Question:
Any time the term “gallons per day,’’ or gpd is used, I wonder if it is referring to design or real gpd. New Hampshire stipulates a capacity of 150 gpd per bedroom, and most understand that this is a “peaked” design figure. If we have real metered water flows to design from, we will at least double this number for design. Maine uses a more realistic 90 gpd per bedroom. Other states have their own numbers. The 150 seems a common figure.
Answer:
Your letter and comments raise some interesting points. You refer to a “design or real gpd” (gallons per day). When the onsite sewage treatment system is designed, average daily sewage flow must be estimated. This is where the design must begin in order to determine the size of the onsite system.
Yes, 150 gpd per bedroom is a common value used to estimate sewage flow. This number is based on an estimated daily sewage flow of 75 gpd per person and each bedroom occupied by two people. The number of bedrooms in the home is then multiplied by 150 gpd to get the estimated average daily sewage flow from the household.
You indicate when metered water flows are available, you double the meter number for design purposes. Perhaps some of the following information will support your practice.
Over the years, a number of studies have looked at water use rates. Most were made before the introduction of toilets having a water use of 1.6 gallons per flush. A publication in 1975 found the single largest water use was from the flush toilet, with the laundry and bath tied for second. The data likely would be different today with the advent of low-flow flush toilets.
Double occupancy?
A number of studies at that same time showed the mean daily per capita water use to be somewhat less than 50 gallons per day. However, the general practice has been to use a value of 75 gallons as the water use rate for design purposes.
Some research scientists found all bedrooms in a home were normally not occupied by two persons. They suggested the occupancy rate for a home is normally one more than the number of bedrooms. The formula they presented was: Estimated daily water use = 75 (BR+1). Using this formula, a three-bedroom home would have an estimated water use rate of 75 x (3+1) or 300 gallons per day.
Another study used a different approach. A base flow of 66 gpd was established for the basic household uses of clothes and dish washing. Then water use by individuals was evaluated. This study led to the formula: Estimated daily water use = 66+38 times the number of persons. Using this formula a three-bedroom house would have an estimated daily water use of 218 gallons per day. Again the occupancy is considered to be the number of bedrooms plus one.
The values from these three different approaches are presented in the following table:
You can see a number of average sewage flow rates can be used to estimate average daily sewage flow. However, it is necessary to select a sewage flow rate to design and size the onsite sewage treatment system. I would suspect your metered flow might be like a Type III home, but the design value like a Type I.
The size of the soil treatment system depends upon the soil characteristics and the average daily sewage flow rate selected. For an example, let’s use a three-bedroom home at 450 gpd requiring 200 lineal feet of trench. The trenches will be 36 inches wide with 12 inches of rock under the 4-inch pipe, 2 inches of rock over the pipe. The rock is covered with geotextile fabric and each trench has an inspection pipe at the drop box and at the end of the trench. Recent figures from an installer friend of mine in Minnesota indicate this trench system would cost around $5,100.
If a Type II home is assumed, a 300-gpd system would need 133 feet of trench, which would cost less, but not in direct proportion because of some basic setup costs. Let’s say $4,000, or $1,100 less.
If a Type III home is assumed, a 218-gpd system would need 97 feet of trench. Again, the cost would be less, but not half as much as the 450-gpd system. Let’s say $3,000, or $2,100 less.
And what will be the cost of this three-bedroom home? The cost certainly varies around the country, but the cost likely will be in the $150,000 to $250,000 range. Does it make good economic sense to try and save a few thousand dollars on the size of the onsite sewage system? If I were the prospective homeowner, I would ask for a full-size system. If I were a developer concerned with costs, I might want to save whatever I can as long as the system lasts beyond the warranty period.
Build to suit
If I were selling an effluent distribution system more costly than trench rock, I might use the argument that my system was more effective than rock. I would argue that the size of my system could be reduced compared to rock. By reducing the size of my system, I could then be cost-competitive with rock-filled trenches.
Actually, what would be happening is less sewage flowing into my soil treatment system than the assumed design flow rate. But if no flow measurement were made, I could state my system was treating sewage at the design rate.
A sewage flow rate has to be assumed to size and design an onsite sewage treatment system. This is the design rate to which you refer. The “real” or actual sewage flow rate can be determined only by using a water meter. Installing a water meter is a small price to pay for evaluating the performance of the onsite system. The meter will tell if a system failure is due to excessive water use.
I have been recommending installing water meters for onsite systems for years. My advice has, for the most part, fallen on deaf ears. If water meter data had been collected over the years for a variety of onsite systems, we would now have a much better estimate of “real” onsite sewage flows.
