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
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:
- 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.
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]
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.
