Bioenergy Potential In California’s Food Processing Residues

Fruit and vegetable processing facilities and wineries are the largest sources of high moisture solid residues potentially available as bioenergy feedstocks.

Ricardo Amón, Mark Jenner and Stephen Kaffka
BioCycle March 2013, Vol. 54, No. 3, p. 47

The California Biomass Collaborative (CBC) at the University of California, Davis (UC Davis) completed a food industry residue assessment in 2011. This study accounts for the liquid and solid fraction of California’s food processing facility organic residues. The wastewater (liquid fraction) varies in quantity and quality by industry sector. The fruit and vegetable processing (F&V) facilities and wineries (grape pomace) are the largest sources of high moisture solid (HMS) residues that are potentially available as bioenergy feedstocks. Most other agriculture and food industry residues are utilized for higher value products.

The residue assessment results for all of the state’s food processing facility organic residues were then used to estimate a technical energy potential equivalent to 560 megawatts (MW) of electricity, with the additional potential to capture 24.5 million Btu (MMBtu) of heat from the combustion process. However, only 43 MW of technical energy potential is available from the F&V and grape pomace HMS residues.

Residue Assessment Methods

The objective of the assessment study was to compile an inventory of high moisture and low solid organic residues generated at food processing facilities during the 2009 production year. The results were used to map the location and magnitude of residues by county in order to calculate the technical potential to generate bioenergy, identify current use and estimate the potential to utilize these resources as bioenergy feedstocks.

The study was organized by market segment, differentiated by commodity and the type of raw materials processed. These included fruits and vegetables, nuts, milk, cheese, meat and poultry products.

Out of approximately 3,000 registered and certified food processing and manufacturing facilities (Dun & Bradstreet, 2007), the assessment results are based on 1,134 facilities with 25 or more employees. Survey data was collected directly and supplemented with secondary source data obtained at Regional Water Quality Control Board Offices (RWQCB) and through waste-water treatment facilities (WWTFs). In addition to collecting wastewater discharge data, the assessment results include data to account for high (HMS) and low (LMS) moisture solid residues. These quantities are used to calculate the technical potential to generate bioenergy from these sources.

Industry data was aggregated from survey responses and the observations obtained from secondary sources, to create a prediction equation to estimate values from the collected waste-water data. Although wastewater data was collected for all observations, HMS and LMS data was not always available for all facilities. To estimate missing or unreported amounts of HMS and LMS, an ordinary least squares model is used. The model predicts HMS and LMS as a function of reported total wastewater volume, the number of workers and the HMS and LMS moisture content from the fruit and vegetable (F&V) processing industry. Resulting high-adjusted R Square values (HMS 0.835; LMS 0.839) provide confidence in estimates of solid residues for surveys with incomplete solids data. (An R Square (R2) is used to describe how well a regression line fits a set of data. An R2 near 1.0 indicates that a regression line fits the data well (Source: Wikipedia).

Quantities Available, Location

Results include almost 80 percent of the wastewater and wet solid residues generated by California’s fruit and vegetable processing facilities that employed 25 or more workers in 2009. The assessment accounts for approximately 26.3 billion gallons of wastewater (equivalent to 175,000 tons of BOD5) and 3.5 million dry tons of solid residues produced annually by the industry sectors investigated (Table 1). The BOD5 (5-day biological oxygen demand) is the amount (mass) of dissolved oxygen needed by aerobic biological organisms to decompose organic material in (or stabilize) wastewater.

Approximately 55 percent of the wastewater is generated by canneries and fruit and vegetable processing facilities, with another 20 percent each from creameries and meat manufacturing facilities. Almond hulls account for nearly 60 percent of solid residues (LMS) with almond and walnut shells contributing another 20 percent.

Many of the rural F&V factories are seasonal and dedicated to processing agricultural raw materials grown within cost-effective transportation supply chain sources. These include, but are not limited to, processing tomatoes, raw milk, fruits and vegetables, nut crops and animal husbandry. Processed and packaged raw materials produced by these facilities are purchased by food manufacturers that make higher value added products. Many of them are located closer to urban areas and transportation hubs like airports, ports and rail yards.

HMS residues are managed according to the location of the food processing facility. If the factory is in a rural area, the wet solid residues commonly are transported and incorporated into agricultural lands or delivered as animal feed. Wastewater at some facilities is pumped and discharged to agricultural lands. The availability of these relatively low cost disposal options allows the F&V HMS residues to be treated as a low value product. Facilities located within urban areas pay higher costs to discharge wastewater to WWTFs and pay tipping fees for solid residue disposal at municipal landfills. Most of the urban-based food manufacturing facilities operate year round as they produce prepared foods, snacks, desserts and beverages.

Underutilized Residues

Currently the utilization of HMS produced at F&V facilities and winegrape pomace as a bioenergy feedstock is underdeveloped. Grape pomace residue is produced at hundreds of small, medium and large wineries distributed around California. Pomace is the fiber that results from the liquid extraction process during the grape crush. Significant amounts of pomace are generated and are mostly composted for use as soil amendments or for bedding purposes. The limited amount of pomace produced at most wineries diminishes the potential to use this material as an on-site bioenergy feedstock. The large geographic dispersion of the wineries limits the option to aggregate these residues, except for the few large wineries where pomace residue is an economic burden. At the same time, entrepreneurs are evaluating the nutritional value of winegrape seeds and pomace to extract colorants, flavors and flours.

The majority of the organic solid residues produced are already being utilized for their second best alternative — as a raw material for a higher value than in its original form. For example, dairy creameries and cheese manufacturers transform milk lactose and cheese whey residues into protein-rich nutritional and functional food products. Dairy farmers offer a high price for almond hulls used as a nutritional component in animal feed. During 2010, almond hulls sold for $120 to $140/dry ton. Other by-products like olive pits and almond and walnut shells can be used as bioenergy feedstocks, while stone fruit pits are valued to produce industrial tools. Animal residues are collected by well-established service providers and delivered to rendering facilities to manufacture industrial products.

The HMS residues at F&V processing facilities and the winegrape pomace at wineries require collection, transportation, treatment and disposal, incurring additional costs. For the most part, these waste residues have yet to find a higher value alternative.

Table 2 provides county data for HMS residues and wastewater BOD loads generated by canneries, dehydrators, and fresh and frozen fruit and vegetable processors in California. It also includes the low moisture solids (LMS) created from the hulling and shelling of almonds and walnuts.

Food processing facilities are distributed throughout the state but concentrated in the Central Valley where most of the agricultural production takes place (Figure 1). Food industry facilities located in the counties of Fresno, San Joaquin, Madera, Kern and Stanislaus generate over 55 percent of the estimated HMS residues. Although some amount of LMS is produced by F&V processors, the largest proportion comes from nut hulling and shelling, with 58 percent produced within the counties of Fresno, Kern, Merced and Stanislaus.

Table 2 and Figure 1 do not include solids data from wine grape pomace, meat and poultry processing residues, as data from these sources was not obtained at the individual county level. A detailed evaluation of winegrape pomace production is available in Section 3.6 Wine and Brewery Industry, Amón, et al., 2012.

Technical Potential To Generate Bioenergy

The technical potential to generate bioenergy from food industry residues was estimated using an Excel-based calculator developed by researchers from the Department of Biological and Agricultural Engineering at UC Davis. This basic tool employs generic feedstock properties and basic assumptions for conversion efficiencies. The energy potential from waste-water flows and solid residues, except for nutshells and hulls, was estimated assuming conversion by anaerobic digestion (AD) to produce biogas. The biogas can be used to fuel reciprocating engine generators (for both heat and power) or to fuel hot water (e.g., hot water boilers are used in the winery industry) and steam boilers. Nutshells and hulls were assumed to be converted via thermal pathways for heat and power.

The technical energy potential was calculated assuming that all identified residues are available for conversion. This is estimated to be 557 MW of electricity, with 24.5 million MMBtu (million Btu, or British Thermal Units) of recovered heat derived from combined heat and power (CHP) systems.

Conclusion And Recommendations

The concept of a residual infers that a by-product from production is available and that it currently has limited or no economic value. The value of current food processing residuals available in California is generally highly variable. There is technical potential to convert food industry residues to energy, but most of this biomass resource is not available. Higher value residues are already utilized for their best current alternative, excepting for the HMS produced by F&V facilities and grape pomace produced at wineries. These residues will remain available because of the seasonal nature of the processing facilities, the wide geographic dispersal of the residue, the relative small amount produced at each location and low cost disposal options.

Because of the mature nature of the cheese whey industry and the industrial infrastructure to collect and process animal fats, these by-products should not be considered an organic residue with potential for bioenergy conversion. Almond hulls could be diverted from animal feed if bioenergy pays a higher price than that currently offered by dairy farms. Dairy farmers would then have to find a lower cost alternative source of animal feed or incur higher feed costs.

Site-specific analysis is needed to determine the economic costs and benefits of installing bioenergy systems at F&V and grape pomace seasonal facilities. These studies could evaluate the greenhouse gas (GHG) effects of current disposal practices and account for the potential of earning carbon allocations from the California Air Resources Board (CARB). CARB’s Cap and Trade program ( will influence the outcome of such analyses, especially for facilities that generate 25,000 or more tons of carbon per year. The on-site biogas resource is regarded as carbon neutral and could earn CARB allocations when generating electricity or as a direct fuel to power steam boilers. County-level results are provided for use by economic development agencies, private sector prospectors and policy makers who need to estimate the potential to generate bioenergy from food and beverage industry resources. The assessment does not include economic analyses, which are more useful when produced for site-specific conditions.

Ricardo Amón and Stephen Kaffka are with the University of California, Davis. Mark Jenner is with World Agricultural Economics and Environmental Services. The full report can be downloaded at the California Biomass Collaborative’s website (


Amón, R., M. Jenner, M., R.B. Williams, H. El-Mashad, S.R. Kaffka, Draft California Food Processing Industry: Residue Assessment. California Energy Commission, 2012. http://

Dun and Bradstreet Reports, Food Processing Industry Database, 2007.

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