Urban water use is presently an open loop system, in contrast to closed loop, natural systems. Water is collected in dams, distributed, polluted, treated and disposed, either to the sea or back to rivers. No municipalities presently purify wastewater for re-use by people, except in emergencies when the energy intensive and expensive process of reverse osmosis - also used to desalinate seawater - has been used on treated sewage water.
If we are to sustainably manage our wastewater we must purify it to vastly improved standards removing all nutrients and pollutants in order to render the water biologically neutral.
To do this we must improve wastewater treatment processes. The present practice of polluting perfectly clean drinking water with human waste products and soaps, then partially treating it before releasing it into the environment is outdated. The nutrients are wasted, either through being dumped in landfills, pumped into the sea, or in the case of defective water treatment works, returned to our water sources with often catastrophic consequences on human and environmental health.
Most of the nutrient in human waste is derived from either fossil fuel sources (chemical fertilisers converted to food, which we convert to waste) or from industrial products such as soaps. All water scarce nations should urgently legislate to forbid the use of high phosphorous soaps, which are radical pollutants. This has already been enforced in many developed nations.
Historically, the rich nutrient component in human waste was a valuable commodity. This 'night soil' fertilised fields to replenish the nutrients absorbed by growing crops. Only by returning to such a closed loop system can we halt our squandering of these twin resources of nutrient and water.
The increasing scarcity and expense of fossil fuels that industrial agriculture depends upon for fertilisers and production, examined against the waste of nutrients produced by humans, clearly represents another open loop system that must be closed.
Instead of simultaneously polluting watercourses and oceans by wasting these precious nutrients, we need to use systems that recapture them so that they can again fertilise our food, directly or indirectly. This is the only logical and practical way to close this loop and to begin to shift towards a concerted and truly sustainable water and sanitation management system. The most efficient systems separate nutrients and purify the water through using the lowest possible energy systems.
One way to achieve this on the micro-scale is through the use of composting toilets. These have been condemned as being culturally insensitive or degrading by some local communities, while others have embraced them as contributing tangible benefits to community health and wealth. This compost, commonly termed 'humanure' is safe to use as a balanced and efficient plant nutrient.
So, if we wish to achieve sustainable cities from a hydrological perspective, we need to re-examine not only our abstraction of water from distant sources but more importantly, its treatment once we have polluted the originally pristine product.
Industries also pollute vast amounts of water as part of the manufacturing process. Our huge mining industry has created major point sources of pollution through acid mine drainage, which then is returned to rivers and streams, usually without adequate treatment, spreading the impact.
For a start, mining and industrial effluent must be separated from the general wastewater process. Each large industry and mining company must be obliged to return water to the environment only when it has been purified. By doing so the true ecological and economic cost of water can be calculated and included in the industrial process.