Membrane Technology for the treatment of pharmaceutical wastewater
Water resources which can be utilized are not infinite and hence have to be protected in order to preserve the public health and lives of the population. Considering the above, the discharge standards for wastewater effluents have been increasingly strengthened to reduce contaminants released into the water environment. Thus there has been an increasing emphasis and endeavor for reduction of total solids (TS) and their subsequent reuse for industrial applications. A high quality of treated wastewater reduces negative impacts on inland waterways, and thus widens the range of possible reuse options.
Some of the conventional treatment technologies such as ion exchange, carbon adsorption etc., have their own disadvantages and cannot be considered reliable for long term solutions. The use of membranes to separate solids and permeate from raw wastewater signifies the main difference between membrane bioreactors (MBRs) and traditional treatment plants. Among the biological processes, MBRs have been regarded as the advanced treatment technology due to their increased effluent quality, operability with high biomass concentrations, compact reactor configurations, wide range of operating conditions such as sludge age and organic loading rates (OLRs) (Marrot et al., 2004).
Table of Contents
Pharmaceutical Products and Wastewater Treatment
World over, there is a very high production and consumption of pharmaceutical products. Pharmaceutical products are generally designed to produce a biological activity on human beings. Manufacturing processes of pharmaceutical products lead to the release of toxic organic compounds and their metabolites into the environment. Most of the above such compounds present in the wastewater and released to the environment indiscriminately, have inherent recalcitrant behavior and are structurally complex organic compounds. Hence, they are not easily removable from such wastewaters by conventional treatment processes. Extensive literature review carried out reveals that substantial studies have been carried out on the treatment of municipal wastewater, highly organic industrial wastewater and synthetic wastewaters using MBRs. However, various aspects of treatment practices still received little or no attention to-date for the application of MBRs for the treatment of pharmaceutical wastewaters, as evident from the overview of various wastewaters investigated by MBRs. Even a few research works reported were on the exclusive treatment of pharmaceutical wastewaters in combination with conventional biological treatment process and MBRs. In the above reported studies the biological processes serve as a pre-treatment for subsequent treatment by MBRs. However, research on the treatment of pharmaceutical wastewaters using MBRs as a single stage of treatment has not been investigated and reported so far. However, if the application of the above treatment is adopted, it eliminates the provision of solid thickening facilities and hence reduces land area required for treatment, apart from other advantages like low maintenance and operation cost of wastewater treatment. In view of the above advantages and the enormous potential for research on an emerging area, attempts were made in the present investigation to comprehensively study the treatment of a real (industrial) pharmaceutical wastewater in a submerged MBR. A ‘membrane’ as applied to water and wastewater treatment is simply a material that allows some physical or chemical components to pass more readily through it than others (Judd. S and C. Judd, 2007). It is thus perm-selective, since it is more permeable to those constituents passing through it (which becomes the ‘permeate’) than those which are rejected by it (which forms the ‘retentate’). An overview of the membrane separation processes is shown in Fig. 1. In view of the above, there exists an enormous potential for research on the treatment of industrial pharmaceutical wastewater using MBRs
Bioaugmentation
Bioaugmentation refers to a method of maintaining sufficient biomass when adequate carbon substrates and nutrients are unavailable in the effluent for biodegradation. Bioaugmentation usually help conventional biodegradation processes work faster, or may provide additional, exogenous biological agents to polluted systems and improve the transformation processes (Bathe et al., 2005; Fantroussi and Agathos, 2005). Bioaugmentation has been demonstrated to enhance the removal of many specific pollutants such as phenols, chloroaniline, chlorobenzoate, resin acid, etc. (Singer et al., 2005; Thompson et al., 2005; Damsa et al., 2007). However, the bioaugmentation dose not always works because the inoculants were washed out from the system (Limbergen et al., 1998; Singer et al., 2005).
Fig. 1 Overview of the membrane separation processes
(Judd and Jefferson, 2003)
Bioaugmentation can be implemented in two ways. One by using acclimated mixed culture microorganisms grown in an enricher reactor, wherein, a primary substrate (ie. the target substrate) and a secondary substrate (ie. a carbon source) which is seeded initially in the enricher reactor. The other method is to use a genetically engineered microorganism (GEM), which is a promising way to remove recalcitrant chemicals from wastewater. For example, Atrazine-degrading GEM obtained from the gene of Pseudomonas Sp. ADP and cloned into plasmid vector PACYC184 and then transformed into Esscherichia Coli DH5 alpha was used for treating of atrazine containing wastewater [Liu et al. (2008)]. However, there is an associated uncertain long-term ecological risk, which has not been systematically investigated and documented. On the other hand, the use of acclimated mixed culture microorganisms for bioaugmentation have the advantages of local and easy availability at very low cost.
Fouling Mechanisms in MBR
The pore size of most MBR membranes is in the range of 0.03 – 0.4 µm. Comparing the particle sludge in the feed sludge with the membrane pore size, the particles can only form a filter cake. The colloids and macro-organics can either form a filter cake or block the membrane pores (complete blocking or standard blocking). The solutes are unlikely to form a filter cake. They may either be absorbed on the membrane pores and result in standard blocking or pass the membrane and end up in the permeate without any interaction with the membrane. The relative contribution of particulates, colloids/macro-organics and solutes to membrane fouling are influenced the filtration flux and hydrodynamic conditions, which determine the tendency of particle deposition. If the flux is high but the cross flow velocity is low, the permeation velocity can be higher than the back transport velocity. The particulate fouling and cake filtration may dominate. However, if the filtration flux is low and the crossflow velocity is high, the permeation velocity can be lower than the back transport velocity and only colloids/macro-organics and solutes may deposit/absorb on the membrane. The role of organic fouling and pore blocking becomes important. However, most full-scale MBRs run under sub-critical flux condition to limit the deposition of particulates and only colloids/macro-organics and solutes may deposit. Many studies concluded that cake filtration is the dominant fouling mechanism in MBRs. It has been reported that the membrane resistance, cake resistance, blocking and irreversible fouling resistance contributed 12%, 80% and 8% to the total resistance, respectively in a submerged MBR using 0.1 µm UF membrane (Lee et al., 2001). Chang and Lee (1998) reported that cake resistance was the major contributor to the resistance of membrane coupled activated sludge systems especially under low sludge age conditions. During the filtration process of MBR, the formed filter cake may function as a dynamical membrane layer and reduce direct contact of foulant with the membrane. In addition, the colloidal/macromolecular organic matter could be rejected/ adsorbed and biodegraded by the dynamic “membrane”. As a result, pore blocking is alleviated and membrane cleaning becomes easier (Lee et. al. 2001).
MBRs are extensively used in well developed countries for treatment of wastewaters. However, research on MBRs and the use of MBRs for treatment of wastewaters is rather rare in developing countries like, India. Substantial studies have been considered on treatment of a variety of synthetic wastewaters, municipal wastewaters, petrochemical and polymeric wastewaters. However, studies on the treatment of pharmaceutical wastewater using MBR, is rather rare. Even in a few reported studies, a combination of conventional biological treatment and MBR has been considered for treatment of pharmaceutical wastewaters. In such studies, the biological processes serve as a pre-treatment for subsequent treatment by MBRs. Research on the treatment of pharmaceutical wastewaters using MBR as a single stage of treatment has not been investigated and reported so far, in spite of specific advantages of such a treatment. In general, mathematical modeling of MBR process has not been carried out extensively, especially, covering all types of wastewaters.
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References
Marrot, B., A. Barrios – Martinez, P. Moulin and N. Roche (2004), Industrial wastewater treatment in a membrane bioreactor: A review, Environmental Progress, 23 (1), 59 - 68.
Judd.S and C. Judd (2007), The MBR Book: Principles and applications of membrane bioreactors in water and wastewater treatment, Elsevier publication.
Chang, I.S. and Lee, C.H (1998), Membrane filtration characteristics in membrane – coupled activated sludge system – the effect of physiological state of activated sludge on membrane fouling, Desalination, 120 (3), 221-233.
Lee, J., W.Y. Ahn and C.H. Lee (2001), Comparison of the filtration characteristics between attached and suspended growth microorganisms in submerged membrane bioreactor, Water Research, 35 (10), 2435 – 2445.
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This article was written by:
R. Saravanane
Assistant Professor, Environmental Engineering Laboratory, Department of Civil Engineering, Pondicherry Engineering College, Puducherry – 605 014, India, E-mail:rsaravanane@pec.edu
The issues in the article are covered in the more extensive research paper on the Treatment of Cephalosporin Anti-biotic Pharmaceutical Effluent in a Submerged Flat sheet Membrane Bioreactor published inEnvironmental Technology, Volume 30, Issue 10 September 2009 , pages 1017 - 1022.
