Heavy metal from my era has inundated TV commercials over the past few years. Classic rock and roll by Led Zeppelin, ACDC, and Judas Priest is the background music for selling cars and cologne, but heavy metal is not so popular when it comes to organics management.
I recently joined in a Facebook conversation in the Solar Cities group regarding food waste recovery using anaerobic digestion at the Reedy Creek Wastewater Treatment Plant in Orlando. It specifically focused on the concept of codigestion. Concern arose over mixing food waste with sewage sludge – it was thought that the value of food scraps was lost due to contamination by or with heavy metals.
Heavy metals are found in the soil, and are also present in food. Arsenic is found in apples and Mercury is found in fish. (In fact, some of us even take vitamins with heavy metal ingredients.) But did you know compost has heavy metals too? It should be no surprise that both biosolids and compost are regulated to control the levels of heavy metals. Composting is not the only way to return carbon and nutrients to the soil – land application of residuals from wastewater treatment digesters (biosolids) is also a viable means of recycling carbon and nutrients.
Reluctance to see food waste codigestion as resource recovery is throwing the baby out with the bathwater or, put another way — “throwing out the baby to save the bathwater.” Environmentally conscientious people should embrace the concept of recycling both food scraps and biosolids together because both are resources and neither should be discarded.
My last post discussed the use of disposers to boost biogas at wastewater treatment plants. One of the reactions I heard was, “Not many plants use anaerobic digestion.” So what is the current infrastructure framework in the U.S.?
Anaerobic Digestion Treatment Plants Doing Major Work
According to the 2008 Clean Watersheds Needs Survey conducted by the Environmental Protection Agency (EPA), 15,618 wastewater treatment plants in the U.S. serve 284 million people. A new database of these plants indicates over 1200 utilize anaerobic digestion (AD). While this may seem like a small number, these plants are treating a large portion of the U.S. wastewater flows. In other words, a few plants are doing most of the work, not dissimilar to the Pareto principle in which 20% of the people do 80% of the work.
When you consider that most of these plants provide treatment for large municipalities, it’s easy to understand why a small group of plants use AD – it’s where most of the sewage is produced! AD is the most efficient process to reduce the volume of biosolids produced, which at the largest plants is massive. But smaller plants are able to handle their equally smaller amount of solids even though it’s at the expense of more energy.
Biogas=Energy. Just, Not Everywhere.
But the greater question is why many other large plants don’t use the biogas produced to make energy.
The Water Environment Research Foundation (WERF) answered that question in a report last year entitled, “Barriers to Biogas Use for Renewable Energy.” In a nutshell, the conclusion was that it’s very expensive to add cogeneration (combined heat and power) equipment and the return on investment can take longer than officials are ready to tolerate. Even so, simple return on investment shouldn’t be the only rationale for whether or not to make and use energy from anaerobic digestion. WERF also recently published “Reframing the Economics of Combined Heat and Power Projects” to help officials consider other metrics for the business case of cogeneration.
Tackling the Expense of Co-Gen
One way of improving the economics is by utilizing co-digestion feedstocks to boost biogas production. Co-digestion is the process by which food waste and other substrates are added to sewage sludge and manure to improve the digestion process. Cities like Oakland and Des Moines are already net energy producers from utilizing co-digestion, and many more, like Philadelphia and Madison, WI may soon follow. Sheboygan, WI has nearly reached the self-sufficiency goal set for its plant. In talking to folks like Dale Doerr, Plant Superintendent for the Sheboygan Regional Wastewater Treatment Plant and a forward-thinking wastewater professional, short-term payback shouldn’t be the deciding factor in whether or not to invest in cogeneration. His plant will be there for decades to come, he said, and investing in equipment to utilize renewable energy is just the right thing to do. The “unwritten policy” at Sheboygan WWTP is to consider energy use whenever an equipment or process upgrade is needed. The plant now uses 20% less energy compared to a baseline figure in 2003 and ranges from 70-90% energy self-sufficiency due to the energy produced on-site.
Co-digestion and Disposers
While co-digestion normally refers to direct addition of substrates to digesters, food waste disposers accomplish essentially the same thing ‒ most of the solids they grind, though tiny, are removed during primary clarification and deposited in the digesters. An aid to digester health and co-digestion, wouldn’t you agree?
Cooperative Co-digestion. Brewery waste has been found to aid digesters; it’s an excellent feedstock. This humorous old ad for Oshkosh Beer claimed beer was an excellent aid to human digestion too. Who knew. (From the oshkosh beer blog)