On-site Wastewater Treatment
July 09, 2004
What is a bigger priority for septic fill...a higher D10 number (say 0.25 mm and up) or a low Cu number (say <4)?
In your opinion and experience, what is a bigger priority for septic fill...a higher D10 number (say 0.25 mm and up) or a low Cu number (say <4)?
There was a paper entitled "The lateral-flow sand-filter system for septic tank effluent treatment" by G.Check et al, published in 1994, Vol 66, No 7 of Water Environment Research.
The researchers tested 3 different grades of sand in "real life" plexi-glass models (similated slices of lateral sand filters) and loaded them with septic tank effluent at 3.3 cm/day and later at 6.6 cm/day. It was interesting how the various sands performed and their corresponding gradations and Ksat values.
In a nutshell, after at least 6 months of loading the results were as follows:
Sand 1:
D10=0.29 mm, Cu=5.9, Ksat = 5.4x10-4 m/s, porosity=30%
Results were: at 3.3 cm/day loading, no biomat formation, infiltration rate = 4800 cm/day.
at 6.6 cm/day loading, no biomat formation, infiltration rate maintained at 4800 cm/day
Good treatment for BOD and bacteria
Sand 2:
D10=0.15 mm, Cu=5.3, Ksat = 1.5x10-4 m/s, porosity=31%
Results were: at 3.3 cm/day loading, 1-2 cm biomat formation, infiltration rate = 240 cm/day.
at 6.6 cm/day, black sulphide layer formed, infiltration rate decreased to only 48 cm/day.
Very good treatment for BOD and bacteria
Sand 3:
D10=0.23 mm, Cu=1.7, Ksat = 4.0x10-4 m/s, porosity=43%
Results were: at 3.3 cm/day loading, 0.5 cm biomat formation, infiltration rate = 240 cm/day
at 6.6 cm/day, black sulphide layer formed, infiltration rate decreased to only 48 cm/day
Very good treatment for BOD and bacteria
Based on the above results, I would say that sand 1 was the best followed by sand 3 with sand 2 being the least preferable.
Even the coarsest sand (#1) performed well for BOD and bacteria treatment and was able to withstand much higher hydraulic loading rates with no biomat formation.
A higher D10 number (and higher Ksat) therefore appears to be more important than having a low Cu number.
Based on the results of the paper, I'm thinking that if necessary, it is preferable to sacrifice the Cu number (perhaps going as high as 6 or 8) if it means getting a D10 in the 0.30 and up range...
In my part of the country (Prince Edward Island, Canada) this comes down to a matter of performance versus economics. We simply do not have local sands which are affordable and strictly meet common D10 and Cu criteria. I am trying to get our local Dept. of Environment to adopt higher standards for septic fill and would be embarrassed to tell you what is currently allowed under our regulations.
If you couldn't get the best of all worlds, what would be your priority for a septic sand? Has anyone had good long term results with higher Cu value sands? I'm assuming that these are still washed sands with low fines content (say <2% passing the #200/0.075 mm sieve) with hydraulic conductivities in the 10-4 m/s range.
The hydraulic loading rate which will be applied is about 0.57 usgpd/sq.ft (2.8 cm/day) based on peak design flows and gravel trench bottom area only.
Any comments are welcome.
Kelly Galloway, P.Eng.
Engineering Technologies Canada Ltd.
Email kelly@engtech.ca
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