| 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. |
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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.
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| 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
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| 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
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| 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
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non-absorbent media
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| 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.
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| 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.
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| 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
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| 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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