I ran across an article recently that discussed the concept of "Urban Mining." When I first heard the term the image that flashed across my mind was an old prospector with a pickaxe over his shoulder, standing with his trusty burro looking at a trash-filled dumpster.
I could picture a comic bubble over his head: "There's Gold in that there trash!!"
Urban mining is just a fanciful term for recycling. Basically prospecting the urban landscape for recoverable materials that can be recycled, reused and turned to profit.
It's certainly not a new concept in the industrial water market, but the wastewater once flushed to the sewer has the potential to contain significant value and recoverable materials that can help offset the cost of treatment systems.
The "value" found in wastewater can take various forms. Simply recycling and reusing process water can help reduce the total volume of water used and can sometimes reduce costs, depending on the level of treatment required.
But in some industries, the temperature of the water has value, whether it be cold or hot. Chilling or heating water takes energy, and recycling the water can help capture residual heat and reduce energy costs.
An interesting new twist is to use wastewater as a heat source for a heat pump system, helping reduce HVAC costs. A large wastewater tank or trunk line tends to have a stable temperature, and can serve as a viable heat source for both heating and cooling systems, as opposed to using a ground-sourced heat pump. In China several large buildings have successfully employed the technology for heating and air conditioning, including a hotel and a 1-million-square-foot train station in Beijing. Similar systems are being piloted in the United States at municipal wastewater plants.
Of course, most wastewater contains some form of solids that must be removed before discharge or reuse. Depending on the industry, those solids may have commercial value. As an example, in a recent article I wrote about washing potatoes to remove starch. The starch was later reclaimed during a treatment step and sold as a stand-alone product.
There are examples across both the municipal and industrial water markets where solids from wastewater treatment are put to use in a variety of ways, including as fuel when dried, to generate biogas, and as fertilizer, among others. One recent innovation is the recovery of phosphorus from wastewater biosolids.
The metals processing industry has long history of reclaiming metals that find their way into rinse water. Depending on the industry, recovered metals could include gold, silver, copper, cadmium, and zinc, among others.
Clearly, if you manage a wastewater treatment system, whatever the industry, it's worth your time to take a hard look at the material that's in the waste stream and how it might be reused. And even if there is no recoverable material in that stream, the water itself might be worth reusing again and again.
Look out the window and see if my old prospector is staring at your wastewater treatment plant. He just might be thinking, "There's Gold in that there wastewater!"