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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. |
| | | ---- |
| 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}} |
| | | ---- |
| 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. |
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| 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> |
| | | ---- |
| ==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.]] |