Turning Sludge into Green
With almost seven billion people on the planet, a very significant fraction of the planet’s organic matter now cycles through the human-centered econosystem. The food we eat, and much of the food we waste, goes through our toilets and garbage disposals into our sewers, and (hopefully) into a wastewater treatment plant. If that plant performs its function well, relatively clean reclaimed water is reintroduced to our natural water systems. What remains is a thickened sludge that contains all of the organic materials. How we dispose of this sludge is an important ecological and economic decision.
In the United States, over half of our sewage sludge is either composted or pelletized and then applied to our national farmlands. This closed cycle is much to be desired, once concerns about heavy metal, pharmaceutical and other chemical residuals are properly addressed. Other choices for disposal include landfills, land reclamation, or incineration.
Incineration utilizes large quantities of natural gas to burn the sludge. Essentially 100% of the carbon contained in the organics is released as CO2 into the atmosphere. Because this carbon is biogenic in origin, it is not counted as human-caused GHG emissions. The substantial CO2 emissions from the natural gas used to incinerate the sludge, however, do contribute to increased GHG concentrations in the atmosphere.
What remains after incineration is a quantity of ash, which contains all the non-combustible elements of the sludge. Most of this is inert silicates and other elements that help make up the bodies of plants and animals. But trace heavy metals will be present as well, often in concentrations that make the ash hazardous material that must be disposed of properly in a landfill. If concentrations are lower, the ash may be used in the production of concrete or other permanent materials.
From the point of view of removing potential toxins from the environment, incineration is a very effective technology – presuming proper incineration where only low levels of toxins are released in the combustion smokestack emissions. From ecosystem and econosystem perspectives, however, much value is lost in incineration. Rather than return nutrients and essential trace minerals to the soil, we break the cycle and sequester them. Our farmlands are supplemented instead with fossil-fuel manufactured fertilizers, and are gradually leached of the trace elements necessary for healthy plants and nutritious food. We are using fossil fuels to incinerate our biosolids, and then using more fossil fuels to make fertilizer. This is not a sustainable solution.
Dumping sewage sludge into a landfill also breaks the natural biological cycle and sequesters the organic and inorganic elements away from the ecosystem. Decomposition of this sludge in the landfill will also generate large amounts of methane gas, much of which will escape to the atmosphere before any methane recovery system can be put into place. The increasing expense of landfills and transportation also makes landfills a poor choice for organics disposal.
Higher population densities combined with a more environmentally aware populace has made Europe the leader in innovative methods for environmentally and economically effective organics recycling. For several decades, European countries have been rapidly adopting variations of a technology known as “anaerobic digestion” to treat their wastewater sludge, increasingly combined with source-separated yard waste and the organic fraction of municipal solid waste.
Although the term “anaerobic digestion” may not be familiar to many people, all of us are intimately familiar with the process because we carry around a similar facility inside our bodies. A wide range of bacterial microbes in the oxygen deprived (anaerobic) interior of our digestive tracts break down the complex organics of the food we eat, and feed our bodies as well as theirs with the nutrients. They do not do a perfect job, however, and our feces still contains abundant, energy rich organic material.
A municipal anaerobic digestion facility utilizes a different set of bacterial microbes that live at much higher temperatures than those in our bodies. Those microbes thrive on our sewage, foodwaste, compostable paper and yard trimmings, generating biogas rich in methane (natural gas), as well as an end product of high-value compost.
The latest generation of “dry” anaerobic digestion facilities are able to process the full range of municipal organics and are most favored for new facilities, delivering two strong benefits to the econosystem: biogas that has value as a “green” substitute for fossil natural gas, and high quality compost that restores soil health and productivity. The relatively high temperature microbial digestion process destroys all pathogens and most complex organics. Trace heavy metals not removed in the wastewater treatment process will remain in the finished compost, but concentrations are much lower than in the highly concentrated ash from incineration.
The EPA has set standards for heavy metals concentrations in compost, but in Europe anaerobic digestion facilities are able to meet far more stringent standards for compost from anaerobic digestion, allowing the use of anaerobic digestion compost on food crops. Standards for copper and chromium, for example, are up to ten times more restrictive than the EPA standard. Concentrations of heavy metals and toxic molecules will of course also depend on levels in the original wastewater, and testing of compost should always be performed regularly. By Europe’s example, however, there does not seem to be a residual toxins risk that outweighs the environmental benefits of recycling our organic wastes.
Water extracted from the anaerobic digestate prior to final aerobic composting is returned to the wastewater treatment plant for settling, oxidization, and testing before being released into the natural watershed. Again, Europe’s experience suggests that this digester effluent does not negatively affect the quality of water ultimately released to the natural watershed.
Combining yard clippings, compostable paper, and foodwaste with biosolids significantly increases biogas production and can improve the quality of finished compost as well, by improving the balance of carbon to nitrogen inside the digester. However, some facilities allow for two simultaneous digestion processes, one with and one without sewage biosolids. This may diminish the actual biological value of both compost products, but a “no sewage” product may have greater economic value as an “organic” compost with essentially no trace contaminants.
Advanced anaerobic digestion is the best technology available for cost-effectively and environmentally recycling municipal organic wastes back into the ecosystem and returning value to the econosystem. It has not been deployed in the United States simply because until recently our economic calculations have not incorporated environmental costs and benefits. When the values of green energy, GHG reductions and organics recycling are added into the equation, anaerobic digesters are the “natural” solution.