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Huge discharge of different organic and inorganic waste compounds into water sources is the prime reason for water pollution. To protect the environment, appropriate biological treatment methods of wastewater with high removal efficacy are needed. To meet this end, indigenously available microbial consortiums were explored for their possible bioremediation efficiency. Cyanobacteria purified from microbial consortium was identified as Desertifilum sp. based on 16 s rRNA gene sequencing, and its biochemical characteristics were determined. High-rate algal pond (HRAP) of 6 m3 volume with dimensions of 3 m × 2 m × 1 m was inoculated @ 0.25% and operated in an open environment at a light intensity of 38,000 to 62,000 lx with a hydraulic retention time (HRT) of 12 days. Results obtained after 12 days showed removal efficiencies of 78.26, 76, 79.55, 4.77, and 58.74% for soluble chemical oxygen demand (sCOD), total chemical oxygen demand (tCOD), biochemical oxygen demand (BOD), nitrates, and total phosphorus, respectively. The results from the study inferred that Desertifilum sp. is a suitable candidate for secondary-stage wastewater treatment without any additional amendment. Moreover, the biochemical composition of the biomass obtained unraveled its potential application in the field of nutraceuticals.

This study evaluated the efficiency of microalgae activated sludge (MAAS) for wastewater treatment by investigating the influence of hydraulic retention time (HRT) on MAAS using batch regime pilot scale photobioreactors at Wupa Wastewater Treatment Plant. The outcome of the study showed that MAAS has a comparably high wastewater treatment performance in comparison with the current Wupa Wastewater Treatment Plant (WWTPA) activated sludge (AS) method and is capable of treating wastewater to the defined Nigerian effluent standards. It was further revealed that treatment performance for most parameters were optimal from HRT 3 (6-day hydraulic retention time). Precisely, total nitrogen (TN), total phosphorus (TP), and BOD 5 had highest removal efficiency at HRT 3 with average total removal of 81.36%, 91.77% and 87.04% respectively. Correspondingly, the average percentage DO increment peaked at HRT 3 with a value of 269.7%. In addition, there was a general deterioration of SVI with increasing HRT, particularly after HRT 2 (4-day HRT). Notably, SVI 30 was significantly good at HRT 1 and HRT 2 with SVI values of 48.6 ml/g and 105.52 ml/g; however, from HRT 3 down to HRT 9 , the SVI 30 became remarkably increases greater than that of HRT 1 and HRT 2 , with values ranging from 685.61 to 1832.46 ml/g, which indicates a badly bulking sludge. The MAAS system is recommended as an attractive alternative to the conventional AS wastewater treatment in Nigeria and by extension West African subregion.

Even though aerobic methane-oxidation coupled to denitrification (AME-D) has been extensively studied, the exact estimation of CH4 utilization during this process still requires better understanding because effective utilization of CH4 is essential in denitrification performance, CH4 emission and economy. This study presents the effect of hydraulic retention time (HRT) on CH4 utilization in an AME-D bioreactor. Stoichiometries for AME-D were newly established by using the energy balance and the thermodynamic electron equivalent model. The theoretically determined CH4 utilized/NO3− consumed (C/N) ratio from the stoichiometry was 2.0. However, the C/N ratios obtained from the experiment varied with increasing tendency as the HRT increased. Specifically, the C/N ratio increased from 1.38 to 2.85 when the HRT increased from 0.5 to 1.0 days, which placed the theoretical C/N ratio at the HRT between 0.5 and 1.0 days. The higher C/N ratio at the longer HRT was associated with a larger CH4 utilization by methanotrophs than denitrifiers. The results obtained in this study together with those obtained in previous studies clearly illustrated that a variety of conditions affect the utilization of CH4 which is essential for optimizing the AME-D process.

The production of biogas from food waste is a good approach to the minimization of food waste and increase in the production of renewable energy. However, the use of food waste as a feedstock for biogas production currently poses a difficulty due to an ineffective hydrolysis process, which is a pretreatment procedure and the initial step of the biogas conversion process. This restriction results from the food waste polymers’ solubilization and breakdown. This has an impact on the volume of biogas produced during the methanogenesis stage. It is essential to increase the biodegradation of organic compounds (OC) during the hydrolysis process to increase biogas generation. This study focuses on the enhancement of biogas production by the anaerobic digestion (AD) of food waste (FW). FW was hydrolyzed by the immobilized biofilm and digested anaerobically in a semi-continuous digester. Four different digesters including the control were prepared. The control digester composed of no hydrolyzed food waste had no immobilized biofilm while the other three digesters had immobilized biofilm-hydrolyzed food waste with inoculum concentrations of 10%, 30%, and 50%. The results showed that the 50% digester had the highest biogas yield of about 2000 mL/500 mL. The 10%, 30%, and control digesters had a biogas yield of 1523 mL, 753 mL, and 502 mL respectively. Thus, the analysis of total volatile solid (TVS) reduction in the digesters with 10%, 30%, and 50% inoculum and the control have increased to 43.4% for the digesters with 30% and 10%, 60% for the digester with 50% inoculum, and only 29% for the control. Total chemical demand (TCOD) removal increased to 29%, 33%, 43%, and 56% for the control, and 10%, 30%, and 50%, respectively for the inoculum-to-feed ratio. From these results, the 50% inoculum-to-feed ratio has shown the highest biogas production and highest degradation based on TVS reduction and TCOD reduction. Based on this study, the biofilm pretreatment method can be considered a promising method for the enhancement of biogas volume and biodegradation. Biogas production was high (2000 mL) for hydraulic retention time (HRT = 20) days but the HRT = 15 days was also able to produce a significant amount (1400 mL) of biogas and the 50% inoculum-to-feed ratio has shown the highest volume of biogas production.

A two-line denitrifying phosphorus removal process (2L-DPR) was established treating low C/N municipal wastewater efficiently in our previous studies, while hydraulic retention time (HRT) is one of the most important factors determining the substrate loading, contact time for biomass, and pollutants and further affect performance of the whole system. Removal and transformation mechanism of organic carbon (C), nitrogen (N), and phosphorus (P) were investigated together with mass balance under various HRTs (6, 9, and 18 h) in the established 2L-DPR process. The results showed that in anaerobic units, the concentration of the main storage products in activated sludge such as poly-hydroxyvalerate (PHV) and poly-hydroxybutyrate (PHB) at HRT of 9 h was higher than that under other HRTs. The highest TN and TP removal efficiency was also achieved under the HRT of 9 h with removal rates of 55.9% and 84.6% respectively. Increasing HRT from 6 to 9 h greatly enhanced TN removal in anoxic and aerobic units; however, HRTs had little influence on COD removal with effluent concentration of 48.6, 49.1, and 48.9 mg/L, respectively. HRT affected phosphorus up-taken in anoxic and aerobic units rather than on the release of phosphorus processes in anaerobic units.

The textile industry is a significant source of nonyphenol and their ethoxylates, which are suggested to be responsible for endocrine disruption in wildlife and humans. This study is a comparison of two conventional wastewater treatment processes in a cotton and a synthetic fiber factory in Vietnam, with regard to the distribution and removal of nonyphenol ethoxylates and nonyphenol throughout each process. Diverse trends in the distribution of nonyphenol ethoxylates in wastewater from factories, distinguished by their raw materials, could be revealed. Primary coagulation might not perfectly facilitate nitrification in the secondary activated sludge process regarding pH. Nevertheless, satisfactory removals were achieved during coagulation and activated sludge processes in both systems. The roles of long hydraulic retention times (21 and 16 h, respectively), low organic loadings (0.1 and 0.2 gCOD/gMLVSS.day, respectively), extended solids retention times (61 and 66 days, respectively), and mixed liquor suspended solids of greater than 2000 mg/L have been demonstrated. The findings provide evidence and a better understanding of nonyphenol ethoxylate and nonyphenol removal efficacy as well as influencing factors in Vietnamese textile wastewater treatment. The results are beneficial for the textile industry in Vietnam regarding investment decisions for wastewater treatment.

In this study, a vertical flow constructed wetland in batch mode was examined for the removal efficiency of simulated agricultural run-off (SAR) using gravel and soil as substrates, planted with Canna, Typha and Eichhornia plant species in single and mixed culture conditions. From this study, it was found that the constructed wetland (CW) planted with Canna + Eichhornia plant species (mixed) showed maximum removal efficiency of the studied parameters, i.e. phosphates, nitrates and ammonical nitrogen to the tune of 99%, 96% and 98%, respectively of simulated agricultural run-off as compared to other studied CWs. Canna and Typha macrophytes showed higher biomass and chlorophyll content, indicating better tolerance in the mixed culture CW system. Treatment efficiency improved when longer hydraulic retention times (HRTs) were used. Maximum treatment efficiency was shown by hybrid CW, which included the properties of the horizontal flow (HF) and vertical flow (VF) type CWs.

Biohydrogen production by fermentation has become a promising technology developed in the world. In this study, diluted normal molasses wastewater is used as the raw material, and biohydrogen production efficiency and operation characteristics of an anaerobic baffled reactor (ABR) are studied. The effect of hydraulic retention time (HRT) on biohydrogen production efficiency and operating characteristics of ABR is extensively discussed. Experimental results showed that methanogen residuals were still observed in the last three compartments under HRT of 24 h and COD(Chemical oxygen demand) concentration of 8000 mg/L. Meanwhile, the first three compartments presented an ethanol fermentation type. The characteristics of butyric acid fermentation in Compartment IV were also enhanced. The average removal efficiency of COD was reduced to 15.4%. The average rates of biohydrogen production and specific biohydrogen production were 12.85 and 360.22 L/kg COD, respectively. The extension of HRT was beneficial to enrich hydrogen-producing acetogens and could increase the production rate of biohydrogen by a factor of 1.65. However, with the decrease in the bioactivity of acidogenic fermentation bacteria, the biohydrogen production efficiency of ABR was significantly reduced when HRT was longer than 30 h. The specific biogas production rate decreased from 191 to 92 L/(kg MLVSS·d). The specific biohydrogen production rate also decreased from 24.34 to 2.7 L/(kg MLVSS·d).

This study determined how the activity and number of nitrogen-converting microorganisms varied with changes in hydraulic retention time (HRT) and the operating regime of aerobic granular sequencing batch reactors (GSBRs) treating high-nitrogen wastewater. Continuously aerated (O-mode) GSBRs were operated at HRTs of 10-, 13- and 19-h. Then the same reactors were operated at identical HRTs but the cycles started with an anoxic phase (A/O mode). To investigate the microbial communities, DNA- and RNA-based relative real-time PCR was used. In all experimental reactors ammonium was fully removed with a removal rate up to 75 mg N–NH₄⁺/(L·h), and nitrification efficiency was above 90 %. The efficiency of the removal of oxidized nitrogen forms decreased with the lengthening of HRT. The study found that variable oxic conditions (A/O mode) in the GSBR cycle stimulated the simultaneous activity of ammonium oxidizing bacteria (AOB), N₂O-reducers, and Anammox bacteria in aerobic granules. With both modes, the activity of nitrogen-converting bacteria was highest with a 13-h HRT. Shortening HRT, resulted in higher chemical oxygen demand and nitrogen loadings, which favored the growth of Anammox microorganisms in granules and caused a decrease in the number of AOB. With all HRTs, the number of Anammox microorganisms was about 1.5-times higher in A/O mode than in O mode.

The anammox granular sludge was enriched by shortening the hydraulic retention time (HRT) (from 27 h to 6.67 h) in the UASB reactor, which was fed with ammonium chloride and nitrite as the substrates, and the effect of different HRTs on the nitrogen removal performance of anammox granule sludge was studied. After 159 d of operation, the total nitrogen loading rate (NLRtn) reached 1.72 Kg/(m3-d), the total nitrogen removal rate (NRRtn) reached 1.33 Kg/(m3-d), and the removal efficiencies of NH+-N (NREnh+ 4-n) and NO2-N (NREno\"2-n) were over 95%. The ratio of ANO2-N/ANHi-N was 1.31 and ANO\" 3-N/ANH+4-N was 0.24, which complied with the chemical reaction stoichiometry of anammox. The colour of anammox granular sludge changed from light red to deep red; the percentage of granular sludge larger than 1.5 mm was the highest, which proportionately accounted for 62.32%; and the surface of the granular sludge was found, via Fourier transform infrared spectroscopy (FTIR), to contain abundant functional groups. The inhibitory effect of substrates (NH+i-N and NO2-N) on anammox was studied via an inhibition kinetics batch test using anammox granular sludge (Day 159) in the UASB reactor, and the test results were fitted in the Haldane inhibition model with correlation coefficients (R2) of 0.9912 and 0.9949.