Fact Sheets

Click the titles for complete EPA resources that document each of the following topics and their role in onsite wastewater treatment systems.


Because of the presence of significant numbers and types of bacteria, enzymes, yeasts, and other fungi and microorganisms in typical residential and commercial wastewaters, the use of septic system additives containing these or any other ingredients is not recommended.

aerobic systems

The activated sludge process is an aerobic suspended-growth process that maintains a relatively high population of micro-organisms (biomass) by recycling settled biomass back to the treatment process. The biomass converts soluble and colloi-dal biodegradable organic matter and some inorganic compounds into cell mass and metabolic end products.

Disinfection Process

The process of disinfection destroys pathogenic and other microorganisms in wastewater. A number of important water-borne pathogens are found in the United States, including some bacteria species, protozoan cysts, and viruses. All pre-treatment processes used in onsite wastewater management remove some pathogens, but data are scant on the magnitude of this destruction.

fixed film process

Fixed-film systems (FFS) are biological treatment processes that employ a medium such as rock, plastic, wood, or other natural or synthetic solid material that will support biomass on its surface and within its porous structure.

Holding tanks/hauling systems

A holding tank or vault receives wastewater from a home or commercial establishment and stores it until it is pumped out and hauled to a receiving/processing facility. Although similar to septic tanks, vaults have no outlet piping and must be watertight.

Intermittent sand filters

The term intermittent sand filter (ISF) is used to describe a variety of packed-bed filters of sand or other granular materials available on the market. Sand filters provide advanced secondary treatment of settled wastewater or septic tank effluent.

Phosphorus removal

There are a large number of processes that can reduce nitrogen and a few that can reduce phosphorus. Most of these phosphorus removal processes are additions to other pretreatment processes that enhance the overall removal of phosphorus

Nitrogen removal

Nitrogen is a pollutant of concern for a number of reasons. Nitrogen in the ammonia form is toxic to certain aquatic organisms. In the environment, ammonia is oxidized rapidly to nitrate, creating an oxygen demand and low dissolved oxygen in surface waters.

Recirculating sand filter

Recirculating filters using sand, gravel, or other media provide advanced secondary treatment of settled wastewater or septic tank effluent.

Sequencing batch reactor

The sequencing batch reactor (SBR) process is a sequential suspended growth (activated sludge) process in which all major steps occur in the same tank in sequential order.

Water softeners

Home water softeners, which periodically generate a backwash that is high in sodium, magnesium, and calcium concentrations, can affect wastewater treatment processes and the composition and structure of the infiltration field biomat and the underlying soil.

High organic wastewater

Because many onsite treatment alternatives are sensitive to organic loading rate, high-strength wastewaters may require additional treatment steps to ultimately meet environmental discharge or reuse goals.