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Advanced Treatment Systems
Advanced treatment systems differ from conventional systems in a number of ways, the primary difference being that they further treat the wastewater before it is dispersed to the soil environment. Advanced treatment systems incorporate carefully designed treatment steps, and create conditions to facilitate a consistently high degree of treatment. Many advanced treatment systems control flow through the system using pumps and timers to avoid overloading the treatment and final dispersal components during periods of high water usage, or “peak flow” conditions, which could occur during a morning rush of activity or when many guests are in the home.

The treatment provided by the advanced treatment system serves to further reduce the “strength” of the wastewater, or the solids, fats, oils, and greases (FOG) that are suspended in the effluent. This step may also contribute to reductions in pathogens and/or nutrients in the wastewater depending on the design and configuration of the system. Systems that function to reduce nitrogen generally recirculate the effluent back to the septic tank or to a separate recirulation step where raw effluent and treated effluent are mixed, creating conditions that facilitate the removal of nitrogen by beneficial bacteria.

Advanced treatment systems are designed as a “treatment train,” or a logical sequence of treatment components to achieve a certain level of treatment, which may be specified by local, state, or regional governing agencies. In Rhode Island, the Department of Environmental Management (DEM), Coastal Resources Management Council (CRMC), and town governments may all have jurisdiction over a given area of land, and may impose differing regulations regarding wastewater treatment.

Technologies are initially chosen based on the level of treatment that is required, as not all technologies will effectively achieve nutrient and/or pathogen reduction. Treatment technologies achieve the best results when receiving wastewater with certain chemical and microbiological characteristics, and so the components are chosen and paired up with this in mind. Site constraints may also dictate the potential use of some technologies. For instance on small lots with existing homes and failed septic systems, the advanced treatment technologies with the smallest footprints are most commonly used as replacement systems. Advanced treatment systems generally require annual or semi-annual maintenance activities in order to function properly; these maintenance activities should be performed by a trained and qualified service provider.

Primary Treatment
Separation of solids, fats, oils, and greases, referred to as primary treatment, is a critical step in the treatment process, as it eliminates portions of the waste that cannot be easily broken down by bacteria living in the system, and would clog the pores providing treatment in subsequent steps of the treatment train. This is the same process that occurs in a conventional two-compartment tank.
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.

Fixed activated sludge treatment


Media filters

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).

absorbent media
non-absorbent media
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.

Final Dispersal Technologies: Drainfield options for advanced systems

Though it is not a requirement to disperse wastewater effluent to an alternative type of drainfield, treated effluent is best dispersed via pressurized lines, either a shallow narrow drainfield or bottomless sand filter (both commonly used drainfields in Rhode Island). In both of these options, effluent continues to be treated as it is dispersed to the native soil. Because effluent from advanced treatment technologies contains very small amounts of solids and organic matter (food for bacteria), it is best introduced to the soil environment in the most biologically active, near-surface soils (tope soil) of a site, where existing soil microbes can further reduce pathogen counts and nutrient loads. Plants rooted in these shallow soils also help to remove nutrients from the effluent before it moves downward through the soils toward the water table. When dispersed into deeper, less active zones of the soil, there may not be enough organic matter (solids) in the effluent to sustain the beneficial bacterial community, and further treatment may not occur to the extent achieved in shallow dispersal.In Rhode Island, approved final dispersal technologies include pressurized shallow narrow drainfields and bottomless sand filters that must be used in conjunction with an I&A technology.

Drip Dispersal
Pressurized lines are used to deliver the effluent to a distribution network of flexible plastic tubing. Water is dispersed from the distribution network through emitters, which control the rate at which effluent is released to the shallow soil environment. The distribution network is generally installed within one foot of the ground surface to maximize soil-based treatment of the effluent. The shallow placement may also serve as a way to make use of treated wastewater for irrigation purposes; this practice is generally regulated by state agencies, and is not allowed in all states.
Low-pressure distribution

Pressure distribution is used to evenly distribute wastewater over the entire surface of the drainfield trenches or bottomless media filters. Water is pumped through small-diameter PVC pipes having small-diameter holes along their length from which the effluent squirts.

Pressurized Shallow Narrow Drainfield

Media filters as drainfield options

Media filters can be configured in a bottomless fashion to serve as drainfield options. In a treatment train, these filters follow an advanced treatment step. They provide an additional final treatment step and serve to disperse treated wastewater to the shallow soil environment. Wastewater is pumped to the technology and Drainfield size is reduced when this configuration is used. Bottomless sand filters are the most frequently used media-based drainfield option, though in some regions peat filters may also be designed as a final dispersal technology.

Bottomless Sand Filter

Pumps, Timers, and Controls: Moving wastewater through the system

The flow of wastewater through the system is controlled in order to prevent peak flows and high flow events (e.g. laundry days and the morning shower rush) from overloading components and thus inhibiting proper treatment of wastewater. Wastewater is stored in either the headspace of the processing tank or a pump tank before it is pumped to the next component. Programmable timers (small computers) turn pumps on and off at prescribed intervals to send small doses of wastewater forward through the treatment train. This small incremental feeding of wastewater helps to achieve consistent treatment and prevents or strongly minimizes peak flow perturbations in the system. Because the doses of water are pre-determined and consistent, the timers and controls enable the use of smaller footprint treatment units, as opposed to the larger systems that would be required to handle occasional peak doses.

Some control panels also incorporate a telemetry system that notify the service provider of alarms and system status through the home’s telephone line. This feature ensures that the system will be attended to in the case that a problem arises. Alarms may occur for a number of reasons

 

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