Upflow Anaerobic Sludge Bed Reactor Research Articles

A new substrate equalization method for optimizing the influent conditions and fluid flow patterns of a multifed upflow anaerobic sludge blanket reactor with mature anammox granules

To improve nitrogen removal efficiency (NRE) and achieve homogenous distribution of anammox sludge and substrate, a new substrate equalization theory and a cumulative overload index was proposed for multifed upflow anaerobic sludge bed (MUASB) reactors with mature anammox granules. The performance and flow patterns of MUASB reactors were investigated under various influent conditions. The results showed that the nitrogen removal performance and stability of MUASB reactors could be optimized by minimizing the cumulative load. The NRE gradually increased from 83.3 ± 2.2 %, 86.8 ± 4.2 % to 89.3 ± 4.1 % and 89.7 ± 1.6 % in feeding flow tests and feeding port tests, respectively. Furthermore, the flow patterns were compared based on residence time distribution and computational fluid dynamics, indicating that a better equilibrium distribution of microorganisms and substrates could be achieved in the MUASB reactors under the lowest cumulative load. Therefore, substrate equalization theory can be used to optimize the nitrogen removal performance of MUASB reactors with low-carbon footprints.

Long-Term Anaerobic Digestion of Seasonal Fruit and Vegetable Waste Using a Leach-Bed Reactor Coupled to an Upflow Anaerobic Sludge Bed Reactor

Fruit and vegetable waste (FVW) generated locally in open (public or wholesale) markets is a valuable resource and should not be considered as waste. The anaerobic digestion (AD) of FVW can minimize landfill disposal and generate renewable energy, thus decreasing greenhouse gas emissions. Moreover, the digestate after the AD of FVW, devoid of antibiotics and animal fats in manure and food waste, may have a high fertilizing value. In this study, FVW mixtures were composed to mimic the real FVW generated in Mediterranean open markets annually. The first goal was to evaluate the biochemical methane potential (BMP) of different size fractions resulting from FVW grinding. Indeed, the FVW was ground and separated into two size fractions, 0–4 mm and 4–10 mm, respectively. The 0–4 mm fraction exhibited a lower BMP but a higher rate constant than the 4–10 mm fraction. The second goal was to first evaluate the BMP of the lumped fraction of FVW after grinding (0–10 mm) via BMP assays and then feed it to a mesophilic two-stage leaching-bed reactor (LBR)-upflow anaerobic sludge bed (UASB) system for almost one year. The BMP of the FVW ranged between 406 and 429 L kg−1 of volatile solids (VS) independently of the FVW production season. The system received an average organic loading rate (OLR) of 3.1 ± 0.7 g VS L−1 d−1. During operation, the LBR gradually transited from acidogenic to methanogenic, and the overall methane yield of the system increased from 265–278 to 360–375 L kg−1 VS, respectively. The proposed technology does not require water addition or liquid digestate removal. Compared to the continuous stirred tank reactor (CSTR) digester technology, the LBR/UASB system is suitable for the anaerobic digestion of FVW. The results of this study can be further used to upscale the proposed technology and contribute to the societal need for affordable and clean energy included in the Sustainable Development Goals (SDGs).

Effects of Fe(II) on anammox-hydroxyapatite system after temperature reduction shock

The effects of Fe(II), as well as the low-temperature shock on the anammox-hydroxyapatite (Anammox-HAP) system, were investigated in the upflow anaerobic sludge bed (UASB) reactor. The nitrogen removal efficiency (NRE) decreased from 84.69 % to 57.85 % with temperature decreasing from 35 °C to 18 °C, and further declined to 46.24 % with Fe(II) concentration increasing from 1.56 mg/L to 5.00 mg/L. The secretion of extracellular polymer substances (EPS) increased by 23.99 % in response to the rising concentration of Fe(II). Meanwhile, the content of heme c and the activity of hydrazine oxidase (HZO) enhanced by 75.52 % and 30.26 %, respectively, while the electronic transport system activity (ETSA) decreased by 56.44 %. Despite the unfavorable influence of Fe(II) and low temperatures, the anammox-related genus Candidatus Kuenenia remained the dominant genus involved in the Anammox-HAP system, with the relative abundance increasing from 30.85 % to 38.54 %. PICRUSt2 was used to predict gene abundances, and genes related to nitrogen metabolism (nirS, nirK) and the iron metabolism (fur, feoB) showed a decreasing trend. In summary, the elevated Fe(II) concentrations at low temperatures potentially hindered the nitrite reduction process, possibly by reducing the abundance of functional enzymes or the capacity for electron transfer.

Changes in the prokaryotic communities occurring along a two-stage anaerobic digestion system treating the organic fraction of solid waste

Two-stage anaerobic digestion is useful for recycling of the organic fraction of municipal solid waste and wastewater; the prokaryotic communities involved in this process were characterized using massive sequencing of 16S rRNA gene amplicons. During the 60-day study the prokaryotic communities were stable in time but were affected by their position in the system. The first stage of solid waste digestion was operated in an anaerobic-hydrolytic leach bead reactor with high organic solids content, the microbial communities were dominated by members of the orders Bacteroidales (62%), Selenomonadales (15%), Lactobacillales (13%), Clostridiales (4%) and Rhodospirillales (1%). At this stage, a vertical stratification acted as a transition zone from aerobic to anaerobic environments modifying significantly the composition and structure of bacterial communities.The second stage occurred in an up-flow anaerobic sludge bed reactor specialized in acidogenesis-methanogenesis, where the leachate of the first stage was co-fed with municipal wastewater. In this stage, the bacterial community was dominated by members of the orders Pseudomonadales, Clostridiales, Burkholderiales, Bacteroidales and Syntrophobacterales (accounting for 52%), while the most abundant orders of the archaeal community corresponded to Methanosarcinales (72%) and Methanobacteriales (13%), both of them with methanogenic capacity.

Application of iron oxides modified volcanic rock in continuous phenol wastewater anaerobic treatment: Performance and underlying mechanisms

In this study, volcanic rocks (VR) or iron oxides modified volcanic rocks (IOVR) were added into the upflow anaerobic sludge bed reactor to serve as the supporting materials for improving phenol containing wastewater treatment efficiency. Results showed that with the presence of supporting materials the anaerobic digestion efficiency was effectively improved. In stage Ⅳ, in VR reactor and IOVR reactor the organics removal efficiency were 97.5% and 120.0% higher than that in control reactor, and the methane production were 71.2% and 105.3% higher than that in control reactor, respectively. Mechanisms exploration indicated that the activity of the electron transport system (ETS) and Coenzyme F420 were obviously improved in both VR reactor and IOVR reactor, which played an important role in organics decomposition and methanogenesis process, respectively. Microbial community analysis indicated that the frequency of functional genomes encoding key enzymes was effectively improved with the existence of supporting materials, especially in IOVR reactor, which meant the total microbial activity was significantly enhanced. Besides, the abundance of syntrophic bacteria (i.e. Clostridium and Longilinea) and syntrophic archaea (i.e. Methanothrix and Methanobacterium) also increased in these two reactors, which meant that syntrophic metabolism was potentially established and played an important role during anaerobic digestion.

Efficient anaerobic digestion of hydrothermal carbonization wastewater via an innovative multistage anaerobic hythane reactor (MAHR): Organic conversion and microbial evolution

Seeking continuous treatment of downstream wastewater is a desperate requirement for the scale-up of hydrothermal technology. In this study, a novel multistage anaerobic hythane reactor (MAHR), which was constructed with an internal downflow biofilter bed and an external up-flow sludge bed, was used to dispose of hydrothermal carbonation wastewater (HTCWW), a by-product of the hydrochar preparation process of microalgae. To verify the performance of MAHR for the actual complex wastewater treatment, a conventional up-flow anaerobic sludge bed reactor (UASB) was used as a comparison. The two reactors were both operated independently in parallel with a hydraulic retention time (HRT) of 12 h and an organic load rate (OLR) of 20 g SCOD/L/d. Both MAHR and UASB achieved 84% COD removal rate and 7.0 ± 0.6 and 5.5 ± 0.3 L/L/d of methane production rate, respectively, when the proportion of HTCWW was increased to 100% of the original wastewater. Specifically, the hythane-producing area (MH) of MAHR was gradually transformed into methane producing area in the process of feeding HTCWW. High-performance liquid chromatography (HPLC) analysis revealed that more small molecule compounds including organic acids and inhibitors were degraded in MAHR. Fourier transform ion cyclone resonance mass spectrometry (FT-ICR MS) analysis demonstrated that more soluble proteins and lipids were degraded to some extent, while the lignin and condensed aromatic compounds increased especially in UASB. A dramatic change was observed in the dominating microbial communities in MAHR and UASB from the Streptococcaceae associated with carbohydrates degradation to the Renkenellaceae involved in both proteins and carbohydrates degradation. At the archaeal level, hydrogenotrophic methanogens replaced acetoclastic methanogens. Redundancy analysis (RDA) further verified the relationship between dominant functional communities and the anaerobic digestion performance of the reactors. The above results indicate that MAHR is a promising configuration for the anaerobic valorization of hydrothermal wastewater.

Effects of carbon sources on phosphorus speciation transformation process, characteristics of extracellular polymeric substances, and microecology in anaerobic systems

The effects of different carbon sources on the transformation process of the form of phosphorus and microecology were examined in an anaerobic system using glucose and L-malic acid as carbon sources. Results showed that for these two carbon sources (glucose and L-malic acid), the average removal efficiencies of chemical oxygen demand by up-flow anaerobic sludge bed reactors were 97.16 % and 88.63 %, and the average removal efficiencies of total phosphorus were 11.38 % and 5.68 %, respectively. The average concentrations of phosphine in anaerobic reactors using glucose and L-malic acid as carbon sources were 160.73 mg/m3 and 90.80 mg/m3 over days 24–40, respectively. When glucose was used as carbon source, the proportions of biologically available phosphorus (i.e., the sum of non-apatite inorganic phosphorus and organic phosphorus content) within total phosphorus in sludge on days 0, 20, and 40 were 76.22 %, 77.90 %, and 85.04 %, respectively. Glucose could better promote the enhancement of dehydrogenase activity than L-malic acid. The carbon source affected the activity of alkaline phosphatase, and the enzyme activity was stronger when glucose was used as carbon source. Kyoto Encyclopedia of Genes and Genomes functional analysis showed that amino acid metabolism and carbohydrate metabolism dominated. On days 0, 30 and 60, the relative abundances of carbohydrate metabolism in microorganisms using glucose as carbon source were 0.26 %, 0.36 %, and 0.34 % higher than those with L-malic acid as carbon source, respectively.

Insights into biodegradation behaviors of methanolic wastewater in up-flow anaerobic sludge bed (UASB) reactor coupled with in-situ bioelectrocatalysis

Granular sludge disintegration and washing out pose a challenge to up-flow anaerobic sludge bed (UASB) reactor treating methanolic wastewater. Herein, in-situ bioelectrocatalysis (BE) was integrated into UASB (BE-UASB) reactor to alter microbial metabolic behaviors and enhance the re-granulation process. BE-UASB reactor exhibited the highest methane (CH4) production rate of 388.0 mL/Lreactor/d and chemical oxygen demand (COD) removal of 89.6 % at 0.8 V. Sludge re-granulation was strengthened with particle size over 300 µm of up to 22.4%. Bioelectrocatalysis stimulated extracellular polymeric substances (EPS) secretion and formation of granules with rigid [-EPS-cell-EPS-] matrix by enhancing the proliferation of key functional microorganisms (Acetobacterium, Methanobacterium, and Methanomethylovorans) and diversifying metabolic pathways. Particularly, a high Methanobacterium richness (10.8%) drove the electroreduction of CO2 into CH4 and reduced its emissions (52.8%). This study provides a novel bioelectrocatalytic strategy for controlling granular sludge disintegration, which will facilitate the practical application of UASB in methanolic wastewater treatment.

Simultaneous production and recovery of volatile fatty acids from fermentation process using an electrochemically assisted up-flow granular sludge bed reactor

A continuous production and in-line recovery of volatile fatty acids (VFAs) was developed by coupling an electrochemical cell (EC) to the recycling line of an up-flow anaerobic sludge bed (UASB) reactor. The use of an easy fermentable feedstock (glucose) was proposed for a fast production of VFAs. The EC allowed the simultaneous recovery of VFAs while producing alkali equivalent and hydrogen brought to the reactor, allowing to decrease caustic addition and promoting VFA production. Duplicate systems were used with glucose at an initial concentration of 15 g/L for Phases I to III, and increased to 60 g/L in a stepwise manner during Phase IV to VI. The EC was connected using 3.5 V to the UASB reactor at Phase III (15 g/L glucose, hydraulic retention time (HRT): 24 h), decreasing by 46 % the VFA concentration in the reactor mixed liquor and the sodium hydroxide consumption by 49 %. The highest VFA concentration in the anode chamber was 4.6 times higher than in the reactor. The highest caproic acid production achieved corresponded to 6.01 g/L during Phase IV (30 g/L glucose, HRT: 24 h). At the end of the trial, the voltage was increased up to 6.0 V and the water addition in the anode chamber was stopped to build up the VFA concentration, reaching 27.80 and 28.05 g/L for EC1 and EC2, respectively. The coupled UASB-EC system showed promising results in terms of good recovery of the VFAs from the fermentation broth, decrease of the alkali demand and caproic acid production.

Energy recovery from wastewater in Mexico: A systematic review

The usage of fossil fuels to generate energy and the lack of wastewater treatment in Mexico are two issues that can be addressed at the same time while developing wastewater treatment technologies that incorporate energy recovery in their process train. We carried out a systematic review based on the PRISMA methodology to identify and review studies regarding energy recovery using wastewater as a substrate in Mexico. Peer-reviewed papers were identified through Scopus, Web of Knowledge, and Google Scholar, using a timeframe of 22 years that represented from 2000 to 2022. After applying the selection criteria, we identified 31 studies to be included in the final review, starting from 2007. The kind of energy product, type of technology used, substrate wastewater, amount of energy produced, and main parameters for the operation of the technology were extracted from the papers. The results show that methane is the most researched energy recovery product from wastewater, followed by hydrogen and electricity, and the technology used to archive it is an up-flow anaerobic sludge bed (UASB) reactor to produce methane and hydrogen. In addition, microbial fuel cells (MFCs) were preferred to produce electricity. According to our data, more energy per kgCOD removed could be obtained with methane-recovering technologies in the Mexican peer-reviewed studies compared with hydrogen recovery and electricity production.

Coexistence of sulfate-reducing and methane-producing populations in upflow anaerobic sludge bed reactor treating lignocellulosic effluent with material balance

Coexistence of sulfate-reducing and methane-producing populations in upflow anaerobic sludge bed reactor treating lignocellulosic effluent with material balance

The Effect of Up-Flow Rate on the Nitrogen Treatment Efficiency and Sludge Characteristics of ANAMMOX Process with Up-Flow Anaerobic Sludge Bed Reactor

Anaerobic ammonia oxidation (ANAMMOX) technology is a novel biological nitrogen removal technology with potential applications for the treatment of nitrogenous wastewater treatment prospects. Most of the literature explores the growth environment of anaerobic ammonia-oxidizing bacteria and total nitrogen removal efficiency but the influence of reactor operating conditions (such as up-flow rate) on the treatment efficiency and sludge growth property of anaerobic ammonia-oxidizing bacteria is rarely discussed. Therefore, the purpose of this study is to discuss the effect of up-flow rate on the treatment efficiency and sludge property of the anaerobic ammonia oxidation treatment procedure adopting up-flow anaerobic sludge bed (UASB) as a reactor. The results show that up-flow rate has a significant effect on sludge concentration and sludge growth rate. The highest sludge concentration and maximum sludge growth rate could be obtained at the up-flow rate of 3.21 m/h. According to the analysis results of the sludge concentration, we speculate that when the flow rate was lower than 3.21 m/h, the sludge particles did not easily collide with each other to produce a larger sludge floc. On the contrary, when the up-flow rate was higher than 3.21 m/h, the larger sludge floc could be decomposed by the shear force. The sludge concentration was reduced by these two reasons. On the other hand, the average total nitrogen volume removal rates in test runs 1 through to 4 were 0.18 g-N/m3/d, 0.19 g-N/m3/d, 0.20 g-N/m3/d and 0.20 g-N/m3/d at up-flow rates from 1.95 m/h to 3.70 m/h, respectively. Therefore, the treatment efficiency was not affected by the up-flow rate in these operating conditions.

Inhibition and recovery of ANAMMOX with Na2SO3: From performance to microbial community analysis

In this study, three concentrations of Na2SO3 solution were used to remove dissolved oxygen in the influent water and enter the up-flow anaerobic sludge bed reactor (UASB). This study investigated the effects of three concentrations of Na2SO3 on the inhibition and performance recovery of the ANAMMOX system and the structure of the microbial community. The results showed that although Na2SO3 had a deoxidizing function, the ANAMMOX system underwent a sulfate reduction reaction after adding Na2SO3, and the reduction product was mainly H2S, which caused the AnAOB to be inhibited by sulfide. The denitrification performance was less than 1/2 of that under normal conditions. The ANAMMOX system was restored by immediately removing Na2SO3 and reducing nitrogen load. After 11, 20, and 34 days, the performance of the ANAMMOX system returned to normal, but low concentrations(45 mg/L) required a long recovery time. High-throughput sequencing analysis showed that the community of the ANAMMOX system changed significantly. Before and after the experiment, the relative abundance of ANAMMOX functional bacteria Candidatus Brocadia decreased from 32.66% to 21.4%, and Candidatus Kuenenia decreased from 7.3% to 4.06%. However, the relative abundances of Armatimonadetes and Ignavibacteriae increased from below 2% to about 20%. At the same time, Thiobacillus and Desulfuromonadales were observed. This paper explores and summarizes the cause of Na2SO3's inhibition of AnAOB, highlights the recovery of AnAOB after inhibition for the stable operation and deepens the understanding of the environmental conditions regulating the ANAMMOX process.

Effect of organic shock loading on anaerobic performance of pumice-reinforced up-flow anaerobic sludge bed for incineration leachate treatment

The research and application of leachate in the treatment of leachate in UASB reactors are increasing, but UASB also has problems such as poor resistance to organic load impact and unstable operation. In order to improve the anaerobic performance stability of an up-flow anaerobic sludge bed reactor (UASB) subjected to high organic loading shock, pumice was added to the leachate from the incineration. The reinforcement effect of pumice on the anaerobic efficiency of the reactor was studied by increasing the influent load. The results showed that with a gradual increase in the influent organic load [11.6‒66.6 kg COD/(m3∙d)] and without the addition of pumice, irreversible acidification took place in the reactor (R1) when the influent load reached 14.51 kg COD/(m3∙d). The average methane output and content were reduced to 39.7 L/d and 66.16%, respectively. In contrast, the reactor (R2) with pumice could still be operated stably when the influent organic load reached 40.04 kg COD/(m3∙d). The COD removal rate reached 91.80%, and the average methane output and content were increased by 42.30% and 15.20%, respectively. The results of analyzing the sludge microbial community structure in the reactor showed that the adding of pumice could selectively enrich the methanogenic bacterium methanosaeta, promote the decomposition of volatile fatty acid (VFA), effectively mitigate the acid inhibition effect of VFA on the anaerobic reactor, and enhance the balance between acidogenic bacteria (Chloroflexi and Proteobacteria) and methanogenic bacterium (Methanosaeta) to a great degree. These results prove that the addition of pumice filler can reinforce the resistance of UASB to organic loading.

Experimental and Mathematical Investigation of Anaerobic Granulate Density via Settling Velocity

The objective of this study was to determine the density of anaerobic granules on different heights of a full-scale Upflow Anaerobic Sludge Bed reactor. The density was defined through the settling velocities of anaerobic granules, measured in a full-scale Upflow Anaerobic Sludge Bed reactor. In this study, granular density was calculated with the measured settling velocities and developed mathematical model. The developed mathematical model is based on the Stokes model. In the experiment, granules were taken from different heights of an Upflow Anaerobic Sludge Bed reactor, from 0.6 to 7.6 m. The granules’ diameters varied between 1 and 5 mm. The granules were taken from six different heights through the reactor. The settling velocity of the active granules (with gas in the granule pore and on the surface of the granule) was measured first. After the active granules’ settling velocity measurement, the granules were placed in a vortex to obtain degassed granules (granules without any gas in the pores or on the surface), for which the settling velocities were also measured later. It is shown that granules’ densities at different heights are independent of the reactor height.

Enhanced anaerobic digestion of post-hydrothermal liquefaction wastewater: Bio-methane production, carbon distribution and microbial metabolism.

Hydrothermal liquefaction (HTL) is a cost-effective and environment-friendly technology for using biomass to produce bio-crude oil. The critical challenge of HTL is its complicated aqueous product containing high concentrations of organics and diverse toxicants. This paper reports the continuous anaerobic digestion of raw and zeolite-adsorbed Chlorella HTL wastewater using up-flow anaerobic sludge bed reactors. The bio-methane production capacity, total carbon distribution and microbial response were investigated. The anaerobic process was severely suppressed when more than 20% raw wastewater was fed; while it showed essentially improved performance till 60% pre-treated wastewater was added. Produced methane contained 17.3% of the total carbon in feedstock, which was comparable with the value (16.7%) when 25% of raw wastewater was added. The metagenomic analysis revealed distinct microbial community structures in different stages and feedstock shifts. The abundance of functional genes was consistent with anaerobic digester performance.

Evaluation of effluent post-treatment by slow filtration and adsorption with activated carbon produced from spent coffee grounds in surfactant removal in sewage treatment

Environmental pollution is a worldwide concern, especially when caused by sewage dumping into water bodies. Many substances are present in industrial or domestic wastewater, causing contamination in superficial water collection. Surfactants stand out for being widely used both industrially and domestically. The use of detergents and many types of surfactants was increased during the Covid-19 pandemic period, a fact observed in the levels in the effluent sample analysis from a Sewage Treatment Plant (STP) - Vila City around 21 and 39 mg L-1 in this period. This work evaluated the surfactant concentrations in the primary and secondary treatment units of the Vila City STP, in the city of Paranavaí-PR.-Brazil. In addition, the use of a post-treatment by slow sand filtration and adsorption by activated carbon produced from spent coffee grounds in the complementary removal of surfactants was proposed. A mixed bed with sand and activated carbon columns was made on a pilot scale, and filtration/adsorption runs were performed simulating slow filtration with rates of approximately 15 m3 m-2 day-1. The parameters used for the efficiency removal evaluation in a pilot plant run were: turbidity (NTU) and surfactant concentrations. The removal of surfactant concentrations was about 9% and 7% in the Upflow Anaerobic Sludge Bed reactors (UASB-RALF) and in the secondary treatment, respectively, at the STP – Vila City units. In the post-treatment proposed by filtration/adsorption, bed columns on a pilot scale plant obtained a reduction of approximately 94% in terms of turbidity (NTU) and 95% in terms of surfactant removal.
 Keywords: activated carbono, coffee poder, effluent, surfactants.

Dynamic anaerobic membrane bioreactor coupled with sulfate reduction (SrDMBR) for saline wastewater treatment

This study investigated organic removal performance, characteristics of the membrane dynamics, membrane fouling and the effects of biological sulfate reduction during high-salinity (1.0%) and high-sulfate (150 mgSO42--S/L) wastewater treatment using a laboratory-scale upflow anaerobic sludge bed reactor integrated with cross-flow dynamic membrane modules. Throughout the operational period, dynamic membrane was formed rapidly (within 5–10 min) following each backwashing cycle (21–16 days), and the permeate turbidity of <5–7 NTU was achieved with relatively high specific organic conversion (70–100 gTOC/kgVSS·d) and specific sulfate reduction (50–70 gSO42--S/kgVSS·d) rates. The sulfide from sulfate reduction can be reused for downstream autotrophic denitrification. 16S rRNA gene amplicon sequencing revealed that the microbial communities enriched in the sludge were different than those accumulated on the dynamic layer. Overall, this study demonstrates that the anaerobic dynamic membrane bioreactor coupled with sulfate reduction (SrDMBR) shows promising applicability in saline wastewater treatment.

Anaerobic Degradation of Dairy Wastewater in Intermittent UASB Reactors: Influence of Effluent Recirculation

This work studied the influence of effluent recirculation upon the kinetics of anaerobic degradation of dairy wastewater in intermittent UASB (Upflow Anaerobic Sludge Bed) reactors. Several laboratory-scale tests were performed with different organic loads in a UASB reactor inoculated with flocculent sludge from an industrial wastewater treatment plant. The data obtained were used for determination of specific substrate removal rates and specific methane production rates and adjusted to kinetic models. A high initial substrate removal was observed in all tests due to adsorption of organic matter onto the anaerobic biomass which was not accompanied by biological substrate degradation as measured by methane production. Initial methane production was about 45% of initial soluble and colloidal substrate removal rate. This discrepancy was observed mainly in the first day of all experiments and was attenuated in the second day. Effluent recirculation raised significantly the rate of removal of soluble and colloidal substrate and methane productivity as compared to literature results for batch assays without recirculation.

Calcium hypochlorite pretreatment improves thermophilic digestion of waste activated sludge in an upflow anaerobic sludge blanket reactor

Anaerobic wasted activated sludge (WAS) digestion has been widely applied to reduce sludge volume and generate bioenergy in the form of methane. However, anaerobic WAS digestion performance is often challenged with poor hydrolysis of biomass cellular structures. In the present study, the feasibility of using calcium hypochlorite (Ca(ClO)2) to improve the thermophilic digestion of WAS was studied. Two thermophilic upflow anaerobic sludge bed (UASB) reactors (one with and one without Ca(ClO)2 pretreatment) were operated for 120 days under low and high organic loading rate (OLR) conditions, corresponding hydraulic retention time (HRT) of 10 days and 6 days, respectively. Both reactors achieved satisfied performance during the studied period. Under the low OLR condition, Ca(ClO)2 pretreatment significantly improved WAS total volatile solids (VS) removal efficiency (from 48.06 ± 2.63% to 57.34 ± 3.54%) and methane yield (from 289.2 ± 27.6 to 362.2 ± 36.7 N mL/g VS). However, no significant improvement was observed under the high OLR condition. g_S1 and g_Fervidobacterium were predominant bacteria in the thermophilic UASB reactor fed with Ca(ClO)2 pretreated WAS. Methanosarcina was dominant archaea in both reactors. The treatment mechanism and application potential of using Ca(ClO)2 to enhance the WAS digestibility were further discussed.