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Best Practices for Designing Large Wastewater Treatment Systems

Best Practices for Designing Large Wastewater Treatment Systems

By Dennis F. Hallahan, PE,

Determining the optimum wastewater treatment system design can be complicated particularly in sensitive watershed areas or where fluctuating daily flows and seasonal temperature fluctuations are a factor. With larger systems come larger liabilities so getting it right from the start of the project is critical. The better the basis of system design is understood, the better the system performance will ultimately be. 

Today, engineers and designers of large systems have options that can handle flows in excess of 1 MGD without having to rely on centralized wastewater treatment plant capacity or even availability. That is an asset however with so many new and effective treatment options and products available, it can take time and research to pinpoint the best one for your projects design flow, location, and special characteristics.

Best Practices for Successful Results

Do the research. Determine the best technology that will meet the permit limits at the best cost. Find existing systems that use the technology under consideration and talk to the managers and designers of those facilities. Also, look closely at sampling data and question the design flow, metering data, and any special system characteristics such as peak flows or peaking events. When existing plant data is available a flow and load report can be an invaluable tool.  When new facilities are being planned the flow and load report will inform preliminary engineering efforts by assembling published literature and regulatory design requirements.

Understand projections for community growth and how they will impact anticipated wastewater treatment needs. Under-sized or nonexistent wastewater treatment facilities can limit future development within a community. Each community has a unique set of circumstances and needs. While centralized sewers offer the possibility for large-scale commercial and residential growth, many communities are more focused on retaining historic and community character. Others simply do not have the funds for large infrastructure improvements. A decentralized treatment approach can enable a community to focus on areas with the most critical need, thereby taking a phased approach to upgrading wastewater treatment community wide. 

Understand the facility type and the wastewater it generates. 

Questions to address prior to system design selection include: 

  • Are there Intermittent or seasonal flows resulting in long resting periods? 
  • Is flow equalization necessary?
  • Is subsurface discharge a viable option for the site? 
  • Will a subsurface discharge allow for more relaxed discharge limits thereby saving the client costs? (Local regulatory requirements may impact this)

Minimizing surface discharges and nitrogen pollution have been leading initiatives to develop new methods of treating wastewater at individual sites as well as community and municipal systems. The engineer of record should assess the shortcomings of the existing wastewater treatment facilities & infrastructure and expected potential of future development within the community.  

Some examples of assessment factors that will impact the decision-making process are: 

  • A wastewater treatment plant which is projected to, or routinely, receives flows in excess of the rated capacity due to combined sewer overflows (CSOs) during precipitation events (less tolerance for these events than in the past)
  • Watershed proximity issues including groundwater pollution and saltwater intrusion for coastal communities
  • Regulatory noncompliance of current facilities or individual residential systems within the community

Deliberately access the treatment technology options. Do not force a solution or settle on a product for your project until all the boundary conditions are known. These include design criteria, site location, size of site, discharge limits, etc. There is no one product to fit all projects, the design engineer is best served by remaining neutral to the system type, keeping all options open so the client can be best served. Any design choice must meet the treatment discharge limits, account for O&M and the skill level of the operators, address O&M costs, consider product lifespan, and bottom-line overall costs for the treatment plant and construction.

Build professional management into any large wastewater treatment system design.

Recognizing the need to advocate advanced wastewater treatment systems of a scale that will support positive development, health officials also recognize and often require these systems to be professionally managed.  Professional management provides more control on the quality of the waste treatment process.  If competent management is available, then utilities may favor this approach as the most cost-effective long-term solution.

Large-scale Treatment System Options

Consider Advanced Onsite Wastewater Treatment Systems (AOWTS). AOWTS high-level treatment strategies and system designs that address nitrogen reduction, watershed protection, and sensitive environments are particularly critical for coastal communities and those where dispersal to surface waters is no longer an option.  Engineers who are embracing these new options find them easily adaptable to various site conditions. Pre-engineered, Alternative Onsite Wastewater Treatment Systems (AOWTS) provide long-term treatment solutions and professional management in many cases. What’s great is that they can be used in multiple small scale or single larger system applications.   

Explore a combination of centralized and decentralized wastewater treatment technologies

Engineers are commonly challenged to design wastewater treatment solutions for communities faced with environmental challenges including CSOs from outdated centralized systems. Often these same communities have limited, if any, budget for infrastructure improvements. In these instances, taking a combined approach that utilizes both centralized and decentralized methodology can be an option to review.

For example, by expanding the wastewater district service area without extending the collection system out to new distant parcels the district can incorporate decentralized strategies by installing a satellite plant with subsurface disposal. These systems often provide collection that moves to a centralized treatment facility then to a large disposal field. STEP systems, for individual homes or businesses can collect effluent in tanks and pump to a centralized treatment facility. Utilizing decentralized treatment strategies in concert with centralized wastewater districts or management structures offers the optimum solution for communities with minimum budgets for infrastructure projects and growing treatment needs. 

This centralized/decentralized strategy and combining of technologies also can work well with large commercial systems with multiple wastewater generating sources. Large businesses and communities no longer need wait or pay exorbitant tap fees to tie-in to existing centralized services. Consultants will perform feasibility studies reviewing options for their clients, and the decentralized solution may yield the most beneficial cost position.

In the case of community wastewater treatment facilities that are reaching or over capacity, adding a bed utilizing subsurface infiltration, such as an engineered chamber system, can extend the life and community investment in the WWTF and have the added benefit of reducing phosphorus and eliminating outfall discharges to bodies of water.

And, most centrally managed decentralized wastewater treatment systems, such as publicly- and privately-owned community systems, are being staffed with trained and educated personnel in the same manner as centralized systems.

Natural processes and functions provide holistic benefits 

When reintroduced into the design of semi-urbanized environments, green infrastructure that uses vegetation and soil to reduce rainwater runoff volume may also reduce air pollution and air temperature through evapotranspiration. This can help to minimize the urban heat island effect, while at the same time providing ground cover that serves a habitat function. Designing with nature can also be seen in a larger sense, as land development that is more sustainable – economically, environmentally, and socially.

Technology in Action

Lauloa Maalaea Resort Utilizes Extended Aeration System to Update Wastewater Treatment

To meet more stringent effluent regulations for a permit update and handle the design flow of 21,000 gallons per day, the Lauloa Maalaea Resort in Hawaii was required to update their wastewater treatment system. The Resort selected a new Delta Extended Aeration Treatment Unit which had to be installed in the same location of the existing below grade system.

The extended aeration process selected for this system utilizes aeration followed by clarification and disinfection. The flow equalization chamber receives the incoming wastewater then duplex pumps discharge the wastewater into the aeration chamber. Duplex positive displacement blowers and an air distribution manifold system supply all the air needs to the system including air diffusers, airlift pumps, and a scum skimmer. The hopper-style clarifier chamber has baffling to prevent short circuiting and to provide the maximum uniform solids settling area. The settled sludge returns from the clarifier floor sludge well to the aeration chamber by the positive sludge return system. Immediately following the clarifier is a plug flow chlorine contact chamber. The influent characteristics were typical domestic waste loadings, with effluent requirements of less than 20 mg/L BOD/TSS.

Delta was faced with the challenge of manufacturing a treatment system that would maintain the footprint boundaries of the existing system, while providing treatment with more stringent effluent quality requirements.  Additionally, given the location and importance of esthetics in this highly traveled vacation area, the owners wanted the system tucked away and virtually unnoticeable by the residents of the resort, so a building was constructed around the unit.


Asking the right questions and drawing from available research and data can make or break the wastewater treatment system design process and ensure long-term performance. Most industry professionals are willing to discuss projects and provide support. Additionally, manufacturers or advanced wastewater treatment technology have dedicated technical support teams that are readily available to provide background and suggest the best solution for your large-scale wastewater system challenge.

Dennis F. Hallahan, PE has more than 30 years of experience with onsite wastewater treatment systems’ design and construction.  Currently Technical Director at Infiltrator Water Technologies, he is responsible for technology transfer between Infiltrator and the regulatory and design communities and consults on product research and testing for universities and private consultants. Hallahan received his MS in civil engineering from the University of Connecticut and his BS in civil engineering from the University of Vermont. He is a registered professional engineer in Connecticut and holds several patents for on-site wastewater products. He can be reached at dhallahan@infiltratorwater.com.