Buildings and Industry

Anaerobic Treatment of Waste in Flexography Industry

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Summary

In the flexography industry, incineration is the most widely used technology in North America to destroy volatile organic compounds (VOCs) in the air exhausted from plants. Among other drawbacks, this technology requires a great deal of energy.

Société de recherche SNC, a subsidiary of SNC-LAVALIN, has developed an original system for treating the air from flexography plants. In the SNC-LAVALIN system, the VOCs are converted by bacteria to methane gas. As a result, this new system brings substantial savings in energy and operating costs for the plant in which it is installed. In addition, the methane can be used to heat the plant.

The Principle

Flexography is a process (widely used to print food wrappers) for continuous printing on a plastic or aluminium film, using ink diluted in a mixture of solvents. Once the printing is completed, the solvents, known as "volatile organic compounds" (VOCs), are separated from the ink by drying in hot air. Incineration, in particular, is the most widely used treatment technology in North America. Incineration consists of heating the entire air/VOC mass to a high temperature to burn the VOCs. The following are a few disadvantages of incineration:

• high operating costs because of the substantial consumption of natural gas and electricity;
  
• problems in deactivating the converter, despite the potential heat recovery provided by the catalytic incinerator;
  
• destructive technology contributing to the greenhouse effect by the formation of more CO₂.

A typical flexography plant discharges around 900 to 1,800 kg of VOCs per day, with an air flow that can reach 100,000 m³/hour.

Société de recherche SNC has developed an original system for treating the air exhausted from flexography plants. The system consists of a water-based purifier (scrubber), a buffer tank and a multi-plate reactor. The VOCs are removed from the contaminated air by the scrubber. The clean air is discharged into the atmosphere, while the water containing the VOCs leaving the washer is taken to the buffer tank, where nutrients are added. This effluent is then directed to the anaerobic reactor, where the VOCs are converted into biogas containing 75% methane. The cleaned liquid effluent is returned to the scrubber (Figure 1). The generation of the biogas could be used within the plant as a source of energy.

In May 1993, a 4-litre reactor was installed on the site and tests were carried out with a synthetic mixture consisting of the same elements as those in the contaminated air. After the tests had confirmed the treatability of the effluents, Société de recherche SNC installed a 900-litre pilot reactor in October 1993. To evaluate performance under actual plant conditions, a duct was connected to the plant air effluent and run to the pilot unit (Figure 1). The air flow rate through the scrubber was approximately 250 m³/h with a liquid flow through the system of 75 l/h.

The performance of the entire system for water soluble solvents under actual plant conditions is outlined in Table 1. The production of gas was very stable at 3,790 l/day which is very close to the maximum theoretical yield possible. The methane content of the biogas was 83% with undetectable amounts of all solvents. The yield of methane is thus 0.66 m³/kg solvent treated in the reactor.

In the case where solvents of low solubility were used, a different configuration was set up. The main alteration was the replacement of the scrubber with an activated carbon absorber, which is a column filled with granulated activated carbon. The air containing the solvents is passed through the column where adsorption of the solvents takes place. The purified air is then released into the atmosphere. Once the column is saturated, the solvent is removed by heating the column with steam, and recovered for reuse in the plant, providing substantial savings. The desorbed solvent vapours are then condensed by cooling and solvents of low solubility can be recovered by decantation (Figure 2). The soluble solvents which remain in the water phase can be treated anaerobically. The activated carbon is ready for reuse.

The SNC-LAVALIN system operates at lower temperatures (35°C) than those of incineration, and thus requires much less energy. The biological aspect of the system makes it a soft technology, resulting in a favourable perception, unlike conventional incineration systems which contribute to air pollution. Biological technologies are more suited to small and medium-sized industrial firms because of the lower operating costs and the simple, soft-technology operation.

In addition to flexography plants, the SNC-LAVALIN process can be applied to a number of water-soluble VOC sources, including print shops and lithography workshops; factories producing adhesives, caulking materials, wallpaper, inks and varnishes; factories producing electrical and electronic components; tanneries; and processing plants for foods, chemicals and petrochemicals.

Economics

From the economic standpoint, the capital and operating costs of the SNC-LAVALIN system for treating VOCs compare favourably with those of the catalytic incineration and thermal processes. Table 2 compares these three systems for treating gaseous effluents of 46,000 m3/h (24 kg/h of solvents). The investment profitability period of the SNCLAVALIN system varies between three and five years, depending on the flow of air to be treated and the technology to be replaced.

Table 1: Performance of the entire system for water soluble solvents under actual plant conditions.

Table 2: Comparison of costs (Canadian Dollars CAD).