2.1 Quantify annual nutrient loads from septic systems

Key Message: Properly sited and maintained septic systems can reduce 30% to 40% of nitrogen inputs. However, system design, age, maintenance, and site-specific hydrogeology significantly affect nutrient loading. Quantifying localized nutrient loads is essential to prioritize upgrades or conversions to central sewer.

Importance

The performance of septic systems depends on multiple site- and system-specific factors:

• Appropriate design and sizing
• Functional condition of the tank and drainfield (maintenance history, age, operation)
• Soil denitrification potential and seasonal depth to groundwater
• Soil denitrification potential and proximity to surface waters

As a result, septic systems vary widely in their potential to impair water quality. Quantifying nutrient loads under different local hydrogeologic conditions is key to prioritizing areas for system upgrades, sewer conversions, and nutrient management planning.

Overview

The Florida Department of Health (FDOH) estimates that about 30% of Florida’s population relies on 2.6 million septic systems. FDEP implements Florida Statutes and regulations applicable to septic systems. Sarasota County Health Department manages permitting and inspection (see Chapter 2.4). Regulations govern siting, installation, and evaluations.

The County has approximately 44,989 septic tanks, with 22,204 (49%) located within the County Public Utilities service area. FDOH data show that approximately 400 septic systems (1.1%) are repaired annually. Improved data collection and metadata are needed to produce reliable nitrogen load estimates and support planning decisions (see Chapter 2.2).

Approach

To estimate nutrient loading, both the number of septic systems and their average nutrient contributions must be known. For demonstration purposes, an estimate of 35,000 septic system parcels in Sarasota County was used, with loading rates based on FDOH and U.S. EPA data (Figure 2.1.1). Assuming an average household size of 2.2, nitrogen and phosphorus contributions are estimated at 19.8 pounds and 4.8 pounds per household per year, respectively. Septic systems that are failing or not functioning properly could be considered at or approaching the average gross nutrient loading for estimating purposes.

FDOH estimates that a properly constructed and functioning septic system can remove between 30% and 40% of the gross nitrogen load (FDOH 2015). This level of nitrogen reduction takes place primarily in the drainfield system where ammonia nitrogen is quickly and readily converted to nitrate through nitrification. However, treatment efficiency depends on many factors including maintenance regimes, the age of the system and most importantly, whether natural denitrification is likely to occur downstream before the effluent reaches a water body. While there is evidence that some areas in Sarasota County may be capable of providing as much as 90% reduction in nitrogen loads from septic systems due to natural denitrification downstream (Suau, unpublished data), a 30% reduction was utilized as a conservative assumption for these calculations. A 30% reduction equates to a net loading of 13.8 pounds per household per year or 483,000 pounds per year county-wide. A 90% reduction would equate to 1.98 pounds per household per year or 69,300 pounds per year county-wide.

A properly constructed and functioning septic system removes about 97% of the gross phosphorus load (Mechtensimer et al., 2017). This equates to a net loading of 0.14 pounds of phosphorus per household per year or 5,040 pounds per year county-wide.

To identify the potential for natural denitrification, water quality data collected monthly over the past six years and published by Sarasota County Government were reviewed for areas predominantly served by septic systems (Table 2.1.1 and Figure 2.1.2).

These data reveal elevated nitrogen levels in some areas, potentially linked to septic system discharges or legacy wastewater treatment loads. In other areas where septic systems are prevalent, yet water bodies remain within water quality thresholds—such as Siesta Ditch, where 100% of parcels are on septic systems—natural denitrification may be reducing nitrogen by as much as 90% (Suau, unpublished data). Understanding localized hydrogeology is essential to inform investment and policy decisions, including in areas using reclaimed water for irrigation (see Chapter 1.2).