By Frank Zeman
Although we often talk about the latest new technology to hit the renewable energy markets, such as thin-film solarx or advanced nuclear technology, sometimes it is worth reexamining an old one. Composting is one of these. Common on the farm, it has been revisited on a larger scale as cities began to look for ways to reduce the amount of material being sent to landfills, the simple logic being that any reduction in the amount sent to a landfill increases its life. This turns out to be much easier than opening a new facility in the face of public opposition. But in addition to the potential for simple waste reduction, there is the potential to harvest energy from this process.
First, let’s look at traditional composting. Composting refers to the aerobic decomposition of organic matter through microbial activity. This mass loss is accomplished through two primary routes: the digestion of organic matter and the evaporation of water. The digestion occurs in three distinct phases: psychrophilic, mesophilic, and thermophilic. The three phases occur in succession as the temperature in the pile increases from 12–21°C (55–70°F) to 21–38°C (70–100°F), with the final phase occurring in the 45–70°C (113–160°F) range. These changes manifest themselves as a gradual increase in temperature that reaches its peak after 2 to 3 days, with a gradual decline in temperature over the next week. The pile is allowed to cure (i.e., slowly decompose) until it becomes relatively stable and easy to handle. This profile results in approximately 25% of the organic material being consumed in the first 4 to 5 days, followed by the reduced rate curing period requiring an additional 3 weeks to reach the 50% level. This spike in activity is visible in the profiles of carbon dioxide (CO2) emissions, oxygen consumption, and heat evolution, which all peak (or trough in the case of oxygen) in the first couple of days. The concentration of the CO2 gases leaving the pile is on the order of 5%, approximately equal to a natural gas power plant. On the whole, composting is generally an outdoor activity occurring over weeks that uses the atmosphere as a source of oxygen and a repository for the CO2, water, and heat produced during the process. This article discusses the recovery of these valuable resources in the context of urban sustainability.
Though the techniques mentioned offer savings in terms of landfill space, there is little in the form of energy or material recovery, leading to the conclusion that the energy value of the waste is not recovered. The question that arises is whether this can be changed. In an age of globalization, one of the few industries that cannot be readily outsourced is waste management. This is unfortunately not the case with electronic waste, which is sent to the developing world for low-tech metal recovery. The organic fraction of municipal solid waste (i.e., food scraps) will always be produced in urban centers, and although it was long ago determined that throwing this material in the streets is unsanitary, there is still room for movement toward sustainability. Perhaps it is time to reimagine the composting process from an urban, rather than a landfill, perspective. This may be even more important should water scarcity continue. Food waste is the wettest waste stream, with a moisture content of 70%, meaning that more than half of its mass is water. This provides a double motive for composting because not only does water occupy volume in a landfill it eventually wants to leave as leachate (effluent requiring treatment). The high water content also explains the low energy content (up to 3,000 Btu/ lb) as water evaporation consumes a large fraction of the energy value. By comparison, natural gas has an energy content of 20,000 Btu/lb. Although this precludes boiler use, the 160 pounds per person per year do add up. For example, a 2,000 square foot home with three occupants requires 500,000 Btu of heat per year, less than the 1.5 million Btu available in their food waste. The challenge is recovering the heat in a safe and pleasant (read odorless) manner. The primary emission of concern is ammonia, which is produced on the order of 4 kilograms per tonne of organic waste.
So the is energy is there, but how do we remove it? At first it seems a simple problem: a hot pile requiring cooling and a simple pipe buried in the pile circulating a heat transfer fluid—either oil or water—would suffice, provided the solids can flow through the system. This could then be connected to a residential hot water heater or thermal mass (e.g., a concrete block) that could be used to store heat for the home. Alternatively, district heating systems could provide the same benefit with greater efficiency (due to scale) and simplified handling. After treatment, the residues would still require transportation to a curing site before application on the land. Even with a paucity of district heating systems in North America, the presence of apartment buildings suggests some opportunities. One advantage of colocating such a system with a boiler would be the ability to use the off gases in the boiler, thereby eliminating pollutants such as ammonia or volatile organics. Technologies such as moving grate systems deployed in waste-to-energy plants could also assist with material handling at larger facilities.
Food waste can be considered a mixture of the inedible fraction of food (e.g., banana peels and spoiled food) or the fraction of edible food wasted. Often, these foods are imported, either from different parts of the country or the world. Given the high fraction of water (not including that lost during irrigation), it is reasonable to consider food a water export. This may be a heightened concern given the preponderance of droughts associated with climate change, especially in arid regions such as the American Southwest or even densely populated regions like the Northeast, where the water system may be at risk due to industrial activity. As a back-of-the-envelope calculation, if the average household contains three people and uses 100,000 gallons of water a year, then about 1% can be supplied from food waste. Although this is a small fraction, it is worth noting that most household water use is sanitary (i.e., for toilets, showers, washing machines, and faucets) and the 1,000 gallons recovered provides the 64 ounces of recommended daily water intake per person. Furthermore, droughts are not annual (yet), so groundwater recharge could also be considered. Recovering the water is more complicated. The heat required for evaporation can be provided in two ways, internally or externally. Internal heat refers to letting the composting process reach a temperature closer to 100°C. To do so, the pile must move beyond the thermophilic region into the hyperphilic region. Here, biological activity is replaced with chemical reactions and the material begins to heat rapidly. The temperature rise is capped by the evaporation of water, which maintains 100°C until all the water is vaporized. The reactions continue in the absence of water, leading to the “spontaneous combustion” seen in hay bales. Though this is a risk, simply blowing cool air through the bed readily mitigates it. Another method could be to boil off the water using solar thermal heat, a ready resource in the Southwest, or waste heat from boilers. Such a system would be unsuitable for subsequent composting, owing to low moisture content, but there would be significant mass reduction with an increase in the energy content to a level suitable for waste-to-energy facilities.
Another potential resource would be the CO2. As it originates from biomass, it would be considered biogenic CO2 and therefore climate neutral. If it could be captured, then it could displace fossil CO2 in the food industry and be used for dry cleaning or other beneficial uses. It could be injected underground as a means of actively removing CO2 from the atmosphere, although the food waste available would make this a very long-term solution. More interestingly, it could be used as a form of hydrogen storage, as methanol or some other higher alcohol. This is currently being practiced in Iceland, where CO2, water, and electricity are cheap but gasoline is expensive. The CO2 capture could be similar to that proposed for the power sector, although again, scale is an economic issue. Alternatively, oxygen could be produced and used as the energy source for the bacteria, as opposed to air. Much research would be needed to verify this route, but in the struggle against climate change, there is no such thing as waste, and biogenic CO2 will be a valuable commodity.
Posted on: November 7th, 2012