Design Flow Sizing for Commercial Systems

Effective client communication and accurate water usage data are keys for developing the best onsite system for unique situations.

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One of the most challenging and rewarding tasks facing wastewater professionals is the design of commercial onsite wastewater treatment and dispersal systems. Residential system design is relatively straightforward as the wastewater flow component is generally accepted. Commercial wastewater flows can vary significantly; it is the design professional’s job to provide the client with a conservative, cost-effective solution that meets local code requirements. Taking the following steps will help to successfully meet that challenge. These items relate to the hydraulic aspect of system design. The organic aspect is important to take into account, but is not discussed here.

Talk to the client

Most codes provide starting values for design flow based on characteristics such as seats, campsites or rooms. Other factors, including hours of operation, peak business times, special events and others can only be learned from the facility owner/operator. An often-overlooked factor is a client’s plans for future expansion. Considering future plans during the initial design phase can prevent problems and disappointments later. If a client’s dreams cannot be accommodated, either due to site limitations or regulatory requirements, it is better they learn this now rather than later.

Water use records

Daily wastewater flow from commercial facilities like restaurants and campgrounds can vary greatly, and need to be considered in the design. There is no substitute for actual water-use data from the facility in question or a similar one. If the facility is served by a public water system, quarterly or monthly data should be readily available. Where a private water supply is utilized, a water meter is a wise investment. Regardless, the collection of at least one month of daily water-use readings is critical to developing an effective treatment and dispersal design.

Additionally, water-use data from existing facilities can reveal problems such as leaks or inefficient dishwashing or laundering processes. Correcting these problems can result in a lower design flow and cost savings for the client connected to a public water utility.

Seasonal or year-round?

Many commercial facilities operate seasonally. Traditional design assumes year-round operation. One way to deal with this is to design as year-round but install only a portion of the dispersal area – typically two-thirds to one-half – keeping the remainder as reserve area. This approach may be acceptable to regulators and minimize installation cost for the client.

Treatment and dispersal

Knowing the actual wastewater flow characteristics is invaluable when designing treatment and dispersal components. A good approach is to design treatment components such as septic tanks and aerobic treatment units (ATUs) to handle the anticipated peak flow. Graphing the water-use data and using a percentile formula (80 to 95 percent, depending on measurement frequency) is an effective way to size the dispersal area. Percentile formulas are found in most spreadsheet programs, which are also useful in graphing water-use data. A design value equal to the 80th percentile means the value is equal to or greater than 80 percent of the data points. Drawing this value on your water-use graph shows how often the daily flow exceeds the design value so adjustments can be made. Monitoring ports in the dispersal allow for easy evaluation of long-term performance.

Flow equalization is another technique to minimize the size of the ATU or dispersal area.  Facilities with large variations between peak- and low-flow days can utilize storage with pumping – employing timer rather than float control – to dose the ATU or dispersal area, evening out daily variations. This needs to be evaluated on a site-by-site basis, as the cost of the extra tankage, pumps and controls are weighed against the cost of the larger ATU or dispersal area.

Knowing the hydraulic loading characteristics of a commercial project is crucial to designing a system that will work for years to come. Installing these systems represents a significant investment and deserves a thorough investigation. I hope these thoughts regarding wastewater design flows will be useful to design professionals. Please feel free to contact me if you would like to discuss this important topic in more detail or have any questions.


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