Rainwater Harvesting Ing. Marco Cremona
Rainwater Harvesting : An Ancient Practice Old limestone-hewn prehistoric Misqa tanks found in the vicinity of the Mnajdra temples on Malta
20 th century developments in RWH in Malta 31 small dams have been constructed across major drainage lines. Total dam capacity is estimated at 154,000 m 3. A number of open reservoirs have been constructed along roads in the 1970s to catch flowing runoff water; their total volume is estimated at 250,000 m 3
20 th century developments in RWH in Malta
Rainwater Harvesting Rain is a free source of salt-free, soft, generallyclean water, that does not require a distribution system but requires a clean, impermeable catchment area e.g. roof The amount of rainwater that can be harvested in Malta in a year is 498 litres per square metre of catchment area assuming a
The biggest problem with rainwater harvesting is the huge seasonal differences in the supply (and unpredictability of the supply). 84% of all rain falling over Malta comes in Winter (45%) and Autumn (39%). Spring contributes 14% while summer only receives 2%. This results in a mismatch between supply and demand, which can be mitigated by storage.
temperature (celsius) rainfall (mm) Malta - Climate Graph 30 160 25 140 120 20 100 15 80 10 60 40 5 20 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec months 0 Average Rain mm Average Temp. ºC
The design storage depends on: Whether storage is required to extend the supply between dry days in the autumn/winter/spring months Whether storage is required to carry over surplus supply capacity in autumn/winter over to spring/summer
Rainwater collected from relatively-clean surfaces can be used as 2 nd class water without treatment. These uses include flushing of toilet cisterns and urinals, for clothes washing, floor washing, topping up of swimming pools (chlorination necessary), for landscaping and for boilers and cooling towers. Indeed, rainwater is superior to mains water when it comes to irrigation (no salt, no chlorine), clothes washing (soft), boilers and cooling towers (soft water, no chloride).
However, rainwater cannot be used for 1 st class water uses (including for showers/baths and wash-hand basins) unless treated, tested and registered with the Environmental Health Unit of the Public Health.
The use of rainwater within a commercial establishment may require a separate water distribution system.
Traditionally, rainwater harvesting is carried out with the use of excavated below-ground lined cisterns. However, retrofit solutions (using above-ground plastic/fibreglass tanks) exist, and provide a payback period of 4-5 years, depending on economies of scale.
Pros: Cheap, sustainable source of relatively-good quality water that can be used as 2 nd class water without any treatment, or 1 st class water with inexpensive treatment (estimated cost of rainwater - 0.05/cubic metre) Reduces vulnerability to water tariffs, electricity and chemical costs No specialised technical expertise required for operation/maintenance Prevents flooding
Cons: Requires significant capital investment in the storage requirement Cannot be the only source of supply, due to seasonal fluctuations and unpredictability Opportunity cost of lost space May require a separate water distribution system
Most suitable: For situations where there is high 2 nd class water demand There is a catchment area of adequate size A rainwater cistern already exists A distribution system for 2 nd class water already exists For situations where the consumer is paying a high price of water, or is using an illegal source of water (seawater, or borehole water) for the flushing of toilets
WSC Groundwater Extraction 16,312,500 WSC Reverse Osmosis 18,000,000 Private Groundwater Extraction 15,750,000 Private Reverse Osmosis 1,125,000 Rainwater Harvesting 2,812,500 Treated Sewage Effluent 2,250,000
Percentage of preference Potential for domestic RWH in Malta 80 70 60 50 40 30 20 10 0 well garage both don't know Preference of a garage or cistern (Section D Q3 of RI) Expressed uses for rainwater in the home
My personal experience with Domestic RWH Completely self-sufficient for over 8 years (2 person household) Combination of rainwater harvesting, greywater recycling and water conservation practices Primary cistern of 6 m 3 ; secondary (back-up) cistern of 30 m 3 ; 90m 2 roof area System designed to drinking water standards