In this article we will discuss about the top four methods adopted for management of wastewater. The methods are: 1. Use of Commercial Blends of Microorganisms in Wastewater Treatment 2. Use of Immobilised Cells in Wastewater Treatment 3. Role of Microorganisms in Metal Removal 4. Application of Recombinant DNA Technology in Waste Treatment.
Contents
Method # 1. Use of Commercial Blends of Microorganisms/Enzymes in Wastewater Treatment:
(i) Bioaugmentation (Use of Blends of Microorganism):
Acceleration of biodegradation of specific compounds by inoculating bacterial cells is called bioaugmentation. Bacterial cells contain specific plasmid which encodes enzymes for degradation of those, compounds.
A variety of plasmids have been reported from Alcaligenes, Acinetobacter, Arthrobacter, Beijerinkia, Klebsiella, Flavobacterium and Pseudomonas. Several genetically engineered strains have been developed exploiting Pseudomonas.
Microroganisms capable of degrading herbicides/other chemicals in industrial water are isolated from wastewater, compost, sludge, etc. Some of the strains may be irradiated to enhance their ability and mutants are selected.
Before their use in the environment they are tested in laboratory for their biodegradation ability. Bioassays are also used to assess the toxicity of the waste water for commercial preparation of microbial seeds. Selected strains are used in large fermentor to get mass culture. Then they are preserved through lyophilization, drying and freezing.
Commercial bioaugmentation products are single culture of consortia of microorganisms with certain degradative properties or their desirable characters. At present most important users are the industrial wastewater treatment plants.
The selected microorganism is added to a bioreactor so that potential for biodegradation of wastes must be maintained or enhanced. Due to trade secrets information on bioformulation of mixture of microbial cultures are not scanty.
Application of bio augmentation includes:
(a) The increased BOD removal in wastewater treatment plants,
(b) Reduction of sludge volume by about 30% after addition of selected microorganisms,
(c) Use of mixed cultures in sludge digestion,
(d) Biotreatment of hydrocarbon waste, and
(e) Biotreatment of hazardous wastes.
The use of added microorganisms for treating hazardous wastes such as phenol, ethylene glycol, formaldehyde has been attempted. Bioaugmentation with parachlorophenol-degrading bacteria decomposed 96% para-chlorophenol in 9 hours. Cells of Candida tropicalis have been used for removal of high concentration of phenol present in freshwater.
Ability of a bioreactor to dechlorinate 3-chlorobenzoate was increased after addition of Desulfomonile tiedjei to a methanogenic upflow anaerobic granular sludge banket. Anoxygenic phototrophic bacteria have also been considered for the degradation of toxic compounds in wastes.
Some demerits of bioaugmentation are:
(a) Need of an acclimation period prior to onset of biodegradation,
(b) A short survival or lack of growth of microbial inocula in the seeded bioreactors, and
(c) Sometimes negative or non-conclusion of some of commercial products.
(ii) Use of Enzymes in Wastewater Treatment:
Several enzymes have been detected in wastewaters such as catalase, phosphate esterases and aminopeptidases. These enzymes can be added to freshwater to improve biodegradation of xenobiotic compounds. For example parathion hydrolases (isolated from Pseudomonas and Flavobacterium) have been used to clean up the containers of parathion and detoxification of wastes containing high concentration of organophosphates.
A little information is available on use of enzymes in wastewater treatment plants. This technology is applied to reduce the production of excessive amount of extracellular polysaccharides during wastewater treatment because overproduction of polysaccharides results in increased water retention with reduced rate of dewatering. Addition of enzyme can degrade these expolymers.
Some specific enzymes (e.g. horseradish peroxidase) can catalyse the polymerisation and precipitation of aromatic compounds (e.g. substituted aniline and phenols). Horseradish peroxidase catalyses the oxidation of phenol and chlorophenols by hydrogen peroxide.
The extracellular fungal laccases (obtained from Trametes versicolor or Botrytis cinerea) can be used for the treatment of effluents generated by the pulp and paper industry because this enzyme can be useful tor dechlorination of chlorinated phenolic compounds or oxidation of aromatic compounds even at adverse environmental conditions such as low pH, high temperature, presence of organic solvents etc.
Therefore, attention has now been paid to use extremozyme of microorganisms that can work at extreme environments also.
Method # 2. Use of Immobilised Cells in Wastewater Treatment:
There are various methods to immobilise the microbial, plant and animal cells such as:
(a) entrapment (immobilisation of cells in polymeric materials e.g. alginate, carrageenan, polyacrylamide polyurethrane foam, etc.,
(b) adsorption (on sand beads, porous silica, porous brick or wood),
(c) covalent binding (using hydroxymethyl acrylate).
Entrapment is the most popular approach.
Immobilised cells have been used for the treatment of various wastes for decontamination of water or wastewater containing natural or xenobiotic compounds and decontamination of soils and aquifers.
Some of the following examples of pollution control has been discussed by Bitton (1999):
(a) Removal of brown lignin compounds:
Brown lignin compounds are found in paper mill effluents. Immobilised white-rot fungus (Coriolus versicolor) can be used to remove this compounds.
(b) Biodegradation of phenolic compounds:
Much information is available on degradation of phenolic compounds by using bacteria. Immobilised cells of Pseudomonas, Arthrobacter and Alcaligens degrade chlorinated phenols.
Bioreactors containing Flavobacterium immobilised in calcium alginate can degrade pentachorophenol at maximum rate (i.e 0.85 mg/g beads/ hours Immobilised tyrosinase can remove rapidly phenol, chlorophenol, methoxylphenol and cresols from fresh water. A polycation derived from chitin (chitosan) can remove efficiently the coloured products from the effluents.
(c) Methane production by Immobilised methanogens:
Anaerobic waste treatment can be enhanced using immobilised methanogens in two-staged bioreactors.
(d) Dehalogenation of chloroaromatics:
Immobilised cells of Pseudomonas sp. removes chlorides linked to aromatic compounds.
(e) Immobilised activated sludge microorganisms:
A high treatment efficiency was achieved with a two step process consisting a reactor containing immobilised activated sludge microorganisms followed by a biofilm reactor.
(f) Use of immobilised algae to remove micronutrients from wastewater effluent:
Immobilised Phormidium or Scenedesmus removes nitrogen and phosphorus from wastewater effluents.
(g) Use of immobilised cells/enzymes in biosensor technology:
Biosensor is a device consisting of a wide range of biological elements and a transducer. Biological sensing elements are immobilised microorganisms, enzymes, nucleic acids or antibody which interacts with an analyze and produce a signal.
This signal is transmitted to transducer which converts it into an electrical signal (Fig. 33.13). Different types of biosensors have been developed for its use in food, clinical, pharmaceutical and wastewater treatment.
Method # 3. Role of Microorganisms in Metal Removal:
Generally physical and chemical methods are used for removal of heavy metals from wastewater such as oxidation, reduction, precipitation, ultrafiltration, etc. Use of microorganisms is an alternative to physical and chemical methods.
There are several microorganisms growing in marine water, fresh water and wastewater. Bacillus licheniformis and Zooglea ramigera have been isolated from activated sludge. They produce extracellular polymer which complex and accumulate metals such as iron, copper, cadmium, nickel or uranium.
The accumulated metals are released from biomass upon treatment with HCI. Fungal mycelia (e.g. Aspergillus and Penicillium) also remove metals from wastewater and offer a good alternative for detoxification of effluents. Bio sorption has shown that Aspergillus oryzae can remove cadmium efficiently from solution (Table 33.5).
Table 33.5 : Microorganisms involved in metal removal from industrial wastewater.
In recent years recombinant bacteria are being investigated from removal of specific metals from contaminated water. For example, a genetically engineered E. coli was developed which expresses H2+ transport system and metallothionein (a metal-binding protein). It was able to accumulate 9 µmol Hg2+/g cell dry weight. Bioaccumulation could not be affected by chelating agents, Na+ Mg2+ and Cd2+.
Mechanism of metal removal:
Microorganisms remove metals by the following mechanisms:
(a) Adsorption (negatively charged cell surfaces of microorganisms bind to the meta ions),
(b) Complexation (microorganisms produced organic acids (e.g. citric acid, oxalic acid, gluconic acid formic acid, lactic acid, malic acid) which chelate metal ions. Biosorption of metals also takes place due to carboxylic groups found in microbial polysaccarides and other polymers
(c) Precipitation (some bacteria produce ammonia, organic bases or H2S which precipitate metals as hydroxides or sulfates. For example, Desulfovibrio and Desulfotomaculum transform SO4 to H2S which promotes extracellular precipitation of insoluble metal sulfides. Klebsiella aerogenes detoxifies cadmium to cadmium sulphate which precipitates on cell surface,
(d) Volatilization (some bacteria causes methylation of Hg2+ and converts to dimethyl mercury which is a volatile compound).
Method # 4. Application of Recombinant DNA Technology in Waste Treatment:
Still this technology is at the stage of infancy due to the lack of knowledge and fear in the society for release of genetically engineered microorganisms (GEMs). But its major use is to detect the pathogens and to increase biodegradation of xenobiotic in wastewater treatment plants.
The major tools of recombinant DNA technology are the nucleic acid probes and PGR to detect pathogens in effluents of wastewater plant. Some pathogens detected by PGR are E. coli, Shigella flexneri Salmonella, Legionella pneumophila, and Pseudomonas aeruginosa. Yersinia, Hepatitis A virus, HIV and Giardia. Moreover, the molecular-based technique must be validated in order to be considered by regulatory agencies.
GEMs are used in several areas of waste treatment such as biomass production, biodegradation of recalcitrant, removal of toxic metals, fermentation (methane and organic acid production), enhancement of enzyme activity, increased resistance to toxic inhibitors (Fig. 33.14).
Recombinant DNA technology is involved in two steps:
(a) Searching out of microorganisms of desired function, and
(b) Transfer of character of desired function to the other microbes relevant to environment.
Such microbe is called genetically engineered microorganism (GEM).
Industrial wastes have harsh environment for the growth and maintenance of GEMs. Environment is made harsh due to extremes of temperature, pH, salinity, oxygen, redox potential and ionic composition.
Special bioreactors are constructed where microorganisms are used to degrade industrial wasters. For example, biofilm reactors are preferred for this purpose because there will be less chance for potential release of GEMs into the environment.
Many plasmids containing strains of Pseudomonas are used to degrade several components of crude oils. Using recombinant DNA technology the level of several enzymes has been increased. These enzymes are tryptophan synthetase, a-amylase, DNA ligase, benzylpenicillin acylase.
These techniques help to improve enzyme stability and catalytic efficiency, increase their substrate range, to create multifunctional hybrid enzymes with improved substrate flux. This would be an exciting area of industrial waste treatment.
However, there is a fear for deliberate release of GEMs into the environment. Therefore, the society must be educated about the possible risk, if any, regarding the GEMs.