a 60% removal efficiency would result in a ROI for this denitrification system of $14.15 and $7.08/pound TN reduced, respectively.

The Clean Waterways Act (2020) creates a wastewater grants program within the Florida Department of Environmental Protection that provides a 50% match to local funds for upgrading septic systems with advanced nutrient removal technologies and connecting septics to central sewer facilities.