This week's edition of Science contained over a dozen articles on a subject that sounds deceptively simple: waste. Human societies produce a dizzying variety of waste purely as a byproduct of functioning, from agricultural waste or discarded electronics to excrement. In a world of finite resources and limited fossil fuels, it's obvious that we have to make better use of our waste. But doing so isn't just a challenge; it's dozens of them.

Organic waste

Take agricultural waste. The edible portions of most plants are generally accompanied by huge amounts of inedible material, primarily in the form of cellulose (a long string of sugars) and lignin (a chemical that links them together). Right now, if this is used at all, it's burned directly as fuel or used for feed. And, right now, efforts directed towards getting more out of it are focused largely on using it for fuel.

But one of the reviews notes that, assuming we can, diverting waste to biofuels may not make economic sense. The glycerol in biological material can be readily converted to fuel, but transforming it to epichlorohydrin produces a product that's three times more valuable (and 10 times more valuable than burning it to generate electricity). And that's a niche chemical. Ethylene and propylene are two of the major industrial chemicals made from fossil fuels; switching to bioethanol as the raw material to make these would currently require about 30 percent of all the land allocated to farming. (Although that could be sharply reduced if we could efficiently produce bioethanol from agricultural waste).

So, even a relatively simple and homogenous waste source requires two things: technology to efficiently convert it into useful materials, and significant thought about what material would be the most useful end point.

Most waste streams aren't so simple. Wastewater is produced by human, agricultural, and industrial use, has at least two distinct components: the water itself, and the waste that contaminates it. Water is an increasingly scarce commodity, but the good news is that we've largely solved that: current wastewater treatment can leave the output safe to drink. The bad news is that we've not gotten over the "ick" factor. So, as one of the articles notes, some locations are pumping treated water underground to replenish aquifers. Doing so provides a level of abstraction that allows people to comfortably drink water later pumped up from the same aquifer.

As for the waste itself, treatment currently consumes roughly three percent of the electrical power produced in the US, most of it spent on aeration of the waste. The waste itself, however, contains the equivalent of about 17GW of potential chemical energy, a roughly equivalent amount.

How best to get that energy out, though? Anaerobic digestion by bacteria can convert it to methane, which can then be used as fuel. Alternatively, some bacteria digest organic material in a way that leaves them with spare electrons that they shunt into the environment. In the right configuration, these can be used to make a sort of biological fuel cell, where the electrons sent off by bacteria are used as electricity. But it's also possible to provide electricity to these cells and get valuable chemicals out. These can range from hydrogen up to more complicated chemicals, although the current efficiencies are lower than standard industrial practices.

Managing metals

Shortages of petroleum and the rising prices they entail, which dominate considerations for handling organic waste, have been front page news for decades. But, in recent years, gold and other precious metals have seen their prices reach record levels, even as China's dominance of rare earth metal supplies have led to worries about trade wars. This should provide a strong impetus for metal recycling, and 18 different metals see half of their volume returned via recycling. But the overall outlook remains mixed.

Some metals, like copper and aluminum, are easy to recycle in part because they're typically used in a pure form, and the addition of minor contamination has little effect on any final products produced with them. That's not the case with many other metals, which need to be separated out from any impurities.

Modern manufacturing techniques have also made matters worse. We've gotten very good at identifying materials that are optimized for special uses, which tends to make most products a complex mix of dozens of raw materials. One example: a typical chip from Intel now contains something like 80 percent of the elements that aren't gasses. That makes separating out any metal for recycling a nightmare. In addition, we've generally managed to minimize the use of the most expensive materials among those, which cuts down on the economic value of the waste stream.

Now, this isn't true of all waste streams. Some items, like aluminum cans, have a high rate of recycling. So do jet aircraft, which have engines that incorporate some rather expensive metals.

But one of the biggest successes is with a metal that isn't especially expensive: lead. About 80 percent of the lead used today is put into lead-acid batteries. Here, the highly toxic and dangerous components have caused the industry to organize a recycling program that captures over 95 percent of the batteries sold. The program is so successful, in fact, that it's the major source of recycled polypropylene, the plastic used for the battery housing.

To reach those heights, however, required a concerted effort by both governments and industry. These ensured that spent batteries could be easily returned at no cost to consumers (who, in some locations, may even have a deposit returned when doing so). The effort also created consequences for the improper disposal of the batteries.

Sadly, it won't be easy to match that sort of effort in other areas. But, in a world with finite resources, we may find doing so becomes an economic necessity.

Science, 2012. DOI: 10.1126/science.337.6095.662 (About DOIs).