Waste activated sludge hydrolysis and short-chain fatty acids accumulation under mesophilic and thermophilic conditions: Effect of pH

The effect of pH (4.0–11.0) on waste activated sludge (WAS) hydrolysis and short-chain fatty acids (SCFAs) accumulation under mesophilic and thermophilic conditions were investigated. The WAS hydrolysis increased markedly in thermophilic fermentation compared to mesophilic fermentation at any pH investigated. The hydrolysis at alkaline pHs (8.0–11.0) was greater than that at acidic pHs, but both of the acidic and alkaline hydrolysis was higher than that pH uncontrolled under either mesophilic or thermophilic conditions. No matter in mesophilic or thermophilic fermentation, the accumulation of SCFAs at alkaline pHs was greater than at acidic or uncontrolled pHs. The optimum SCFAs accumulation was 0.298 g COD/g volatile suspended solids (VSS) with mesophilic fermentation, and 0.368 with thermophilic fermentation, which was observed respectively at pH 9.0 and fermentation time 5 d and pH 8.0 and time 9 d. The maximum SCFAs productions reported in this study were much greater than that in the literature. The analysis of the SCFAs composition showed that acetic acid was the prevalent acid in the accumulated SCFAs at any pH investigated under both temperatures, followed by propionic acid and n-valeric acid. Nevertheless, during the entire mesophilic and thermophilic fermentation the activity of methanogens was inhibited severely at acid or alkaline pHs, and the highest methane concentration was obtained at pH 7.0 in most cases. The studies of carbon mass balance showed that during WAS fermentation the reduction of VSS decreased with the increase of pH, and the thermophilic VSS reduction was greater than the mesophilic one. Further investigation indicated that most of the reduced VSS was converted to soluble protein and carbohydrate and SCFAs in two fermentations systems, while little formed methane and carbon dioxide.     

Biological short-chain fatty acids (SCFAs) production from waste-activated sludge affected by surfactant

Short-chain fatty acids (SCFAs), the preferred carbon sources for biological nutrient removal, are the important intermediate products in sludge anaerobic fermentation. Sodium dodecylbenzene sulfonate (SDBS) is a widespread used surfactant, which can be easily found in waste-activated sludge (WAS). In this investigation, the effect of SDBS on SCFAs production from WAS was investigated, and the potential of using fermentative SCFAs to promote enhanced biological phosphorus removal (EBPR) was tested. Results showed that the total SCFAs production increased significantly in the presence of SDBS at room temperature. At fermentation time of 6 days, the maximum SCFAs was 2599.1mg chemical oxygen demand (COD)/L in the presence of SDBS 0.02g/g, whereas it was only 339.1mg (COD)/L in the absence of SDBS. The SCFAs produced in the case of SDBS 0.02g/g and fermentation time 6 days consisted of acetic acid (27.1%), propionic acid (22.8%), iso-valeric acid (20.1%), iso-butyric acid (11.9%), n-butyric acid (10.4%) and n-valeric acid (7.7%). It was found that during sludge anaerobic fermentation, the solubilization of sludge particulate organic-carbon and hydrolysis of solubilized substrate as well as acidification of hydrolyzed products were all increased in the presence of SDBS, while the methane formation was decreased, the SCFAs production was therefore remarkably improved. Further investigation showed that the production of SCFAs enhanced by SDBS was caused mainly by biological effects, rather than by chemical effects and SDBS decomposition. With the fermentative SCFAs as the main carbon source, the EBPR maintained high phosphorus removal efficiency ( approximately 97%).     

Pre-acidification in anaerobic sludge bed process treating brewery wastewater

The effect of pre-acidification on anaerobic granule bed processes treating brewery wastewater was the focus of a comparison study employing two configurations, (a) a single stage upflow anaerobic sludge bed (UASB) and (b) an upflow acidification reactor in series with a methanogenic UASB. The pre-acidification reactor achieved 20±4% SCOD removal and 0.08±0.003 L of methane produced per gram of SCOD removal at a hydraulic retention time (HRT) of 0.75–4 h. Butyric acid was not detected and short chain fatty acids (SCFAs) were mainly acetic and propionic acids. The acidification ratio was about 0.42±0.02 g SCFAs as COD/g of influent COD.

Both systems’ critical loading rate to achieve 80% COD removal was established at 34–39 kg COD/m³ of total sludge bed volume per day. SCOD removal efficiency of 90±3% was achieved by both systems at an organic loading rate of 25±1 kg COD/m³ of total sludge bed volume per day, indicating that the installation of an acidification reactor had no effect in terms of the maximum granular activity, biomass granulation and the settleability of granules. At an organic loading rate of 67 kg COD/m³ of total sludge bed volume per day at an HRT of 1 h, the series system outperformed the single UASB by a removal of 62 compared to 57%.     

The mesophilic and thermophilic anaerobic digestion of coffee waste containing coffee grounds

The anaerobic digestion of waste water containing significant levels of coffee grounds was assessed in mesophilic and thermophilic batch studies and CSTRs fed daily. A 58% reduction in VS was seen in both batch studies. Proximate compositional analysis showed that the waste had a high lipid component (26–33%). Levels of lipid, hemicellulose, α-cellulose and lignin were determined before and after digestion. These components were reduced as follows: lipid by 87% in the mesophilic study and 65% in the thermophilic study, α-cellulose by 51% in both mesophilic and thermophilic batch studies, hemicellulose by 22% in the mesophilic studies and 64% in the thermophilic studies. The lignin component was not reduced in either study. Mesophilic continuous digestion was achieved at a loading rate of 1.3 kg COD m⁻³ day⁻¹ (25 day HRT) for 99 days. Addition of sodium bicarbonate alone was not sufficient for long term anaerobic digestion. Addition of Ca(OH)₂, nitrogen, phosphorus and trace elements, however, gave successful digestion with COD and VS removal of 60% and a gas production rate of 0.34 11⁻¹ day⁻¹ (65–70% methane). Low levels of TVFA and high levels of bicarbonate alkalinity were present. Thermophilic digestion could be established at 1.6 kg COD m⁻³ day⁻¹ (20 day HRT) with the addition of sodium bicarbonate alone, or Ca(OH)₂ with nitrogen, phosphorus and trace elements. However long term digestion could not be established beyond 50 days without a increase in TVFA occurring.     

Characterization of soluble organic matter of waste activated sludge before and after thermal pretreatment

Microwave (MW) irradiation and conventional heating (CH) at 96 °C was successful in disrupting the complex waste activated sludge (WAS) floc structure and releasing extra- and intra-cellular biopolymers, such as protein and sugars from activated sludge flocs into soluble phase along with solubilization of particulate chemical oxygen demand (COD). Soluble CODs of CH and MW-irradiated WAS were 361±45% and 143±34% higher and resulted in 475±3% and 211±2% higher cumulative biogas productions (CBP) relative to the control at the end of 23 days of mesophilic anaerobic digestion, respectively. Ultrafiltration (UF) was used to characterize the soluble molecular weight (Mw) distributions of control (unpretreated), CH and MW-irradiated WAS. Depending on the Mw fraction, the range of substrate volumetric utilization rate increases from anaerobic digesters was between 94% and 84% for CH and 26–113% for MW compared to the control for the first nine days of the digestion. Digesters treating high Mw (>300 kDa) materials resulted in smaller biodegradation rate constants, k, indicating that microorganisms require a longer time to utilize high Mw fractions which are most likely cell wall fragments and exopolymers.     

Preliminary trial on degradation of waste activated sludge and simultaneous hydrogen production in a newly-developed solar photocatalytic reactor with AgX/TiO2-coated glass tubes

A solar fluidized tubular photocatalytic reactor (SFTPR) with simple and efficient light collector was developed to degrade waste activated sludge (WAS) and simultaneously produce hydrogen. The photocatalyst was a TiO₂ film doped by silver and silver compounds (AgX). The synthesized photocatalyst, AgX/TiO₂, exhibited higher photocatalytic activity than TiO₂ (99.5% and 30.6% of methyl orange removal, respectively). The installation of light collector could increase light intensity by 26%. For WAS treatment using the SFTPR, 69.1% of chemical oxygen demand (COD) removal and 7866.7 μmol H₂/l-sludge of hydrogen production were achieved after solar photocatalysis for 72 h. The SFTPR could be a promising photocatalysis reactor to effectively degrade WAS with simultaneous hydrogen production. The results can also provide a useful base and reference for the application of photocatalysis on WAS degradation in practice.     

Investigation of the impacts of thermal pretreatment on waste activated sludge and development of a pretreatment model

This study investigated the impacts of high pressure thermal hydrolysis (HPTH) pretreatment on the distribution of chemical oxygen demand (COD) species in waste activated sludge (WAS). In the first phase of the project, WAS from a synthetically-fed biological reactor (BR) was fed to an aerobic digester (AD). In the second phase, WAS from the BR was pretreated by HPTH at 150 °C and 3 bars for 30 min prior to being fed to the AD. A range of physical, biochemical and biological properties were regularly measured in each process stream in both phases. The COD of the BR WAS consisted of storage products (X_STO), active heterotrophs (X_H) and endogenous decay products (X_E). Pretreatment did not increase the extent to which the BR WAS was aerobically digested and hence it was concluded that the unbiodegradable COD fraction, i.e. X_E, was unchanged by pretreatment. However, pretreatment did increase the rate of degradation as it converted 36% of X_H to readily biodegradable COD (S_B) and the remaining X_H to slowly biodegradable COD (X_B). Furthermore, X_STO was fully converted to S_B by pretreatment. Although pretreatment did not change the VSS concentration in the downstream aerobic digester, it did decrease the ISS concentration by 46 ± 11%. This reduced the total mass of solids produced by the digester by 21 ± 8%. A COD-based HPTH pretreatment model was developed and calibrated. When this model was integrated into BioWin 3.1^®, it was able to accurately simulate both the steady state performance of the overall system employed in this study as well as dynamic respirometry results.     

Production of polyhydroxyalkanoates in open, mixed cultures from a waste sludge stream containing high levels of soluble organics, nitrogen and phosphorus

In this study, the production of polyhydroxyalkanoates (PHAs) from waste activated sludge (WAS) was evaluated. PHAs were produced from fermented WAS pretreated via high-pressure thermal hydrolysis, a stream characterised by high levels of nutrients (approximately 3.5 g N L⁻¹ and 0.5 g P L⁻¹) and soluble organics. PHA-storing organisms were successfully enriched at high organic loading rates (6 g COD_sol L⁻¹ d⁻¹) under aerobic dynamic feeding in sequencing batch reactors at a sludge retention time of 6 d with a short feast length less than 20% of the cycle, and a maximum substrate concentration during feast of 1 g COD_VFA L⁻¹. The biomass enrichment, characterised by a decrease in species evenness based on Lorenz curves, provided a biomass that accumulated 25% PHA on a dry-biomass basis with yields on VFA of 0.4 Cmol Cmol⁻¹ in batch tests. The PHA consisted of ∼70 mol% 3-hydroxybutyrate and ∼30 mol% 3-hydroxyvalerate, and presented high thermal stability (T_d = 283-287 °C) and a molecular mass ranging from 0.7 to 1.0 × 10⁶ g mol⁻¹. Overall PHA storage was comparable to that achieved with other complex substrates; however, lower PHA storage rates (0.04-0.05 Cmol PHA⁻¹ Cmol X⁻¹ h⁻¹) and productivities (3-4 Cmol PHA L⁻¹ h⁻¹) were probably associated with a biomass-growth and high-respiration response induced by high levels of non-VFA organics (40-50% of COD_sol in feed) and nutrients. PHA production is feasible from pretreated WAS, but the enrichment and accumulation process require further optimisation. A milder WAS pretreatment yielding lower levels of non-VFA organics and readily available nutrients may be more amenable for improved performance.     

Interactions between methanogenic,sulfate-reducing and syntrophic acetogenic bacteria in the anaerobic degradation of benzoate

Effect of sulfate on the anaerobic degradation of benzoate was investigated by using the chemostat-type reactors at 35°C. The benzoate concentrations were equivalent to 1250–10000 mg.l⁻¹ in COD (chemical oxygen demand) and the sulfate concentrations were equivalent to 167–1670 mg.l⁻¹ in sulfur (S). Interactions between the methane-producing bacteria (MPB) and sulfate-reducing bacteria (SRB) were dependent strongly on the ratio of COD/S in wastewater. The MPB consumed 99% of the available electron donors at COD/S ratio of 60, but consumed only 69% at ratio of 1.5, and 13% at 0.75. The biochemical reactions and the bacterial composition in the biomass were also governed by the COD/S ratio. At high COD/S ratios (3.0 or higher), benzoate was degraded mainly to methane via acetate and hydrogen/formate. The degradation of benzoate required the syntrophic association between the hydrogen-producing acetogens such as Syntrophus buswellii and hydrogen-consuming MPB, plus Methanothrix-like MPB. On the other hand, at low COD/S ratio (1.5 or lower), benzoate was consumed mainly by SRB, converting sulfate into sulfide and suppressing the methane production. The anaerobic degradation of benzoate was partially inhibited when sulfide concentration was high.     

Synergetic pretreatment of sewage sludge by microwave irradiation in presence of H2O2 for enhanced anaerobic digestion

A microwave-enhanced advanced hydrogen peroxide oxidation process (MW/H₂O₂-AOP) was studied in order to investigate the synergetic effects of MW irradiation on H₂O₂ treated waste activated sludges (WAS) in terms of mineralization (permanent stabilization), sludge disintegration/solubilization, and subsequent anaerobic biodegradation as well as dewaterability after digestion. Thickened WAS sample pretreated with 1 g H₂O₂/g total solids (TS) lost 11-34% of its TS, total chemical oxygen demand (COD) and total biopolymers (humic acids, proteins and sugars) via advanced oxidation. In a temperature range of 60-120 °C, elevated MW temperatures (>80 °C) further increased the decomposition of H₂O₂ into OH^• radicals and enhanced both oxidation of COD and solubilization of particulate COD (>0.45 micron) of WAS indicating that a synergetic effect was observed when both H₂O₂ and MW treatments were combined. However, at all temperatures tested, MW/H₂O₂ treated samples had lower first-order mesophilic (33 ± 2 °C) biodegradation rate constants and ultimate (after 32 days of digestion) methane yields (mL per gram sample) compared to control and MW irradiated WAS samples, indicating that synergistically (MW/H₂O₂-AOP) generated soluble organics were slower to biodegrade or more refractory than those generated during MW irradiation.     

Athermal microwave effects for enhancing digestibility of waste activated sludge

A bench scale industrial microwave (MW) unit equipped with fiber optic temperature and pressure controls within pressure sealed vessels successfully simulated conventional heating (CH, in water bath). By identical temporal heat temperature profiles for waste activated sludge (WAS) samples, evaluation of the athermal effects of MW irradiation on WAS floc disintegration and anaerobic digestion was achieved. In a pretreatment range of 50-96 degrees C, both MW and CH WAS samples resulted in similar particulate chemical oxygen demand (COD) and biopolymer (protein and polysaccharide) solubilization and there was no discernable MW athermal effect on the COD solubilization of WAS. However, biochemical methane potential (BMP) tests showed improved biogas production for MW samples over CH samples indicating that the MW athermal effect had a positive impact on the mesophilic anaerobic biodegradability of WAS. BMP tests also showed that despite mild inhibition in the first 7d, MW acclimated inoculum digesting pretreated (to 96 degrees C) WAS, produced 16+/-4% higher biogas compared to the control after 15 d of mesophilic batch digestion. However, initial acute inhibition was more severe for non-acclimated inoculum requiring recovery time that was two times longer with only 4+/-0% higher biogas production after 17d. Inoculum acclimation not only accelerated the production of biogas, but also increased the extent of the ultimate mesophilic biodegradation of MW irradiated WAS (after 15-27 d).     

Recovery of nitrogen and phosphorus from alkaline fermentation liquid of waste activated sludge and application of the fermentation liquid to promote biological municipal wastewater treatment

In previous publications we reported that by controlling the pH at 10.0 the accumulation of short-chain fatty acids (SCFA) during waste activated sludge (WAS) fermentation was remarkably improved [Yuan, H., Chen, Y., Zhang, H., Jiang, S., Zhou, Q., Gu, G., 2006. Improved bioproduction of short-chain fatty acids (SCFAs) from excess sludge under alkaline conditions. Environ. Sci. Technol. 40, 2025-2029], but significant ammonium nitrogen (NH₄-N) and soluble ortho-phosphorus (SOP) were released [Chen, Y., Jiang, S., Yuan, H., Zhou, Q., Gu, G., 2007. Hydrolysis and acidification of waste activated sludge at different pHs. Water Res. 41, 683-689]. This paper investigated the simultaneous recovery of NH₄-N and SOP from WAS alkaline fermentation liquid and the application of the fermentation liquid as an additional carbon source for municipal wastewater biological nitrogen and phosphorus removal. The central composite design (CCD) of the response surface methodology (RSM) was employed to optimize and model the simultaneous NH₄-N and SOP recovery from WAS alkaline fermentation liquid. Under the optimum conditions, the predicted and experimental recovery efficiency was respectively 73.4 and 75.7% with NH₄-N, and 82.0 and 83.2% with SOP, which suggested that the developed models described the experiments well. After NH₄-N and SOP recovery, the alkaline fermentation liquid was added to municipal wastewater, and the influence of volume ratio of fermentation liquid to municipal wastewater (FL/MW) on biological nitrogen and phosphorus removal was investigated. The addition of fermentation liquid didn't significantly affect nitrification. Both SOP and total nitrogen (TN) removal were increased with fermentation liquid, but there was no significant increase at FL/MW greater than 1/35. Compared to the blank test, the removal efficiency of SOP and TN at FL/MW = 1/35 was improved from 44.0 to 92.9%, and 63.3 to 83.2%, respectively. The enhancement of phosphorus and nitrogen removal was mainly attributed to the increase of influent SCFA, or rather, the increase of intracellular polyhydroxyalkanoates (PHA) which served as the carbon and energy sources for denitrification and phosphorus uptake. The addition of alkaline fermentation liquid to municipal wastewater, however, increased the effluent COD, which was caused mainly by the increase of influent humic acid, not protein or carbohydrate.     

进水COD浓度及C/N值对脱氮效果的影响

进水COD浓度及C/N值是影响系统反硝化效果的两个重要参数,为此研究了不同进水COD浓度在不同C/N值条件下的脱氮效果。结果表明:进水COD为150mg/L和200mg/L左右时,脱氮率随C/N值的增加而增加,而进水COD为100mg/L左右时,系统的脱氮率随时间增加而降低;进水COD浓度<200mg/L时,反应条件相同、C/N值相同而进水COD浓度不同,系统的脱氮率也不相同,进水COD浓度高,则脱氮率也高;当进水有机碳源浓度较低时,需要以进水COD浓度及C/N值共同来表示系统的脱氮能力。     

Ultrasonic enhancement of waste activated sludge hydrolysis and volatile fatty acids accumulation at pH 10.0

Volatile fatty acids (VFA), the preferred carbon source for biological nutrients removal, can be produced by waste activated sludge (WAS) anaerobic fermentation. However, because the rate of VFA accumulation is limited by that of WAS hydrolysis and VFA is always consumed by methanogens at acidic or neutral pHs, the ultrasonic pretreatment which can accelerate the rate of WAS hydrolysis, and alkaline adjustment which can inhibit the activities of methanogens, were, therefore, used to improve WAS hydrolysis and VFA accumulation in this study. Experiment results showed that the combination of ultrasonic pretreatment and alkaline adjustment caused significant enhancements of WAS hydrolysis and VFA accumulation. The study of ultrasonic energy density effect revealed that energy density influenced not only the total VFA accumulation but also the percentage of individual VFA. The maximal VFA accumulation (3109.8 mg COD/L) occurred at ultrasonic energy density of 1.0 kW/L and fermentation time of 72 h, which was more than two times that without ultrasonic treatment (1275.0 mg COD/L). The analysis of VFA composition showed that the percentage of acetic acid ranked the first (more than 40%) and those of iso-valeric and propionic acids located at the second and third places, respectively. Thus, the suitable ultrasonic conditions combined with alkaline adjustment for VFA accumulation from WAS were ultrasonic energy density of 1.0 kW/L and fermentation time of 72 h. Also, the key enzymes related to VFA formation exhibited the highest activities at ultrasonic energy density of 1.0 kW/L, which resulted in the greatest VFA production during WAS fermentation at pH 10.0.     

Addition of an aerated iron‐rich waste‐activated sludge to control the soluble sulphide concentration in sewage

Hydrogen sulphide emission in sewers is associated with toxicity, corrosion and odour and also yields considerable costs. The purpose of this study was to evaluate whether the soluble sulphide concentration in raw sewage can be controlled by dosing an iron‐rich waste‐activated sludge (WAS) or an iron‐rich aerated waste‐activated sludge (AWAS). An average soluble sulphide elimination of 99% was achieved at an iron‐rich AWAS to sewage ratio (v/v) of 16%, whereas dosage of iron‐poor AWAS at the same ratio decreased the soluble sulphide in the raw sewage by only 53%. Our lab‐scale tests suggest that dosing iron‐rich AWAS to sewage did not affect the chemical oxygen demand (COD) and total ammonia nitrogen (TAN) removal as well as the nitrification efficiency in the receiving activated sludge system. The results indicate that iron‐rich AWAS dosage is a feasible technique to remediate the sulphide problem in sewers.     

Hybrid reactor for priority pollutant nitrobenzene removal

The performance of a hybrid reactor, comprising of trickling filter and activated sludge process, in treating nitrobenzene wastewater was investigated. Acetate induced cells of mixed consortia was acclimatized with gradual increase of nitrobenzene concentration up to 90 mg/l in 100 days using sodium acetate as co-substrate and considering COD and nitrobenzene concentration as paramount parameters for assessing the growth of biofilm and acclimation. A removal of 60-95.80% COD and 80-90.23% nitrobenzene was observed during acclimation. During hydraulic retention time (HRT) studies, the optimum HRT was found to be 29.55 h at which a maximum of 95.83% COD and 97.93% nitrobenzene removal was observed. Other studies included optimization of C:N ratio, substrate:co-substrate ratio, effect of shock loading and estimation of volatilization losses. The optimum C:N ratio was found to be 100:20 at which maximum 97.93% removal of nitrobenzene was observed. At optimum HRT (29.55 h) and optimum C:N ratio (100:20) optimum substrate:co-substrate ratio was found to be 1:33. From the shock load studies it can be concluded that the system can withstand shock load up to two times of usual nitrobenzene concentration. A loss of 9.44% nitrobenzene was observed due to volatilization and mass balance gave an efficiency of 87.49% biological removal of nitrobenzene.     

Identification of trigger factors selecting for polyphosphate- and glycogen-accumulating organisms in aerobic granular sludge sequencing batch reactors

Nutrient removal performances of sequencing batch reactors using granular sludge for intensified biological wastewater treatment rely on optimal underlying microbial selection. Trigger factors of bacterial selection and nutrient removal were investigated in these novel biofilm systems with specific emphasis on polyphosphate- (PAO) and glycogen-accumulating organisms (GAO) mainly affiliated with Accumulibacter and Competibacter, respectively. In a first dynamic reactor operated with stepwise changes in concentration and ratio of acetate and propionate (Ac/Pr) under anaerobic feeding and aerobic starvation conditions and without wasting sludge periodically, propionate favorably selected for Accumulibacter (35% relative abundance) and stable production of granular biomass. A Plackett-Burman multifactorial experimental design was then used to screen in eight runs of 50 days at stable sludge retention time of 15 days for the main effects of COD concentration, Ac/Pr ratio, COD/P ratio, pH, temperature, and redox conditions during starvation. At 95% confidence level, pH was mainly triggering direct Accumulibacter selection and nutrient removal. The overall PAO/GAO competition in granular sludge was statistically equally impacted by pH, temperature, and redox factors. High Accumulibacter abundances (30–47%), PAO/GAO ratios (2.8–8.4), and phosphorus removal (80–100%) were selected by slightly alkaline (pH > 7.3) and lower mesophilic (<20 °C) conditions, and under full aeration during fixed 2-h starvation. Nitrogen removal by nitrification and denitrification (84–97%) was positively correlated to pH and temperature. In addition to alkalinity, non-limited organic conditions, 3-carbon propionate substrate, sludge age control, and phase length adaptation under alternating aerobic-anoxic conditions during starvation can lead to efficient nutrient-removing granular sludge biofilm systems.     

Effect of COD/SO4ratio and Fe(II) under the variable hydraulic retention time (HRT) on fermentative hydrogen production

The effect of chemical oxygen demand/sulfate (COD/SO₄²⁻) ratio on fermentative hydrogen production using enriched mixed microflora has been studied. The chemostat system maintained with a substrate (glucose) concentration of 15 g COD L⁻¹ exhibited stable H₂ production at inlet sulfate concentrations of 0-20 g L⁻¹ during 282 days. The tested COD/SO₄²⁻ ratios ranged from 150 to 0.75 (with control) at pH 5.5 with hydraulic retention time (HRT) of 24, 12 and 6 h. The hydrogen production at HRT 6 h and pH 5.5 was not influenced by decreasing the COD/SO₄²⁻ ratio from 150 to 15 (with control) followed by noticeable increase at COD/SO₄²⁻ ratios of 5 and 3, but it was slightly decreased when the COD/SO₄²⁻ ratio further decreased to 1.5 and 0.75. These results indicate that high sulfate concentrations (up to 20,000 mg L⁻¹) would not interfere with hydrogen production under the investigated experimental conditions. Maximum hydrogen production was 2.95, 4.60 and 9.40 L day⁻¹ with hydrogen yields of 2.0, 1.8 and 1.6 mol H₂ mol⁻¹ glucose at HRTs of 24, 12 and 6 h, respectively. The volatile fatty acid (VFA) fraction produced during the reaction was in the order of butyrate > acetate > ethanol > propionate in all experiments. Fluorescence In Situ Hybridization (FISH) analysis indicated the presence of Clostridium spp., Clostridium butyricum, Clostridium perfringens and Ruminococcus flavefaciens as hydrogen producing bacteria (HPB) and absence of sulfate reducing bacteria (SRB) in our study.     

Nitrous oxide production in high-loading biological nitrogen removal process under low COD/N ratio condition

Effects of influent COD/N ratio on N2O emission from a biological nitrogen removal process with intermittent aeration, supplied with high-strength wastewater, were investigated with laboratory-scale bioreactors. Furthermore, the mechanism of N2O production in the bioreactor supplied with low COD/N ratio wastewater was studied using 15N tracer method, measuring of reduction rates in denitrification pathway, and conducting batch experiments under denitrifying condition. In steady-state operation, 20-30% of influent nitrogen was emitted as N2O in the bioreactors with influent COD/N ratio less than 3.5. A 15N tracer study showed that this N2O originated from denitrification in anoxic phase. However, N2O reduction capacity of denitrifiers was always larger than NO3(-)-N or NO2(-)-N reduction capacity. It was suggested that a high N2O emission rate under low COD/N ratio operations was mainly due to endogenous denitrification with NO2(-)-N in the later part of anoxic phase. This NO2(-)-N build-up was attributed to the difference between NO3(-)-N and NO2(-)-N reduction capacities, which was the feature observed only in low COD/N ratio operations.     

SBR无厌氧段生物强化除磷的诱导研究

采用SBR工艺处理人工配水,考察了进水COD及氨氮浓度、C/N值、好氧时间对诱导无厌氧段生物强化除磷的影响.结果表明,当以醋酸钠为碳源、进水COD和氨氮分别为100和5mg/L、C/N值为20时,对在A/O运行方式下表现为厌氧释磷、好氧超量吸磷的SBR,逐渐缩短其厌氧时间且保持好氧时间为135 min后,好氧吸磷现象并不会消失,仅是吸磷量略有降低.该除磷现象的发生是系统微生物经过特定诱导的结果.