L.W. Hulshoff Pol

Anaerobic treatment of sulphate-rich wastewaters

Until recently, biological treatment of sulphate-rich wastewater was rather unpopular because of the production of H2S under anaerobic conditions. Gaseous and dissolved sulphides cause physical-chemical (corrosion, odour, increased effluent chemical oxygen demand) or biological (toxicity) constraints, which may lead to process failure. Anaerobic treatment of sulphate-rich wastewater can nevertheless be applied successfully provided a proper treatment strategy is selected. The strategies currently available are discussed in relation to the aim of the treatment: i) removal of organic matter, ii) removal of sulphate or iii) removal of both. Also a whole spectrum of new biotechnological applications (removal of organic chemical oxygen demand, sulphur, nitrogen and heavy metals), recently developed based on a better insight in sulphur transformations, are discussed.

Advanced anaerobic wastewater treatment in the near future

New insights into the anaerobic degradation of very different categories of compounds, and into process and reactor technology will lead to very promising new generations of anaerobic treatment system, such as ‘Expanded Granular Sludge Bed’ (EGSB) and ‘Staged Multi-Phase Anaerobic’ (MPSA) reactor systems. These concepts will provide a higher efficiency at higher loading rates, are applicable for extreme environmental conditions (e.g. low and high temperatures) and to inhibitory compounds. Moreover, by integrating the anaerobic process with other biological methods (sulphate reduction, micro-aerophilic organisms) and with physical-chemical methods, a complete treatment of the wastewater can be accomplished at very low costs, while at the same time valuable components can be recovered for reuse.

High-Rate Anaerobic Waste-Water Treatment Using the UASB Reactor under a Wide Range of Temperature Conditions

(1984). High-Rate Anaerobic Waste-Water Treatment Using the UASB Reactor under a Wide Range of Temperature Conditions. Biotechnology and Genetic Engineering Reviews: Vol. 2, No. 1, pp. 253-284.

Long-term competition between sulfate reducing and methanogenic bacteria in UASB reactors treating volatile fatty acids

The competition between acetate utilizing methane-producing bacteria (MB) and sulfate-reducing bacteria (SRB) was studied in mesophilic (30 degrees C) upflow anaerobic sludge bed (UASB) reactors (upward velocity 1 m h-1; pH 8) treating volatile fatty acids and sulfate. The UASB reactors treated a VFA mixture (with an acetate:propionate:butyrate ratio of 5:3:2 on COD basis) or acetate as the sole substrate at different COD:sulfate ratios. The outcome of the competition was evaluated in terms of conversion rates and specific methanogenic and sulfidogenic activities. The COD:sulfate ratio was a key factor in the partitioning of acetate utilization between MB and SRB. In excess of sulfate (COD:sulfate ratio lower than 0.67), SRB became predominant over MB after prolonged reactor operation: 250 and 400 days were required to increase the amount of acetate used by SRB from 50 to 90% in the reactor treating, respectively, the VFA mixture or acetate as the sole substrate. The competition for acetate was further studied by dynamic simulations using a mathematical model based on the Monod kinetic parameters of acetate utilizing SRB and MB. The simulations confirmed the long term nature of the competition between these acetotrophs. A high reactor pH (+/-8), a short solid retention time (<150 days), and the presence of a substantial SRB population in the inoculum may considerably reduce the time required for acetate-utilising SRB to outcompete MB.

Effect of NaCl on thermophilic (55°C) methanol degradation in sulfate reducing granular sludge reactors

The effect of NaCl on thermophilic (55degreesC) methanol conversion in the presence of excess of sulfate (COD/SO42-=0.5) was investigated in two 6.5L lab-scale upflow anaerobic sludge bed reactors inoculated with granular sludge previously not adapted to NaCl The effect of NaCl on thermophilic (55degreesC) methanol conversion in the presence of excess of sulfate (COD/SO42-=0.5) was investigated in two 6.5L lab-scale upflow anaerobic sludge bed reactors inoculated with granular sludge previously not adapted to NaCl. Methanol was almost completely used for sulfate reduction in the absence of NaCl when operating at an organic loading rate of 5 g COD L-1 day(-1) and a hydraulic retention time of 10 h. The almost fully sulfidogenic sludge consisted of both granules and flocs developed after approximately 100 days in both reactors. Sulfate reducing bacteria (SRB) outcompeted methane producing archaea (MPA) for methanol, but acetate represented a side-product, accounting for maximal 25% of the total COD converted. Either MPA or SRB did not use acetate as substrate in activity tests. High NaCl concentrations (25 g L-1) completely inhibited methanol degradation, whereas low salt concentrations (2.5 g NaCl L-1) provoked considerable changes in the metabolic fate of methanol. The MPA were most sensitive towards the NaCl shock (25 g L-1). In contrast, the addition of 2.5 g L-1 of NaCl stimulated MPA and homoacetogenic bacteria

Sulphate reduction by aggregates of sulphate-reducing bacteria and homo-acetogenic bacteria in a lab-scale gas-lift reactor

Abstract Microbiological aspects of biological sulphate reduction in gas-lift reactors were studied. Hydrogen and carbon dioxide were used as energy and carbon sources. Biomass retention was obtained by aggregation and natural immobilization on pumice particles. Biological sulphate reduction on H 2 /CO 2 appeared to be applicable within a pH range of 5·5–8·0 with an optimum near pH 7·5. The pH affected aggregate configuration and diameter. At pH 7·0, the average Sauter mean diameter of the aggregates was 1·5 mm. Moreover, phase-contrast and SEM microscopy showed highly branched aggregate surfaces. A pH increase led to increased surface irregularity without affecting the particle diameter. A pH decrease caused a decreased surface irregularity and changed the aggregate Sauter mean diameter from 1·50 mm at pH 7·0 to 2·26 at pH 5·5. However, the pH did not have a significant effect on the biomass composition. Examination of the bacterial composition of the aggregates by phasecontrast microscopy, SEM microscopy, as well as enrichments, showed that at all pH values Desulfovibrio species and Acetobacterium species were the most abundant micro-organisms.

Effect of the inoculation with Desulforhabdus amnigenus and pH or O2 shocks on the competition between sulphate reducing and methanogenic bacteria in an acetate fed UASB reactor

Abstract The competition between acetate utilising sulphate reducing bacteria (ASRB) and methane producing bacteria (AMB) was studied in an upflow anaerobic sludge bed (UASB) reactor (superficial liquid upflow velocity of 1 m/h) treating an acetate and sulphate mixture (COD:sulphate ratio 0.5) at 30°C and pH 8. The competition was characterised in terms of reactor performance (COD balance) and by microbiological analysis (16S rRNA profile) of the biomass. The following measures of steering the competition between ASRB and AMB were studied: (a) the addition of a pure culture of an acetotrophic SRB ( Desulforhabdus amnigenus ) to the reactor, and (b) the use of shock treatments (pH drop or air exposure). Addition of D. amnigenus was not successful, even after operating the reactor under batch conditions for 14 days. The change of pH from 8 to 7 caused an increase of the free hydrogen sulphide (FS) concentration in the reactor from 5–10 to 90–125 mg S-FS/l. This exerted a small effect on the competition between ASRB and AMB, and the amount of acetate consumed by SRB increased around 5%. A short-term air exposure (24 h) strongly deteriorated the reactor performance and about 20 days were required to recover to previous conditions. However, ASRB were able to outcompete AMB in the subsequent days of operation. The 16S rRNA characterisation of the biomass showed that Desulfobacterium or relatives and Desulfotomaculum acetoxidans -like bacteria were responsible for the sulphidogenic degradation of acetate in this reactor.

Thermophilic sulphate reduction in upflow anaerobic sludge bed reactors under acidifying conditions

The feasibility of thermophilic (55°C) anaerobic sulphur removal from partly acidified wastewater was investigated using two 6.5-l upflow anaerobic sludge bed (UASB) reactors (R1 and R2). Both reactors were inoculated with a mixture of mesophilic sulphidogenic sludge and thermophilic methanogenic sludge (ratio 1:1) and were fed with a sucrose:propionate:butyrate mixture in a chemical oxygen demand (COD) ratio of 2:1:1. Initially, reactor R1 was supplied with this feed supplemented with a high sulphate concentration (COD/SO42− ratio of 1.33), while the feed of reactor R2 contained low sulphate levels (COD/SO42 ratio of 6.67). The reactors were operated at a hydraulic retention time of 3.7 h and the imposed volumetric organic loading rates ranged from 4.9 to 19.8, and from 4.9 to 46.5 g COD l−1 day−1 for R1 and R2, respectively. A complete acidification of sucrose occurred in both R1 and R2. The extent of sulphate reduction depended on the imposed COD/SO42− ratios. R1, when operating at an organic loading rate (OLR) and sulphate loading rate (SLR) of 19.8 g l−1 day−1 and 14.8 g l−1 day−1, respectively, achieved a maximum sulphate reduction efficiency of 50%. In the case of a COD/SO42− ratio of 6.67 (R2), sulphate reduction efficiencies exceeding 95% were achieved at an OLR and SLR of 46.5 g l−1 day−1 and 7.0 g l−1 day−1, respectively. In both reactors, the effluent sulphide concentrations were always below 400 mg l−1, of which ∼90% was present as undissociated H2S (under the given conditions — pH 5.8–6.1 and 55°C). The incomplete sulphate reduction in R1 could be attributed to the limited availability of required reducing equivalents. The biogas (including CH4 and CO2) production rates in R1 were very low, i.e. 0.5 l biogas l−1 reactor day−1, resulting in negligible amounts (<10%) of H2S stripped from the reactor liquid. Introduction of N2 as an additional strip-gas (flow rate ∼20 l l−1 day−1) into R1 resulted in an almost complete H2S removal. In R2, the biogas production rates reached ∼3 l l−1 day−1 at an OLR of 38.5 g l−1 day−1. This resulted in a H2S stripping efficiency of ∼50%.

Use of hydrophobic membranes to supply hydrogen to sulphate reducing bioreactors

This paper reports on the application of hydrophobic membranes to supply the gaseous substrates hydrogen/carbon dioxide (H2/CO2)to a sulphate reducing bioreactor. For this, two flat 0.016m2 sheets of flouroplast microporous (0.45μm) membranes were inserted in a 3.6 dm016m3 bioreactor for the supply of H H2/CO2 gas as small gas bubbles. Thebioreactor was operated at 30 °C and pH 7.0 and was also equipped with an external ultra filtration module for biomass retention. At a sulphate loading rate (SLR) of 1.32 g SO42- dm-3 day-1 and a hydraulic retention time (HRT) of 61 h, a sulphate reduction rate (SRR) of 0.90 g SO42- dm-3 day-1 was achieved. When the influent sulphate concentration was reduced from 3.36 to 0.75 g SO42- dm-3 by lowering the HRT to 10.3 h (SLR of 1.75 g SO42- dm-3 day -1), the SRR dropped to 0.22 g SO42- dm-3 day -1. The lower sulphate reduction efficiency was most probably caused by a too short biomass-substrate contact time or by irreversible sulphide inhibition. Mass transfer limitation of H2 and improper mixing of the reactor liquid were shown not to contribute to the lowsulphate reduction efficiency.

Effect of the Liquid Upflow Velocity on Thermophilic Sulphate Reduction in Acidifying Granular Sludge Reactors

The effect of the superficial liquid upflow velocity on the acidifying and sulfate reducing capacity of thermophilic (55°C; pH 6.0) granular sludge bed reactors treating partly acidified wastewater was investigated. A comparison was made between a UASB and an EGSB reactor, operated at an upflow velocity of 1 m.h−1 and 6.8 m.h−1, respectively. Both reactors were inoculated with a mixture of mesophilic sulphidogenic, thermophilic sulphidogenic and thermophilic methanogenic sludge (ratio 2:1:1). They were fed a synthetic wastewater containing starch, sucrose, lactate, propionate and acetate and a low sulphate concentration (COD/SO4 2− ratio of 10). At the end of the experiment, the sulphate level of the influent was slightly increased to a COD/SO4 2− ratio of 8. The reactors were operated at a hydraulic retention time of about 5 h and the imposed volumetric organic loading rates (OLR) ranged from 4.9 to 40.0 g COD l−1d−1. When imposing an OLR of 40.0 g COD l−1d−1, the acidification efficiency dropped to 80% and the sulphate reduction efficiency decreased to 50% in the UASB reactor. In the EGSB reactor, the sulphate reduction efficiency dropped to 30% directly following the OLR increase to 40 g COD l−1d−1, but recovered rapidly to 100% (at an OLR of 35 g COD l−1d−1) until the end of the experiment. In the UASB reactor, there was a net acetate and propionate production. At the higher organic loading rates, propionate was converted to n-butyrate and n-valerate. These back reactions did not occur in the EGSB reactor, in which an active methanogenic population developed, leading to a net acetate removal (up to 50%) and a high gas loading rate (up to 8.5 l l−1d−1). In both reactors, the effluent sulphide concentration was always below 200 mg l−1, of which about 90% was present as undissociated H2S (under the given conditions - pH 5.8–6.1 and 55°C). The biogas (including CH4 and CO2) production rates in the UASB were very low, i.e. < 3 l biogas l−1 reactor d−1, resulting in negligible amounts (<20 %) of H2S stripped from the reactor liquid. In the EGSB reactor, the biogas production rates reached up to 8.5 l l−1d−1, resulting in H2S stripping efficiencies up to 75%.