Over the past few months, we have received questions about estimating flows for larger system designs. We thought it would be a good idea to explain in a general way some of the approaches to determining design flows for systems that serve multiple residences or establishments, defined as a cluster system.
The definition of “larger” system varies from state to state. In our Minnesota state code, large is defined as having flows between 5,000 and 10,000 gpd. A good case can be made that flows up to 5,000 gpd cover most single residence and small business flows. Flow levels are used to determine the qualification requirements for individuals or companies to design, install and maintain systems. As always, you need to know state code and how it addresses these issues.
The upper end of flows is set by federal requirements. Flows exceeding 10,000 gpd require an NPDES (National Pollutant Discharge Elimination System) permit. These systems must be designed by a professional engineer. Federal rules can also impact the midsized and smaller systems since any system serving two or more residences or other establishments serving more than 20 people are covered under Class V injection well standards. There are specific reporting and inventory requirements for these systems. Each state deals with these requirements a little differently.
A bigger challenge
Estimating flows for larger systems is more challenging than single-family residences, but the general methods are the same. Flow estimates for residences are based on the number of bedrooms, residence size in square feet and/or water-using devices.
In Minnesota, it was recognized that there are differences between groups of established residences and new housing developments. Part of this recognition was due to high numbers of formerly seasonal dwellings converted to full-time residences on lakeshores. Resort cabins were sold off individually on small lots. At conversion time, if there was not already a collections system to a shared treatment system, there needed to be following conversion.
Design flow in this case is determined by taking the total flow from the 10 highest dwellings plus total flow from the remaining dwellings, then multiplying that number by 0.45. This recognizes that all dwellings in these situations will not have the same level of usage. New housing development flows are based on determining the total number of bedrooms in the development and multiplying by 110 gallons per bedroom. This is less than the standard 150 gallons per day per bedroom estimation and reflects the fact that when there are multiple residences involved, there is variation in use and occupancy.
For establishments other than residences, tables provided by U.S. Environmental Protection Agency or the state regulatory authority are often used. An important consideration when using estimates from tables is they reflect maximum flows; as a service provider if you see average daily flows over 70 percent of the estimated values for extended periods, you should be concerned. Soil treatment units will not accept those levels of use over the long term.
Another option if the facility is already in place is to measure flow, ideally during a time of maximum capacity or use. The minimum time to measure flows is probably one week. Maximum capacity is used to make sure that the system is sized large enough to accept the larger flows. If the time period is longer, such as a month, it could include times of much lower use that would bring the average down, allowing a smaller system. But it would not be large enough during busy periods. Where we live, this is a common occurrence for bars and restaurants — very busy during peak tourist times, not so much when it is just locals.
Ideally, during the week when flows are measured, times and levels of peak flows throughout the week can be determined. Peak flows can be used to determine whether to increase the size of the soil treatment area and whether using flow equalization is a good design option. Bars and restaurants often have variations in patronage during the week. Generally, more flow occurs on weekends or special events during the week, such as “Taco Tuesday.”
Don't forget strength of waste
Using the average daily flow, along with knowledge of peak weekly flow events, can help with sizing soil treatment areas and determining storage needed to provide flow equalization. It can also help establish a management and maintenance routine for the system. Management may involve splitting the soil treatment area into sections, storing effluent and establishing dosing sequences run by timers to distribute the effluent more evenly through the week.
The only aspect we have dealt with is flow volume; the designer and installer should recognize that waste strength is also an important component of flow estimation and design parameters. We have used restaurants and bars as examples in the estimation process. These typically have sewage that is considered high-strength waste. The waste is higher in biochemical oxygen demand (BOD), total suspended solids (TSS), and fats, oils and grease (FOG). System design must account for this and incorporate components to reduce these levels before discharge to the soil treatment area.
A final area of consideration that we do not usually worry about for individual residences — but can be important for cluster systems — is the potential for infiltration in the sewer collections system. The longer the collections sewer and the more tanks and treatment devices involved, the higher the probability of infiltration in the system. This should be estimated and incorporated into sizing the final treatment area. Each state will view this somewhat differently, so know your numbers.
