Pretreatment Technologies: The advanced treatment step
An advanced treatment step serves to further reduce solids and FOG concentrations in the wastewater, and may have the potential to reduce nutrient concentrations. This step can be added to the system in two different ways:
1. An additional compartment within a single processing tank – the tank would have separate compartments dedicated to the initial settling and separation of solids and FOG, and then for additional treatment.
2. A separate component located after the septic tank and before the drainfield option.
Treatment is achieved by running the wastewater through any of a variety of technologies. Technologies approved in Rhode Island can be broken down into three categories: aerobic treatment units, media filters, and sequencing batch reactors. Each category is described below – links to detailed information on each technology are below. Technologies listed below are currently approved or seeking approval for use in Rhode Island, or are technologies that have been used in Rhode Island Demonstration Projects carried out by the University. Iinformation on advanced treatment technologies that are approved for use in other New England states can be found at the appropriate State regulatory websites
Aerobic treatment units
Aerobic treatment units (ATUs) rely on air injection systems and blowers to create an oxygenated (aerated) environment, which is able to support bacterial populations that break down organic material. This aeration process produces an effluent that is lower in total suspended solids (TSS) and biochemical oxygen demand (BOD) than that of conventional systems. The injection of air into the ATU agitates the wastewater, so solids are readily mixed with the bacteria, facilitating digestion of the organic matter by bacteria. There is a step in the process where any settled solids and bacteria are returned back to the aerobic portion of the tank for mixing and additional treatment. A clarification step is also a part of the treatment, allowing for solids and bacteria to settle out of the wastewater prior to distribution to the drainfield, helping to ensure that effluent leaving the ATU contains as few solids and organic matter particulates as possible.
Media filters consist of a lined or watertight structure containing specified media, which serves as a surface for bacteria to colonize and for biochemical and physical treatment processes to occur. As the wastewater trickles through the media bed, the organisms growing on the media treat it by breaking down organic matter and consuming nutrients in the effluent. This filter is never saturated with wastewater, and the presence of air promotes the establishment of beneficial aerobic microorganisms. The media may be absorbent (such as peat or textile media) or non-absorbent (sand or plastic).
Sequencing batch reactors
A sequencing batch reactor is a specific type of treatment unit that uses a single treatment tank to perform both aeration and clarification (solids settling). The cycle begins as wastewater enters the tank. The full tank is aerated for biological treatment. After aeration, the mixing system is stopped, and the solids are allowed to settle. Clarified effluent is decanted from the clear zone in the tank. The cycle is completed when the system moves into an idle period for development of anaerobic conditions to facilitate potential nitrogen removal; the vendor of this technology has not approached the RI DEM to seek State approval of this technology for nitrogen removal.