Air Pollution Control Residue Treatment Process

Category: Environment

Mitchell & Associates Consulting Engineering Inc. and

APEX Residue Recovery Inc., Burlington, Ont.

In Europe, Japan, Canada and the U.S. there are approximately 950 large scale municipal incinerators processing an estimated 110 million tonnes of solid waste per year. A large portion of these incinerators use lime-based (dry or semi-dry) air pollution control (APC) systems. Though these systems make the emitted air from the incinerators less harmful, as a byproduct of their processes they generate about 2.4 million tonnes of solid hazardous residue (flyash) which requires treatment or disposal. The current average cost to treat or dispose of the residue is U.S. $200 per tonne. This amount translates into an expenditure of U.S. $480 million per year worldwide.

In 1996 Mitchell & Associates was approached to provide project management and design engineering services for a working demonstration-scale module of an innovative process to treat air pollution control residues. Projections indicated that the APC Residue Treatment Process could be an environmentally safer alternative to treatments such as solidification, chemical stabilization and thermal treatment, and also could compete on an economic basis. The APC Residue Treatment Process concept was developed by Compass Environmental Inc. who had done bench and pilot scale chemical studies and testing.

Mitchell & Associates reviewed the considerable chemical and technical data Compass had compiled in order to design and provide project management for producing a working module of the system. The built unit was successfully demonstrated at an Ontario incinerator site between August and December 1998. Filter cake from the APC treatment system meets Ontario Regulation 347 Leach Test limits, and the results have been forwarded to the Ontario Ministry of the Environment for a system-wide permit.

Treatment process

The innovative aspect of the process is that the hazardous constituents of the residue — soluble lead and salts — are extracted and recovered separately from the other constituents and sent into recyclable byproduct streams. The remaining filter cake can be sent to a controlled landfill, or, depending on the degree of treatment, can be used in other construction applications.

The process concept was based on the following aims:

changing the classification of air pollution control residues (soluble lead and salt) from hazardous to non-hazardous for disposal or re-use.

creating a highly concentrated metal precipitate capable of being recycled through existing smelting operations.

creating a calcium chloride solution which meets commercial grade specifications.

The process incorporates three distinct phases:

washing and rinsing of the residue

precipitation of metals from the wash/rinse water solution

concentration of the resultant calcium chloride solution through evaporation to commercial standards.

Building the demonstration unit

The consulting engineers had to design a complete operating prototype unit, including evaporation, at 20 per cent of the scale of a typical full scale municipal incinerator. It had to incorporate all the major features of a full scale system. Proven process control equipment was to be used wherever possible; special or custom design components were a last resort. Moreover, the design had to be flexible enough to be able to be resized to a number of different municipal incinerator capacities (no standards exist for these), and the unit had to be mobile so that it could be transported and demonstrated on the site.

In eight weeks, the engineers provided a detailed scope, layout and cost estimate for the design and construction of a module that met the above objectives.

For mobility purposes the unit was built on a standard 13.7 metre (45 ft.) flat bed trailer with a 4.8 metre mezzanine addition for the wash/rinse phase equipment. Modular design and soft vinyl enclosures were used to ease the construction process and reduce costs. The construction crew had improved access because they could work at grade level. Each module incorporated spill control piped to a common header collection system so that the demonstration unit would present a minimal environmental risk while on site. The control architecture was PLC-based to test automatic control parameters for full-scale installations.

The project was completed from project approval through to commissioning within the projected 5 1/2 months at the estimated cost of $339,900. Natural Resources Canada and Environment Canada provided part of the funding.

After 18 months of testing, the results of the APC Residue Treatment Process have been analyzed and published, indicating that with minor modifications it will meet all the technical and commercial objectives. The process is in the final stages of receiving patent protection for Canada, the U.S., Europe and Japan. APEX Residue Recovery Inc. has been established as a corporate vehicle to commercialize the technology on a global basis.

Award winners: Mitchell & Associates Consulting Engineering (prime consultant/project managers). Project team leaders: Frank C. Mitchell, P.Eng. (project manager), Ken M. Smith (design/construction manager), George Wright (electrical), Benny Szeto (CAD). APEX Residue Recovery Inc. (owner), Steven Sawell (president), Stephen Hetherington (vice-president development)

Other key players: Flamboro Technical Services (mechanical contractor), Canral Electrical (electrical contractor)