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january 08, 2008 03:09pm
immobilized cell oxidation process
Today, we'll talk about advanced
wastewater treatment technology.
Let's begin.
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Amongst the advanced technologies, immobilized cell oxidation process has been used more successfully for the treatment of wastewater. Immobilized cells have been defined as cells that are entrapped within or associated with an insoluble matrix. Mattiasson discussed six general method of immobilization: covalent coupling, adsorption, biospecific affinity, entrapment in a three dimensional polymer network, confinement in a liquidliquid emulsion, and entrapment within a semi permeable membrane.
Under many conditions, immobilized cells have an advantage over either free cells or immobilized enzymes. By preventing washout, immobilization allows a high cell density to be maintained in a bio-reactor at any flow rate. Catalytic stability is greater for immobilized cells and some immobilized microorganisms tolerate higher concentration of toxic compounds than do their non-immobilized counterparts.
One partial disadvantage of immobilization is the increased resistance of substrates and products to diffusion through matrices used for immobilization. Owing to the low solubility of oxygen in water and the high local cell density, oxygen transfer often becomes the rate limiting factor in the performance of aerobic immobilized cell systems. Thus when aerobic cells are used, aeration technique bears a very important consideration in bioreactor design technology
Advanced 'Immobilized Cell Reactor' employing aerobic cells, has been recommended for the treatment of tannery wastewater. This technology comprises of immobilization of chemo-autotrophs, oxidation of dissolved organics in water and filtration of treated water. The activated carbon serves as a matrix to facilitate selective solute transfer, enhanced bio film attachment or restricts the permeation of microorganisms to the downstream.
**Advanced 'Immobilized Cell Reactor' technology for treatment of wastewater**
The concepts reinforced in this technology are:
1. immobilisation of organisms in the carrier matrix will prevent 2. accessibility of enzymes to the substrate is increased by reducing the mean free path of the bio catalyst to the substrate 3. reduce the cellular synthesis by using the organisms with low-yield coefficient
In Advanced 'Immobilized Cell Reactor' technology, the carrier matrix used is activated carbon of low surface area. The characteristics of carbon is presented in table 1. The bacteria immobilized in anoxic zone can fragment the organics into simpler compounds and the bacteria in oxic zone perform oxidation of organics. In addition to bacterial oxidation, catalytic oxidation is also facilitated at the active sites of the carbon matrix. The heat of combustion of organics released at the active sites will be used for excitation of organic molecules to cross over the activation energy barrier, which normally determines the rate of any chemical reaction.
The freedom of movement of molecules is also restricted at the surface of adsorbent as they are anchored at the sites. Thus energy expenditure towards translational motion, which is considered to be the major component in the orientation of molecule, is lowered to maximum extent. The partially oxidized organic molecule is aerobically oxidized with low heat of combustion by aerobic organisms immobilized at the mouth of the pores. Thus, the energy available for cellular synthesis is decreased and consequently the biomass production is decreased. Since the organisms are in immobilized state, the expenditure of energy towards diffusion of organic molecules and oxygen from the bulk liquid to cellular matrix is very minimum compared to that in suspended growth system.
Hence, the conservation of energy in the immobilized state, enhances the rate of degradation of organics in wastewater is much greater than in suspended growth system. The elimination of micropores in the carrier matrix avoids the loss of active sites by irreversible bonding with organic molecules in aqueous environment.
Therefore, the number of active sites available for oxidation of organic compounds remains a constant. Thus, the rate of removal of dissolved pollutants in wastewater is nearly constant.
Merits and demerits of Advanced 'Immobilized Cell Reactor'
technology:
The treatment of domestic wastewater through Advanced 'Immobilized Cell Reactor' system has many advantages as listed below: Less land requirement Less electrical and mechanical equipment Less detention period (1 - 4 hrs.) Less power consumption (about 30% of the conventional consumption) Aeration tank is not required No foaming problem No addition of micro / macro nutrients No biomass production No secondary settling Tertiary treatment is not required Positive response to achieve discharging standards (BOD < 30 mg/l, COD < 250mg/l) Complete removal of color and odor Possibilities to reuse the treated effluent Provisional to handle the additional load by adding more number of modules Need not work on holidays Treated effluent can be used for agricultural / recreational purposes Investment cost for domestic wastewater treatment is only
75 % of the conventional one Payback period of Advanced 'Immobilized Cell Reactor' system is 26 months towards savings on electrical power and chemical consumption The treated wastewater supports the growth of vegetative plants
Demerits of Advanced 'Immobilized Cell Reactor' technology
Permeability is less than that of sand filters Maximum organic loading rate allowed is only 1.2 kg 2 COD/m of Advanced 'Immobilized Cell Reactor' reactor. Performance of Advanced 'Immobilized Cell Reactor' reactor is limited by the presence of suspended solids in wastewater. Anaerobic treatment is an essential unit of operation before proceeding to Advanced 'Immobilized Cell Reactor' reactor. This is to reduce the viscosity of wastewater and eliminate colloidal solids. Multiple modules is required to handle huge volumes instead of a single module.
The same advanced immobilized cell reactor technology can be used in different industries like leather, textiles, sago, chemicals, pharmaceticals, domestic water treatment, etc.
Before I sign off today, I'd like to ask you to take a closer look at the All About Wastewater Treatment (Institutional/ Corporate Edition) eBook whose contents has been described at allaboutwastewaterandtreatment.com
