Rethinking rainwater harvesting for SuDS

Mark Manning of SDS discusses rainwater reuse in the context of SuDS regulations, and says that architects could be mistakenly rejecting such approaches as an important option for sustainable drainage

Whenever an architect receives a design commission that includes sustainable drainage, their creative thoughts are likely to turn first to appealing blue/green infrastructure. But could a rainwater harvesting system be justified as part of the Sustainable Drainage System (SuDS)?

Even where there is limited space for above-ground SuDS ponds or swales, the opportunity for reuse is often missed. Why? Because our minds are set to view rainwater as a waste product to be discarded and quickly conveyed away. If someone throws paper, glass or recyclable plastic bottles onto the ground, we are enraged. Yet, all too often, we plan to let a valuable resource slip through our fingers. Excess stormwater has become a nationwide waste problem; it causes flooding and sewage overflow pollution as it flows away into the overburdened drainage network and into our rivers.

SuDS design guidance
Most architects know there is a design hierarchy to follow when selecting and specifying SuDS components. However, many guidance documents produced by Local Authorities and Water Companies either don’t include, or quickly demote, rainwater reuse. Instead, infiltration is frequently top of the list of methods to select from.

In England, the Non-Statutory Technical Standards (NSTs) for designing SuDS on new developments currently make no mention of water saving, nor does the corresponding Planning Practice Guidance. Yet, the UK’s ‘design bible’, CIRIA C753 The SuDS Manual, actually mentions rainwater harvesting 105 times. Using surface water as a resource is listed as the number one design criteria and the document makes it clear that reusing water should be a priority consideration over other drainage methods.

The London Plan follows the Manual’s best-practice approach – a policy which is leading to significantly more commercial and residential water reuse schemes. In Wales, the Statutory National Standards for Sustainable Drainage Systems also set out as a first priority that surface water runoff is collected for reuse.

Why should architects and their clients go the extra mile to design water reuse into sustainable buildings? Scientists have shown that global warming is leading to more extreme weather conditions, with longer dry periods interspersed with intense, heavy rainfall. On the one hand, parts of England are predicted to run out of water in as little as 20 years without concerted water efficiency improvements. On the other, around 5.2 million properties in England, one in six, are still at risk of flooding.

Rainwater reuse and flood control are just two sides of the same water management coin. Harvesting rainwater from roofs for toilet flushing or irrigation not only makes water-saving sense, it

holds back (attenuates) floor water like other forms of drainage. Planned on a national scale, implementing property-level water reuse solutions would significantly reduce the predicted shortfall in demand for mains water, while also providing much-needed flood risk protection.

So why haven’t rainwater reuse systems become more mainstream? The required regulatory and policy frameworks have been missing, especially for residential developments, since the withdrawal of the Code for Sustainable Homes in 2015.

Smart, dual-purpose rainwater systems
The picture is brighter in the commercial sector, where the promise of top BREEAM ratings, together with planning conditions that impose demanding per-person water use limits, have resulted in some outstanding schemes, especially in London. SDS has pioneered the use of digitally-controlled rainwater management for some of them with its Intellistorm water reuse system, for example, at the prestigious Southbank Place development in London.

The system principle removes another historic barrier to rainwater reuse – the consideration of what happens to the excess water when the rainwater collection tank is full. In the past, most conventional designs have required a second tank to avoid flooding and meet discharge consents.

Dual-purpose rainwater management schemes use weather prediction data to control the overall levels of rainwater for both attenuation and reuse in a single storage area. When a storm is predicted, the water level can be lowered automatically to create sufficient capacity to accept rainwater and avoid surface water flooding. Water collected from the roofs of one or more buildings can be stored in intelligent tanks, and the resulting attenuation managed at the building level, or engineered over a defined catchment. SDS is currently conducting a trial to intelligently control the levels of smart water butts collecting rain from the roofs of 30 residential properties so that attenuation and surface water discharge can be managed across a catchment.

So, next time you get a brief for a sustainable building, why not consider a smart rainwater management system?

Mark Manning is national specifications manager at SDS