In a “wet” Rain Harvesting system (also called a “charged” system), the pipes from your gutters go down the wall and underground, then up into your tank.
Because they travel underground and sit below the level of your tank inlets, these pipes remain full of water even during periods of time without rainfall.
Many Rain Harvesting systems are designed as wet systems because the size of the building and the location of the tank make it impractical to have pipes running directly from your gutters to your tank. Wet systems are also preferred by some people who consider them to be more aesthetically pleasing than “dry” systems.
Managing your wet system
Without proper management and precautions, wet Rain Harvesting systems can become a breeding ground for disease-carrying mosquitoes and can facilitate anaerobic fermentation.
To solve these problems, it’s necessary to use appropriate screens and filters to mosquito-proof your pipes, and to drain the water from your wet system pipes – effectively transforming it into a “dry” system”.
Draining your pipes
Anaerobic fermentation is a natural process that occurs when leaves and other organic materials decompose in an oxygen-starved environment such as your water-filled pipes.
This process releases thiols and hydrogen sulphide (“rotten egg” smell).
Unless you take preventative steps, water in your pipes that’s been fouled by anaerobic fermentation is pushed into your tank when it rains, contaminating the cleaner water you’ve already harvested.
The best way to prevent anaerobic fermentation from contaminating your rainwater is to drain your water-filled pipes (also called “charged lines”) between each rainfall event so there’s no water for leaves to decompose in.
This can be accomplished by installing an in-ground first flush diverter, which slowly drains the water from your pipes. Alternatively, you can program a flow controller to automatically drain the water from your pipes or use a sliding gate valve to manually release this water.
Mosquito-proofing your pipes
Mosquitoes breed in stagnant water, making your wet system’s water-filled pipes an enticing place for them to call home.
As mosquitoes can spread a range of diseases (including dengue fever, ross river fever, malaria and ebola) and contaminate your rainwater, it pays to keep them out.
In many jurisdictions (including some Australian states), pipes that hold water must be screened to prevent mosquitoes for entering and breeding in them.
Non-corrosive metal screens and filters with an aperture (hole size) of not more than 1mm can effectively keep mosquitoes out of your wet system pipes.
Mosquito-proof rain heads such as Blue Mountain Co’s Leaf Eater range can be installed on downpipes to keep mosquitoes out of your pipes at the gutter end. At the tank end, where your water enters your tank inlets, insect-proof screens or flap valves can stop mosquitoes entering your pipes. A plumbed tank inlet solution such as Blue Mountain Co’s Maelstrom is another effective way to keep mosquitoes out of your wet system pipes.
The right system for you
If you have a large roof area and lots of downpipes, your tank is located further away from your home or you simply don’t want a collection of visible pipes that directly connect your roof to your tank, a wet system may be the right choice for you.
So long as you mosquito-proof your pipes and drain them between rainfall events, you can be comfortable you’ve taken the necessary precautions to overcome the risks most commonly associated with wet rain harvesting systems.