August 2013, Vol. 25, No.8

Small Communities

Centralized or decentralized

Top questions communities should ask when addressing future wastewater needs

Dennis F. Hallahan

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 sewer. 

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 treatmen

New decentralized 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.  

Performance data 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 development project. 

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 treatment. 


1. 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. 


2. What are the anticipated wastewater treatment capacity needs?  

With decentralized 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. 


3. What are the current wastewater treatment challenges?  

Leaking sewers, 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. 


4. 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.  


5. 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. 


6. 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  

Sustainable development 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   


Individual 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  

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. 


Dennis F. Hallahan is technical director at Infiltrator Systems Inc. (Old Saybrook, Conn.).Treatment options