Viscosity of domestic sewage?

[AggieCE]

Does anyone know where I could find "typical" values for viscosity of domestic sewage. I imagine that the viscosity would vary greatly, but I'm wondering how much different it is than for drinking water. When calculating headloss in a force main, I've always used Hazen-Williams or Darcy-Weisbach, but for D-W the friction factor, f, is dependent on the Reynolds # (at least for lower Re). Re is inversely proportional to the dynamic viscosity, thus f varies with the viscosity. So, I'm wondering how much of a difference this makes, if any. Anyone ever looked at this or have any ideas?

 

[DeltaCascade]

Excellent question. I just have some ideas...

"Wastewater Engineering" handbook by Metcalf and Eddy doesn't even mention sewage viscosities. It does mention
typical total solids content 350-1200 ppm, and that of septage (eg: septic tank sludge) as 5-10%, averaging 4%.

Possibly, effect of temperature on viscosity far outweighs effect of dissolved and suspended solids in typical domestic sewage. 1200 ppm is only 0.1%, afterall.

In the Moody diagram, curve of f vs Re, the curve flattens out at high Re. Even if viscosity is significantly different from pure water, the effect of this difference on f, the forcemain pressure drop and design economics, may be small (depending on your system).

Perhaps this is why sewage viscosities were not, and pure water viscosities (as a function of temeprature) were, mentioned in my handbooks.

Hopefully, short of actually measuring your stream's viscosity, one more experienced can comment further.

 

[stanier]

Generally the viscosity for raw sewage used in calculations is as for water because of the highj water content. Sewage sludge however is s different animal. Data for sewage sludges is available from the Mono Pump software.

 

[Poe]

Most likely you won't be moving sewage sludge at high Reynold's numbers, so your viscosity, and thus your kinematic viscosity become very important when calculating head loss. Generally you need to oversize the pipe. Unless your Total Solids is below 1-2%, I wouldn't use the same viscosity as water. If you are realy concerned (designing pipe lines for sewage/sludge) you can get a viscometer out of any scientific supply catalog and test the material you are dealing with.
I do know that sludge between 10-12% TS is roughly 2500 cp (2500 times that of fresh water) and is non-linear as you reduce the solids content.
Maybe a little late, but hope it helps

 

[marktb]

Sewage is treated as water for the purpose of calculating head loss in pipes.

Sludges are non-newtonian fluids and follow a completely different set of rules

 

Head loss multiplication factors for different sludge types and concentrations are given in Metcalf&Eddy,Waste water engineering page 790.

 

[dutchman]

As I worked a long time on a WWTP generally you can you use the viscosity of water. Please take into account that you must have prefiltered water (water coming from the rough filter installation. Water viscosity at 20 oC

Good luck

 

[AggieCE]

Thanks for all of your input. I appreciate the responses!

 

[Chris73]

AggieCE,

I usually take 'v' = 1.3*10^-6 [m^2/s], that is the kinematic viscosity (dynamic is [N*s/m^2] - do not take that!). For WWTP in moderate climate that number covers most sewage (and low solid content sludge) applications.