Anaerobic digestion of industry waste sought

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  • Anaerobic digestion of industry waste sought Although anaerobic microbial diges-tion plays an important role in mu-nicipal sewage treatment, the tech-nology really hasn't caught onin the U.S., at leastfor treatment of indust r ia l wastes. That may be changing, however, as environmen-tal concerns make the anaerobic ap-proach increasingly attractive for in-dustrial applications.

    One organization working to pro-mote the adoption of anaerobic di-gestion (among other things) is the Iowa Biotechnology Byproducts Con-sortium (BBC). BBC consists of facul-ty members from the University of Iowa and Iowa State University and staff from the Cedar Rapids water treatment system. What brings them together is a common interest in winning valuable materials from the by-products of biotechnology and agribusiness operations.

    BBC is governed by a triumvirate: Rex Montgomery, professor of bio-chemistry and associate dean at the University of Iowa's College of Med-icine; Richard R. Dague, professor of environmental engineering at Iowa State; and George Milligan, in charge of water pollution control for the city of Cedar Rapids. They note that the U.S. Department of Agriculture has provided more than $2 million for the consortium and its work.

    Anaerobic treatment of biotech-nology by-products is only one part of a BBC program that also encom-passes characterization of waste streams, bioseparations, bioremedia-tion, microbial transformations, and livestock and soil amendment appli-cations. However, it was anaerobic treatment that was the subject of a recent two-day symposium, "Anaer-obic Methanogenic Processes Ap-plied to Industrial By-Products," or-ganized by BBC and held in Cedar Rapids.

    Al though the symposium ad-dressed an Iowa problem, the topic is one of national and even interna-tional significance: Overloading municipal water treatment plants is a matter of concern wherever there are biotechnology industries. And the symposium was international in scope, with several speakers and

    participants coming from European and Latin American countries.

    In many instances, the organic components of industr ial waste streams are too dilute to be worth recovering. However, they may be present in amounts worth convert-ing to methane, using an appropri-ate bioconversion process. Solid by-products from food processing, live-stock slaughtering, and livestock feeding operations can also be suit-able feeds for bioconversion. The methane, even if not produced in commercial quantities, can provide energy and thus reduce the overall costs of waste treatment.

    Several symposium speakers point-ed out the advantages of anaerobic treatment. These include the ability to treat a great variety of organic wastes, low sludge production (com-pared with aerobic digestion), low energy requirements, an enclosed system that facilitates odor control, low nutrient requirements, and a high-grade energy by-product.

    Indeed, anaerobic digestion has been used successfully to stabilize wastewater sludges for more than 60 years, according to Gene F. Parkin, a professor of civil and environmental engineering at the University of Iowa. But, he adds, the process is still considered by some to be unreliable. Part of the problem is that plant en-gineers and personnel don't under-stand the process well enough and haven't been trained sufficiently, re-sulting in unsatisfactory operations.

    There's more to it than that, how-ever. During most of the preceding 60 years, anaerobic digestion was used largely to stabilize raw domestic

    sewage sludge accumulated during primary sedimentation. Lately, Par-kin says, the process has been in-creasingly used on chemically treated sludges, biological sludges produced by aerobic digestion processes, and sludge mixtures containing industri-al wastes, each application present-ing different problems for system de-signers.

    Reduced to essentials, anaerobic digestion of organics takes place in three stages. The first involves hy-drolysis, liquefaction, and fermenta-tion, which are needed to convert complex wastes to simpler materials that can pass through bacterial cell walls. In the second stage, hydrogen and acetic acid are formed. Finally, those substances are converted to methane. According to Parkin, five groups of bacteria are thought to be involved in the various reactions.

    One of the main aims in design of anaerobic digestion plants is to achieve rapid flow-through of liq-uids but at the same time maximize solids retention time. As the liquid flow rate increases, however, there's an increasing tendency for solids (in-cluding the bacteria that are doing the work) to wash out of the reactor.

    Several symposium presentations dealt with ways of preventing solids washout in continuous-flow sys-tems. The approaches include baf-fled reactors, anaerobic filter reac-tors (which immobilize the bacteria on some heavier medium), the up-flow anaerobic sludge blanket pro-cess (which depends on the devel-opment of dense granules that settle rapidly), and hybrids of two or more of these and other approaches.

    Batch reactor prolongs solids retention time

    Biogas recycle Biogas

    4T

    ut Feed 1

    Feed

    Decant ports

    Super-' natant

    .Settled biomass

    k

    React Settle Decant

    Effluent

    October 28, 1991 C&EN 21

  • Science/Technology

    Iowa State's Dague and associates have taken a different tack, going to a batch approach. Dague notes their "anaerobic sequencing batch reactor" operates analogously to simple anaer-obic contact reactors, except that bio-mass separation and liquid clarifica-tion take place within the reactor, not in an external clarifier. The sequence has four steps: feed, react, settle, and decant. The internal clarification step obviates the need to degasify the ef-fluent, Dague says. Since the partial pressure of the biogas remains con-stant within the reactor, the tenden-cy for biomass solids to float is mini-mized. That leads to rapid solids set-tling and the ability to process large liquid volumes while maintaining long solids retention times.

    Dague notes that the project was initiated to provide an economically viable solution to the difficult prob-lem of treating swine manure. Add-ing value to pigs is, after all, one of Iowa's biggest biotechnology-based industries. But decaying swine ma-nure smells awful. "Lawsuits arising from environmental problems associ-ated with confinement swine-feed-ing facilities are becoming common,"

    Dague observes. However, the batch reactor should also be useful for oth-er industrial applications.

    Other symposium speakers ad-dressed the increasing use of anaer-obic processes for industrial waste-water treatment outside the U.S. For example, Peter Weiland, an engi-neering professor at the Technologi-cal Institute of the German Federal Agricultural Research Center in Braunschweig, noted that the num-ber of anaerobic industrial treatment plants in Germany grew from only 12 in 1982 to more than 60 in 1990.

    Similarly, Adalberto Noyola, a civil engineering professor at the National Autonomous University of Mexico, Mexico City, reported that the number of modern anaerobic di-gesters in Latin America has risen from essentially zero in 1983 to more than 100 in 1990. Brazil has been the leader in installing anaero-bic digesters, Noyola says.

    Ross E. McKinney, an environ-mental engineering professor at the University of Kansas, pondered the future of anaerobic treatment in the U.S. Most of the theory and studies on high-rate anaerobic treatment

    systems (HRATS) came out of U.S. universities, says McKinney, who is one of the pioneers in the field. But most of the "real-world" HRATS ac-tion is in Europe and Latin America. The U.S. has fewer than 10% of the world's full-scale HRATS plants. And U.S. equipment makers are "still not sure it's here to stay."

    Much of the reluctance of indus-try to switch to anaerobic methods results from the way the municipali-ties do business. Typically, munici-palities have to deal with separate consulting engineers, equipment suppliers, and building contractors. Industry doesn't want to deal with separate entities, McKinney says; it wants turnkey systems. And it finds the consultants "don't really under-stand how industry works."

    But, McKinney concludes, "the vise is starting to squeeze," and in-dustry will be forced to go to anaero-bic treatment process to meet ever stricter regulations. Moreover, the field could provide a golden oppor-tunity for chemical and process engi-neers, "if they'd only learn the mi-crobiology and biochemistry."

    Ward Worthy

    U.S. Postal Service launches stamp set depicting planetary exploration

    At a ceremony in Pasadena, Calif., ear-lier this month, the U.S. Postal Service introduced a set of 10 commemorative stamps depicting planetary exploration. The ceremony was held at Jet Propul-sion Laboratorywhich has played a central role in U.S. planetary explora-tion effortsand featured remarks by JPL director Edward C. Stone and post-master general Anthony M. Frank.

    The stamps are sold in a $5.80 booklet containing two panes of 10 29-cent stamps. Each pane shows the nine planets in the solar system and

    Earth's Moon, all but one planet ac-companied by an unmanned spacecraft that has explored it.

    The planets are depicted in order of increasing distance from the Sun. In the top row (left to right) are the Mariner 10 spacecraft approaching Mercury's or-bit, Mariner 2 encircling Venus, Landsat orbiting above Earth, Lunar Orbiter ex-ploring the Moon, and Viking Orbiter speeding toward Mars. In the bottom row (left to right) are Pioneer 11 col-lecting data from Jupiter, Voyager 2 photographing Saturn and racing on to

    encounters with Uranus and Neptune, and Pluto "Not Yet Explored."

    Stamps depicting space-related top-ics are among the most popular with the public, and the Postal Service is promoting sales of the new set as a fo-cus of National Stamp Collecting Month in October, under the theme "Journey to a New FrontierCollect Stamps!" It has enlisted as spokesman Leonard Ni-moy, who played Mr. Spock in the orig-inal Star Trek science fiction TV series and movies. "Stamp collecting is logi-cal," Nimoy notes.

    22 October 28, 1991 C&EN

    Anaerobic digestion ofindustry waste sought

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