Lime Pretreatment Research Articles

Comparison of pretreatment strategies for enzymatic saccharification and fermentation of barley straw to ethanol

Barley straw used in this study contained 34.3% cellulose, 23.0% hemicellulose and 13.3% lignin (moisture, 6.5%). Several pretreatments (dilute acid, lime and alkaline peroxide) and enzymatic saccharification procedures were evaluated for the conversion of barley straw to monomeric sugars. The maximum release of sugars (glucose, 384 mg; xylose, 187 mg; arabinose, 32 mg; total sugars, 604 mg/g; 94% of maximum theoretical sugar yield) from barley straw (10%, w/v) was obtained by alkaline peroxide (2.5% H(2)O(2), pH 11.5) pretreatment (35 degrees C, 24 hours) and enzymatic saccharification (45 degrees C, pH 5.0, 120 hours) after diluting 2 times before adding a cocktail of three commercial enzyme preparations (cellulase, beta-glucosidase and hemicellulase) each at the dose level of 0.15 ml/g of straw. Dilute acid and lime pretreatments followed by enzymatic saccharification generated 566 mg (88% yield) and 582 mg (91% yield) total sugars/g of barley straw, respectively. The yield of ethanol from the dilute acid pretreated and enzymatically saccharified barley straw hydrolyzate (23.7g sugars/L) was 11.4g/L (0.48g/g available sugars, 0.26g/g straw) by the mixed sugar utilizing recombinant Escherichia coli strain FBR5 in 17 hours. The ethanol yields were 11.4 and 11.9g/L from 24.4 and 26.2g sugars/L obtained from lime and alkaline peroxide pretreated barley straw, respectively. No inhibition of fermentation occurred by any of the three pretreatments under the conditions used.

Summary of findings from the Biomass Refining Consortium for Applied Fundamentals and Innovation (CAFI): corn stover pretreatment

The Biomass Refining Consortium for Applied Fundamentals and Innovation, with members from Auburn University, Dartmouth College, Michigan State University, the National Renewable Energy Laboratory, Purdue University, Texas A&M University, the University of British Columbia, and the University of California at Riverside, has developed comparative data on the conversion of corn stover to sugars by several leading pretreatment technologies. These technologies include ammonia fiber expansion pretreatment, ammonia recycle percolation pretreatment, dilute sulfuric acid pretreatment, flowthrough pretreatment (hot water or dilute acid), lime pretreatment, controlled pH hot water pretreatment, and sulfur dioxide steam explosion pretreatment. Over the course of two separate USDA- and DOE-funded projects, these pretreatment technologies were applied to two different corn stover batches, followed by enzymatic hydrolysis of the remaining solids from each pretreatment technology using identical enzyme preparations, enzyme loadings, and enzymatic hydrolysis assays. Identical analytical methods and a consistent material balance methodology were employed to develop comparative sugar yield data for each pretreatment and subsequent enzymatic hydrolysis. Although there were differences in the profiles of sugar release, with the more acidic pretreatments releasing more xylose directly in the pretreatment step than the alkaline pretreatments, the overall glucose and xylose yields (monomers + oligomers) from combined pretreatment and enzymatic hydrolysis process steps were very similar for all of these leading pretreatment technologies. Some of the water-only and alkaline pretreatment technologies resulted in significant amounts of residual xylose oligomers still remaining after enzymatic hydrolysis that may require specialized enzyme preparations to fully convert xylose oligomers to monomers.

Physical and chemical characterizations of corn stover and poplar solids resulting from leading pretreatment technologies

In order to investigate changes in substrate chemical and physical features after pretreatment, several characterizations were performed on untreated (UT) corn stover and poplar and their solids resulting pretreatments by ammonia fiber expansion (AFEX), ammonia recycled percolation (ARP), controlled pH, dilute acid, flowthrough, lime, and SO 2 technologies. In addition to measuring the chemical compositions including acetyl content, physical attributes determined were biomass crystallinity, cellulose degree of polymerization, cellulase adsorption capacity of pretreated solids and enzymatically extracted lignin, copper number, FT-IR responses, scanning electron microscopy (SEM) visualizations, and surface atomic composition by electron spectroscopy of chemical analysis (ESCA). Lime pretreatment removed the most acetyl groups from both corn stover and poplar, while AFEX removed the least. Low pH pretreatments depolymerized cellulose and enhanced biomass crystallinity much more than higher pH approaches. Lime pretreated corn stover solids and flowthrough pretreated poplar solids had the highest cellulase adsorption capacity, while dilute acid pretreated corn stover solids and controlled pH pretreated poplar solids had the least. Furthermore, enzymatically extracted AFEX lignin preparations for both corn stover and poplar had the lowest cellulase adsorption capacity. ESCA results showed that SO 2 pretreated solids had the highest surface O/C ratio for poplar, but for corn stover, the highest value was observed for dilute acid pretreatment with a Parr reactor. Although dependent on pretreatment and substrate, FT-IR data showed that along with changes in cross linking and chemical changes, pretreatments may also decrystallize cellulose and change the ratio of crystalline cellulose polymorphs (Iα/Iβ).

Short‐term lime pretreatment of poplar wood

Short-term lime pretreatment uses lime and high-pressure oxygen to significantly increase the digestibility of poplar wood. When the treated poplar wood was enzymatically hydrolyzed, glucan and xylan were converted to glucose and xylose, respectively. To calculate product yields from raw biomass, these sugars were expressed as equivalent glucan and xylan. To recommend pretreatment conditions, the single criterion was the maximum overall glucan and xylan yields using a cellulase loading of 15 FPU/g glucan in raw biomass. On this basis, the recommended conditions for short-term lime pretreatment of poplar wood follow: (1) 2 h, 140 degrees C, 21.7 bar absolute and (2) 2 h, 160 degrees C, and 14.8 bar absolute. In these two cases, the reactivity was nearly identical, thus the selected condition depends on the economic trade off between pressure and temperature. Considering glucose and xylose and their oligomers produced during 72 h of enzymatic hydrolysis, the overall yields attained under these recommended conditions follow: (1) 95.5 g glucan/100 g of glucan in raw biomass and 73.1 g xylan/100 g xylan in raw biomass and (2) 94.2 g glucan/100 g glucan in raw biomass and 73.2 g xylan/100 g xylan in raw biomass. The yields improved by increasing the enzyme loading. An optimal enzyme cocktail was identified as 67% cellulase, 12% beta-glucosidase, and 24% xylanase (mass of protein basis) with cellulase activity of 15 FPU/g glucan in raw biomass and total enzyme loading of 51 mg protein/g glucan in raw biomass. Ball milling the lime-treated poplar wood allowed for 100% conversion of glucan in 120 h with a cellulase loading of only 10 FPU/g glucan in raw biomass.

Lime Pretreatment of Sugarcane Bagasse for Bioethanol Production

The pretreatment of sugarcane bagasse with lime (calcium hydroxide) is evaluated. The effect of lime pretreatment on digestibility was studied through analyses using central composite design (response surface), considering pretreatment time, temperature, and lime loading as factors. The responses evaluated were the yield of glucose from pretreated bagasse after enzymatic hydrolysis. Experiments were performed using the bagasse as it comes from an alcohol/sugar factory (non-screened bagasse) and bagasse in the size range from 0.248 to 1.397 mm (screened bagasse) (12-60 mesh). It was observed that the particle size presented influence in the release of fermentable sugars after enzymatic hydrolysis using low loading of cellulase and beta-glucosidase (3.5 FPU/g dry pretreated biomass and 1.0 IU/g dry pretreated biomass, respectively).

Operational Lessons Learned During Bioreactor Demonstrations for Acid Rock Drainage Treatment

The US Environmental Protection Agency’s Mine Waste Technology Program (MWTP) has emphasized the development of biogeochemically-based treatment technologies for mitigation of acid rock drainage (ARD). Progressive technology demonstrations by the MWTP over the past 15 years have resulted in improved operation of sulfate-reducing bacteria (SRB) bioreactors. Although using SRB to treat ARD is now fairly widespread, it was uncommon in the early 1990s when the MWTP used this innovative biotechnology. The first and longest running demonstration was an in situ bioreactor installed within the flooded subsurface workings of the Lilly/Orphan Boy Mine in 1994. The second project, at the Calliope Mine, compared the performance of several SRB bioreactor configurations and operational attributes, including lime pretreatment and reactor temperature. The third demonstration, at the Golden Sunlight Mine, consisted of two treatment steps with a recycle stream. The fourth project was an investigation of existing bioreactor designs and resulted in an improved bioreactor configuration. Significant findings included: (1) a mineshaft could be used as a long-term, in situ bioreactor, (2) SRB thrive in temperature extremes, (3) sulfide recycle effectively avoids contact of ARD with bacterial populations, and (4) ideal bioreactor substrate provides short-term and long-term nutrients, good support matrix, and enhanced permeability.

Production of renewable bioproducts and reduction of phosphate pollution through the lime pretreatment and acidogenic digestion of dairy manure

Acidogenic digestion with lime pretreatment was proposed as a means of reducing soluble reactive phosphorous concentrations in dairy cow manure while producing calcium carboxylate salts as a chemical platform for production of fuels and chemicals via the MixAlco Process. When concentrated in small geographic areas, large dairy operations can have environmentally harmful impacts on their watersheds, due to excess nutrients causing eutrophication of local water bodies. In Central Texas, as in many other areas, phosphorous contamination from confined animal feeding operations is a leading cause of human induced eutrophication. It was found that lime pretreatment followed by acidogenic or methanogenic anaerobic digestion led to substantial reductions in soluble reactive phosphate (SRP) levels while also producing potentially valuable products. Dairy manure filtrate was analyzed for SRP before and after the process steps of lime pretreatment and anaerobic digestion. An average reduction of SRP by 89% was observed through the two step process for lime pretreatment coupled with acidogenic digestion. Acidogenic digestion was demonstrated to produce mixed organic acids at mesophillic temperatures and primarily acetic acid at thermophillic temperatures. Using the MixAlco Process, the carboxylate salts of these organic acids could potentially be converted into marketable fuels and chemicals. © 2008 American Institute of Chemical Engineers Environ Prog, 2009

Lime pretreatment, enzymatic saccharification and fermentation of rice hulls to ethanol

Abstract Rice hulls used in this study contained 35.6±0.1% cellulose and 12.0±0.7% hemicellulose. The maximum yield of monomeric sugars from rice hulls (15.0%, w/v) by lime pretreatment (100 mg g−1 hulls, 121 °C, 1 h) and enzymatic saccharification (45 °C, pH 5.0, 72 h) using a cocktail of three commercial enzyme preparations (cellulase, β-glucosidase and hemicellulase) at the dose level of 0.15 ml of each enzyme preparation g−1 hulls was 154±1 mg g−1 (32% yield). The lime pretreatment did not generate any detectable furfural and hydroxymethyl furfural in the hydrolyzate. The concentration of ethanol from lime-pretreated enzyme-saccharified rice hull (138 g) hydrolyzate by recombinant Escherichia coli strain FBR5 at pH 6.5 and 35 °C in 19 h was 9.8±0.5 g l−1 with a yield of 0.49 g g−1 available sugars. The ethanol concentration was 11.0±1.0 g l−1 in the case of simultaneous saccharification and fermentation by the E. coli strain at pH 6.0 and 35 °C in 53 h.

A Comparison between Lime and Alkaline Hydrogen Peroxide Pretreatments of Sugarcane Bagasse for Ethanol Production

Pretreatment procedures of sugarcane bagasse with lime (calcium hydroxide) or alkaline hydrogen peroxide were evaluated and compared. Analyses were performed using 2 x 2 x 2 factorial designs, with pretreatment time, temperature, and lime loading and hydrogen peroxide concentration as factors. The responses evaluated were the yield of total reducing sugars (TRS) and glucose released from pretreated bagasse after enzymatic hydrolysis. Experiments were performed using the bagasse as it comes from an alcohol/sugar factory and bagasse in the size range of 0.248 to 1.397 mm (12-60 mesh). The results show that when hexoses and pentoses are of interest, lime should be the pretreatment agent chosen, as high TRS yields are obtained for nonscreened bagasse using 0.40 g lime/g dry biomass at 70 degrees C for 36 h. When the product of interest is glucose, the best results were obtained with lime pretreatment of screened bagasse. However, the results for alkaline peroxide and lime pretreatments of nonscreened bagasse are not very different.

Evaluation of electrodialysis for the treatment of a hazardous leachate

The chloride concentration of a hazardous leachate of approximately 22,000 mg/l is too high for the direct discharge requirement of less than 1000 mg/l into the municipal sewerage system. Electrodialysis (ED) was selected amongst various other alternative technologies for the desalination/concentration of the leachate. Lime pretreatment of the leachate was superior to caustic soda treatment for the removal of scale-forming chemicals (Ca, Ba, Sr, Fe, Mn). Membrane fouling took place during ED treatment. However, membrane fouling should be restored with polarity reversal and mechanical cleaning of the membranes. Batch ED tests have shown that ED performance remained more or less constant during eight desalination/concentration runs. The electrical energy consumption for ion transport and brine volume varied between 9.6 and 11.4 kWh/m 3 feed and 17 and 35% of the treated feed, respectively. A very good quality water could be produced with ED treatment of the leachate. The electrical conductivity of the leachate could be reduced from 5,490 to 139 mS/m (97.5% removal). Chloride was reduced from 21,957 to 345 mg/l (98.4% removal). Arsenic was reduced from 8.8 to 1.37 mg/l. The capital cost for a 100 kl/d ED plant is estimated at US$ 0.600 million. Operational costs (ion transport) are estimated at US$ 0.38/m 3 leachate.

A Comparison Between Lime and Alkaline Hydrogen Peroxide Pretreatments of Sugarcane Bagasse for Ethanol Production

Pretreatment procedures of sugarcane bagasse with lime (calcium hydroxide) or alkaline hydrogen peroxide were evaluated and compared. Analyses were performed using 2(3) factorial designs, with pretreatment time, temperature, and lime loading and hydrogen peroxide concentration as factors. The responses evaluated were the yield of total reducing sugars (TRS) and glucose released from pretreated bagasse after enzymatic hydrolysis. Experiments were performed using the bagasse, as it comes from an alcohol/sugar factory and bagasse, in the size, range from 0.248 to 1.397 mm (12-60 mesh). The results show that, when hexoses and pentoses are of interest, lime should be the pretreatment agent chosen, as high TRS yields are obtained for non-screened bagasse using 0.40 g lime/g dry biomass at 70 degrees C for 36 h. When the product of interest is glucose, the best results were obtained with lime pretreatment of screened bagasse. However, the results for alkaline peroxide and lime pretreatments of non-screened bagasse are not very different.

Enzymatic hydrolysis and fermentation of lime pretreated wheat straw to ethanol

AbstractBACKGROUND: The objective of this work is to develop an efficient pretreatment method that can help enzymes break down the complex carbohydrates present in wheat straw to sugars, and to then ferment of all these sugars to ethanol.RESULTS: The yield of sugars from wheat straw (8.6%, w/v) by lime pretreatment (100 mg g−1 straw, 121 °C, 1 h) and enzymatic hydrolysis (45 °C, pH 5.0, 120 h) using a cocktail of three commercial enzyme preparations (cellulase, β‐glucosidase, and xylanase) at the dose level of 0.15 mL of each enzyme preparation g−1 straw was 568 ± 13 mg g−1 (82% yield). The concentration of ethanol from lime pretreated enzyme saccharified wheat straw (78 g) hydrolyzate by recombinant Escherichia coli strain FBR5 at pH 6.5 and 35 °C in 24 h was 22.5 ± 0.6 g L−1 with a yield of 0.50 g g−1 available sugars (0.29 g g−1 straw). The ethanol concentration was 20.6 ± 0.4 g L−1 with a yield of 0.26 g g−1 straw in the case of simultaneous saccharification and fermentation by the E. coli strain at pH 6.0 and 35 °C in 72 h.CONCLUSION: The results are important in choosing a suitable pretreatment option for developing bioprocess technologies for conversion of wheat straw to fuel ethanol. Copyright © 2007 Society of Chemical Industry

English

Over the past 15 years, MSE Technology Applications has conducted several notable technology demonstrations of biologically based technologies to treat acid rock drainage (ARD). These projects have progressively evolved under the U.S. Environmental Protection Agency’s Mine Waste Technology Program (MWTP) and have resulted in significant advances in the development of bioreactor application of sulfate-reducing bacteria (SRB) technology. In this paper, summary information from four separate demonstration projects will be presented for the purpose of providing overviews of the bioreactor design parameters and the operation and development of each bioreactor system. Test methods and data analysis information for each project is not fully provided within this paper, as it is available from other sources. Summarized treatment results will be presented in this paper for three field-demonstrations. Additionally, results of one laboratory design project will be presented. A different bioreactor configuration was employed for each of the four projects. The first design to be presented will be an in situ bioreactor. This configuration was installed within the flooded subsurface workings of the Lilly Orphan Boy Mine in Montana and was operated between 1994 and 2005. The second design to be presented will be a set of on-site SRB bioreactors that were configured in parallel at the Calliope Mine in Montana. These test bioreactors allowed various operational attributes to be evaluated including lime pretreatment and temperature. The configuration of the third design to be presented will be a set of both anaerobic and aerobic bioreactors in staged fashion at the Surething Mine in Montana. This bioreactor design has been in operation since 2001 and shows the comprehensive applicability for biological treatment of ARD. The focus of the last project to be presented will be an investigative approach to bioreactor design. This resulted in a proposed bioreactor configuration to effectively treat ARD by reducing dissolved sulfate and heavy metals concentrations. In general, MWTP results from these four bioreactor configurations show that SRB bioreactors are effective for passive ARD treatment. ______________________ 1 Paper presented at the 7 th International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006, St. Louis MO. R.I. Barnhisel (ed.) Published by the American Society of Mining and Reclamation (ASMR), 3134 Montavesta Road, Lexington, KY 40502 2 Suzzann Nordwick, Senior Engineer; Marek Zaluski, Staff Engineer; and Brian Park, Senior Engineer, MSE Technology Applications, Inc., Mike Mansfield Advanced Technology Center, 200 Technology Way, P.O. Box 4078, Butte, Montana 59702, USA, and Diana Bless, United States Environmental Protection Agency, National Risk Management Research Laboratory, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268, USA 7 th International Conference on Acid Rock Drainage, 2006 pp 1410-1420 DOI: 10.21000/JASMR06021410

Improvement of crossflow microfiltration performances for treatment of phosphorus-containing wastewater

In order to improve the removal efficiency of phosphate and mitigate membrane fouling, lime softening as a pretreatment for ceramic membrane microfiltration of phosphorus-containing wastewater was investigated. The effects of pretreatment on filtration flux, particle size, permeate quality, membrane fouling characteristic, and the recovery of membrane permeate flux after cleaning were studied. At the lime dosage of 680mg·L −1, the removal of phosphate increased from 11.0% without pretreatment to 99.7%, and the permeate flux was about 60% greater than that when lime was not used. The permeate obtained from softening and microfiltration met the National Discharge Standard and could be reused under very safe conditions. Pretreatment increased particle size, which mitigated membrane fouling by reducing the extent of pore plugging and forming a more porous cake on the membrane surface. The result was validated by ESEM micrographs. With the same cleaning method, the recuperation of permeate flux is approximately 18% greater with lime pretreatment than without pretreatment, which may be due to the differences in morphology of the deposited cake and the degree of irreversible fouling.

Effect of structural features on enzyme digestibility of corn stover

Corn stover was pretreated with excess calcium hydroxide (0.5 g Ca(OH) 2/g raw biomass) in non-oxidative and oxidative conditions at 25, 35, 45, and 55 °C. The enzymatic digestibility of lime-treated corn stover was affected by the change of structural features (acetylation, lignification, and crystallization) resulting from the treatment. Extensive delignification required oxidative treatment and additional consumption of lime (up to 0.17 g Ca(OH) 2/g biomass). Deacetylation reached a plateau within 1 week and there were no significant differences between non-oxidative and oxidative conditions at 55 °C; both conditions removed approximately 90% of the acetyl groups in 1 week at all temperatures studied. Delignification highly depended on temperature and the presence of oxygen. Lignin and hemicellulose were selectively removed (or solubilized), but cellulose was not affected by lime pretreatment in mild temperatures (25–55 °C), even though corn stover was contacted with alkali for a long time, 16 weeks. The degree of crystallinity slightly increased from 43% to 60% with delignification because amorphous components (lignin, hemicellulose) were removed. However, the increased crystallinity did not negatively affect the 3-d sugar yield of enzymatic hydrolysis. Oxidative lime pretreatment lowered the acetyl and lignin contents to obtain high digestibility, regardless of crystallinity. The non-linear models for 3-d hydrolysis yields of glucan ( Y g), xylan ( Y x), and holocellulose ( Y gx) were empirically established as a function of the residual lignin ( L) for the corn stover pretreated with lime and air.

Delignification kinetics of corn stover in lime pretreatment

Corn stover was pretreated with excess calcium hydroxide (0.5 g Ca(OH) 2/g raw biomass) in non-oxidative and oxidative conditions at 25, 35, 45, and 55 °C. The delignification kinetic model of corn stover used three first-order reactions with following forms: W L = 0.09 · exp(−∞ · t) + 0.28 · exp(− k 2 · t) + 0.63 · exp(− k 3 · t) in non-oxidative pretreatment; W L = 0.16 · exp(−∞ · t) + 0.27 · exp(− k 2 · t) + 0.57 · exp(− k 3 · t) in oxidative pretreatment. The first term corresponds to the initial phase, which is essentially infinite at the time scale of the reaction (weeks). The second and third terms correspond to the bulk and residual phases of delignification. The activation energies for delignification in the oxidative lime pretreatment reactions were estimated as 50.15 and 54.21 kJ/mol in the bulk and residual phases, respectively, which are similar to the Kraft delignification of bagasse, but much less than in Kraft delignification of wood.

Lime pretreatment and enzymatic hydrolysis of corn stover

Corn stover was pretreated with an excess of calcium hydroxide (0.5 g Ca(OH) 2/g raw biomass) in non-oxidative and oxidative conditions at 25, 35, 45, and 55 °C. The optimal condition is 55 °C for 4 weeks with aeration. Glucan (91.3%) and xylan (51.8%) were converted to glucose and xylose respectively, when the treated corn stover was enzymatically hydrolyzed with 15 FPU/g cellulose. Only 0.073 g Ca(OH) 2 was consumed per g of raw corn stover. Of the initial lignin, 87.5% was maximally removed. Almost all acetyl groups were removed. After 4 weeks at 55 °C with aeration, some cellulose and hemicellulose were solubilized as monomers and oligomers in the pretreatment liquor. When considering the dissolved fragments of glucan and xylan in the pretreatment liquor, the overall yields of glucose and xylose were 93.2% and 79.5% at 15 FPU/g cellulose. The pretreatment liquor has no inhibitory effect on ethanol fermentation.

Treatment of Mine Water for Sulphate and Metal Removal Using Barium Sulphide

The integrated barium sulphide process consists of: preliminary treatment with lime, sulphate precipitation as barium sulphate, H2S-stripping, crystallization of CaCO3, and recovery of barium sulphide. Our tests showed that during lime pre-treatment, sulphate was lowered from 2 800 mg/L to 1 250 mg/L by gypsum crystallization; metals were precipitated as hydroxides. The BaS treatment then lowered sulphate to less than 200 mg/L. Sulphide was lowered from 333 to less than 10 mg/L (as S) in the stripping stage, using CO2 gas for stripping. The stripped H2S-gas was contacted with Fe (III)-solution and converted quantitatively to elemental sulphur. The alkalinity of the calcium bicarbonate-rich water was reduced from 1 000 to 110 mg/L (as CaCO3) after CO2-stripping with air due to CaCO3 precipitation. Fe (II), after sulphur production, was re-oxidized to Fe (III) using an electrolytic step. The running cost of the BaS process is R2.12/m3 (US$1 = SAR6.5) for the removal of 2 g/L of sulphate.

Potential groundwater contamination by fluoride from two South African phosphogypsums

A study to investigate the fluoride content in two South African phosphogypsums, Kynoch and Omnia, and the potential threat to water sources was undertaken. Kynoch and Omnia phosphogypsums were found to consist of fluoride in the region of 0.12% and 0.03% by mass, respectively. The phosphogypsum samples were mixed with ZnO and anhydrous Na2CO3 and fluxes thereof prepared at 900°C. The cooled melt was dissolved in deionised water and the fluoride content of each sample determined using the ISE method. These unwashed samples were compared with purified phosphogypsum samples, prepared through a leaching process using deionised water, lime or diluted sulphuric acid solutions. The fluoride content for melts of the purified samples was also determined by the ISE method. The amount of fluoride leached from Kynoch and Omnia phosphogypsums when treated with water was 9% and 22% respectively. Lime pretreatment of Kynoch and Omnia phosphogypsums leached out 37% and 6% of fluoride respectively. On the other hand, the diluted sulphuric acid solution leached out in excess of 70% of the original fluoride from both Kynoch and Omnia phosphogypsums. The presence of fluoride, a species of environmental concern, in South African phosphogypsum as indicated in this study, indicates the need to have waste disposal facilities constructed in such a manner that safeguards the environment. The study also indicates that regular monitoring of fluoride in the environment in the vicinity of phosphogypsum waste disposal sites has to be undertaken.

Treatment of acid and sulphate-rich effluents in an integrated biological/chemical process

A novel chemical/biological process is described in which sulphate and sulphide are removed simultaneously during biological treatment. Partial sulphate removal is achieved during chemical pre-treatment. In the biological stage sulphate is reduced to sulphide in a complete-mixed reactor through addition of sucrose or ethanol as a carbon and energy source. Sulphide is oxidised by allowing oxygen to enter the system in a controlled way. The experimental investigation of the process showed that sulphate and sulphide could be removed simultaneously due to co-existence of sulphate-reducing bacteria and sulphur oxidising bacteria. The volumetric sulphate reduction rate in a complete-mixed reactor, with sucrose as an organic carbon and energy source, amounts to 12.4 g SO4/(ℓ.d). The rate of biological sulphate removal was found to be directly related to the square root of sulphate, COD and VSS concentrations respectively, and inversely proportional to sulphide concentration. The practical value of simultaneous sulphate and sulphide removal is that only one stage is required for removal of both sulphate and sulphide; a conventional completemixed reactor can be used; and sulphate can be removed in a consistent way to below 200 mg/ℓ (as SO4) due to the stability of the process. By combining the biological stage with CaCO3- neutralisation and/or lime pre-treatment, the chemical cost can be reduced. Sulphate, associated with the over-saturated fraction after treatment with CaCO3 or lime, can be removed through gypsum crystallisation. In the integrated sulphate removal process (CaCO3- neutralisation, lime treatment and biological stages), sulphate can be removed from 9 200 mg/ℓ (typical sulphate concentration of coal discard leachate) to 2410 mg/ℓ, 1 230 mg/ℓ and 205 mg/ℓ (as SO4) in the various stages respectively. The chemical cost with the integrated process amounts to R2.94/m3, versus R12.44/m3 when all the sulphate is removed using the biological stage only. Similarly, the cost for treating magnesium sulphaterich mine water amounts to R1.92/m3 for the integrated process, versus R3.11/m3 for biological treatment only. WaterSA Vol.30 (2) 2004: 183-189