November 2008, Vol. 20, No.11

Problem Solvers

Problem Solvers

Keeping Up With the Population Surge

Problem: Population growth strained Florida plant’s capacity, as well as its ability to meet stringent effluent limits.

Solution: New plant handles increased flow, saves energy, and can be controlled remotely.

Sunshine, a low cost of living, and proximity to Orlando have made Sanford, Fla., home for a growing number of people. The population of this central Florida city has grown during the past decade, and the City of Sanford North Water Reclamation Facility (SNWRF) and 100-year-old wastewater collection system have felt the effects. Meeting effluent requirements became a challenge for the plant, and because the utility provides reclaimed water for irrigation, golf courses, parks and the like, effluent quality is a priority.

“Many people are moving to Florida,” said Charlie Turner, utility plant manager for the new Sanford South Water Resource Center (SSWRC). “Our flows got too high based on growth in the area. So we had to expand or build another plant. We decided to build a new plant.”

According to Benjamin Fries, vice president of project engineering firm CPH Engineers (Sanford), the city has been replacing sections of the old wastewater pipelines for the past 15 years to reduce infiltration and inflow. Fries said in an e-mail that the city’s combined sewers were removed in 1990, and a vacuum collection system was installed in the historic district, which reduced flows to the collection system by approximately 20%.

“We were concerned with the potential high-flow conditions that could be introduced at the SSWRC, due to local high groundwater conditions, the fact that the city lies along the southern shore of Lake Monroe, and the age of the collection system,” Fries explained. “Selecting the proper treatment components — particularly the biological treatment system — that could handle the potential surges in flows and loading was critical.”

While SNWRF has only primary treatment, the new SSWRC includes both secondary and tertiary treatment to handle the increased flow more effectively. Given Florida’s propensity for hurricanes, engineers had to select technologies that could easily be adapted to handle heavy wet weather events.

Working with the City of Sanford and CPH Engineers, SSWRC decided to install technologies for secondary and tertiary treatment that would efficiently treat 7570 m3/d (2 mgd), with the opportunity to expand the plant, if necessary.

“Our ultimate goal and objective for the SSWRC was to develop a state-of-the-art treatment facility, designed using the latest treatment technologies, that could be operated with the minimum amount of staff and could eventually be operated remotely from the existing SNWRF with just day trips out to the SSWRC to collect samples and verify the operation of the treatment systems,” Fries said.

After evaluating several biological nutrient removal and tertiary treatment technologies, the city and CPH Engineers decided upon technologies from Kruger (Cary, N.C.), an environmental treatment manufacturer and a subsidiary of Veolia Water Solutions & Technologies.

At the new plant, secondary treatment consists of Kruger’s Bio-Denitro™ system, final clarifiers, and return activated sludge and waste activated sludge pumping. The system comprises one influent distributor, two automated effluent weirs, two 5.5-kW (7.4-hp) submersible mixers, and four 44.7-kW (60-hp) brush aerators.

The system provides the “flexibility for advanced nutrient removal without the need for separate anoxic basins or internal recycle pumps,” Fries said.

Michele Kline, product manager of Biological Treatment Systems for Kruger, explained that the Bio-Denitro system uses phased isolation ditch technology for nitrification and denitrification.

“SSWRC is designed as a two-ditch system,” Kline said. “The system’s operation is based on the concept of phasing, where the ditches alternate between aerobic and anoxic conditions.” A programmable logic controller enables the system to run automatically from a remote location, Kline added.

Todd Hathaway, product manager for Hydrotech Filtration for Kruger, said having customized controls is vital in an area such as Sanford, where storms and hurricanes can dramatically change the inflow. “Hurricane flow can send in two to three times more flow than normal,” he said.

At press time, Florida was in the midst of hurricane season. While the new plant handles an average peak of 4542 m3/d (1.2 mgd), according to Turner, Tropical Storm Fay, which hit Sanford in August, nearly doubled that.

“During the tropical storm, we were treating just under 3 mgd [11,355 m3/d] and had no problems whatsoever,” Turner said.

Fries said this secondary treatment system also was beneficial, as based on the reactor dynamics involved and the detention time (17 hours at design annual average daily flow) provided, significant biochemical oxygen demand reduction and volatile suspended solids reduction would occur — the latter allowing the treatment facility to be designed and operated with a digestion system, either aerobic or anaerobic, significantly reducing capital and operating costs. Fries explained that the waste activated sludge is sent to a holding tank, “which acts as a buffer to convey the biosolids to a belt filter press system and Class A solids management system.” The new treatment facility has consistently produced Class AA biosolids in accordance with 40 CFR 503 and Florida Department of Environment Protection regulations, according to Fries.

A future requirement in the Sanford region, Fries said, will be the reduction of phosphorus and nitrogen levels in reclaimed water to advanced wastewater treatment levels. “This biological treatment technology will allow the city, with a small future expansion, to meet the 3-mg/L TN [total nitrogen] and 1.0-mg/L TP [total phosphorus] limits,” he said.

Tertiary treatment for the Sanford plant includes two Hydrotech Discfilters, which are installed below ground in concrete basins. According to information on the Kruger Web site, water flows into the disc filters from a center drum, and the clean water flows into a collection tank. A countercurrent backwash system cleans solids that accumulate on the inside of the media.

This particular filter system offers a smaller footprint due to its design. Instead of being laid out horizontally, the filters are stacked vertically. Fries said this system has a footprint of about 10% that of other conventional filters.

“In a lot of these locations, plants are near housing developments, so the footprint has to be small,” Hathaway explained.

Turner said ease of maintenance was an important factor when considering options for tertiary treatment. The Discfilter, Hathaway said, “is partially submerged, allowing access for routine maintenance. Media can be accessed without having to dewater the tank.”

Cost savings and efficiency have improved for the Sanford Water Reclamation Facility since the installation of secondary and tertiary treatment. Several factors helped contribute to this.

One is the oxidation ditch used in the secondary treatment. Kline said that with rotors and mixers included in the system, during the phasing option, rotors can be turned off while mixers are kept on, “allowing water to continue to move to complete the process, while mixers run at lesser horsepower. Automating the process in general helps for [energy savings].”

“You can dial down the system as needed,” Hathaway added.

The total project capital cost was $17.1 million. The City of Sanford was able to tap into the Florida Department of Environmental Protection State Revolving Fund Loan Program to finance the project.

Water Volumes

Subsurface Hydrology

George F. Pinder and Michael A. Celia (2006). John Wiley & Sons Inc. Wiley Interscience, 350 Main St., Malden, MA 02148, 468 pp., $77.80, hardcover, ISBN 0-471-74243-0.

This book provides a sound groundwork for subsurface hydrology, starting with the physical attributes of the subsurface environment and the associated terminology. Fundamental equations and descriptions are introduced to define the interaction between the solid and fluid phases. Discussions include a basic description of geology, collecting field data, presenting information for interpretation, and encountering subsurface contamination. The authors cover not only the basics but also practical information used in the profession and some experiments to demonstrate the attributes of this multiphase environment.

After introducing the subject, the authors take the time to develop equations in a manner that is easy to follow and guide the reader into various scenarios, demonstrating how to solve for groundwater flow problems. These discussions include applying various analytical methods to results obtained in the field and developing quantitative models. The latter part of the book is devoted to subsurface contamination, including remediation and the chemistry component that must be considered for such problems.

This book is primarily suited for advanced undergraduate or graduate level students in science and engineering, as well as any scientist or engineer who works with groundwater issues. The authors present the subject of subsurface hydrology in a comprehensible format, starting with basic concepts and progressing to more complex ideas. There are numerous examples, illustrations, and footnotes as clarification to help the reader visualize the scenarios. An introduction and summary are provided in each chapter that aid the reader in locating a particular topic. Problem questions at the end of each chapter reinforce what was covered; sufficient references are provided for further study.

The book is not nearly as well suited as a source about geology. Terminology in geology is quite specific, because what is being described is not limited to the processes of hydrogeology. Geological terminology tells of structure, age, composition, and transmutation, while the language of hydrogeology is by its nature more rudimentary when it comes to formation description — for example, aquitard or aquifer, terms that describe resistance or conductivity of water and that are essentially oblivious to the overburden or rock type. There are several terminology usages — for example, fractured rock being used to refer to blocks of soil or rock — in this book that would not be construed as correct in a geological context. The authors also present a brief summary of geological history that could have been written better to show how basic geological processes and resultant changes over time correlate to the hydrodynamic characteristics observed, which is the primary importance of the geological setting in terms of groundwater movement. There is a section on field investigations and technical practices, some of which would be considered inaccurate — for example, collecting a soil core with a split-spoon sampler (which uses hammer blows) to obtain an undisturbed sample, instead using of a Shelby tube. These shortcomings, however, do not interfere with the overall concepts and mathematical principles presented for groundwater.

Paula MacRae is a geologist–hydrogeologist in the Environmental Site Investigation and Remediation section of TRC Companies Inc. (Millburn, N.J.).