Sewer Flowrate Advice Please!!.

[jcanning]

I need to design a system which will allow me to dispatch the sewage waste from twenty houses into a system, which will only allow me an entry flow rate of 1.12 litres\sec.
Assume the houses to be 3 bed semis.

The BS 5572 code give me an approximate flow rate of 5.1 litres\sec for twenty houses, which is not much good to me. So I need another angle, which will also comply with the regulations.

Can I base my calculation on the average waste produced per person per day, factoring this to find the peak flow per second per person and so on?? If not I'd still appreciate the bad news, so that I might move on.

 

[KRSServices]

Are you sure about those flows? I used a conservative number of 450 l/c/d @ 3.5 persons per house with a peak factor of 4 and infiltration of 33 l/mm of pipe/km I get about 1.57 l/sec. What is the restriction of 1.12 l/sec of the system pipe? I do not have a copy of the BS 5572 code and am therefore not familiar with the methodology you used to get your flow of 5.1 l/s. 5.1 l/s (my numbers reversed works out to be about 1,600 l/c/d. That's an incredibly high flow rate.

In any event, you may wish to consider a retention system to provide continuous outflow at a continuous flow rate and sized to provide for the peak hour flows. Odours are always a problem with this type of system (wetwell), particularly in a residential area.

KRS Services

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[jcanning]

Thanks for the reply. The 5.1 l\sec comes from a table in that code of practice. It's the only methodology I have to go on, but I do agree it seems very large.

We are dispatching into a private system, into which we are only entitled to dispatch 1.12 l\sec. I am unfamiliar with your methodology, so I have a few questions.

Is 450 l\c\d an average waste per person per day?
The infiltration of 33 liters per mm of pipe per KM seems very large. I'm not sure that I understand you properly.

I have read that an average of 225 Litres per person per day is the European average for the flow rate in sewers.
Is this too low or do I need to factor it in my design?
Does the design below make sense?
What infiltration rate should I use.

I appreciate your help.

Average of 225 Litres per person per day. ( Reference below )
This average is the European average for the flow rate in sewers.

This average ought to be factored depending on the inhabitants socio-economic status. Middle class dwellings can be expected to produce a volume roughly equal to the average volume of waste.

Factoring this above average by 1.4 gives a value
of 315 L\person\day

Maximum hourly flow is calculated by factoring the average daily flow by 0.06 .

Maximum flow rate per person per hour = 315 * 0.06 =
19 L per Hour. = 0.00525 L \ sec.

Max allowable flow rate = 1.12 L\ sec.

1.12 / 0.00525 = 214 people.

Therefore 214 people are required to contribute to produce the max allowable flow rate.

Assume an average of 7 people per house.

214 houses / 7 inhabitants = 30.57.

Allowable number of houses = 30 houses.

 

[KRSServices]

I suggest very strongly that you undertake the services of a professional engineer to properly design your sewer collection system because your evaluation and calculations are flawed. I say this because you can seriously under-design or over-design your system with the methodology you described which could either cause some longterm headaches or cost the owner needless expense. In retaining the services of a P.Eng. (or PE in USA), it would be money efficiently spent.

Firstly, the number used to determine the average flow rate is a function of the community's water use. The municipality should have some numbers which can be used as a guideline to start with. You can also contact some of the local engineers involved in the design of residential and multi-family subdivisions because they typically have some good numbers to use. Water rationing versus unlimited supply will proportionately impact the nuber used to determine the average sewer demands per person. The peaking factors that you will either be prescribed by the municipality or will have to be developed based upon a number of factors. Again, most experienced subdivision development engineers have these numbers readily available to them and will vary depending on the landuse, water usage, statistical trends and municipal bylaws/statutes.

Secondly, census data that I typically use in my designs rarely exceed 3.5 persons per single family dwelling unit. Multi-family units utilize higher density populations, but I believe 7 persons per dwelling unit is needlessly excessive.

Infiltration/exfiltration relates to the inflow of groundwater/leakage of the effluent into the system. over time, in a situation where groundwater could present a problem, infiltration should be considered. In most cases, an infiltration factor is prescribed by the municipality or utility owner. Again, these facotrs are generally specific to the local area, have stood the test of time and are generally accepted. An experienced engineer will have these numbers as well.

I am not certain as to how you are arriving at your "factors", but suffice it to say, that I have more questions and confusion than confidence in your results. One other point, the "system" is also going to have to be designed correctly in terms of hydraulics (services, mainline, tie-in, min/max. velocities, etc.).

My apologies for sounding terse, but I think that perhaps you lack sufficient experience in this particular area of design and there could be some serious ramifications for errors or bad assumptions. Good Luck

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