What are Wet Rainwater Harvesting Systems?

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What Are Wet Rainwater Harvesting Systems?Wet rainwater harvesting systems can provide an aesthetically pleasing solution to your rainwater collection needs. With a little careful planning, they can provide a plentiful source of high-quality drinking water. In this article, we explain how wet systems work, the advantages of wet systems and how to ensure the system remains clean and robust.

What are Wet Systems?

Wet systems work by first collecting water off the roof into gutters. Water is then fed into airtight downpipes that meet underground and flow into one large pipe. This pipe continues underground until it reaches a ‘riser pipe’ where water pressure forces water up the pipe from below, until it’s high enough to feed a storage tank.

Because water needs to fall from the gutters and then rise again to feed the storage tank, there needs to be a significant drop from the gutters to the tank. This is to build the water pressure required to force water up the riser pipe and into the storage tank.

This set-up means that when it stops raining, there isn’t enough water in the system to build pressure. Water can’t be forced up the riser pipe and so it remains in the underground pipes. This is why it’s called a ‘wet’ system.

Advantages of a Wet System

The great advantage of the wet system is that is looks much neater than a dry system. This is because there are minimal pipes visible, usually only the gutters and downspouts. The awkward joints and lengths of piping needed to join the different downpipes together are dealt with below ground. This is perfect for difficult building arrangements, for example if your water tanks are positioned far from your building, or there is an obstacle in the path to reach your storage tank. Where your roof is large or is a complex shape that isn’t easily gathered into one or two downspouts, a wet system avoids the messy array of piping across the front of your building that you might get with a dry system.

The disadvantage of the wet system is the potential for standing (stagnant) water. Pressure is needed to force water back up the riser pipe and into the storage tank, so when there isn’t enough water in the system to provide that pressure, water simply stands in the pipes. This can lead to problems with contamination from insects, frogs and bacteria for example. To overcome these problems, wet systems can be fitted with diverters that separate contaminated and clean water. The most useful diverter for a wet system is an in-ground diverter.

Cleaner Water: How In-ground Diverters Work

The overall concept of a diverter is that when it rains they ‘divert’ the first-flush of contaminated water into a separate reservoir. This first-flush of water contains the contamination you might have from the roof or within pipes if there hasn’t been any rain to wash them for a while. The reservoir in the diverter is sized so that it holds a little more than the volume of contaminated water you will have in your system. Clean water can then flow directly into the storage tank.

In a wet system with an in-ground diverter installed, the first-flush water flows along the downpipes until it reaches the bottom of the riser pipe. This is where the diverter is installed, at the lowest point in the system. Instead of beginning to fill the riser pipe, the first-flush water flows into the reservoir of the diverter. Once the reservoir in the diverter is full of the dirtier water, the water that follows should be clean. The flow to the reservoir is stopped by a floating ball which seals the entry to the reservoir when it is full. Clean water now flows up the riser pipe and into the storage tank.

The diverter allows a slow trickle of water to leave its reservoir at the other end. This is so the reservoir is allowed to empty after it stops raining, ready for the first-flush from the next rain event.

Where the in-ground diverter end cap is installed above ground, this effectively turns your wet system into a dry system. This is because all the water remaining in the pipes after it stops raining enters the reservoir and slowly drains. This leaves no standing water in the system making it ‘dry’ and so reduces the potential for contamination.

The in-ground diverter is a very effective addition for wet systems. There are also a number of other types of diverters available to ensure that you water supply remains clean, depending on your situation and choice of system.

Why Choose a Wet System Setup?

  • Well planned wet systems provide the advantages of being aesthetically pleasing and producing high-quality drinking water.
  • They are a little more complex than dry systems, and need some thought up-front to execute properly. However, once you have done this initial planning, you will be left with a neat, robust system that can provide you with the clean rainwater you need.