Flowmeters improve biogas production process

Thermal mass flowmeters from Sierra Instruments are delivering critical readings at a Santa Cruz wastewater treatment facility in California.

These readings indicate exact digestive status while controlling the final step of precisely blending the cleaned and compressed biogas with natural gas.

Santa Cruz Wastewater Treatment 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, wastewater treatment facilities 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.

Valuing responsive customer service and low maintenance costs, Santa Cruz Wastewater decided to change vendors in the spring of 2002 and chose Sierra Instruments.

As a result of improved performance, all of its meters were replaced with Sierra's immersible thermal mass flowmeters by 2007.

Producing healthy amounts of biogas each day in digesters is part of Santa Cruz Wastewater Treatment's renewable energy programme.

Acting like a large stomach, the solids from the wastewater treatment process are 'digested' in four large domed digesters, and then heated to 98deg 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 per cent methane and 40 per cent CO2, are produced.

Facility operators use a supervisory control and data acquisition (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 such as pH levels, alkalinity ratios, temperatures and feed rates to increase the production of digester gas.

Since digester gas applications operate at relatively low pressures, it's essential that instrumentation has the ability to remain accurate in those conditions, a capability that orifice plates lack.

Immersible thermal mass flowmeters create virtually no pressure drop and accurately measure these low flow rates.

With Sierra's thermal technology installed, Santa Cruz Wastewater can measure true mass flow directly, with 40:1 rangeability, an accuracy of +/-1 per cent of reading, plus 0.5 per cent full scale and repeatability of +/-0.2 per cent of full scale.

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