After reading this article you will learn about the examples for biotreatment of industrial effluents.
(a) Biodegradation of Pollutants:
Biological treatment of effluents is a long-established practice in many countries, but some constituents of these effluents are calcitrants and, thus, not amenable to conventional treatments. Biotechnology helps in overcoming this problem.
Following example will illustrate the use of biotechnology in biodegradation of pollutants:
(i) In USA a company-“Bio Technica” is using lignin degradation for treatment of substances like polychlorinated biphenyls and ditoxin.
(ii) In Europe, companies like ICI and Ciba-Geigy are working on enzymatic detoxification to breakdown substances viz., cyanides, and also the by-products from the synthesis of S-triazine herbicides.
(iii) Microbial transformation of biarylethers, cyclic biarylethers, biarylketones, halogenated bibenzodioxins and dibenzofurans are used to avoid the problem of release of effluents with pollutants. For instance, Pseudomonas strains have been isolated, which selectively deoxygenate the 1, 2 positions of substituted biarylethers and biarylketones
(iv) Microbial degradation of chloro, dichloro and trichloromethanes and carbonate chloride is also used to deal with the problem.
The different methods of effluent treatment processes are shown in Fig. 29.2:
(b) Toxic Site Reclamation:
Since the toxic constituents of industrial effluents are known in most cases, their treatment is easy. However, the waste disposal sites are more difficult to deal with, since their chemistry is often complex.
Although incineration (drying and then burning to ashes in a furnace) or chemical treatments are suggested for treatment of these sites, biotechnological approaches involving biodegradation are considered more attractive.
Companies like Bio Technical are working on treating polluted sites in situ. In this connection, it is interesting to note that the first patent ever awarded was given to Dr. Ananda Chakraborty for developing a microbe designed to biodegrade spilled oil. There has been a debate on whether or not to release genetically engineered microbes for treatment of toxic sites.
In view of the above, “Occidental Chemical Corporation” company of USA is conducting their work on effluents from toxic waste sites in closed reactors. One such experiment was conducted at Hyde Park landfill site at the north-west corner of Niagara, USA. This site occupies just over 6 hectares of land and contains an estimated 73,000 tonne of chemical waste.
The site was used from 1953 to 1975 to dispose- off a wide range of waste materials including chlorinated organic materials. For treatment of this site, a clay cover was placed on the landfill in 1978 and a leachate collection system was installed around the perimeter in 1979.
From this system, 230 m3 of leachate per week was shipped to a nearby treatment facility in Niagara where the treatment involved carbon absorption, thus making the effluent suitable for discharge. This solution was not found suitable on a long term basis.
Therefore, the Hyde Park waste-water was subjected to bio treatment in sequencing batch reactors, where biodegradation rate of some more persistent chemicals was accelerated by adding bacterial strain isolated from the landfill site.
Further genetic manipulation of these organisms produced more than 100 recombinant bacteria, which have been tested for their usefulness in reactors. It was demonstrated that significant cost savings will be possible if SBR treatment is used before carbon absorption stage.
(c) Removal of Spilled Oil and Grease Deposits:
Oil spills from oil tankers on land surface and from distant oil spills have been recognised as a major environmental hazard. This spilled oil is believed to not only destroy the habitats of aquatic animals and fish, but also create health problems for local residents and causes long term damage to the environment.
Therefore, remediation efforts in this connection have been made, using chemical dispersants. These chemical dispersants are believed to cause major pollution problems in shallow water due to their toxic nature. Now attempts are being made to use microbiological degradation of oil.
In order to achieve this objective, following methods have been used:
(i) Use of a mixture of bacterial strains:
A mixture of bacterial strain has also been used to clean oil-contaminated water reserves and water supplies. This could be illustrated using the following example. When 800,000 gallons of oily water once accumulated in the bottom of the ship “Queen Mary“, there was a danger that this oily water will be discharged at the harbour causing death to marine life and spoiling beaches.
When a mixture of bacterial strains was introduced in the bottom of the ship, the oil was decomposed within 6 weeks. A mixture of bacterial strains is also being used for removal of grease deposited on the inner side of tanks in a ship. This technique may also be used for cleaning deposits of grease in pipes and vessels of a variety of industries.
(ii) Use of Oleophilic fertilisers:
In early years, application of oleophilic (oil-loving) fertilisers as food for oil-utilizing microbes was considered useful, since this would allow rapid growth and multiplication of indigenous microbes, thus speeding up the biodegradation process for removal of oil.
When 6,000 gallons of petrol leaked from a company in Pennsylvania in USA, it posed a serious health problem due to contamination of underground water supplies. Addition of oleophilic fertilisers on the sites expedited the biodegradation of petrol. In recent years, oil- utilised microbes—Pseudomonas aeruginosa—have been developed through genetic engineering.
(d) Reducing Environmental Impact of Herbicides, Pesticides, and Fertilisers:
Majority of chemical herbicides, pesticides, and fertilizers that are used commercially cause environmental hazards.
Following solutions to this problem of pollution are being tried under biotechnology programmes:
(i) Weed Control & Herbicides:
Herbicides are being developed which will be environmentally safe. To allow the use of these herbicides for crop protection programmes, genetically engineered herbicide resistant plants have been produced in a number of crops.
These plants are being field-tested. Similarly, genetically engineered insect-resistant plants have also been successfully produced in some crops, thus raising the hope for reduced use of insecticides in future.
(ii) Pest Control and Bio Pesticides:
Bacterial pesticides are also being developed. Heliothis complex, which lives in close association with plant roots, consists of 2 major crop pests— budworm and ball-worm.
Biological insecticides against both these insects are being prepared by transfer of a gene from Bacillus thuringiensis (Bt) into either a naturally occurring soil bacterium, or in a strain of Pseudomonas. Bt is already in the market since 1970 and in future these will be modified through techniques of genetic engineering and will be utilised against a variety of insects.
Several companies in USA—including Ecogen, Mycogen, Repligen and Zoe Con are involved in development and marketing of biological pesticides. Among these, Mycogen kills recombinant bacteria and applies them to the leaves of the crop plants.
(iii) Viral Pesticides:
Have also been successfully developed and used for control of pest. These pesticides are environmentally safe and avoid risk of toxicity. These pesticides can also be used against the pest strains which have become resistant to chemical pesticides.
A number of entomopathogenic viruses belonging to the family Baculoviridae (or nuclear polyhedrosis viruses = NPV) and family Reoviridae (cytoplasmic polyhedrosis virus – CPV) have been used as safe and effective pesticides. These viruses kill specific pest species and have no adverse effect on useful insect pollinators, warm-blooded mammals or even man.
Some viral pesticides, with the name of pests they control are:
To reduce the impact of excess chemical fertilizers in the field of agriculture, the bio-fertiliser is a potential tool.
(e) Biosensors to Detect Environmental Pollutants:
Biosensors represent biophysical devices which will detect the presence and measure the quantities of specific substances in a variety of environments. These specific substances may include sugars, proteins or hormones in human body, pollutants in abiotic components of the environment including air/soil and water and a variety of toxins in the industrial effluents.
In designing a biosensor an enzyme or an antibody or even microbial cells are associated with microchip devices which are used for quantitative estimation of a substance.
A biosensor equipment consists of:
(i) A biologically sensing agent.
(ii) A device for collecting the product obtained from the interaction between the substrate and the biosensor.
(iii) A device for measuring the quantity of this product, thus indirectly giving an estimate of the substrate.
The first reported application of biosensor dates back to 1860 when starch content was estimated with the help of malt extracts. Biosensor today is an analytical tool which consists of biological material in intimate contact with a transducer (Fig. 29.3).
It combines high selectivity of the biological/chemical material with high accuracy of solid state devices. However, interfacing poses difficult problems. The biological material can be membrane, enzyme, antibody/antigen, receptor, protein, intact cells, tissue or whole organ. It is necessary that the analyte should reach the reaction site in the biological material.
The reaction center can recognize the analyte and generate signals which can be measured by an appropriate transducer. The choice of transducer depends upon the signal generated by the biological material. The signal can be heat generated, light emitted/absorbed, pH change, electrons released/absorbed or chemical species produced.
The signal may or may not be amplified within a transducer. Weak signals require amplification prior to detection. Data processing can be done on a computer or recorded directly with a plotter. The first modern biosensor was assembled by Clark and Lyons (1962), who modified oxygen electrode to sense glucose concentration. Since then a variety of biosensors have been developed.
Biosensors can be classified according to transducers employed.
The transducers are of different types:
d. Field effect transducers
f. Microbial biosensor
g. Enzyme biosensor
Electrochemical transducers (potentiometric or amperometric) are most popular because of their familiarity to chemists and biologists as well as simplicity of operation. It has added advantage that the same device can be used to sense different types of redox molecules, provided their redox potentials are distinctly different from one another.
In optical biosensors, light is focused on a membrane containing reagents which can combine with target molecules diffusing through membrane. As a result, reagents can absorb or fluorescence, which is, measured and thereby concentrations of target molecules are determined. There are two types of sensor materials used—bifurcated, and coated fiber devices.
Biosensors based on bio analytical calorimetry rest upon the fact that most of the biological reactions are exothermic. Most of the reactions are associated with high molar enthalpy changes (20-100 KJ per mol) in a single enzymatic step. An enzyme immobilized electrode coupled to calorimetric device forms thermometric sensor.
These are simple in operation, insensitive to optical properties of sample and possess high specificity. The most important aspect of this device is that it can be integrated with continuous flow systems and can easily detect 0.01 mM concentrations.
Semiconductor-based devices are ragged and can be mass-fabricated at low costs. The device consists of a chemically sensitive layer, integrated with solid state electronic circuits. It can be designed as a field effect transistor (FET) or in form of a capacitor. FET based device has built-in ampliation and associated electronic equipment is less complicated. It is immune to electromagnetic disturbances.
Conductometric or impedimetric measurements offer yet another possibility. The device can have either two terminal or four terminal measuring system. It is desirable that polarization of electrode and contamination of electrode surface due to oxide film formation should be minimum.
The electrodes should have large area and possess reversible electrochemistry. Au, Pt, Pd, Ti, Al, Fe electrodes are commonly used. These sensors are not popular to biologists and industries as the theory behind these measurements is unfamiliar to the researchers.
Microbial biosensors consist of a number of coupled or uncoupled enzymic steps. These are less sensitive to inhibitors and highly tolerant of variations in pH and temperature. These are cheaper and have long life times.
But there are several disadvantages viz.,
(i) Less selective and difficult to maintain;
(ii) Response times are long and return to baseline at slower rate;
(iii) Sensors are highly susceptible to contamination.
The immobilization of enzyme can be attained by physical entrapment, chemical attachment or by preparing thin solid films. Physical entrapment is a mild method and does not adversely affect catalytic activity of the enzyme.
This can be achieved by immobilizing enzyme-substrate complex as a whole. Chemical attachment can be of ionic or covalent linkage type. The covalent coupling should not involve amino acid residues which are essential to catalytic activity. Lastly, the use of biofilm for immobilization of enzyme is also very effective. Such enzyme immobilized film is packed in a dialysis bag for effective biosensor functioning.