Chapter 2 Activities
2. Septic and other Onsite Sewage Treatment & Disposal Systems
Introduction
Onsite sewage treatment and disposal systems (OSTDS) are designed to treat and dispose of effluent on the same property where the wastewater is generated. Septic systems are the most common type of OSTDS in Florida, consisting of a collection tank connected to a shallow underground drainfield (Figure 2.1). Throughout this chapter, the term “septic system” is used to refer generally to OSTDS.
Figure 2.1. Traditional domestic onsite sewage treatment and disposal system, or septic system, showing the flow path for wastewater from an individual home into a septic tank then into a leaching field and groundwater. Source: EPA
Properly designed, sited, installed, operated, and maintained septic systems can effectively reduce pathogens and nutrients. Critical siting factors include maintaining adequate distance from surface waters, providing enough separation between drainfields and groundwater, and ensuring suitable hydrogeologic conditions that support nitrogen removal. These factors are essential to prevent nitrate pollution in surface waters, groundwater, and springs.
Conventional septic systems typically reduce about 30% to 40% of nitrogen inputs (Toor et al., 2011b). In these systems, organic nitrogen is converted to ammonia, then to nitrate. Further denitrification may occur if the leachate passes through slow-moving, anaerobic, carbon-rich soils where natural denitrifying bacteria can convert nitrate to harmless nitrogen gas. Newer designs incorporate engineered environments that enhance this process, including in-ground nitrogen-reducing biofilters, nitrogen-reducing aerobic treatment units, or Performance-Based Treatment Systems, which can reduce more than 90% of nitrogen inputs (FDOH 2019).
The effectiveness of a septic system can also depend on its age. Florida law currently requires at least a two-foot vertical separation between the drainfield bottom and the seasonal high water table; Sarasota County requires three feet. Before 1983, only a one-foot separation was required. Flooded drainfields compromise treatment by allowing pathogens to contaminate groundwater and eliminating the aerobic zone required for nitrification. For this reason, it is especially important to assess the nitrogen impact potential of older systems in coastal and waterfront areas.
Requiring advanced systems for new installations and incentivizing upgrades of older systems can help reduce nutrient pollution to nearby waterways. In areas with failing septic systems and available central sewer infrastructure, connecting to sewer service is often the most effective solution.
Activity 1:
Quantify Annual Nutrient Loads from Septic Systems
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.
Activity 2:
Convert or Upgrade Underperforming or Failing Septic Systems in Priority Areas
Underperforming or failing septic systems can impact water quality of priority waterbodies. Replacing these systems with central sewer service can protect and improve water quality. Where septic-to-sewer conversion is not feasible or is cost-prohibitive, replacing or upgrading systems with advanced technologies can also deliver water quality benefits.
Activity 3:
Construct Denitrification Barriers in Priority Areas Where Septic System Upgrades or Conversions to Central Sewer are Unlikely
Denitrification barriers installed along the edge of priority water bodies can intercept and reduce nitrates from septic systems carried by groundwater.
Activity 4:
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 ensure they function properly to protect public health and the environment.
Activity 5:
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.
