Enhanced granulation and methane recovery at low load by downflow sludge circulation in anaerobic treatment of domestic wastewater

De novo anaerobic granulation with varying organic substrates: granule growth and microbial community responses

Anaerobic granulation from dispersed inoculum is recognized as a slow process. However, studies under saline conditions have shown that adding complex proteinaceous substrates can accelerate this process. To explore whether this holds true also under non-saline conditions, we conducted a 262-days experiment with four lab-scale upflow anaerobic sludge blanket reactors inoculated with digested sewage sludge. Each reactor received a synthetic feed containing varying amount of carbohydrate/protein substrate: glucose (RGlu), acetate/tryptone (RAc+Try), glucose/tryptone (RGlu+Try), and glucose/starch (RGlu+Sta). Development of granules with different influent composition was monitored with macroscopy, analysis of the extracellular polymeric substances, and microbial diversity. Granulation was faster in reactors RGlu+Try and RGlu+Sta. Increasing granule diameters positively correlated with the occurrence of bacteria from Muribaculaceae and Lachnospiraceae families, suggesting their involvement in de novo granulation. Granules of RGlu+Try also had high relative abundances of both fermenting bacteria (e.g. Lactococcus, Streptococcus, Trichococcus) and bacteria involved in the oxidation of volatile fatty acids (Smithella, Acetobacteroides). The results of this study provide a basis for strategies to enhance the sludge granulation rate in practice when granular inoculum is not available. Specifically, supplementing small amounts of waste protein during reactor start-up can be effective.

Enhancement of electron transfer via magnetite in nitrite-dependent anaerobic methane oxidation system

Nitrite-dependent anaerobic methane oxidation (n-DAMO) is a novel denitrification process that simultaneously further removes and utilizes methane from anaerobic effluent from wastewater treatment plants. However, the metabolic activity of n-DAMO bacteria is relative low for practical application. In this study, conductive magnetite was added into lab-scale sequencing batch reactor inoculated with n-DAMO bacteria to study the influence on n-DAMO process. With magnetite amendment, the nitrogen removal rate could reach 34.9 mg N·L-1d-1, nearly 2.5 times more than that of control group. Magnetite significantly facilitated the interspecies electron transfer and built electrically connected community with high capacitance. Enzymatic activities of electron transport chain were significantly elevated. Functional gene expression and enzyme activities associated with nitrogen and methane metabolism had been highly up-regulated. These results not only propose a useful strategy in n-DAMO application but also provide insights into the stimulating mechanism of magnetite in n-DAMO process.

Hydraulic characterization and start-up of a novel circulating flow bio-carriers

High-quality biofilm carriers are crucial for the formation of biofilm, but problems such as slow biofilm growth on the carrier surface have been troubling a large number of researchers. The addition of a carrier changes the flow state in the reactor, which in turn affects the microbial attachment and the quantity of microorganisms. Also, aerobic microorganisms need to use dissolved oxygen in the water to remove water pollutants. In this paper, a novel recirculating flow carrier with a hollow cylinder structure is proposed, with a certain number of hollow inverted circular plates placed at equal distances inside. In this paper, the hydraulic residence time, aeration volume, and the spacing of the inflow plates of the recirculating flow biofilm carrier, which are three important factors affecting the hydraulic characteristics of the reactor, are first investigated. At the same time, it was compared with the common combined carrier to find the optimal operating conditions for the hydraulic characteristics. Secondly, a reactor start-up study was carried out to confirm that the new recirculating flow biofilm carrier could accelerate the biofilm growth by changing the hydraulic characteristics. The results showed that under the same conditions, the hydraulic properties of the reactor were better with the addition of the recirculating flow carrier, with an effective volume ratio of 98% and a significant reduction in short flows and dead zones. The stabilized removal of COD, NH3-N, and TN in the reactor with the addition of the recirculating flow carrier reached about 94%, 99%, and 91% respectively, at the beginning of the 15th day, which effectively proved the feasibility of the recirculating flow carrier.

Study on the feasibility of carbon source recovery by upflow anaerobic sludge blanket in simulated municipal wastewater

The influence of hydraulic retention time (HRT) on the granulation process, methane-producing capacity, microbial community structure, and pollutant removal efficiency of an up-flow anaerobic sludge blanket (UASB) with simulated municipal wastewater at a mesophilic temperature was investigated. The carbon recovery capacity of the anaerobic fermentation of municipal wastewater at mesophilic temperatures is one of the problems to be investigated for the realisation of carbon neutrality in municipal wastewater treatment plants. In this study, the HRT was gradually shortened (24-6 h), and the effluent chemical oxygen demand (COD), ammonia nitrogen, pH, volatile fatty acid concentration, and specific methanogenic activity (SMA) were investigated. The sludge morphology, the particle size distribution of the different HRT, and changes in the microbial community structure were determined by scanning electron microscopy, wet screening, and high-throughput sequencing. The results indicated that even if the COD concentration was only 300-550 mg/L, with a decrease in HRT, the proportion of granular sludge in the UASB still exceeded 78 %, and the COD removal efficiency reached 82.4 %. The SMA of granular sludge increased with an increase in the size of granules and was 0.289 g CH₄-COD/(g VSS d) at an HRT of 6 h, but the proportion of dissolved methane in the effluent accounted for 38-45 % of the total methane production and the proportion of Methanothrix in UASB sludge was 82.44 %. In this study, dense granular sludge was obtained by gradually shortening the HRT to start the UASB, and the lower effluent COD reduced the load of subsequent treatment processes, which could be used as a low carbon/nitrogen ratio influent for activated carbon-activated sludge, activated sludge-microalgae, and partial nitrification-anaerobic ammonia oxidation processes.

Municipal wastewater treatment with corrugated PVC carrier anaerobic baffled reactor

The anaerobic baffled reactor (ABR) is a promising solution for decentralized wastewater treatment due to its low operation cost as compared to the activated sludge process, but it requires comparatively higher hydraulic retention time (HRT). This ultimately increases land requirement, capital and construction cost of treatment plant. This study investigates performance of ABR using polyvinyl chloride (PVC) corrugated pipe as carrier media to improve biomass retention capacity and treatment performance of reactor with the aim to reduce HRT. Comparative performance of two ABRs with and without carrier media was analyzed under mesophilic conditions (35 ± 1 °C) for organics and total suspended solids (TSS) removal at HRTs of 24, 18, 12, 8, 6 and 4 h. Results showed that at HRTs of 24-08 h, the organics removal performance of the carrier anaerobic baffled reactor (CABR) was better than ABR and was in the range of 77-81% for CABR as compared to 64-70% for ABR. However, on further decrease in HRT to 6 h, CABR sustained the treatment with organics removal of 80%, while ABR performance reduced to 58%, creating a performance difference of 38%. Average total suspended solids (TSS) removal was in the range of 76-83% at all HRTs for both reactors. Therefore, this study identified CABR with PVC carrier media as an effective low-HRT reactor for organics and SS removal with less land area requirement.

Effect of corn stover hydrochar on anaerobic digestion performance of its associated wastewater

Hydrothermal carbonation (HTC) is an effective method to enhance the fuel quality of biomass in a subcritical water environment, but generates large amounts of wastewater (HTCWW), which was converted through anaerobic digestion (AD) into methane in this study. However, the toxic and refractory substances contained in HTCWW tended to cause operation instability of the AD system. The solid product in HTC of corn stover (CS), named CS hydrochar, was modified with KOH immersion and then added to the AD reactor to improve the methanogenic performance. The results showed that the optimum dosage of modified hydrochar (MCH) was 15 g/L, and the COD removal rate was increased by 19.3% and methane yield was increased by 42.3%-301 mL/g-COD, as the pore and the oxygen-containing functional groups of MCH provided colonization points for microorganisms, and also enhanced the electron transfer efficiency among methanogenic archaea. In addition, the increased alkalinity of MCH due to alkaline modification increased the pH buffering capability, and accelerated the consumption of acetic acid and butyric acid in the early AD stage (0-8 days) and propionic acid in the late AD stage (12-18 days), which then alleviated the organic acid accumulation and reduced the lag period by 2 days. The adverse effects of toxic and refractory substances of HTCWW on the AD performance were also decreased due to the adsorption of MCH at the beginning of the AD process, and latterly the adsorbed substances could be degraded by the microorganisms colonized on the MCH surface. The finding of this study showed AD is a feasible method to recover organic energy contained in HTCWW, and the associated hydrochar can be used as an effective promoter for the AD of HTCWW.

Application of Fenton sludge coupled hydrolysis acidification in pretreatment of wastewater containing PVA: Performance and mechanisms

Hydrolysis acidification (HA) is widely used in pretreatment of macromolecular refractory wastewater to improve its biodegradability. However, because the biological activity could be inhibited by macromolecular substances to a certain extent, its application is limited. In this study, polyvinyl alcohol (PVA), as a classic macromolecular pollutant in TPD wastewater, was treated by the Fenton sludge-coupled HA process to investigate the effects of Fenton sludge addition on the HA performance and identify the probable mechanisms behind it. The results showed that approximately 40% of macromolecular PVA was hydrolyzed into small molecular substances with molecular weight (Mw) < 10⁵ in the Fenton sludge-added reactor. Meanwhile, acidification efficiency (AE), volatile fatty acid production increased by 20.8% and 92.05 mg/L with Fenton sludge addition. The values of BOD₅/COD changed from 0.091 of influent to 0.26 and 0.32 of effluent from the simple HA process and Fenton sludge addition HA process, respectively. These results proved that biodegradability was improved by the two processes and the Fenton sludge addition had a positive effect on HA. Further analysis found that 2-lines ferrihydrite involved in Fenton sludge might serve as an electron acceptor to participate in extracellular respiratory. Besides, the Fe²⁺ observed a positive effect of the sludge characteristics in agreement with the higher activity of dehydrogenase and extracellular polymeric substances (EPS) production. This study suggested that Fenton sludge can be recycled and used as an iron source to enhance HA for industrial wastewater pretreatment.

Effect of circulation and micro-aeration on sludge characteristics and microbial community in an ABR for treating traditional Chinese medicine wastewater

The effects of circulation reflux and micro-aeration on the performance of a modified anaerobic baffled reactor (ABR) for treatment of traditional Chinese medicine (TCM) wastewater were evaluated. The characteristics of anaerobic sludge and microbial community structure in the modified ABR were also investigated. The results indicated that with conditions of reflux ratio of 1, reflux ratio of 2, reflux ratio of 2 with micro-aeration, and reflux ratio of 3, the modified ABR achieved an average COD removal efficiency of 90%, 87.7%, 87.8%, and 88.4%, respectively. In addition, the NH₃-N average removal efficiency was 45.1%, 50%, 55.9%, and 55.4%, respectively. The analysis of excitation-emission matrix (EEM) fluorescence spectra of soluble microbial products (SMP) and extracellular polymeric substances (EPS) showed that there were tyrosine-like, aromatic protein-like, and coenzyme F₄₂₀ substances in the sludge. The EPS were analysed by the Fourier transform infrared spectroscopy (FTIR), which showed that aromatic compounds were partially degraded, while the protein and polysaccharide compounds increased in each compartment of the modified ABR. Interestingly, the microbial community of anaerobic sludge analysis results showed that Chloroflexi was the dominant in the first, third and fourth compartments. Meanwhile, Levilinea and Methanothrix were the dominant species in the first and third compartments at the genus level.

Granulation process in an expanded granular sludge blanket (EGSB) reactor for domestic sewage treatment: Impact of extracellular polymeric substances compositions and evolution of microbial population

In this study, an expanded granular sludge blanket (EGSB) reactor was used for the treatment of low-strength domestic sewage and the sludge granulation process was systematically investigated. At an optimized hydraulic retention time (HRT) of 5 h, up-flow velocity (V_up) of 1.9 m/h, and organic loading rate (OLR) of 2.16 kg COD/m³/d, the average COD removal efficiency was 71.5 ± 2.3%. Completely granular sludge can be observed after 107 d of continuous operation. Analysis of the distribution and composition of the extracellular polymeric substances (EPS) indicates that the tightly bound EPS (TB-EPS) content shows an increasing trend, while the loosely bound EPS (LB-EPS) content did not significantly alter after the granular sludge was formed. The three-dimensional excitation-emission matrix technique (3D-EEM) confirms that aromatic protein-like substances are of key importance to sludge granulation. High-throughput sequencing analysis indicates that the metabolism shifted from hydrogenotrophic (Methanobaterium) to aceticlastic methanogens (Methanosaeta) during sludge granulation.

Enhancing anaerobic treatment of domestic wastewater: State of the art, innovative technologies and future perspectives

Recent concerns over public health, environmental protection, and resource recovery have induced to look at domestic wastewater more as a resource than as a waste. Anaerobic treatment, owing to attractive advantages of energy saving, biogas recovery and lower sludge production, has been suggested as an alternative technology to the traditional practice of aerobic wastewater treatment, which is energy intensive, produces high excess of sludge, and fails to recover the potential resources available in wastewater. Sewage treatment by high-rate anaerobic processes has been widely reported over the last decades as an attractive method for providing a good quality effluent. Among the available high-rate anaerobic technologies, membrane bioreactors feature many advantages over aerobic treatment and conventional anaerobic systems, since high treatment efficiency, high quality effluent, pathogens retention and recycling of nutrients, were generally achieved. The objective of this paper is to review the currently available knowledge on anaerobic domestic wastewater treatment for the mostly applied high-rate systems and membrane bioreactors, presenting benefits and drawbacks, and focusing on the most promising emerging technologies, which need more investigation for their scale-up.

State of the art on granular sludge by using bibliometric analysis

With rapid industrialization and urbanization in the nineteenth century, the activated sludge process (ASP) has experienced significant steps forward in the face of greater awareness of and sensitivity toward water-related environmental problems. Compared with conventional flocculent ASP, the major advantages of granular sludge are characterized by space saving and resource recovery, where the methane and hydrogen recovery in anaerobic granular and 50% more space saving, 30-50% of energy consumption reduction, 75% of footprint cutting, and even alginate recovery in aerobic granular. Numerous engineers and scientists have made great efforts to explore the superiority over the last 40 years. Therefore, a bibliometric analysis was desired to trace the global trends of granular sludge research from 1992 to 2016 indexed in the SCI-EXPANDED. Articles were published in 276 journals across 44 subject categories spanning 1420 institutes across 68 countries. Bioresource Technology (293, 11.9%), Water Research (235, 9.6%), and Applied Microbiology and Biotechnology (127, 5.2%) dominated in top three journals. The Engineering (991, 40.3%), China (906, 36.9%), and Harbin Inst Technol, China (114, 4.6%) were the most productive subject category, country, and institution, respectively. The hotspot is the emerging techniques depended on granular reactors in response to the desired removal requirements and bio-energy production (primarily in anaerobic granular sludge). In view of advanced and novel bio-analytical methods, the characteristics, functions, and mechanisms for microbial granular were further revealed in improving and innovating the granulation techniques. Therefore, a promising technique armed with strengthened treatment efficiency and efficient resource and bio-energy recovery can be achieved.