SEPTIC VS. SEWER: A COST COMPARISON FOR COMMUNITIES IN SARASOTA COUNTY, FLORIDA
Abstract:In April 1997, the Sarasota Board of County Commissioners agreed that aging septic systems and small wastewater package plants were factors contributing to the pollution of Phillippi Creek, a major tributary to Sarasota Bay which has been designated as a National Estuary. In 1998, planning efforts were initiated whereby a total of sixteen (16) communities, within the urbanized, unincorporated area of Sarasota County were identified as requiring improvements to existing wastewater treatment practices to improve the water quality of Phillippi Creek and Sarasota Bay. Within the 50+ square mile watershed, a total of 14,000 parcels were utilizing septic systems, typically older systems situated on small parcels with sandy soils and a high groundwater table. These systems discharge wastewater volumes of approximately 3 million gallons per day (gpd) to the subsurface environment.
An assessment of available and applicable onsite wastewater treatment system (OWTS) upgrades and collection system technologies was completed to develop alternatives to improve the current wastewater treatment and disposal practices in these sixteen communities. Based on the assessment, cost comparisons of the various alternatives were made to determine whether existing OWTS should be upgraded or replaced by central sewer systems to provide needed water quality improvements in Phillippi Creek and Sarasota Bay. Cost analyses were performed based on the range of residential lot sizes in the area and included the following categories: Low Density (> 0.5 acre average lot size), Medium Density (0.25 – 0.5 acre average lot size), and High Density (< 0.25 acre average lot size). Three of the sixteen communities were selected as represent ative communities for low, medium, and high density communities, respectively.
The assessment of OWTS upgrade alternatives was completed based on their relative costeffectiveness, treatment performance and land area requirements within the specific limitations of the study area. The capital and O&M costs for the selected alternatives were estimated based on information obtained from equipment manufacturers and local contractors, recent bid information, and general engineering experience. All treatment system sizes were based on a 3- bedroom single family residence with a flow of 300 gallons per day (gpd).
Wastewater collection alternatives were reviewed on the basis of numerous factors including technical feasibility, compatibility with the existing infrastructure in the project area, public acceptance, and cost of implementation. Three sewer collection alternatives were selected for detailed cost analysis: (1) conventional gravity sewers, (2) low pressure/grinder pump systems, and (3) vacuum sewers. Conceptual layouts for the same three communities (low, medium and high density) were developed for all collection alternatives and detailed cost analyses were performed.
Based on a comparison of estimated uniform annual cost per connection, the most cost effective alternative for a community depended significantly on density of development. The collection system costs for the different communities varied widely, not only because of the effects of development density, but also due to the difference in the total number of connections and existing street layouts used in the analyses. The vacuum collection system was the most costeffective alternative for both the medium and high density areas, while the OWTS alternative (septic tank with mounded drainfield) was found to be the lowest cost alternative for low density areas. Results of this analysis were utilized for further definition of the collection system requirements under the preliminary design phase of the project. While selection of a wastewater alternative based on density was found to appropriate, this methodology did have limitations where development density is non-uniform and not contiguous. These situations required further detailed analysis during final design, considering existing infrastructure and the individual densities of sub-areas.
Considering the relatively dense urban development in the project area, Sarasota County selected central sewer collection systems as the design alternative for all 16 communities within the Phillippi Creek Study Area, with vacuum collection chosen for approximately 80% of the areas. The design, construction, and operation of central sewers proved to be the most costeffective option for improving the current wastewater treatment and disposal practices in the Phillippi Creek study area.
Document Type: Research Article
Publication date: January 1, 2003
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