Difference between revisions of "Rainwater harvesting: Sizing and modeling"

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*Filter efficiency (''e'') can be reasonably estimated as 0.9 pending manufacturer’s information.<br>
*Filter efficiency (''e'') can be reasonably estimated as 0.9 pending manufacturer’s information.<br>
*In a study of three sites in Ontario, STEP found the annual ''C<sub>vol, A</sub>'' of the rooftops to be around 0.8 [http://www.sustainabletechnologies.ca/wp/home/urban-runoff-green-infrastructure/low-impact-development/rainwater-harvesting/performance-evaluation-of-rainwater-harvesting-systems-toronto-ontario/]. This figure includes losses to evaporation, snow being blown off the roof, and a number of overflow events.
*In a study of three sites in Ontario, STEP found the annual ''C<sub>vol, A</sub>'' of the rooftops to be around 0.8 [http://www.sustainabletechnologies.ca/wp/home/urban-runoff-green-infrastructure/low-impact-development/rainwater-harvesting/performance-evaluation-of-rainwater-harvesting-systems-toronto-ontario/]. This figure includes losses to evaporation, snow being blown off the roof, and a number of overflow events.
 
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Five percent of the average annual demand can be estimated:
Five percent of the average annual demand can be estimated:
<math>D_{0.05} = P_{d} \times n\times 18.25</math>
<math>D_{0.05} = P_{d} \times n\times 18.25</math>
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*''P<sub>d</sub>'' is the daily demand per person (L)
*''P<sub>d</sub>'' is the daily demand per person (L)
*''n'' is the number of occupants}}
*''n'' is the number of occupants}}
 
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Then the following calculations are based upon two criteria:
Then the following calculations are based upon two criteria:
#A design rainfall depth is to be captured entirely by the RWH system.
#A design rainfall depth is to be captured entirely by the RWH system.
#The average annual demand (''D'') is greater than the average annual yield (''Y'') from the catchment.  
#The average annual demand (''D'') is greater than the average annual yield (''Y'') from the catchment.  
When \(Y_{0.05}/D_{0.05}<0.33\), the storage volume required can be estimated:
When \(Y_{0.05}/D_{0.05}<0.33\), the storage volume required can be estimated:
<math>V_{S} = A_{c} \times C_{vol,E}\times R_{d} \times e</math>
<math>V_{S} = A_{c} \times C_{vol,E}\times R_{d} \times e</math>
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*Careful catchment selection means that the runoff coefficient, for an individual rainstorm event (''C<sub>vol, E</sub>'') should be 0.9 or greater.
*Careful catchment selection means that the runoff coefficient, for an individual rainstorm event (''C<sub>vol, E</sub>'') should be 0.9 or greater.
       
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Finally, when \(0.33<Y_{0.05}/D_{0.05}<0.7\), the total storage required can be estimated by adding ''Y<sub>0.05</sub>'':
Finally, when \(0.33<Y_{0.05}/D_{0.05}<0.7\), the total storage required can be estimated by adding ''Y<sub>0.05</sub>'':
<math>TotalStorage = V_{S} + Y_{0.05}</math>
<math>TotalStorage = V_{S} + Y_{0.05}</math>
 
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==STEP Rainwater Harvesting Tool==
==STEP Rainwater Harvesting Tool==
[[File:RWH_tank_capacity_table.jpg|thumb|500 px|Quick reference table generated using STEP RWH tool, (data for the City of Toronto (median annual rainfall 678 mm). Optimal cistern size is that providing at least a 2.5% improvement in water savings following an increase of 1,000 Litres in storage capacity.]]
[[File:RWH_tank_capacity_table.jpg|thumb|500 px|Quick reference table generated using STEP RWH tool, (data for the City of Toronto (median annual rainfall 678 mm). Optimal cistern size is that providing at least a 2.5% improvement in water savings following an increase of 1,000 Litres in storage capacity.]]

Revision as of 18:59, 20 February 2018

Schematic diagram of the inputs and outputs to a rainwater harvesting cistern

Simple[edit]

Five percent of the average annual yield can be estimated:

Where:

  • Y0.05 is five percent of the average annual yield (L)
  • Ac is the catchment area (m2)
  • Cvol, A is the annual runoff coefficient for the catchment
  • Ra is the average annual rainfall depth (mm)
  • e is the efficiency of the pre-storage filter
  • Filter efficiency (e) can be reasonably estimated as 0.9 pending manufacturer’s information.
  • In a study of three sites in Ontario, STEP found the annual Cvol, A of the rooftops to be around 0.8 [1]. This figure includes losses to evaporation, snow being blown off the roof, and a number of overflow events.

Five percent of the average annual demand can be estimated:

Where:

  • D0.05 is five percent of the average annual demand (L)
  • Pd is the daily demand per person (L)
  • n is the number of occupants

Then the following calculations are based upon two criteria:

  1. A design rainfall depth is to be captured entirely by the RWH system.
  2. The average annual demand (D) is greater than the average annual yield (Y) from the catchment.

When \(Y_{0.05}/D_{0.05}<0.33\), the storage volume required can be estimated:

Where:

  • VS is the volume of storage required (L)
  • Ac is the catchment area (m2)
  • Cvol,E is the design storm runoff coefficient for the catchment
  • Rd is the design storm rainfall depth (mm), and
  • e is the efficiency of the pre-storage filter.
  • Careful catchment selection means that the runoff coefficient, for an individual rainstorm event (Cvol, E) should be 0.9 or greater.

Finally, when \(0.33<Y_{0.05}/D_{0.05}<0.7\), the total storage required can be estimated by adding Y0.05:


STEP Rainwater Harvesting Tool[edit]

Quick reference table generated using STEP RWH tool, (data for the City of Toronto (median annual rainfall 678 mm). Optimal cistern size is that providing at least a 2.5% improvement in water savings following an increase of 1,000 Litres in storage capacity.

The Sustainable Technologies Evaluation Program have produced a rainwater harvesting design and costing tool specific to Ontario. The tool is in a simple to use Excel format and is free to download.
Connect the Drops.PNG

STEP Treatment Train Tool[edit]

TTT.png
Rainwater harvesting: TTT