R.A. Pandey

Production and Recovery of Lactic Acid for Polylactide—An Overview

In the recent past the ultimate disposability of synthetic plastics has been a greater environmental concern, and it has triggered the R&D efforts in the designing of material with an environmentally friendly life cycle by integrating material design concepts with ultimate disposability, resource utilization, and conservation. Traditionally, all plastics have been manufactured from nonrenewable petroleum resources, and these plastics are nonbiodegradable. Conventional disposal methods include incineration and secured landfill, which are associated with many environmental problems, such as production of dioxins. The continued depletion of landfill space and problems associated with incineration have led to the development of biodegradable plastics such as polylactides (PLA), which are manufactured from lactic acid that in turn is produced from starch. Although production processes for lactic acid and PLA are well known, very few processes have been commercialized and still the cost of PLA is not competitive with synthetic plastics. The crux of the PLA production technology is the fermentative production of optically active lactic acid and its recovery. Many processes are reported in the literature and through patents for the recovery of optically active lactic acid and still offer an extensive scope for research and development. This article critically reviews the production and recovery processes for lactic acid and PLA production.

Flue gas desulfurization: Physicochemical and biotechnological approaches

Various flue gas desulfurization processes —physicochemical, biological, and chemobiological—for the reduction of emission of SO2 with recovery of an economic by-product have been reviewed. The physicochemical processes have been categorized as “once-through” and “regenerable.” The prominent once-through technologies include wet and dry scrubbing. The wet scrubbing technologies include wet limestone, lime-inhibited oxidation, limestone forced oxidation, and magnesium-enhanced lime and sodium scrubbing. The dry scrubbing constitutes lime spray drying, furnace sorbent injection, economizer sorbent injection, duct sorbent injection, HYPAS sorbent injection, and circulating fluidized bed treatment process. The regenerable wet and dry processes include the Wellman Lords process, citrate process, sodium carbonate eutectic process, magnesium oxide process, amine process, aqueous ammonia process, Berglau Forchungs process, and Shells process. Besides these, the recently developed technologies such as the COBRA process, the OSCAR process, and the emerging biotechnological and chemo-biological processes are also discussed. A detailed outline of the chemistry, the advantages and disadvantages, and the future research and development needs for each of these commercially viable processes is also discussed.

Desulfurization of Gaseous Fuels with Recovery of Elemental Sulfur: An Overview

Emphasis on environmental concerns due to emission of sulfur species through utilization of gaseous fuels containing high sulfur constituents has led to the development of a number of physicochemical and biological processes for desulfurization of gaseous fuels. This article summarizes information on the development of liquid and gaseous redox processes and biological processes for desulfurization of gaseous fuels with concomitant recovery of elemental sulfur and generation of clean gaseous fuels for various applications. The future needs for research have also been incorporated in the article for the development of efficient desulfurization processes for cost-effective management of sulfur emission into the environment.

Treatment of waste gas containing low concentration of dimethyl sulphide (DMS) in a bench-scale biofilter.

Biological treatment of dimethyl sulphide (DMS) was investigated in a bench-scale biofilter, packed with compost along with wood chips, and enriched with DMS degrading microorganism Bacillus sphaericus. The biofilter could remove 62-74% of the inlet DMS, at an optimum loading of 0.484 g/m(3)/h with optimum empty bed contact time (EBCT) of 384 s and an average moisture range of 65-70%. The biodegradative products of DMS were sulphide, thiosulphate and sulphate. Evaluation of microbiological status of the biofilter indicated the presence of other bacterial cultures viz. Paenibacillus polymyxa, and Bacillus megaterium, besides B. sphaericus.

Biological treatment of gaseous emissions containing dimethyl sulphide generated from pulp and paper industry.

A bench scale biofilter packed with compost and wood chips seeded with potential DMS degrading culture (Bacillus sphaericus) could efficiently remove DMS from ambient air with removal efficiency (RE%) of 71 ± 11 at an effective bed contact time (EBCT) of 360 ± 20s with loading rate in the range of 4-28 gDMS/m(3)/h. Further, the same biofilter operated for the treatment of vent gas generated from a P&P industry indicated DMS removal of 61 ± 18% at optimal EBCT of 360 ± 25s with a loading rate in the range of 3-128 gDMS/m(3)/h.

Treatment of waste gas containing diethyldisulphide (DEDS) in a bench scale biofilter

Waste gas containing diethyldisulphide (DEDS) is generated from various industries including pulp and paper, refinery, rayon and molasses based distilleries, etc. DEDS has odour threshold detection with an average concentration of 10(-9)mg/m(3) at 25 degrees C. DEDS is toxic to bacteria, fungus and also to mammals when exposed for a long period. Waste gas containing DEDS require proper treatment prior to discharge into the environment. DEDS containing waste gas was treated in a biofilter, packed with compost along with wooden chips and enriched with DEDS degrading microorganisms. The biofilter could remove DEDS to the extent of 94+/-5% at a loading of 1.60 g/m(3)/h with an empty bed retention time of 150s. At optimal operating conditions, the average moisture content required by the biofilter was in the range of 60-65%. The biodegradative products of DEDS were thiosulphate and sulphate.

Reduction of NOx in Fe-EDTA and Fe-NTA solutions by an enriched bacterial population

An enriched biomass was developed from municipal sewage sludge consisting of three dominant bacteria, representing the genera of Enterobacter, Citrobacter and Streptomyces. The biomass was used in a series of batch experiments in order to determine kinetic constants associated with biomass growth and NOx reduction in aqueous Ferrous EDTA/NTA solutions and Ferric EDTA/NTA solutions using ethanol as organic electron donor. The maximum specific reduction rates of NOx in Ferrous EDTA and Ferrous NTA solutions were 0.037 and 0.047mMolesL(-1)d(-1)mg(-1) biomass, respectively while in Ferric EDTA and Ferric NTA solutions were 0.022 and 0.024mMolesL(-1)d(-1)mg(-1) biomass, respectively. In case of Ferric EDTA/NTA solution, the kinetic constants associated with reduction of Ferric EDTA/NTA to Ferrous EDTA/NTA were also evaluated simultaneously. The maximum specific reduction rates of Ferric EDTA and Ferric NTA were 0.0021 and 0.0026mMolesL(-1)d(-1)mg(-1) biomass. The significance of these observations are presented and discussed in this paper.

Physicochemical and Biochemical Approaches for Treatment of Gaseous Emissions Containing NOx

Nitrogen oxides (NOx) are the cause of severe environmental problems such as acid rain, smog formation, an increase in ground-level ozone, depletion of the ozone layer, and global warming and can indirectly affect human and animal health. Considering its severe polluting aspects, many approaches have been utilized so far for the development of a technology that efficiently removes NOx from the industrial gaseous emissions. These control techniques can be broadly classified as primary and secondary techniques. Primary control techniques modify the existing combustion methods to limit the production of NOx, which includes various physical and chemical approaches, whereas the secondary NOx control techniques involve chemical reduction of NOx in flue gas using a chemical reducing agent such as ammonia or urea, reacting on a specially engineered catalyst surface or by absorption of the NOx into a special chelating liquid, and then reducing the chelate-NOx complex to regenerate the chelate using chemical and biochemical approaches, which involve compost biofilters, trickling bed biofilters, packed bed reactors, and several other types of bioreactors. The overall efficiency of the process depends on the absorption efficiency of the chelating agent, the denitrification capacity of the microorganism and the process parameters and physicochemical conditions. The authors highlight the essential features of various physicochemical and biochemical NOx control strategies and techniques. Further, extensive research and development efforts are recommended to improve existing technology for effective NOx control.

Biological treatment of waste gas containing mixture of monochlorobenzene (MCB) and benzene in a bench scale biofilter.

The paper outlines treatment of waste gas containing monochlorobenzene (MCB) and benzene in a mixture using biofilter packed with compost and woodchips seeded with Acinetobacter calcoaceticus. The biofilter could treat waste gas containing MCB and benzene effectively with an efficiency of (99+/-5%) and (97+/-6%) at optimal empty bed contact time (EBCT) of 3 min with a loading of 57 g/m(3)/h of MCB and 2g/m(3)/h of benzene. At optimum loading of MCB and benzene, the biofilter showed total bacterial count of 13 x 10(5)CFU/g of compost, while the MCB and benzene degrading bacterial count was 71 x 10(4)CFU/g and 5 x 10(4)CFU/g compost respectively. The experimental removal efficiency of MCB and benzene were in good agreement with the model predicted value.

Bench Scale Evaluation of a Chemo-biochemical Process for Desulphurization of Gaseous Streams Containing Hydrogen Sulphide

A Chemo-biochemical process for desulphurization of a gaseous stream containing hydrogen sulphide (H2S) with concomitant recovery of elemental sulphur was investigated on bench scale. The process operates in two stages. In the first stage, H2S present in a gas stream is oxidized to elemental sulphur by ferric sulphate. In the second stage, ferrous sulphate produced in the process, after separation of sulphur, is biologically oxidized using the microbial culture of Thiobacillusferrooxidans isolated from the sediment of a river flowing through a coal bed containing high pyritic sulphur. Hydrogen sulphide in the gas stream was oxidized in a packed bed reactor containing Raschig rings of uniform size. The gaseous stream containing a mixture of nitrogen and H2S (7.31% v/v) could be oxidized effectively in the presence of ferric ion. The bench scale unit, operated in a continuous mode, indicated that an optimal load of 0.1755 kmol m-3 d-1 of H2S and 0.386 kmol m-3 d-1 of ferric ion are required to be maintained in order to achieve the H2S removal efficiency of more than 99%.The ferrous sulphate produced in the process could be biologically oxidized to ferric sulphate using Thiobacillus ferrooxidans immobilized ona rotating biological rope contactor (RBRC). A ferrous ion load of 0.01 kmolm-3 d-1, considering the recirculation, could be converted intoferric ions with an efficiency of 99.12% at a hydraulic retention time(HRT) of 0.15 day.