Cities and towns throughout North America are facing the
challenge of dealing with large volumes of wastewater discharged, often with
minimal treatment, into concentrated locations, such as rivers and other
natural waterways. Watersheds and groundwater supplies are critical areas under
close scrutiny when in the proximity of any potential runoff or pollutant
stream. Municipalities continuously need to closely regulate the capacity and
the quality of their infrastructure systems, as well as how expansion or change
to these systems will affect the surrounding area.
Centralized sewers often
are seen as the preferred solution; however, they are a primary contributor to
the discharge to surface water problems. Centralized treatment also contributes
to urban sprawl and the resulting loss of community identity that follows the
The centralized approach
to wastewater treatment involves collection, treatment, and discharge,
typically as a point source. Unfortunately, the collection system can have
inflow and infiltration from stormwater. This is the case whether the
collection system is old or new, and the additional flow can inundate the
treatment facility, possibly exceeding its capacity and contributing to
combined sewer overflows.
With a decentralized
approach to wastewater treatment, groundwater is extracted, used, and treated
onsite; then it is returned near its point of origin to recharge the aquifer.
Decentralized systems can treat to the same degree as centralized systems,
regardless of flow — from a small residence to a facility or community
discharging 1900 m3/d (500,000 gal/d) — some systems even exceed
3800 m3/d (1 mgd).
Advantages of decentralized treatment
wastewater treatment technologies that use natural approaches provide suitable
long-term treatment solutions and better development practices. They require
less land and can be more cost-effective than centralized systems.
available from test centers and universities make these systems increasingly
popular with local health officials. Wastewater treatment for sites with
difficult soils and tough terrain or large recreational and commercial
developments are now feasible with decentralized systems. With a decentralized
approach, the same treatment technologies utilized by large-scale water
resource recovery facilities (WRRFs) are available for smaller flows. This includes
membrane bioreactors, which have become widely specified in the last decade.
Additionally, owners and developers do not have to wait for sewer extensions to
reach their site or for the WRRF to be expanded to move forward with a
However, every community
is unique, and needs vary greatly. Below are some questions for communities to
consider before making the choice between centralized and decentralized
What should the community look like several years down the road?
Sewers offer opportunity
for community expansion and business growth, such as high-rise hotels and
increased housing density. In some communities, rapid growth is the goal;
however, with such growth can come increased traffic, traffic lights at each
intersection, strip malls, and a loss of community character. Each community
has to choose its own path to determine what future to strive for. This
community planning decision is at the core of selecting the best sustainable
wastewater treatment plan for the future.
What are the anticipated wastewater treatment capacity
treatment, a community can focus on only treating the areas of town that are
causing a problem or have the potential to do so. This allows for smaller
design flows, smaller disposal areas, and, therefore, lower costs. This also
places the financial burden on properties where issues are seen or anticipated.
Centralized sewers do not offer this functional and selective capability.
What are the current wastewater treatment challenges?
overburdened systems, underfunding, watershed issues, groundwater pollution,
and regulatory noncompliance all could be at hand. Officials have to know and
understand the problems in the community and have evidence and proper
documentation of the situation that can be posted and/or left in the town hall
for people to review. They then should present how the proposed solution
specifically would address the defined problem. Making the motives crystal
clear to townspeople will ensure proper support.
How long will it be before the new system can be operational?
Even when under
regulatory pressure, cities should not rush to construction without proper
vetting of the alternatives. There can be dramatic differences in cost and even
what the solution will ultimately mean for community growth.
What wastewater treatment options are available?
Basically, the choice is
threefold: decentralized, centralized, or a melding of the two. When evaluating
the options, it is important for the community to hire an expert in each model;
otherwise, the solution selected may be bias-directed toward only one of the
options. “Treatment options” (p. 81) lists some common treatment alternatives.
What are the true costs?
Short-term costs include
expenditures for design, permitting, legal responsibilities (such as easement
acquisition, operations and maintenance, covenants, creation of a district),
land purchase, and construction. Long-term costs include outlays for operations
and maintenance, licensed operators, billing structure, district vehicles
(specialized trucks), and specialized equipment.
Cost savings is a
definite advantage of the decentralized approach. In some areas in the U.S.,
the average cost per unit to connect to a new wastewater treatment facility or
sewer extension is between $54,000 and $60,000, according to the document On-Site
Wastewater Treatment Programs in the Watershed – A Status Update issued by
the New York City Department of Environmental Protection Section Chief of
Community Planning Michael J. Meyer.
This is unfathomable for
most communities. With “free” grant money from the government unavailable,
towns and ultimately the individual homeowner must bear the real cost of the
sewer or the treatment alternative. This means it is critical to calculate the
true cost of any approach before a decision is made.
Sustainability is about balance
through sustainable designs for water and wastewater infrastructure is a means
of accomplishing balance. The traditional answer in the past for most
communities was to move toward centralized wastewater collection and treatment.
As funding for these systems has become scarce and effective alternatives more
prevalent, decentralized treatment is growing in the market as the solution of
choice for many communities.
It is important that as
the technology of onsite systems advances, those involved in evaluating these
systems become better educated regarding performance, advances, and options so
that systems are designed to desirable long-term operation standards and the
correct structural capacities. When it comes to town infrastructure and
specifically wastewater treatment, engineers, contractors, regulators, town
officials, and homeowners all share the same goal: systems to perform well
today and for years to come.
Decentralized treatment systems
Onsite septic systems come in many sizes and design configurations, including trench and bed designs, sand filters, and mound systems. The introduction of highly adaptable plastic has expanded the possibilities for specialized system designs and treatment needs.
Highly adaptable and effective, chambers are a key element in septic systems’ evapotranspiration beds, community (cluster) systems, constructed wetlands, and large-scale water resource recovery facilities (WRRFs).
Community and commercial systems
Community or cluster systems are a sustainable option that can be monitored and managed effectively by independent contractors, developers, or town officials. They are cost-effective and easily can be hidden and landscaped to integrate into their surroundings.
Product and design advances provide improved decentralized treatment in commercial and community systems, including streamlined collection and increased storage capacity to meet peak flows. On sites with adequate space, the application of engineered wetlands can result in substantial operations and maintenance cost savings, especially for systems that must operate over long periods. Engineered wetlands are unique from other treatment processes in that they employ vegetation as part of the treatment process and require minimal energy input.
Centralized treatment systems
Municipal water resource recovery facilities
Conventional WRRFs collect, treat, and discharge wastewater as a point source. They are one of the many options that should be explored in any community wastewater treatment plan. A good way to do this is for the wastewater committee personnel and town politicians to visit similar-size communities that have WRRFs to learn about challenges they may be facing, including sustainable development and plant overflows, and to understand their operating budget and sewer fees.
Smart sewers are a new storage application within a centralized system. Many sewer systems are at or beyond capacity, and it is too costly to expand the WRRF. Established facilities are constricted to their peak design flow, which typically only occurs during a short period each day. Smart sewers enable operators to store some of the peak flow in large tanks at the facility or in individual tanks installed at each home or commercial facility and release it during low-flow periods. This can extend the life of the WRRF without requiring an investment in upgrades.
Combined decentralized and centralized treatment solution
Water resource recovery facility extensions
Providing cost-effective and ecologically sound wastewater treatment when the WRRF is at capacity and growth is strong is a challenge faced by design engineers. Extending the life of WRRFs by adding an auxiliary disposal field or other extension to provide effective treatment in communitywide wastewater treatment systems is an example of how centralized and natural treatment systems can work in concert.