Rainwater Harvesting Systems: A Full Guide

Along with gas and electricity, water bills are expensive and ever-increasing, and if any money can be saved, it’s always a definite bonus.

With this in mind, in this project we take a look at rain water harvesting systems and where they can be used to reduce your water bills.

Rainwater Harvesting: The Basics

Rainwater harvesting involves collecting rainwater that would typically run off your roof and go down the drain, and using it for practical purposes. Instead of relying on treated mains water, which costs money and energy to produce, you can use harvested rainwater for tasks like flushing toilets, watering gardens, and washing your car.

The easiest way to collect rainwater is from roofs, though driveways and other surfaces can also work. The water is directed to a storage tank, where it’s filtered to remove debris before being stored for later use.

It’s important to note that rainwater is non-potable, meaning it’s not safe for drinking. However, it’s perfect for non-consumable uses like flushing toilets, watering gardens, and even running washing machines.

Rainwater Harvesting Points to Think About

Rainwater harvesting is straightforward, but there are key factors to consider:

• Collection Area: The size of your collection area determines how much water you can gather. If you’re collecting from surfaces other than your roof, ensure your system can filter out contaminants like oil or animal waste.
• Tank Size: Choosing the right tank is crucial, as it’s the most costly part of the system. Consider these factors:
  • Collection area size
  • Annual rainfall in your location
  • Water losses from gutters and filters
  • Your water usage needs, typically around 5% of the potential collection
• Start Small: Before investing in rainwater harvesting, consider easier and cost-effective water-saving options like low-flow taps, aerated showers, rainwater butts, and low-flush toilets.

For basic rainwater harvesting, a 1000-litre IBC tank is an excellent choice. These durable plastic cubes, often with a pallet base for easy transport, are affordable and versatile, making them a practical water storage solution.

What Does Potable Mean?

The term “potable” refers to water that is safe for human consumption. In simple terms, potable water is clean and free from any contaminants that could cause you harm if consumed. This isn’t just for drinking water, this also includes water used for cooking, bathing, and other household uses where contact with the body might occur.

To be considered potable, water must meet a certain set of safety standards outlined by the DWI (Drinking Water Inspectorate) and other authoritative bodies.

These standards ensure that the water is free from harmful bacteria, viruses, chemicals, and other pollutants. Regular testing is conducted to ensure that water sources, whether they come from wells, rivers, or public water systems, remain safe and reliable for everyday use.

If you’re unsure whether your water is potable, it’s important to get it tested by a certified laboratory. This is particularly crucial in rural areas where private wells might be the primary water source. Regular maintenance and testing can prevent potential health risks, ensuring that your water remains safe to drink.

What Type of Water Storage Tank Do I Need for Rainwater Harvesting?

Choosing the right water storage tank for rainwater harvesting is crucial to the efficiency and success of your system. The type of tank you need depends on several factors, including the size of your collection area, the amount of rainfall for your area, and your intended water usage.

• Tank Size: The size of your tank is one of the most important considerations. A larger roof or collection area means you can capture more rainwater, so in turn you’ll need a bigger tank to store it. Also, think about the average rainfall in your area. If you live in a region with heavy rainfall, you may need a larger tank to capture and store as much water as possible. However, if you plan to use the water primarily for gardening or outdoor cleaning, a smaller tank might be better.
• Material: Water storage tanks come in various materials, including plastic, steel, and concrete. Plastic tanks, like the popular 1000-litre IBC tanks, are lightweight, durable, and cheap to buy, making them a great choice for most residential rainwater harvesting systems. Steel tanks are more durable and can be better suited for larger, long-term installations. Concrete tanks are extremely durable and often used for underground storage, but they are more expensive and require professional installation.
• Installation Location: Where you plan to place your tank is also crucial. Above-ground tanks are easier to install and maintain, but they can take up large amounts of space and might also be an eyesore. If aesthetics or space are concerns, consider an underground tank. Underground tanks are hidden from view and can store larger amounts of water, but they are much more costly to install and maintain.
• Budget: Your budget will be a key factor in your choice of tank. While larger and more durable tanks offer better long-term value, they come with higher upfront costs. Smaller, plastic tanks are more affordable and easier to install, making them a good option for those starting small or working with limited funds.

Ultimately, the best tank for your rainwater harvesting needs will balance your storage needs, space, and budget. Whether you opt for a simple plastic tank or a more complex underground system, the right tank will help you make the most of the water you collect.

What Type of Lid Should I Use for Rainwater Harvesting?

Choosing the right lid for your rainwater harvesting tank is essential for ensuring water quality and maintaining the efficiency of your system. The lid you select not only protects your water from contamination but also helps prevent evaporation and reduces the risk of algae growth.

• Secure Fit: This is essential to prevent leaves, debris, insects and any other contaminants from polluting your water.
• Construction Material: These can vary and include metal, plastic and fibreglass. The latter 2 are both cost effective, lightweight and do not rust, whereas metal lids are generally more durable and suited to larger tanks.
• UV Resistant: UV rays from the sun can help to promote algae growth, so it is essential whatever lid you go for is UV resistant and prevents sun light from entering the tank. This is especially the case if your tank is above ground.
• Access: Think about how often you will need to access your storage tank for maintenance and to check water levels as also how much of the tank you will need to access at a time. This will help you to decide what type of tank and lid you will need to go for.

How Rainwater Harvesting Works

1. Collection and Filtration: Rainwater is collected from a roof or hard surface and directed into a storage tank. A filter blocks leaves and debris, while a smoothing inlet minimises sediment disturbance at the bottom of the tank.
2. Pumping Water for Use: The stored water is pumped out through a secondary filter that removes floating particles. This non-potable water is delivered via separate pipes from your mains supply. You can use either a submerged or external suction pump for this process.
3. Automated Control System: A control unit (in more advanced systems) monitors the tank’s water level and system performance. If the water level is too low, it switches to mains water, ensuring continuous supply. An air gap prevents rainwater from contaminating the mains supply.
4. Tank Maintenance: An overflow trap automatically skims off floating debris into the storm drain when the tank overfills, typically twice a year. Rodent barriers and a non-return valve keep pests and contaminants out.
5. Surface Collection Options: Rainwater can also be collected from permeable pavements. In this case, an additional oil trap and filtration system are required to handle potential oil and animal waste contamination.

Note: As we have said, if water is not filtered and cleaned correctly and then verified for human consumption, it is regarded as non-potable and should not be consumed. However, it can be used for activities such as washing cars, cleaning clothes, flushing toilets etc.

If the water you have collected is not potable, then it should never be mixed with mains water!

Installation Costs

Choosing the right tank size is crucial since it’s the most expensive part of a rainwater harvesting system. The tank should overflow at least twice a year to flush out floating debris, balancing cost with storage capacity.

Installation costs for domestic rainwater harvesting systems vary widely. The Environment Agency estimates costs range from £2,500 to £6,000, depending on system size. For an average-sized family home it is estimated that the cost will be somewhere between £2000 and £3000. Installing a system during other building work can significantly reduce expenses, especially when retrofitting plumbing from the tank to areas like toilets.

The most basic, entry level system would effectively be a water butt, which can be purchased for as little as £30, whereas the next level up, a 1000 litre LBC tank can be purchased new for about £150 and second hand for as little as £50!

Payback depends on maintenance and water costs. Maintenance is minimal—typically just cleaning the filter every three months. In hard water areas, rainwater systems can extend the life of appliances like washing machines, as rainwater is soft and reduces lime scale buildup.

With a fully-fledged system, the storage tank keeps rainwater oxygenated to prevent algae growth, with sediment settling at the bottom. While this water isn’t suitable for drinking, the tank can be topped up with mains water during dry spells to avoid drying out.

To maintain water quality, consider these best practices:

• Use an overflow siphon to remove floating debris.
• Install a rodent barrier to keep animals out.
• Opt for floating extraction to draw water from just below the surface, where it’s cleanest, along with an extra filter.
• Regularly clear gutters, as bird and animal droppings or leaf litter can contaminate the system.

Benefits of Rainwater Harvesting

The average person uses about 150 litres of water daily. By using rainwater instead of mains water for toilet flushing, you can save around 39 litres per day, roughly 26% of your water usage. You can achieve even greater savings by using rainwater for laundry and outdoor activities.

Rainwater harvesting systems can reduce your reliance on treated mains water by up to 50% in homes, and up to 80% in commercial buildings.

While grants for home installations aren’t available yet, commercial properties can benefit from Tax Relief Schemes (ECA) for approved equipment listed on the Water Technology List.

Installing a Rainwater Harvesting System

There are three main types of rainwater harvesting systems. Choose the one that suits your needs:

• Pumping water directly from storage tanks to points of use.
• Using gravity to feed water from storage tanks to points of use.
• Pumping water to an elevated cistern, then feeding it by gravity to points of use.

The most critical part of the installation is sizing the tank, as it’s the most expensive component. To determine the right size, use this formula:

Annual rainfall (mm) x collection area (m²) x drainage coefficient (%) x filter efficiency (%) x demand (5%)

Annual rainfall (mm): Rainfall varies across the UK, from 6000mm in the highlands to 500mm in East Anglia and London.

Collection area (m²): This is the area that catches rainwater, typically the size of your roof base.

Drainage coefficient (%): This accounts for water loss due to evaporation or overflow. Typical values are:

• Pitched roof: 90%
• Pitched roof with tiles: 80%
• Flat roof with gravel: 80%

Filter efficiency (%): This indicates how well the filter separates debris from water. If unspecified, assume 90% efficiency.

Demand (5%): This represents the percentage of annual rainwater that your household will actually use, typically 5%.

For UK installations, follow BS 8515 guidelines for design, installation, and maintenance of rainwater harvesting systems, applicable to both new builds and retrofits. Relevant Building Regulations include Part H and Part G.

Ensure external taps connected to untreated rainwater have handles removed and are clearly labeled to prevent drinking. Use distinct coloured pipework for rainwater to avoid cross-connections with potable water. Suppliers offer marking tape and warning labels for this purpose.

Although maybe quite expensive to get setup initially, over time you will inevitably save yourself a fair bit using a rainwater harvesting system, so if you are looking to reduce your bills, especially water usage, then a rainwater harvesting system is well worth considering.