Water Useage vs Sewage Generation

[EngWade]

Question: A colleague of mine found a published standard somewhere that says water useage is typically approximately equal to 75% of sewage generated. Does this sound right to the rest of you? I thought this "rule of thumb" sounded like it was meant to be taken from the perspective of a wastewater treatment plant. He is applying it to a small commercial site. Theoretically, I can see how sewage generated must always be greater than water useage, but the 25% difference seems a bit high - unless it is just being conservative for the sake of design - which makes sense to me.

 

[MiketheEngineer]

That is the way I pay my sewer bill - but not sure what the percentage is. Seems to me it would be the other way around - a lot of water is used for plants, car washing, house washing, etc. that would not end up in the sewer. Maybe some rain runoff gets in sewers. Theoretically, not allowed around here.

 

[bimr]

About 60 to 85% of the water usage becomes wastewater with the lower percentage applicable to semi arid regions of the southwestern US. Metcalf & Eddy

http://books.google.com/books?id=nA... as a percentage of wastewater volume&f=false

These numbers are too general to be used as a design basis.

1. The numbers do not include I/I.
2. The numbers do not account for conservation which may easily reduce the water usage by 15 to 20%.
3. The design values that are used will have to be approved by the regulatory agency that will review the project.
4. The percentages do not reflect the water usage of specific climate regions.
5. The percentages do not reflect what type of project that you have. Different land uses have higher water consumption than others.

 

[greenone]

I have a food processing facility that uses a great deal of water in the product, plus it irrigates, plus they have ww pumped out of settling tanks, so they only sewer about 60% of their water intake.

Really depends on the type of commercial facility involved.

 

[concretemasonry]

Splitting cost between water supply and sewerage vary wildly depending on the location.

Where I live now and have lived previously (VA and MI). The billing was based on the rates established for the assumed combination of that gave an over-all rate. In a couple of cases, separate meters could legally be installed so there was two readings and a different rate was applied to the exterior water since it was not required for sewage treatment.

In some countries the newer homes and apartments have separate waste water systems (one for "black" water and one for "gray" water that can be reused without treating. I never asked the question on how they could come up with a split between the "black" and "gray" water or if a percentage was assumed (rule of thumb).

If you have a single facility, it might be wise to look into the rates and uses before making any design decisions.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[dicksewerrat]

Some times the sewage rate is based on the water usage for the coldest time frame. Not much lawn watering, car washing and pool filling. By the way pool water must be put into the sanitary sewer.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[beej67]

When I was a kid, (16 yr old engineering intern) one of my duties was to run around to industrial facilities who used an inordinately large amount of water for their products. (think Soda Bottling Company) They had meters of domestic water used for their product, and we read the meters, did an analysis, and filed the difference between their water meter and their product usage with the water company, to base their sewer bill on.

This:

the 25% difference seems a bit high - unless it is just being conservative for the sake of design - which makes sense to me.

..seems backwards to me. The more conservative thing to do, when designing sewers, is to assume the sewer is closer to the water usage, not lower than the water usage.

Regardless, I think we need to understand what sort of study your colleague is trying to put together. In my experience, when you're designing sewer, you design it for a load that's based on usage, and your sewer company or municipality gives you a table to go off of to determine load. For small commercial it's usually based on square feet, unless it's a restaurant, in which case it's based on tables. Residential is based on beds, etc. Then you design around that standard sewer load instead of manufacturing your own load to design for.

On a couple of rare occasions I've worked for very sophisticated retail clients who had years worth of water metering data, and we would derive our own loads for their projects based on their metering data instead of using the community standard. Each of these designs had to be walked through with the municipal reviewer though, and they had to buy in to what we were selling. I do not recall "reducing" their water metering load by a fraction for sewer, though, because their data alone was lower than the community standard, so we didn't want to push it.

Hope this is helpful.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[bimr]

Note that putting in a smaller pipe only saves on the cost of the pipe. The pipe installation cost is much more expensive. So you are really not saving that much by downsizing.

 

[beej67]

Yes bimr, but in the case of a downstream capacity analysis, showing that existing pipe is adequate can save you huge amounts of money.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com