Rainwater Harvesting Systems – A Practical Guide

Rainwater harvesting systems may sound technical and complicated but they are actually one of the simplest and oldest ways for self-supply of household water and for a garden or crop irrigation.

Rainwater harvesting refers to the process of collecting, storing, and redistributing the water from rainfalls for your personal use. This can be done and has been done in many different ways throughout history – wells, shafts, boreholes, barrels, aquifers, and other reservoirs.

The standard uses for harvested rainwater typically include gardening and crop irrigation, livestock care, residential heating, and even domestic use under certain conditions and water treatments.

What is a rainwater harvesting system?

When talking about a rainwater harvesting system in this article, we’re going to talk specifically about modern and professionally manufactured systems sold for residential and commercial use. You can also make a DIY rainwater harvesting system from barrels, a tarp, a well or other methods and we’ve talked about those here as well.

As far as commercially sold rainwater harvesting systems are concerned, these are several main types of systems you’re likely to encounter: direct & indirect pumped harvesting and indirect gravity or gravity only harvesting. All of those systems collect water from the roof and drain pipes of your home or other buildings and redirect it toward a storage tank. The differences lie in how the water is collected and redistributed.

Direct pumped rainwater harvesting systems

Rainwater harvesting system explained

These systems store the rainwater in underground tanks after passing it through intricate filtration systems. This system is usually easy to install and is mostly used for domestic purposes. The water gets to the tank with the help of both gravity and often a pump, and is then redistributed from the tank to your desired appliances via another pump. If that pump is inside the tank, the system is called a “Submersible direct pumped rainwater harvesting system” and if the pump is located by the appliances and “sucks” water out of the tank, it’s a “Suction direct pumped rainwater harvesting system.”

Indirect pumped rainwater harvesting systems

The storage tanks of these systems are not located underground but can be placed anywhere on your property. Sometimes, there are multiple tanks, both underground and inside the building. The benefit here is that the water is redistributed via a booster pump inside the main tank which offers pressurized supply. This system offers great flexibility in how you can use the collected rainwater.

Indirect gravity rainwater harvesting systems

The storage tanks of these systems are usually placed in a high level header tank, typically located by or on the building’s roof. The rainwater collected from the roof and the drains is redirected to the tank via a pump, however, it can be redistributed from the tank using only the force of gravity. This can save you a lot of energy consumption as this system only needs to pump the water once.

Gravity only rainwater harvesting systems

These are the most economical and efficient rainwater harvesting systems you can get and the easiest to install as well. They don’t work for any situation, however. Gravity only systems use only the natural force of gravity as they collected the rainwater from your roof and drainpipes by just placing the storage tank underneath the drainpipes.

After that, the water is also redistributed to your home and property via gravity alone as long as the water tank is located higher than the garden or appliances it needs to supply. The tank needs to be situated perfectly under the rainwater drainpipes and above these things it has to supply.

Gravity only systems are most commonly used on two- or three-storage homes and are especially useful for gardening and groundfloor water supply. Essentially, these systems are a better and professionally-made variant of the DIY rainwater barrel rainwater harvesting method.

Stages in a rainwater harvesting system

The basic rainwater harvesting system stages are the same for all of the above systems regardless of the specific water supply or water redistribution methods they use:

  1. Collecting the rainwater. All systems do this by collecting the water from the roof and drainpipes of your home or other buildings. The larger the roof, the more water can be collected.
  2. Filtration. Even when the rainwater isn’t collected for a drinking water supply – which it usually isn’t – good rainwater harvesting systems will still pass it through water filters to make sure it’s clean of debris and certain contaminants.
  3. Storage. All rainwater harvesting systems collect rainwater in one or more storage tanks for later use. This tank can be underground, on the roof, even inside the building.
  4. Redistribution. Whether that’s done via gravity or by pumps, the rainwater is redistributed to wherever it’s needed – your garden, crops, a heating system, or anywhere else.

Design recommendations for rainwater harvesting systems

No rainwater harvesting system is always better than the others – they are all designed for different circumstances and preferences. If you want to minimize your energy expenditure and if your needs allow for it, gravity only systems are easy to install, easy to use, and incredibly cheap.

If you’re going to use the rainwater for many different purposes inside your home and around your property, you’ll likely want the superior control and flexibility of an indirect pumped rainwater harvesting system.

If you want to minimize expenses but still have better control over the stored water, indirect gravity systems are a good choice.

And if you want something simple and straightforward but your property or needs don’t allow for a gravity-only system, a direct pumped system is the most common choice.

Rainwater quality

Untreated rainwater can vary a lot in its contents. Aside from debris, it can also contain all types of contaminants. That’s why it’s important to install adequate filtration systems inside your rainwater harvesting system. The filters you’re going to want will depend not only on what you want to use the water for but also on what type of rain falls in your area.

Is a rainwater harvesting worth it?

To answer this in detail you’ll have to calculate how much water you’re going to need for a given period of time (month or year, for example), how much it would cost to use municipal water, how much it would cost to install the rainwater harvesting system of your choice, how much energy it’s going to cost for the same period, and how long you intend to use it overall.

Most of the time, big rainwater harvesting systems with significant energy consumption won’t be worth it if you only need them for minor household water use. However, if you want to supply a large indoor heating system, a garden, or a crop field, they are almost always more than worth it.

Can you drink rainwater?

It wouldn’t be a good idea to drink unfiltered rainwater even if you know it’s not overly polluted. If the water filtration in your rainwater harvesting system is good, however, and you know it accounts for all pollutants present in the rainwater, then it can be safe to drink.

At the end of the day, rainwater is the same as the groundwater municipal water suppliers harvest, it’s just in a different stage of the hydrologic cycle.

How do you filter rainwater?

A good rainwater harvesting system will use multiple different water filters to make sure that the water is safe for whatever it’s going to be used for. The filters you’ll see in a good system include:

  • Cartridge filters. These filters are the workhorse of a rainwater filtration system and they do most of the heavy lifting. They include a series of different filters inside of them such as activated carbon filters, ceramic filters, sediment filters, alkaline ionizer filters, and/or a reverse osmosis membrane.
  • Chlorine. Chlorine is widely used as a water disinfectant in all municipal water and public water systems such as schools, pools, and others. That’s because it eliminates many water-borne diseases such as hepatitis, cholera, typhoid, dysentery, and others. It does have side effects but those are counteracted by activated carbon filters.
  • Ultraviolet (UV) light filters. UV light works by penetrating the cell walls of live organisms inside the water and preventing their reproduction. This reduces the harmful pathogens inside the water. Such filters are usually installed at the end of the filtration system after all other filters have already done their job.

How much does a rainwater harvesting system cost?

This depends on many different factors – what type of system you want to install, how many and how large its components have to be (i.e. the gutter outlets, the gutter mesh, the size of the container tank, the type of pumps, the size and number of pipes, and so on), where you live, what the construction costs are going to be, and more.

The tank, pump(s), and filter(s) are usually the most costly components. Many water tanks and bases will vary from several hundred to several thousand dollars, and most pumps and filters are usually in the 3-digit range.

The additional expenses such as screens, pipes & fittings, diverters, mesh, outlets, screens, and others can also pile up quickly, however.

Overall, a basic system can cost in the low thousands (~$2,000) and an advanced system will usually cost over $10,000. 

What happens when your rainwater tank is empty?

Many rainwater harvesting systems are set up in such a way that if/when the water tank is in danger of running dry, it’s fed a small amount of mains water to keep it full. This is mostly done for underground or near ground level tanks.

Roof tanks usually don’t have this precaution and if they run dry the user simply needs to switch to using mains water for the time being.

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