Off-mains drainage consultants and suppliers
Your trusted partner across all domestic, commercial and industrial drainage solutions.
The Off Mains Tank Store
Shop our range of tanks delivered directly to site.
Drainage Consultancy
Regulatory compliance services, expert advice and practical support.
Installation, Service & Repair
Design, install and maintain new or existing drainage systems.
Featured Products from the Off Mains Tank Store.
The Garantia Barrica Rainwater Barrel is a decorative water butt shaped like a traditional wooden barrel — a practical rainwater storage solution that earns its place in the garden rather than being tucked out of sight. Manufactured by Graf, sold under their Garantia range, it connects to your downpipe and collects naturally soft rainwater for garden irrigation. Built from 100% recycled plastic, with a removable, child-proof lid and two 3/4" threaded connections for tap fitting and full water extraction.
Key features
Rustic barrel design – wood-effect finish suits traditional and naturalistic garden settings
Removable child-proof lid – easy access for watering cans and buckets; simplifies cleaning
Two 3/4" threaded connections – one for the tap, one for full drainage using a universal hose set
Brass tap included – no drilling required; connects via the threaded fitting
100% recycled plastic construction – UV-stable and weather-resistant
Available in 260L and 420L capacities
Connecting to your downpipe
A downpipe diverter is required separately — add one to your order below. If you're connecting to a square downpipe, you'll need the Square Downpipe Diverter specifically.
Per Graf's installation guidance: "The water butts do not come predrilled. This product is very easy to use and can be drilled in by anyone. Simply attach the drill bit onto your drill and begin drilling – remember you must drill the hole in the water butt at the same level as the hole in the downpipe! As long as the holes drilled in both the water butt and the downpipe are level, the water will continue down the downpipe when the water butt is full."
The Garantia Extra Slim Stone-Effect Wall Tank delivers 400 litres of rainwater storage in one of the slimmest profiles available — just 280mm deep. Manufactured by Graf, sold under their Garantia range, it's the practical choice for gardens, patios, or balconies where space is tight but storage capacity still matters. Built from 100% recycled plastic, it's UV-stable and weather-resistant throughout.
At 1840mm tall and only 280mm deep, this tank fits flush against a wall without eating into your outdoor space — and the stone-effect finish means it doesn't need hiding away.
Key features
Ultra-slim profile – 280mm deep, fits against virtually any garden wall
400L capacity – serious storage without the footprint
100% recycled plastic construction – UV-stable and weather-resistant
Stone-effect finish – blends into urban or rural garden settings
Brass-coloured aqua-quick plastic tap included – easy access for watering cans or hoses
Setup kit included – everything needed to connect to your downpipe, except the diverter (see below)
Connecting to your downpipe
A setup kit is included, but a downpipe diverter is required separately — add one to your order below. If you're connecting to a square downpipe, you'll need the Square Downpipe Diverter specifically.
Per Graf's installation guidance: "The water butts do not come predrilled. This product is very easy to use and can be drilled in by anyone. Simply attach the drill bit onto your drill and begin drilling – remember you must drill the hole in the water butt at the same level as the hole in the downpipe! As long as the holes drilled in both the water butt and the downpipe are level, the water will continue down the downpipe when the water butt is full."
The Diamond DMS range of sewage treatment plants offer a simple, reliable and discreet solution for off mains properties housing 1 up to 20 people. Typical uses include domestic homes and small commercial businesses such as farm shops, B&Bs and glamping sites. The standard tank models come supplied with a weather proof kiosk and low air pressure alarm beacon and blower inside.
The patented Diamond design not only gives you high quality process, but also provides you with a discreet, efficient and cost effective solution for the longevity of the plant.
The DMS2 is suitable for properties housing 1 to 6 persons with a tank capacity of 2270 L. See other capacities here.
Key Features
Longer emptying cycles (up to 3 years desludge) - due to the highly efficient aeration process which breaks down organic matter at a reduced sludge growth rate.
No internal electrical or mechanical moving parts
Handles Seasonal Fluctuations - Efficient performance even at just 10% capacity — perfect for sites with variable occupancy or future growth in mind.
Fully tested, accredited and certified - under European Standard BS EN12566-3 2014 approval which allows a CE mark.
Meets consent standards - of 20mg/l BOD; 30mg/l SS; 20mg/l NH4 as a minimum.
Part H2 UK - of building regulations.
Designed using the British Water Code of Practice Flows and Loads - to ensure that the tank is correctly sized.
Additional published test results:* - total ‘P’ mean average removal 52%, mean concentration 3.8mg/l - total ‘N’ mean average removal 52.2%, mean concentration 25mg/l
Odourless plant
Flushest lids to the ground - creating perfect integration in to the landscape.
Compact in design - keeps the excavation to a minimum and installation simple.
Range of invert depths - for complicated sites, allowing more discharge options
Operating Principle
The wastewater flows into the main tank and enters directly into the bio-chamber. The bio-chamber is a zone where the solids circulate continuously; this is achieved by air rather than mechanical or electrical parts inside the tank.
The draft tube enables the solids to be drawn back up into the central aeration chamber and re-circulated through the treatment process.
The treated liquid then exits through the dip pipe and is discharged safely in to the environment, meeting the required consent standard.
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Compliance with regulations
Our services fully align with Environmental Agency legislation, British Water Standards and all relevant building and planning guidelines.
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Service you can trust
A family business since 1995 we’re trusted by our clients to provide expert knowledge, efficient service and value for money.
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Commitment to sustainability
Focused on providing responsible solutions that demonstrate a commitment to protecting the environment.
Our Partners
We’re proud to have longstanding relationships with the UK’s most trusted manufacturers and local companies.
News
Trusted by our customers
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Karen and Mat
“Thanks so much for your positivity and support and for making this seem super easy, it really does make the world of difference.”
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Vanessa
“I cannot thank you enough for your effort, professionalism, and friendliness in helping us get to a resolution over the septic tank. It is not often, that you come across such an excellent service where someone goes above and beyond.”
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Neville
“Thanks for giving me such a good overview of what we need to do to make our tank compliant.”