Sewer Flows mimimum lpcd 1

[BRIS]

We have a World Bank project to rehabilitate/improve water supply and wastewater to 7 towns in a developing country. The Client has aspirations of piped water and main sewer systems. Current water consumption is as low as 20 lpcd. (litres per capita per day). Limited resources mean that even after rehabilitation supply is unlikely to exceed 50 lpcd. It is suggested for a self cleansing foul sewer system a reliable water supply and a minimum consumption of 100 lcpd are basic requirements (WHO).

I am looking for any guidance, references, papers etc. on minimum foul sewer inflows to give relative maintenance free self cleansing operation.

 

[LHA]

For self-cleansing, the system must reach a minimum scour velocity of 2 fps during peak inflow.

The 20 lpcd is probably an average inflow rate, use that to design treatment facilities, not conveyance.

Use Qpeak for conveyance, apply a PF between 1 to 4.5, depending on population, usually between 3 and 4. This takes the Qdesign to 60-80 litres.

But even using the 20 lpcd, the system might work. If the system is designed for 100 lpcd capacity, but only achieves 20 lpcd inflow, v/vf will be about 0.78. As long as v at Q/Qf is 2 fps or greater, the system should scour.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[BRIS]

Iha thanks for your reply but perhaps I have not explained my problem. The concern is not the self cleansing velocity but how much liquid is needed to move the solids. The following is a quotation from a WHO conference on water and wastewater management in 2001. - The flow of 100 Lpcd appears high to me ??

- "Conventional sewerage systems are designed as waste transportation systems in which water is used as the transportation medium. Reliable water supply and a consumption of 100 Lpcd are basic requirements for problem free operation of conventional sewerage systems" - .

With a supply of only 20 Lpcd (5 gals/day/capita) waste would comprise practically only solids.
#
cheers

 

[bimr]

You are describing an aspiration that most people have, but that is probably unaffordable for your client. The lack of sufficient and reliable water supply (and/or power supply) will probably preclude the use of the system that is proposed.

You probably want to investigate the Shallow Sewerage system. A toilet, usually in-house, flushed using lower volumes of water than either conventional sewerage or septic tanks, to smaller diameter sewers laid at flatter gradients and shallower depths between dwellings on a block. On-site shallow inspection chambers are also provided.

Look at the South Africa examples here:

http://web.mit.edu/urbanupgrading/waterandsanitation/levels/provide-san-serv.html

http://www.dwaf.gov.za/dir_ws/content/lids/PDF/Technical.pdf

 

[BRIS]

bmr thanks for youir reply

1) You are correct the project is not financially sustainable but the client does not understand. The World Bank are dumping money on non sustainabale projects!.

2) Septic tanks and small bore effluent piping is a good technical solution.
- thanks for the links