These large underground water tanks are prefitted with a 3P Flat Filter so that rainwater from a nearby roof can be pre-filtered before it is stored. 

Installing an underground rainwater harvesting tank (with capacities from 6,000L to 20,000L) for garden or plant irrigation in the UK offers several strategic and practical advantages:

• Free water for irrigation purposes
• Drought resilience
• Mains water conservation
• Rainwater is better than mains water for plants as it doesn't contain chlorine or hard minerals
• Space efficiency
• Keeps harvested water cool therefore minimising algae growth

About the Large Underground Water Tank

These large underground water tanks provide reliable and durable rainwater storage. All of the tank sizes are approximately 1.9m wide and 2.4m tall (only the length changes for different capacities). They are designed to meet latest UK and European standards. Their heavy duty shells enable installation in all ground conditions.

Built to Last
Manufactured from high-quality GRP using virgin, unfilled resin (no chalk or fillers), these shallow horiztonal tanks feature a consistent wall thickness for exceptional structural strength and durability. This premium construction also makes the tanks significantly lighter, making on-site handling and installation easier—while enhancing resistance to hydrostatic pressure during use.

Superior Protection
Each tank is coated with a chemically resistant gel-coat that safeguards the internal structure from water and chemical exposure. This ensures long-term performance and reliability.

Unmatched Longevity
When properly installed the underground rainwater tank has a design life of 50 years and a 25-year structural guarantee.

About the 3P Flat Filter - High-Efficiency Rainwater Filtration

Designed for direct installation inside rainwater storage tanks, the 3P Flat Filter provides highly efficient filtration with zero height difference between the inlet and wastewater outlet—meaning no head loss across the filter. Filtered cascades straight into the tank through the stainless steel filter, making it perfect for garden watering, irrigation systems, and other non-potable applications.

Key Benefits:

• No Calmed Inlet Required – water enters the tank gently without disturbing settled debris
• Flat, large-surface filter ensures high filtration efficiency at all flow rates
• Self-cleaning design – leaves and debris are automatically flushed to waste
• Low maintenance – cleaning frequency depends on rainwater quality and solid load

Technical Specifications:

• Connection capacity: Suitable for roof areas up to 800m²
• All connections: DN 150 (160mm OD)
• Height difference: 0mm between inlet and outlet
• Filter mesh: Stainless steel, 0.7 x 1.7 mm

Further information can be found on our Flat irrigation Filter web page.

Optional Pump Kit

Further information about the Pump Kit suitable for these tanks can be found on the Rainwater Tank Underground Pump Kit page. The Kit includes an XAJE80B SteelPump (pre-fitted with a Mac3 Key Recycled Float Switch) and 2m Pump to Turret connector.

Optional Floating Intake

Floating Intake Filters draw water from about 6" (150 mm) below the surface—where it’s cleanest—keeping floating debris out of the pump suction hose. Unlike fixed filters that sink to the tank bottom and clog with sediment, these filters stay in the clear water zone.

By pre-straining water before it reaches the pump, they extend pump life. Fixed strainers often block because particles remain stuck in their mesh, and they pull water from sediment-heavy areas near the bottom.

Floating Intake Filters stay mobile, so water movement helps clear particles from the strainer, ensuring consistent, high-volume flow. Each assembly includes a brass non-return valve (NRV) as standard.

For this optional pump kit you will need a 1" version with a 30cm or 1m length of hose. 

Other recommended products

Pressure Vessel

Also called expansion vessels, pressure accumulators, or surge arrestors.

These vessels extend pump life by storing pressurised water, reducing pump starts during short uses like handwashing or dips and leaks.. For small demands, water comes from the vessel, refilling automatically without engaging the pump. The pump only runs for larger draws, such as filling a bath or showering.

Even the smallest Pressure Vessel will save thousands of unnecessary starts each year and therefore prolong the length of life of your pump.

Mains Water Top-Up

A Mains Top-up Controller like the RainForce Home (TCA7000) automatically tops up your rainwater tank with mains water when levels run low, ensuring uninterrupted supply for toilets and other appliances—even in dry spells.

Delivery

Price includes delivery to UK Mainland addresses. Please contact us for a competitive delivery quote to other locations.

Customer must be able to off-load the tank from the delivery vehicle.