Last month, we covered estimating of average daily sewage flows for single-family residences. For other types of structures, the picture gets a little hazier from our perspective. The same per-person estimate is used, but other characteristics of the business or establishment may provide a better picture of the water use patterns and amounts. Examples are per seat, per meal, per car stall, or per square foot of area. We have seen all of these and combinations used to estimate flows. The characteristic that best fits the establishment should be used.

Most state codes have a table of values for different kinds of activities or establishments that are used to estimate the total average daily flow for design purposes. They are typically based on published values in established engineering publications. It has been our observation and it has been noted by others in publications that these values tend to err on the high side. Some are based more on peak flows rather than averages. For nonresidential situations, this is where the safety factor for estimated flows is built into the calculation.

As with residential flows, codes vary by state even though the numbers are probably based on the same studies and publications. A couple of examples for Minnesota and Arizona: For a bar and lounge, each state determines the estimated flow based on numbers of seats. Minnesota uses 30 gallons per seat and Arizona 36 gallons per seat. For a restaurant, the Minnesota number is 8 gallons per seat while in Arizona it is 7 gallons. Each state makes an addition to the estimate on a per-employee basis (Minnesota is 15 gallons per employee and Arizona 20 gallons per employee).

ACCURATE PROJECTIONS

Both states allow some deviation from these numbers based on criteria such as hours of operation, size in terms of square feet, and the type of operation: a restaurant serving alcohol versus one that does not, for example. Just as with changing residential flow estimates, the system designer is responsible for making a case to alter the numbers to the state or local regulatory personnel.

Probably the best way to get an estimated flow for a nonresidence is to monitor flow and wastewater characteristics from the establishment before making design decisions about any of the system components. If it is a new establishment, data collected from a nearby establishment with similar characteristics would be best.

In Minnesota, there is a specified method to gather flow data. It is determined by averaging measured daily flows for a seven-day period where the establishment is operating at its maximum capacity. Getting accurate readings requires a close working relationship with the owner and an understanding of how flows change during each day and during different days of the week.

Anyone working in this industry recognizes estimating average daily flows only tells part of the story. There is usually a lot of flow variation during any day or week for residences and nonresidences alike. Understanding peak- and low-flow times and amounts needs to be accounted for in the design process.

For residences, two peak-flow periods are in the morning and evening. There can be large variations during the week due to work and school patterns, and whether there is in-home business activity. This information may require some additional system components or characteristics incorporated into the design, such as using a timer system to regulate flow. In other establishments, knowledge of flow patterns can change the entire system design and approach.

SEWAGE STRENGTH VARIES

Wastewater characteristics should be monitored. BOD and suspended solids levels in the effluent should be determined. For residences, we assume waste generated is of typical domestic sewage characteristics. With proper flow estimation, septic tank sizing and regular maintenance, septic tank effluent should have a BOD concentration of about 175 mg/L and a suspended solids level of less than 60 mg/L. This is incorporated into the soil sizing factors, which along with estimated flow determines final soil dispersal area size.

Nonresidential wastewater characteristics are often very different from residential numbers. This is most notably the case in restaurants and bars. BOD values for restaurants are often 3 to 5 times higher than typical residential wastewater. In addition, levels of fats, oils and grease are two to five times higher in restaurants.

Unless a change is made in the pretreatment choice during the design process, the soil treatment area will be subjected to these higher levels and result in faster development of a more resistant biomat leading to premature hydraulic failure. A method of pretreatment will need to be incorporated into the design and installation that brings organic loading more in line with residential waste. Use of media filters and aerobic treatment units are two ways to provide the additional treatment.

With this deeper look at estimating flow, you’ll see there are variations and the opportunity to make different estimates based on residence and other establishment characteristics. But you must approach each situation with caution and have a sound basis either from published studies or local data.

As always, this should help all of us recognize that reducing water use and wastewater flows, and the quantity of organic loading will improve both the efficiency and longevity of our systems.