Can WWTPs go from Energy Drain to Energy Gain?
As many of us know, the wastewater treatment process uses a lot of energy, and with increasing economic pressures on wastewater utilities, facility managers are looking for high strength waste to help them become energy neutral — even net energy producers. The creation of energy would not only provide relief from normal operational expenses, it could help demonstrate sustainability to the surrounding communities.
One such utility, Milwaukee Metropolitan Sewerage District, cleans billions of gallons of wastewater every year at two reclamation facilities that serve 1.1 million people in 28 communities. MMSD wants to achieve energy neutrality by 2030 and they see food waste as an integral part of it.
This isn’t another fantasy. In the first quarter of 2016, MMSD received over 200,000 gallons of high strength food waste slurry at one of their two facilities.
The slurry is virtually contaminant-free and readily biodegradable, which enables water resource recovery facilities like MMSD to make more biogas to produce onsite energy. As a result, they are able to help offset operational expenses by reducing their dependence on purchased electricity.
The slurry is high in solids compared to sludge pumped into municipal digesters – about 10-12% TS. But it is pumpable and contains very high volatile solids — over 90% on average. The high COD content and readily biodegradable composition is a result of the food waste being held in a storage tank at the waste generator for 7-10 days.
Plants say, “Keep it coming”
The treatment plants already accepting food waste slurry say, ‘We want more of this!’ It has very high volatile solids content (more than 90 percent) and it’s rapidly biodegradable. The methane potential testing we’ve done in-house shows that the organics are converted to methane in five to 10 days. It doesn’t take the normal 20- to 30-day detention time that a digester requires to process wastewater sludge.
If you’re thinking it’s too good to be true, you should take a look at this video. Feel free to drop me a note if you have questions or need convincing you’re not dreaming.
A certain person I know well claims that, for his psychological well-being, an occasional fine cigar is beneficial. That determination is based on weighted values known only to him, and arguably with willful and sublime ignorance of the values held by, say, his wife.
Spouses aside, individuals are pretty much free to deem whether something is beneficial to them or not and take it from there. But in public policy, others get to decide what is beneficial for the rest of us.
This post is about the “beneficial reuse” of biosolids, which got me thinking about who declared their reuse to be so, and how, where, and when it’s considered a good thing.
How biosolids come to be. Wastewater treatment relies on both physical and biological processes to clean water. Primary clarifiers physically remove most of the organic materials simply through settling them out as primary sludge. The remaining soluble organic matter that passes on to the aeration tanks is biologically converted by bacteria. The bacteria multiply, so to keep their population in check, a portion of them is “wasted” out of the system to digesters along with primary sludge. Sludge digestion is either done aerobically or anaerobically (some sludges are incinerated), but under the EPA 503 Rule, the end product becomes “biosolids.” Read more
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)