Onsite sewage treatment and disposal systems (OSTDS) are designed to treat and dispose of effluent on the same property that produces the wastewater. Septic systems are the most common type of OSTDS in Florida and consist of a collection tank connected to a shallow underground drainfield (Figure 2.1). Hereafter, the term septic system will be used to generally refer to OSTDS.
Figure 2.1. Traditional domestic onsite sewage treatment and disposal system, or septic system, showing the flowpath for wastewater from an individual home into a septic tank then into a leaching field and groundwater. Source: EPA
Septic systems can provide effective treatment of pathogens and nutrients if properly designed, sited, installed, operated, and maintained. Appropriate siting to ensure adequate distance from surface waters, adequate depth between drainfields and groundwater, and suitable hydrogeologic conditions for nitrogen removal is critical to preventing nitrate pollution of surface water, groundwater, and springs.
Conventional septic systems typically reduce about 30% to 40% of nitrogen inputs (Toor et al., 2011b) between the tank and drainfield where organic nitrogen is converted to ammonia then nitrate. Additional denitrification may occur naturally if the drainfield leachate flows slowly through anaerobic carbon rich soils that allow natural denitrifying bacteria to convert nitrate to harmless nitrogen gas. Newer septic designs incorporate this denitrifying environment into the septic system using advanced technologies such as in-ground nitrogen-reducing biofilters, nitrogen-reducing aerobic treatment units, or nitrogen-reducing Performance-Based Treatment Systems, thereby reducing 90% or more of nitrogen inputs (FDOH 2019).
The age of the septic system can also influence its effectiveness. Current Florida law requires a protective depth of two feet between the bottom of the drainfield and the seasonal-high water table; Sarasota County Government requires three feet. Prior to 1983, only one foot was required. If groundwater floods the septic drainfield, in addition to pathogens contaminating groundwater the aerobic treatment layer critical for processing ammonia to nitrate can be reduced or eliminated. Therefore, understanding and addressing potential nitrogen impacts from septic systems installed in coastal and waterfront areas before 1983 may be particularly important.
Requiring advanced systems on new permitted installations and encouraging upgrades of older conventional systems with advanced technologies will help reduce nutrient pollution to nearby waterways.
Quantify annual nutrient loads from septic systems
Properly sited and maintained conventional septic systems can reduce about 30% to 40% of nitrogen inputs. But all septic systems are not equal in their potential to impair surface water quality. Many factors affect nutrient loading from septic systems, including design, age and size; soil types and distance from drainfields; and how well the system has been maintained. Quantifying localized nutrient loads will help to prioritize areas most in need of upgrades or conversions to central sewer.
Quantify cost and effectiveness of nutrient reduction options for septic systems
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.
Improve knowledge about the location and status of septic systems and prioritize areas for upgrades or conversions
There are conflicting estimates of the number of septic systems in Sarasota County. Clarifying how many septic systems exist, where they are located, and their functional status is fundamental to managing their potential environmental impacts.
Require periodic inspection and maintenance of septic systems and develop incentive programs to facilitate compliance
Once a septic system is approved in Florida, ongoing inspection or maintenance is not required. Because septic systems can deteriorate over time, legislation that requires periodic inspection and maintenance is essential to ensuring they function properly to protect public health and the environment.
Deliver Targeted Education and Incentives to Improve Operation and Maintenance of Septic Systems and Encourage Upgrades for Enhanced Nutrient Treatment
Regular septic system maintenance is economical compared to other costs of home maintenance and to central sewer fees. Education is needed to encourage proper maintenance of systems, along with incentives to upgrade to newer technologies with improved nutrient reduction.