Bringing in the rain

Yorkshire Water went through a pretty torrid time a couple of years ago. The company was accused of "mismanagement of resources" by Greenpeace; with media reports stating 25% of water was being lost through pipe leaks. As roads filled with tankers ferrying resources from Keilder Water to Yorkshire, attention turned to water conservation policies.

The Inland Revenue is one major water user that took the initiative to reduce its reliance on potable water. The organisation decided to install a rainwater collection system for its office in Shipley, Yorkshire, as an environmentally sound way to manage toilet flushing.

The building is one of the Inland Revenue's two main centres. It lies immediately adjacent to the River Aire, with the Leeds to Liverpool canal flanking the opposite side of the site. The building was constructed specifically for the government department in the mid-1970s and consists of two main buildings – a four-storey office block and a two-storey computer production block – joined by a low level building which houses the facilities management team and some offices. The entire structure is flat-roofed with an area of 11 000 m².

During 1998, while acting as a consultant for the Inland Revenue, building services engineer IBSEC identified the building as a potential site for a rainwater collection scheme. "The department as a whole," states IBSEC's associate energy manager, John Blears, "takes a very progressive view of environmental issues, and is presently trying for ISO 14 001 accreditation for the site." This was a consideration in deciding to accept the scheme.

IBSEC carried out a feasibility study into the environmental and economic consequences of the proposal in October 1998. The company won the contract by tender in February 1999, beating three other invitees.

The system

Installation of the system was relatively simple and took less than six weeks to complete, everything being in place by July 1999. IBSEC carried out the full design work, with local firm Jeffery Robinson in charge of installing the scheme.

The entire system is sited within a previously empty area of the two-storey building's basement. Two 30 000 litre glass-reinforced plastics tanks lie around 20 m apart, gathering the rainwater for use in the building's 95 toilets.

The structure of the building and layout of its original services have been fully utilised for the scheme and this contributed to the ease of installation. It also lowered the capital costs.

Use of the collected rainwater from the car park and external landscaping was rejected at the feasibility study stage due to the possibilities of pollutants, and concentration was placed on the roof collection.

Rain landing on the roof seeps through a 50 mm layer of pebbles to a weatherproof protection foam sublayer. The water runs naturally over this slightly sloping surface to roof gullies distributed around the roof. Mesh filters at the entrance of these gullies remove any matter such as leaves, and the water flows via downpipes into the basement of the building. The system is simple but effective. The water falls by gravity, with the vertical drainpipes connecting into a single 250 mm cast-iron horizontal pipe in the basement. This runs through the length of the building, gathering the water for discharge directly into the River Aire.

With this already in place, it was a simple matter to tap into the main discharge pipe at the required places. No alterations were needed to the original drainage pipework and changes to the building were also avoided. This gave the additional advantage of the installation being possible without disruption of work in the offices – a critical factor to the Inland Revenue.

The tanks were supplied in 1 m² sections which were positioned on a base, bolted together and sealed on-site. Flow and return connections are made from these into the 250 mm pipe, with the flow inlet positioned 500 mm above the tank bottom and the return outlet 500 mm higher and around 200 mm from the top of the tank. These positions were carefully chosen to minimise any debris from entering the supply water to the toilets: a filter at the entrance to the tank removes any large pieces of dirt which have passed through the roof level filters while any small pieces entering the tank are allowed to settle on the bottom. Lighter fragments such as leaves float to the top to be removed via the overflow.

Plastics pipes have been used for all new pipework due to the acidic nature of the rainwater. IBSEC looked at using copper pipes but was concerned about reports that the material may corrode under pressure. Also, the pvc pipes were a relatively cheap option.

Generally, both tanks fill simultaneously, unless instructed otherwise by the Satchwell building energy management system. However, tank one naturally fills quicker as it taps into the drainage pipe at an earlier point. The period from rain falling on the roof to entering the tank takes around one to two days.

With the inconsistency of supply that comes from using rainwater, it was necessary to maintain the original mains supply from two cold water storage tanks at roof level. In the larger building, the bulk of the toilets are in a central core. The design of these also assisted the engineers: a walk-in service duct dividing the male and female toilets holds the toilet cisterns and pipework, therefore modifications could again be carried out with minimum disruption.

The rainwater supply system is pressurised to 3 bar. A small pressure vessel, 1750 W duty and standby pumps and a water meter are the only plant required in addition to the tanks, valves and controls. The pumps operate so as to maintain the required pressure, depending on how much water is being used in the system.

Motorised butterfly valves on the flow and return pipes control the water flow around the system, switching between tank and mains water depending on the rainwater levels. The motorised valves were selected also as a precaution against backflow from the River Aire into the system, the basement lying below river level at high tide.

System maintenance

Little maintenance is required for the system. As the rainwater is being used simply for flushing toilets (determined by the quality of the water and lack of suitable process requirement on the site), water regulations simply require that it must be completely separate from the mains supply so there is no risk of cross-contamination between the sources.

The mesh filters used are sufficient for cleaning purposes, with no chemical treatment required due to the final purpose of the water. These filters are cleaned regularly.

Temperature sensors in the tanks are used to monitor for Legionella. An annual chlorination of the tank is also carried out, and monthly water samples are analysed by an on-site treatment specialist.

Savings so far

In theory, having analysed the rainfall patterns for the region, IBSEC estimates that a potential of 8 million litres/y of rainwater falls on the roof. However, in reality the company suggests the practical figure is around 4 million litres.

At the time of visit the reading on the water meter stood at 691 m³. This relatively low use has been due to a lengthy problem with one of the valves in the system, which was passing water. Initial reports from the manufacturer indicated that the actuator was faulty and this was replaced several times before it was determined it was actually a fault in the butterfly valve which was burning out the actuator.

The fact that this problem was rectified in July 2000 meant that the fruitful 1999 winter season was missed and as IBSEC's John Blears says, "It's early days to say whether targets are being met."