Scrub solution treatment

Scrub solutions used for off-gas cooling or cleanup, together with all condensates and purge streams, should be collected, treated and returned to the scrubbing process. There shall be no direct pathway for active liquids to the environment. The scrub solution system blowdown should be treated by a radioactive liquid waste treatment system. In some instances the blowdown may be processed (e.g. dried) in the incineration system. 

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The treatment of amine-containing scrubbing solutions requires a nitrification/denitrification stage. These treatment steps can also be carried out in an external biological waste water treatment plant. An alternative to discharge is the recovery of the amines. 

When using acid scrubbers, the amine is transferred to the scrubbing solution, which then needs further treatment. However, recovery of the amine from the solution is possible. This is discussed in Section 4.6.5. 

The weak acid or scrubbing solution bleed represents a disposal issue and is usually sent to an effluent treatment facility where the acid is neutralised with lime to form gypsum and the dissolved metals are precipitated. The filter cake can be returned to the sinter plant feed where the CaO content will be useful as a flux addition. The filtrate solution containing chlorides can be discarded, provided... 

The dry off-gas treatment systems are simpler and more economical to operate. These systems may preferably be used if there are no specific reasons for the incorporation of wet scrubbers. The dry systems keep the off-gas temperature well above the dewpoint so that corrosion by acid gas condensation can be minimized. A dry system may be selected when processing tritiated wastes (subject to regulatory conditions and to tritium concentrations) since it will release tritium, together with the dry-filtered off-gas, into the envirotunent. This practice may present a lesser radiological hazard when compared with the aqueous scrubbing of tritium and the consequent need for the handling and conditioning of the tritiated scrub solutions. 

The operation of both the dry and wet off-gas treatment systems generates secondary radioactive wastes. These typically include filters and filtrafitm materials, adsorption materials, and in the case of wet scrubbers, liquid scrub solutions and blowdowns. Dry scrubbers would produce secondary wastes in the form of solid neutralizing agents and their residues, probably mixed with fly ash. Some of these secondary wastes (both solid and liquid) may be processed by incineration or drying in the same incineration system. Others need to be handled by a separate volume reduction system or disposed of with or without immobilization. The ash removed from the various collection points in the off-gas treatment system also needs to be treated and disposed of with the bulk of the ash collected from the combustion system. 

A bioscrubber for air treatment consists of two separate units (1) a scrubbing tower into which water is sprayed, often over a medium that disperses the scrubbing solution, and air is blown counter to the flow of the scrubbing water, and (2) a biological wastewater treatment unit in which the impurities absorbed from the air are degraded, often with an activated sludge treatment process (see Chapter 5, Section 5.12 and Figure 5.9). This process is most suitable for removal of water-soluble air pollutants. 

For continuous processes the catalytic reactor, or a hybrid process if satisfactory chemical dosing equipment is already installed, appear to be a near-optimum solution still for many installations. At moderate hypochlorite concentrations, economic benefit does accrue from using the catalyst in-loop rather than end-of-pipe, but these benefits may be offset by any required investment in heat-exchange capability. At concentrations above 10 wt% the integration of decomposition into the scrubbing process is beneficial to the overall cost base of hypochlorite treatment. 

The SW-400 soil washing unit is an ex situ, treatment technology that removes contaminants from soil using biodegradable chemical surfactants and/or coUoidals. The water-based, soil washing process mechanically and chemically scrubs excavated soUs. Contaminants are removed from soils by suspension or dissolution within the wash solution. The SW-400 is mobile and transportable and can operate either as a stand-alone technology or in combination with others. The SW-400 system is currently commercially available. 

The acid gases, usually CO2 and H2S, are removed by scrubbing with diethanolamine or monoethanolamine solutions and possibly an additional caustic treatment. Older processes used caustic solutions of 5-15 wt% NaOH followed by a water wash. Of course, the spent caustic creates a disposal problem it must be neutralized with acid and then properly disposed of according to prevailing pollution and hazard waste standards. Different column configurations have been proposed, but usually large scrubber towers with well over 30 valve-type trays are used. 

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