BioCycle November 2008, Vol. 49, No. 11, p. 52
Water-based separation/preparation and two-stage anaerobic digestion system are the basis of a new technology.
Melvin S. Finstein and Yair Zadik
THE defining characteristics of municipal solid waste (MSW) are heterogeneity, variability, abrasiveness and wetness. The task of recovering material and energy from such a problematic mixture has historically challenged both biological and thermal systems. A new technology designed to address the difficulties posed by processing mixed MSW integrates the functions of water-based separation/preparation and two-stage anaerobic digestion, terminating in a reactor of the Upflow Anaerobic Sludge Blanket (UASB) type. Water is the integrating medium, derived from the moisture content of the waste, which is about 30 percent.
The ArrowBio system incorporates a number of return loops; the major return loop connects the two ends. At the back-end, biological action frees the moisture content of the waste, generating liquid (water). A portion of this is returned to the front-end for separation/preparation of the fresh MSW input. Excess water is discharged.
Within the front-end, return loops provide multiple opportunities for size reduction of particulates to pass through screens with successively smaller openings. These particulates are recovered as recyclables, removed from the system as nonprocessible/nonbiodegradables (landfillables) or passed to the back-end for biological treatment.
In 2003, following extensive developmental work at the laboratory, pilot and small commercial scales, a one-line plant (150 tons/day) opened at the Tel Aviv, Israel, MSW transfer station. The front-end physical separation/preparation stage is under a roof in an open-sided building. The back-end biological stage is adjacent to the building.
Another ArrowBio plant is a key component of the comprehensive Macarthur Resource Recovery Park (MRRP) in suburban Sydney, Australia – a project developed by WSN Waste Solution (www.wasteservice.nsw.gov.au). The ArrowBio facility is designed to process 300 tons/day of mixed waste.
Plant construction was completed in July 2008 and is in the final stages of commissioning. This includes charging the digester tanks with “starter culture” consisting of solids from an existing anaerobic digestion facility. Thereafter, the microbial community is self-replicating using the MSW biodegradable organics received from the front-end. Mechanical components on the front-end are being tested individually and then collectively, to ensure functional integration. Gradually increasing amounts of MSW are processed, leading to the design throughput of 300 tons/day. Full operation will commence in early 2009.
A building houses the reception hall and the front-end of the system (separation/preparation). The biological back-end of the system consists of two sets of bioreactors and auxiliary tanks (one set per separation/preparation line). The two smaller tanks are acidogenic reactors; the larger tank is the UASB reactors. Each pair serves one separation/preparation unit.
The facility is a one-module plant consisting of two parallel lines, each conservatively rated at 11 tons/hour. A two-shift operation with 13.5 productive hours has a throughput of 300 tons/day. The end of the second shift is dedicated to cleaning and preventative maintenance.
The front-end of the system exploits gravitational separation in water to recover nonbiodegradable recyclables (metals, glass, rigid and film plastic) and to remove nonprocessible materials (stones, grit). The action of gravity is abetted by mechanical, hydraulic, electromagnetic and pneumatic devices. The dissolving power of water brings soluble biodegradable organics into solution. Insoluble biodegradables are size-reduced to foster microbial colonization. The well-isolated biodegradable organics, in solution and in the form of finely divided particulates, are pumped to two-stage anaerobic digestion at the biological back-end of the system.
The functions of the first two reactors are liquefaction (microbial dissolution of particulates), hydrolysis (large molecules split) and acidogenesis (formation of organic acids). The function of the terminal reactor is methanogenesis (formation of methane) via UASB digestion. This two-stage sequence follows well-established scientific logic (e.g., syntropic proximity, differential growth rates, maximization of specific population densities) proven in widespread practical application – treatment of high-strength wastewater.
Prevention of pollution is embedded at many levels of the system. For example, water-based separation/preparation suppresses dust and odor – the latter because most odorous compounds are highly soluble in water and biodegradable.
The technology has been short-listed in response to separate Requests for Proposals from both the City of Los Angeles and Los Angeles County. Several thermal technologies have been short-listed as well. The primary energy product from a process like ArrowBio is biogas (methane, carbon dioxide and little else). Conversely, the primary energy product of the thermal systems is syngas, a complex mixture of various classes of organic chemicals, though mainly carbon monoxide, hydrogen and methane. Markets for the syngas are limited primarily to steam for district heating and generation of electricity. Markets for biogas, on the other hand, are much broader, including boiler fuel as is, electricity generation, upgrade for pipeline or vehicle fuel, and higher synthesis into ammonia, alcohols, aldehydes, etc. The integrated materials and energy recovery process is transparently a “green technology.”
Melvin Finstein is Professor Emeritus Rutgers University and head, ArrowBio USA (firstname.lastname@example.org). Yair Zadik is CEO of Arrow Ecology & Engineering Overseas (email@example.com).
November 24, 2008 | General
Integrated Materials And Energy Recovery From Municipal MSW
BioCycle November 2008, Vol. 49, No. 11, p. 52