Freda R. Hawkes

Sustainable fermentative hydrogen production: challenges for process optimisation

Abstract This paper reviews information from continuous laboratory studies of fermentative hydrogen production useful when considering practical applications of the technology. Data from reactors operating with pure cultures and mixed microflora enriched from natural sources are considered. Inocula have been derived from heat-treated anaerobically digested sludge, activated sludge, aerobic compost and soil, and non-heat-treated aerobically composted activated sludge. Most studies are on soluble defined substrates, and there are few reports of continuous operation on complex substrates with mixed microflora to produce H2. Methanogenesis which consumes H2 may be prevented by operation at short hydraulic retention times (around 8– 12 h on simple substrates) and/or pH below 6. Although the reactor technology for anaerobic digestion and biohydrogen production from complex substrates may be similar, there are important microbiological differences, including the need to manage spore germination and oxygen toxicity on start-up and control sporulation in adverse circumstances during reactor operation.

Colour in textile effluents – sources, measurement, discharge consents and simulation: a review

This paper aims to review the problem of colour in textile effluents, the different classes of dyes available and their contribution to the problem. Through new regulations, pressure is being placed on water companies all over the world to reduce the amount of colour in sewage effluent. Dyes exhibit low toxicity to mammals and aquatic organisms and therefore colour consents are normally applied for aesthetic and industrial reasons rather than for prevention of toxicity. The absorbance, ADMI values and concentrations of dyes in effluent are examined here with particular reference to reactive azo dyes used in cotton processing. Colour consents, the problem of colour in textile wastewaters and the importance for research in this area are also discussed. Dye concentrations of 0.01 g dm−3 up to 0.25 g dm−3 have been cited as being present in dyehouse effluent, depending on the dyes and processes used. ADMI values ranged from 50 to 3890 units for the dyeing of cotton. It was concluded that 1500 ADMI units was a reasonable value to aim for when simulating coloured effluents. Simulated textile effluents may be used for research purposes. These should resemble real wastes as closely as possible, but it is often difficult to replicate the ADMI values, absorbance and spectra of real effluents. The concentrations of dye used in simulated effluents examined in literature varied from 0.01 g dm−3 to 7 g dm−3. As absorbance and ADMI values change with the types of dye used, it is difficult to relate these values to dye concentrations. A concentration of 0.18 g dm−3 of a Red or Yellow dye or 0.43 g dm−3 of a blue dye would provide an ADMI of approximately 1500 units and fits within the range of dye concentrations presented in literature. A dye mixture simulating colour in a real textile effluent is suggested and some limitations of simulating actual wastewaters discussed. © 1999 Society of Chemical Industry

Enhancement of hydrogen production from glucose by nitrogen gas sparging

The effect on hydrogen yield of N2 sparging was investigated in non-sterile conditions using a hydrogen-producing mixed culture previously enriched from soya bean meal. A continuous stirred-tank reactor (CSTR) at 35°C and pH 6.0 was operated on a mineral salts-glucose (10 g l−1) medium at a hydraulic retention time (HRT) of 8.5 h, and organic loading rate of 27.02 g glucose litre reactor−1 day−1. Results are reported from an 8 week period of continuous operation, and the enrichment culture gave stable results over an extended period. A hydrogen yield of 0.85 moles H2/mole glucose consumed was obtained after 5 HRT, the gas produced being 53.4% H2. With N2 sparging at a flow rate approximately 15 times the hydrogen production rate, the hydrogen yield was 1.43 moles H2/mole glucose consumed. The specific hydrogen production rate increased from 1.446 ml hydrogen min−1g−1 biomass to 3.131 ml hydrogen min−1 g−1 biomass under sparging conditions. It is suggested that hydrogen partial pressure in the liquid phase was an important factor affecting hydrogen yield. Energy could be recovered as hydrogen from processes generating volatile fatty acids for fine chemicals and liquid bio-fuels or from acidification reactors preceding normal anaerobic biological treatment of sugary wastewaters.

Azo-dye degradation in an anaerobic-aerobic treatment system operating on simulated textile effluent.

Abstract Decolorisation of azo dyes during biological effluent treatment can involve both adsorption to cell biomass and degradation by azo-bond reduction during anaerobic digestion. Degradation is expected to form aromatic amines, which may be toxic and recalcitrant to anaerobic treatment but degradable aerobically. Methods for the quantitative detection of substituted aromatic amines arising from azo-dye cleavage are complex. A simple qualitative method is suggested as a way in which to investigate whether decolorisation is actually due to degradation, and whether the amines generated are successfully removed by aerobic treatment. Samples from a combined anaerobic-aerobic system used for treating a simulated textile wastewater containing the reactive azo dye Procion Red H-E7B were analysed by high-performance liquid chromatoraphy/ultraviolet (HPLC-UV) methods. Anaerobic treatment gave significant decolorisation, and respiration-inhibition tests showed that the anaerobic effluent had an increased toxicity, suggesting azo-dye degradation. The HPLC method showed that more polar, UV-absorbing compounds had been generated. Aerobically, these compounds were removed or converted to highly polar compounds, as shown by HPLC analysis. Since the total organic nitrogen (TON) decreased aerobically as organic N-containing compounds were mineralised, aromatic amine degradation is suggested. Although only a simple qualitative HPLC method was used, colour removal, toxicity and TON removal all support its usefulness in analysing biotreatment of azo dyes.

Anaerobic treatment of textile effluents: A review

The treatment of textile waste water is commonly carried out using biological (mainly aerobic) and physico-chemical systems. However, anaerobic bioreactors can be used to at least partially treat these effluents and provide a number of significant advantages. The most attractive feature for the treatment of textile effluents is the decolourisation of many dyes under the reducing conditions present in an anaerobic reactor. Laboratory-scale results on this particular topic are here reviewed. A second major advantage of anaerobic processing is its ability to treat wastestreams with high organic loads such as the effluents from the desizing and scouring operations currently employed in the textile manufacturing industry. Reports on successful, full-scale and pilot-scale plants are also reviewed and some limitations are discussed.

Modular tubular microbial fuel cells for energy recovery during sucrose wastewater treatment at low organic loading rate.

Energy recovery while treating low organic loads has been investigated using longitudinal tubular microbial fuel cell (MFC) reactors. Duplicate reactors, each consisting of two modules, were operated with influent sucrose organic loading rates (OLRs) between 0.04 and 0.42 g COD/l/d. Most soluble COD (sCOD) removal occurred in the first modules with predominantly VFAs reaching the second modules. Coulombic efficiency (CE) in the second modules ranged from 9% to 92% which was 3-4 times higher than the first modules. The maximum energy production was 1.75 W h/g COD in the second modules at OLR 0.24 g/l/d, up to 10 times higher than the first modules, attributable to non-fermentable substrate. A simple plug flow model of the reactors, including a generic non-electrogenic reaction competing for acetate, was developed. This modular tubular design can reproducibly distribute bioprocesses between successive modules and could be scalable, acting as a polishing stage while reducing energy requirements in wastewater treatment.

Development of a static headspace gas chromatographic procedure for the routine analysis of volatile fatty acids in wastewaters.

An optimised procedure has been developed for the routine analysis of volatile fatty acids in wastewater matrices, using static headspace gas chromatography with flame ionisation detection. Factors such as sample volume, sample pre-treatment and the time and temperature of sample equilibration have been included in an optimisation model designed to provide maximum detector response for acetic, propionic, iso- and n-butyric and iso- and n-valeric acids in the concentration range 0-1000 mg/l. Optimal headspace conditions were observed when equilibrating at 85 degrees C for 30 min, using a 2.0 ml sample volume with the addition of 1.0 ml of NaHSO4 (62%, w/v) into standard 22.3 ml vials. 2-Ethylbutyric acid was used as an internal standard. The suitability of ordinary least squares regression and weighted least squares regression models for the purposes of calibration and quantification were investigated. A weighted least squares linear regression model applied to the heteroscedastic data provided lower detection limits, e.g. 3.7 and 3.3 mg/l for acetic and propionic acids.

Two-stage anaerobic co-digestion of waste activated sludge and fruit/vegetable waste using inclined tubular digesters

The anaerobic co-digestion of a 10% total solids (7.4% VS) waste activated sludge/fruit and vegetable mixture with approximately 25% of the VS arising from the fruit and vegetable waste was studied in duplicate two-stage systems. Acidogenic CSTRs and methanogenic inclined tubular digesters operated at 30°C achieved stable anaerobic digestion at an overall system loading rate of 5.7 kg VS m−3 d−1, overall HRT of 13 days (3 day acidogenic HRT, 10 day methanogenic HRT), with 40% VS destruction and a system biogas yield of 0.37 m3 kg VS−1 added. The biogas methane content was 68% and bicarbonate alkalinity in the methanogenic stage was over 4000 mg CaCO3 l−1, although TVFA levels were relatively high at 1300 mg l−1. By increasing the overall system HRT to 17 days (system OLR 4.3 kg VS m−3 d−1) with the methanogenic HRT increased to 13 days, the average TVFA in the methanogenic stage was reduced to 300 mg l−1 and the overall VS destruction was 44%. Using these results an embodiment design was developed for a full-scale plant. The duty cycle was such that HRTs could vary from 4 to 26 days in the case of the acidogenic stage and 10 to 65 days in the case of the methanogenic stage.

Comparative performance of anaerobic digesters operating on ice-cream wastewater

Pilot-scale anaerobic digesters were operated on ice-cream wastewater for over three years. The performance of four reactor designs, an anaerobic filter, contact process and UASB of capacity 5 m3, and a 0.5 m3 fluidised bed, was compared. The anaerobic filter, with a 3.3 m3 Pall ring bed, operated stably at organic loading rates (Bv) around 6 kg COD m−3d−1, giving total COD removals around 67%. The contact process gave consistently good total COD removals of 80%, but was limited by the poor performance of the settling compartment. Mixed liquor suspended solids did not rise above 3 kg m−3, and this reactor operated at Bv of 1 kg COD m−3d−1. The fluidised bed reactor, operated on sand or granular activated carbon support media at Bv of 4 and 2 kg COD m−3d−1 respectively, gave about 60% total COD removal, but difficulty was experienced with GAC break-up. The UASB reactor gave the poorest performance, (approximately 50% total COD removal at a Bv of 2 kg COD m−3d−1) since successful granulation was not achieved. The anaerobic filter was also operated for 9 months after removing half of its packing material, but the former loading rate could not be achieved without instability. Poor biomass retention in all reactors, possibly related to the fat content of the wastewater, was the limiting factor in performance. Using experience gained at pilot scale, a full-scale upflow anaerobic filter was subsequently installed on an ice-cream factory site. This reactor showed similar performance to the pilot-scale filter.

COMPARISON OF MESOPHILIC AND THERMOPHILIC UPFLOW ANAEROBIC SLUDGE BLANKET REACTORS TREATING INSTANT COFFEE PRODUCTION WASTEWATER

Abstract Stable anaerobic digestion of settled instant coffee wastewater was achieved for over 100 days in mesophilic (35°C) and termophilic (55°C) UASB reactors. Thermophilic upflow anaerobic sludge blankets (UASBs) were seeded with mesophilic granules and converted to thermophilic operation by raising the temperature to 55°C in one step. Successful thermophilic operation was achieved within 28 days. Both mesophilic and thermophilic UASBs achieved stable digestion at organic loading rates (OLR) of up to 10 kg COD m−3 d−1 (hydraulic retention times (HRT) 24 h). Higher OLRs to the mesophilic resulted in reactor failure due to increasing total volatile fatty acid (TVFA) levels. The thermophilic UASB achieved stable operation at an OLR of 11.4 kg COD m−3 d−1 (21 h HRT) but an OLR of 13.3 kg COD m−3 d−1 (18 h HRT) saw a rise in TVFA from 80 to 600 mg l−1. COD reduction in the thermophilic UASB was slightly lower at all OLRs, achieving a 70% COD reduction compared to a 78% COD reduction in the mesophilic UASB. The average TVFA levels were low, but somewhat higher in the thermophilic UASB than the mesophilic UASB (100 mg l−1 compared to 25 mg l−1). It is concluded that either mesophilic or thermophilic digestion could be used successfully to treat settled instant coffee production wastewater in UASB reactors.