Harnessing Biogas for Energy Savings

For the Santa Cruz Wastewater Treatment Facility (WWTF), dependability matters. Since 1989, this state-of-the-art WWTF, nestled in a shallow canyon in the coastal city of Santa Cruz, Calif., has been leading the way for renewable energy programs and environmental protection in the West.

Santa Cruz WWTF generates its own electric power by using internal combustion engines that burn a mixture of methane gas produced by the sewage digestion process and purchased natural gas, producing 7.4 million kWh of electric power per year.

Historically, WWTFs around the globe have used differential pressure devices for their gas measurements. These devices proved to be expensive to maintain, clogged easily and required a separate pressure and temperature transducer and flow computer to deliver true mass flow. In the late 1980s, Santa Cruz WWTF made the switch from these costly, maintenance-ridden differential pressure devices to thermal technology, which measures true mass flow directly with one instrument and has excellent turndown and accuracy at low flows, as well as a lower cost of ownership.

Valuing responsive customer service and low maintenance costs, in spring 2002, Santa Cruz WWTF decided to change vendors, ultimately choosing Sierra Instruments. By 2007, the WWTF had replaced all of its meters with Sierra’s immersible thermal mass flowmeters.

Capturing Biogas

The cornerstone of Santa Cruz WWTF’s unique renewable energy program is producing abundant, healthy amounts of biogas each day in its digesters. Acting like a large stomach, the solids from the wastewater treatment process are “digested” in four large, domed digesters, heated to 98°F for optimal digester gas production.

When methane-forming bacteria breaks down the acids in the digester, large amounts of anaerobic digester gas made up of 60% methane and 40% CO2 are produced. Facility operators, the “brains” of this stomach, use their SCADA computer system to check that each digester is producing optimal amounts of digester gas. If the digester gas readings are low, the stomach is upset, so the operators will check various process parameters (e.g., pH levels, alkalinity ratios, temperatures and feed rates) to increase the production of digester gas.

“We are not able to make operational adjustments if we don’t have the right accuracy of our gas meters,” said Senior Plant Operator Dave Meyers. “These gas readings are the first indicator of the health of the stomach.”

Because digester gas applications operate at relatively low pressures, it is essential that instrumentation has the ability to remain accurate at these low pressures. Immersible thermal mass flowmeters create virtually no pressure drop and accurately measure these low flow rates, whereas orifice plates could not be used at all. With Sierra’s thermal technology installed, Santa Cruz WWTF can measure true mass flow directly and have outstanding 40:1 rangeability and an accuracy of ±1% of reading, plus 0.5% full scale and repeatability of ±0.2% of full scale.

Fueling Cogeneration System Engines

According to Al Locatelli, co-gen and standby generator specialist, preparing this raw biogas into fuel for internal combustion engines is no simple task, but it is what makes Santa Cruz WWTF unique. Its Waukesha 7042 GLD 820-kW engine, coined Cogeneration 1, burns a mixture of digester gas and purchased natural gas that is mixed with air to lower the BTU levels closer to digester gas. This biogas enters a manifold system, goes through a scrubber to remove the hydrogen sulfide (a harmful greenhouse gas) and enters a chiller unit to remove the moisture from the gas.

Compressed from 7 in. of water column to 3 psi, the gas enters a second carbon scrubber to remove siloxanes, which can harm the engines. In the plant’s static inline mixer, this cleaned and compressed biogas is mixed with the diluted natural gas to maintain the optimal gas blend of 80% digester gas and 20% purchased natural gas. If the mass flow rate of digester gas decreases, more diluted natural gas enters the blend, ensuring that little digester gas has to be flared off.

After passing through a particle filter, this mixed gas is ready to fuel up the Waukesha 7042 GLD engine. To improve engine performance, a high BTU shot of natural gas is injected into the precombustion engine chamber using admission valves to ignite the lean fuel mixture in the cylinders. The thermal meters provide a more efficient way to troubleshoot engine problems and optimize efficiency, according to Locatelli.

By reusing its methane, Santa Cruz WWTF prevents 41 tons of CO2 emissions from polluting our atmosphere. At times, however, especially when Cogeneration 1 is down, the plant must flare off extra biogas that needs to be measured. Mike Sanders, facility operations manager, said that the accuracy of instrumentation is critical not only for process control purposes but also for reporting through regulatory permit requirements to the Monterey Air Board. Soon this will also include the AB32 greenhouse gas requirements—measuring digester gas produced, digester gas burned in the cogeneration engines and digester gas flared to the atmosphere.

Energy Cost Savings

By using the sophisticated SCADA system and the ION meter, which automatically calculates the WWTF’s monthly energy bill, Meyers makes important energy-saving decisions each day. Using about 1.3 MW, the plant cannot run solely on the 820 kW produced by Cogeneration 1.

Between noon and 6 p.m. Monday through Friday from May to October, energy rates jump from 11 cents per kWh to $11 per kWh, which includes on-peak demand charges. To break these peak demand times, at 11:30 a.m. Meyers and other operators fire up their Waukesha 3521 GLD lean-burn engine, coined Cogeneration 2, which produces 440 kW of energy but runs solely on purchased natural gas at 14 cents per kwH.

Meyers and other operators save Santa Cruz WWTF money on energy by choosing to pay for the natural gas to run Cogeneration 2 during peak demand times rather than importing expensive energy. “The bottom line for us is power,” Meyers said. “It’s an energy savings balancing act.”

Solutions in Action

With millions of dollars in instrumentation systems, nothing pleases operators more than to hear nothing about their instrumentation.

“We just want something that works. We need a turnkey solution, as they say. Sierra does that,” Meyers said.

With its cogeneration system, photo voltaic solar panels and heat-recovery system used to heat the digesters, Santa Cruz WWTF is one step ahead of the “green” curve, always striving to optimize energy usage, decrease its carbon footprint and improve energy production and savings with the latest technology.

Scott Rouse is industrial product manager for Sierra Instruments. Rouse can be reached at [email protected] or 800.866.0200.