Using Rainwater Harvesting For Multi-Dwellings

By Marcus Bicknell

Rainwater can be used indoors and outside if drinking quality mains water is not required. Almost a third of our daily water is literally flushed down the toilet so, by using rainwater for the washing machine and in the garden (for irrigation, cleaning and water features etc) about half of the average mains water consumption can be saved. For the individual household, this concept is easy: collect rainwater from the roof, filter it, store it, preferably in an underground tank, and when necessary then pump it to where it is needed. In multiple dwellings there are various configurations possible within this basic concept.

Rainwater Management Systems

The most straightforward solution is to have one system per dwelling unit and this is obviously possible in property developments with detached homes. However to gain from economies of scale, it is often cheaper to have large capacity storage to serve several dwellings. As the whole point of rainwater harvesting is that rain is "free", it is not feasible to try and control the amount of rainwater used from a shared tank by the dwellings supplied by it. It is assumed that each dwelling's use of rainwater will be much the same. In a water shortage, residents must water their gardens sparingly.

With a shared tank, the issues of how to pay for the electricity to power the pump, and how to pay for the mains water (should the rainwater run out) have to be dealt with. The developer needs to anticipate that the rainwater harvesting system, or parts of it, should be owned by the landlord company and render a long term service under contract to the owner/occupant. So, if a pump is shared, it would have to be connected to a landlord's electricity supply (like street lights and other shared services) and billed back to the occupants in the normal way.

In whatever scenario, rainwater is collected off the roof and stored in one or several linked underground storage tanks. Underground, the water stays cool and fresh (no bacterial action under 12 degrees).

Thereafter, there are two principal methods of distributing rainwater to where it is needed, and there are some variances within each principal method. These two methods are direct and indirect feed. Either direct or indirect, each rainwater system has to incorporate a mains back facility in the event that the storage tank runs out. The choice can effect how electricity and mains water costs are evaluated.

Direct Feed with Submersible Pump

The submersible pressure-sensitive pump sits on the bottom of the underground rainwater storage tank. The different appliances are directly plumbed to the output on the pump. When water is drawn, for example, a toilet is flushed, the pump starts. When no more water is being drawn, the pressure builds up in the network of pipes and the pump stops pumping.

In this layout, the simplest and cheapest way of providing mains water when rainwater runs out is to admit mains water to the underground tank. When the float switch in the tank shows the tank is empty, a mains-electric solenoid tap opens to admit mains water through a tundish (WRAS requirement for air gap) into the underground tank. When the water level has risen a few inches, the float switch rises and turns the solenoid tap off. The bulk of the tank is left empty to accept the next rain shower.

This adapts well to shared systems, i.e. taking rainwater from a communal underground tank to each property. Several homes in a small area (2 to 8 homes typically, semi-detached or within 2-10 metres of each other) drain their roofs to one underground tank. One pump can serve all the homes connected; at 1000 watts and 4 to 5 bar of pressure, the pump will give mains-like pressure with some minor drop-off when several homes are using it full blast. It is also possible to have a pump per dwelling in the shared storage tank which avoids landlord metering of electricity use. But this puts up the initial equipment cost.

Advantages
- Cheap to purchase and install
- A shared storage tank scenario needs only one pump serving all dwellings


Disadvantages
- No rainwater if there is a power cut or the pump fails
- Pump hunts to maintain pressure and uses a lot of electricity


In a shared, direct feed, storage tank scenario, a) the rainwater, b) the electricity for the pump, and c) the mains water for backup must all three be shared. There must be a landlord's meter for electricity and mains water.

Indoor pump: rain manager

The German market has for many years preferred to place the pressure-sensitive pump in the house, sucking from the tank and delivering water to the appliances. A mini header tank, mounted in the same console, is used to provide mains water backup if the rainwater runs out. Both pump and mini header are housed in a suitcase-sized console which is wall-mounted in the ground floor or basement. These rain managers take up space, pump can be heard, and are more expensive than other options. Furthermore, each dwelling needs its own rain manager and, therefore, the pump cost cannot be shared. They could be a good option for up-market individual homes where there is no roof space for a gravity-feed tank.

Advantages
- In a shared system, the electricity (one pump per dwelling) and mains water backup are both billed through the dwellings' own meters
- Pump is accessible, mini header tank for mains backup is integrated
- Control of rainwater even if there is no roof space


Disadvantages
- No rainwater if there is a power cut or the pump fails
- Pump hunts to maintain pressure and uses a lot of electricity
- Pump noise
- Expensive to purchase and to run
- A rain manager is required in each dwelling (no option for shared pump)


Gravity Feed with Plain header tank


Instead of supplying directly, the pressure-sensitive pump in the main storage tank supplies a plain header tank in the roof space and keeps it full using a classic ball cock and float. If the rain runs out, a lower ball cock and float admits mains into the header tank across an A-B gap. The disadvantage of this is that, as in direct feed and rain manager options, the pump works whenever water is drawn. Also the rainwater in the header tank could go stale in higher temperatures and periods of disuse (occupant on holiday).

Gravity Feed with Smart header tank


The smart header tank has an electric float sensor at the high water level and one at empty. They are connected to a computer controlled set of electric solenoids wall-mounted at the ground floor so that the header tank is left to empty completely before being filled, so reducing pump cycling. Almost 5 times less electricity is needed. The tank can also be programmed to be flushed out and refilled with mains water to avoid stale water.

Advantages
- In a shared system, as with direct feed, one pump can serve multiple header tanks, or there can be a pump per dwelling
- Mains water backup (smart header tank and controller per dwelling)
- Is billed through the dwelling's own meter
- Minimal pump cycling reduces power consumption and wear and tear on pump
- Water from header tank even during power cuts
- Manual as well as automatic mains back-up
- Can program mains water for indoor appliances if drought and hosepipe

- Price is lower than that of Rain Managers and other mini header-tank systems
- Pump in rainwater storage tank so, unlike rain managers, no pump noise in the house


Disadvantages

- Difficult to accommodate if there is no roof space
- Gravity provides less pressure and slower flow to toilet or washing machines (a tap for the garden can be provided by teeing off prior to the controller)


Choice Of Storage Tanks For Multiple Dwellings


For large capacity storage tanks for shared use of rainwater, the choice is between one piece laid-up fibre glass or steel framed and GRP sectional tanks or much smaller tanks linked together. The large tanks tend to be expensive to manufacture, transport and install. They also often require long advance order delays and the booking of a date for assembly on site by the manufacturer. These large tanks have to be transported from the factory in one piece on a low loader and slow convoy, with accompanying expense.

Mass produced two-piece tanks such as the Carat Range from Otto Graf GmbH in Germany are cheaper to manufacture, transport and install than one piece laid-up fibre glass or steel framed and GRP sectional tanks. The moulding process of the small tanks keeps the size to 6500 to 7000 litres when underground and 1-piece tanks above ground to around 10.000 litres. However, these multiple moulded tanks can be easily linked together in parallel, making the total volume required.

Rainwater Harvesting Limited supplies rainwater harvesting systems, storage tanks, pumps, filters and management systems to households and commercial businesses. RainWaterHarvesting.co.uk is a large information resource offering valuable technical information and advice in addition to 1000s of product specifications. Download the invaluable rainwater harvesting tank size calculator.

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