4 Water Conservation
4.1 Community and domestic users
Water conservation for community and domestic supply relate to initiatives that either:
- reduce the amount of water required by the end consumer or
- reduce the amount of water lost from the reticulated network (non revenue water or leakage).
With reference to Table 2.2. Typical Breakdown of Demand, in Section 2.2.2.2, this table illustrates that on an average demand day some of the most significant water uses are from domestic demand (35 percent) and leakage (20 percent). However these two uses have the potential to be significantly larger as described in the following paragraphs.
During a hot/dry summer the water demand will reach a peak similar to the peak day demand, approximately 2.5 times higher than average. This additional demand is primarily due to hosepipes for garden watering and cleaning. At these times domestic demand may grow to 75 percent or greater. The means of conserving water from this type of use are generally water use restrictions. Some forms of drip irrigation may be effective in reducing the overall water used for garden watering; however these technologies have a capital cost to be borne by the home owner and they must be maintained.
Leakage or non-revenue water is the other significant demand that can be minimised. Depending upon the state of the infrastructure this may represent between 20 to 50 percent of the total demand. Small communities are unlikely to have reticulated networks requiring sophisticated waste minimisation strategies such as pressure management and active flow control systems. Leak detection and maintenance of infrastructure is likely to be the only practical measure that can be applied. Most local authorities will have some form of leak detection and asset maintenance plan already in place.
The water and sanitary services assessment due to be completed by councils in June 2005 may allow councils to determine the amount of wastage currently experienced in their communities.
4.2 Industry, commerce, tourism and recreation
Increased industrial water use efficiency may be achieved by:
- improvements in process technology as new capacity is added to existing plants or new plants are established
- company internal wastewater minimisation policies and environmental audits require a more stringent compliance standard
- voluntary waste minimisation processes that provide an economic advantage either due to cost of raw materials that can be recovered or a secondary product market is found for waste products
- increasing the amount of water reuse within the process or treated water being used for other applications such as municipal supply or irrigation.
Limitations to the degree of water efficiency may result from:
- difficulty in making existing processes more water efficient, as retrofitting may be prohibitively expensive
- more comprehensive water and waste minimisation techniques may not be justifiable in the present economic environment
- the food processing industry requires a high level of disinfection for compliance with health standards and this limits the applications for water recycling/reuse.
Quality of the end product is a principal health and safety matter that has significant bearing on the amount of water required per tonne of product. Water minimisation strategies incorporating water reuse must ensure that a very high disinfection standard is achieved to prevent contamination. Although disinfection processes are commercially available they are relatively new to New Zealand industrial applications and expensive.
Micro- and ultra-filtration technologies are disinfection processes currently available that may allow some process wastewater streams to be recycled. Principally used for water treatment and supply applications, they are becoming more widely used in New Zealand and are capable of producing potable water from a variety of waste streams. The cost of implementation is also reducing and typical operating costs may range from $2.00/m3 to less than $0.70/m3 depending on the volume of water being processed.
Most industries using the water volumes required to make ultra-filtration economic will have significant issues with pre-treatment and this will drive the cost of water reuse higher. Pre-treatment processes are required to prevent membrane fouling and optimise efficiency.
Logically there may be the potential for reuse of some very large industry waste streams either within the plant or for municipal supply and irrigation.
