SDS Limited                            WQR0067


Written evidence submitted by SDS Limited

About SDS Limited

SDS Limited is the leading UK provider of sustainable drainage products to prevent flooding and pollution.

As well as manufacturing and installing water attenuation and filtration systems, the company is investing in the development of innovative autonomous dual benefit rainwater and greywater re-use systems (RWH and GWH).

These systems can be installed at any scale and used to hold back rainwater during peak flows to protect combined sewage networks and prevent emergency overflows into watercourses and bathing waters. Rainwater re-use meanwhile provides additional tank capacity and helps conserve potable supplies by replacing tap water with rainwater or grey water for non-potable tasks such as toilet flushing, vehicle washing, irrigation or other industrial purposes.

The existing barriers to RWH and GWH are well explored in this report commissioned for WaterWise.

RWH systems should be considered a valuable component of a SuDS system. They catch rainfall at source; they are useful even for small quantities of rainfall and they offer multiple benefits including water conservation.

How can consumers be persuaded to change their behaviour to minimise pollution?

Overflowing sewers is a legacy of our combined sewerage network which takes both water and sewage away from homes and business in a single pipe. Replacing the combined network with a fully separated system would cost billions, cause enormous upheaval, and may still not provide all the capacity we will need under future climate change scenarios.

Everyone who sleeps under a roof contributes some surface water to the combined sewerage network. However, for most people, overflows of untreated stormwater from combined sewers is an invisible problem.

It is, in fact, a societal problem that we should all take some responsibility for – consumers, business and government. It is not simply the water company’s problem.

There are well documented actions that individuals can take to reduce their impact. Avoiding sewer blockages is one. We could also do more to attenuate rainwater on our own properties but instead, front gardens are disappearing at an alarming rate - more than 4.5 million of them contain no plants at all, and a quarter of front gardens are now totally paved over. Cumulatively this has a major impact on combined sewer capacity. Urban creep typically occurs in the older built environment, which is more likely to be connected to combined sewers, and therefore causes more issues than newer developments which must do more to attenuate surface water on site and only discharge to separate surface water sewers.

The best action for individual consumers to minimise pollution, besides preventing blockages, is for them to reduce the amount of surface water they contribute to the sewerage network. With less surface water to pump around the sewerage network, water companies not only have more capacity but they also reduce the cost and carbon cost of the service.

This could be through the mass adoption of rainwater harvesting (RWH) and greywater harvesting (GWH) systems.

Latest developments in rainwater harvesting

Rainwater harvesting is often overlooked in SuDS design because dumb systems in particular have limited benefits as attenuation systems. A ‘dumb’ water butt is generally only useful once in a winter unless and until it is emptied again – often not until the spring. Dumb water butts are a good way to remind people to be waterwise but are of limited use as a storm attenuation asset.

More useful are intentionally leaky water butts which gradually release a percentage of the water back into the sewerage network whenever they are full. They are however prone to blockages as the leaky valve by necessity is a very small diameter pipe.

A water tank that is plumbed in to flush toilets with rainwater is constantly creating capacity for more rainwater. In one pilot area in Exmouth in Devon, rainwater tanks plumbed in to flush toilets and run a washing machine replaced on average 142 litres of tap water per dwelling, per day, enabled the re-use of 50,000 litres of rainwater and held back 66m3 of water per property per year from the combined sewer.

Smart water butts or larger tanks (smart RWH systems) autonomously create capacity based on an algorithm and incoming data feeds such as the precipitation forecast. Releases of water can be made back into the combined sewer (or to a SuDS feature or a watercourse) when it is dry, or at staggered times, or in response to a sensor alert. These systems can be fitted at any scale to almost any building.

A smart RWH system is by its nature more expensive than a dumb RWH system. However, the costs are coming down and they offer additional benefits at least commensurate with the increased capital cost. 1,000 smart RWH systems installed in a flooding hotspot controlled to optimise storage and flatten the peak of a storm could rival a large stormwater tank in terms of storage and costs.

But they are not directly comparable to stormwater storage because they also provide other benefits which stormwater tanks do not, such as rainwater for re-use; performance data for the installer; and, potentially, information for building users as a water conservation behaviour change tool. In addition, in many urban locations new end-of-pipe stormwater tanks cannot be constructed due to lack of space, topographical constraints or ground conditions.

When incorporated into a well designed rainwater management plan for a new development, a district system with a central tank could supply houses, attenuate stormwater and, when it reaches capacity, overflow to raingardens or other SuDS features to help encourage biodiversity. 

Partnership models that share the costs and benefits of smart RWH between stakeholders

SDS Ltd has successfully worked with water companies and their customers to share the costs and benefits of installing smart RWH systems. The water company pays the capital cost and the homeowner takes forward the liability and maintenance. The water company gains the stormwater storage space while the homeowner gets the use of the water and savings on their water bill. The smart RWH then autonomously optimises its level to ensure attenuation benefits for the water company whilst ensuring the homeowner also has a supply of rainwater.

This model has been relatively popular, with around 30-40% of homeowners keen to take up the opportunity and appreciative of the benefits to the environment of saving water. Larger systems (800L+) that replace potable water with rainwater for toilet flushing also save the homeowner money – in one project in the South West Water region, that amounted to £200 per customer per year on average. For the financial incentive to work it is of course imperative that the homeowner is on a metered supply and will see the commensurate savings in their water bill. This is another good reason for universal water metering. We envisage that with further improvements to the product and a wider roll-out, participation could be increased.

We are exploring other models, for example, partnerships with developers to help them control site run-off rates, and sustainable developments where houses will attract a premium for ongoing utility bill savings and environmental credentials. We would also like to trial the use of smart RWH as a farm improvement within Defra’s ELMS tests and trials.

Incentivise smart RWH with discounts on water bills.

Smart RWHs have the potential to hold back significant quantities of water from the combined sewer network. By optimising performance based on weather forecast data, they can intervene at critical points in the flow of water through pipes. At present the main domestic incentive is through savings on water bills, which for garden-use only smart RWHs can be minimal. However, if the surface water element of the water bill was reduced, even partially, to reflect the installation of a smart RWH, it is likely this would help increase take-up and give homeowners another reason to get on board. The actual amount saved in many cases is less important than the principle.

Incentivise developers

The London Plan has successfully incentivised rainwater and grey water re-use by prioritising water in its planning policies. Planning permission is only granted if these technologies have been properly considered.

Since its launch about 20% of large new developments in London have invested in GWH and RWH systems, often utilising the Intellistorm system from SDS, a package dual RWH GWH plant that can be incorporated into building designs. The Intellistorm system captures, filters, treats, re-uses and balances the system for dual benefits and is ideal for hotels and leisure centres which produce a lot of grey water from showers, baths and wash-hand basins. Systems like this can offer substantial savings on water bills for the building’s operators.

Smart district RWH systems could also be much more widely incentivised in new developments. This would make better use of excess water than simply attenuating it. However, more work needs to be done on the delivery model which ensures ongoing maintenance and benefit to the homeowner.



SDs also manufacture a range of water filtration systems that are designed to meet surface water runoff quality control objectives.

Used either on their own or together in a variety of configurations, these products encompass a range of proven treatment mechanisms, in the process removing soluble pollutants and 95% of suspended solids and increasing the lifespan of any vegetative SuDS systems.

By using the SDS Aqua-Swirl™ swirl chamber for pre-treatment, followed by a filtration chamber for secondary treatment, SDS Aqua-Filter™ maximises the removal of suspended solids by up to 80%.

The SDS Aqua-Xchange is an engineered pollution control and enhanced filter media that uses ionic exchange and filtration to remove soluble and solid pollutants from surface water runoff.

These products used in conjunction with a well designed SuDS system can remove most harmful elements of surface water run-off from ever reaching rivers or the sea.


Smart RWH and GWH are not the whole solution to CSO pollution. However, they should be considered as an important piece in the jigsaw puzzle. Smart RWH offers multiple benefits, but uptake will require incentivising through regulation. Universal metering, adjusting planning policies and enabling water companies to vary surface water charges are key levers available to the Government.

Encouraging and incentivising smart RWH and GWH would help to increase capacity in the sewerage network without major costly infrastructure improvements. It would also enhance public engagement with a problem that can otherwise be ‘out of sight and out of mind’.

Lorna Devenish

Market Development Manager



February 2021