| Small cities, townships and counties face many complex issues when it comes to wastewater treatment (Pinkham et al. 2004) . Although centralized sewers are perceived as the preferred solution to wastewater problems, communities that have centralized sewers discharging to surface waters have been identified as sources of numerous pollution problems (EPA 2004) . Loss of community identity due to the urban sprawl that follows the sewer is another challenge faced by these communities (Sparks 2005) . Cluster systems have proved to be an invaluable method to provide communities with high quality, cost-effective wastewater management while protecting the character of the community.
There are many barriers to cluster onsite wastewater treatment systems, such as uninformed public, regulatory, and design communities. However, in retrospect, the biggest barrier may have developed due to the Clean Water Act itself. As a result of heavy federal funding under the EPA's construction grants program, the whole wastewater industry has been geared toward the big treatment works and sewer networks. Resources, regulations, permits, and design knowledge have all been directed to expand and maintain this infrastructure model (Kreissl and Suhrer 2005) .
While this effort has been very effective in reducing point-source pollution, many communities in the US still do not have sewer service (Wallace et al. 2005) . A paradigm shift has begun in the industry with the realization that there are other alternatives (Hallahan and Wallace 2001) . This shift is due to many reasons, the biggest of which is economic; the "free lunch" is over. The construction grants program that went along with the Clean Water Act is gone; today's grants are of much smaller amounts and include revolving fund loans that are required to be paid back. Another factor is the advent of technological innovation that allows small-scale treatment facilities to treat to a high standard, and associated products that allow more efficient infiltration. When these innovations are combined with a reliable entity to provide quality management, operation, and maintenance (MOM), cost-effective, environmentally responsible wastewater systems can be implemented.
The following case studies illustrate the cost-effectiveness of the cluster model. These examples demonstrate the fact that technology exists today to produce a high-quality effluent at an effective cost.
The objectives of these projects were similar to any other wastewater project; designers needed to deliver a cost-effective solution that protects public health and the environment. The solutions provided were novel and unique. In the absence of regional sewers, cluster onsite wastewater treatment systems were designed for high water-usage projects. The systems consisted of advanced treatment from constructed wetlands with discharge to the subsurface via large-scale infiltration trenches, thereby replenishing groundwater supplies.
Development of Cluster Systems: Lake Elmo, MN
Lake Elmo, located in the Twin Cities (Minneapolis and St. Paul) metropolitan area of Minnesota, is a municipality with a population of 6,863. In many ways, Lake Elmo is a rural hamlet in the middle of a fast-paced metropolitan area of nearly 2.6 million people. Regional sewer was not available to Lake Elmo until 1992. Up to that time the entire community was served by onsite wastewater systems.
Lake Elmo has purposely restricted its growth through zoning ordinances and land planning activities. In a desire to preserve open space and thereby protect the rural character of the community, Lake Elmo refused to connect to the regional sewer. The community believed that along with regional sewer came high-density development to pay for the high cost of "Big Pipe" infrastructure. This conflicted with growth plans created by the regional planning agency, the Metropolitan Council. The resulting legal battle went all the way to the Minnesota Supreme Court, with the result that Lake Elmo was forced to accept regional sewer along the Interstate 94 corridor and limited use elsewhere (Sparks 2005) . The remaining areas of the community are able to retain their current wastewater systems.
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PHOTO: S. WALLACE |
| Constructed wetland treatment facility at The Fields of St. Croix, Lake Elmo, MN. |
In 1995, local developer Robert Engstrom proposed a development near the "Old Village," the central area of Lake Elmo. His plan, called The Fields of St. Croix, was to mirror the Old Village with large tracts of open space surrounding a cluster of homes to be served by a central water and sewer system. No ordinances were in existence to accommodate such a request, and wastewater treatment was a concern. After months of work with the city and state, the development was able to proceed with the first state-permitted subsurface flow wetland in Minnesota.
This was the beginning of open-space developments using decentralized wastewater technology within the community of Lake Elmo. Currently there are eight wetland treatment systems in the area treating in excess of 118,000 gpd. These systems employ both subsurface-flow and vertical-flow systems. Infiltration chambers have emerged as the preferred method of effluent disposal in this glacial till environment.
Since the initial systems in 1998, cluster development has emerged as a preferred infrastructure model. Table 1 is a compendium of systems operating in Lake Elmo.
Capital Costs of Cluster Wastewater Systems
In order for cluster systems to be considered an attractive infrastructure option, they must be able to "hold their own" when compared against individual onsite systems and regional sewer. The issue is not what the systems cost, but what is the relative cost between a cluster system and competing alternatives. The cost of onsite treatment systems vary widely across the US, and can vary 40% or more within a particular state (Williams et al. 2005) .
USEPA has clearly demonstrated that misapplying an infrastructure model—such as attempting to regionalize low-density areas, or install onsite systems in urban areas—is not cost-effective (EPA 1997) . However, in Lake Elmo, both individual onsite systems and regional sewer are viable development models, and both are being used in different areas of the city. In this case, how do cluster systems stack up financially against the other choices? Table 2 offers some clue.
Clusters vs. Onsite Treatment Systems
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PHOTO: NORTH AMERICAN WETLAND ENGINEERING |
| An infiltrator chamber pressure-dosed bed under construction. |
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PHOTO: S. WALLACE |
| Completed wetland system at The Fields of St Croix Phase II. |
Compared to individual onsite systems, cluster systems have an immediate disadvantage in the cost of the collection system. Building a sewer network is not a cost developers face when using individual onsite systems. The cost of the collection system must be offset by economies of scale in the treatment and disposal systems. However, in the case of Lake Elmo, individual onsite systems primarily required mounds and cost between $12,000 and $14,000 per system. Comparing those costs to the systems presented in Table 2, it is clear that the cluster systems offer cost savings.
However, cluster systems offer other financial advantages over individual onsite systems. For example, cluster systems allow lots to be positioned for optimal market value, and not just based on where the good soils are. Cluster systems can be designed to preserve open space. Due to the open space and smaller lot sizes, cluster system developments can have a reduced amount of roadway construction, thereby resulting in lower construction costs; this cost savings is not reflected in the cost comparisons. Finally, cluster systems are much more practical to operate from a MOM standpoint.
Inspection of Table 2 also shows two other trends. First, larger cluster systems tend to deliver lower per-lot costs than small developments, as economies of scale provide financial benefit. Experience in Lake Elmo and other areas in Minnesota indicate that the "cross-over" point between individual onsite systems and cluster systems is around seven homes. For developments smaller than seven homes, individual onsite systems will likely be more cost-effective, assuming that soil conditions on each lot can support an onsite system. For developments larger than seven homes, cluster systems are likely to be more cost-effective, and the relative cost savings from a cluster system will increase as the development size increases.
Secondly, systems constructed earlier in time tend to have lower per-lot costs than newer systems. This is due to factors such as inflation but also represents changes in regulations. For instance, the sizing criteria for large septic tanks have increased three-fold in Minnesota since 1998. These trends of inflation and regulatory changes would add costs to each of the systems (onsite, cluster, and sewers).
Clusters vs. Regional Sewer
If cluster systems are cost-effective (within a reasonable range of development sizes) compared to onsite treatment systems, how do they compare to regional sewer? It is difficult to make direct cost comparisons, because for regional sewer, the costs are spread among multiple governmental jurisdictions, each of which may choose to subsidize sewer fees through other revenue sources.
In the Twin Cities, the regional sewer and associated treatment plants are operated by Metropolitan Council Environmental Services (MCES). MCES currently has a sewer availability charge (SAC) of $1,350 ( MCES 2004) . This is the "wholesale" cost for sewage capacity for a single-family home in the regional network. However, this does not pay for the collection systems operated by the individual municipalities that are under MCES's jurisdiction. In reality, municipalities take MCES's base charge for SAC and add on their own costs. Additional funds are collected by the municipalities to deal with sewer maintenance, pumping stations, and correction of systemic infiltration/inflow (I/I) problems. Within the Twin Cities region, SAC charges vary between $1,500 and $15,000 per home, with a median value of about $7,500.
SAC fees do not pay for the sewer within the development, or the cost of connecting the home to the development. In Lake Elmo, sewer costs are approximately $3,500 per home, and connecting the house to the sewer costs about $500 for new construction. This indicates that the "average" cost of regional sewer is approximately $11,500 per home. Again, it appears that cluster systems in Lake Elmo are cost-effective, in part because they do not have the I/I maintenance expenses of the regional sewer network.
Operation, Maintenance, and Management Costs
The key to success of any wastewater system is proper management. Lake Elmo chose a model where the homeowners association for each development would own the wastewater system. Lake Elmo's belief was that the residents directly served by the wastewater system would have the greatest interest in making sure the system was operated properly. Lake Elmo's experience with homeowners associations has been mixed. Homeowners associations were often slow in securing a maintenance entity. When they did hire an operator, the management entity put the minimum of effort into understanding the systems they were operating. Many associations were reluctant to fund routine maintenance work and set aside adequate funds for replacement of worn-out components. This eventually led to operational problems.
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In order to address these concerns, the city began to hold regular meetings with all of the homeowners associations so they could share knowledge and expertise. A single management company (EcoCheck Inc.) began to operate most of the systems. This standardized operations and led to proactive management. EcoCheck began to work with associations to set realistic budgets to cover all aspects of system management. Monitoring, operation, and maintenance (MOM) costs are summarized in Table 3.
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| Drainfield construction, Carriage Station. |
PHOTO: NORTH AMERICAN WETLAND ENGINEERING |
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PHOTO: NORTH AMERICAN WETLAND ENGINEERING |
| Infiltrator bed under construction in Whistling Valley development, Lake Elmo, MN. |
MOM costs vary based on the size of the development, the type of treatment and infiltration method used, and permit-driven monitoring requirements. Typically, a range of costs is presented to homeowners because some assumptions must be made about how inflation and interest rates will vary over time. A major difference between MOM costs borne by the homeowners association and municipal sewer charges is that capital replacement is funded through up-front contributions from the homeowners. This "pre-payment" plan ensures that adequate funds are in the bank when major repairs are needed. This is in sharp contrast to the municipal model, in which money is borrowed via bonds when repairs are needed. Because the homeowner associations in Lake Elmo are funding capital replacement costs up front, their monthly service charges are 40% to 50% higher than comparable municipal service fees.
In the Twin Cities region, the median charge for municipal sewer services is $15.50 per month (MCES 2004) . The average cost for the cluster system presented in Table 3 is $41.18 per month. However, if capital replacement costs are factored out, the actual portion of the homeowners bills that go to fund ongoing operation and maintenance is approximately $20.59 per month. While regional sewer may cost slightly less from an O&M standpoint, the difference is not nearly as great as an initial inspection of Table 3 might indicate.
Conclusions
The Lake Elmo experience has demonstrated that decentralized cluster wastewater systems are cost-competitive with both individual onsite systems and regional sewer. From a capital cost standpoint, cluster systems can compete with both regional sewer and individual onsite systems. In terms of operating costs, the biggest difference is that the homeowners associations are putting money away up front for future capital replacement, whereas a regional sewer entity is required to bond or borrow the money when funds are needed.
SCOTT D. WALLACE, P.E., is vice president of North American
Wetland Engineering. DENNIS F. HALLAHAN, P.E., is
technical director of Infiltrator Systems Inc.
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- September/October 2005
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