BioCycle February 2005, Vol. 46, No. 2, p. 30
The Sacramento Municipal Utility District – the nation’s sixth largest – develops effective ways to turn biomass from green waste collections, manure digesters and bioreactors into clean power.

CALIFORNIA’S Sacramento Municipal Utility District (SMUD) is the sixth largest municipal utility in the nation and has long been known for its progressive outlook on renewable energy. SMUD’s Solar Program has resulted in almost 10 megawatts of renewable solar power from over 900 residences, businesses and utility scale photovoltaic (PV) installations supported by the utility. Besides being blessed with an abundance of sunshine, California is also blessed with another resource in large supply – biomass. Because of its diverse geography, California is the largest food producer state in the nation. Even though biomass is one of the oldest forms of energy, it has once again gained importance in the recent search for renewable energy in California.
PART OF A PORTFOLIO
California generates the majority of its electricity from natural gas and large hydropower projects. Legislation in 2002 obligated California’s investor owned utilities to meet a Renewable Portfolio Standard (RPS) acquiring 20 percent of their electricity from (non-large hydropower) renewable fuels by 2017, while California’s municipal utilities develop their own plans voluntarily. SMUD adopted a goal to achieve 10 percent renewable power by 2006 and 20 percent by 2010. That’s over 2,400 gigawatt-hours of electricity annually. SMUD purchases rather than generates much of its energy, so to achieve RPS goals, SMUD issued a Request for Offer for a combination of conventional and emerging renewable technologies in 2004. This resulted in numerous proposals for wind, solar, geothermal, small hydropower and 15 proposals for various types of biomass energy. While SMUD is negotiating purchase of many of these renewable resources, mostly located outside of its boundaries, it is also working to develop several local biomass to energy projects.
One of the core values adopted by SMUD’s governing board is to provide local benefits to customer/owners while maintaining the lowest utility rates possible. Biomass offers some unique advantages that can help meet those goals:
Local biomass projects often serve a dual purpose by turning local wastes and residues into energy. This not only reduces waste locally, but also supplants use of fossil fuels.
Environmental benefits of turning a waste stream into usable products can be numerous, from preventing contamination of land and water, to reducing air emissions from the waste itself or from transporting the waste. Renewable energy projects often provide the financial motivation to collect and channel the waste into a better use.
Costs of biomass projects are often borne by several sources, making it relatively low cost electricity. For instance, revenues from tipping fees, pollution cleanup fees, or from the sale of products made from the residue, such as compost, can offset the cost of conversion to electricity.
IT ALL ADDS UP
SMUD launched a Regional Biomass Fuels Inventory for Sacramento plus seven contiguous counties to assess just what local biomass resources were available. “We look at the size and sustainability of the fuel source and at current practices for collection or disposal, with a goal of matching high quality supplies of fuel with best available conversion technologies,” says Ruth MacDougall, project manager heading the biomass program. “We want to be able to estimate their energy potential, costs and environmental impact so we can make cost / benefit comparisons.”
The inventory is conducted by way of phone and email surveys. It’s a challenging project because of the enormous scope of biomass resources and the difficulty getting much of the information. Waste producers are often reluctant to divulge the details of their operations, but the inventory is already revealing opportunities that could turn into projects in the future.
TECHNOLOGY MATCHING
In parallel with the Regional Biomass Fuels Inventory, an assessment of Biomass Conversion Technologies is underway by the University of California at Davis (UC Davis). A research team has evaluated technologies capable of converting biomass to electricity through various methods: thermal gasification, biological digestion, pyrolysis, and combustion. Key factors they look for are: conversion efficiency, environmental performance including air emissions, public acceptance, and flexibility to use different types of feedstock. Commercial factors are also considered, such as: are they mature technologies, what number of systems are installed, how many years operating, are they coming down on the cost curve.
Overall, the potential for biomass to energy in Sacramento is mostly in municipal solid wastes (MSW), due to its largely urban environment. Sixteen percent of the energy potential in MSW is being captured in landfill gas to energy projects but 46 percent of the energy potential is in MSW that is “diverted,” and potentially could produce energy.
In addition, the Sacramento region collects about 300,000 tons per year of garden refuse or “green waste”. Sacramento Waste Authority (SWA) is actively seeking to site a 100,000 annual ton composting facility. SMUD is investigating the potential for composting green waste in an anaerobic digester, a technology that can produce high value compost, and also generate biogas that can be converted to renewable electricity.
To explore this opportunity further, SMUD hired RIS International to evaluate mature anaerobic digestion technologies and examine commercial feasibility using Sacramento costs and fees. Over 70 anaerobic digesters are in operation in Europe converting over 12 percent of their municipal solid waste to biogas. Most facilities have been operating for five to 10 years. Europe has unique economic conditions that make anaerobic digestion very cost-effective, such as high tipping fees (as high as $90/ton) and high prices for renewable energy (as high as 15 cent/kilowatt-hour). The U.S. is seeing upward pressure on the cost of standard waste disposal practices due to rising fuel costs and dwindling landfills, which may create more favorable economic conditions. Other factors may also create an atmosphere of cooperation that could support a demonstration project in Sacramento.
PARTNERING WITH PROJECT OWNERS
SMUD sees great value in partnerships and is partnering with several biomass facilities. For example, SMUD is helping local dairy farms implement anaerobic digester systems that turn manure into renewable energy. In a pilot program last year, SMUD offered dairies incentives and help with the applications to USDA for cost share on their digester projects.
Three local dairies were awarded funding from USDA, and the projects are getting underway this year. One dairy farmer hopes to enhance his biogas production with additions of food waste or whey. This practice can make the overall project more efficient and provide a pathway for otherwise problematic wastes.
KEEPING AHEAD OF THE CURVE
One way that SMUD has maintained its leadership in the electric utility industry is by making it a practice to keep abreast of clean, distributed generation technology. SMUD has partnered with the California Energy Commission on a program to research, develop and demonstrate methods to make renewable energy technologies more diverse and affordable. One of the highlights of this effort under the Public Interest Energy Research (PIER) Program, is the Accelerated Anaerobic Digestion in a Yolo County Landfill. This landfill is operated as a bioreactor, in which leachate is recirculated back into the landfill. This technique speeds up methane production in the earlier years of gas collection, making energy recovery more cost-effective and efficient. Several innovative research projects planned for next year will test the impact of different biocovers on gas collection and operation as a peak energy generator.
Continuing improvement of animal manure digester systems is vital given new environmental regulations coming into effect in California. A few projects planned are a feasibility study to determine the economic advantage of using a community generator for colocated dairy digesters and a side-by-side test of advanced lean-burn engine technology on biogas.
“While it’s important to keep an eye to the future in technology development, we still keep our feet planted firmly on the ground by implementing tried and true technology. We’re excited about the prospect for biomass to add significantly to our portfolio of renewable energy in the coming years as economic and environmental factors line up in the favor of using our waste more wisely to produce ‘green’ renewable energy,” concludes MacDougall.