The City of Phoenix can take after metropolises around the globe and utilize the warm (and hot) water that goes down our sewer pipes to create renewable thermal energy resources by tapping into the network of regional sewer lines.

To paraphrase the writings of others: In sewer heat recovery, nonclogging filters and pipes with heat exchangers transfer thermal energy from the wastewater stream to water-to-water heat pumps. The relatively stable temperature of the wastewater stream makes these heat pumps very efficient because it takes less energy to heat water that is already warm. Buildings using these systems then have a separate loop of clean water that circulates in the buildings where efficient radiators heat or cool the living and working spaces in the building. This approach is also used to provide domestic hot water for cleaning, showers, and other hot water uses. The same principles that heat water can also be used to cool water by rejecting unwanted heat back into the sewer lines.

As anyone who has ever turned a tap on in Phoenix in the months from May-September knows, water already comes out hot, so we can use that 'waste water' to create more energy without burning more fossil fuels and doing so more cheaply than conventional energy generation.

I've attached a link to an article from the Urban Land Institute describing the process in greater detail for commercial and residential uses.