Physical verification of household rainwater tank systems
5.5 summAry And conclusIon
In response to an identified knowledge gap in post tank installation monitoring, this chapter has presented a generic rainwater tank installation compliance audit protocol, and described its application in SEQ, Australia, to assess compliance of rainwater tank systems and validation of water savings and quality objectives required by QDC MP 4.2.
This chapter has summarised the results of applying the audit protocol in the assessment of household rainwater systems of 223 Class 1 detached dwelling sites with mandated rainwater tanks in SEQ. The audit identified installation factors which may impact adversely on achieving the 70 kL/hh/yr water savings target for detached dwellings in SEQ.
The three main installation requirements of QDC MP 4.2 relate to: rainwater tank storage capacity, roof catchment area and connection to appliances. The results of the on-site assessment have shown that:
• Installed storage capacity is mostly greater than the required 5000 L, although 16% of sites inspected had storage volumes less than 5000 L.
• Roof catchment area does not meet requirements in 40% of cases, either by having <100 m2 or <50%
of total roof area (whichever is the lesser), connected to the tank.
• Connection to toilets, washing machine and external tap meets requirements in most cases.
The study has concluded that in estimating effective tank volume from manufacture’s claimed volumes, claimed volume should be reduced by about 10% to account for the dead volume below the outlet level. In addition, the active storage volume should be further reduced due to the water level setting on the pressure
switch and the operating levels of the float valve, with an overall total reduction in effective tank volume of about 20% if pump water level cut-off factors are also included. In the absence of information on claimed volumes, storage volumes can be calculated using in situ physical tank dimensions (after allowing for dead volumes etc.), but should first be reduced by about 12% to account for volume lost due to tank shapes which generally round off edges and are not perfectly square or round.
Of greater concern is the higher level of non-compliance in roof area catchment connected to the rainwater tank system, which we report with a reasonable degree of confidence. Even relaxing the level of compliance to within a 20% variance does not significantly improve the compliance. Certainly, in any policy and management intervention to improve the yield from rainwater tank systems in SEQ, this aspect of increasing the connected roof catchment area is the single most important physical installation factor to be considered. This level of non-compliance in connected roof area has implications for potable water savings across the region, most likely resulting in lower actual savings than those predicted, and thereby reducing the time period of deferred infrastructure investment. Specification of the area of the roof to be connected, rather than the percentage of roof area, is a more easily achievable target in situations when average roof area is greater than 200m2.
The practical reasons for this low compliance probably stem from construction phase issues such as cost, difficulty in getting the downpipes to connect to the tanks past windows, doors and other obstructions, and the aesthetics of having horizontal pipes running along external walls. Clearly, it is important to consider the overall design of the house when planning rainwater tank installation so that the anticipated benefits can be realised.
Charged systems, where the downpipes are usually full of water and run along the ground or beneath the house before rising up to discharge into the tank, are often used to achieve the maximum roof area connection. A caution is given here in that several sites with charged systems had the potential of cracking and delivering water to the house foundations. Silting up with leaf debris over time is also a problem if adequate inspection is not carried out and cleaning points are not installed.
The lack of detailed inspections by a dedicated agency of all aspects of rainwater tank installation has the potential to both reduce the anticipated savings and to cause health and safety problems in the future.
Inspection of connected roof area is probably the single most important aspect as this seems to be most problematic in terms of compliance.
QDC MP 4.2 also includes requirements for water quality protection through prevention of contaminants and vermin from entering the rainwater tank, prevention of mosquito breeding and prevention of contamination of reticulated town water supply. Except for the installation of backflow devices that prevent contamination of reticulated town water supply, requirements are adequately met. The results of the on-site inspection have shown that:
• Screened downpipe rainhead installation largely meets requirements.
• First flush device installation were infrequent but were not required as rainwater was generally not connected to showers, wash basins or hot water services.
• Mosquito breeding prevention device installation meets requirements.
• Vermin trap installation mostly meets requirements.
Backflow prevention device installation does not meet requirements, with only 45% of those rainwater tanks with switching valves having a backflow device installed. However, all tanks with a trickle top-up system (23%) had an air gap present between the overflow and the trickle top-up device which serves the purpose of a backflow prevention device.
The results indicate a high level of compliance with QDC MP 4.2 (P5), which states the requirement for a continuous back-up supply of potable water to internal fixtures supplied from a rainwater tank using either a trickle top-up or an automatic switching device.
In applying the rainwater tank installation compliance audit protocol in SEQ, a number of shortcomings were identified. The protocol needs further enhancement in relation to the measurement of underground tank systems, and the timing of the assessment in relation to rainfall events is important so that measurement occurs when conveyance pipes are sufficiently charged with water.
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