October 25, 2006 | General

Converting Biosolids To A Renewable Fuel

BioCycle October 2006, Vol. 47, No. 10, p. 32
A California sanitation district selects a technology that converts high moisture residuals into high-grade, sustainable power.
Michael D. Moore, Layne Baroldi, Deirdre Bingman and Ray Kearney

WITH traditional biosolids management options diminishing in California, the Orange County Sanitation District (OCSD), in conjunction with CH2MHill, studied innovative bio-solids management technologies as part of its long-range biosolids management plan and RFP process. The result of this comprehensive strategy was for OCSD to contract with EnerTech Environmental, Inc. (EnerTech) to manage approximately one-third of its biosolids production utilizing their SlurryCarb process at a facility to be constructed in Rialto, California.
The SlurryCarb process offers an innovative approach to treating and recycling biosolids that takes advantage of the organic content to produce a fuel with an energy content of approximately 7,000 Btu/lb. This regional option requires approximately two-thirds less energy than a conventional dryer and destroys pathogens, organic pollutants and requires less fuel for transportation than all other options.
OCSD is a public agency responsible for collecting, treating and recycling wastewater and biosolids for over 2.5 million people in Orange County, California. The District strives to recycle its biosolids using sustainable options while protecting public health and the environment. In 2003, OCSD was the first agency in the nation to obtain the National Biosolids Partnership’s (NBP) certification for its Biosolids Environmental Management System (EMS) after being verified by an independent third-party auditing firm assigned by the NBP. OCSD has three primary goals guiding the management of biosolids.
The goals, adopted in OCSD’s 1999 Strategic Plan and the 2003 Long-Range Biosolids Management Plan (LRBMP), are summarized as follows: Implement multiple management options, under suitable separate and various contracts, to maximize interim and long-term residuals beneficial reuse while maintaining reliability; Strive for 100 percent biosolids recycling; and Maintain at least one in-county or regional management option.
Unfortunately, the sustainability of agricultural land application and landfilling of biosolids in California is uncertain. Both options face numerous political challenges, including alleged health concerns and mounting nonscience-based regulatory restrictions that make it clear that biosolids generators need to convert to both Class A biosolids “products” and utilize energy production technologies in order to have a sustainable biosolids management program.
Managing both bulk Class A and Class B biosolids in agriculture will continue to be more restrictive and costs will continue to increase as current options are eliminated. For example, in southern California, management fees for biosolids land application have risen from $25/ton in the mid-1990s to well over $40/ton today. The cost to create sustainable “product-quality” Class A biosolids or energy is expected to further increase to $80/ton. Dwindling biosolids management capacity in agriculture and the associated increase in cost have made the biosolids-to-energy opportunities become attractive as a sustainable biosolids management option that wastewater agencies should explore to protect against the effects of weakening agricultural markets. Fortunately, a great deal of effort has been invested by the private sector in developing biosolids-to-energy technologies to allow wastewater agencies to participate in these “noncropping” markets of direct energy production.
Power generation from anaerobic digester gas at wastewater agencies is well known; however, this recovers only a portion of the energy value of the biosolids. Direct energy production refers to the creation of power generated by the combustion or oxidation of biosolids through either the creation of fuel char, pyrolysis, gasification, or incineration. Distinctions between the technologies include:
Pyrolysis: Conversion or cracking of biosolids at high temperatures, in the absence of oxygen. The process is highly endothermic, but usually produces a char and sometimes an oil that have heating value. The products depend on the temperature at which the process is conducted. Pressures are typically in the range of 1,000-3,000 psi.
Gasification: Combination of complete combustion and pyrolysis. The products are combustible gases, which usually have a fairly low heating value, tars, oils and a char with a heating value. This process has been conducted in multiple hearth furnaces with biosolids to produce a gas which is subsequently combusted in the afterburner to provide the needed temperature to lower hydrocarbon emissions.
Incineration: Complete combustion is the oxidation of organics in the presence of sufficient oxygen for complete combustion. The net fuel production depends on the heating value and the moisture content of the feed substrate.
OCSD’s Long-Range Biosolids Management Plan (LRBMP) identified the top five biosolids markets and products that can be generated for these markets and the most viable product manufacturing processes and recommended an implementation plan. The markets and products included horticulture, energy production, and silviculture. The processes included composting, heat drying, energy recovery and organo-mineral fertilizer manufacturing.
OCSD implemented one of these markets in April 2004 by amending our contract with Synagro to commit 200 tons/day (for up to 20 years) to a new aerated static pile compost facility to be built in Kern County, California.
In July 2004, OCSD issued five separate Requests for Proposals (RFP) for managing 200 wet tons/day of biosolids via either energy production, off-site drying, on-site drying, drying with thermally-remediated soil, or through composting. The RFPs were issued to more than 100 potential vendors. OCSD received nine proposals including at least one proposal for each of the five RFPs issued. The Phase 1 portion of the evaluation compared like technology proposals. The best proposals from each group then were evaluated in Phase 2, to determine the “best value.”
The first phase of the review and selection process involved reviewing and comparing each proposal for eight major elements including experience, project team, financial capabilities, project schedule, siting, permitting and site viability, technology viability, and product marketing. The review criteria used as the basis for OCSD staff recommendations to the Board of Directors in Phase 2 included an analysis of the risks, the total life cycle cost and environmental concerns and impacts.
Concurrent with the RFP process, OCSD identified the need to obtain independent input from interested residents and stakeholders about OCSD’s biosolids management program. To address this need, OCSD create a Biosolids Advisory Committee (BAC) to: Examine OCSD’s LRBMP and obtain meaningful input on how to best achieve sustainable biosolids management program; Assist in the review of possible management options for biosolids; Meet the requirements of the National Biosolids Partnership Environmental Management System’s requirement to have “meaningful community involvement” in the planning for biosolids management; and Submit to OCSD’s Board of Directors their recommendations as to preferred approach to managing biosolids.
The BAC members were selected through an application process based on qualifications such as geographic representation, expertise and group representation. Members of the BAC represented homeowner groups, regional business interests, environmental organizations, public policy experts, economists, engineers and technical professionals. The BAC’s function was as an advisory group with direct access to OCSD’s Board of Directors. The Project Plan for the BAC consisted of four phases – education, review of vendor proposals, analysis and consensus building, and report writing with presentation to the Board of Directors. The BAC spent three months in weekly meetings reviewing presentations from staff and discussing the options.
RFP & BAC Results
The proposal from EnerTech received the highest overall score from both OCSD and the BAC. It was selected as both the best value to OCSD and the lowest risk option. The BAC specifically recommended that the Board of Directors proceed with the EnerTech proposal subject to a more limited contracting period, (e.g., upwards of ten years with “off-ramps”) should other technologies become more viable for an in-county solution. The BAC also suggested that prior to proceeding with the EnerTech proposal, OCSD should attain assurances this technology can be scaled-up to handle the amount of biosolids they are proposing.
Technical viability and risk were important factors in evaluating the proposals for both OCSD staff and the BAC, and although the SlurryCarb process is a new technology, much of the equipment used in the process is common in the biosolids industry. OCSD staff and consultants evaluated the technology and observed EnerTech’s Process Development Unit in Conyers, Georgia. They concluded the SlurryCarb process is a viable biosolids-to-energy technology.
The SlurryCarb process is a new technology that will physically and chemically convert biosolids into a fuel product at a significantly lower cost than conventional drying and burning. Using heat and pressure, the process carbonizes the organic matter in biosolids and ruptures cell walls to release bound water. The resulting slurry dewaters to 50 percent total solids by centrifugation, and is then dried using approximately two-thirds less energy than conventional drying. The energy savings result in approximately a one-third reduction in overall cost compared to traditional drying technologies. The process consists of seven basic steps:
Biosolids Preparation: Biosolids at 20 to 30 percent by weight solids are received and macerated until all particles are less than a half-inch in diameter.
Slurry Pressurization: Prepared biosolids are slurried and pressurized above the vapor pressure of water at the desired reaction temperature. Pressurization prevents the water from boiling during reaction and ensures that the carbonization reactions occur in liquid phase.
Slurry Heating: Specially designed heat exchangers raise the temperature of the slurry to the range of 400°to 450°F.
Reaction: Cellular structure of biosolids ruptures and the carboxyl groups of the organic molecules break off and are released as carbon dioxide gas in a reaction known as decarboxylation. This reaction causes the biosolids to become hydrophobic. The reaction is stopped short of pyrolysis.
Dewatering/Drying: After partial cooling and depressurization, the carbonized slurry is centrifuged to about 50 percent total solids. The dewatered slurry is then dried to 90-plus percent total solids. This final product, called E-Fuel, has a heating value of approximately 7,000 Btu/lb.
Filtrate Processing and Recycle: Centrate and dryer condensate are processed to remove contaminants via wastewater treatment. Resulting effluent is discharged with a portion being recycled into the process.
Utilization: E-Fuel is used as an alternative to fossil fuels.
E-Fuel, has been certified as a renewable fuel by the California Energy Commission and the Green-e Renewable Electricity Certification Program and has combustion characteristics similar to lignite coal. E-Fuel can help states meet their renewable energy goals and can be utilized in cement kilns, oil boilers, gasifiers, fluidized beds and most other coal-burning facilities.
It is an ideal feedstock for cement kilns where the ash (containing high levels of calcium, iron, silica, and alumina) can be incorporated into the cement product, reducing the quantity of cement feedstock required.
Rialto, California Facility
EnerTech has designed a 675 wet tons/day SlurryCarb facility in Rialto, California. The plant is located adjacent to the City of Rialto wastewater treatment plant. Construction is underway and operations are scheduled to commence in early 2008. The Rialto SlurryCarb facility will produce approximately 140 tons/day of E-Fuel, which will be used as a renewable fuel in lieu of coal in a local cement kiln. This beneficial use will allow the cement kiln to reduce ore consumption and lower emissions of fossil-fuel based greenhouse gases.
In addition to biosolids supplied by OCSD, the Rialto facility will receive biosolids from the County Sanitation Districts of Los Angeles County, and the cities of San Bernardino, Riverside, and Rialto. The plant will provide these wastewater agencies complete biosolids recycling and avoid the environmental perceptions associated with agricultural land application in this region.
Michael Moore, Environmental Assessment Division Manager, Layne Baroldi, Legal and Regulatory Affairs Liaison, and Deirdre Bingman, Principal Environmental Specialist, are with the Orange County Sanitation District. Ray Kearney is Vice-President of EnerTech Environmental, Inc. Mike Moore is giving a presentation on this project at BioCycle’s Sixth Annual Conference on Renewable Energy From Organics Recycling, October 30-31, November 1, 2006 in Minneapolis. See pages 15-17 of this issue for details.

Sign up