Rainwater harvesting

Question 1

What is RWH?

Answer 1

o Collection of water from impervious surfaces
o Storing of water
o Using the water over time
 Domestic use
 Agricultural use
 Groundwater recharge

Question 2

Why RWH?

Answer 2

o Reducing soil erosion
o Reduces stormwater runoff volume and peak
o Eases the stress on aquifers as supply and/or recharges GW
o Simple and inexpensive systems
o Mostly healthier and less contaminated compared to surface and ground water
o More sustainable water supply for rural and urban communities
 Low impact
 Healthy
 Accessible

Question 3

• RWH applications

Answer 3

o Irrigation
o Personal hygiene
o Toilet flushing
o Laundry
o Drinking water
o Cooking
o Industrial/car wash

Question 4

What controls RWH design?

Answer 4

Quantity and quality of demand

Question 5

What are the components of a RWH system?

Answer 5

o Roof (catchment)
o Gutters and pipes
o Leaves filter
o First flush tank
o Cistern
o Post-treatment

Question 6

How does the first flush tank look?

Answer 6

 Can be a barrel (V >50 L) or a PVC pipe (V < 50 L)

Question 7

What to consider for the cistern location?

Answer 7

• Weight of cistern and water
• Is gravity possible to use for water transport?
• Accessibility for construction and maintenance
• Distance from roof – shorter the better
• Distance from use
• Trees that can shade the tank? Prevents algae

Question 8

What to consider for the cistern design?

Answer 8

• Filling the cistern from top or bottom
• Connection of multiple cisterns
  o Parallel
  o Series
• Flush valve at bottom to remove accumulated sediments
• Withdrawal tap and connector pipe (outlet) elevated from bottom to not disturb sediment and old water
• Humidity should be kept within cistern – structural issues otherwise
• Air inflow – prevents high pressure, cracks and leakage
• Overflow pipe

Question 9

Describe boiling as a post-treatment?

Answer 9

• Aim: disinfection
• Boiling for one minute and adding an additional one minute for every 1000 meters of elevation above sea level
• Pros
  o Effective on all pathogens
  o Easy to use and inexpensive
• Cons
  o Energy consumption (electricity, gas, biomass, etc.)
  o On-going cost for fuel
  o Deforestation (using wood or charcoal)
  o Air pollution (if not using clean energy)

Question 10

Describe solar disinfection as a post-treatment?

Answer 10

• Aim: disinfection
• Mechanism: UV radiation from the Sun
• Usable for water with low turbidity
• PET bottles exposed to direct sunlight for at least six hours on a sunny day and at least two full days when semi-cloudy
• Pros
  o Effective on all pathogens
  o Very simple and almost free setup
• Cons
  o Time consuming (can take 2 days)
  o Not-working on overcast days
  o Non-turbid water required

Question 11

Describe chlorine as a post-treatment?

Answer 11

• Aim: disinfection
• Chlorinated sodium salts (NaDCC) e.g. sodium hypochlorite
• Mechanism: NaDCC release hydrochloric acid (HCl) which kills MOs
• Pros
  o Very effective on bacteria
  o Very quick effectiveness
  o Inexpensive
• Cons
  o Regulated chemical dose (Over-dosing is harmful esp. for children)
  o Not very effective on protozoa & helminths
  o Changes the taste of water
  o May be corrosive on specific uses (e.g. metals)

Question 12

Describe bio-filter as a post-treatment?

Answer 12

• Aim: Particles (turbidity) and Pathogen removal
• Biologically active slow-sand filtration (biofilm formation on sand)
• Mechanism: Physio-chemical filtration and biological processes
• Pros
  o Very effective on larger pathogens (protozoa and bacteria)
  o Can be made from local materials for many years
  o Simple setup with no moving parts
  o Inexpensive ($80 / 20 lit)
• Cons
  o Somewhat effective on viruses
  o Difficult to transport/ An in-site setup
  o Takes a long time to develop biofilm (biologically activated

Question 13

Describe pot filter as a post-treatment?

Answer 13

• Aim: Particle (turbidity) and Pathogen removal
• Main component: clay + combusted organics (activated carbon-AC)
• Mechanism: Physical filtration + Absorption + disinfection
• Capacity: 1-3 lit/hr; Practically usable for 2 years
• Pros
  o Effective on all pathogens
  o Simple setup with no moving parts
  o Inexpensive
• Cons
  o Water quality depends on manufacturer
  o Somewhat effective if colloidal silver is not used
  o Low performance in case of high turbidity

Question 14

Describe candle filtration as a post-treatment?

Answer 14

• Aim: Particle (turbidity) and Pathogen removal
• Components: hollow cylinders made from clay + AC (combusted organics e.g. sawdust, grain husk) + antibiotic chemicals (colloidal silver)
• Mechanism: Physical filtration + Absorption + disinfection
• Capacity: 2-8 lit/hr; Practically usable for 3-5 years
• Pros
  o Effective on all pathogens
  o Simple setup with no moving parts
  o Inexpensive
• Cons
  o Water quality depends on manufacturer
  o Somewhat effective if colloidal silver is not used
  o Low performance in case of high turbidity

Question 15

Name three more high-tech post-treatment methods

Answer 15

 Microfiltration followed by UV-light radiation
* Removal of 1-5 micron solids
* Filters need to be cleaned or replaced after a while
* High performance disinfection by UV if there is low levels of turbidity
* UV has less corrosion risk to exposed metal than Chlorine
 Reverse osmosis
 Distillation
* Highly effective on all kinds of contamination removal
* RO needs pre-treatment filters such as pre sedimentation and granular active carbon (GAC)
* Outlet water needs to be equilibrated before use (adding minerals/bypass water)