C+S April 2021 Vol. 7 Issue 4 (web)

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 fluctu- ating 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 as- set 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 tech- nology 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 develop- ment within a community. Each community has a unique set of circum- stances 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 lead- ing initiatives to develop new methods of treating wastewater at indi- vidual 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.



April 2021

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