Septic tanks in South Florida are a source of land-based pollution, adding excess nutrients to our waterways when not properly cared for.
How Septic Systems Work
Septic systems are used in approximately one-third of all homes in Florida. This serves as an on-site wastewater treatment system where access to sewers is not available .
Water from the household runs through a drainage pipe into a septic tank, a watertight container buried underground. The tank holds the wastewater long enough for the solids to settle to the bottom of the tank and the grease and oil floats to the top as scum . In the septic tank, microorganisms go through the process of anaerobic digestion, producing gases such as methane, hydrogen sulfide, and sulfur dioxide. When the septic tank is pumped, the sludge will be taken to a municipal sewage treatment plant or landfill where it will readily decompose when exposed to oxygen and aerobic bacteria.
The primary effluent, liquid waste or sewage, will then move from the septic tank to the drainage field. Biological mats form around the field and assist with the biological filtration of the effluent before it passes into the soil and ultimately, the environment. The effluent is discharged through pipes onto these porous mats that allow the wastewater to filter through to the soil while further being treated by organisms living in the mats .
As the wastewater percolates through the soil, dissolved components continue to be digested by bacteria in the soil. Harmful coliform bacteria, viruses, and nutrients are all removed from the wastewater as it flows through the soil. The soil is able to filter out some of the nutrients that are present in the effluent, but not all. Nutrient-rich elements such as nitrogen and phosphorus can still be present after the wastewater has been through all of these filtering processes.
Septic in Miami-Dade
Due to climate change, rainstorms and rising sea levels are an impending threat to groundwater levels and septic tanks. Ground saturation from rainwater and rising sea levels can push the groundwater upward, with nowhere to go except back into the septic system. This can cause a system failure and the contents of the septic tank to come out from drains inside the home. This is especially common in areas that are low-lying and close to the ocean.
Soils in Southern Florida are relatively unsuitable for septic tanks because they consist of porous materials. Bacteria and microorganisms in the soil provide the majority of wastewater treatment in septic systems. The wastewater leaving septic systems and ultimately entering into groundwater and coastal waters contains nitrogen and phosphorus; nutrients that cause eutrophication and can destroy marine life and cause harmful algal blooms .
Regular maintenance of septic units is crucial to keep the systems functioning properly. Every few years, tanks need to be pumped to remove the solid sludge that builds up. If a tank is too small or has too much solid material built up, large particles can move from the tank to the drain field and clog the soil . If the pipes do not become blocked, the clogged soil prevents wastewater from draining downward and the soil above the drain field will become saturated. The water above the drain field has not been treated properly and may contain raw human sewage.
Environmental Impacts of Septic Systems
If the soil becomes blocked and wastewater containing raw human sewage saturates the soil above the drain field, this wastewater can reach a stream or storm drain, both directly and by rainwater runoff, where it then can travel to coastal waters.
Nitrogen and phosphorus are naturally occurring nutrients vital to aquatic ecosystems. These nutrients support the growth of algae and aquatic plants, providing food and habitat for small organisms in the water. These organisms then become a food source for larger predators, including animals used for human consumption.
Excessive amounts of nitrogen and phosphorus cause algae to grow faster than the ecosystem can sustain. These algal blooms can reduce or eliminate the amount of oxygen in the water while also blocking the amount of sunlight penetrating the water’s surface. This can negatively impact a wide variety of species. Some algal blooms are harmful to humans as well. The blooms produce toxins that can cause sickness and neurological damage if contact is made with polluted water, tainted fish or shellfish, or contaminated drinking water. Algal blooms can also kill marine mammals and shorebirds that rely on fish as a food source .
Algal blooms also can cause hypoxic areas or “dead zones.” These are areas with little to no oxygen where aquatic life cannot survive, and they often occur after an algal bloom when the excessive amounts of algae and other organisms die from lack of oxygen and nutrients. As the algae dies and sinks to the bottom, oxygen is able to return to the system, but the bacteria that are decomposing the algae consume most of it . The continued lack of oxygen results in a dead zone.
What Can You Do to Lessen the Impact?
Ideally, all properties would be connected to the sanitary sewer system. The system would need additional underground pipes and maintenance to make this happen. Until this can be done, there are ways to keep your septic system running efficiently and properly maintained to prevent leaks and contamination of storm drain water.
- Have the system pumped out and inspected by a professional every three years
- Grow grass or small plants above the system to hold the drain field in place
- Ensure that the system is installed so that rainfall and surface water flow away from the system
- Do not put grease or non-biodegradable materials down the toilet or sink
- Do not flush paint thinners, polyurethane, anti-freeze, pesticides, disinfectant, water softeners or other strong chemicals
- Do not flush indigestible materials such as diapers, cigarette filters, feminine products, cat litter, or plastic
- Do not plant trees within 30 feet of your system or park/drive over the system
- Do not do all machine washing in one day, this could overwhelm the system with excess wastewater 
- Septic Tanks/ Sanitary Sewer System
- How Your Septic System Works
- Sources of Nutrients in Biscayne Bay
- Septic System Impacts
- Nutrient Pollution
- Dead Zones
- House Septic System Installation