Small Sewer Network

My first thought is installing a few small pump stations with force mains (there are numerous companies that sell premade stations, just specify the flow conditions) or local septic systems if possible. I wouldn't go for any cleaning or maintenance advice as a gravity network that is designed to have less than scour velocity will only be a constant headache and likely become a public health problem rather quickly (Personally I'd never put my name on a set of plans designed that way.)

Do you really think a sewer system drains all the way to the septic tank on every flush? Personally, I doubt it. I would expect the watery fluids would flow gradually to the tank, so set the system up with a constant slope, that is standard.

For 'self cleaning' I expect the waste to move in slugs pushed along by water that backs up. To expect a low flush toilet of a gallon and a half to completely evacuate a long run of four inch pipe is silly, in my opinion.

I've seen documentation of systems that used smaller daimeter pipe in South America at the edges of the network to obtain increased flow rates and apparent depth.

What is the "grade" of your system? That's the important factor.

Also, How have you determine the theoretical flow rate? I hope that it's not pipe cross section.

Have you considered relocating the final collection location? There may be a better topography solution that provides greater fall.

If you're aiming for 2% grade, then that's 10m fall you need if the final station is at one end. (However, it's only 5 m fall if the final station is at the middle, assuming you have absolutely level ground.)

First of all, thx everyone for their answers

Just an Engineer, what's ment by "grade", is it some sort of classification of the system?

I've determined the flows based on the population using the objects. In fact, these are 3 small workshops with 8-15 people using them. I tried with 2 options - 55 l/day/person and 95 l/day/person which are some recommended values of sanitary loads for offices and workshops. The proposed diameter is DN 250, I tried to use DN 150 but to no satisfying end... The topography of the location is quite flat, and actually the puropose of this project is to collect all of this sewage in one location so that it could be filtered in a single filter and than released to the nearby river.

More info, possibly a rough diagram with grade information, budget, etc. With the limited information you provided I'm seeing a common collection area that pumps out to an outlet with a slope that will provide your target speed.

Are you chlorinating this effluent, or are you dumping raw sewage into the river? Is there any processing besides 'filtering'?

Hi Fredski, could this provide the info you asked:

I think the sources are too far away from each other to have the common collecting area.

Mike k, I'm almost sure that this filter has some chemicals being injected in it, but I dunno the details, because it's clients filter ;).

By "grade", I mean the slope of the pipe that induces the flow of effluent to the end point. For sewage/wastewater it is absolutely essential that the slope be consistent with no low sections that allow pooling of the material. (You can check that the pipe has been laid straight with mirrors on a sunny day. The mirror at one manhole shining sunlight intot he pipe and a mirror at the other end to see the light coming through the pipe. (simple but effective)

You should go back and check your calcs again. 150mm pipe is used to service quite large subdivisions with many more people and higher loadings than you indicated. (We would move to 200mm for 800 people and 250 for around 1100.)

You might consider something like the following.

Average daily water flow (assuming 8 hours use by the factories) will be ...??/hr.

Peak daily water flow (assuming comfor breaks taken at meal times) will be 7 x the average daily water flow.

I'm certain that a 150mm diameter pipe will cope with the volumes that you indicate as the pipe volume for the serviced properties is greater than the total daily water volume you indicated.

For your slope, you should try to get 2% fall, but 1% could be used. You need access points so that you can clean the lines if a blockage occurs. If you myust have your sump at the extreme end of the line, then 2% fall means a 10m deep sump for your pump and a fairly deep trenth to that point. If the sump and pums were at the middle then the sump will only need to be 5m deep (and also your trenches for pipe will be easier to make.)

You might want to use alternate technology that is already available. Each property could use a dedicated "private pump station" that pumps into a shared line to your treatment works. These have small (100L) pits and only pump when the level is deep enough for the pump. This system would not require accurate slope of the pipe, as the pumps move the material through the line. (shallow trenches and shallow sump) but have other issues relating to how "stagnant" the water can get inside the pipes, especially sitting over weekends. (Look up "packaged pump stations", "grinder pumps" etc.)

Good luck.

Thanks Just an Engineer, this was sort of advice I was looking for.