rAInwAter system set-up

Rainwater supply systems are typically designed for residential, commercial and industrial applications. A rainwater system comprises a series of components including tanks, pumps, flow control devices, switching valves, back-up systems, and ancillary components such as pressure vessels and filters, as listed in Table 6.1 (Retamal et al. 2009; Hauber Davidson et al. 2010; Umapathi et al. 2012). A more comprehensive review of key system components and configurations adopted in rainwater supply can be found in Retamal et al. (2009).

In urban settings, the most common system adopted consists of an above ground rainwater tank with a fixed-speed pump (either external or submersible) and a trickle top-up or an automatic switching valve to allow mains water back-up. Figure 6.2 shows a typical set-up, with rainwater flowing from the roof into the tank after passing through a leaf-guard to remove leaves. The bottom of the tank is connected to an external fixed-speed pump equipped with an automatic switching device for mains water back-up. The pump provides rainwater for non-potable, fit-for-purpose water use applications such as toilet flushing, laundry cold water taps and garden irrigation. When the tank is full, rainwater overflow is discharged to the stormwater drain.

table 6.1 Components that can be adopted in a rainwater system.

component description

Pump Pumps water from the rainwater tank to the dwelling under pressure. There are various types: Fixed-speed external or submersible, variable-speed, jet pump, and so on (Retamal et al. 2009).

Rainwater tank Tanks are supplied in a range of shapes and volumes including above ground, below ground and bladder tanks.

Back-up system Ensures continuous water supply. There are two types: (a) trickle top-up which fills a tank with mains water via a float activated mechanical valve; or (b) automatic mains switch valve that sources water directly from mains water if the rainwater tank falls below a threshold water level measured with a pressure transducer.

Pressure vessel Device that releases water from a vessel whilst maintaining the pressure in the line from the pump to the vessel above the threshold value that would otherwise activate the pump. They are particularly useful to prevent pump activation for small water demand events such as hand washing or water leakage. They come in a range of sizes from 3 L to >100 L. Pressure vessels used in residential dwellings typically have small volumes (<10 L).

Filter Device used for removal of particulate matter before supply to the dwelling.

Resistance to flow increases as they become clogged.

Header tank Balancing storage between a rainwater tank and the end use of a dwelling. It is located above the ceiling line and is filled by the pump and supplies water to end uses by gravity.

Figure 6.2 Example of a typical residential rainwater system set-up with an external fixed-speed pump equipped with an automatic rainwater to mains water switch. The rainwater flow path into the dwelling is shown by the blue arrows. (Source: Umapathi et al. 2012).

Fixed-speed pumps are much more common than variable-speed pumps as they are significantly lower in cost (Retamal et al. 2009; Tjandraatmadja et al. 2012). Most importantly, a fixed-speed pump draws almost the same amount of electrical energy irrespective of the water flow rate. The typical energy

associated with pump operation is characterised by three stages, as shown in Figure 6.3: (a) Start-up as the inertia of the pump motor is overcome; (b) Operation at steady flow and pressure; and (c) Over-Run as the flow is stopped and the system is re-pressurised to the set pressure. The start-up stage is characterised by a transient peak in power lasting less than 1 second, followed by constant power draw during the steady water supply regime (Retamal et al. 2009; Tjandraatmadja et al. 2012). The energy required for the start-up, operation and over-run stages are a function of the design of each pump. This is illustrated in Figure 6.4 which shows the energy consumed during each of those stages for three pumps during the filling of a toilet cistern after a half and a full flush. In Figure 6.4, the start-up energy contributed to a larger share of pump A’s total energy use than for pumps B and C, whilst for pump C, the over-run energy was higher than for the other pumps. Analysis of 11 pumps sold in Australia showed that the over-run energy can contribute from 5% to 25% of the total energy required to fill a toilet cistern after a full flush depending on the pump model and its pressure settings (Hauber-Davidson & Shortt, 2011).

Figure 6.3 Typical operation profile for a fixed-speed pump showing the power consumption, pressure and flow delivered by the pump at the start-up, operation (flow) and over-run of pump operation. (Source:

Tjandraatmadja et al. 2011).

Rainwater tanks adopted as a supplementary water supply system are usually fitted with a back-up system, that is, a mechanism to switch to the mains water when the tank water level falls below a threshold level set by a float or pressure transducer. There are two types of back-up systems commonly used: trickle top-up and switching device. A trickle top-up system (Figure 6.5a) uses a mechanical float device within the storage tank, so that when the water level reach a threshold, a mechanical valve opens and mains water is trickled into the tank to maintain water supply to the household. A trickle top-up system has adverse energy implications as mains water is depressurised when it enters the tank, after which it is re-pressurised by the

household pump. A switching device (Figure 6.5b) uses a pressure sensor to measure the water level in the tank, and once a threshold value is reached, it signals a solenoid valve to open which connects directly to the mains water supply. The switching device is placed on-line after the pump and does not depressurise the mains water supply. However, energy is used to operate the sensor circuitry and the stand-by energy use of the devices in the market vary substantially. In some cases, the non-pumping related energy (stand-by and over-run time) can contribute up to 20 to 40% of the total energy consumption depending on pump type and operation (Water Conservation Group, 2010). However Umapathi et al. (2013) examined 20 dwellings with rainwater tanks with the two different mechanisms and found a slightly higher energy consumption for the trickle top-up system (1.59 kWh/kL) compared to the switching device (1.46 kWh/kL).

Figure 6.4 Energy use during supply of rainwater to a toilet cistern by three different pumps A, B and C with motor capacities of 0.2 kW, 0.55 kW and 0.75 kW respectively. (Source: Tjandraatmadja et al. 2011).

Figure 6.5 Illustrative diagram for a rainwater tank system working on: (a) a ‘Trickle top-up’ mechanism;

and (b) a ‘Rainwater Switch’ mechanism.

The energy usage for rainwater supply is mainly dependent on the pumping of water from a rainwater tank to the end uses in a building (Retamal et al. 2009). Hence, thoughtful system design and the proper selection of ancillary components can help to reduce the energy usage for rainwater pumping.