end use water requirements

The number of times a pump starts, the duration of its operation, and the flow and volume of water supplied will vary with the water use pattern for each individual appliance, and with the pump settings.

Water end uses in a household have specific service requirements for volume, pressure, flow and pattern of water supply. Table 6.2 provides a summary of typical operating requirements for appliances such as washing machines, dishwashers, showers and taps. Water fittings such as taps, showerheads and irrigation devices are designed to operate within a pre-set flow rate range. Likewise, household appliances, such as washing machines, toilet cisterns and dishwashers are designed to operate within a specific range of flow and pressure conditions. In addition, in Australia water efficient appliances and fittings that comply with the Water Efficiency Labelling Scheme (WELS) are designed to use less water and/or supply water at lower flow rates than non-WELS appliances.

table 6.2 Service and design parameters for water appliances and fittings. (Adapted from Tjandraatmadja et al. 2013).

Dishwasher 30–150² 800–1000¹ <4.1⁴ 16⁴

Toilet cistern 35–50²

Note: ¹Manufacturer specifications, ²Standards Australia (1999), ³Australian Government (2011), ⁴Measured (adapted from Tjandraatmadja et al. 2011), ⁵Measured by Roberts (2005), ⁶Water Efficiency Labelling Scheme (WELS) is a water efficient rating scheme for all major water using fittings and appliances in Australia (Australian Government, 2011 (http://www.waterrating.gov.au/index.html)), ⁷Measured by Water Conservation Group (2010), n.a. Not applicable.

The most common end use applications for rainwater in dwellings are toilet flushing, washing machine supply and outdoor irrigation. Toilet flushing and washing machines typically require low flows, less than 6 L/min and 13 L/min respectively, and limited volumes of delivery, up to 4 to 6 L for a toilet cistern, 40 to 60 L for a front loader washing machine, and 100 to 150 L for a top loader washing machine (Tjandraatmadja et al. 2012).

The pattern of water supply also varies with end use (Tjandraatmadja et al. 2012). For instance, washing machines require a pump to stop and start multiple times during a wash. Tjandraatmadja et al. (2012) showed that the wash cycle in a top loader washing machine comprised two to three major water supply episodes of 40 ± 18 L each, and four to five short water top-up episodes in the spray cycle (Figure 6.10a).

The rainwater pump operated on average for 21 ± 4 min. In comparison the wash cycle of a front loader washing machine was characterised by three to four water supply episodes of 11 ± 0.5 L each, and up to five small injections of water (Figure 6.10b). The rainwater pump operated on average for 5 +/−2 min during the cycle. In contrast filling a toilet cistern is a short duration single event where the pump operated for 0.5 min to 1 min depending on a half or a full flush, (Figure 6.10c).

Laboratory and in-situ studies show that the flow rate associated with end uses is the key influence on the energy intensity of pump operation, with low flow end uses typically using more energy per kL than high flow uses. In addition, Hauber-Davidson has also shown that pumping energy is barely affected by the pumping distance to the various end uses supplied, in a double storey house (Water Conservation Group, 2010). Retamal et al. (2009) estimated that the energy intensity associated with individual end uses varied from 0.4 to 2.9 kWh/KL for pumps with motor sizes between 0.5 to 0.9 kW. Talebpour et al.

(2011) examined the energy associated with individual end uses in five dwellings equipped with a popular pump and switching valve system and measured energy intensities from 1.04 to 1.67 kWh/kL for garden irrigation and toilet flushing respectively. Hauber-Davidson and Shortt (2011) determined the energy for the operation of a washing machine, hose spray for outdoor irrigation, toilet cistern and a hot-cold water mixing tap and found specific energies ranging from 1.13 to 4.73 kWh/kL. Tjandraatmadja et al. (2012) verified that the specific energy for rainwater supply to various appliances (dishwasher, toilet cistern, top and front loading washing machines and header tank) depended on the end use and on the pump motor capacity, ranging from 0.39 kWh/kL to fill a header tank with a 0.2 kW pump, to 5.3 kWh/kL for a dishwasher with a 0.75 kW pump, as shown in Figure 6.11.

Hauber-Davidson and Shortt (2010) and Tjandraatmadja et  al. (2012) have shown that the typical operating requirements for rainwater end-uses in urban dwellings (toilet flushing, washing machine and irrigation) are below the best efficiency point for the majority of pumps adopted in rainwater systems.

This is shown in Figure 6.12, which reports on the service flow requirements for typical end uses, and the associated specific energy requirements for pumps of 3 different sizes (0.2 to 0.75 kW). Figure 6.12 shows more generally that low flow water supply events of short duration, such as toilet flushing and opening a tap briefly for hand washing, result in higher specific energy use than longer duration events with higher flows, such as filling a washing machine or garden irrigation. In comparison the best efficiency point for most pumps was achieved at flow rates greater than 25 L/min.

Ferguson (2011) verified, in a one-year study of 52 dwellings constructed after 2009 in Sydney, that 40% of rainwater pumping events are short duration events supplying volumes of 2.5 to 9.5 L, such as toilet flushing, whilst less than 20% required over 100 L (for irrigation and washing machine use), which are characterised by higher flow rates. Similarly, toilet flushing was also identified as the predominant end use for rainwater in Melbourne studies (Hauber-Davidson & Shortt, 2011).

Thus, a fixed-speed pump will often operate inefficiently and outside its optimal range for many of the end uses. Consequently, it is almost impossible to select a fixed – speed pump that is suitable for all water uses using the pump curve alone (as shown in Figure 6.6a).

Figure 6.10 Pump energy use and water supply flow and pressure patterns during operation of (a) Top loader washing machine, (b) Front loader washing machine and (c) Filling of a toilet cistern. Note that the x-axis (time) does not have the same scale for all three events. (Source: Tjandraatmadja et al. 2012).

Figure 6.11 Specific energy requirements for rainwater supply to a dishwasher, washing machine, toilet cistern and a header tank using pumps of motor capacities ranging from 0.2 to 0.75 kW. (Source:

Tjandraatmadja et al. 2012).

Figure 6.12 Relationship between specific energy and flow required for rainwater uses in a dwelling.

(Source: Tjandraatmadja et al. 2012).

Yet, pump motor size also matters as demonstrated in Figure 6.11. In Figure 6.12, pumps of various motor sizes fulfilled the individual service requirements of the common household end uses. In addition Pump B was tested also with an automatic water mains switch (i.e., when the tank is empty mains water is provided to the dwelling). In particular, the smaller size pumps were able to deliver a satisfactory level of service at a lower specific energy (Hauber-Davidson & Shortt, 2010; Ferguson, 2011; Tjandraatmadja et al. 2012, 2013).

Cunio and Sproul (2009) showed that under-sizing a pump runs the risk of limiting the flow rate and pressure of the rainwater supply. However, the common perception that a ‘one size bigger is better’ for pumps has resulted in many pumps being oversized and not optimised for their operating range, particularly in urban areas where rainwater supply is connected to limited end uses instead of to the entire household demand. However the quality of service (flow rate & pressure) does not vary markedly between a 0.2 kW and a 0.75 kW pump for supply to end uses such as toilet cisterns and washing machines (Tjandraatmadja et al. 2012). Most household rainwater users typically only notice the performance of their rainwater pump when they water the garden and expect a high flow rate and pressure similar to their mains water supply.