The document summarizes a study on the impact of diverting the first flush of rainfall on water quality and reliability in rainwater harvesting systems in rural Rwanda. Field tests showed that diverting the first 1mm of runoff reduced turbidity by 50% while having a small impact (less than 2%) on system reliability. Simulation results found diverting the first flush after three consecutive low rainfall days increased reliability by 15-30% while maintaining a high level of water quality. The study recommends diverting the first 1mm of runoff to provide cleaner water with minimal impact on water supply reliability.

1. The Impact of First Flush Removal on Rainwater Quality and Rainwater Harvesting Systems’ Reliability in Rural Rwanda Kelly C. Doyle – Montgomery Associates: Resource Solutions Dr. Peter Shanahan – Massachusetts Institute of Technology May 17, 2010

5. Average Monthly Rainfall Distribution Rainfall Depth (mm)

6. Rainwater Tanks 2007

8. The first portion of the total discharged storm volume which contains the main proportion of the pollutant load during a storm event (Bertrand-Krajewski et al. 1998) What is the First Flush?

9. Constant Volume Diverter Storage tank Gutters Diversion pipe Removable cap Tank inlet Courtesy of Matt Stevenson

12. New Clinic Roof (CL) First setup: 0.66 mm rain Second setup: 1.34 mm rain Roof material: new iron sheets Location: close proximity to road

13. Rusty Roof House (HS) Roof material: old, rusty iron sheets Location: far from vehicle road Captures first 1.45 mm of rain

14. Potato House (PT) Roof material: old clay tiles Location: moderate distance from vehicle road Captures first 1.61 mm of rain

16. Turbidity Reduction Trends-CL

17. Turbidity Reduction Trends-HS

18. Turbidity Reduction Trends-PT

20. Recommendation to divert 1st mm of runoff: Turbidity reduction

21. Storage-Reliability-Yield (SRY) Simulation Model q = time-based reliability d f = # days demand is not met n = total days in study

23. Effect of FF Diversion on Reliability in Bisate 7 days 4 days 6 days Additional days not meeting demand with first flush diversion 1.7% 1.0% 1.5% Reduction in reliability from diversion scheme 87% 30% 91% 15% losses, 1 mm diversion after three consecutive days each with <1 mm rain 89% 31% 92% 15% losses, no diversion Trackers’ House Primary School Health Clinic Scenario Reliability (%)

26. Questions?