Direct potable reuse

The option of direct potable reuse is the most technically demanding and societally contentious. In direct potable reuse the effluent of a wastewater treatment plant is routed directly to the intake of a drinking-water treatment plant. Because of the seemingly closed-loop cycle this process achieves, it is often called ``toilet-to-tap'' (WRA, 2001). However, a number of issues prevent widespread adoption of this technology:

• Direct potable reuse is technically demanding because wastewater requires more extensive treatment prior to re-introduction in the drinking water plant. Typically, wastewater is discharged to receiving bodies of water such as lakes and rivers; directly cycling the wastewater back into drinking water requires physical and chemical treatment surpassing that necessary for surface water discharge (Baumann and Dworkin, 1978).

• Direct potable reuse is societally contentious because of the negative associations of wastewater. Although many communities already practice ``indirect potable reuse'' because their drinking water intake lies downstream of another municipality's wastewater plant, the idea of direct reuse is often more upsetting. Citizen group reactions in areas where direct potable reuse has been proposed tend to be strongly negative (WRA, 2001).

• While some of the initial upset over direct reuse can be attributed to a general ignorance of the realities of water treatment, direct potable reuse does suffer some serious questions regarding health and hygiene. The dilution of pollutants by receiving bodies of water in traditional water treatment plays a significant role in cleaning the water. A system that loops back a large quantity of its water volume has the risk of concentrating pollutants over time. While EPA-limited pollutants and pathogens are closely monitored, there are other potential problem chemicals whose effects are unknown. For example, many medications are excreted from the body and are detectable in wastewater. Such chemicals are not on the list of monitored pollutants, but would certainly be present in recycled wastewater (NRC, 1998).

In summary, direct potable reuse is the most extreme case of water recycling, and is at present used only in water-critical situations. While toilet-to-tap systems do work in theory, and some pilot plants are presently being monitored, they are generally an option of last resort.