Essay on Bioremediation of waste water, sewage and industrial effluents

Severally known as bioremediation, biorestoration, bioreclamation and biotreatment, it refers to using biological agents to remove toxic wastes from the environment.

This word has a broad based meaning. The method is practiced in cleaning contaminated soil, water and air and not just polluted water and sewage. However, pollutants containing water, such as waste water, domestic sewage, agricultural runoff and industrial effluents are predominantly decontaminated by this method.

Bioremediation of waste water, sewage and industrial effluents

Waste water and domestic sewage contain degradable organic compounds. Bacteria, Protests and other microorganisms are constantly at work, breaking down this organic matter. However, the industrial sewage and runoff water from agricultural land contain complex synthetic organic compounds, not easily degraded by microorganisms.

In the presence of these synthetic molecules, most of the microorganisms die, while a few only survive by using the pollutants as the carbon source. The harmful complex organic pollutants are degraded into simple harmless compounds.

These microorganisms need to be supplied with fertilizer (nutrients), oxygen and other essential elements for their growth, which consequently enhances the rate of chemical degradation.

Bioremediation process is carried out in three steps: primary, secondary and tertiary. In the primary step, the coarse particles are removed.

The secondary process consists of aerobic microbial digestion in an open bioreactor. The organisms multiply and grow forming a biomass known as sludge. The sludge is passed on to an anaerobic bioreactor for its anaerobic digestion to biogas and manure. The tertiary process is optional. It consists of chemical precipitation.

The effectiveness of this process depends on the number of microorganisms coming in contact with the organic pollutant molecules. Therefore, the process is carried out in a constantly stirred open bioreactor, supplied with fertilizers.

An alternative to the aforementioned bioreactor is known as percolating or trickling filter bioreactor. The polluted liquid is allowed to flow on the surface of stone, gravel or a plastic sheet, on which the microorganisms are immobilized. Another innovation in the waste water treatment is a deep shaft fermentation system.

The deep shaft is a hole in the ground, divided to allow the cycling and mixing of the waste water, air and microorganisms.

In countries, receiving high annual hours of sunlight, an algal-bacterial bioreactor is developed. It results in clean water and algal-bacterial biomass. The biomass fish used in biogas formation or as animal feed. In some countries, artificial wetlands are created for the treatment of urban sewage runoff and industrial effluents.

The contaminated soil and sludge are treated by two methods: ex situ (off site) and in situ (on site). In the ex situ method, the contaminated water or soil is removed from its normal location and decontaminated, while in the in situ method, the treatment is carried out in its location.

Chemical Degradation

Inoculated indigenous microorganisms cannot degrade organic chemicals, since their growth is inhibited by these chemicals. Aromatic compounds with halogens and benzene rings are very resistant to bio-degradation. Aromatic hydrocarbons containing more than five rings degrade very slowly or not at all in the natural environment.

The half life period of a few aromatic chemicals is known. Benzopyrene, a five ring compound has a half life period of 200-300 weeks. Pyrene, a four ring compound and naphthalene, a two ring compound have 34-90 weeks and 2.4-4.4 weeks of half life periods, respectively. From this data, it is evident that such chemicals, if present in the environment, will degrade very slowly.

Effective treatment requires an inoculation of genetically engineered bacteria. Several species of the soil bacterium, Pseudomonas are known to be very effective candidates in this respect. They are known to degrade over 100 organic compounds. The genes encoding the enzymes for these derivative pathways reside on chromosomes or plasmids.

Halogenated aromatic compounds are resistant to degradation. Dehalogenetion is necessary before degradation. The enzyme dioxygenase replaces the halogen atom by a hydroxy group. Non-halogenated compounds degrade into catechol and protocatechuate. These are broken down into acetyl co-enzyme A, succinate, pyruvate and acetaldehyde. These products are broken down into CO and H₂0 in normal metabolic pathways.

Many bacteria, algae and fungi are known to degrade polycyclic aromatic hydrocarbons (PAHs). Some cyanobacteria such as Nostoc, Anabaena, Aphanocapsa and Oscillatoria oxidize naphthalene. Bacteria that can degrade a wide variety of toxic chemicals are highly desirable.

Therefore, the degradative pathways are engineered to a desired end. As mentioned earlier, degradative pathways are plasmid-encoded. For example, TOL plasmid degrades toluene and xylene. PJP3 plasmid degrades the herbicide, 2, 4-dichlorophenoxyacetic acid.

A bacterium possessing a suitable degradative pathway is produced by transferring a suitable enzyme encoding plasmid from a donor. Normally plasmids are transferred from a donor to a recipient by conjugation. If the endogenous plasmid and the transferred plasmid have homologous regions, recombination occurs producing a recombinant plasmid encoding enzymes for multiple degradative pathways.

Bioremediation of oil spill

The first genetically engineered bacterium, that could degrade crude petroleum, was generated by Ananda Chakrabarty and his coworkers in 1970s.

They transferred camphor degrading plasmid (CAM) into a bacterium harbouring octane degrading plasmid (OCT). Both the plasmids were incompatible. (Note: Compatible plasmids do not undergo recombination. They replicate and are maintained as separate molecules. Incompatible plasmids have homologous regions and therefore, undergo homologous recombination forming a recombinant plasmid.).

They underwent homologous recombination and formed a CAM-OCT recombinant plasmid. This bacterial strain could degrade both camphor and octane. Another bacterial strain with xylene degrading plasmid (XYL) received a naphthalene degrading plasmid (NAH). Both were compatible, therefore, coexisted separately.

When the CAM-OCT recombinant plasmid was transferred to the strain carrying the XYL and NAH plasmids, it resulted in one strain harbouring enzyme capabilities for four different degradative pathways. Chakrabarty oBTained the first US patent for a genetically engineered microorganism.

Pseudomonas putida contains the gene encoding the enzyme, toluene dideoxygcnase. When this gene is transferred into an Escherichia coli, the transformed cell oxidizes trichloroethylene, benzene, toluene, xylene, naphthalene and phenol. This is one among a few of the success stories on oil spill cleaning in the marine environment.

Recombination product of naphthalene (NAH) I xylene (XYL) and octane (OCT)- camphor (CAM) degrading bacterial strain.

Bioremediation of heavy metal pollution

Several metals (magnesium, manganese, copper, iron, molybdenum, cobalt and selenium) are required by living organisms in trace amount for the maintenance of structure and metabolic functions, while heavy metals (arsenic, cadmium, lead and mercury) are highly toxic even at their lowest concentrations. The main source of heavy metals in the environment is smelters, power plants and vehicular emissions.

Many bacteria have evolved mechanisms for neutralizing the adverse effects of the heavy metals. In a majority of cases, the detoxifying pathways are catalyzed by enzymes that are plasmid encoded.

They employ two detoxifying mechanisms: bio-accumulation of the heavy metals in an inaccessible form such that it is not available for its action and chemical transformation of a more toxic compound to a less toxic compound. Some bacterial genes have already been identified, which function in the resistance to heavy metals like mercury, cadmium, cobalt, zinc, chromium, copper, lead, tin, arsenic and tellurium.

Bioremediation of solid waste

Landfill technology

Solid waste accounts for an increasing problem in the urban society. Although a part of this is recycled into paper, plastic etc., disposal of this type of waste has caused a serious problem for municipalities and city administration.

Due to repeated dumping of unused materials in open air, a filthy atmosphere is created in and around the human settlement.

The toxic run off from this site may enter into settlements and cause serious health problems. Therefore, these wastes are recommended to be dumped in low-value low land and covered periodically by soil. This method is known as land filling. Land filling is essentially an anaerobic process.

Current regulations enforce that landfill sites must be water and air tight, such that the toxic materials do not contaminate the underground water table or flood the residential areas by the rain water runoff. The land fill operation is viewed as a giant bioreactor with the evolution of biogas. In most land fill sites, a vent is created for the collection of biogas.

Composting

Composting is an aerobic microbial process that converts solid organic wastes into materials that can be used for benefits or returned safely to the environment.

It is in effect solid substrate fermentation. Readily decomposable organic wastes are used as substrate in this method. These substrates include, domestic sewage, agricultural and food industry wastes.

Bioremediation of radioactive wastes

Uranium is used as the fission material in nuclear facilities. The uranyl ion [U0₂]²⁺ is a common soluble radioactive contaminant present in the environment in and around the nuclear facility. Microorganisms can immobilize this soluble uranyl ion into insoluble uraninite (U0₂) in three ways (1) A cytochrome- c hydrogenase of Desulfovibrio vulgaris can reduce uranyl ion to uraninite, (2) Deinococcus radiodurans can change uranyl ion to uraninite and (3) An acid phosphates N from Citrobacter sp can precipitate uranyl ion as hydrogen uranyl phosphate.