By Michael S. Stadnyckyj
The Arapahoe County Water and Wastewater Authority (ACWWA) operates the Lone Tree Creek Wastewater Treatment Plant (WWTP) on the outskirts of Denver, Colo. The plant installed membrane technology to achieve phosphorus discharge limits of < 0.2 mg/L and expand capacity in a "Just-in-Time" strategy to reduce current and future capacity expansion costs.
The Lone Tree Creek WWTP was constructed in the early 1980s as a 0.2 mgd oxidation ditch system. Over the years the plant has undergone several upgrades including the construction of Rapid Infiltration Basins (RIB), Sequencing Batch Reactors (SBR) and aerobic digesters. In the early 1990s the oxidation ditch clarifier was modified and put into operation to chemically remove phosphorus.
As the facility reached its flow capacity of 0.8 mgd, the ACWWA began to investigate alternative treatment methods to achieve discharge limits of 0.2 mg/L phosphorus, a new limit of 10 mg/L total nitrogen and to expand capacity.
Proposals were received for two treatment methods. The first was the conversion of the existing SBR process into a conventional activated sludge (AS) process, followed by an advanced treatment process consisting of chemical precipitation, sedimentation and filtration.
The second method was the conversion of the SBR process into a membrane bioreactor (MBR) process. This process consists of immersing hollow-fiber membranes into the aeration basin. The membranes combine clarification, aeration and filtration into one process step, which takes place in a single SBR basin.
Both methods were piloted at the Lone Tree Creek WWTP. In selecting a treatment process, the ACWWA had several key parameters which were given special consideration: the quality of the effluent, ease of expanding capacity while minimizing plant footprint, and the ability to accommodate rapid growth of the surrounding community.
The method which best met the ACWWA's needs was the MBR process, and in 1997 a contract was awarded to Zenon Environmental Inc., Toronto, Canada, to provide the necessary equipment to upgrade the facility. The initial capacity of the MBR process was 1.2 mgd.
Upgrade Strategy
For the initial upgrade, only one of the two SBR basins was required. The MBR process typically operates at MLSS concentrations between 10,000 and 15,000 mg/L, allowing for reduced hydraulic retention times and increased solids retention times as compared to a conventional activated sludge basin. If required, this higher MLSS operating environment would allow the ACWWA to increase the capacity of each SBR tank by up to four times. The second SBR tank was taken off-line until the second phase of the upgrade.
Several modifications were made to the WWTP to accommodate the MBR process. The main alterations included: upgrading the existing coarse screen to a 3 mm fine screen, the construction of an equipment room next to the process tanks to house the membrane pumping equipment, and the replacement of the existing jet aeration system with a fine bubble diffuser grid.
Within the SBR process tank, a concrete baffle was constructed to create a plug flow anoxic zone prior to a completely mixed aerobic zone.
MBR Treatment Process
The MBR process consists of four major process steps:
- pre-treatment screening,
- anoxic zone treatment,
- aerobic zone treatment and
- immersed membrane filtration.
Once the raw sewage has been screened, it is conveyed to an influent pump station that feeds the anoxic zone. The anoxic basin has a volume of 112,000 gallons and has three, 4 hp submersible mixers. Ferric chloride is added to the anoxic zone for phosphorous precipitation, typically 75 mg/L.
The aerobic basin is operated as a suspended growth activated sludge basin with a volume of approximately 186,000 gallons. Nitrification is achieved in the basin as well as phosphorus precipitation through further ferric chloride addition.
The immersed membranes are placed in the aerobic basin and replace the secondary clarifier and media filtration steps of a traditional tertiary treatment AS system. The membranes occupy approximately half of the aerobic basin.
Within the aerobic zone, individual membrane modules are combined to form cassettes. At the Lone Tree Creek WWTP, there are currently 12 cassettes with 12 modules each.
Filtration is achieved by drawing effluent through the surface of the hollow-fiber membrane under a low-pressure suction of -1 to -9 psi. The pores on the membrane surface form a positive barrier to prevent solids from entering the effluent stream. Once drawn to the lumen of the membrane fiber, treated water is further drawn to either top or bottom water chambers of the membrane module and conveyed to the main effluent discharge pipes.
The surface of the membrane fiber is kept clean through two primary methods, aeration and back-pulsing. Diffused air is introduced from the bottom of the membrane module via an air supply connection. Air bubbles travel up the surface of the membrane fiber, removing any solids that may have adhered, the aeration also creates a recirculation pattern within the process tank, minimizing the settling of solids. At pre-set time intervals, the membranes are back-pulsed. This is accomplished by briefly reversing the flow of effluent through the membrane to remove any particles that may have obstructed the pores during membrane operation.
A Programmable Logic Controller monitors the entire system and makes adjustments to pump speeds and valves to ensure the plant is operating at optimal performance.
Expansion Strategy
The MBR method offers the advantage of staged expansion to coincide with the service needs of the community. Simply adding immersed membrane modules/cassettes to the aeration tank through predetermined guide slots, and ancillary equipment hook-ups, increases process capacity. As no major facility alterations are required, process capacity can be increased in a "Just-in-Time" method, reducing unused capacity that provides little benefit to the authority.
Conclusion
The membrane system has allowed the Lone Tree Creek to meet required effluent discharge limits and easily expand process capacity to match the surrounding community's growth, reducing capital and operating costs.
Total nitrogen concentrations have been consistently less than 10 mg/L and generally less than 6 mg/L. Effluent total phosphorus concentrations have been consistently less than 0.2 mg/L and frequently less than 0.1 mg/L. The high effluent quality produced by the MBR since commissioning has allowed the ACWWA to expand it's wastewater reuse plans within the county for landscape irrigation and to protect the local recreational lake from potential degradation from excess phosphorus load.
