ExordiaGlobal Ltd

Sizing is based on the number of people the property serves (population equivalent), not the number of bedrooms. As a rule of thumb, a 6-person treatment plant suits most family homes, but you should always check the manufacturer's flow-rate tables and consult your installer if you have holiday lets, annexes or high occupancy. Undersizing leads to poor treatment and more frequent servicing.

If you're discharging to a soakaway or watercourse, you'll usually need to register the discharge with the Environment Agency (or NRW in Wales), and building regulations approval is required regardless of discharge point. New installations discharging to surface water must generally use a sewage treatment plant rather than a septic tank under the General Binding Rules. Your installer or building control officer can confirm what applies to your specific site.

GRP (glass-reinforced plastic/fibreglass) tanks are rigid, very strong and well suited to larger capacities or where ground conditions demand extra structural integrity, such as high water tables. Polyethylene (plastic) tanks are lighter, more cost-effective and easier to manoeuvre on site, making them popular for smaller domestic installations. Both are durable and corrosion-resistant; the right choice usually comes down to site access, ground conditions and tank size.

Only if the tank is specifically rated for traffic loading — standard domestic tanks are designed for pedestrian or light garden areas only. Trafficked-rated tanks typically require a reinforced concrete surround or slab to spread the load, and installation instructions must be followed precisely. Always confirm the load rating with the manufacturer before siting a tank anywhere vehicles will pass over it.

Most underground tanks need a excavated, level base — often with a pea-shingle or concrete bed depending on the manufacturer's specification and local ground conditions — ready before the tank arrives. You should also check access for delivery vehicles and any crane or offloading equipment needed, as GRP and larger polyethylene tanks can be too heavy to move by hand. Backfilling generally needs to happen promptly and evenly to avoid the tank flexing or floating in high water tables.

Rainwater harvesting systems need periodic checks of filters, pumps and float switches, plus an occasional tank inspection to check for sediment build-up at the base. Most residential systems only need a full clean-out every few years, but it's worth checking inlet filters more regularly, especially after heavy leaf fall in autumn. A well-maintained system will keep water quality suitable for its intended non-potable uses, such as toilet flushing or garden irrigation.