Title:
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Water conservation impact on solid transfer in sewers
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The scope of the study was to investigate the impact of water conservation on several
aspects of solids transfer during low flows in sewerage systems which service small
catchments. A newly constructed test-rig allowed for several test regimes in a small
catchment, using simulated faeces. The test-rig encompassed WCs/bathrooms, lateral
sewers, junctions, three branch sewer types, including a wide egg-shaped pipe and a main
sewer which were all adjustable for height and gradient. Testing addressed gaps in the
research including simulated human faeces, impact of wipes, cistern height adjustment,
effects of non-invert level junctions, single occupancy dwellings, white goods discharge,
bath and shower discharge patterns, degradation of human faeces and variable gradients.
Original contributions are:
1. Simulated human faeces has been developed; this organic material is known as "The
Buncrana".
2. Degradation of human faeces can take place in at least eight stages; this was also
verified for the Buncrana.
3. Physical conditions can exist to provide unlimited solid transfer distance of given
organic material, in contrast to work of previous researchers.
4. For a given solid and a 6.0 litre flush, limiting solid transfer distances (LSTD) can be
established in a 30.0 m long 160 mm OD PVC pipe but not in the equivalent wide egg
shaped sewer/pipe. The LSTD values are based on a single solid that is not combined
with toilet paper or toilet wipes.
5. The wide egg-shaped pipe/sewer has advantages over circular pipes and at a 1/50
gradient can transfer the Buncrana a minimum of 30.0m with a maximum of 3 No.
6.0 litre flushes. For the same gradient and flush conditions the equivalent 160 mm
OD PVC results in transferring the Buncrana circa 10.30511" However solid
degradation was inhibited in the wide egg-shaped pipe.
6. Sewer pipeline configuration is key; a "minimum distance to convergence point"
must be established to maximise use of water other than WC flush and prevent it bypass
solids.
7. A methodology for the scoping of a code of practice for low flow testing has been
drafted.
8. Risk of sewer blockages from single occupancy dwellings arises from adhesion of
human faeces to the sewer.
This thesis has recommended significant further research to provide innovation in the
design of sewers to cater for water conservation.
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