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Rainwater Harvesting Guide (2023)

Are you curious to learn more rainwater harvesting and its types?

Why is it important to harness rainwater?

What does it mean by rainwater harvesting and why is it especially necessary today?

I’ll address all your questions about rainwater harvesting in this guide.

Let’s get started.

Rainwater Harvesting Basics​

Let us look at the exact process followed in rainwater harvesting systems.

Dive right in!

Hi there! Are you wondering…

Why is it important to harness rainwater?

What does it mean by rainwater harvesting and why is it especially necessary today?

Let me explain!

The collection and storage of rainwater in your property for future use is essentially called rainwater harvesting. Rainwater harvesting is one of the most important water conservation methods.

Rainwater can be collected from impermeable areas and roofs. It helps prevent surface water run-off.

The collected water can be diverted to a tank, pit, or reservoir where it can be stored for future use. It can also be treated and purified for later use.

So, what is the process followed in a rainwater harvesting system?

The water collected from the rooftops and other impermeable surfaces pass through a filter which essentially removes the dirt and contaminants before sending to a holding tank that could be placed either underground, above the ground, or on the side of the property.

The size of the holding tank can vary according to the space available. This tank would be usually fed by the gutters surrounding the property if it is a rainwater harvesting system for domestic purposes.

As seen above, to observe the level of water in the holding tank, a control unit is used. Additional details such as the pressure of the pump and the water temperature are also provided by some rainwater harvesting systems.

Depending on the end-use and the size of the rainwater harvesting system, several filters may be used.

The harvested water is taken to the points of use by a dedicated pump system.

In cases of emergencies like low rainfall, a backup piping system may also be required.

In the UK, the harvested water from the rainwater harvesting system is not drinkable without further treatment. So, necessary steps should be taken accordingly.

Components of a Rainwater Harvesting System

There are different components available in the rainwater harvesting system. They are

  • Collection pipe
  • Tank
  • Treatment
  • Pump
  • Cistern
  • Distribution pipe
  • Controls

Let’s understand them in more detail.

Rainawater Harvesting Process
Image copyright: CIRIA – Rainwater and greywater use in buildings: Best practice guidance

Let’s see the function of each component one by one

Collection pipe

The pipe which is used to collect rainwater from the sources like roofs and impermeable areas is called the collection pipe. The pipe that is normally used is ABS plastic waste pipe or low-pressure PVC pipe.


The collection tanks for packaged rainwater systems are mostly made of plastic.

Materials like concrete, steel, or brick can also be used provided. They need to scaled and protected against disinfectants or corrosive effects of the stored water.

The tank should be designed in such a way that the inflow of contaminated groundwater must be prevented.

Underground tanks must be designed to resist backfill pressures, vehicular loads, and surcharge loads.

These tanks must be periodically cleaned and maintained for efficient operation.

In the case of buried tanks in areas prone to surface water flooding, access covers should be raised to prevent the inflow of contaminated water due to surface water run-off. You will need a flood risk report to be able to define the risk to the soakaway clearly.. You will need a flood risk report to be able to define the risk to the soakaway clearly.. You will need a flood risk report to be able to define the risk to the soakaway clearly.. You will need a flood risk report to be able to define the risk to the soakaway clearly..

A screened air vent should be installed in the tank to avoid the build-up of harmful gases.

The excess water overflows to a soakaway or the surface water sewers. In this case, using an appropriate backflow prevention device is mandatory which otherwise could lead to an inflow of contaminated water entering the system.

Tanks that are situated above the ground are prone to freezing. Tanks and associated pipes could start to crack and result in leaking if they are not properly insulated.


Treatment can be divided into three categories.

(i) Filters
(ii) Biological processes
(iii) Disinfection system

(i) Filters

In a rainwater harvesting system, filters can be potentially located at four points.

  • Before the collection tank
  • Inside the collection tank
  • Between the cistern and the collection tank
  • Between the collection tank and the points of use

These filters must be easy to clean. Filters play a significant role in the operation of the rainwater harvesting system. When there is a problem with the filter, the operation of the rainwater harvesting system could be affected.

(ii) Biological processes

This process is mostly applied to large rainwater harvesting systems by reducing the growth of microorganisms in the water. This reduces the biological oxygen demand. The nutrients are used up and the microorganisms can’t grow. 

(iii) Disinfection system

The disinfection system in a rainwater harvesting system can be applied potentially at three points.

  • Directly to the collection tank
  • Directly to the cistern
  • Between the collection tank and the cistern

This system, applied to some rainwater harvesting systems, is used to disinfect microbiological contamination.

Chemical disinfection must be applied to the collection tank to treat the stored water.

Biological treatment must be applied before using chemical disinfection. These chemical disinfectants may also harm the membrane filters. The parts of the system should be selected in such a way that they can resist the chemicals used to treat the water. For instance if used as drinking water. 

The life of the parts can be reduced by the use bromine and chlorine chemicals.

High residual chemicals can be present in the reclaimed water that can negatively impact plant or animal life. It can also cause potential damage to the skin. Systems should be designed to avoid these problems.

In hot spaces, gases build up from chemicals. If the disinfectant is applied in large amounts, it can lead to a pungent odor. However, when the disinfectant is applied in correct amounts, it functions effectively by reducing microbiological risk and does not lead to an extreme pungent odor.


Water is moved using a pump into the building for those rainwater harvesting systems that use a collection tank below or at the ground level.

These pumps must resist corrosion and should not continually switch on and off as it may result in overheating.

If a filter is used to pump water, it must be maintained regularly which otherwise would reduce the life of the pump or create substantial head build-up.

To prevent the inflow of debris, submersible pumps can be placed above the floor tank.


If the mains water supply is connected to the storage cistern, an air gap arrangement between the mains inlet and the spillover level must be made. This air gap must be adequately large.

There must also be a warning pipe that should indicate tank overflow conditions.

When the collection tank is empty, the cistern may have a float valve to allow makeup from the mains water. When the collection tank is full, the cistern may have a high-level switch to stop the collection tank pump.

Distribution pipe​

High-pressure plastic pipes like PVC, ABS, PEX are normally used to connect the pump and the cistern and for reclaimed water distribution.

Some plastics become fragile when they are exposed to sunlight over time. So care must be ensured to select highly resistive ones.

Because of the corrosive nature of chemicals in water, the use of copper pipes is not advised.


Using electronic controls can be expensive and consume electricity.

Some rainwater harvesting systems are controlled by mechanical float valves and only a low-level switch with no other electric controls.

Rainwater harvesting systems should have a control panel so that the user can check the system visually.

Sometimes, the users can be completely unaware of the fault in the system. Hence, the control panels must contain alarms for common fault scenarios.

Types of Water Harvesting Systems

There are different types of rainwater harvesting systems

Each one comes with it’s own pros and cons

Let’s look at each one of them in detail!

Image copyright: renewableenergyhub

There are eight main types of rainwater harvesting systems as listed below.

  • Waterbutt
  • Direct pumped – Submersible
  • Direct pumped – Suction
  • Indirect gravity
  • Indirect pumped
  • Gravity only
  • In-ground storage
  • Retention ponds

1. Waterbutt​


The simplest rainwater harvesting system that can be used is the waterbutt or a small storage tank.

The water that is collected from different sources such as the roofs and other impermeable surfaces and the natural rainwater is filtered and stored in a waterbutt.

This water in the tank can be used for a variety of purposes like car washing, gardening, etc.

The volume of this tank could be anywhere between 50L and 1000L.

A water tank with a higher capacity is suitable for stables, farms, etc.

A water tank with a capacity of less than 1000 L is suitable for domestic property in a high rainfall area that needs water in less quantity.

Normally, a good quality waterbutt will cost around £150.

A rain saucer can be paired along with the waterbutt for the efficient collection of rainwater.

2. Direct pumped – Submersible​

This submersible pump is located inside the underground tank and is the popularly used rainwater harvesting system.

As an easy-to-install rainwater harvesting system, this pump is mostly used for domestic properties and commercial installations of small size.

This pump can help pump directly to washing machines and toilet cisterns.

In case the tank is on the verge of running dry, mains water is supplied to the tank in small amounts to maintain supply.

In the case of commercial projects, dual pump arrangements are used.

The tank size ranges from 1000 L to 22,000 L depending on the end-use and property size.

3. Direct pumped – Suction

In this rainwater harvesting system, the pump is located inside the control unit in the property.

As the pump is not situated inside the tank, cleaning the filter and the pump becomes easier.

The backup from the mains is also dealt with by the suction pipe and so mains water need not be directed to the underground tank.

4. Indirect gravity

In the indirect gravity type rainwater harvesting system, the header tank is pumped with the harvested water. Then, they are used to feed the outlets using gravity.

In this system, the pump works only when the header tank has to be filled.

The mains water is not supplied to the harvesting tank, instead, it is supplied to the header tank.

5. Indirect pumped

The indirect pumped rainwater harvesting system is similar to indirect gravity type rainwater harvesting systems.

It does not depend on gravity to feed the outlets. The pressurized water is provided with the help of a booster pump.

One difference between the indirect pump and indirect gravity type rainwater harvesting systems is that in the indirect pump type, the internal tank can be located in the building at any level.

The booster pumps can be used to meet the pressure and flow requirements of the building.

6. Gravity only​

This is a rarely used rainwater harvesting system as this system does not use pumps and operates purely based on gravity.

The tank must be placed at a higher level compared to the outlets but at a lower level compared to the gutters.

Since this type of rainwater harvesting system depends on gravity to supply the outlets, this system is a very energy-efficient option.

7. In-ground storage​

In areas where rainfall is widely present in one season, underground tanks are commonly found.

Another plus point is that water inside these tanks when present below the frost line, does not freeze.

To feed the water to the outlets, an electric pump is connected to these tanks.

The evaporation rate is less as these tanks are insulated.

8. Retention ponds

Surface runoff water is collected by the retention ponds.

By using processes like sedimentation, decomposition, soil filtration, and solar disinfection, the water quality is improved.
The end-use of water collected by this method is livestock watering, irrigation, groundwater recharge, etc.

The bottom of this pond is covered with mud or sometimes it may be lined with concrete.

Choose the right Rainwater Harvesting System

Finding the right rainwater harvesting installer for your property might be tricky.

There are three main details to consider.

1. Find out if your site is suitable

2. Decide on the type and size of the rainwater harvesting system you are going to install

3. The budget

Let’s get right into the details.

1. Find out if your site is suitable​

You may have to analyse the spacing for the underground tank, a measure of rainfall in the area, capacity for a raised tank, requirements for installing the pipe system, roof space type, and area for rainwater collection.

If you’re planning to install an underground water tank in the garden, you will have to concentrate on the area and space beforehand for installation and excavation.

For commercial premises, rainwater harvesting from the roof should be the better option.

The factors for installing a pipe system for rainwater harvesting should also be considered.

When it comes to rooftops, mostly flat roofs have more efficiency when compared to sloped ones in that they have a larger area from where water can be collected potentially.

2. Decide on the type and size of the rainwater harvesting system you are going to install

We have already seen the different types of rainwater harvesting systems above. Now, let us see how to calculate the size of a rainwater harvesting system that you need.

Before deciding the size of the tank, it is important to consider the following factors.

  • The area you live in – To calculate the approximate amount of rainfall in your area
  • Number of people living in the property
  • Roof size – To roughly determine the amount of rainfall that could be potentially collected
  • Amount of water needed for the garden
  • Number of bedrooms on your property
  • Amount of drought protection needed

For domestic use, the following details can help if you are looking for a simple guide.

You may require a 1500 litre tank for 1 to 2 occupants if you are aiming for limited or nil garden irrigation.

You may require a 3000 litre tank for 2 to 4 occupants if you are aiming for limited or nil garden irrigation.

You may require a 5000 litre tank for 4 to 6 occupants if you are aiming for occasional garden irrigation.

You may require a 7500 litre tank for more than 4 occupants if you are aiming for regular garden irrigation.

You may require a 15000 litre tank for more than 4 occupants if you are aiming for extensive garden irrigation.

If the harvested rainwater is going to be used for garden maintenance, you can consider the following based on a water butt of 100 litres supplying a water can of 1 litre.

100 days of supply is provided when 1 water can is used per day.

50 days of supply is provided when 2 water cans are used per day.

33 days of supply is provided when 3 water cans are used per day.

25 days of supply is provided when 4 water cans are used per day.

3. The budget

The most important point to consider when it comes to choosing the right rainwater harvesting system for your property is the budget.

For an average-sized home, the equipment cost for installing the rainwater harvesting system can range between £2-3,000.

The waterbutt which is the simplest rainwater harvesting system can cost as outlined below.

  • For a 200 litre container, the cost can vary between £70-250.
  • For a 1000 litre container, the cost can be around £450.

These are generally used for gardening and no pipe systems are connected to them.

The one with the pumps can cost around £750 for a 700 litre capacity system including filters.

A fully operating rainwater harvesting system can cost anywhere between £2,000 and £3,000, according to the UK Rainwater Harvesting Association(excluding the cost of installation). The amount of piping and the roof conversion for the collection of water can also play a significant role in the overall cost.

A commercial rainwater harvesting system of the same size as the domestic one will usually cost around the same(physical installation included). However, large rainwater harvesting systems for commercial establishments can provide great benefits mostly in a couple of years.

The water meter is usually installed for free when your service provider is contacted.

4. Installers​

When looking for a rainwater harvesting system installer for your property, consider the following factors.
(i) The number of years they have been in business
(ii) Reviews and testimonials
(iii) Liability insurance covers
(iv) Attachment to trade unions

To find out the list of potential installers and manufacturers of rainwater harvesting systems, visit this link –


Let me address some of the frequently asked  questions related to rainwater and rainwater harvesting!

Where can I use the recycled rainwater?

Recycled rainwater has several uses as listed below.
Washing clothes using a washing machine
In the garden, for watering plants
Toilet flushing
Washing cars
Cleaning building exteriors
Note: It is important to note that the water collection process used in a rainwater harvesting system for domestic and commercial establishments in the UK is not fit for food preparation, drinking, or bathing.

What are the benefits of using a rainwater harvesting system?

Using a rainwater harvesting system reduces the use of mains water. So only less amount of water is required to be drawn artificially.
It also reduces the load on the drainage systems which in turn reduces the effect of flooding wherein run-off water is channeled into the tanks. This is particular good system in areas where the only mains available are combined sewer systems.
It also reduces the water bills and loads on water systems which otherwise would have feed the house. 

What are rainwater harvesting tanks made of?

Most rainwater harvesting tanks available in the market are made using polyethylene plastic.
Larger ones are made using polyethylene which is in turn surrounded by something similar to a steel case.

What are rainwater harvesting kits?

From small tanks to large ones, a range of rainwater harvesting kits is available from the installers and manufacturers. 
These systems can be installed by oneself if one possesses sufficient knowledge on the subject. 
The main elements to consider while using rainwater harvesting kits are if the size is suitable, the tank size, and if you require below-ground or above-ground harvesting kits.

What is the legislation associated with rainwater harvesting systems?

There is not much legislation that relates to rainwater harvesting.
The installation of water harvesting systems are seen as positives at planning stage.
One significant condition concerning rainwater collection is that the collected rainwater is to be labeled as ‘non-potable to ensure it is not drunk. 

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Argemiro Rivera, <b>BEng(Hons) MSC CEng MICE MCIWEM C.WEM CEnv</b>
Argemiro Rivera, BEng(Hons) MSC CEng MICE MCIWEM C.WEM CEnv

Passion for Water, Flood Risk & Sustainability
I love to dig into complex flood risk and water engineering projects and deliver them. I focus on delivering reliable flood risk information to protect livelihoods and the environment.

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