Wastewater Treatment Facility Renovations, Dahlonega, Georgia
ACEC/GA Engineering Excellence Award Winner
Nestled in the foothills of the Blue Ridge Mountains 70 miles north of Atlanta lies the picturesque city of Dahlonega. Like so many charming small towns across America, Dahlonega’s scenic allure is made possible by a complex network of infrastructure that has at times struggled to keep pace with population growth and changing environmental regulations. The city’s wastewater treatment facility (WWTF) was originally constructed in the mid-1970s with a maximum capacity of 0.72 million gallons per day (mgd). The original plant included an influent comminutor, oxidation ditch, a single secondary clarifier, chlorine disinfection, aerobic digester, and sand drying beds. A small upgrade in the 1980s replaced the comminutor.
With increasing flow and aging equipment originally intended for different treatment limits, the facility began to experience operational issues in early 2014. Two notices of violation (NOVs) for ammonia were handed down by the Georgia Department of Natural Resources, Environmental Protection Division (EPD) in July 2014: one for exceeding the monthly average and one violation for exceeding the weekly permit limits. There were also nine NOVs associated with phosphorus between January and August: six violations of monthly limits, two violations of weekly limits, and one violation of monthly mass limits. In August alone, the plant received nine total NOVs associated with the effluent total phosphorus and ammonia nitrogen.
After meeting with the EPD in September 2014, the City of Dahlonega agreed to a Consent Decree which required them to develop a Corrective Action Plan within 90 days detailing improvements and upgrades to the WWTF to mitigate and eliminate the reoccurring violations and provide a schedule for implementing the corrective actions. The city retained WK Dickson to prepare the plan as well as conduct an overall assessment of the WWTF.
WK Dickson provided overall analysis, planning, environmental, permitting, design, and construction administration services for the WWTF improvements. Our initial evaluation of the facility focused on addressing and correcting the ammonia and phosphorus NOVs revealed that the ammonia nitrogen permit violations were directly related to a break in the aeration header within one of the SBR aeration basins. This required the affected basin to be drained for repairs and the SBR system to be operated in the remaining basin in one continuous flow. Drainage of the basin also revealed other potential issues with the diffuser system, accumulations of solids within the basin, and potential leaks with the flexible piping from the decanter.
Phosphorus permit violations were attributed to operational issues with the traveling bridge filter and chemical feed system. The filter was approaching approximately 20-years of service and the manufacturer was no longer in business. This made replacement parts difficult to obtain, resulting in long lead times for repairs with the filter out of service for extended periods. In the facility’s early years when permit limits were higher, the filter provided a solid dependable feature in the plant but asking it to operate to the performance levels of the current permit conditions proved to be too much. Difficulties were experienced in achieving an efficient filter backwash to maintain the filter hydraulic capacity.
Following development of preliminary cost projections, the following improvements were selected by the project team for implementation as part of the corrective action plan:
- Aeration System: The existing blower motors were replaced with high efficiency motors with variable frequency drives (VFDs) to control blowers based on SBR dissolved oxygen level, and leaking gaskets in the aeration header system were replaced.
- SBR System - Accumulations of grit and solid materials within SBR basins were removed. The existing SBR aeration system was replaced with stainless steel headers and coarse bubble diffusers for increased reliability and efficiency, and the flexible piping at the decanters was replaced. The existing SBR control system was also upgraded to a new PLC with capability for providing anaerobic, anoxic phases into SBR phases and control blowers based upon dissolved oxygen levels within basin. The SBR electric valve actuators were also replaced in the influent and effluent piping.
- Effluent Filters: A chemical flocculation basin was added immediately ahead of the disk filters to improve chemical reaction time and floc formation. The existing traveling bridge filters were replaced with new cloth media mini disk filters within the existing filter structure.
- UV System: The aging UV equipment was replaced with a new dual bank UV system, providing redundancy.
- Biosolids System: The existing belt press sludge pump was replaced. The belt press was also replaced with a 1.7-meter press to provide additional capacity and to reduce required operation time. The dewatered sludge screw conveyor was also replaced.
- The chemical feed system was converted back to the original sodium hydroxide feed to eliminate the handling and mixing of lime. The alum and caustic chemical pumps were replaced with duplex, flow proportional pumps, and new alum pumps and piping were added to permit alum feed to the aerobic digesters prior to decanting to reduce phosphorus recycled back to plant influent.
Implementation of these improvements proved challenging on several fronts. Bypass pumping of the influent pump station was required while modifications to the influent pumps were completed. During replacement of the SBR control system, the SBR system had to be manually operated, rather than automated as is typical. One SBR basin was operated in continuous flow mode during repairs to other SBR basin, and SBR basin bottom solids deposits had to be dewatered to achieve enough solidification for landfill disposal. A temporary mini disk filter system was used during modifications to the filter system, and a temporary chlorination/ dichlorination system was used during modifications to the effluent UV system. Solids inventory and staging replacement of existing sludge dewatering belt press system was carefully coordinated with provisions for a contract dewatering system if needed.The commitment of the contractors, operators and suppliers to timely management and installation of these features was critical to the construction process.
As construction progressed and the team gained access to new plant components, additional needs were identified. Flexible piping at effluent decanters had leaks, permitting leakage of mixed liquor solids into the effluent during decant operation and needed to be replaced. Work to replace the influent pump retravel cables revealed a need to replace the influent pump base elbows and discharge piping within the wet well. The dewatered sludge system screw conveyor had significant wear and was nearing failure and needed replacement. Removal of the mechanical decanters for replacement of the flexible piping permitted a detailed inspection leading to the conclusion to replace the existing decanters. Additionally, it was determined that the SBR walls required minor repairs of areas just below the normal waterline.
In recent years, scientists and engineers alike have begun to conclude that excess phosphorus and ammonia nitrogen discharged into natural waterways via wastewater treatment plants and agricultural runoff, have contributed to an increase in algal blooms. Coupled with warmer temperatures, these blooms become toxic to fish and other creatures living in and along the waterways. When small municipalities are hit with fines and violations for excess nutrient runoff, it is often difficult to make the necessary corrections without the necessary financial resources or the expertise of specialty infrastructure engineers. WK Dickson began to recognize the connection of nutrient density and toxicity to wildlife years before these issues started making headlines.
WK Dickson’s solution of SBR process modifications to incorporate dissolved oxygen control, chemical system modifications, new flocculation and cloth mini disk filter system modifications have achieved compliance with the effluent phosphorus permit limits well below federal mandate standards. An average effluent phosphorus of 0.11 mg/L has been achieved in compliance with the monthly average permit limit 0.13 mg/L with no weekly maximum violations for 24 straight months since completion. Additionally, the new SBR blower system with variable frequency drives and dissolved oxygen control system have successfully reduced the operational power costs of for the facility approximately 25% from $142,000 in 2015 to approximately $104,400 in in 2018. The flow pacing of chemical feeds is also providing a more efficient system of delivery, and the operational improvements now allow the plant staff to focus their time and energy on proactive operations, yielding added return on investment to the rate payers.
This capital project was a successful venture thanks to a sincere collaborative effort on the part of the city of Dahlonega, plant operations staff, the Georgia EPD, the contractor, subconsultants, and the design team. Even though there were seven additive changes orders during construction, there were also five deductive change orders to the project, ultimately delivering a complete project with all the functionality that the operations staff requested ahead of schedule and below budget. The upgrades and modifications now complete at the WWTF will enable Dahlonega to remain a city rich in charm and history, with the infrastructure necessary to serve thousands of visitors and students each year.