Location: Horse Cave, Kentucky
Project Type: Industrial BOD Roughing
Completion Date: May 2012
950 m3/day (0.25 MGD)
- 9 mg/L Total Ammonia-Nitrogen
- 30 mg/L BOD
Project Background & Challenges
The town of Horse Cave, Kentucky, is located in southern Hart County between the Barren County line and the border of Cave City. Initially settled in 1840 and incorporated in 1864, the town received its name following the establishment of a railroad station, under the landowners condition that it be called Horse Cave, for the large cave located nearby.
Currently, Horse Cave is comprised of approximately 3000 people. Owing to it’s early railroad connection, it was once a major centre for agricultural commerce for the town and surrounding area, primarily tobacco. However, since the 1970’s, several factories have also relocated to the area, specializing in industries such as single-use foodservice products, salad dressing, bread/rolls, geothermal system products and automotive accessories.
However, with the increase in major manufacturing in the area, comes both an increase in wastewater output and also a change in the content of that wastewater. The existing wastewater treatment plant (WWTP) wanted to reduce the biological oxygen demand (BOD) in the wastewater from the salad dressing manufacturer by adding a pre-treatment stage for reducing biochemical oxygen demand (BOD) ahead of the main treatmment plant.
The Nexom Answer
The primary objective for the pretreatment facility in Horse Cave was to reduce the biological oxygen demand (BOD) in wastewater directly after the plant’s existing dissolved air flotation (DAF) system.
The existing system in Horse Cave was initialluy designed to meet a 24 hour BOD concentration of 350 mg/L. So when the city imposed a 300 mg/L (5-day average) limit on the industrial flow to the municipal plant, the Hart County Industrial Authority and Scott and Ritter Construction, Inc, explored various pretreatment options before ultimately choosing the BioPortz Moving Bed Biofilm Reactor (MBBR).
The BioPortz MBBR was chosen for its flexibility as a stand-alone process or as part of a larger process, as well as its ability to reduce BOD, Total Suspended Solids (TSS) and Fats, Oils and Gases (FOG) on a small footprint.
As a pretreatment process, the MBBR in Horse Cave is dedicated exclusively to treat the wastewater from the salad dressing manufacturer, and therefore is used for food waste only. Average daily flow ranges from 0.160-0.200 MGD.
To accomplish the required amount of treatment, Nexom supplied the wastewater treatment plant (WWTP) with 6,684 ft3 of BioPortz media, which is the equivelent to 1,196,436 ft2 of total surface area, at a protected surface area to volume ratio of 179 ft2/ft3. The media was constructed of virgin high-density polyethylene (HDPE) with carbon black added for additional UV protection, and containing a specific gravity of 0.96. This would provide for a 47% fill at Minimum Operating conditions (specifications require no more than 50% fill at this operating volume).
Upgraded System Performance
The Horse Cave MBBR system receives flow for only 8 hours a day but discharges flow 24 hours a day, which allows it to act like its own activated equalization tank. Due to the source and nature of the influent, it is also nutrient deficient, so the facility adds nitrogen and phosphorus to the wastewater. Average wastewater temperature rages from 15-35°C.
The influent is mechanically screened prior to aerated equalization storage and pH adjustment with 50% caustic soda (adjust from 5 to 7 pH). Wastewater is then pumped to diffused air flotation units where, aided by coagulant and flocculant addition, FOG are separated from the wastewater. The wastewater is then pumped to the MBBR for primary biological treatment.
From there, the MBBR effluent is pumped to a sequencing batch reactor (SBR) for final biological treatment. SBR effluent is then discharged to an effluent holding tank for eventual discharge to the main municipal WWTP.
A supervisory control and data acquisition (SCADA) system was installed to help control the MBBR tank level. The MBBR is filled with DAF effluent during the eight-hour workday. The MBBR then discharges to a pre-set minimum level to the SBRs during the workday and overnight so that is ready for ‘re-fill’ the next morning. During the fill period, the influent wastewater contains 1,750 mg/L of BOD, and 350 mg/L of TSS. The rate of discharge is considerably higher during the fill period than during the non-fill period. During the fill period, the tank discharge at a peak rate of 340 gpm, compared to during non-fill conditions, when the tank discharges wastewater at a peak rate of 170 gpm. Due to this constant fill & discharge of high strength wastewater, the facility often wastes SBR sludge to the MBBR to help maintain a stable biomass.
Since the biomass was fully established mere weeks after the system came online, it has averaged 98.46% removal with average effluent BOD of only 127 mg/L.