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.
Those involved with wastewater treatment are aware of the challenges faced through ever-tightening regulations regarding nutrient discharge to waterways. The Chesapeake Bay and Lake Erie are two examples of waterbodies having received media attention for issues such as hypoxia (low oxygen levels) which is related to nutrients. Even though experts agree that 60-75% of the nutrients come from agriculture, wastewater treatment has been identified as the solution provider to this sizable problem. In some cases, limits imposed on municipal wastewater treatment plants are lower than what the available amelioration technology can achieve. And it’s not uncommon to expect virtually unlimited investment in the search for solutions.
But most lay people are unaware of nutrient issues, also known as eutrophication, let alone the efforts underway to address them, such as water quality trading and adaptive management. Fewer people, including wastewater treatment professionals are unaware, as I shared three years ago, using disposers can reduce nutrients discharged from treatment plants.
Water quality trading and adaptive management
What is water quality trading and adaptive management? Both are strategies instituted by regulators to help municipalities meet compliance with low nutrient discharge limits. According to the Wisconsin DNR, water quality trading is used to offset treatment plant discharges, to comply with permit limits. It pertains to reducing nutrients from other sources in the watershed instead of reducing the effluent discharge. Adaptive management, on the other hand, focuses on achieving water quality criterion (acceptable background levels) and emphasizes overall reduction. How much success either offers depends on the measurement used to gauge them.
Both are being forced on municipal treatment plants in an effort to reduce nutrients coming from agriculture. In fact, most of the water quality trading and adaptive management strategies are specifically employed in the agriculture sector and yet – they are paid for by municipalities.
Image: World Resources Institute
Food production thanks to fertilizer
U.S. farmers are tremendously successful at food production largely due to fertilizer use (nutrients) which, by the way is a limited resource (see Phantastic Phosphorus). This overproduction via fertilizer ends up polluting waterways. Then the wastewater sector has to pay for remediation. We all need to understand the price for the luxurious grocery store selection we expect. This abundance is one of the reasons rates are increased by the treatment plant. Unfortunately, we live in a society that largely takes food availability and its consequences for granted. So what is to be done? It’s a big problem requiring big solutions, right? Maybe not.
Starting small instead of big
Some argue that small-scale steps taken by small agriculture can be more productive than grand initiatives and costly treatment plant investments. This makes sense, especially when the cost of remediation at the treatment plant using the “best available technology” is much more expensive and yields a much lesser reduction – a concept explained by Maria Laukkanen and Anni Huhtala in their paper, “Optimal Control of Nutrient Pollution in a Coastal Ecosystem: Agricultural Abatement vs. Investment by Wastewater Treatment Capacity.” However, I still struggle with placing the burden of managing the reduction of nutrients on one industry, when a majority of the problem comes from another.
Sharing my passion for disposer usage to divert food waste from landfills invariably generates questions about what can and can’t put down a disposer – even in the professional settings where I make presentations. Last week the questions were about onion skins from one person, and cantaloupe rinds from another. Of course both can go down a disposer – easy as can be, I told them. As long as you use it in the right way.
But over time I’ve learned that people do not use their disposer as much or as effortlessly as they could. They mainly use the appliance to clean up, and anxiety over potential plumbing problems leads them to follow the old adage, “better safe than sorry.”
I believe this is either from experiencing problems firsthand or from hearing about them after an episode of improper disposer use. All too often, people use one in a way that is completely opposite to instructions.
Some people fill up the disposer chamber with food scraps, start the unit, and then turn on the water. This sends a “slug” of food into the plumbing trap all at once. Others don’t use any water at all. Either way is using the appliance improperly and invites issues.
The steps to proper use are few — turning the water on, then the disposer, and then gradually adding food scraps into the disposer, grinding them until nothing is left in the chamber. You can tell by the sound when grinding is complete. Then make sure to keep the water on for a few additional seconds to carry the food waste through the trap and internal plumbing. Simple.
But many people are even more expert at ignoring or dismissing instructions.
During a recent series of tests in five cities we provided pretty extensive education and use instructions to participants getting them accustomed to using their new appliances in the right way. Through direct contact we learned that about 30% of household food waste gets processed in disposers. While this is substantial, there is potential for disposers to keep much more out of landfills. And it’s easy to do successfully when following directions.