In order to identify the most appropriate stormwater use scheme option for a particular study site, the possible options need to be identified. Section 3.3.1 described the method and components used to generate stormwater use scheme options that could theoretically be implemented. This method was based on a simplistic tree diagram utilising all of the component options to determine a vast range of stormwater use scheme options.
To reduce the number of scheme options to be analysed, Section 3.3.2 described screening based on environmental, social and economic issues that were used to reduce the number of feasible options. This screening was implemented as appropriate for all study sites. Additional screening tools are required to further reduce the number of stormwater use scheme options to be analysed in detail. Additional screening based on technical and non-technical issues are carried out on a case by case basis. Sections 4.3.1 to 4.3.6 provide a summary of the technical criteria that causes stormwater use scheme options to be deemed infeasible or inferior. These criteria are integrated throughout the steps of the decision making framework.
4.3.1 REGULATORY PROHIBITIONS ON THE USE OF STORMWATER FOR END USES
The first criterion to consider is whether there are regulatory prohibitions on the use of stormwater for certain end uses. In particular, regulations may prohibit stormwater being used for potable end uses. The type of end uses covered under potable end uses and the areas where there is significant consumption of water in the household needs to be verified. Potable end uses generally include water supplying kitchen taps and hand basins. However, all indoor cold water taps and taps supplying water to showers and washing machines may also be categorised as potable end uses.
4.3.2 INSUFFICIENT QUANTITY OF WATER
Another possible criterion that limits the number of feasible stormwater use scheme options is an insufficient quantity of stormwater to meet the proposed end uses. This is particularly important if stormwater is the sole source of water, rather than being supplemented from existing water supplies. Insufficient quantities of water can be due to not enough rainfall in the area, insufficient runoff or an inadequate capture system. Factors such as infiltration capacity and evaporation influence the amount of rainfall that becomes runoff. Inadequate capture systems can be due to small inlets, losses from infiltration trenches or open systems resulting in excessive evaporation or seepage. Losses in the capture systems can also be due to storage sizes being too small or incompatible demand patterns resulting in large quantities of runoff becoming overflow. There are a number of solutions to overcome the problem of insufficient quantity of water to meet end use demand. These solutions are based on either collecting more
stormwater or otherwise reducing demand. Improved collection systems to minimise evaporation or exfiltration can result in larger volumes of water being collected. Alternatively, the catchment area can be extended to capture more runoff.
If insufficient quantities of stormwater are available, end use demand needs to be reduced. End use demand can be reduced by not supplying all end uses. Alternatively, reliability of stormwater supply can be reduced so that end use demand is only partially met. This requires stormwater to be supplemented with the existing water supply system or other water sources.
4.3.3 END USE DEMAND NOT MATCHING STORMWATER RUNOFF
The temporal and spatial pattern of the end use demand influences the likely success or otherwise of a stormwater use scheme. Temporal variability means there are different demands throughout the day, as well as throughout the year. Irrigation demand often has large temporal variability. In areas with large variability between summer and winter rainfall patterns, more irrigation is required in summer when there is less rainfall and vice versa in winter. This is not the case for every location, as some areas have relatively constant rainfall throughout the year, and therefore irrigation demand is fairly constant.
Spatial variability of demand can also be constant or variable. Spatial variability in terms of household water use is dependent on householder behaviour, and is very unpredictable on an individual basis. However, water use and demand can be more easily generalised on larger scales. To reduce end use demand, householder behaviour can be influenced and changed through education and regulations. Education can change behaviour of using showers or washing machines. Regulations can change behaviour either through water restrictions or enforcing the use of water efficient appliances and equipment. Strategies to change end use patterns through regulations and education can be included in the development of a stormwater use scheme and would minimise over design of the scheme.
4.3.4 STORMWATER OPTIONS NOT APPROPRIATE FOR STUDY SITE
There are some cases where there is no demand for certain end uses, for example community scale irrigation areas. Alternatively, an area consisting of only flats may
have no gardens and therefore no irrigation demand. In each of these cases the end use which has no demand is not included in any of the calculations and decision making processes.
4.3.5 INSUFFICIENT SPACE OR FUNDS FOR TREATMENT
Under this criterion, the amount of treatment required is considered. The amount of treatment influences the quality of water produced, which governs the end uses that can be matched. If there is only space or funds for minimal treatment, end uses which require low-quality water can only be examined. If space is the only limiting factor, small sized treatment systems can be considered and are discussed in Section 4.4.3.1. This is a particular issue in existing urban areas where space is a limiting factor for implementation of stormwater use schemes.
4.3.6 INSUFFICIENT SPACE FOR STORAGE
Space can be a limiting factor in terms of available storage size. Where there is insufficient space to construct a storage system, either all end uses can not be met, or end use demand patterns need to be selected that more closely follow stormwater yield. Otherwise, the percentage of the end use that can be met must be reduced. This means the reliability of stormwater supply decreases.