However, several issues were raised as to the high cost and lack of information on technical aspects of the process [ 22 ]. The combination of AOP as chemical pretreatment and biological processes was found particularly useful in enhancing the biodegradability and was recommended as a successful technology for industrial wastewater treatment. The combination of microwave irradiation with AOPs has also been discussed in detail [ 15 ]. The choice of a pretreatment method cannot be generalized as it depends upon several factors, including the type of contaminants, real conditions and process costs [ 23 ].
Many of the aforementioned methods have been recognized as efficient techniques for the treatment of RWW and to offer a lot of featured advantages such as energy efficiency, safety and environmental compatibility [ 3 , 13 ]. However, most of these physiochemical methods suffer from noticeable drawbacks such as high capital and operating costs.
Ultimately, most of them do not destroy the contaminant, but rather transfer it to another phase, which results in the formation of harmful by-products [ 13 , 16 ]. Formation of chlorinated organic compounds has been reported during some electrochemical applications, and activated carbon adsorption was recommended as a further polishing treatment to remove them [ 5 , 12 ]. Also formation of intermediate by-products like catechol and hydroquinone was detected after ozonation of phenolic compounds [ 22 ].
Although AOPs have emerged as effective methods, which offer a chance for the mineralization of various biorefractory organics [ 14 ], the running cost of many AOPs is still relatively high [ 15 ]. Also, they were reported for the formation of by-products and to be limited to treating wastewater with relatively low COD concentration [ 24 ]. Many other methods which are relatively cheap and easy to operate are characterized by strict technical limitations, in terms of operating conditions and effluent hydraulic rates e. Fenton and membrane applications , low efficiencies and excessive sludge generation e.
On the other hand, some applications are limited by the hazards associated with them like in ozone utilization, being an unstable gas [ 2 ].
Others are difficult to be commercialized for real-time RWW treatment, and large-scale industrial applications seem to be lacking. In view of the above discussion, it is essential to search for more viable alternatives that can be utilized in novel biological treatment systems. As for the several mixed processes that have been proposed recently, a lot of these treatment schemes not only have noticeable advantages but also have important drawbacks. These problems can be sorted in two main areas: the first relates to all economic aspects including the high cost needed for the implementation of these techniques; the second includes all technical issues related to the resources needed for the transformation from very toxic compounds to environmentally compatible ones [ 22 ].
Biodegradation is the decomposition of organic substances by microorganisms into metabolic by-products with lower toxicity.
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Enzymes play a catalytic role in this process, where a chemical is converted stepwise into end products through various intermediates. This transformation is called mineralization [ 25 ]. Biodegradation is a cost effective and environmentally compatible option that is often preferred, thanks to the possibility of complete mineralization [ 26 , 27 ].
Because of the aromatic structure of many organic compounds e.
However, several microorganisms have the capability to utilize these compounds for their metabolic activities as carbon and energy sources. Biological transformation has been recognized as one of the key solutions to deal with environmental pollution caused by many problematic organic contaminants. In this regard, the use of pure and mixed cultures of organisms is considered a favorable and most promising approach [ 28 ]. Many strains of bacteria, fungi and algae have the ability to degrade toxic organic substances.
Bacterial cultures of Pseudomonas genus are the most commonly utilized biomass for the biodegradation of organic contaminants, with special interest paid to Pseudomonas putida due to its high removal efficiency [ 29 ]. However, a main drawback in bioprocesses is the inhibition of the enzymatic activity at high substrate concentrations. Under certain conditions, organic material can be decomposed aerobically or anaerobically [ 30 ].
Conventionally, aerobic processes are preferred. Aerobic microorganisms grow faster and are more efficient because they can achieve complete mineralization of toxic organic substances to inorganic constituents CO 2 , H 2 O [ 31 ].
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This is in addition to low associated costs [ 16 ]. On the other hand, the end products of biochemical reactions in anaerobic processes often produce esthetically displeasing colors and fouling odors in water [ 2 ]. Therefore, there is a limited interest in the utilization of anaerobic microorganism for the degradation of organic waste. However, there have been several studies in this regard [ 32 — 34 ].
Since most biological treatment studies have used aerobic biomasses, discussion in the following sections of this chapter will focus on aerobic biodegradation. Detailed reviews on the biodegradation of some organic compounds can be found in the literature [ 27 , 35 — 37 ]. Biodegradation is a multivariable process, which is affected by a combination of many biotic and abiotic factors, including pH, temperature, oxygen content and availability, microbial abundance and substrate concentration [ 26 , 27 ].
The chemical structure of aromatic compounds plays a key role as reflected by the number, type and position of substituents on the aromatic ring and degree of branching.
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The greater the number of substituents in the structure, the more toxic and less degradable it is. Metabolic processes are dominated by the catalysis of enzymes, which are particular to each type of biomass and reaction. A metabolic reaction is ultimately a process of energy conversion. Little is known about the biodegradation mechanism by fungi and algae; so the following is a brief discussion of this mechanism by aerobic bacteria, as typically represented by the biodegradation of aromatics.
In aerobic biodegradation, enzymatic attack on the aromatic ring is initiated by oxygen. A typical pathway for metabolizing phenols phenol is a basic structural unit for a variety of synthetic organic compounds is to hydroxylate the ring by the enzyme phenol hydroxylase, form catechol and then open the ring through ortho- or meta- oxidation. Thus, phenol hydroxylase is the first enzyme and catechol is a basic intermediate in the degradation pathways of many aromatic substances. In the ortho- pathway, the aromatic ring is cleaved by the enzyme catechol 1,2-dioxygenase C12O. In the meta -pathway, the ring is cleaved by the enzyme catechol 2,3-dioxygenase C23O.
The ring is thus opened and then degraded [ 27 ]. C12O and C23O designate two different orientations as to how the ring cleavage can occur. However, the biodegradation of many aromatics proceeds through the ortho -cleavage pathway after the formation of catechol because the meta -cleavage results in the formation of dead end metabolites from catechol; the enzyme gets inactivated by the accumulation of a toxic intermediate [ 38 ].
As a rule, conversion of catechol does not follow the meta -cleavage pathway, and generally, the ortho -cleavage pathway is required for the complete degradation of many aromatic organics [ 39 ]. Discussion of biodegradation pathways and mechanisms can be found in [ 27 , 35 , 36 , 40 ]. Kinetic studies indicate how effectively a bioprocess is functioning.
This is essential to improve process control and removal efficiency [ 31 ]. The main step in modeling a biodegradation process is to relate the specific growth rate of the biomass to the consumption rate of the substrate [ 26 ]. Different kinetic models have been used to describe the dynamics of microbial growth on phenols [ 35 ]. There are two most common models for cell growth during the biodegradation: the Monod model and the Haldane model, represented by Eqs.
The first model neglects the substance inhibition, whereas the second model is the most widely used since it accounts for the inhibitory effect of toxic material and it is mathematically simple [ 26 , 41 ]. It is noteworthy that when K i is very large, the Haldane equation reduces to the Monod model.
Although cell immobilization by entrapment offers significant advantages, it is problematic in terms of diffusion limitation owing to the resistance imposed by the protective structure [ 42 ]. Also, in the design and modeling of bioreactors, it is equally important to assess external mass transfer coefficients for the transfer of substrate from the bulk phase to the surface of the biofilm.
Diffusion limitations considerably affect the intrinsic reaction kinetics. Therefore, kinetic models for degradation in biofilm reactors are complex and difficult to develop. Several studies addressed this topic and proposed comprehensive diffusion-reaction models, which account for mass transfer limitations [ 42 , 43 ]. In light of the fact that biodegradation of aromatic organic pollutants has a unique environmental and ecological impact, there is a rising interest in this area of biotechnology and a strong motivation to develop efficient strategies and novel cost-effective methods for RWW management.
This section will briefly review some of the achievements on this track. Immobilization is an effective technique that is usually used for many purposes, mainly the protection of the biomass from toxicity effects of the contaminant, in addition to the ease of separation and reutilization of the biomass [ 35 ]. Other advantages were highlighted in the previous section.
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Bacterial activity can be increased by various immobilization methods like bead entrapment, carrier binding, encapsulation, cell coating and film attachment [ 44 ]. Polyvinyl alcohol PVA is a synthetic polymer that shows attractive properties to be used as a carrier in bacterial immobilization; it is inexpensive, nontoxic, water soluble and has excellent electrical insulation [ 45 ]. The effectiveness of these gel pellets with immobilized P.
Emphasis was placed on the contaminant uptake per mass of PVA, which is of particular economical importance in the design of RWW treatment processes. Compared to natural biodegradable Ca-alginate, which has long been used in the area of biocatalysts, PVA gel is more durable with lower resistance to mass transfer due to its porous structure. In view of that fact that biodegradation is in essence a series of enzyme-catalyzed reactions, there has been an increasing attention to the direct application of immobilized robust enzymes obtained from degrading microorganisms for the treatment of contaminated wastewater.
Enzymatic treatment of organic pollutants is documented in a few recent studies, which addressed novel, practical and inexpensive immobilization methods. Attention in recent years has focused on developing aerobic granules in sequencing batch reactors SBRs [ 50 ].
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