Southside Shopping Centre Rainwater Harvesting

In the run up to this years Green Apple Awards at the Houses of Parliament on the 12th November, we will be reviewing some of the amazing award winning projects from last year.

Southside Shopping Centre Rainwater Harvesting, Wandsworth


Aware of our drain on such a precious utility and inspired by the rainwater cascading off of our multi-levelled roof, our aim was to try to reduce our impact on the environment, take advantage of this natural resource and to reuse it in areas where our demands for mains water were greatest. For us, this is our public toilets with 8.5m potential users a year.

We also wanted to further reduce our effect on our community by not having to treat the harvested water by chemical processes. With no landscaping on-site this limited its use to the flushing of toilets and urinals only.

However this is still an area where high volumes of water are used considering that the flush of a toilet consumes substantially more water than the average hand wash. It was also important to us to be able to reuse as much of the existing water delivery system as possible.

Additional objectives were to reduce our reliance on the mains water system, help with redirecting storm water from an already overloaded local system and in general to promote the use of a sustainable energy source within our organisation. We hopefully also wanted to inspire others.


We designed and introduced a system that is proving to be successful in conserving the valuable commodity, water and providing Southside with a sustainable energy source.

We knew from the very beginning that the realisation of the rainwater harvesting system in reducing our reliance on mains water was very much going to be dependent on the weather. Bearing this mind and having experienced record dry periods over the last 12 months, we are still on target to reduce our mains water consumption by 14% in 2011.

We were successful in modifying the system with minimal disruption to existing services. We ensured that the public toilets remained open during the installation process. We promoted sustainability within our community through marketing the rainwater harvesting initiative via our website, Facebook and consumer awareness posters, and by using a Did You Know? prompt in our periodic consumer research surveys.

We inspired and encouraged other centres and companies to consider introducing such environmental initiatives through presenting our project at internal regional and national conferences. Furthermore, we encouraged industry colleagues to visit the site and see for themselves the relative simplicity of the installation and the environmental benefits that come with it.

We reused of much of the previous water delivery system with only minor pipework modifications and additional plant being required.


Inspired by watching the vast volumes of water cascading off of our various roof levels during rainy periods, in 2009 we started researching the ability of being able to harvest this natural commodity.

Initially this was not about economics but the ability for us to be able to lessen our impact on our local community and the environment.

Southside Shopping Centre benefits architecturally from having large roof areas at varying levels. There also is an existing 32,000 litre tank on the lowest section of roof, which via the mains water system already supplied the portable and flushing water for our public toilets and security staff facilities.

The challenge was to collect the rainwater via the roof-and to divert it to the existing lower level tank. Then we had to get the water to the flushing systems of the public and staff facilities and re-divert the mains for portable use only.

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With the assistance of our M&E consultants, Associated Building Maintenance, a feasibility study was successfully completed and the following design guidelines were established, considering the water would be used for flushing toilets and urinals only:

● Retain existing roof water tank

● Reconfigure all toilets and urinal water supplies to accept the rainwater feed

● Installation of a filtration system suitable for the water’s final use

● Segregate the portable water systems (hand basins)

● Reconfiguration of mains water supply to feed the hand basin system only

● Plant room equipment revision to disconnect the existing pressurised pumping system

● Introduce an automatic water make-up system for the storage tank in times of drought

● Modification of existing rain water pipework from the cinema roof to the storage facility

● Installation of a lower wattage submersible pump system within the storage tank.

The existing tank would be retained and the roof chosen as the harvesting field was the cover above our restaurant and cinema complex, spanning approximately 6,500 square metres. Additionally the drainage pipework serving this area was closest to the tank and was the easiest run to redirect.

We completed basic calculations to establish the volumes of rainwater potentially available based on the average rainfall in London of 600mm per annum. We estimated that the total rain fall available was circa 3,800 cubic metres (rainfall x area = volume) which equalled to 119 full tanks a year. Southside annual mains water consumption was at the time approximately 7,500 cubic metres, so a possible saving of 50% could be achieved. However considering that reducing water consumption costs was not our main motivator, we saw anything harvested as being a positive result.

As the existing storage tank had previously been split in two to reduce its capacity the first element of the works was to reclaim the space and increase the size of the unit back to 32,000 litres. This was a simple enough task.

Next was to install the new pipework from the tank to the rainwater run along with the first stage external self- cleaning filter to remove any solids brought down from the roof. The means of taking the water from the tank to the toilets followed, whereby a submersible pump system was brought in to replace the less energy efficient pressurised units within the plant room.

Second stage filtration occurred through a floating mesh intake within the tank and a third process through the submersible pump itself. All filters are reusable after periodic cleaning. However from the regular inspections conducted of the tank, the first filter is doing excellent work of removing the debris before water enters the container.

Towards the latter stages of the project the mains waterfeed was disconnected and redirected to serve only the hot and cold hand basin and staff kitchen taps. Existing heat trace systems were reused for all external pipework.

For drought periods when the tank runs out of water and to ensure the toilet facilities can continue to operate, a make-up facility was introduced. When the water level falls below a certain depth, a sensor activates a solenoid valve which will top up the tank using the mains supply.

However this is restricted to no more than 5% of the tank;s capacity at any one time. Current water regulations do not allow any direct or indirect contact between mains and any other type of water, so an ancillary make up system was introduced.

All in all, it was a very challenging and innovative concept.


The overall cost of the project was £22,500, which was partly funded by the scheme’s service charge budget and the remainder directly by the property owners.


The system went live in January 2010 and we did experience some initial teething problems, which we quickly resolved. As a result, we achieved a 4.6% increase in the scheme’s overall footfall in the last year and that put us on target in 2011 to reduce our mains water consumption by 14%.

Based on water charges of £14,935 in 2010 ,the financial saving we are expecting to achieve in 2011 is £2,100. We are currently looking at a payback period of 11 years but

we expect this to be reduced considering the system has gone live during one of the driest periods on record. Of course, this gives us even more reason why we should be conserving water at source!

The people who have benefited from the installation include the local community through our conservation of this valuable commodity and the redirection of rainwater from the already overloaded storm drainage system.

The property owner has benefited from being able to provide a sustainable energy source for its customers and retailers and to prove their commitment to reducing their impact on the environment by introducing such initiatives. Also the staff at Southside are extremely proud to be associated with this innovative project.

We are currently investigating the possibility of linking in further drainage pipework from our other roof areas and widening our rainwater harvesting field. Furthermore we are looking to introduce a stand alone much smaller version as part of the management office refurbishment due to be complete in August 2011. It will be scaled down but based on the same principles of design.


Well, you certainly cannot rely on the British weather, good or bad. But if you put your mind to it and persevere, then anything is possible!