Going Their Separate Ways

A closer look at the advantages of cluster sewage systems, a decentralized sanitary sewer system option for larger, low-density population areas

This article explores the nature, utilization and differences between centralized and decentralized sanitary sewer systems. It explores how and why the majority of sanitary wastewater is routed to centralized sewer systems, and some of the benefits and problems associated with centralized sewers.

By strict definition, centralized sewers collect sewage from several different locations (residential, commercial and industrial) over a relatively large geographical area, and through a network of underground laterals and manifolds, transport the untreated sewage to a large centralized treatment plant for treatment and discharge. The ultimate goal of centralized sewers is to transport sewage from every customer in a particular region to a centralized treatment plant.

Decentralized sewers refer to a network of individual onsite treatment systems and/or a network of smaller wastewater treatment plants serving several customers, but not an entire community. In other words, a centralized system is usually comprised of a large collection system plumbed into one large treatment plant serving a relatively small, high-density area. Similarly, as illustrated in decentralized systems are typically comprised of multiple small treatment systems with a more limited collection system serving a larger, low-density area.

One decentralized sanitary treatment alternative is the cluster sewage system, which typically services larger, low-density population areas. The latter part of this article focuses on this alternative, more specifically on the potential benefits of decentralized cluster sewage systems.

This article does not address typical problems with centralized systems or individual onsite treatment systems, such as groundwater inflow and infiltration (I/I) or failing leachfields. Nor does this article discuss the pros and cons of the large number of technologies that addresses sewage collection, treatment and discharge (a number of these publications exist and are attainable through the Internet). Rather, the information presented below is specifically directed at some of the benefits of a generic cluster-type system installed in new developments located in a rural suburban area.

Since, according to the 2000 census, the majority of areas across the United States that are experiencing the highest rate of growth are classified as rural suburban, we are specifically focusing on the applicability of decentralized cluster systems in these areas. Additionally, since the cluster system concept is relatively new, some of the political and technical hurdles that have emerged are also addressed.

Why Centralized Sewers?
Many of the centralized sewers in service today are a direct result of the Clean Water Act (CWA) passed in 1972. Of course, before enactment of the CWA, centralized sewers existed, but there was no regulatory mechanism i.e., the U.S. Environmental Protection Agency (EPA), established in 1970 that oversaw their operation or enforced discharge standards. One section of the CWA enacted the National Pollutant Discharge Elimination System (NPDES) permit program, which regulates discharges from wastewater treatment facilities to surface waters of the United States. At that time, many of the discharges did not meet NPDES standards. Prior to enactment of the CWA, treatment of sewage was not as important as transporting the sewage to an adequately sized surface water body so that it could be discharged without causing a nuisance (i.e., dilution was the solution).

In order to meet the new standards, upgrades and/or replacements had to be performed on many of the collection and treatment systems that were in use. Additionally, many of the smaller communities that did not have centralized systems invested in new state-of-the-art collection and treatment systems. The new systems and upgrades were expensive, requiring large capital expenditures, and the majority of capital funds were derived from the Federal Construction Grants Program (FCGP). The FCGP provided publicly owned treatment works (POTWs) with at least 50 percent or more (sometimes as high as 90 percent) in matching funds for capital improvements. During the 1970s and 1980s, the FCGP provided over $60 billion in matching funds to POTWs to upgrade and/or replace treatment and collection components (i.e., lift stations, interceptor sewers, trunk lines, treatment systems, etc.). However, in 1990, funding for the FCGP was terminated and the Clean Water State Revolving Fund (CWSRF) was initiated. Under the CWSRF states were given a specified amount of money that could be used for capital costs associated with improvements and upgrades. The major difference was that the money taken from the CWSRF was a low interest loan, not a grant. As a result, the POTW was/is responsible for reimbursing the fund.

Benefits of Centralized Sewers
It is undeniable that the enactment of the CWA, in conjunction with centralized sewers, significantly decreased many of the environmental problems that existed in the late 1960s. Prior to enactment of the CWA, only one-third of the surface waters in the United States were considered safe for swimming and fishing; today, two-thirds of all waters are considered safe for swimming and fishing. A large contribution to the improvement of surface waters resulted from centralized sewers and the CWA. Some of the principle advantages associated with centralized sewers are listed below:

  • Centralized sewers allowed residential, commercial and industrial development to proceed at a more rapid rate. During any land development process, one of the first hurdles to overcome is addressing the sanitary sewage a development will produce. If the desired location of the proposed development has access to a centralized sewer, the developer simply has to pay the tap-in fee and the problem is solved.
  • Centralized sewers are oftentimes large, complex and expensive; therefore, one of the major advantages is that they allow focused management. The consistent operation of any electromechanical, synthetically made apparatus requires constant maintenance and oversight.
  • Centralized sewers are usually operated and maintained by public entities/municipalities (i.e., county, city, township, etc.) that employ licensed, qualified operators.
  • Centralized treatment facilities are easier for regulatory agencies to monitor and oversee. For example, regulatory oversight over one 10-million-gallons-per-day (GPD) facility is easer than overseeing 10 1-million-GPD facilities.

Problems Associated With Centralized Sewers
In the last 10 to 15 years, some unforeseen financial and environmental problems associated with centralized sewers have emerged and no clear solutions have been put forth. Some of these problems are listed below:

  • Financially, the operation and maintenance of centralized treatment systems is more than initially predicted. The financial impact of centralized sewers is most evident in small to midsized communities that lack the high population densities found in urban areas
  • Maintaining and repairing centralized sewers in highly populated areas is typically cumbersome and logistically difficult, and the sanitary sewer infrastructure may be extremely difficult and inconvenient to access.
  • Currently, it is estimated that in the next 20 years anywhere from $21 billion to $122 billion will be needed for capital and O&M (operations and maintenance) costs associated with existing centralized sewers. To date, the federal government has not set aside any funds to cover these costs, and no federal, state or local agency has any workable plan for raising the necessary funds.
  • After underground gravity sewers were installed, they were "out of sight, out of mind," with the expectation that the concrete and or plastic piping would last forever and never leak. However, during rain events many of the underground pipes suffer from groundwater and surface water I/I problems that can quickly flood the pipes and overwhelm the treatment system. These types of problems have historically been associated with combined sewers (i.e., sanitary and stormwater sewers plumbed together). However, with the degradation of sewer lines over time, areas with separate sanitary and storm sewers also have observed a significant increase in I/I problems. Replacement and/or refurbishment of the underground pipes is expensive, time consuming and inconvenient since many of the pipes are under active streets, along or in creek beds and in close proximity to other subsurface utilities.
  • Environmentally, centralized sewers have helped some aspects of the environment while inadvertently harming others. As referenced above, surface water quality has improved since enacting the CWA; however, groundwater resources are being withdrawn faster than they are being replenished. Over half (53 percent) of the U.S. population derives its drinking water from groundwater, but 75 percent of the population uses centralized sewers that discharge to surface waters. As a result, potable water supplies derived from groundwater aquifers are not replenished.
  • Groundwater can also be withdrawn and/or contaminated from leaky sewer pipes and man ways. During drier periods, untreated sewage can exfiltrate from pipes and contaminate groundwater, while during wet periods groundwater can infiltrate into the pipes and not only overwhelm the treatment plant, but also decrease groundwater levels.
  • Even though the surface water quality has improved, the majority of treatment plants still discharge effluent high in nitrates that can damage ecosystems. The EPA, along with other regulatory agencies, recognized the need to decrease nitrates from treatment plant discharges. However, the cost to retrofit existing treatment systems that do not address nitrates with tertiary filters used to decrease nitrates would be cost prohibitive.
  • In many areas, rapid development has resulted in exceeding the original design capacity of the centralized system. As a result, some centralized systems have difficulty complying with permitted effluent requirements.
  • Centralized sewers allow land development to occur at a rapid pace, which unfortunately has resulted in decreased open space and uncontrolled growth. During the 1970s and 1980s, very little money in the private and public sector was allocated toward land planning. Consequently, the majority of large cities are suffering from varying degrees of urban sprawl. Of course, centralized sewers are not the only source of urban sprawl; however, if sewers are not easily accessible, developers and communities are forced to put more thought into sustainable land development.
  • Large treatment plants will occasionally suffer problems that are inherent with any electromechanical system (i.e., pump/motor failures, chemical spills, large rain events, etc.). Since large treatment plants collect and treat large amounts of sewage, there exists the potential for a more significant environmental impact if a problem arises. As a result, instead of a localized environmental impact, the impact could be regional.

Eliminating centralized sewers is obviously not a practical or viable option. In high-density population areas with a large tax and/or rate base, a centralized sewer system is most probably the best and only cost-effective way to address sewage. Conversely, in a rural, low-density area a series of onsite sanitary treatment systems is the most viable alternative to address sewage. This is because the residences are geographically spaced further apart, and each lot usually has adequate land for an individual onsite discharge. In other words, using a centralized sewer in a low-density, rural setting makes as much sense as using multiple onsite treatment systems in a high-density, urban setting. The difficult or gray area lies in how to address sewage collection and treatment in a suburban rural setting.

Since the end of World War II and the installation of the interstate highway system, a large portion of the residential population has vacated the urban areas and moved outward to the suburbs. According to the 2000 census, the majority of counties across the United States that are experiencing the highest rate of growth are classified as rural suburban. Along with the residential population, a variety of commercial establishments (i.e., restaurants, strip malls, grocery stores, etc.) followed. Running centralized sewers from subdivision to subdivision in these lower-density population environments has proven to be less efficient and more expensive than in a high-density environment. For example, more lift stations are often required in suburban areas since the sewage must be transported a further distance for treatment. Conversely, individual onsite treatment systems are usually not appropriate since the small commercial or residential lot sizes do not allow adequate space for a properly functioning onsite treatment system. In many suburban areas, centralized sewers may service one subdivision, while another subdivision, less than 1 mile away, with the same lot size and zoning restrictions, may use a series of onsite systems.

Given the apparent lack of planning and random placement of onsite vs. offsite (centralized vs. decentralized) treatment systems in suburban areas, there exists an obvious need for change. Of course, anyone can sit back and find fault and criticize the shortcomings of the existing wastewater treatment infrastructure; and it is obvious that insufficient funds have been set aside to maintain, much less expand, the majority of existing centralized systems. However, it is important to observe some of the mistakes and shortcomings in planning, management and design that have occurred in the last 30 years and learn from them so that they are not repeated when new communities are developed.

What is a Cluster Sewage Treatment System?
Before discussing the benefits of cluster sewage systems, it is important to understand exactly what a cluster system is. Cluster systems are commonly used in developments with more than 10 homes. Subdivisions with more homes (i.e., greater than 1,000 homes) can often use multiple cluster systems, depending on site conditions. The basic concept of a cluster system is that the homes inside a small community discharge to a treatment plant located inside the development that treats and discharges that effluent onsite (although cluster systems can discharge to surface waters). Some smaller cluster systems are configured to perform primary treatment at individual residences using septic tanks. Following the primary treatment, the solids-free liquid can then be easily transported, using gravity, vacuum or low-pressure sewers, to a small secondary treatment plant.

From the treatment plant, the effluent can be land applied inside the confines of the development using one or a combination of technologies (i.e., spray/drip irrigation). Traditionally, land application of liquid sewage effluent has been used as the final step in the treatment process. For example, traditional subsurface leachfields, Wisconsin mound or land application systems have served two purposes, which are to provide tertiary treatment by removing nutrients (i.e., nitrogen, phosphorus) and to act as a discharge mechanism. Ideally, using a cluster system, the treatment plant can be designed so that the effluent it produces needs little or no treatment. In other words, it is intended to produce a high water quality effluent by adding a supplemental treatment stage, and the land application process is solely a discharge device, not a treatment mechanism.

Benefits of a Cluster System
Well-designed and properly installed, cluster sewage systems can be environmentally friendly, create communities that preserve high-quality green space and be cost-effective for developers and municipalities.

Cluster sewage systems are inherently more environmentally friendly than centralized sewers since they represent a more sustainable development approach. Centralized sewer systems collect sewage generated from several individual areas and transport it to a large treatment plant. The treatment plant is often times located several miles from the source, and effluent from the plant is discharged to surface water. The concept described above does not represent a sustainable development model. Cluster systems generate, collect and treat sewage and discharge-treated effluent inside the same development, which has much less of an impact on the surrounding area. Additionally, by using treated effluent (i.e., gray water) in place of potable water for irrigation of common areas, water resources are not depleted.

Inevitably, all wastewater treatment systems, no matter what the size, will malfunction. Often times when a small individual onsite system (i.e., septic tank/leach field) malfunctions, the problem is not discovered until the owner suffers an inconvenience, at which time the environment may have already been impacted and repairs/replacements is costly. Despite the fact that individual onsite treatment and discharge systems are technically and environmentally viable if properly designed and installed, their "out-of-sight-out-of-mind" nature can result in numerous individual septic system failures due to lack of maintenance. Conversely, malfunctions at large centralized treatment plants are usually quickly remedied. However, since the treatment plant treats and discharges a large quantity of wastewater, it is almost impossible to contain the problem; and the volume of inadequately treated or untreated wastewater discharged into surface water can have a significant environmental impact. Properly designed and managed cluster systems (discussed later) are not immune to failure. However, unlike a small onsite system, the problem can be discovered and addressed quickly; and unlike a large centralized system, the environmental impact is not as drastic since the flow quantities are less and the system discharges onsite.

Most individual onsite treatment systems (i.e., septic tanks and leachfields) are limited by soil type and topography and typically require 2-acre to 5-acre lots (depending on zoning). Within a 100-acre development, the soil, topography, depth to bedrock and groundwater conditions can vary significantly. As a result, cluster systems allow designers to choose the most suitable location in a development to locate the treatment and, more importantly, the discharge systems. Instead of being limited by small individual lots that have separate systems, the system designer has more flexibility and can utilize the best features of an entire development.

From a land-development perspective, cluster systems allow developers to create larger, higher-quality green spaces. For example, instead of subdividing 100 acres into 20 5-acre lots, the developer/land planner could set aside 20 1.5-acre lots for homes, 10 acres for a sewage treatment and land application area and the remaining 60 acres could be left undeveloped for public green space. Since the developed area (i.e., house lots) is smaller in size, there is less environmental impact on the land being developed. Additionally, the land development and infrastructure costs are less, since the lots occupy a smaller area.

Ideally, any mechanical wastewater collection and treatment system should be operated by a competent, accountable municipal and/or private entity. Proper operation and management of treatment systems help to dramatically improve the longevity and performance of any system.

The absence or presence of sewers affects a community's ability to grow. In order for communities to grow they must increase sewage capacity, which is accomplished by running additional sewer laterals to unsewered areas and/or by increasing the size or building new treatment plants. In order to pay for the capital improvements, the municipalities often times borrow the money from sources such as the CWSRF, which eventually must be paid back. Smaller cluster systems allow developers to pay for not only the collection system, but also the treatment and discharge system. Following construction and start up, the entire system can be transferred to the local municipality, which will maintain and operate it. The developer is reimbursed for the capital costs by collecting the sewer tap-in fees paid by the home owners or builders, and the municipality receives a state-of-the-art collection and treatment system. The municipality can then charge usage fees to pay for the operation and maintenance of the system. Since the capital cost for the system has already been paid for by the developer, the municipality does not have to service any long-term debt.

Technical and Political Obstacles for Cluster Systems
With all the apparent benefits that a cluster sewage system possesses, one would speculate that there should be more systems currently in use. However, that is obviously not the case. Although the cluster system concept is gaining acceptance, the majority of system designers and regulatory personnel prefer centralized sewers. The reasons for this preference are simple:

  • They are familiar with the components and management structure associated with centralized sewers. On a technical level, the majority of collection, treatment and discharge components that comprise a cluster system are individually relatively straight-forward and simple. However, the true technical challenge is correctly applying the right combination of technologies to achieve a low-capital, low-maintenance and environmentally friendly system from collection point to discharge point.
  • It is generally perceived to be easier to manage and/or regulate one large treatment plant vs. several (five to six) smaller plants.
  • Many states do not possess specific regulatory guidelines, policies and administrative framework for design, permitting and operation of cluster systems, particularly with onsite discharge.

Design engineers who have been designing and installing centralized treatment systems for the last 30 years are oftentimes reluctant to embrace new concepts and technologies for several reasons. Treatment system designers are often intimidated by the perceived professional liability associated with using alternative technologies or concepts. Historically, several small treatment systems installed in the last 30 years have failed prior to their anticipated life expectancy. Sometimes the systems were poorly designed or improperly installed. However, the majority of failures resulted from either an inadequate, or in some cases totally absent, proper preventative maintenance. In some cases, the failure of these poorly maintained systems was blamed on the design of the system. This fact again underscores the importance of utilizing a competent, reliable entity that will maintain and operate the system consistently.

Typical political hurdles that stifle any new sewerage concepts such as cluster systems include the following:

  • Most large-lot zoning codes do not allow small lots even though the arrangement of the entire development does not exceed the zoning. For example, if a 100-acre development is zoned for 5-acre lots (i.e., 0.2 homes per acre), the developer could build 20 2-acre lots and leave 60 acres as green space. As a result, the total 100-acre development would not violate the pre-approved density.
  • Some municipalities do not allow private developers to collect tap-in fees for sewers. The financial incentive vanishes if the developer can not recoup the capital costs to build the cluster system.
  • Regulatory officials and municipalities are under the impression that several smaller treatment and collection systems are more difficult and more expensive to operate and maintain. However, within the last 15 years the capability, reliability and cost of telemetry devices and remote monitoring technologies has significantly improved, which makes multiple system monitoring and operation possible from a centralized location.
  • In some areas of the country where potable water is abundant, the general public has not fully accepted using gray water for irrigation.

When initially designing a cluster system, it is important to obtain the early involvement of the permitting regulatory agency, the entity responsible for long-term operation and maintenance of the system (i.e., local municipality), the developer, the land planner and the design engineer. Currently, regulatory agencies, municipalities and system designers have a unique opportunity to explore decentralized sanitary treatment concepts in rural suburban areas (such as cluster systems), or continue to embrace the centralized sewer concept. If the later is chosen, one of two scenarios may occur. Either the public will suffer by having to pay for the improvements necessary to operate and maintain a large centralized sanitary system, or the environment will suffer because of inadequate treatment and subsequent discharge. One must question whether it is even possible to maintain, much less expand, our existing centralized treatment systems to maintain pace with population growth and economic development without the utilization of alternative technologies. The hope of this publication is to illustrate that the selection of the most appropriate wastewater disposal option depends on numerous variables, and the concept that "one method fits all" (i.e., centralized sewers) is overly simplistic and antiquated.

This article originally appeared in the 05/01/2004 issue of Environmental Protection.

About the Author

Patrick T. Gates is the vice president of T. M. Gates Inc. He received his BS from Michigan State University. He can be reached at (513) 248-1025.

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