Direct Potable Reuse: Crossing the Barrier
Pandering to public perceptions can often lead to expensive and somewhat absurd decisions. Take the case of indirect potable reuse of water. Is it not a waste to purify wastewater to such advanced levels, blend it with water of lower purity and then let it get treated all over again at drinking water treatment plants to supposedly make it fit for drinking? Or to take tertiary treated effluent and make it recharge groundwater through layers of soil pulling in contaminants all over again?
It is established best practice in many parts of the world, notably California, Israel, Singapore, Australia and others to implement the multi-barrier concept which puts many barriers between treated wastewater and the human systems. In addition to the multiple barriers in the treatment process, a dual distribution system carrying water and treated wastewater in separately marked pipes with the latter being used for non-potable uses such as washing and irrigation has been a desirable objective of water reuse programmes. The entire business of non-potable and indirect water reuse is centred on the public distaste for drinking toilet-water.
However, there is a significant cost associated with the need to provide separate piping and storage systems for reclaimed water. Dual distribution systems are prohibitively expensive and are difficult to implement in densely populated urban areas. As experts point out, people are anyway drinking wastewater in an unplanned manner since cities are discharging their treated and even untreated wastewater (in developing countries) into rivers which are used as a source of raw water downstream. Whether it is the Colorado, Rhine, Thames, Yang-tse, Mekong or Ganga, wastewater has been a part of our drinking water supplies.
Direct potable reuse (DPR) of wastewater should not be such a repugnant proposition today if one considers that advanced methods of treatment are able to remove contaminants to extremely low levels and have become cheaper than before. As droughts become more acute spurred by climate change, the only reliable source of water is often only wastewater.
Windhoek in Namibia has been practicing DPR since the 60s. Two cities in Texas, USA (Big Spring and Brownwood) beset with chronic water shortages are also set to go the toilet-to-tap way very soon.
Advanced treatment processes have evolved over time and commonly include microfiltration, reverse osmosis and advanced oxidation. Major innovations are improving the cost and efficiency of these processes.
Direct potable reuse of wastewater will bring another aspect into sharp focus – the detection of personal care products and pharmaceuticals, which end up in wastewater. With advanced instrumentation, nowadays, it is possible to detect contaminants even in trace concentrations. Indicator constituents can also be measured in place of direct measurements to ensure the reliability of performance of key unit processes.
More importantly, when we know we have to ingest whatever we use in our daily lives, it becomes our responsibility to manufacture and use products that are amenable to easy removal by treatment processes. Stringent discharge regulations with regard to personal care products and trace constituents will be needed if DPR becomes more commonplace.
Perhaps drinking our wastewater is just the bitter medicine we humans need to live in greater harmony with nature. Only when we know our chemical-laden shampoo, moisturiser, toilet cleaners and detergents are going straight to our drinking water supplies will we be forced to rethink our lifestyle.