Water Views - recycling and saving

Water consumption can be reduced by minimizing personal use and by recycling untreated grey water (waste from shower, basin, bath and washing machine) around the home.

Grey water may be diverted by a licensed plumber for subsoil (garden) use without Council approval. However, treatment and storage of grey water for other purposes such as flushing toilets or car washing requires Council permission. The alternatives to formal storage connected to dedicated plumbing involve retaining water in the bath or shower (if suitably designed), and informally diverting washing machine and perhaps dishwasher outflows to suitably sized containers. In each of these cases, re-use typically requires a significant amount of bucketing.

"Institutional recycling" is a process where an institution accepts and cleans grey water, before releasing it to the environment or piping it to customers for limited uses.

Institutional recycling of grey water is expensive and commonly impracticable other than for large (commercial/industrial/education) sites or new subdivisions. This is because the process necessitates installation of dedicated sets of pipes for transporting grey water from houses to a recycling plant and returning treated grey water to homes. These external (to the home) costs exceed other options discussed below and involve major disruption within well-established communities. Furthermore, to allow recycled grey water to be piped to the cistern, washing machine, and other points in the home where it might be useable, there are internal costs related to dedicated grey water plumbing, and deployment of differently coloured taps to distinguish drinking water from non-potable grey water.

Reticulated potable recycling (RPR; sometimes called "indirect potable recycling") is a much better alternative. RPR involves treating water from the sewerage treatment plant (including secondary treated storm water and sewerage ready for discharge to the ocean) by means of "state of the art" water treatment in a Membrane Reverse Osmosis plant, including micro filtration pre-treatment and ultraviolet light post-treatment steps. Water from such a plant is very pure and has a risk of infection of about 100 thousand to a million times less than from our current tap water; that is, RPR water is at least a hundred thousand times more safe.

The RPR product can be recycled to a dam reservoir, where it becomes heavily diluted by catchment-derived rainwater.
This blended water, after re-treatment to improve the quality of the dominant rainwater component:
RPR water has been used for many decades in city water supplies in Europe and USA. In comparison with desalination, the capital cost is lower because the water treated is far less corrosive, and the running costs are also lower. This is because the total dissolved solids in seawater are 35-350 times greater than the feed to the proposed reverse osmosis plant, so the power cost to remove them by desalination is at least 3 and up to 6.25 times greater. Inevitably, this higher power consumption for desalination induces an equivalently higher environmental impact through greenhouse gas production. But the over-riding benefit is that most of our precious water gets reused and we become less reliant on catchment rainfall.

Is it safe? Most definitely yes! Reverse Osmosis removes hormones, pesticide residues, organic molecules and chemical species as tiny as sodium ions. The product is purer than your current drinking water.

Is it expensive? It is much cheaper to produce than desalinated water. Desirably, sewerage treatment plants and RPR units should be sited to avoid excessive pumping distances.

Is there a power disincentive? There is a power cost, but it is small. At a consumption of 285 litres per person per day (average for detached dwellings, 2003), the power cost per year of the water consumed would be between 15% and 25% of the cost of running a 300-litre (6star rated) refrigerator. (285 litres is 3.5 times Rosemary's daily consumption).

What about rainwater tanks? Sydney has about 1.5 million freestanding homes. Simplistically, the $2,510 million cost of a 500ML/day desalination plant could instead pay $2510 up front towards plumbing-in a tank in two-thirds of the freestanding houses in Sydney. The running cost of the desalination plant would approximate another $165 million a year, and at $2510 per home this could subsidise an additional 65,000 tanks per year. Plumbed-in tanks provide more water at less cost than desalination (the worst option!), but they are less efficient and less environmentally effective than RPR because they depend on rainfall, are not suited to every home, and the water, unlike recycling, is used once and thrown away. Of course, if the 'treasury' can afford it, plumbed-in tanks together with RPR is ideal.

The information in this article is derived from a critical review of the Sydney Metropolitan Water Plan by the Society's Groundwater subcommittee. The review will shortly be placed on the Society's website.

Ian Ernst,

Groundwater Subcommittee,
Blue Mountains Conservation Society.