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Sewer Network and Treatment System

I'm Redesigning a sewer network and treatment system for a remote orphanage.

I need support to evaluate the minimum gradient for 100, 150 and 200mm diameter pipes.

I have worked out the corresponding gradients of 0.012, 0.006 and 0.004.

I need to know if these are acceptable for normal pipes ks factor 3, or if I need to secure pipes with lower ks factors.

Furthermore, I have planned for basic primary treatment and secondary treatment.

This comprises of three tanks each one having a one day’s retention time.

The first two tanks are for primary settlement while the next is for secondary anaerobic processing. Is this acceptable?

Regards, Stephen

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Have very little information to go on but here is our advice:

Collection pipe work: Assuming that the pipe work relates to gravity collection sewers: Normally for sewers running part-full (not surcharged) DN 100 min slope 1:100 DN 150 min slope 1:150 DN 200 min slope 1:200

For pumping mains different criteria apply.

Roughness k=3 for untreated sewage irrespective of material as slime is developed such that flow does not see material of construction.

Wastewater treatment: The proposed retention times do not look like conventional design. For example typical design requirements fro primary settlement tanks would normally be based on a sewage retention time of 1.5 to 2 hours maximum with a surface loading of 30 to 45 m3/m2.d at max flow.

The treatment objectives seem to be removal of solids and perhaps some anaerobic treatment of the solids. This would seem to require a design such as septic tanks where solids settlement will occur and the settled solids will receive some anaerobic treatment before removal and disposal of solids. However, as no flow data are provided it is not possible to comment, as septic tanks may not be applicable to larger flows.

An alternative would be the provision of a pond system with perhaps an anaerobic pond providing solids removal and secondary/tertiary ponds treating the effluent from the anaerobic ponds such that the desired effluent quality can be achieved.

Both of these options could give rise to odours, although that from septic tanks can be readily contained. The use of ponds assumes that land is available for this form of treatment.


Andy (via James)

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2014-01-13 09:51:53 -0500
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Last updated:
Jan 20 '14