The Falls Lake Algal Turf Scrubber® (ATS) pilot was implemented for the City of Durham, North Carolina, to evaluate the suitability of ATS technology for nitrogen and phosphorus load reduction through direct treatment of Falls Lake water. The pilot was located downstream of Interstate 85 and upstream of the Hickory Hill boat launch, where lake water was pumped through a 500-foot-long ATS floway and discharged back to Falls Lake.
Falls Lake ATS pilot floway adjacent to Falls Lake near the Interstate 85 crossing in Durham, North Carolina. The pilot evaluated attached-algae treatment for nutrient reduction under site-specific lake and watershed conditions.
Facility Summary
Facility: Falls Lake Algal Turf Scrubber® Pilot
Location: Durham, North Carolina
Technology: Algal Turf Scrubber® / attached-algae floway
Scale: 28,000 GPD; 500-foot-long by 1-foot-wide floway
Status: Pilot testing completed
Initial Operation: 2015
Source Water: Falls Lake surface water
Operating Context: Water-supply reservoir; nutrient management strategy; direct lake-water treatment
Application: Nitrogen and phosphorus load reduction, suspended solids reduction, biomass recovery, and full-scale performance projection
Owner / Sponsor: City of Durham Stormwater Services
HydroMentia Role: Pilot system design, installation support, operation, monitoring, data evaluation, ATSDEM modeling, and final performance reporting
Operating Context
Prior to the pilot, HydroMentia and Biohabitats prepared an Algal Turf Scrubber® Feasibility Study for the City of Durham. That study evaluated potential ATS applications along Ellerbe Creek, Little Lick Creek, and Falls Lake; identified and ranked candidate pilot and full-scale sites; and modeled nutrient-load reduction and treatment-cost scenarios. The Falls Lake pilot was subsequently implemented to generate site-specific field data for direct lake-water treatment and to refine full-scale performance projections.
Falls Lake is a water-supply reservoir in the Neuse River basin. The City of Durham commissioned the Falls Lake ATS pilot after identifying the need for cost-effective nutrient reduction approaches to help address Falls Lake nutrient management requirements. The pilot was intended to evaluate whether ATS technology could provide meaningful nitrogen and phosphorus load reduction through direct treatment of Falls Lake water.
The selected pilot site was located near the Falls Lake Interstate 85 crossing, downstream of I-85 and upstream of the Hickory Hill boat launch. The location provided direct access to Falls Lake water, available site access, and power. Water was pumped from the lake to the top of the ATS floway, passed across the shallow sloped treatment surface, and returned to the lake approximately 35 feet downstream of the intake.
The pilot floway was 500 feet long and 1 foot wide. The system was operated as a single-pass ATS unit, with a target flow of approximately 20 gallons per minute. Flow was introduced on July 29, 2015, followed by a start-up and stabilization period during which the attached algal community developed on the floway surface. Water quality and biomass monitoring continued through December 2016.
Operational Significance
The Falls Lake pilot is significant because it tested ATS technology in a temperate reservoir setting with seasonal temperature variation, variable lake nutrient concentrations, high rainfall conditions, and operational challenges associated with lake-level flooding. These conditions provided a very different evaluation environment than HydroMentia’s large Florida ATS facilities.
Total metered flow to the ATS floway was 12.6 million gallons at a mean flow rate of 19.5 gallons per
minute for the Monitoring Period. The mean flow rate for the CY2016 Monitoring Period was 19.2 gallons
per minute. Flow was directed to the ATS for a period of 448 days during the Monitoring Period.
During 2016, harvested biomass productivity averaged 18.9 dry grams per square meter per day, with warm-season productivity substantially higher than cool-season productivity. Warm-season 2016 productivity averaged 26.3 dry grams per square meter per day.
The system reduced total phosphorus during the monitoring period and showed meaningful nitrogen reduction during warm-season and late-cool-season operation. Annual nitrogen performance was affected by early-2016 flooding, pump shutdowns, dry-out events, snow and ice cover, and the resulting temporary loss or disruption of the algal turf community.
The pilot also demonstrated suspended solids reduction. During 2016, total suspended solids were reduced by approximately 54%, and during one high-suspended-solids event, an established algal turf reduced TSS from 67 mg/L to 3 mg/L. This finding was important because the ATS floway functioned not only as a biological nutrient recovery system, but also as a physical filtration and solids capture surface when the algal turf community was well established.
The Falls Lake final report used pilot data to project load reductions and treatment costs for larger 10 MGD and 25 MGD ATS facilities. Based on 2016 pilot performance, projected annual load reductions ranged from approximately 2,856 to 7,140 pounds of nitrogen per year and 504 to 1,260 pounds of phosphorus per year for the 10 MGD and 25 MGD facility sizes. The project therefore provided the City of Durham with field data, full-scale modeling, and life-cycle cost estimates for evaluating ATS as a potential nutrient management tool.
Photographs
Falls Lake ATS pilot intake from Falls Lake.
The Falls Lake pilot provided valuable information on seasonal performance, start-up and stabilization, and operational resilience under high-rainfall, flooding, and cold-weather conditions.
View Falls Lake ATS photo gallery.
Additional facility photographs are available in HydroMentia’s Facebook photo archive. Facebook may require visitors to sign in to view the complete gallery.
Reports and Publications
The Falls Lake Algal Turf Scrubber® Pilot Program Final Report was prepared for City of Durham Stormwater Services and completed in June 2017. The report summarizes pilot installation, operations, water-quality performance, biomass productivity, suspended solids reduction, ATSDEM modeling, treatment cost projections, and biomass management options.
Technical Report: Falls Lake Algal Turf Scrubber® Pilot Program Final Report — June 11, 2017
Additional reports and publications related to Algal Turf Scrubber® technology and facility-scale applications are available in HydroMentia’s ATS Library.
Related Facilities
Related HydroMentia ATS facilities and demonstrations include Osprey Marsh ATS, Egret Marsh ATS, S-154 ATS, Maryland Port Administration Algal Flow-Way, and other full-scale and pilot-scale systems.