Protecting the Florida Keys with advanced wastewater treatment
Clayton McCormack, P.E.
Palm Bay, Florida
The Florida Keys are a chain of approximately 800 independent islands located off the southeastern tip of Florida, representing the most southerly point of the continental United States. Home to a complex and dynamic marine ecosystem, the Keys include the world's third largest coral reef system and offer a natural beauty that attracts visitors from around the world. Supporting major fishing and tourism industries, the reef and the entire marine ecosystem are the lifeblood of the Keys; protecting their existence and vitality is critical to the economic and environmental future of the islands. Development, population growth, and tourists threaten the Keys' health and future existence.
The majority of the Keys are served by onsite treatment systems and small packaged treatment plants. An estimated 23,000 onsite systems (including septic fields, aerobic treatment units, and illegal cesspools) primarily serve single-family residences and small commercial establishments, while approximately 250 small wastewater treatment plants serve condominium and apartment complexes, resorts, motels, restaurants, and other larger commercial developments. Each of these onsite systems and treatment plants provide minimal if any nutrient removal, with effluent from these facilities typically containing nutrient levels of about 20 milligrams per liter (mg/L) of total nitrogen (TN), and 5 mg/L of total phosphorus (TP).
Extensive scientific study of the deterioration of the reef and the entire marine ecosystem has revealed that one of the principal causes of the Keys' unhealthy state is the elevated level of human-derived nutrients in the surrounding canals and nearshore waters. Nutrients, comprised of nitrogen and phosphorus, are found in high levels in raw sewage, partially treated and secondary treated wastewater discharged to the ocean. To improve the water quality of the nearshore waters and the coral reefs in the Keys, the State of Florida and Monroe County are undertaking an initiative requiring existing wastewater treatment plants to be upgraded to meet stricter advanced treatment effluent limits. In populated areas where no sanitary sewer services are currently available, entirely new centralized collection systems and advanced treatment plants are being required to be installed by 2010. This initiative is projected to cost in excess of $400 million.
The North Key Largo Utility Corporation is being faced with the challenge of developing a solution to these tighter regulatory requirements. The authority provides water and wastewater services to the Ocean Reef Community in North Key Largo, Florida. Their existing 0.55 million gallon per day wastewater treatment plant, last expanded in the early 1990s, utilizes a conventional extended aeration treatment process that meets typical secondary effluent permit limits. The utility has been challenged to upgrade their existing wastewater treatment plant to achieve advanced wastewater treatment (AWT) standards.
In Florida, AWT standards require nearly complete removal of nitrogen and phosphorus from wastewater to achieve a total nitrogen level of 3 mg/L or less and total phosphorus of 1 mg/L or less in the treated effluent. These limits are extremely difficult to achieve. Due to multiple potable water conservation measures, it is common for the ammonia concentration of the raw sewage to be greater than 75 mg/L. The challenge to meet strict effluent limits is even greater for the North Key Largo Utility Corporation because there is very little space available onsite to expand their wastewater treatment plant with additional aeration tanks.
To produce a higher quality of treated effluent within the same approximate footprint, the utility will be upgrading their treatment plant using an advanced membrane biological reactor (MBR) treatment technology. This newly emerging wastewater treatment technology, when used in conjunction with an Enhanced Nutrient Removal activated sludge biological treatment system, will achieve the required level of effluent quality within their existing tank space. In an MBR system, membrane filters are submerged in an aerated biological reactor. The membranes have a pore size from 0.035 to 0.40 microns, which is considered between micro and ultra filtration. This level of filtration allows high quality effluent to be drawn through the membranes eliminating the sedimentation and filtration processes typically used for conventional wastewater treatment. Using the MBR allows the biological process to operate at a much higher mixed liquor concentration (10,000 mg/L) and can dramatically reduce the volume of the biological treatment tanks necessary to achieve treatment.
Existing aeration basins will be retrofitted into a membrane bioreactor treatment system to meet Advanced Wastewater Treatment effluent limits.
Several operational factors must be addressed when upgrading to a membrane biological reactor type treatment technology. Fine screening (down to 2 mm opening) must be used to protect the membrane from possible damage. The membrane filters require routine cleaning every 30 to 60 days using a backflush of sodium hypochlorite to restore the permeability of the membranes if clogging of the fine membrane pores occurs. Plant operators need to be aware that MBRs are highly automated and therefore their operation is more complex than a conventional type of treatment.
In the case of the North Key Largo Utility project, the existing aeration basins are being retrofitted into a four-stage Bardenpho biological system to achieve nearly complete biological nitrogen removal. Nitrogen in typical domestic wastewater is generally in the form of organic nitrogen and ammonia. During treatment, a certain class of bacteria converts the organic nitrogen into ammonia form. Nitrifying bacteria then oxidize ammonia to nitrate under aerobic conditions. Nitrate is a stable form of nitrogen and not easily removed from the wastewater without converting to another form. Denitrification, which must occur in an anoxic (no oxygen) environment, will convert the nitrates to gaseous nitrogen, which is released to the atmosphere. The existing aeration basins will be converted into anoxic and aerated zones to achieve the required nitrogen removal.
Odor concerns for uses surrounding the wastewater treatment plant will be addressed by covering all treatment tanks with an FRP cover system and conveying gases from the treatment process through an odor control unit. The small footprint of the treatment process allows covering the tanks to be cost effective. This approach will ensure there will be no odors or other objectionable gaseous emissions from the treatment plant.
The membrane bioreactor has several distinct advantages over a conventional activated sludge system that make most regulatory agencies look favorably on MBRs, especially for small plants in environmentally-sensitive areas. These include:
Since the equipment can be more costly than conventional plants, it may not be appropriate technology for every application. The North Key Largo Utility Corporation has selected to upgrade their treatment plant using membrane bioreactor technology to achieve a higher level of treatment within the same footprint as their existing treatment plant.
Clayton McCormack is a Project Manager with Wade Trim, a consulting engineering firm with offices throughout seven states. He has 12 years of experience designing wastewater treatment facilities for municipalities. He can be reached at (321) 728-3389 or firstname.lastname@example.org.