The Seminole Tribe of Florida (STOF) consists of five reservations throughout South Florida. Due to growth, concerns over water quality and increasing regulatory requirements, all of the existing treatment facilities at four of the reservations are in the process of replacement with current technology.
The STOF is implementing an overall capital improvement program to all of the water and wastewater treatment facilities at the Immokalee, Big Cypress, Hollywood and Brighton reservations. Currently, two facilities are operating; one is ready for bids; and the fourth is on hold pending resolution of the disposal of effluent, reject water and other side streams.
During the course of planning and designing the four water treatment facilities for the tribe, disposal of the membrane reject water and other treatment side stream flows became an issue, due to extremely rigorous requirements pertaining to environmental impacts. The plants for the four reservations have capacities ranging from 0.10 to 1 mgd. Source water comes from wells at each site, and membrane purification results in concentration of dissolved constituents in the reject flows to between four and five times their original strength, depending on recovery. This stream, as well as the initial startup well purge stream (to reduce and stabilize SDI), contained high levels of hardness, minerals and miscellaneous dissolved and suspended components. These waste streams required disposal and were originally to be processed through the wastewater treatment facilities (WWTF). However, an absence of nutrient value combined with the strength of dissolved constituents would impose an unacceptable hydraulic and inert material load on the facilities, so another alternative was sought.
The following alternatives were evaluated:
• Surface water (point) discharge;
• Land application as spray
or percolation;
• Subsurface disposal by deep well injection;
• Ocean outfall; and
• Co-disposal with treated waste- water effluent.
Alternative evaluations included discussions with various regulatory agencies concerning permitting viability, relative economic impact on overall project costs and long-term risks. Each alternative had advantages and liabilities that were evaluated before the final recommendation was made to the Seminole Tribe.
Variations on land application were eventually selected at three of the four sites, and the final site is still being evaluated.
Immokalee Reservation
The first plant to enter operation is located at the Immokalee Reservation in Collier County. This facility has a membrane water treatment plant and an extended aeration wastewater treatment plant located on a common site, which serve the entire population of the reservation. At this facility, groundwater had background total dissolved solids (TDS) of 250-300 mg/L and operated at a 75% recovery. This produced a reject stream TDS concentration of 1,185 mg/L. The wastewater treatment plant is designed for an average daily flow of 100,000 gpd with effluent disposal to three 1.6 ac percolation ponds. TDS in the wastewater effluent was anticipated to be 150 mg/L, and concentration of the combined flow 407 mg/L. The final project design blended the water plant reject stream with the wastewater plant effluent and applied the combined flow to the percolation ponds. As the site was over 50 mi from the west Florida coast, an ocean outfall was not considered feasible. Deep well injection was rejected due to the lack of negative impacts and much lower cost of the percolation pond alternative.
Big Cypress Reservation
The Big Cypress water treatment facility in Hendry County has recently been placed into service. The wells for this facility produced water with TDS of 616 mg/L, and the plant operates at a recovery of 80%. This results in a reject stream concentration of 3,060 mg/L. The original design anticipated the reject water would be conveyed to sanitary sewer and processed through a new wastewater treatment facility.
However, the anticipated facility was delayed, and an interim solution for reject disposal was pursued. Disposal in a large storm water detention pond was considered, as well as an interim disposal in a canal adjacent to the treatment site. The TDS concentration and a concern over a buildup of solids due to proximity of the shallow water wells to the pond eliminated this alternative.
Temporary disposal in the adjacent canal was then pursued and ultimately abandoned due to environmental issues and costs associated with required dilution of the TDS concentrations in the reject water. Ultimately, the plant startup was delayed while a new forcemain was constructed from the water treatment facility to the existing wastewater treatment plant, where it will be blended with treated effluent and discharged to the existing percolation pond.
The long-term solution involves rerouting the flow to a proposed wastewater treatment facility and five percolation ponds, totaling 12 ac of the area. The final configuration will blend the reject stream with 0.4 mgd of highly treated wastewater effluent. This results in a TDS of 732 mg/L for the blended stream. This decision was based on the availability of land, the acceptable dilution ratio from the WWTF effluent and higher cost for other alternatives. The option for ocean outfall was prohibited by distance from a suitable discharge point and cost.
Brighton reservation
The Brighton water treatment facility is located in Glades County and is designed for a capacity of 1 mgd, with an anticipated bid date in mid-2005. Low well yield in the vicinity of the plant required seven supply wells over a large area to supply enough raw water, and TDS concentration averaged 500 mg/L. Due to quality problems with the groundwater (iron, manganese, sulfides and color), a dual treatment process was required using chemical conditioning and microfiltration followed by nanofiltration. With an 80% recovery, and including preliminary treatment side streams, the total plant waste concentration had 2,530 mg/L of TDS. The resulting side stream quality prohibited land application without significant dilution, and distance from the wastewater treatment facility made blending with effluent unattractive. As a result, this facility considered percolation ponds or spray irrigation with dilution from either dedicated wells or with nearby surface water. A number of considerations, including readily available land area, resulted in the final decision to spray the diluted water plant side streams on a 70-ac tract. While percolation ponds would have required significantly less area, a high groundwater table coupled with restrictions on siting made the spray option the most practical solution.
The previously mentioned low well yield near the water plant made obtaining dilution water from the canal significantly more economical than adding the number of wells required to get effective dilution. Dilution water will be pumped from the nearby canal with onsite storage of seven days effluent for wet weather. The quantity of dilution water will limit TDS in the land application to 700 mg/L or less.
Hollywood reservation
The Hollywood project includes the design of new water treatment and wastewater treatment facilities on a site currently occupied by similar facilities. The site is located in Broward County and currently houses a 0.5 mgd packaged steel wastewater treatment facility and a 0.5 mgd water treatment facility, both recently purchased by the STOF. Wastewater effluent and water treatment side streams are discharged to two percolation ponds located on an adjacent Florida Power and Light right-of-way.
As with the Brighton project, the quality of the groundwater source is poor, and the treatment process designed utilizes a preliminary coagulation and filtration step followed by nanofiltration. This results in a higher quantity of side stream flow and relatively high TDS in the combined flow. This raised concerns about the resultant TDS concentration when blended with the wastewater treatment facility effluent. The concentration exceeded 700mg/L and could affect the groundwater quality. This concern was tempered by the existing facility, which had been in service over 25 years, discharging both wastewater plant effluent and spent brine from the regeneration of the zeolite water filters. This combination has not resulted in any observed problems with surrounding water quality.
General conclusions
Experience on these projects has shown the cost-effectiveness of surface percolation as a means of reject water and water plant side stream disposal. The viability is dependent on the quality of the disposal stream and the availability of sufficient land area to limit loading rates to acceptable levels. Costs relative to ocean or deep well disposal are much lower, depending on a number of factors such as land availability and cost, as well as the necessity for dilution and associated costs.
For the STOF projects, land application proved to be the least expensive and time-consuming and was chosen for three of four facilities. The Hollywood facility would have also benefited from use of surface disposal had the land been available.
In executing the designs of the four facilities, research has shown that deep well injection and ocean outfall are the most expensive alternatives.
In southern Florida, point discharge alternatives are limited due to the nature of many surface waters having limited flow and low ability to dilute and disperse contaminants discharged. In a number of instances, designs will have to consider deep well injection or possibly ocean outfalls when other alternatives are not available.
RSS: Roads & Bridges Articles
