Optimum Hydraulic Retention Time Research Articles

Optimum hydraulic retention time, the best microbial community genus, and carbon sources in woodchip denitrifying bioreactors: a mini review

Optimum hydraulic retention time, the best microbial community genus, and carbon sources in woodchip denitrifying bioreactors: a mini review

Significantly Enhanced Nitrate and Phosphorus Removal by Pyrite/Sawdust Composite-Driven Mixotrophic Denitrification with Boosted Electron Transfer: Comprehensive Evaluation of Water-Gas-Biofilm Phases during a Long-Term Study.

Further reducing total nitrogen (TN) and total phosphorus (TP) in the secondary effluent needs to be realized effectively and in an eco-friendly manner. Herein, four pyrite/sawdust composite-based biofilters were established to treat simulated secondary effluent for 304 days. The results demonstrated that effluent TN and TP concentrations from biofilters under the optimal hydraulic retention time (HRT) of 3.5 h were stable at <2.0 and 0.1 mg/L, respectively, and no significant differences were observed between inoculated sludge sources. The pyrite/sawdust composite-based biofilters had low N2O, CH4, and CO2 emissions, and the effluent's DOM was mainly composed of five fluorescence components. Moreover, mixotrophic denitrifiers (Thiothrix) and sulfate-reducing bacteria (Desulfosporosinus) contributing to microbial nitrogen and sulfur cycles were enriched in the biofilm. Co-occurrence network analysis deciphered that Chlorobaculum and Desulfobacterales were key genera, which formed an obvious sulfur cycle process that strengthened the denitrification capacity. The higher abundances of genes encoding extracellular electron transport (EET) chains/mediators revealed that pyrite not only functioned as an electron conduit to stimulate direct interspecies electron transfer by flagella but also facilitated EET-associated enzymes for denitrification. This study comprehensively evaluates the water-gas-biofilm phases of pyrite/sawdust composite-based biofilters during a long-term study, providing an in-depth understanding of boosted electron transfer in pyrite-based mixotrophic denitrification systems.

Characterization and Performance Evaluation of Anaerobic Night Soil Biodegrading Inoculum

The efficiency of anaerobic night soil (human excreta) biodegradation depends on the characteristics of the inoculum used. This anaerobic microbial inoculum (AMI) contains a microbial consortium, including rate-limiting methanogens whose growth depends on various physicochemical and biological parameters necessitating the inoculum characterisation essential for optimum night soil (NS) biodegradation. The inoculum contained 24.567 g/L (±0.07) total solid, 5.022 (±0.57) g/L total dissolved solid, 18.148 (±0.0.058) g/L volatile solid (VS), and 7.0±0.2 pH. The Gas chromatography analysis confirmed the presence of a small amount of volatile fatty acid (VFA), (≤ 4 mM except propionate) showing rapid conversion of VFA to methane, and 55% methane. Further, the presence of methanosaetaceae group of methanogens was detected through Taqman probe-based real-time PCR. The inoculum performance was evaluated for NS biodegradation in semicontinuous mode of feeding in four 2 L anaerobic biodigesters (2D HRT, 4D HRT, 5D HRT, and 10D HRT) with varying hydraulic retention time (HRT) (2, 5, 8, and 10 days, respectively) at mesophilic temperature (35°C). After 5 HRT, VS reduction for 2D HRT and 5D HRT was &lt;40% while for 8D HRT and 10D HRT was &gt;45-50% and &gt;45-60%, respectively. The methane content was &lt;20% in biogas for 2 and 5 days HRT while for 8 and 10 days &gt;45-50% and 50-65% methane, respectively. The % VS reduction and methane content showed the overfeeding of NS in biodigesters 2D HRT and 5D HRT while 8D HRT showed towards optimum and 10D HRT optimum with improved VS reduction and methane content.

Evaluation of the optimum conditions of an anaerobic Down-flow Stationary Fixed Film reactor for swine wastewater treatment

The Down-flow Stationary Fixed Film (DSFF) reactor is a downstream anaerobic reactor combined with a fixed layer of adhesive media that is capable of handling large organic loads, has cheap investment costs, and has a shorter treatment time than other conventional anaerobic technologies. In this study, the DSFF reactor was used to investigate its potential to remove contaminants from swine wastewater. The optimal operating conditions of the DSFF reactor including hydraulic retention time (HRT), organic concentration, and nutrient ratio were investigated. The obtained results showed that the COD removal efficiency increased from 50% to 79.721% when the influent COD enhanced from 500 to 3000 mg/L. The COD removal efficiency was 85.66%, 61.71% and 50.81% at three different nutritional ratios of C/N/P:250/1.5/1, C/N/P:250/7.5/1, and C/N/P:250/7.5/0.2, respectively. The overall results revealed that the optimal HRT is 12 hours, the optimal COD concentration is between 2000 mg/L and 3000 mg/L, and the optimal nutrient ratio is C/N/P:250/1.5/1. The DSFF reactor effectively manages BOD and phosphorus removal while handling a range of contamination levels in swine wastewater. Furthermore, there is no significant sludge loss and the operation mode is simple. This reactor is potentially appropriate for implementation in pig farming households.

Biological denitrification performance of a novel sulfur-slow-release carbon source mixed filler

Long-term operation of the sulfur autotrophic denitrification (SAD) process leads to pH reduction, nitrite accumulation, a low nitrogen removal efficiency and excessive sulfate in the effluent. Two slow-release carbon source (SRC) fillers (reactors S2 and S3) consisting of different ratios of hydroxypropyl methyl cellulose, disodium fumarate, polyhydroxy fatty acid ester and Fe3O4 powder were designed to couple SAD with heterotrophic denitrification by using SRC and sulfur particles (reactor S1) as components, combining the advantages of both processes while avoiding secondary pollution. During reactor operation, S3 showed the best denitrification performance. Under the optimal hydraulic retention time of 4 h, different influent nitrate concentrations of 39.92 ± 0.59 mg/L and 20.22 ± 0.45 mg/L achieved average nitrate removal efficiencies up to 88.01 ± 3.62 % and 94.28 ± 2.52 %, and the average denitrification strength were 8.78 mg N/(L·h) and 4.77 mg N/(L·h), respectively. 16S rRNA gene analysis at the genus level showed that the coupled reaction had the highest relative abundance of Thiobacillus associated with SAD, and SAD dominated the coupled system.

Optimizing hydraulic retention time of high-rate activated sludge designed for potential integration with partial nitritation/anammox in municipal wastewater treatment

The integration of high-rate activated sludge (HRAS), an effective carbon redirection technology, with partial nitritation/anammox (PN/A) is a novel AB treatment process for municipal wastewater. In this study, an airlift HRAS reactor was operated in the continuous inflow mode for 200 d at a wastewater treatment plant. The balance between potential PN/A system stability and peak HRAS performance under decreasing hydraulic retention time (HRT) was optimized. Energy consumption and recovery and CO2 emissions were calculated. The results showed that the optimal HRT suitable with the PN/A process was 3 h, achieving 2–3 g/L mixed liquor volatile suspended solid, 67.8 % chemical oxygen demand (COD) recovery, 81 % total COD removal efficiency, 2.27 ± 1.03 g COD/L/d organic loading rate, 62 % aeration reduction, and 0.24 kWh/m3 power recovery potential. Such findings hold practical value and contribute to the development of the optimal AB process capable of achieving energy autonomy and carbon neutrality.

Pilot composite tubular bioreactor for outdoor photo-fermentation hydrogen production: From batch to continuous operation

A novel 70 L composite tubular photo-bioreactor was constructed, and its photo-fermentation hydrogen production characteristics of batch and continuous modes were investigated with glucose as the substrate in an outdoor environment. In the batch fermentation stage, the hydrogen production rate peaked at 37.6 mL H2/(L·h) accompanied by a high hydrogen yield of 7 mol H2/mol glucose. The daytime light conversion efficiency is 4 %, with 37 % of light energy from the sun. An optimal hydraulic retention time of 5 d was identified during continuous photo-fermentation. Under this condition, the stability of the cell concentration is maintained and more electrons can be driven to the hydrogen generation pathway while attaining a hydrogen production rate of 20.7 ± 0.9 mL H2/(L·h). The changes of biomass, volatile fatty acids concentration and ion concentration during fermentation were analyzed. Continuous hydrogen production by composite tubular photo-bioreactor offers new ideas for the large-scale deployment of photobiological hydrogen production.

Co-digestion of filter cake, biogas effluent, and anaerobic sludge for hydrogen and methane production: Optimizing energy recovery through two-stage anaerobic digestion

This study assesses a two-stage anaerobic digestion process designed to efficiently recover energy through hydrogen and methane production by co-digesting filter cake (FC), biogas effluent (BE), and anaerobic sludge (AS) from the sugar and ethanol industry. The optimal proportions of FC, BE, and AS were determined in batch fermentation experiments using Design Expert software, which identified a suitable ratio of 31.05:28.95:0.00 (g VS/L), respectively. The results indicated that hydrogen production in the first stage could occur solely through the hydrogenic bacteria present in the BE and FC mixture, without the need for AS as an inoculum. This optimized FC:BE ratio was then applied in a semi-continuous fermentation system, achieving a hydrogen production rate of 193.6 mL H2/L.d and a hydrogen yield of 9.8 mL H2/g VS at an optimal hydraulic retention time (HRT) of 3 d. In the subsequent second stage, the effluent from the hydrogen reactor was used for methane production. This stage achieved a methane production rate of 422.0 mL CH4/L·d and a methane yield of 140.2 mL CH4/g VS, with an HRT of 20 d. Overall, the two-stage process exhibited an impressive energy output, peaking at 5.17 kJ/g VS. This suggests the potential for an annual electricity generation of 194,655 MWh and an estimated reduction of 85,668 tCO2eq/year in greenhouse gas emissions. This study highlights the efficiency of the two-stage anaerobic digestion process for harnessing energy through the co-digestion of FC and BE, without the need for additional AS inoculum. The findings demonstrate the potential for sustainable energy recovery from industrial waste streams while mitigating environmental impacts.

Comparison of Artificial Neural Fuzzy Inference System (ANFIS) and Response Surface Methodology (RSM) Model in Predicting the Outlet Flow Rate of Passive Treatment System Column

Acid mine drainage (AMD) is one of the major environmental problems the mining and mineral processing industries face. Treatment of AMD involves active and passive treatment. In the long term, passive treatment is the most effective way to treat acid mine drainage, but it can be expensive. if handled properly. Therefore, the study of flow rate in a passive treatment system is one of the important ways to identify optimum hydraulic retention time to ensure the maximum percentage of heavy metal removal can be achieved while keeping the cost to a minimum level. This study focused on developing and comparing the Response Surface Methodology (RSM) model and Artificial Neural Fuzzy Inference System (ANFIS) model to predict the outlet flow rate of the passive treatment system column based on three parameters inlet flow time, thickness of peat soil bed, and inlet flow rate. The RSM model was created by Design-Expert software whereas MATLAB created the ANFIS model with 80% of data used for the model training and 20% of the data for model testing. The models' performances were compared in terms of statistical errors (AAPE, RMSE, R2, STD, minimum error, and maximum error). It was found the ANFIS model has performed better in predicting the outlet flowrate with R2 value of 0.99 RSM model with the value of 0.97. The inlet flow rate was an insignificant parameter affecting the outlet flow rate of the passive treatment column. From the 3-D surface response plot, the highest outlet flow rate is predicted to be 524 mL/min.

Nitrogen removal and performance deterioration in digested effluent treatment by partial nitrification-anammox (PNA) process based on aeration sedimentation integrated microaerobic reactor (ASIMR)

The existing microaerobic reactors for PN/A process have the drawbacks of complex structure and hard to scale-up in full scale application. Aiming at these problems, we have designed new aeration sedimentation integrated microaerobic reactor (ASIMR). The feasibility of ASIMR for PN/A processes and the optimal HRT were explored by gradually reducing the hydraulic retention time (HRT). When the HRT was 4.45 h and the NLR was 3.98 kg·(m3·d)-1, ASIMR achieved the highest nitrogen removal performance, with the ammonia nitrogen removal efficiency (ARE) and total nitrogen removal efficiency (NRE) of 90.5 % and 79.2 %, respectively, and the total nitrogen removal rate (NRR) could reach 3.15 kg·(m3·d)-1. When HRT decreased to 3.60 h with the NLR of 4.76 kg·(m3·d)-1, the performance deterioration was observed, with ARE and NRE decreasing to 35.9 % and 12.4 %, and NRR decreasing to 0.59 kg·(m3·d)-1. The main reasons for the decrease in nitrogen removal performance were overhigh hydraulic load in the sedimentation zone which led to the loss of nitrifying sludge. The loss of nitrifying sludge in turn reduced the efficiency of ammonia nitrogen removal resulting in the accumulation of free ammonia (FA) which inhibited the activity of nitrogen removal functional bacteria. Although reactor performance deteriorated at too low HRT, when the HRT was much than 4.45 h, ASIMR could achieve a good performance for nitrogen removal owe to the combined action of reactor design, enriched nitrified and anammox sludge as inoculum, and dissolved oxygen control. Except good nitrogen removal performance, ASIMR has another advantage, such as simple structure, low cost for manufacture, and more suitable for engineering scale-up in full scale application.

Mitigating short-circuits through synergistic temperature and hydraulic retention time control for enhancing methane yield in continuous stirred-tank reactors

Temperature and hydraulic retention time (HRT) control are crucial for mitigating the “short-circuit” issue and enhancing methane production in continuous stirred-tank reactors. This study focuses on the combined effects of temperature and HRT on methanation via operating the reactors at different temperatures (35 °C, 45 °C, and 55 °C) and HRT (30, 25, and 20 d) for 116 days. The results demonstrated that the digester conducted at 45 °C and an HRT of 30 d unexpectedly yielded the maximum average methane production of 296.08 ml/g volatile solids (VS). In contrast, the optimal HRT for the 35 °C and 55 °C digesters were 25 and 30 d, with a methane yield of 241.88 ml/g VS and 240.9 ml/g VS, respectively. High-throughput sequencing shows that Firmicutes, Bacteroidetes, Themotogae, Chloroflexi, and Acidobacteria dominated 90–98 % of the total bacteria. Temperature exhibited a significant influence on shaping microbial ecosystems, indicating that temperature, rather than HRT, is the primary factor to consider. Additionally, metagenomic analysis reveals that the levels of genes involved in acetoclastic and hydrogenotrophic methanogenic pathways were enhanced at 45 °C, probably improving methane yield. These findings provide new insights into “short-circuit” mitigation and strategies to strengthen methane generation from biomass.

Valorization of sugarcane leaves and co-digestion with microalgal biomass to produce biofuels and value-added products under the circular economy and zero-waste concepts

This study investigates the production of biofuels and value-added products by co-digesting sugarcane leaves and microalgal biomass, aligning with circular economy and zero-waste principles. Hydraulic retention time (HRT) on continuous hydrogen production in a continuous stirred tank reactor was investigated. An optimal HRT of 12 h maximizes hydrogen production rate (3,983 mL-H2/L·d) and yield (99.6 mL-H2/g-VS). Microbial analysis using 16S rRNA amplicon sequencing on the Illumina platform indicates diverse communities, prominently featuring Streptococcus (43.4 %) and Clostridium (31.3 %) genera. The process efficiently utilizes residual components—acidified slurry, hydrogenic effluent, and solid residue—for methane and hydrogen generation, yielding up to 22.2 kJ/g-VS in total energy. The solid residue from hydrogen production exhibits composting properties and serves to produce vermicompost and densified refuse-derived fuel. This approach exemplifies the adept utilization of raw materials within a circular economy and zero-waste framework, yielding no waste throughout the entire process.

Effect of HRT on nitrogen removal from low carbon source wastewater enhanced by slurry and its mechanism

Insufficient carbon sources in low-carbon wastewater have consistently hindered nitrogen removal performance. This work utilized slurry after anaerobic digestion as an additional carbon source, and the optimal hydraulic retention time (HRT) parameters were obtained through adjustment. The findings revealed that, under HRT conditions of 24–12 h, the discharge standards of wastewater treatment plants (WWTPs) could be met. It had been proven that HRT 12 h was the more cost-effective parameter for application in WWTPs, and the removal efficiencies for COD, NH4+-N, NO3–-N, and TN reached impressive levels of 82.43%, 95.24%, 77.49%, and 86.70%, respectively. However, the HRT 6 h struggled to achieve efficient wastewater treatment. Mechanistic analysis demonstrated that shortening the HRT resulted in considerably higher microbial richness, diversity, and evenness, causing notable shifts in microbial community structure. Additionally, a shortened HRT reduced the enrichment abundance of key bacterial phyla (Proteobacteria, Bacteroidetes) and genera (Hydrogenophaga, Filimonas, Meganema) that played an important role in nitrogen and organic matter removal. Furthermore, a shortened HRT increased hydraulic shear forces, impeding the enrichment of carbon and nitrogen metabolism pathways, and reducing the abundance of essential genes associated with glycolysis (HK, pfkA, FBA, GAPDH) and nitrogen metabolism processes (narG, narH, narI, norB, nirS, nosZ). It was noteworthy that the microbial community under the HRT 6 h condition carried the highest risk of microbial pathogenicity. This work revealed the optimal parameters, and carbon and nitrogen metabolism mechanism of wastewater treatment systems with slurry as a carbon source for the first time, and provided novel ideas and insights for low-carbon wastewater treatment.

Characterizing the role of hydraulic retention time on nitrate removal indices in denitrifying bioreactors by nonlinear models

Denitrifying bioreactors (DNBRs) are a sustainable and cost-effective practice commonly used at the edge of fields to reduce nitrate from agricultural runoff. The hydraulic retention time (HRT) is a crucial variable that affects nitrate removal rate (NRR, g N m−3 d−1), nitrate removal efficiency (NRE, %), and nitrate concentration reduction per length (Nrd, mg N L−1 m−1). In this study, two nonlinear models, the developed Michaelis-Menten (MM) model and the Mitscherlich (MT) model, were developed to characterize the relationship between nitrate removal indices (NRR, NRE, and Nrd) and HRT. This study first utilizes nonlinear models to quantitatively understand the relationship between NRR, NRE, Nrd, and HRT. To verify the models, eight experiments were conducted under different conditions, including different scales (laboratory and field), media (woodchip, woodchip+biochar, woodchip+silage leachate, woodchip+biochar+silage leachate), and influent nitrate concentrations (6.8–70 mg N L−1). The results showed that the MT model outperformed the MM model and MT could accurately characterize the nitrate removal changes with HRT and provide the optimal HRT (HRTO). Overall, the model could be beneficial for designers and practitioners to optimize nitrate removal.

Sulfur-pyrite-limestone biological filter for simultaneous nitrogen and phosphorus removal from wastewater treatment plant effluent: Interaction mechanisms of autotrophic and heterotrophic denitrification

To efficiently remove nitrogen(N) and phosphorus(P) from wastewater treatment plant effluent (WWTPE) with insufficient carbon, sulfur-pyrite-limestone autotrophic denitrification (SPLAD) reactor and gravel heterotrophic denitrification (GHD) reactor were constructed. Results showed that SPLAD was successfully started after 8 days and was faster than GHD. Under optimal hydraulic retention time (30 min) and backwash cycle (2 d), SPLAD achieved higher removal rates for TP, TN, NO3−-N, NH4+-N at 70.49 %, 90.35 %, 92.82 %, 79.66 %, respectively, surpassing those of GHD. The N removal primarily occurred through nitrification and denitrification with a proportion of 2.42–2.51 between heterotrophic and autotrophic denitrification, while P removal predominantly relied on the combination with Fe2+/Fe3+ generated by FeS2 autotrophic denitrification. Microbial community analysis revealed that Thiobacillus (17.86 %), Thioalbus (7.30 %) and Geothrix (0.81 %), belonging to autotrophic denitrification bacteria using S0/FeS2 as electron donors to reduce NO3−-N, were existed exclusively in SPLAD. Besides, the denitrification genes, such as nitrate reduction (narG, narZ, nxrA, and narH, narY, nxrB) and nitrite reduction (nirB and nirD), were higher in PSLAD than that in GHD. High abundance of sulfur oxidation genes (soxA, soxX, soxB, soxY and soxZ) and iron oxidation gene (ccoN) for autotrophic denitrification were also found in SPLAD. This study highlights the practical engineering application potential of the sulfur/pyrite/limestone involved mixotrophic denitrification process due to its simplicity, cost-effectiveness, and efficient N and P removal.

Water supply sludge-based ceramsite denitrification filter: Pollutant removal and microbial community characteristics

Using water supply sludge to prepare ceramsite for sewage treatment is one of the methods to realize resource utilization. In this study, water supply sludge-based ceramsite was prepared, and analyzed the preparation conditions, reaction conditions, and characterization of water supply sludge-based ceramsite. The results showed that water supply sludge-based ceramsite had the best adsorption performance (for NO3−-N and PO43−-P were 31.2 % and 64.5 %) when the mass ratio of water supply sludge to kaolin was 85:15, the firing temperature was 1150 °C, and the firing time was 10 min. The removal efficiencies of chemical oxygen demand (COD) and total nitrogen (TN) were 67.0 % and 79.9 %, respectively, with the water supply sludge-based ceramsite as denitrification biofilter carrier, when the optimal hydraulic retention time (HRT) and carbon‑nitrogen ratio (C/N) were 0.5 h and 2:1. However, the removal efficiency of total phosphorus (TP) gradually decreased. During the biofilm formation of denitrification biofilter, the microbial community richness remained basically stable, but the microbial diversity decreased in the later stage of biofilm formation. The main dominant microbial communities were Acinetobacter and Methylophilaceae; and the relative abundance of amino acid metabolism genes increased, with the extension of biofilm formation time.

Effects of hydraulic retention time and influent nitrate concentration on solid-phase denitrification system using wheat husk as carbon source.

Solid-phase denitrification shows promise for removing nitrate (NO3--N) from water. Biological denitrification uses external carbon sources to remove nitrogen from wastewater, among which agriculture waste is considered the most promising source due to its economic and efficiency advantages. Hydraulic retention time (HRT) and influent nitrate concentration (INC) are the main factors influencing biological denitrification. This study explored the effects of HRT and INC on solid-phase denitrification using wheat husk (WH) as a carbon source. A solid-phase denitrification system with WH carbon source was constructed to explore denitrification performance with differing HRT and INC. The optimal HRT and INC of the wheat husk-denitrification reactor (WH-DR) were 32 h and 50 mg/L, respectively. Under these conditions, NO3--N and total nitrogen removal rates were 97.37 ± 2.68% and 94.08 ± 4.01%, respectively. High-throughput sequencing revealed that the dominant phyla in the WH-DR operation were Proteobacteria, Bacteroidetes, and Campilobacterota. Among the dominant genera, Diaphorobacter (0.85%), Ideonella (0.38%), Thiobacillus (4.22%), and Sulfurifustis (0.60%) have denitrification functions; Spirochaeta (0.47%) is mainly involved in the degradation of WH; and Acidovorax (0.37%) and Azospira (0.86%) can both denitrify and degrade WH. This study determined the optimal HRT and INC for WH-DR and provides a reference for the development and application of WH as a novel, slow-release carbon source in treating aquaculture wastewater.

Continuous lactate-driven dark fermentation of restaurant food waste: Process characterization and new insights on transient feast/famine perturbations

The effect of hydraulic retention time (HRT) on the continuous lactate-driven dark fermentation (LD-DF) of food waste (FW) was investigated. The robustness of the bioprocess against feast/famine perturbations was also explored. The stepwise HRT decrease from 24 to 16 and 12 h in a continuously stirred tank fermenter fed with simulated restaurant FW impacted on hydrogen production rate (HPR). The optimal HRT of 16 h supported a HPR of 4.2 L H2/L-d. Feast/famine perturbations caused by 12-h feeding interruptions led to a remarkable peak in HPR up to 19.2 L H2/L-d, albeit the process became stable at 4.3 L H2/L-d following perturbation. The occurrence of LD-DF throughout the operation was endorsed by metabolites analysis. Particularly, hydrogen production correlated positively with lactate consumption and butyrate production. Overall, the FW LD-DF process was highly sensitive but resilient against transient feast/famine perturbations, supporting high-rate HPRs under optimal HRTs.

Promoting Circular Economy in the Palm Oil Industry through Biogas Codigestion of Palm Oil Mill Effluent and Empty Fruit Bunch Pressed Wastewater

This research aimed to investigate the biogas production and circular economy perspective in the palm oil industry through codigestion of oil palm empty fruit bunch (EFB) pressing wastewater and palm oil mill effluent (POME). The EFB pressing method constitutes an alternative new technology used to extract the remaining oil, increasing palm oil product; however, it produces highly polluted wastewater. Batch experiments were carried out at 35 °C to investigate the optimal ratios of EFB wastewater, inoculums, and POME. The optimal condition was 45% POME + 50% seed + 5% EFB wastewater. This condition was then used in semicontinuous fermentation where the optimal hydraulic retention time (HRT) totaled 25 days. The accumulated biogas was 18,679 mL/L while the accumulated methane totaled 6778 mL/L. The methane content was 62%, and the COD removal efficiency was 67%. The sludge produced from the HRT 25-days digester complied with the organic compost standard which could be further used to nourish the soil. An economic analysis of the EFB pressing project revealed a higher internal rate ratio with shorter payback compared with the conventional process. These results provide information on the circular economic approach to promote sustainable palm oil processing.

Enrichment strategies of heavy metals in the O/H/O process composed of biological fluidized bed for wastewater treatment: A case study of Cu and Zn

Municipal sludge can be utilized as a resource despite its polluting nature. However, the presence of heavy metals lowers its resource value, and their uniform dispersion in conventional biological processes because of sludge reflux poses enormous difficulties for the subsequent separation/recovery. In this study, the enrichment strategies of Cu and Zn in simulated wastewater based on a new biological process named oxic-hydrolytic & denitrification-oxic (O1/H/O2 or O/H/O) combination were investigated and the influence mechanisms of pH, chelate, operating parameters, and denitrification reaction control were illustrated. Results showed that the amount Cu and Zn in the sludge of H reactor exceeded those of both O1 and O2 when pH of O1＜6.5. The maximum sludge-water partition coefficients (Kd) of Cu and Zn in H reactor were 3128 and 3421 times when the influent concentration of NH3–N was at a certain value (210 mg/L NH4Cl for Cu and 140 mg/L NH4Cl for Zn). This was ascribed to the control of nitrification rate, ammonification/nitrosation in the O1 reactor, anaerobic ammonia oxidation combined with autotrophic denitrification in the H reactor, optimal hydraulic retention time (HRT) and sludge retention time (SRT). Under the pre-set conditions of water quality and operation parameters, the precipitates in the sludge were found to be Cu(OH)2, Zn(OH)2, CuS, ZnS which were theoretically and instrumentally characterized. In sum, the joint regulation of biological combination and biochemical reaction will provide new enrichment/separation strategies for heavy metals in municipal sludge management.