The objective of an on-site wastewater treatment system is to treat wastewater and return it to the environment so that risks to health are not created, the impact on ground and surface water is minimized, and the environment is not harmed.
On-site wastewater treatment systems use a septic tank or treatment plant for initial treatment. The household plumbing collects waste water and sends it to the septic tank working compartment which acts as a separation chamber. Heavy particles separate from the wastewater and settle to the bottom to form a sludge layer. Lighter particles, mainly soap and grease, separate and float to the top to form a scum layer. Using a baffle device the clearest liquid from the center of the tank flows by gravity to the effluent dosing chamber. A pump or siphon in the effluent dosing chamber will deliver the effluent to the final soil treatment component.
There are other options for initial treatment components, including manufactured package sewage treatment plants, textile filter systems and incineration devices. These components, like a septic tank, will receive all the wastewater generated by the facility they serve. These systems will produce cleaner effluent and are considered to be advanced treatment systems. Cleaner effluent does not clog the soil pore spaces of the final treatment component as easily as septic tank effluent.
All initial treatment devices reduce the amount of organic material, dirt, grease, etc. However, disease-causing organisms ( pathogens ) are not destroyed by initial treatment and it’s bacterial action. In an initial treatment, methane gas and hydrogen sulfide gas are produced in the septic tank.
The sludge and scum separated out in the initial treatment stage will need to be pumped out on a regular basis.
The final treatment and recycling of effluent back to groundwater occurs in the soil. Natural processes and soil bacteria will remove or alter the pollutant and pathogens in wastewater. With suitable soil and adequate separation distances from water tables this effluent will safely return to the groundwater.
The soil treatment of effluent occurs mostly through the action of aerobic bacteria. Aerobic bacteria require oxygen if they are to do their job. In treating sewage they must have food and water (effluent), are to breathe and suitable environment in which to live (soil). Thus a sufficient depth of suitable soil is needed to allow the use of treatment field trenches to return effluent to the soil. The Alberta standard of practice requires 5 feet of this suitable soil below the bottom of the treatment field trench for septic effluent and 3ft for class 1 plant or advanced treatment effluent. Treatment field trenches are typically 2 ft wide by 2 ft deep and will likely total more than 400 lineal feet. In areas not having the required depth of suitable soil, imported suitable fill material will be needed to construct a treatment mound or if adequate forested areas are on site an LFH At-grade system may be used.
Pressure distribution is preferred for all systems. A pressure distribution system is a carefully designed network of small pipes with small holes called orifices, connected to a pump. Periodically the pump sends doses of effluent under pressure to the network of pipes. The pipes are totally filled during each dosing cycle to ensure a uniform volume of effluent is distributed from each orifice. This intermittent dosing allows the effluent to pass into the sand layer or ground and then time for air to reach the soil before the next dose. Pressure distribution is superior to gravity distribution and is required for a mound or an at-grade system. Pressurized distribution makes use of the whole mound or treatment field area thus creating a longer life of the final treatment component.