What would happen to a filled-up garbage bag if it were dumped and agitated in water? As its contents fall out, the plastics would float, the organics absorb water and begin decomposition while metals and other heavy items such as glass would sink. Now consider applying this design to the millions of tonnes of municipal solid waste produced each year and you have the ArrowBio waste technology concept.
In early June, the 3rd International Environmental Technology Conference and Trade Show was held in Tel Aviv, Israel. One of the six environmental sites toured was the ArrowBio processing plant. It’s appropriately built on the same site as Tel Aviv’s garbage mountain, located at Hiriya, where municipal solid waste has been dumped for decades, growing to 60 meters in height and width and 120 meters in length. Still serving as a waste transfer station, Hiriya is the ideal place to showcase the company’s unique features and benefits.
Central to the patented process is the floatation tank where unsorted (or sorted) waste is simply dumped on the adjacent tipping floor and pushed into the tank where the garbage bags are agitated and opened with cutting blades. (Blades are used rather than a shredder so that batteries or cleaning product containers are not compromised, thereby causing toxics to be released into the solution). This hydro-mechanical system uses specific gravity for separation assisted with a paddle wheel. A conveyor belt brings floating material to a slow-speed shredder that reduces it into smaller pieces. The materials are then conveyed across a trommel screen. Unsorted plastics are baled (and mostly sold to markets in China).
Heavy materials such as rocks, glass, ferrous and non-ferrous metals, sink to the bottom of the tank where they are separated using a drag chain. The metals are sorted using magnets and an aluminum precipitator and sold to recycling facilities. The glass is mechanically separated using an optical sensor. The remaining heavy residue is inert and comprises between 5 to 10 per cent of the total volume brought into the plant. It’s used for road and construction fill.
The soluble materials, which tend to be biodegradable, stay in the floatation tank and then enter the filtration systems to pulverize the material into a watery organic solution. All biodegradables such as paper, organics, cardboard, etc. go through this process in order to prepare the waste for further treatment.
The biological treatment is composed of a two-step process. First, the material enters an acetegenic bioreactor process where biological hydrolysis splits certain molecules into readily metabolized parts (i.e., simple sugars and organic acids). Secondly, the rich mixture of organic liquid is transported to the methanogenic reactor for gasification through natural fermentations and other biological reactions.
ArrowBio uses an advanced variant of anaerobic digestion that, while unique to the ArrowBio process for the treatment of waste, is commonly used in the treatment of heavy sewage sludge. Known as Upflow Anaerobic Sludge Blanket Digestion (UASB), this process results in a much faster and thorough degradation of biological materials, producing a finished soil amendment product and methane-rich biogas. The biogas is collected in the upper part of the methanogenic reactor in a specially designed compartment. The gas is then re-circulated by a compressor and re-injected close to the bottom of the reactor. This assures permanent agitation without mechanical devices. The 70 to 80 per cent pure methane gas is also collected and used in a CAT power system (which produces 1.1 MW at this time). Half the energy is used onsite and the other half is sold to the grid. At maximum capacity, the system would only need 20 per cent of the green electricity produced.
Although ArrowBio constantly recycles its own water from the digesters to the floatation tank, due to the moisture content of the organics entering the system, the process is a net exporter of water. The excess is used for irrigation (useful in dry Israel) or sent to the municipal wastewater system. Due to the nature of the system, no odor controls are needed as the floatation tank suppresses them.
The system can handle disposable diapers, textiles, leachate from landfill (which it accepts from the Hiriya mountain), manures and agricultural residues. The system also accepts landfill gas recovered from the Hiriya mountain. The process can also work in any climate. (Colder climates would require an enclosed building under negative air-pressure.)
The system is scalable. The pilot plant in Hiriya can currently work to a capacity of 70,000 tonnes of waste per year and produce seven million cubic meters of methane gas. The plant is scalable to serve between 70,000 and 100,000 people. This is the company’s second plant. The first was in Hadera — a smaller pilot plant and part of its nine-year research history. The ArrowBio project has been chosen to handle the greater Dan region’s waste (a region of four million people).
In an age where it’s becoming increasingly difficult to site landfills, and where tipping fees and electricity prices will only increase, this system showcases how we could use our own garbage (that others don’t want in their backyards) to produce much-needed electricity in an environmentally responsible and non-smog causing manner.
Shai Spetgang, B.E.S., CEPIT manages the Ontario Environment Industry Association (www.oneia.ca). His attendance at the conference was as a guest of the Israel Economic Mission to Canada, the Canadian branch of the Ministry of Industry and Trade of Israel. Learn more about the ArrowBio system at www.arrowbio.com or www.hiriya.co.il