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Determining Design Flow For Restaurants Based On Measured Daily Water Usage & Attendance Records

Determining Design Flow For Restaurants Based On Measured Daily Water Usage & Attendance Records

Determining Design Flow For Restaurants Based On Measured Daily Water Usage & Attendance Records

I am working on a project that currently has an existing septic system with a design flow of 1,500 gpd. 1,500 gpd was based on the number of occupants and employees (250 occupants x 5 gpd/occupant + 10 employees x 25 gpd/employee = 1,500 gpd). This is an indoor trampoline facility. Our client plans on expanding his existing building because he has ran out of room to accommodate his jumpers, which will result in 100 additional occupants and 12 employees. In addition, he plans on adding a restaurant with 100 seats. The restaurant will be single service and sort of similar to Hardee's, McDonalds, Bojangles, Burger King, Subway, Pizza Hut etc. and the menu will consist of the items below.

1. Pizza
2. Fries
3. Hamburgers/cheese burgers
4. Hot dogs
5. Soda drinks
6. Wings

We have looked at the client's water usage over the last 12 months and found that our client at the most is only using 683 gpd instead of 1,500 gpd. I have met with local health department and state review agency and they indicated that we can apply for flow reduction based on actual water usage data. The client is currently taking daily water usage readings, which I will use to compute a peaking factor and then hopefully we can expand his existing building without modifying his existing septic system by just multiplying 683 gpd by 40% increase in attendance and the peaking factor, which I expect to stay below the original 1,500 gpd design flow.

My question is about the restaurant itself. The restaurant will have to be on a separate standalone system. The state onsite wastewater branch has a table of what the design flow per seat in the restaurant should be. The state onsite wastewater rule book lists 40 gpd per seat as the design flow for restaurants but this number is based on the assumption that the restaurant is full service. It does not have a value for single service restaurant. Our restaurant will be single service. Another branch of state government, for gravity sewers, does have a value of 20 gpd per seat for single service restaurants. If worst comes to worst I plan on citing that table in my report and submitting it for design flow confirmation with the state onsite wastewater branch.

However, I would like to see if there is any way that I could use the water usage data that is being currently collected for trampoline facility and somehow adjust it to come up with a value less than 20 gpd per restaurant seat. The restaurant will be serving kids and parents that come to the trampoline facility. There is already an existing septic system in the trampoline building so I feel like if I design the restaurant based on 20 gpd per, then I will be sort of double dipping. I can't think of a good way of using water usage and attendance data for the trampoline facility and extrapolating and adjusting that data to come up with a reduced design flow for the restaurant without making lots of assumptions that the state review agency would heavily scrutinize. I have contacted state agency but they have not been very helpful and basically told me that I can submit what I want and once they get it, then they will review it. Their rules don't tell you how to adjust water usage data to come up with a reduced design flow for restaurants. All it says is "further adjustments shall be made" without giving out specific factors or the like.

So that brings me here. Is anyone aware of any books/literature regarding how reduced restaurant design flows per seat are determined based on the actual measured water usage data and attendance records? I am guessing those numbers of 40 gpd per seat for full service restaurant and 20 gpd per seat for single service restaurant are somehow based on meal frequencies and the like but I am stuck at this point and simply do not know what adjustment factors to apply to my water usage data that I can reasonably back up with reputable sources. I have tried contacting similar trampoline facilities with restaurants trying to see if I could obtain their water usage and attendance records but they all blew me off since they do not want to help out their competition in any way. I appreciate any help that I can get.

RE: Determining Design Flow For Restaurants Based On Measured Daily Water Usage & Attendance Records

North Carolina design standard:

"This Rule shall be used to determine wastewater flow rates for all systems covered by this Subchapter unless alternate
criteria are provided by a program specific rule and for flow used for the purposes of 15A NCAC 02H .0105."


Limit the number of seats. You are not talking about a restaurant, just a snack bar.

RE: Determining Design Flow For Restaurants Based On Measured Daily Water Usage & Attendance Records

With a restaurant, I would think that employees and patrons would wash their hands more frequently than if they were associated with the trampoline facility. After all board of health across the nation require employees to wash their hands after using the bathrooms. In my mind, I would use the 40gpd as my design number for the restaurant septic system. The local board of health will have the final say so and I am certain its members will stick to the 40 gpd requirement.

RE: Determining Design Flow For Restaurants Based On Measured Daily Water Usage & Attendance Records

in Vermont....
you can design water based on metered demand or rule-based demand. BUT, you must design on-site sewage based on the numbers in the rule. i think it is good policy.

are you looking at composting toilets?

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