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Cluster Systems - Advanced
Treatment and Community Character
Guest article by Dennis F. Hallahan, P.E., Technical Director,
Infiltrator Systems Inc.
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Cluster systems have gained much
recognition as a viable solution as small cities, towns and
counties face complex wastewater management issues. Interest in
these systems continues to grow due because they provide
communities with high quality, cost efficient wastewater
treatment, while protecting community character and reducing the
(Photo: Snake River in
Jackson Hole required an onsite wastewater treatment system
which would preserve the natural habitat. Infiltrator Systems’
chambers were installed to minimize required leach field
The Cluster Systems of Today
There are many definitions of a
cluster system. The most used is that cluster systems serve a
number of structures with more than one and as many as hundreds
of connections. Also known as community systems, cluster systems
are a treatment option when traditional onsite systems (a single
system for each structure) or the extending of sewers are not
The decision regarding which type
of system to use remains based on a site’s soils, groundwater
and bedrock elevation, topography, and regulatory requirements.
Similar to individual onsite systems, cluster systems typically
treat and dispose of effluent within the development limits.
Subsurface, non-point discharges are the norm; however surface
discharge is also an option. One of the benefits with cluster
systems is that they offer a wide range of methods available to
collect, treat, and dispose of the effluent. This allows the
designer to choose the most cost-effective technology that is
appropriate for the community.
The opportunities and benefits
presented to communities by the cluster system approach are
significant. Many communities have master plans that detail the
growth patterns within the community with the primary goal of
maintaining community character. In many cases, these plans
become idealistic goals once a conventional sewer system is
installed. Historically, growth has followed sewering resulting
in urban sprawl. Cluster systems allow controlled and
sustainable development. The community can have the best of both
worlds by maintaining its character and increasing its tax base.
This sustainable development model is gaining attention with
community designers, developers and regulators nationwide.
Environmentally sensitive sites
are especially well suited for cluster systems. The treatment
component of the cluster system can be designed to treat to a
higher standard than a conventional onsite wastewater system. It
is extremely valuable in the removal of constituents of concern
such as nitrogen in saltwater bodies or phosphorous in
freshwater ecosystems. In addition, the treatment and dispersal
components can be located in a more environmentally protective
location. This might mean installing the cluster system up and
out of the flood plain, in a location of deeper/better soils, or
a greater distance away from a body of water.
If a cluster system has a
subsurface disposal component then that can be an environmental
improvement over a surface, point discharge approach. With water
scarcity a major concern even in the eastern United States,
cluster systems represent a method for better management of
groundwater resources. Water is a valuable resource and cluster
systems allow the replacement (recycling) of the water within
the same watershed. This can sustain the water balance within
the watershed and avoid inter-basin transfers of water.
Management – The Most Vital
There are many components
critical to the success of any wastewater management system
including siting, design, installation, and management.
Management is just as vital as all of the others, but
traditionally it has been the Achilles Heel for cluster systems.
Public perception may be the biggest barrier prior to
installation, but once the system is installed, management
becomes the vital component.
As is the case with large
municipal wastewater treatment plants, operations and
maintenance (O&M) is critical to the success of a cluster or
community system. Due to size efficiencies and differing
regulatory requirements, cluster systems may offer less
intensive O&M than traditional sewers and are therefore more
economical to operate. Also, cluster systems served by small
treatment plants typically do not require a full-time operator.
These small treatment plants (up
to 500,000 gpd) only require visits by an operator 2-3 times per
week for 2-3 hours. As a result, one trained operator can manage
several facilities. Advances in technology via web-based remote
monitoring or telemetry have raised system management to a new
level of efficiency and automation. The system can be monitored
around the clock. If there are problems, the provider is
As part of the local approval
process, the developer can contract for system management for
the initial startup period (1-2 years). This allows the system
to get up and running well. Then, the system can be turned over
to the municipality or district. At that time, the municipality
(or existing utility) has a new system paid for by the developer
as source of income to fund the necessary O&M.
O&M considerations should play a
large role up front in the design of the system. For example,
the design of a cluster system could include features such as:
- Easy access to system
components that require inspection, maintenance, and
- Reliable products and
materials with a long design life and manufacturers’
- Watertight components such
as tanks, pump chambers, manholes, piping, fittings and lids
to prevent surface or ground water infiltration.
- Watertight riser lids
secured with a specialized tool to help prevent unauthorized
entry. The lids may vary in color such as green to blend
well with surrounding vegetation.
- Risers flush with the grade
to allow for easy access without yard excavation. This
prevents further damage or maintenance from to soil, rocks
or other material entering the system.
- Pumps labeled for
identification and labels coinciding with the labeling on
the control panel.
- Metal detection tape
(locator tape) to assist in locating buried system
components such as pressure lines that do not have riser
- Vegetative cover that is
maintained if the system disposes of effluent onsite via a
subsurface disposal field. The vegetation (typically grass)
can serve many purposes such as evapotranspiration, nutrient
uptake, and soil erosion prevention.
Cluster System Barriers
There are barriers to the
acceptance, approval, and installation of cluster system
designs. Among these are the uninformed public, regulatory
officials, and the design community. The Onsite Wastewater
Treatment Manual (EPA 2002) lists several barriers including:
- Public misperceptions that
centralized sewage treatment plants perform better, protect
property values, and are more acceptable than decentralized
- Prescriptive requirements
outlined in state onsite regulations that discourage local
jurisdictions from utilizing a decentralized approach.
- Liability laws that may
discourage the use of innovative systems.
- Funding difficulties brought
about by various grant guidelines and loan priorities that
prevent communities from accessing resources to fund
construction of alternative wastewater treatment systems.
- Lack of a suitable O&M
- A subsurface discharge
component that may require a sizeable parcel of land.
Systems – The “new” option
Cluster systems are not a new
concept, but they have become more viable given the availability
of advanced wastewater technology. The biggest developments have
been chambers, which can save critical space and be installed
much easier and quicker resulting in lower costs. Also, drip
systems are particularly effective on sites with limitations
such as high groundwater or high bedrock.
(Photo: A drainfield
utilizing Infilltrator Systems’ chambers serves over 200
individual and clustered residential homes along the Columbia
River in Quincy, WA.)
The drip system does require
pretreated effluent, a larger footprint, and specialized
installation, whereas chamber systems can be sized to accept any
level of treated effluent. And, as previously discussed, the
tide is turning in favor of viable management entities. The
private and public sector have recognized that due to
technological advances, this can be a source of income.
Cluster systems represent an
additional option for a community, but barrier that remains is
the resistance to change. Economic conditions however, have made
resistors more receptive to new approaches. As loss of funding
continues to be an issue, engineers and communities will be
forced to be more resourceful with their financial resources and
innovative with their wastewater management solutions. In this
scenario, cluster treatment will move to the forefront of the
options that will provide excellent treatment at a reasonable
cost for sustainable development.
About our author
Mr. Dennis Hallahan received his
MS in civil engineering from the University of Connecticut and
his BS in civil engineering from the University of Vermont.
Dennis is a registered professional engineer in Colorado and
Connecticut. He also holds patents for several onsite wastewater
Dennis has over sixteen years of
experience in the design and construction of onsite wastewater
treatment systems. He has authored several articles for onsite
industry magazines and has given numerous presentations
nationally on the science and fundamentals of onsite wastewater
treatment systems. Dennis joined Infiltrator Systems Inc. of Old
Saybrook, Connecticut in 1999 and is currently their Technical
For more information contact
Dennis F. Hallahan, P.E.
Infiltrator Systems Inc.
6 Business Park Road
P.O. Box 768
Old Saybrook, CT, 06475
Telephone: (860) 577-7100
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