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Biological Processes For The Removal Of Organic Xenobiotics From Wastewaters

Congress: 2015
Author(s): Adamu Rasheed, Davide Dionisi
University of Aberdeen1

Keyword(s): Sub-theme 13: Non-conventional sources of water,
Abstract

Davide Dionisi * and Adamu A. Rasheed

Materials and Chemical Engineering group, School of Engineering, University of Aberdeen, Aberdeen, AB24 3UE, UK

*Corresponding author. Tel: +44 (0)1224 27281; E-mail address davidedionisi@abdn.ac.uk

Introduction

This study is aimed at developing biological processes for the removal of organic xenobiotics from wastewaters. The benefits include; reducing the presence of organic xenobiotics in raw water bodies such as groundwater, rivers, lakes, sea, where treated wastewaters are ultimately discharged; developing a more environmentally friendly processes for the removal of organic xenobiotics from wastewaters as well as improving the scientific knowledge on biological wastewater treatment processes for xenobiotics removal.

Xenobiotics are defined as man-made synthetic chemicals which are foreign to the biosphere (Dionisi, 2014; Bosma et al. 2001). Primary xenobiotics include pesticides, polycyclic aromatic hydrocarbons (PAHs), halogenated aromatic and aliphatic compounds, pharmaceuticals, azo compounds, chlorinated and polychlorinated biphenyls (CBs and PCBs), etc. In a broader sense the term xenobiotics also include the naturally occurring substances such as petroleum hydrocarbons and natural hormones that are present in some water bodies at concentrations higher than their natural levels mainly due to human and industrial activities. Xenobiotics may be present in wastewaters due to releases from manufacturing processes or from households. Commonly household products such as cosmetics, pharmaceuticals and detergents contain a wide range of xenobiotics, e.g. surfactants, biocides, oils, fragrances. They may also be released to the environment by food-processing industries (Marsden et al. 2001). Thus, xenobiotics have become one of the greatest problems of modern world-wide society today due to their extreme resistance to natural degradation and toxicity to the ecosystem.

Xenobiotics are often treated by physiochemical methods such as adsorption on activated carbon or other adsorbents, chemical oxidation, membrane filtration, incineration, etc. Though these techniques are often effective, they are characterized by only transferring the substance from one phase to the other (e.g. a solid phase for activated carbon adsorption) without actually removing it from the environment. They are also very expensive and in some cases may generate toxic products (e.g. PAHs can be released to the atmosphere during incineration of industrial wastewaters) (Dionisi, 2014).

Microbial biodegradation is an interesting alternative for the removal of organic xenobiotics from the environment. It is effective, minimally hazardous, economical, versatile and environmentally-friendly. By the means of biodegradation, microorganisms can be utilized to oxidize organic substances into non-toxic products such as carbon dioxide and new microorganisms. When present in the influent of conventional biological wastewater treatment plants, many xenobiotics are only partially removed because they are metabolised in the presence of a more readily biodegradable compound serving as the primary growth substrate for the microorganisms. Thus there is lack of scientific knowledge on the biodegradation of xenobitics as primary growth substrate in biological wastewater treatment facilities. Therefore this study will investigate the biodegradability of the most common xenobiotics found in wastewater as primary growth substrates for microorganisms. This represents the first step towards closing these gaps in our knowledge of their treatment.

Methods/Materials

Glucose and three (3) model xenobiotics were studied: phenol, p-cresol and bisphenol A (BPA) as primary growth substrates in a biological reactor. The research was carried out at lab scale in glass bioreactors inoculated with 100 mg/l of mixed microbial cultures from soil. The first step was to run the bioreactors with a single substrate (100 mg/l) serving as the only carbon source for the microorganisms. The concentration of the substance was monitored over a length of time.

Results and Discussion

Glucose was studied because of its readily biodegradable nature. It is used as a test to determine whether the microbial community is active, and if so, able to degrade organic substrates.

Table 1: Biodegradation time of glucose, phenol, p-cresol and BPA

The compounds here are said to be degraded if 95% decrease in their concentration is achieved over a period of time.

From the table 1, glucose was degraded within 2 days of inoculation. This concludes that the environment (pH, temperature, oxygen and nutrients) is favourable for the growth of microorganisms. It also concludes that the microbial community used is active and hence will be able to degrade other carbonaceous compounds.

Phenol and p-cresol were also degraded within a week of inoculation. It also shows that although phenol and cresol are biodegradable, the microbial community needed some time to acclimate and develop the enzyme necessary for their biodegradation, hence the longer degradation time.

BPA showed no sign of degradation after 150 days. However, the removal of glucose, phenol and p-cresol is evident that the microorganisms will eventually develop the abilities to degrade BPA. The degradation of p-cresol over a short time may also suggest that the rate limiting step for BPA degradation is hydrolysis of the molecule (BPA is basically a combination of two p-cresol molecules). So once the BPA molecules are hydrolysed into simpler products like p-cresol, the microorganisms will be able to quickly degrade the compound.

Conclusion

In conclusion, degradation of BPA was not achieved because the contact between the microorganisms and BPA is probably too short for the development of enzymes which will hydrolyse and then degrade the molecules.

The BPA reactor will continue to run until degradation is achieved. Once it's achieved, the xenobiotic-degrading microorganisms will be inoculated into a continuous process where important parameters (e.g maximum growth rate and yield) will be used to design an activated sludge process capable of removing these xenobiotics from wastewater effluents.

1. Bosma,T., Harms., Zehnder, A. (2001). Biodegradation of xenobiotics in environment and technosphere, in: Biodegradation and Persistance, Springer, 163.

2. Dionisi, D. (2014). Potential and limits of biodegradation processes for the removal of organic xenobiotics from wastewaters. ChemBioEng Reviews, 1, 67.

3. Marsden, LM.W., Mackay, D.W. (2001). Water quality in Scotland: the view of the regulator. Sci Tot Environment, 265, 369.

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