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Potential impact of water conservation and saving measures

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					Potential impact of water conservation and saving measures on downstream collection systems
John Blanksby Pennine Water Group

The nature of sewage
• Water • Gross solids
– – – – – Faeces Toilet tissue Sanpro products Leaves Litter

• Sediments

Development of sewerage systems
• Prior to the industrial revolution, the majority of the population lived in small towns and villages and London was the only major conurbation in the UK • Some culverting of watercourses had taken place so as to make movement in the cities more easy, but those piped drains that had been constructed were for surface water only, as solid waste was disposed of by different routes

Development of sewerage systems
• Improved understanding of the causes and prevention of disease resulted in sewerage systems that were based around the surface water drainage systems that had previously been constructed • Initially communal water closets and latterly individual households were connected to the surface water sewers converting them into combined sewers. These networks were improved and enhanced as the urban areas grew and roof and highway drainage was also directed into the new sewers

Development of sewerage systems
• The result of this was that rivers, which had previously been relatively unpolluted, rapidly became highly polluted and a source of disease themselves • As a response to the situation, sewage treatment processes were developed and new sewers were constructed, intercepting the discharges into the rivers and carrying the dry weather flows to the treatment plant

Development of sewerage systems
• However, the treatment plant could not handle storm flows and so these flows were diverted to the rivers at the points of interception. The devices that control this process are called Combined Sewer Overflows (CSO) • CSOs were also built at points where the sewerage systems were so overloaded that flooding occurred. • CSOs can be a major source of pollution to the aquatic environment

Development of sewerage systems
• In the early years of the twentieth century separate drainage systems were installed in more rural areas and were also used in green field developments where no existing drainage system existed • Separate drainage systems have two pipes, one for carrying sanitary waste, known as the foul sewer and one for carrying surface water, known as the surface water sewer • These systems can be effective in controlling the flows to treatment, but they can be subject to wrong and cross connections and the surface water draining from streets tends to be highly polluted

Development of sewerage systems
• Ongoing development in urban areas has caused overloading of systems which were constructed over 100 years ago • The old approach of relieving the system by creating new overflows is now being superseded by other approaches including flow reduction and flow attenuation, the former being removal of flows from the system altogether and the latter being the use of storage to limit flows passed downstream

Development of sewerage systems

Foul sewers
• Foul sewers are designed to carry a small multiple of dry weather flow so as to provide adequate capacity for the peak diurnal rate of foul sewage and a degree of infiltration • It would be typical for foul sewers to have the capacity to carry four times the average dry weather flow. • This means that providing there are no wrong connections from surface water drainage systems and that the pipe is constructed to a reasonable standard of workmanship so as to exclude too much infiltration, that the pipe will never run full

Upstream flow conditions
• At the upstream end of the system, flow is intermittent, because not sufficient properties are connected to produce a continuous discharge • At the upstream end of the system, there is a large amount of solid faecal material and toilet tissue, but this is broken down as the material passes down the system.

Upstream flow conditions
• The transportation methods for these materials may be described as floating and sliding dam by Littlewood & Butler

Combined sewers
• Combined sewers are designed to carry both fouls and surface water and therefore need to be sufficiently large as to prevent flooding in wet weather conditions • This has a significant effect on the size of the sewers, as the discharge during wet weather conditions may be many times that during dry weather • As a result of this, during dry weather, velocities are generally low, giving circumstances where deposition may occur

Sediment deposition

Downstream flow conditions

Water conservation measures
• Some have positive benefit, but others have adverse effects • The removal of solids from the waste stream is beneficial
– Reduced blockages and reduced handling at STW inlets

• The reduction of water may be seen to have both beneficial and adverse effects
– Reduced energy for transporting materials – Reduction in pipe sizes and tank sizes at STW

Current research
• Hydraulic capacity of sewers
– Large body of research

• Transportation of gross solids
– Littlewood and Butler – UKWIR 01/WW/08/10 - Characteristics of Sewage Particles

• Sediment transport
– Empirically based modelling and small scale laboratory experiments

Future research
• Large scale controlled testing
– – – – – Sediment transport Gross solids transport Different diameters Different gradients Impact of laterals


				
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