In situ bioremediation:
- In situ bioremediation is the toxic removal technology where micro-organisms feed on contaminants and dissolve contaminants for biotransformation.
- Biotransformation is a very difficult process.
- The potential advantages of in situ bioremediation are:
- Minimal site disruption
- Continuous treatment of contaminated soil and ground water
- Minimal exposure of public, site personnel
- Economical
- There are two types of in situ bioremediation:
- i) Intrinsic in situ bioremediation
- ii) Engineered in situ bioremediation.
i. Intrinsic in situ bioremediation:
- Intrinsic bioremediation is the process of converting environmental pollutants into the non-toxic forms through the inherent abilities of naturally occurring microbial population.
- This process is most effective in the soil and water as these biomes always have high chance of being fully contaminated by contaminants and toxins.
- This process is usually employed in underground places as such underground petroleum tanks.
- There is escalating attention on intrinsic bioremediation for control of all or some of the contamination at waste sites.
- The natural ability of micro-organisms to degrade the contaminants should be examined and tested at laboratory and in field trails prior its use for intrinsic bioremediation.
- There are several conditions of site that promotes intrinsic bioremediation.
- These conditions are:-
- Flow of ground water throughout the year
- Carbonate minerals to buffer acidity produced during biodegradation
- Dispense of electron acceptors and nutrients for microbial growth
- Absence of toxic compounds
- The other environmental factors such as pH, concentration, temperature and nutrient availability decides whether or not biotransformation takes place.
- The microbial growth during bioremediation of the waste is hindered by presence of metals such as Hg, Pb, As and cyanide at toxic concentration.
- Degradation of pollutants using bacteria in ground water relies on the type and concentration of compounds, electron acceptor and time period for which bacteria was exposed to contamination.
- Therefore, capacity of bacteria used to degrade contaminants must be determined in laboratory by microbial studies prior to use.
ii. Engineered/accelerated in situ bioremediation:
- Despite the good results, intrinsic bioremediation may not be suitable when site conditions are not matching with the microbial growth requirement.
- It is because, intrinsic bioremediation is a slow process as growth and availability of micro-organisms are not adequate, there is limited capacity of electron acceptor and nutrients, cold temperature and high concentration of contaminants.
- In these conditions, engineered in situ bioremediation is employed.
- Engineered in situ bioremediation accelerates the desired biodegradation reactions by enhancing growth of more micro-organisms under optimum physico-chemical growth conditions. Oxygen and electron acceptors (e.g., NO31– and SO42–) and nutrients (e.g., nitrogen and phosphorus) increase microbial growth in surface.
Limitations of in situ bioremediation:
- Following are limitations of in situ bioremediation.
- Tedious as compared to other remedial methods.
- Direct exposure to existing environmental factors results seasonal variation of microbial activity and lack of control of these factors.
- Difficulty in utilisation of treatment additives such as nutrients, surfactants and oxygen. The micro-organisms act properly only when the waste materials serves to generate more cells. If the native micro-organisms fails in executing biodegradation, genetically engineered micro-organisms may be added to the site during in situ bioremediation.
Ex-situ bioremediation:
- Ex situ bioremediation includes elimination of waste materials and their collection from the contaminated site or place to assists microbial degradation.
- Ex situ bioremediation technology includes most of demerits and limitations as it is expensive process due to costs associated with solid handling process, such as excavation, screening and fractionation, mixing, homogenising and final disposal.
- Contaminated material may be either in liquid or solid form.
- On the basis of phases of contaminated materials under treatment ex situ bioremediation is classified into two part as per following :
- i) Solid-phase system (involving land treatment and soil piles), i.e., composting.
- ii) Slurry-phase systems (including treatment of solid-liquid suspensions in bioreactors).
i. Solid-phase treatment:
- Solid-phase system includes organic wastes present in solid form (e.g., leaves, animal manures and agricultural wastes), and problematic wastes (e.g., domestic and industrial wastes, sewage sludge and municipal solid wastes).
- The traditional clean-up method involves the processing of the organic materials and production of composts which may be used as soil conditioning.
- example of solid phase treatment system is composting
Composting process:
- Composting is a solid-phase biological treatment process thus target compounds must be either solid or a liquid related with a solid matrix.
- The hazardous compounds needs to be biologically transformed. For this, the waste material should be treated prior so that biological treatment potential should boost.
- This is done by adaptation of several physical, chemical and biological factors.
- The hazardous wastes must be solubilised for the easy availability to the micro-organisms.
- The hazardous compounds and soil organic matters act as source of carbon and energy for micro-organisms.
- Enzymes secreted by micro-organisms during growth phase are responsible to degrade toxic compounds.
- Availability of water, O2, inorganic nutrients and pH, enhance the rate of decomposition of hazardous compounds.
- If there is site-specific conditions or low substrate-density, non-hazardous carbon sources that can enhance microbial growth and enzyme production can be added to compost.
- Presence of sufficient amount of water stimulates microbial growth. Addition of inorganic nutrients enhances microbial growth and rate of decomposition of hazardous wastes.
- It has also been noted that a pH range of 5.0–7.8 enhanced the highest rates of degradation of hazardous wastes. But lignin degradation has been recorded the most rapid at pH of 3.0–6.5. This shows that optimal pH levels can be species, site and waste specific.
i. Slurry-phase treatment:
a) Aerated lagoons:
- Slurry-phase lagoon system which is almost identical to aerated lagoon is used for treatment of small common municipal wastewater.
- For boosting growth of micro-organisms, nutrients and aeration are dispensed to the reactor.
- Mixers are fitted to mix different components and form slurry, whereas surface aerators supplies air needed for microbial growth.
- Depending upon requirement, the process may be used as single-stage or multistage operation.
- The limitation of slurry phase lagoon system is that this reactor is not suitable for treatment of waste containing volatile materials or components.
b) Low-shear airlift reactors (LSARs):
- Low shear airlift reactor has been developed in order to overcome the limitations of slurry phase lagoon in case of volatiles containing waste.
- The LSARs are employed when waste consists of volatile components; tight process control and when increased efficiency of bioreactors are needed.
- LSARs are like cylindrical tank which is made up of stainless steel.
- In this bioreactor pH, temperature, nutrient addition, mixing and oxygen can be regulated as per need.
- Impellers are mounted on shaft to accomplish the need and driven by motor set up at the top.
- The rake arms are linked with blades which is used for resuspension of coarse materials that seem to settle on the bottom of the bioreactor.
- Air diffusers are also arranged along the rake arm. Airlift serves to bottom circulation of contents in reactor.
- Baffles sustains the hydrodynamic behaviour of slurry-phase bioreactors.
- Contaminated material should be pre-treated using size fractionation of solids, soil washing, milling to decrease particle size and slurry preparation.
- To stimulate the rate of biodegradation, some surfactants such as anthracene, pyrene, perylene, etc., are added to waste. These act as co-substrate and are utilised as carbon and energy source. Co-substrates also increase the production of beneficial enzymes.
Factors affecting slurry-phase biodegradation:
- Following factors play vital role in slurry phase biodegradation:
- pH (optimum 5.5–8.5)
- Moisture content
- Temperature (20–30°C)
- Ageing
- Mixing
- Nutrients (N, P, micronutrients)
- Microbial population.
- Reactor operation (batch and continuous cultures).