Potential impact of water conservation and saving measures on downstream collection systems

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Potential impact of water conservation and saving
measures on downstream collection systems

• John Blanksby
• Pennine Water Group

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The nature of sewage

• Water
• Gross solids
• Faeces
• Toilet tissue
• Sanpro products
• Leaves
• Litter
• Sediments

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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

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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

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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

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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

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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

8
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

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Development of sewerage systems
10
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

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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.

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Upstream flow conditions

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

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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

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Sediment deposition
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Downstream flow conditions
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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

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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

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Future research

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

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