Very often discussions about renewable energy focus on solar and wind power, and other sources fall into the background. To be sure, these two will form a significant part of the new system, but they are not all there is. Aside from geothermal energy, tidal energy, and various forms of crop-based biofuels, we also have an abundant form of energy available that is directly proportional to the number of people living nearby.
Using biological waste for energy is nothing new - People have been burning cow dung as an alternative to wood for centuries; but using human waste to generate electricity or cooking gas is a fairly recent concept, and has only really been developed in the last couple decades. Initially, it was used as a source of cooking fuel for small-scale farmers in developing nations, but it has recently taken hold around the world as a way to handle sewage and generate power and profit simultaneously.
In 2010, a sewage treatment plant on the Thames in England finished a six-month remodel that cost them a grand total of $3.9 million, and is now generating over $23 million worth of electricity every year. That's a pretty good return on investment, and this technology is only just getting off the ground.
Indeed, Thames Water is not alone. A quick google search for "Sewage biomethane" turns up dozens of results from all across Europe, with sewage providing power to heat homes, generate electricity, and even run cars and busses. Turning our attention closer to home, this brilliantly simple technology is being employed in the United States as well, though on a smaller scale.
Right now, we face multiple crises. A warming climate threatens our nation's food supply, infrastructure, and coasts, and expenses are rising in a sluggish economy; there are a number of ways forward, and we should be taking advantage of all of them. Turning human waste, which is currently an expense in both power and money, into a source of power and money just makes sense, and to be honest, it should have been done years ago.