Aqueous waste treatment

In most cases treatment of aqueous waste aims at splitting it into two streams: (a) a small fraction of concentrate containing the bulk of radionu­clides; and (b) a large part, the level of contamination of which is sufficiently low to permit its discharge to the environment or for recycling [11]. Effec­tive liquid treatment separates as much of the radioactive contamination as possible from the waste in a concentrated form. Generally, the radioac­tive concentrate requires additional conditioning prior to disposal. Aqueous treatment processes are usually based on conventional physical and chemi­cal treatment principles with individual characteristics of the waste to be considered. Failure to account for the chemical and biological nature of aqueous waste may result in inadequate treatment and/or conditioning and could even damage the waste processing facilities. Detailed descriptions of the technologies can be found in Refs [11, 15-18] . Historically, three tech­nologies have mainly been applied to treat aqueous waste, namely chemical precipitation, ion exchange and evaporation. Membrane processes such as reverse osmosis, nanofiltration, ultrafiltration and microfiltration are now also successfully used and demonstrating good performance. In each case, process limitations due to corrosion, scaling, foaming and the risk of fire or explosion in the presence of organic material should be carefully consid­ered, especially with regard to the safety implications of operations and maintenance. If the waste contains fissile material, the potential for critical­ity should be evaluated and eliminated to the extent practicable by means of design and administrative features.

The objective of a chemical precipitation process [15] is to remove radio­nuclides from liquid waste by the use of an insoluble finely divided solid material. The insoluble material, flocculate or floc is generally, but not nec­essarily, formed in situ in the waste stream as a result of a chemical reaction. The use of these processes concentrates the radioactivity present in a liquid waste stream into a small volume of wet solids (sludge) that can be sepa­rated by physical methods from the bulk liquid component. Chemical pre­cipitation is suitable for the waste which is low in radioactivity, alkaline in pH and contains a significant salt load. This process is simple and relatively inexpensive in terms of the plant and its operation but it requires good understanding of the process chemistry and strict consideration of process parameters. The process may be limited by the activity level.

Ion exchange is a standard method of liquid clean-up [16]. The ion exchange materials are insoluble matrices containing displaceable ions, which are capable of exchanging with ions in the liquid passing through by reversible reaction. Organic and inorganic, naturally occurring and syn­thetic ion exchangers have found their specific fields of application in dif­ferent purification and liquid waste treatment processes. If the waste is relatively free of salts, mildly acidic in pH and requires a decontamination factor of around 100 or so, ion exchange may be a good choice. This process is more expensive than chemical treatment — especially when special purpose resins are used — but has a wider range of application with regard to radioactivity concentration.

The limitation of conventional filtration and ion exchange is that colloidal particles, some radioactive, pass straight through to the product (treated) water. Colloidal particles containing 58/60Co, 54Mg, 55Fe and 125Sb are typical examples. Ultrafiltration is capable of removing these particles completely and has been adopted at a number of sites to complement the existing conventional filtration/ion exchange systems.

Membrane processes [ 17] are successfully used as one or more of the treatment steps in complex waste treatment schemes, which combine con­ventional and membrane treatment technologies. For example, electrodialy­sis is a well-established membrane technology that has been used widely for the desalination of brackish water and also to separate monovalent ions from multivalent ions. These combined systems offer superior treatment capabilities, particularly in instances where conventional methods alone could not perform a similar task as efficiently or effectively. They are capable of producing high-quality treated effluents with an acceptably low level of residual radioactivity for discharge, or for recycle and reuse. The concen­trate waste stream containing the removed radioactivity invariably needs further processing by evaporation or other means to facilitate final condi­tioning to a solid waste form suitable for intermediate storage and disposal. When applying membrane technologies, the selection of the membrane material, its configuration and the operating parameters are critical. A wide variety of membranes are commercially available with different operational characteristics [17] [ The choice of a membrane must be based not only on performance data (salt rejection, flux), but also take into account the inter­action of the membrane with the feed solution and whether this will lead to stable operation and minimal fouling (a process where deposits on surface or into pores of membrane cause performance degradation).

Evaporation is a proven method for the treatment of liquid radioactive waste providing both good decontamination and good concentration [18] . Water is removed in the vapour phase of the process leaving behind non­volatile components such as salts and most radionuclides. There could be situations when waste volumes are somewhat high, having a low salt content but a considerably higher activity level; in this event evaporation is used to reduce the waste volume to a concentrate and also to obtain a high decon­tamination factor (of the order of a few thousand). However, the process can be limited by the presence of volatile radionuclides, and also it is energy-intensive.

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