Their main business may be cleaning up water for drinking and other human purposes. But today, some water and wastewater treatment facilities are also turning themselves into small-scale power plants. The idea is to use processes that take place at these facilities to generate electricity, which, in turn, helps to power their operations, thereby cutting overall energy costs.

One technology available to make this possible is anaerobic digestion. In an anaerobic digestion process, microorganisms break down the organic material in wastewater in an oxygen-free environment. The process yields “biogas” consisting mainly of methane and carbon dioxide. The methane can be combusted to produce electricity as well as compressed natural gas (CNG).

Anaerobic digestion is not new, but companies such as Cleveland-based Quasar Energy Group are trying to move the technology forward. Quasar’s anaerobic digestion R&D is a collaborative effort with the Ohio Agricultural Research and Development Center at Ohio State University. The company is offering technology that it hopes will appeal to the thousands of older municipal wastewaster treatment plants in the U.S. that are in need of upgrades.

Net Energy Producer

In Quasar’s anaerobic digestion system, microorganisms are fed organic waste from wastewater and other sources. The digestion process converts the waste into biogas that consists of about 60% methane and 40% carbon dioxide, according to Clemens Halene, Quasar’s chief operating officer.

A digester tank used to produce biogas. Source: Quasar EnergyA digester tank used to produce biogas. Source: Quasar EnergyThe biogas, which has about 60% of the heating value of natural gas, can be used to power a natural-gas engine modified to run on low-BTU gas, says Halene, an engineer who designs Quasar’s equipment. That gas-powered engine drives a generator that produces electricity.

In addition Halene says, the carbon dioxide in the biogas can be separated from the methane to produce a natural gas equivalent. This can be stored in a pipeline and withdrawn when needed to supply fueling stations for vehicles that run on CNG.

Halene estimates that the energy needed to run a plant using the company’s anaerobic digestion technology is around 10% of what the plant can produce, leaving a lot of excess power that can be used for other purposes. So what's the potential impact of the technology, should it be widely adopted? "About 3% of the electricity produced in the U.S. is used to power wastewater treatment plants," Halene says. “We could cover that and also generate another 3%.”

On the downside, gas from anaerobic digestion usually isn’t very clean. Halene says that hydrogen sulfide in the gas can double or triple maintenance costs for a combustion engine or turbine. To address that problem, Quasar developed what Halene claims is “a very inexpensive way of scrubbing the gas,” which eliminates the extra maintenance costs.

Quasar’s waste-to-energy technology is considered environmentally friendly, which potential customers like — as long as they don’t have to pay more for it. “Even though you’re green, you can’t be more expensive,” Halene says. So for the last eight years, his company has been working to make anaerobic digestion more affordable. Today, he claims, capex and operating costs associated with the Quasar system are lower than those of other wastewater treatment options.

Currently, Quasar has 16 anaerobic digestion projects under way. One is operating a facility for a wastewater treatment plant in Wooster, Ohio.
Kevin Givins, utilities manager for the plant, says that the plant’s electric bill for August 2013 was almost $31,000, compared with a little over $300 one year later, after the digester system was installed.

The facility has been generating surplus electricity, which will soon be sent to a nearby municipal water plant. “On a typical day, we should be able to produce enough power to cover the energy needs of both plants,” Givins says.

Rather than buy Quasar’s equipment, Wooster is paying the company a monthly fee to operate and maintain its anaerobic digestion facility. Quasar is bringing third-party waste to the facility and charging fees for taking that waste, called a "tipping fee." According to Givins, the extra waste is needed because municipal waste doesn’t contain enough organic material to produce a sufficient amount of biogas to power a treatment plant.

“In the six years we operated our own cogeneration system before Quasar got involved, I don’t think we made enough money to even pay for the maintenance we had to do on that equipment,” he says. “You need high-strength waste from somewhere to produce enough methane to really make the process worthwhile.”

Modular System for Field Use

Another treatment option based on anaerobic digestion comes from an MIT spinoff called Cambrian Innovation. Designed for wineries, breweries and other food and beverage producers, the Boston firm’s EcoVolt system uses electrically active organisms to produce both electricity and heat while treating wastewater.

The anaerobic treatment that takes place in the EcoVolt reactor is enhanced by a process called electromethanogenesis. In this process, electrodes are coated with electrically active microbes that convert organic wastewater pollutants into electricity. The electricity then is sent to other electrodes coated with different microorganisms, which convert the electricity, ambient protons and carbon dioxide into methane.

Gas exiting the treatment units is sent to a cogeneration system that converts it to electricity and heat for use onsite. The treated wastewater exits the reactor with 80-90% of the pollutants removed, Cambrian claims, allowing it to be reused in plant processes such as cleaning and heating.

Designed for road shipment, the standard EcoVolt product is a prefabricated modular system that can handle wastewater flows of up to 300,000 gallons per day and produce up to 400 kW of power, according to Cambrian. For those buying the system (Cambrian also offers leasing and other financing options), the payback period is typically just a few years, says Justin Buck, the firm’s chief technology officer.

As with the Quasar system, the EcoVolt must be designed to meet the challenges of combusting biogas. According to Buck, these challenges include dealing with hydrogen sulfide, which is almost always a major issue, and siloxane, which can be a problem depending on the process and materials involved. A corrosive gas, hydrogen sulfide can damage piping, while siloxane can turn into small particles that are harmful to microturbines and reciprocating engines.

Different strategies can be used to handle these troublesome substances. One is to select components and materials that are more resistant to them. Another, Buck says, is to clean the biogas with a conditioning unit, which is usually the preferred approach for dealing with siloxane.

Cambrian launched the EcoVolt in 2013. The company publicly discusses two EcoVolt projects, both involving breweries. One is Lagunitas Brewing Co., a craft brewery in Petaluma, Calif. Lagunitas transports more than 50,000 gallons of wastewater per day to the local treatment facility, requiring over 3,000 truck shipments a year.

To eliminate these shipments, Lagunitas opted for an EcoVolt system designed to treat 20,000 gallons of wastewater per day to meet the company’s sewer discharge requirements. (Expandable units will allow the capacity to increase to 180,000 gallons per day.) In addition, the system can produce 130 kW of renewable electricity enabling it to meet up to 20% of the site’s baseload electricity needs, Cambrian says.

In-pipe Pressure

Anaerobic digestion isn’t the whole story when it comes to treatment-plant power generation. One option currently on the market taps the power of in-pipe pressure. Excess pressure builds up in pipes when water is stored at elevations that are higher than those who use it. Drinking water processors normally install pressure-reduction valves (PRVs) to relieve this pressure, producing heat in the process.

Instead of waste heat, however, this pressure can be turned into clean electricity. A New York City firm called Rentricity Inc. offers what it calls in-pipe hydrokinetic power-generation systems. The systems use pumps that operate in reverse to capture pressure normally dissipated by PRVs in drinking water systems. The pressure of the gravity-fed flow of water drives an impeller that powers a generator that produces an electric field, says Frank Zammataro, Rentricity’s president.

Unlike solar and wind power systems, in-pipe hydro technology must be added to a plant’s supervisory control and data acquisition system and monitored on a regular basis. On the other hand, Zammataro says that in-pipe hydro will produce electricity a much higher percentage of the time than systems that depend on the sun shining or the wind blowing.

Zammataro says users will probably get “a decent payback” from in-pipe hydro if their wholesale electricity cost is above $0.06 a kilowatt hour. Payback periods may be anywhere from 5-10 years, he says.

In its search for customers, Rentricity has been targeting domestic drinking water pipelines because of the higher pressures in those system. So far, the company’s technology has been installed at five facilities. One is a water treatment plant in Keene, N.H. This facility has a gravity-fed water system with a PRV that reduces pressure in a line descending from a water-storage reservoir. A pair of Rentricity turbine generators operate in parallel with the PRV, generating 62 kW of electricity, according to Zammataro.

The systems, installed at the inlet of the facility, have made the plant energy neutral, Zammataro says. “We believe this might be the first energy-neutral water treatment plant in the world powered by its own inflow of gravity-fed water.”

If results like this can be achieved at a reasonable cost, more water and wastewater facilities will no doubt be looking for ways to turn their processes into power.