EWB Water supply by Guna Hewa P236

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EWB -Water supplybyGuna Hewa P2-36
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General Considerations

• Demand
• In-house (lpcdprojected population), ex-house
  (gardening), fire needs (20l/s4hours)
• Quality requirements
• Potable (drinking water quality) and non-potable
  water (washing, toilet flushing)
• Sources
• Groundwater (bore water), surface water (service
  basin to store 1wk demand, re-cycled water (need
  to consider the cost), rainwater harvesting (need
  to consider the seasonality of rainfall when
  sizing the tank)

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• Distribution needs (pumped-pipe systems)
• Head requirements, pipe material, operating
  costs, elevated tanks (1 day supply)
• Maintenance low maintenance
• Other issues
• Effects of urbanization (floods),
• rainfall patterns,
• combine sewer and stormwater systems
• Peoples perception, community involvements
• Cost
• High labour, low cost, low technology

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Effects of flood

• Sri Lanka and the southern state of Tamil Nadu in
  India receive almost 60 of their rainfall from
  the North East Monsoon during October to February
  and the existing drainage systems are overloaded
  and overflow.

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Main Causes of Flooding Rainfall in excess
of capacity for which drainage system is designed
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In reality

• Substantial garbage/solids deposits in drains
• major impact on drainage capacity

• - Theoretical concept of self-cleansing
  velocities is practically inapplicable to the big
  size solids (construction debris and solid waste)
• - Frequent flooding is inevitable where rainfall
  intensities are high

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Effect of urbanizationon quality of water

• Most storm drains carry sewage in dry weather
• 70-80 diseases related to water contamination
  and poor sanitation
• Human excreta associated with more than 50
  diseases
• Safe disposal required

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Post-Flood Sanitation measures

• Accumulated solid waste and debris removal

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Rainwater Harvesting
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Design objective

• Yield per unit storage. - where there is an
  additional source (s)
• Security of supply - where rainwater is the
  primary source
• Minimizing the cost per unit supply (In
  Australian practice)

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Selecting the tank size
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Selecting the tank size

• For the range of tank sizes being assessed a
  linear yield/storage relationship can be derived
  between the minimum and maximum tank sizes or by
  determining the average yield/unit storage
  increments.
• At some point along the yield/storage curve the
  slope (yield/unit storage) will be equal to the
  linear relationship.
• At this point a smaller tank size will result in
  increasing benefit and a larger tank will result
  in a decreasing benefit with respect to the
  linear relationship.

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The Model Output
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Input variables

• Rainfall
• Connected roof area
• Demand
• in-house and
• Irrigation
• Losses
• Initial
• First flush
• Evapotranspiration
• Overflow

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Initial losses

• Initial loss is defined as the amount of initial
  rainfall depth that is retained by the roof and
  conveyance system
• It accounts for wetting and absorption by the
  roof material and dead storage in the gutters and
  piping system
• The magnitude of initial loss can be influenced
  by the evaporation characteristics between
  rainfall events
• Therefore, in this model, an initial loss of
  0.5mm, 1.0mm and 1.5mm is allocated for Iron,
  terracotta concrete roofing

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First flush

• Generally, the first portion of roof runoff is
  considered to have a relatively high proportion
  of suspended solids and associated pollutants,
  particularly after long dry spells or in
  environments where pollution buildup is rapid.
• It is a good practice to avoid collection of the
  initial runoff with a first flush device.
• Typically a volume of approximately 20 to 60 L is
  accounted for first flush (Australian Capital
  Territory, 2005)

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Case study - Colombo

• Average annual rainfall of over 2000mm.
• Concentrated heavy rainfall pattern
• 60 of the rainfall results from Monsoon during
  and the existing drainage systems are overloaded
  and overflow.
• High variability of rainfall
• Mean annual daily rainfall is as low as 6.3mm,
  events as high as 285 mm of rainfall per day,
  which is equivalent to a 1 in 30 year annual
  maximum rainfall

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In-house and ex-house demand

• Daily demand- per capita daily demand could vary
  from 25 to 100L with an average of 50L
  (http//www.rainwaterharvesting.com
• average number of household size is 4.3.
  Department of Census and Statistics (2003) the
  Frequent non-registered occupants
• Based on the above information, in-house daily
  demands varied from 100 to 200 L (Lpd)
• A constant value of 150l/day is accounted except
  for monsoonal periods

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Connected roof area

• Typically vary from 25m2 to 75m2 while more
  established residences could have roof areas as
  high as 150m2 or greater
• Roof cover is mainly manufactured from asbestos
  (49.5), tiles (27.6), metal sheets (10.2) and
  concrete (8.4) while the rest counts for
  unclassified and Cadjan (coconut tree-leaves)
  roofing (Dept of Census and Statistics, 2003)

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Recommended sizes of Roof Runoff Storage tank
for a range of demand rates and roof areas
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Summary results

• For a household with a roof area of 25m2 and
  daily demand of 200 L, the recommended storage is
  2000 L.
• Resulting average water savings are around 40
  kL/year, which is equivalent to 54 of the annual
  household usage.
• On average there could be 120 days with no supply
  from the tank.
• 50 reduction of 10-day dry spells within the
  last 15 yearsThis represents a 50 improvement in
  the security of supply.

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Water Supply System Design
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Distribution pipe network system

• A water supply distribution system consists of a
  complex network of inter-connected pipes, service
  reservoirs and pumps which deliver water from the
  treatment plant to the consumer.
• Water demand is variable, both by day and by
  season, supply in contrast, is relatively
  constant.
• Consequently, the distribution system system must
  include storage elements and must be capable of
  flexible operation
• Water pressure within a distribution system are
  normally kept between maximum allowable head of
  about 70m and minimum head of about 20m. This
  range ensures that sufficient flow occurs meeting
  demand, and that undue leakage due to excess
  pressure does not occur.
• The topography of the demand area plays a
  important role in the design of the distribution
  system

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EPANET for pipe network analysis

• EPANET performs extended period simulation of
  hydraulic and water quality behaviour within
  pressurized pipe networks
• Output include flow in each pipe, pressure head
  at each node, height of water in each tank, and
  concentration of a chemical species throughout
  the network

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EPANET

• Steps in using EPANET to model water distribution
  systems
• Draw a network (see section 6.1 in the manual)
• Edit properties of the objects that make up the
  system (section 6.4)
• Describe how the system is operated (section 6.5)
• Select a set of analysis options (section 8.1)
• Run a hydraulic/water quality analysis (section
  8.2)
• View the results (Chapter 9)

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Pumps

• It is often necessary to use pumps in pipeline
  systems to overcome large friction and other
  losses

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Energy grade line with pump
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Performance curves

• The head supplied by a pump is a function of the
  pump at a given speed
• The total head vs. discharge and efficiency vs.
  discharge curves are referred to as performance
  curves or characteristic curves
• The performance curves are obtained from pump
  manufacturers

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Example Performance Curve
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Pump Power

• The power consumed by a pump when delivering a
  discharge Q (m3/s) at a head Hm (m) with a
  combined pump/motor efficiency ? is