Gwinnett County,  GA

Ardurra (fka Constantine Engineering) was a major consultant for the project team that completed the 2017 Sewer Master Plan for the Gwinnett County Department of Water Resources (GCDWR). Gwinnett County has a long history of providing high quality service and wastewater treatment to customers through periods of record growth and change. The Gwinnett County wastewater collection system includes over 2,715 miles of sanitary sewers, 278 miles of pressurized force mains, 220 pump stations, and three water reclamation facilities. The flows are conveyed by a very complex and interconnected wastewater collection system which provides great operational flexibility and redundant capacity. 

The purpose of the Sewer Master Plan was to evaluate the existing and historical population and wastewater flows and to develop future population and wastewater flow projections that can be used to anticipate infrastructure needs and the associated funding requirements. The planning documents provide a guide to wastewater management that helps Gwinnett County consider a wider range of wastewater collection and treatment options to address their wastewater needs, including the construction, operation, maintenance, and financing of future improvements to the wastewater collection and treatment system.   

The specific objectives of the project were as follows: 

• Identify current wastewater system deficiencies and needs 
• Estimate future growth and wastewater flows 
• Develop conveyance and treatment capacity to support future growth 
• Coordinate needs and plans with Metro North Georgia Water Planning District 
• Develop a Capital Improvements Plan for 2017-2050 

Some of the goals and objectives that were achieved in the master plan were: 

   1. Optimize the sewer system to minimize risk, reduce power and chemical costs 
   2. Plan for sewer capacity proactively to support economic development activities and future growth  
   3. Develop strong Asset Management Programs  
   4. Coordinate with stakeholders to support the County’s objectives and the Unified Plan  
   5. Employ responsible long-term planning and financing for major project
   6. Consider regional opportunities for leadership and coordination with the Metro Water District and the State  
   7. Protect public health, quality of life, and the environment and comply with existing regulations  

Projected Growth and Wastewater Flows  

In the Sewer Master Plan, wastewater flow projections were developed based upon the existing flows (including both the average day sewer flow and groundwater infiltration components), as well as the incremental projected flow increases resulting from the anticipated population growth in each of the planning periods. The sequence of tasks used to develop the population and flow forecasts included the following: 

• Review existing land development plans and population forecasts 
• Develop Gwinnett County historical wastewater flows and peaking factors 
• Consider and adjust flows for existing residences with septic tanks 
• Develop population forecasts and apply per capita wastewater flows 
• Geocode existing customers 
• Allocate growth per Traffic Analysis Zone (TAZ) populations 
• Apply peaking factors for design wastewater flows 

Based upon these evaluations it was decided to use the Gwinnett County Transportation Planning Traffic Analysis Zone (TAZ) estimates as the basis for population growth projections for this current wastewater plan. TAZ units are small geographical areas used by the Georgia Department of Transportation (GDOT) for estimating populations and traffic requirements for each county.  The population estimates within the zones are constructed using census data, and growth patterns are based on socio-economic factors. The GDOT TAZ data for Gwinnett County were imported into the GIS wastewater system database, which allowed population growth projections and wastewater flow projections to be evaluated at the TAZ level.    

Historical wastewater flows at each of the treatment plants were analyzed to develop realistic flows per TAZ and flows per capita, including peaking factors for maximum flows. After adjusting for populations served by septic systems, these historical flow ratios and population projections were then used to develop estimated future flows for Average Day Flows, Maximum Month Flows, and Peak Hourly Flows. The maximum month average day wastewater flows (MMADF) for Gwinnett County are estimated to increase from about 55 million gallons per day (MGD) in 2015 to approximately 150 MGD by 2050. 

Assessment of Needs 

The current needs for the wastewater system were developed by reviewing operating data, making site visits to pumping stations and treatment facilities, interviewing operations and maintenance staff, evaluating system capacities (current and future) using hydraulic modeling of sewer flows, and discussing restrictions and goals with management. Gravity collection sewers and force main pressure pipes were evaluated using the InfoSWMM hydraulic model.  Gravity sewers experiencing flows beyond the pipe capacity were flagged and upgraded as needed in subsequent model runs. 

To accommodate the increase in future wastewater flows, a strategy of flow management and treatment location was developed as a planning tool.  Furthermore, the restrictions in effluent disposal had to be resolved through modified and/or additional permitting. To facilitate this evaluation and make planning decisions, Ardurra developed a flow management model to monitor future increase in flow, needed increases in treatment capacity at specific facilities, and needed increases or modifications in effluent discharge capacity; improvements and capacity upgrades were set in the model to occur just in time but certainly before future flows exceed capacity. 

Evaluation of Improvement Alternatives  

Alternatives for improvements needed to address the existing deficiencies and future capacity requirements for growth were developed and evaluated by Ardurra and the project team. The evaluation of alternatives and recommended improvements were reviewed by stakeholders and prioritized into a Capital Improvements Plan (CIP). The final CIP resulted in more than $2 billion in planned improvements over the planning period. Ardurra also assisted with a funding schedule and phased rate increase strategy to anticipate cash flow increases needed to sustain the improvements.