About the author: Abbott is marketing manager, Water Purification Solutions, Severn Trent Services. She can be reached at 215/997-3733 or by e-mail at [email protected].
For many years, heavy rainfalls in Birmingham, Ala., caused flow at the Village Creek Wastewater Treatment Plant to increase from a normal 30 mgd to more than 360 mgd. Any flow beyond approximately 80 mgd had to be bypassed to the Village Creek waterway, which feeds Bayview Lake.
To address the problem, a pilot filtration study was commissioned by the Environmental Services Department of Jefferson County, Ala., and by consulting engineers Gary L. Owen & Associates of Birmingham. A pilot filter plant containing two 10-sq-ft filters, each housing a 5-ft depth of large, rounded sand filter media, was supplied by Severn Trent Services.
In operation since mid-2003, the $300 million peak-flow treatment facility has dramatically improved wastewater quality in Alabama’s largest city and passed the sternest test to date when Hurricane Ivan drenched the surrounding area with nearly 10 in. of rain in September 2004.
Pilot testing
Pilot testing of the deep-bed sand filtration system took place in the summer of 2000. The pilot program lasted more than two months while a variety of flows, solids loadings and operating techniques were investigated. Operations focused on loading the pilot filters with about 100 mg/L TSS wastewater at 10 gpm/ft2 of filtration area. This represented applied loading rates that were two to four times higher than previously experienced and required developing new ways to maintain filter flow.
During testing, monthly average effluent limits were 30 mg/L TSS and 25 mg/L carbonaceous biochemical oxygen demand. The pilot plant consistently met these limits during repetitive testing. The best effluent and longest filter runs were achieved with no chemical addition.
Conventionally operated filters would have quickly plugged with such high influent flow and solids loadings, so during the pilot study, new methods of operating and backwashing were developed to keep the water moving. These methods are now patented.
One proprietary operating method, called SpeedBump, involved stopping effluent flow from the first filter, reversing the flow using the backwash pumps for a short period (about one minute), then proceeding immediately to the next filter while flow was restored to the first filter, and so on. Closing influent valves during this process was optional, depending on the initial water level in the filter. This continuous process provided a surprisingly effective reduction in the backup wastewater.
According to Dave Slack, general manager of Severn Trent Services’ office in Tampa, Fla., “Originally, our customer at Village Creek didn’t believe the SpeedBump process would be an important part of the operation. But after seeing the system in operation, he recognized its significance. It allowed water levels in the filters to drop below the overflow point for a period of 20 minutes to as much as two hours. The technique was key to achieving reasonable filter run times when handling high wastewater flows and heavy solids loadings.”
Further testing was done to simulate the first flush of the collection system in the first few hours of a major rain event. This led to the development of new backwashing methods to allow for continuous application of 200 to 500 mg/L TSS undiluted raw wastewater to the filters at about 5 gpm/ft2.
With this very high loading, filter run time was cut to about an hour, even with 20-minute interval SpeedBumping.
A short air-water backwash of only a few minutes was enough to expel most of the solids causing the backup and get the flow going again. This proprietary method, called SpeedWash, has been automated in the large filter plant to proceed continuously, seamlessly and quickly from one filter to the next, completing the circuit in sufficient time to operate at elevated solids loading conditions.
The first flush treatment tests produced effluent quality of about 40 mg/L TSS, below the projected weekly permit average of 45 mg/L.
Plant start-up
The new plant was put into operation in July 2003. With deep-bed sand filters, the system is capable of handling the base flow plus flow from a second new bio plant, which became operational in October 2004. The system also includes a 340-mgd pump station; 20 surge basins with a total capacity of 90 million gal that can handle up to six hours of peak flow; a second new biological treatment plant; and ultraviolet (UV) disinfection.
The plant includes 22 deep-bed filters, each with a 1,160-sq-ft filtration surface area. The filters are laid out in two trains with room for expansion. The new control methods and innovative piping designs (a technology package referred to as StormMaster) allow continuous, rolling execution of up to two simultaneous air-water backwashes and four water-only backwashes at the same time, divided among the two filter trains to control head loss and expel excess solids during wet weather events.
These events peak at up to 360 mgd for four hours, with elevated flows persisting for as much as 24 hours. However, between storms, the plant is successfully polishing 60 mgd of final effluent for discharge to Village Creek. A portion of the peak flow continues to be handled by the existing biological treatment plant. Dirty backwash water is sent to thickeners in the biological treatment plants. The full-scale, heavy-duty filter design has been successful in large new high-flow plants in other locations and in heavy industrial and steel mill services all over the world. In addition to handling wet weather flows, the filters serve a valuable role of effluent polishing for many municipalities.
Plant features
One key feature of the filters is the 6-ft-deep bed of large, rounded sand, which allows high flow rates, large storage capacity for heavy solids loadings and long run times. Self-flocculating mechanisms for solids lodged within the filter media reduce the need for chemical addition.
A second key feature is the filter underdrain. Rows of arched concrete blocks rest on the filter floor, leaving large passages for water to flow under and between rows. This design, called a T-block system, is extremely resistant to biological fouling, making it ideally suited for wastewater applications. The blocks also protect the backwash air-distribution system.
A third key feature is a system of stainless steel backwash air laterals under alternate rows of T-blocks. The system precisely meters air under the entire bottom of the filter, allowing for even, effective backwashing. The design stands up well to heavy-duty conditions and provides zero maldistribution. Many plants have used this system for more than 20 years with no major maintenance or loss of media.
“Simultaneous air-water backwashing provides far superior cleaning capability and much lower water consumption than separate air scouring and high-rate water washing,” Slack said. “This is important in a large, heavily loaded filter plant. Backwash water consumption during rare peak flow operation is calculated to be about 10% of forward flow, but only 1 to 2% for normal operation. A field survey found that other filter types were averaging 11% of forward flow for backwash, even for normal operations. As a result, the Village Creek plant is benefiting from reduced recycle flows and treatment costs.”
Adjacent to the new filter plant are 24 surge basins that hold the first flush of undiluted wastewater with the highest concentration of pollutants for later processing. This water may be applied directly to the filters at the operator’s discretion. The new biological treatment plant also takes in some of the surge flow directly. All wastewater treated by the new facility also passes through the new filters.
Final effluent is treated by UV disinfection before discharge to Village Creek. UV disinfection is greatly enhanced when supplied with filtered water, resulting in improved kill effectiveness, reduced power draw and greatly decreased tube maintenance.
Weathering Ivan
On Sept. 16, 2004, the plant received a significant test when Hurricane Ivan roared through the state. Ivan was still a tropical storm as it passed through Birmingham and Jefferson County during the late afternoon and early evening. While peak wind gusts were significantly reduced by then to just 45 to 55 mph, the storm dumped nearly 10 in. of rain on the area.
The new facility handled the torrential rainfall according to its design. Rain infiltration into the collection system caused flows to rise rapidly. A 182-mgd flow was applied to the filters, which they handled without difficulty. The SpeedBump cycle was utilized, reducing head loss in the filters as planned. Later in November, the plant experienced three consecutive days of high flow ranging from 187 to 137 mgd.
“The effluent TSS in each instance—during Hurricane Ivan and during the three consecutive days in November—ranged from 1 to no more than 12 ppm TSS,” Slack said. “In some cases, the use of the patented SpeedBump feature lowered the cell level from 89 to 55%, establishing its criticality.”
The peak-flow facility in Birmingham, using deep-bed filter technology, has helped the plant not only improve wastewater quality, but benefit the Village Creek waterway and Bayview Lake—even during a hurricane.
