Quantalux Blog

News, events, observations, industrial and otherwise, from a Quantalux point-of-view.

Energy at Wastewater Treatment Plants - A Short Primer

On the topic of wastewater, most people are understandably of the “flush and forget” variety. But today’s wastewaster professionals recognize that in the long term, our approach to sewage treatment needs to be less about “processing” wastewater, and more about “recovery” of  limited resources such as water and energy from the incoming waste. Ultimately, the goal is to transform wastewater treatment plants into “Utilities of the Future”, where advanced technology is used to recover energy, water and nutrients from wastewater. (1)

The need for energy recovery is particularly relevant to wastewater facilities since electricity use at wastewater treatment plants accounts for 25–40 percent of a facility’s operating budget (2). Innovative ideas on energy recovery can be implemented even today at those facilities that use a process called anaerobic digestion (AD) to process sewage sludge.  A key advantage of AD  is the production of biogas as the sewage is digested, and many treatment plants already use their biogas to generate renewable electricity or process heat. A recent analysis by Quantalux asked the question:

How can a WWTP maximize the value of their anaerobic digester?

The answer to this question has two parts:

First, the plant should identify additional organic waste (called “co-feedstocks”) to process in the anaerobic digester in order to increase biogas production.  As the figure on the right shows, co-feedstocks such as food waste, FOG (fats, oils and greases), sugar water and dairy waste contain substantially more bioenergy content than sewage.  In this case, a 8% food waste fraction (by mass) results in 23% of the total biogas production.

Co-feedstock processing can –and does — make financial sense. For example, the wastewater plant in Flint, Michigan has an active program to accept food waste from local farms and businesses. They not only produce far more biogas than from sludge alone (at least 50% more), but the facility also earns money from the tipping fees to accept the co-feedstocks.

Second, a WWTP can implement an energy storage strategy that reserves energy when electricity is cheap (off-peak periods) and release the energy when electricity is expensive (on-peak periods.)  This strategy is especially valuable when a WWTP is paying so-called “demand charges” to the local utility in addition to their consumption charges.  If the WWTP generates their own electricity during peak periods to offset electricity from the local utility, the facility gets twice the benefit: it pays less for electricity during peak periods, and also decreases the peak electrical demand, thereby lowering the demand charge for subsequent months.

This strategy (sometimes referred to as peak shifting) is certainly not new, but is especially appealing since a wastewater facility sees a predictable variation in the sewage flows into the plant. People in a city get up in the morning, follow their morning routine, and after a few hours, the water from showers, baths, toilets, etc. arrives at the treatment plant.  Quantalux analyzed data from the Flint Michigan waste treatment plant and found that the daily inflows were remarkably consistent. Certainly there will be times when the plant needs to process a surge in the inflow, but in general, an energy storage strategy is a natural fit for a wastewater treatment facility.

Deploying biogas storage is the first step in this strategy. Biogas is produced 24/7, and a portion of the nighttime biogas production can be held in storage bags, and used to produce electricity during the on-peak window (typically 11AM to 7PM). Biogas storage bags are relatively inexpensive, so the cost/BTU for this storage is modest. In fact, our analysis shows that only 4 hours of storage is needed since the maximum electrical consumption only occurs during the first portion of the on-peak window. However, a study that trades-off biogas storage volume, electrical generator capacity and on-peak/off-peak electrical rates (and demand charges) is essential.

In addition to biogas storage, there are a number of other ways that a facility can implement storage. As grid-scale batteries drop in price, a hybrid biogas/battery approach becomes more cost-effective. In fact, batteries and biogas are complementary in the sense that biogas is well-matched to equipment with constant electrical demand, whereas batteries are the ideal electrical source for equipment with periodic peak current demand. Furthermore, large-scale capacitors (another energy storage device) can be installed at the plant to correct the power factor, further saving the plant in their electrical charges. Finally, the plant can change their operational plan to save energy by storing the sewage that arrives during the day in buffer tanks or tunnels, and then processing the material during the nighttime when electricity is cheaper. For plants with excess capacity (such as WWTPs in the upper Midwest), this approach can be readily implemented

Energy storage strategies can result in substantial savings for a waste treatment plant, and in the case of operational storage, can be achieved with little capital investment.  Best of all, facilities can start to save money on their energy costs right away using established and reliable storage solutions.

Interested in more data on this topic? Contact us at info@quantalux.com and we’ll be happy to chat.

(1) For a comprehensive look at the future of wastewater, download the Blueprint for Action, Water Resources in the Utility of the Future. See https://www.wef.org/globalassets/assets-wef/direct-download-library/public/03—resources/waterresourcesutilityofthefuture_blueprintforaction_final.pdf

(2) Energy Efficiency in Water and Wastewater Facilities – A Guide to Developing and Implementing
Greenhouse Gas Reduction Programs.  US EPA, 2013, See https://www.epa.gov/sites/production/files/2015-08/documents/wastewater-guide.pdf

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