February 23, 2005 | General

Digester Plant Operates At Tel Aviv Transfer Station (Israel)

BioCycle February 2005, Vol. 46, No. 2, p. 70
The 70,000 ton per year anaerobic facility makes use of vats of circulating water to separate incoming feedstocks prior to a biological phase for gas recovery.
Melvin S. Finstein

AFTER extensive development involving laboratory, pilot and small commercial-scale steps, a 70,000 ton per year anaerobic digestion plant began operations in early 2003 at the Tel Aviv, Israel MSW transfer station. Using an ArrowBio module, the facility has two components: A front-end physical element for separation and preparation purposes; and a back-end biological system with an Upflow Anaerobic Sludge Blanket (UASB) digestion. Currently, source separation is at an early stage of development in the system’s design which calls for feedstocks from the collection truck to be tipped into vats of circulating water. Water in the vat is constantly “refreshed” through exchange with the back-end biological element. The source of the water is the moisture content of the waste, typically comprising around 30 percent of the weight. In the water vat, the nonbiodegradable and biodegradable fractions are separated gravitationally. Other benefits of tipping into water include dust suppression and the neutralization of odors delivered with “ripe” loads.
The functions of the separation and preparation elements are two-fold – to remove traditional recyclables (e.g., noncompliance bottles and cans) and other nonbiodegradables, while simultaneously isolating the biodegradable for UASB digestion. The system includes a large settling tank, cyclone at the terminal end of a film plastic removal system, large trommel screen and control room.
In the separation/preparation vat, the watery flow carrying the heterogeneous mixture of MSW materials follows multiple pathways that are, by design, complex and overlapping.
Overflow from the water vat, screened to exclude large items, passes though smaller enclosed trommel screens and thence, according to partitioning criteria, to large and small settlers. In the settlers, grit is separated from organics and removed from the system. Meanwhile, larger floaters and buoyancy-neutral items are lifted to a slow speed shredder and thence to the large trommel screen. The “overs” from this trommel consist mostly of film plastic and are removed at a pneumatic station. The “unders” (material that passed through screen) are washed into a nonmechanical device for further solubilization and size reduction.
Nonbiodegradables are recovered for recycling as secondary material commodities, and soluble and particulate organics come into solution or fine suspension, including food sticking to containers and the contents of unopened diapers. The latter are disrupted in the processing train, freeing the feces, urine and cottony absorbent. Insoluble biodegradable organics (e.g., nonsource-separated food-tainted paper products, tough fruit rinds) get increasingly soggy and fragmented, ultimately to the point of passing screens of selected sizes. The organics, now in watery isolation, are pumped to the biological element. In turn, return water from the biological element refreshes the separation/ preparation water vat.
Biological Element
The organic flow first enters acidogenic bioreactors for several hours of preliminary treatment. There, readily metabolized substances already in solution are fermented (e.g., sugars fermented to alcohols), while certain complex molecules are biologically hydrolyzed to their simpler components (cellulose to sugar, fats to acetic acid). The overflow, rich in such intermediate metabolites, then enters the UASB bioreactor. UASB digestion is a generic, technology specifically designed for treatment of high strength wastewaters, such as in dairy and candy manufacture and other industries.
The Tel Aviv facility needs only two acres to process 70,000 tpy. Supernatant is pumped to the physical separation/preparation element as needed for makeup water, or to an aerobic tank for polishing if necessary. Water may be stored or used immediately as in irrigation. The solids are dewatered for use as stabilized organic soil amendment.
Some of the biogas is used to fire boilers to maintain UASB digestion at its optimum temperature of ~ 95°F (35°C). Otherwise, depending on site-specific circumstances, the gas fuels an electrical generator via a storage tank. Waste heat from the generator contributes to the maintenance of digestion temperature.
Mel Finstein, Professor Emeritus of Environmental Science, Rutgers University, represents the ArrowBio Process in the United States. E-mail: finstein@envsci.rutgers.edu.

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