Two is better than one: using UV in pre-treatment for removal of organics in industrial effluents.
Laboratory simulations of both UV and biological treatment aid in determining whether or not a specific target organic contaminant is susceptible to UV light (i.e., "Can the molecule absorb light either directly or through a catalytic process?"), and whether or not that UV exposure enhances the biological degradation of the target organic contaminant. In the laboratory, these systems are fed with fresh river water for several weeks to develop a mature biofilm representative of the industrial region under investigation.
In the design of a UV effluent treatment system combined with biological processes two major considerations must be taken into account. The first is the wastewater quality and its variability. The second is the nature of the discharge permit and the regulated discharge quality on that permit. Two-stage, or tandem, processes can achieve relatively high efficiencies of contaminant mineralization at reactor residence times considerably shorter than required for single-step chemical or biological reactors. As well, integration of two-stage wastewater treatment processes has the potential to reduce water treatment costs versus single stage systems. This is due to the increased variations in reactor residence time combinations and generally lower volumetric costs for biological versus chemical reactors. As shown in research conducted, partial chemical oxidation of a toxic wastewater may dramatically increase its biodegradability. This approach tends to be more effective in terms of overall cost, time, and level of organics removal than attempting to destroy all the organics in wastewater by chemical means. In general, the results of two-stage degradation described herein support the theory that this tandem configuration may be beneficial to overall contaminant and toxicity reduction at industrial outfalls, especially where biologically persistent compounds are anticipated.
To illustrate the value of two-stage, or combination, degradative processes, the kinetics of resin acid mineralization were assessed using two-stage laboratory scale systems. Resin acids are contaminants known to resist biodegradation both in industrial treatment systems and in receiving waters. These compounds constitute a major class of environmental toxins derived primarily from pulp and paper processing of softwoods, although lumber processing and natural weathering also contribute resin acids to river systems.
A study area in Germany was chosen for its location in a traditional pulp and paper milling region of the former German Democratic Republic. The River Saale is the Elbe's major tributary flowing through the states of Thuringia and Sachsen-Anhalt and receives organics inputs from several industrial facilities including pulp and paper mills.
Using the previously described laboratory scale reactors, single-stage UV degradation, single-stage biological degradation, and two-stage UV-biological degradation schemes were evaluated and compared for rate of degradation, degree of mineralization, and residual concentration of the target resin acids. The results indicate that photolysis by either UV254 or broad band UV/vis radiation is a powerful degradation method for the target compounds and also is an effective pre-treatment method for resin acid biodegradation. Application of the pre-treatment UV stage essentially doubled the subsequent biological degradation rate observed without UV exposure. In all cases investigated, the UV254 provided a more rapid photodegradation rate than the UV/vis source. The bacterial toxicity of the aqueous resin acid solutions as measured with Microtox luminescence assays decreased with exposure time. Consequently, the two-stage process applied to the resin acids did not generate any notable amounts of toxic intermediates and/or the intermediates formed were further degraded into compounds of lower toxicity than the parent molecules.
The results of the research conducted indicate that minor modifications to industrial water treatment and/or natural sunlight-mediated degradation processes are sufficient for reducing resin acids to levels below agricultural and ecological significance. With tandem photo- and biological treatment at pulp and paper mills, as well as in-situ degradation by solar radiation and natural biofilms within natural river courses, resin acid inputs can be reduced in both concentration and toxicity to near undetectable levels. Since the northern flows of the River Saale are used extensively for agricultural irrigation, concerns related to toxicity and contamination may be alleviated by the implementation of upstream two-stage industrial effluent treatment processes. Implementation of UV pre-treatment in existing industrial effluent treatment systems may increase bioavailability and reduce shock loading to the microbial communities responsible for degradation. For these reasons, tandem degradation is anticipated to be effective for removal of compounds seemingly recalcitrant to biodegradation alone. Furthermore, the interactions of these reactions represent environmental pathways of organic pollutants as they are exposed to solar radiation and bacteria ubiquitous to receiving waters.
ASAE student member Dena W. McMartin, P. Eng., is on the faculty of the engineering department, University of Regina, 3737 Wascana Parkway, Regina, SK Canada S4S-0A2; 306-585-4703; fax 306-585-4855; Dena.McMartin@uregina.ca.
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|Author:||McMartin, Dena W.|
|Publication:||Resource: Engineering & Technology for a Sustainable World|
|Date:||Jan 1, 2004|
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