Mixing Systems Increase Digester Gas Production in Moreno Valley, Calif.

The Moreno Valley Regional Water Reclamation Facility (WRF) in southern California uses four 48-ft-diameter, 22-ft sidewater depth digesters, with a total active volume of about 1.2 million gal.

In 2001, the gas mixing systems were replaced with the Vaughan Rotamix mixing system. Each mixing system consists of a Vaughan Chopper Pump and four Rotamix nozzle assemblies sized and oriented to provide a dual-zone rotational mixing pattern. These pumps provide the flow necessary to give 90% complete mixing in 30 minutes—Vaughan's primary design criterion.

At this facility, digester gas production is a reliable measure of digester performance. A high-pressure digester gas storage sphere is utilized to allow the engine-blowers to operate a portion of the day fully loaded on digester gas.

Before the digester mixing project was installed, the digester gas production was sufficient only to operate the engine-blowers for about 12 hours per day. Therefore, any increase in digester gas production means less natural gas purchases. On average, the measured digester gas production increased about 80% with the new mixing system. At an average cost of $0.50/therm for natural gas, the increase in digester gas production saves about $60,000 per year in natural gas purchases. This also reduces the operational cost of the dewatering facility, as well as the hauling and disposal costs.

In an effort to minimize electrical energy consumption, each of the newly installed Rotamix systems at Moreno Valley are operated only when the digesters are being fed and for a short period afterwards. On average, about 1.5 of the four mixing systems are continuously operating. The measured gas production data indicates the digesters are performing well with this mixing schedule. The net reduction in energy consumption due to pump mixing is about 100 kW, which at $0.15/kWh is a savings of about $130,000 per year.

Besides improving digester performance, the project resulted in some significant, easily quantified, operating cost savings due to reduced electrical and natural gas energy consumption. Together these savings amount to about $200,000 per year. With a total installed capital cost of $400,000, the simple payback period is two years.

The project summary and conclusions are:

• • The overall average increase in measured digester gas production, seen after installation of the external pump mixing system and flow nozzles, is about 80 percent.
• • Operation of the pump mixing systems only during and just after digester feeding appears to be sufficient and significantly reduces electrical energy consumption.
• • The increase in digester gas production and reduction in mixing energy creates sufficient cost savings to give the project a two-year simple payback period.