Optimisation of sulphate reduction in a methanol-fed thermophilic bioreactor

Several methods were tested to optimise sulphate reduction and minimise methane formation in thermophilic (65 degrees) expanded granular sludge bed reactors fed with a medium containing sulphate and methanol. Lowering the pH from 7.5 to 6.75 resulted in a rapid decrease of methane formation and a concomitant increase in sulphate reduction. The inhibition of methane formation was irreversible on the short-term. Lowering the COD/SO4(2-) ratio (COD: chemical oxygen demand) from 6 to 0.34 (g/g) rapidly favoured sulphate reduction over methanogenesis. Continuous addition of 2 g L(-1) 2-bromoethanesulphonate was ineffective as complete inhibition of methanogenesis was obtained only for two days. Inhibition of methanogens by sulphide at pH 7.5 was only effective when the total sulphide concentration was above 1200 mg S L(-1). For practical applications, a relatively short exposure to a slightly acidic pH in combination with operating the reactor at a volumetric methanol-COD loading rate close to the maximum volumetric sulphide-COD formation rate.     

Anaerobic biodegradation of estrogens--hard to digest.

Although many publications are available on the fate of estrone (E1), 17beta-estradiol (E2) and 17alpha-ethynylestradiol (EE2) during aerobic wastewater treatment, little is published on their fate under strictly anaerobic conditions. Present research investigated the digestibility of E1 and EE2, using digested pig manure, granular UASB sludge, UASB-septic tank sludge and activated sludge as inocula. Besides, actual concentrations were measured in a UASB septic tank treating black water. Under anaerobic conditions E1 is reduced to E2 but the extent of this reduction depends on type of inoculum. No significant loss of the sum of E1 and E2 and of EE2 was observed. Adsorption was responsible for a 32-35% loss of E1 and E2 from the liquid phase in the UASB septic tank and the effluent still contained considerable concentrations of respectively 4.02 microg/l and 18.79 microg/l for E1 and E2 with a large fraction present in conjugated form. No EE2 was detected in the UASB effluent.     

Sulfide inhibition of the methanogenic activity of granular sludge at various pH-levels

The effect of sulfide on the formation of methane from acetate in granular sludge originating from a UASB reactor has been determined using a new batch anaerobic toxicity assay. The assay is based on measurement of the methane concentration in the closed head space of a serum bottle, thus allowing operation at constant pH and without loss of sulfide via off-gases.

Sulfide toxicity appeared to be correlated with the free hydrogen sulfide concentration in the pH range 6.4–7.2. However this correlation did not hold at pH = 7.8–8.0. Free hydrogen sulfide concentrations leading to 50% inhibition were 250 mgS l⁻¹ in the pH range 6.4–7.2 and 90 mgS l⁻¹ at pH = 7.8–8.0.

The high tolerance for sulfide toxicity exhibited by the granular sludge can probably be attributed to the existence of a pH gradient in the sludge granules leading to an increased internal pH.     

Oxidative detoxification of aqueous bark extracts. Part II: Alternative methods

The most important problem associated with the high-pH autoxidative detoxification of methanogenic toxins in debarking wastewater was its limited application to the aqueous extractives of bark from specific species. The autoxidative polymerization of coniferous bark extracts exhausted prior to complete removal of the toxic oligomeric tannins. As a result, highly toxic spruce bark extracts were only partly detoxified. Birch bark extracts were not detoxified, even though effective polymerization of the oligomeric tannins occurred. The non-tannin toxins in birch bark extracts and toxic intermediates produced by high-pH autoxidation of these extracts were responsible for the poor detoxification results. These problems can potentially be resolved by applying alternative oxidation methods. In this study a high level of detoxification was obtained for all bark species tested by short-term destructive oxidations with H₂O₂, or by long-term aerobic biological treatments which caused high levels of polymerization. The applicability of high-pH autoxidation was also made feasible for all species of bark by applying a granular active carbon treatment after the autoxidation. The active carbon was able to adsorb the residual toxic oligomeric tannins of autoxidized spruce bark extracts and the non-tannin toxins in autoxidized birch bark extracts. Additionally, a potential disadvantage of the autoxidative detoxification method is the formation of colored end products. Although these compounds are non-toxic and are non-biodegradable, their elimination from the wastewater would be necessary if discharge norms concerning color and non-biodegradable COD must be fulfilled. The destructive oxidation with H₂ O₂ produced non-toxic low-molecular-weight compounds which did not have much color. The autoxidation and long-term aerobic biological treatments produced highly colored humic end products which could be eliminated by calcium precipitation.     

Fate of oestrogens during anaerobic blackwater treatment with micro-aerobic post-treatment.

The fate of oestrone (E1), 17beta-oestradiol (E2) and 17alpha-ethynyloestradiol (EE2) was investigated in a concentrated blackwater treatment system consisting of an UASB septic tank, with micro-aerobic post-treatment. In UASB septic tank effluent a (natural) total concentration of 4.02 microg/L E1 and 18.69 microg/L E2, comprising the sum of conjugated (>70% for E1 and >80% for E2) and unconjugated forms, was measured. During post-treatment the unconjugated oestrogens were removed to below 1 microg/L. A percentage of 77% of the measured unconjugated E1 and 82% of E2 was associated with particles >1.2 microm in the final effluent implying high sorption affinity of both compounds. When spiking the UASB septic tank effluent with E1, E2, EE2 and the sulphate conjugate of E2, removal in the micro-aerobic post-treatment was >99% for both E2 and EE2 and 83% for E1. The lower removal value for E1 was a result of (slow) deconjugation during the treatment, and in the final effluent still 40% of E1 and 99% of E2 was present in conjugated form. The latter was the result of incomplete deconjugation of the spiked E2(3S) in the post-treatment system.     

Phosphate inhibition on thermophilic acetoclastic methanogens: a warning.

The inhibitory effect of phosphate on acetoclastic-methanogens was investigated for three different thermophilic (55 degrees C) anaerobic consortia. When 70 mM of phosphate was tested, acetoclastic methanogens was completely inhibited in "Eerbeek" sludge which is dominated by Methanosaeta-like methanogens. For the "Hoogezand" sludge the specific methanogenic activity dropped by 79%, indicating that any of the acetate-consuming methanogens present in the sludge was more resistant to the phosphate applied. The results demonstrated that the inhibitory effect of phosphate may affect both methane production rate and final methane concentration and might also be time dependent. This study indicates that the degree of inhibition is species-dependent, and even more resistant species may be affected during long-term experiments. Such inhibition is a matter of concern for researchers since misleading conclusions might be taken from growth and specific methanogenic activity tests when considerable concentrations of phosphate buffer are used and no interference is expected.     

Bioflocculation of mesophilic and thermophilic activated sludge

Thermophilic activated sludge treatment is often hampered by a turbid effluent. Reasons for this phenomenon are so far unknown. Here, the hypothesis of the temperature dependency of the hydrophobic interaction as a possible cause for diminished thermophilic activated sludge bioflocculation was tested. Adsorption of wastewater colloidal particles was monitored on different flat surfaces as a function of temperature. Adsorption on a hydrophobic surface varied with temperature between 20 and 60 degrees C and no upward or downward trend could be observed. This makes the hydrophobic interaction hypothesis unlikely in explaining the differences in mesophilic and thermophilic activated sludge bioflocculation. Both mesophilic and thermophilic biomass did not flocculate with wastewater colloidal particles under anaerobic conditions. Only in the presence of oxygen, with biologically active bacteria, the differences in bioflocculation behavior became evident. Bioflocculation was shown only to occur with the combination of wastewater and viable mesophilic biomass at 30 degrees C, in the presence of oxygen. Bioflocculation did not occur in case the biomass was inactivated or when oxygen was absent. Thermophilic activated sludge hardly showed any bioflocculation, also under mesophilic conditions. Despite the differences in bioflocculation behavior, sludge hydrophobicity and sludge zetapotentials were almost similar. Theoretical calculations using the DLVO (Derjaguin, Landau, Verweij and Overbeek) theory showed that flocculation is unlikely in all cases due to long-range electrostatic forces. These calculations, combined with the fact that bioflocculation actually did occur at 30 degrees C and the unlikelyness of the hydrophobic interaction, point in the direction of bacterial exo-polymers governing bridging flocculation. Polymer interactions are not included in the DLVO theory and may vary as a function of temperature.     

Anaerobic sewage treatment in a one-stage UASB reactor and a combined UASB-Digester system

The treatment of sewage at 15 degrees C was investigated in a one-stage upflow anaerobic sludge blanket (UASB) reactor and a UASB-Digester system. The latter consists of a UASB reactor complemented with a digester for mutual sewage treatment and sludge stabilisation. The UASB reactor was operated at a hydraulic retention time of 6h and a controlled temperature of 15 degrees C, the average sewage temperature during wintertime of some Middle East countries. The digester was operated at 35 degrees C. The UASB-Digester system provided significantly (significance level 5%) higher COD removal efficiencies than the one-stage UASB reactor. The achieved removal efficiencies in the UASB-Digester system and the one-stage UASB reactor for total, suspended, colloidal and dissolved COD were 66%, 87%, 44% and 30%, and 44%, 73%, 3% and 5% for both systems, respectively. The stability values of the wasted sludge from the one-stage UASB reactor and the UASB-Digester system were, respectively, 0.47 and 0.36g CH(4)-COD/g COD. Therefore, the anaerobic sewage treatment at low temperature in a UASB-Digester system is promising.     

Anaerobic stabilisation and conversion of biopolymers in primary sludge--effect of temperature and sludge retention time

The effect of sludge retention time (SRT) and process temperature on the hydrolysis, acidification and methanogenesis of primary sludge was investigated in completely stirred tank reactors (CSTRs). The CSTRs were operated to maintain SRTs of 10, 15, 20 and 30 days at process temperatures of 25 degrees C and 35 degrees C. The rates of hydrolysis and the biodegradability of primary sludge were assessed in batch reactors incubated at 15 degrees C, 25 degrees C and 35 degrees C. The results revealed that the major amount of sludge stabilisation occurred between 0 and 10 days at 35 degrees C and 10 and 15 days at 25 degrees C. Hydrolysis was found to be the rate limiting-step of the overall digestion process, for the reactors operated at 35 degrees C and 25 degrees C, except for the reactor operated at 10 days and 25 degrees C. At the latter conditions, methanogenesis was the rate-limiting step of the overall digestion process. Proteins hydrolysis was limited to a maximum value of 39% at 30 days and 35 degrees C due to proteins availability in the form of biomass. The biodegradability of primary sludge was around 60%, and showed no temperature dependency. The hydrolysis of the main biopolymers and overall particulate COD of the primary sludge digested in CSTRs were well described by first-order kinetics, in case hydrolysis was the rate-limiting step. Similarly, the hydrolysis of the overall particulate COD of the primary sludge digested in batch reactors were described by first-order kinetics and revealed strong temperature dependency, which follows Arrhenius equation.     

The contribution of biotic and abiotic processes during azo dye reduction in anaerobic sludge

Azo dye reduction results from a combination of biotic and abiotic processes during the anaerobic treatment of dye containing effluents. Biotic processes are due to enzymatic reactions whereas the chemical reaction is due to sulfide. In this research, the relative impact of the different azo dye reduction mechanisms was determined by investigating the reduction of Acid Orange 7 (AO7) and Reactive Red 2 (RR2) under different conditions. Reduction rates of two azo dyes were compared in batch assays over a range of sulphide concentrations in the presence of living or inactivated anaerobic granular sludge. Biological dye reduction followed zero order kinetics and chemical dye reduction followed second-order rate kinetics as a function of sulfide and dye concentration. Chemical reduction of the dyes was greatly stimulated in the presence of autoclaved sludge: whereas chemical dye reduction was not affected by living or gamma-irradiated-sludge. Presumably redox-mediating enzyme cofactors released by cell lysis contributed to the stimulatory effect. This hypothesis was confirmed in assays evaluating the chemical reduction of AO7 utilizing riboflavin, representative of the heat stable redox-mediating moieties of common occurring flavin enzyme cofactors. Sulfate influenced dye reduction in accordance to biogenic sulfide formation from sulfate reduction. In assays lacking sulfur compounds, dye reduction only readily occurred in the presence of living granular sludge, demonstrating the importance of enzymatic mechanisms. Both chemical and biological mechanisms of dye reduction were greatly stimulated by the addition of the redox-mediating compound, anthraquinone-disulfonate. Based on an analysis of the kinetics and demonstration in lab-scale upward-flow anaerobic sludge bed reactors, the relative importance of chemical dye reduction mechanisms in high rate anaerobic bioreactors was shown to be small due to the high biomass levels in the reactors.