Low Chemical Oxygen Demand Removal Research Articles

Impact of feeding regimes on the performance of anaerobic sequencing batch membrane bioreactor treating wastewater from an urban drain

ABSTRACT This work evaluated an anaerobic sequencing batch membrane bioreactor (AnSBR-MBR) equipped with waste-based ceramic membranes for treating low-strength real drain wastewater. Two feeding regimes, viz. settled feed (Phase 1) and mixed feed (Phase 2) were compared. The biological performance (chemical oxygen demand (COD) and phosphate (PO43-) removal) and filtration performance (total suspended solids (TSS) removal and membrane fouling) were investigated. The reactor characteristics in terms of solids content, sludge settling behaviour, and extracellular polymeric substances (EPS) were also assessed. In Phase 1, the settled feed (with the absence of particulate organic matter) resulted in low volatile suspended solids (VSS; 0.23 ± 0.15 g/L), causing low COD removal (28.38 ± 8.14%). In the mixed feed regime in Phase 2, VSS increased (1.82 ± 0.60 g/L) due to the organic content in the particulate matter, which gave microorganisms additional food, resulting in increased COD removal (47.42 ± 2.81%); this phase was also characterised by lower membrane fouling because of the lower EPS content. Marginally higher TSS removal was obtained as well in Phase 2 (96.72 ± 3.26%) than in Phase 1 (90.48 ± 8.14%). This work demonstrates that AnSBR-MBR performance is superior with mixed drain wastewater containing suspended solids and thus pre-settling is not recommended.

Effects of contact/stabilization time ratio on organic carbon-methane conversion in high-rate contact stabilization system

The high-rate contact stabilization (HRCS) process is a promising technique for recovering carbon from wastewater. However, its practical application faces challenges due to its low chemical oxygen demand (COD) removal efficiency, which requires further optimization. The objective of this study was to investigate the optimal process conditions for enhancing COD removal efficiency and carbon recovery rate (CRR) in the HRCS process. To achieve this, a sequencing batch reactor (SBR) was utilized to maximize the feast-famine regime. Although the HRCS process exhibited a lower COD removal efficiency compared to the high-rate activated sludge (HRAS) process in SBR mode, it demonstrated a 16.3 % higher CRR. The influence of the contact time to stabilization time (tc/ts ratio) on both COD removal efficiency and CRR was investigated. The optimum condition was achieved at a tc/ts ratio of 0.18, which resulted in a COD removal efficiency of 60.2 ± 3.0 % and a CRR of 0.417 ± 0.041 g-CODCH4/g-CODinf (CODCH4: produced methane as COD, CODinf: influent COD). The microbial communities in HRAS and HRCS exhibited simplification compared to conventional activated sludge. Distinct shifts were observed in response to changes in solids retention time and tc/ts ratio. Under harsher operating conditions, Lactococcus spp. became the predominant species, achieving the greatest dominance at a tc/ts ratio of 0.18. This is because they produce more extracellular polymeric substances and polyhydroxyalkanoates, which improve organic capture. The operating conditions identified in this study are anticipated to make a significant contribution to practical applications aimed at enhancing carbon recovery.

Development of a Kinetics Based Onsite Treatment System for Faecal Sludge Treatment and Resource Recovery

The management of faecal sludge in cities without a sewerage system is a challenging problem that requires immediate attention. The high organic and nutrient content, pathogen load, and pharmaceutical residues in faecal sludge necessitate special attention for its treatment to ensure environmental and public health safety. Onsite treatment of faecal sludge with a focus on resource recovery is a sustainable approach to its management. However, studies in these areas are limited, particularly in the Indian context. In this study, the aim is to generate primary data on the current status of faecal sludge management in a model region without a sewerage network. Additionally, kinetic parameters were examined to design an anaerobic bioprocess and an engineered system for resource-oriented faecal sludge treatment. One of the wards in Thiruvananthapuram Corporation, Kerala was selected for the study. Fresh faecal sludge collected from an apartment complex was used for the experimental studies. Anaerobic treatment of fresh faecal sludge was validated in bench-scale experimental bioreactors using different combinations: with mixing without inoculum, with mixing with inoculum, without mixing without inoculum, and without mixing with inoculum. The primary focus was on monitoring the organic removal efficiency as Chemical Oxygen Demand (COD) of these combinations. Among the four combinations tested the units "without mixing with inoculum" and "with mixing with inoculum" demonstrated COD removal efficiencies of 20.89% and 18.91%, respectively, with a first-order rate constant of 0.0008 hr-1 for both combinations. The other combinations exhibited comparatively lower COD removal efficiency and rate constants. Since the combinations with mixing and inoculum showed higher COD removal efficiency, either of them can be adopted for design purposes. Based on these results, a combined anaerobic-aerobic treatment unit is proposed for the treatment of faecal sludge.

Propidium monoazide - polymerase chain reaction reveals viable microbial community shifts in anaerobic membrane bioreactors treating domestic sewage at low temperature

An anaerobic membrane bioreactor (AnMBR) treated domestic sewage at 15 °C under different hydraulic retention time (HRT) conditions (6, 12, 16, and 24 h). Propidium monoazide (PMA)-PCR excluded microorganisms without intact cell membranes, focusing on the viable microbial community in anaerobic digestion. The results showed that the 6-hour HRT had poor treatment performance: low chemical oxygen demand removal efficiency (below 80%) and high mean trans-membrane pressure and flux (15 kPa and 9.4 L/(m2 h)). Comparatively, PMA-PCR combined with next-generation sequencing improved the identification of microbial changes compared to conventional 16S rRNA gene sequencing. HRT influenced microorganisms in the hydrolysis and acid-production stages, including carbohydrate-degrading bacteria such as Bifidobacterium and Prevotella 1. Remarkably, a comparison with an AnMBR at 25 °C showed Proteobacteria to be the main cause of membrane fouling in the low-temperature AnMBR, with most operational taxonomic units negatively correlated with HRT and solids retention time.

Free-draining two-stage microbial fuel cell integrated constructed wetlands development using biomass, construction, and industrial wastes as filter materials: Performance assessment

This study reports organic, nutrient, and coliform removal performances in free-draining, three parallel two-stage microbial fuel cell (MFC)-based wetlands that were filled with organic, construction, industrial waste materials and planted with Canna indica. Two hydraulic load rates, i.e., 260 (Phase I) and 520 mm/d (Phase II), were employed. Mean ammoniacal nitrogen (NH4-N), total nitrogen (TN), total phosphorus (TP), coliform removal percentages of the organic, construction, industrial waste materials packed two-stage MFC-based wetland systems ranged between 88 and 96%, 83 and 94%, 59 and 78%, 72 and 100%, respectively, throughout the two operational periods. Chemical oxygen demand (COD) removal percentage in construction, industrial waste materials packed systems ranged between 66 and 79%. The organic waste media packed system achieved low COD removal (33%) during the first operational period that improved substantially (84%) during the last phase. Wastewater organic, nutrient removals were achieved by plant uptake, media-based adsorption, and microbial transformation. Nitrogen, phosphorus accumulation percentages in the plants tissue (with respect to total removal) ranged between 0.3 and 64%, 0.2 and 6%, respectively. Nitrogen, phosphorus concentration in the used media ranged between 0.4 and 13.5 g/kg, 0.7 and 1.1 g/kg, respectively. Maximum voltage and power density production across the construction waste jhama brick packed MFC-based wetland (i.e., 119 mV and 86 mW/m2, respectively) exceeded the bioenergy production of organic, industrial waste materials packed wetlands. Input organic load increment adversely impacted voltage production across all MFC-based wetlands. The carbon composition of the waste media partially supported electrochemical removal mechanisms. This study demonstrates a relatively stable pollutant removal performance of the free-draining construction, industrial waste media packed two-stage MFC-based wetlands despite input pollutant load increment.

Effect of antibiotic mixtures on the characteristics of soluble microbial products and microbial communities in upflow anaerobic sludge blanket

Two upflow anaerobic sludge blanket reactors (UASBs) were used to investigate the effects of three antibiotic mixtures (erythromycin, sulfamethoxazole, and tetracycline) on reactor performance, soluble microbial products (SMPs) composition and microbial community. One reactor (UASBantibiotics) was fed with antibiotic mixtures, whereas another reactor (UASBcontrol) was used as a control without the addition of antibiotic mixtures. Compared with those in UASBcontrol, UASBantibiotics show lower chemical oxygen demand removal efficiency and biogas content. A higher removal efficiency of antibiotic mixtures was obtained in first few stages in UASBantibiotics. The SMPs composition of effluent from the two reactors did not differ significantly, and the main components were protein-like substances, which produced higher fluorescence intensity in UASBantibiotics. Gas chromatography-mass spectrometry analysis revealed that the main compounds identified as SMPs (<580 Da) were alkanes, aromatics and esters, with only 20% similarity of SMPs between UASBantibiotics and UASBcontrol. Antibiotics had a significant effect on the microbial community structure. Notably, in UASBcontrol, hydrogenotrophic methanogens, key microorganisms in anaerobic digestion, had an obvious advantage at all stages compared with UASBantibiotics, whereas acetoclastic methanogen exhibited the opposite pattern. The above results demonstrated that antibiotic mixtures influenced the effluent quality during anaerobic treatment of synthetic wastewater, resulting in changes in the microbial community structure. This study clarified the effect of antibiotic mixtures on the operation of UASBs. It could contribute to identifying potential strategies for improving effluent quality in anaerobic treatment.

Different methods applied to remove pollutants from real epoxy paint wastewater: modeling using the response surface method

ABSTRACT Comparing between classical and advanced methods for the treatment of industrial wastewater is extremely important to optimize factory operations and cost, and reduce the effects on environment. In the present study, some classical and advanced processes were assessed for chemical oxygen demand (COD) removal, the biggest problem in the wastewater from a factory producing epoxy paints. The response surface methodology was used for optimization. While the chemical coagulation (CC) and classical Fenton (CF) processes were used as the traditional methods, the electrocoagulation (EC) and electro-Fenton (EF) processes were used as the advanced methods to treat wastewater. As a result, the highest COD removal efficiencies were obtained in EF (97.8%) and CF (88.6%), respectively. Low COD removal efficiencies (43.5% and 47.5% respectively) were achieved in coagulation-based processes (CC and EC). Operating cost for CF calculated as 2.15 €/m3. Considering in terms of treatment efficiency, sludge volume and operating cost, CF is the most suitable option for the management of epoxy paint wastewater (EPW). These results also show that Fenton-based methods provide higher removal yields for the treatment of EPW. In other words, it is stable wastewater and the presence of hydroxyl radicals is required for its successful management.

Simultaneous nitrification/denitrification and desulfurization of wastewater polluted with ammonium, COD and sulfide: effectiveness of a new up-flow vertical hybrid reactor.

Industrial wastewater discharges pose an environmental risk. Here, the effectiveness of an up-flow vertical hybrid system, operating with synthetic and industrial wastewater was investigated, as a new approach to perform nitrification/denitrification and desulfurization within a single reactor. The hybrid reactor is divided in two reaction zones, the oxic and anoxic. The removal of chemical oxygen demand (COD), ammonium, and sulfide was investigated, highlighting changes in microbial diversity. The reactor was evaluated at hydraulic residence time (HRT) of 1.6days, and its performance throughout 180days is presented in four stages. In stages I-II, high COD and ammonium removal was obtained with synthetic wastewater. In stage-III, sulfide-rich synthetic wastewater did not alter the system, attaining COD, ammonium, and sulfide removal efficiencies of 81, 99.5, and 99.7%, respectively. In the last stage, a mixture of effluents was fed into the reactor at loading rates of 277mg COD/L-d, 46.5mg NH4 +-N /L-d, and 15mg HS--S /L-d. Sulfide and ammonium removals were 100% and 99.9%, respectively. However, low COD removal was observed, being of 51%, and the system removed 97% in terms of BOD5. The structure and microbial diversity also changed. Sulfide feeding, induced the proliferation of sulfur oxidizers like Thiomiscropira and Thiobacillus. Industrial wastewater enhanced the abundance of Pseudomonas (15.53%) and favored the proliferation of new bacteria of the genus Truepera (2.98%) and Alicyclipilus (7.56%). This is the first study reporting simultaneous nitrification/denitrification and desulfurization to remove ammonium, COD and sulfide from complex industrial wastewater using an up-flow vertical hybrid reactor.

Efficiency of pollutants removal in treated palm oil mill effluent (TPOME) using different concentrations of sodium alginate-immobilized Nannochloropsis sp. cells

Palm oil mill effluent (POME) has high chemical oxygen demand (COD), thus requires effective treatments to environmentally benign levels before discharge. In this study, immobilized microalgae cells are used for removing pollutants in treated palm oil mill effluent (TPOME). Different ratios of microalgae beads to TPOME concentration were examined at 1:2.5, 1:5, and 1:10. The biomass concentration and COD removal were measured through a standard method. The color of the cultivated microalgae beads changed from light green to darker green after the POME treatment for 9 days, hence demonstrating that microalgae cells were successfully grown inside the beads with pH up to 9.84. The immobilized cells cultivated in the POME at 1:10 achieved a higher biomass concentration of 1.268 g/L and a COD removal percentage of 72% than other treatment ratios. The increment of the ratio of microalgae cells beads to POME concentration did not cause any improvement in COD removal efficiency. This was due to the inhibitory effect of self-shading resulting in the slow growth rate of microalgae cells which responsible for low COD removal. Therefore, this system could be a viable technology for simultaneous biomass production and POME treatment. This will contribute to research efforts toward the development of new and improved technologies in treating POME.

Novel low-cost activated algal biochar as a cathode catalyst for improving performance of microbial fuel cell

Microalgae derived activated biochar (ABC-900) was synthesized by thermal activation of biochar at 900 °C and the activated biochar was characterised to ascertain the applicability as low-cost electrocatalyst in microbial fuel cell (MFC). Characterisation of ABC-900 using energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed presence of intrinsically doped nitrogen and phosphorus. Cyclic voltammetry exhibited that ABC-900 is an active electrocatalyst, which follows four electron pathway for oxygen reduction as estimated from Koutecky-Levich plot. Three MFCs, using electrocatalyst ABC-900 (MFC-ABC), Pt-C (MFC-Pt) and third MFC with bare carbon black coated cathode (MFC-CB), were operated for a period of 90 days using synthetic wastewater in batch mode (72 h) having chemical oxygen demand (COD) of 3 g L−1 (carbon source: sucrose). Comparable COD removal efficiencies were observed in MFC-ABC (79.5 ± 5.1%) and MFC-Pt (79.0 ± 6.7%), while MFC-CB exhibited lower COD removal (73.5 ± 9.1%). Comparable maximum power density was achieved for MFC-ABC (12.86 ± 0.35 W m−3) and MFC-Pt (13.52 ± 0.05 W m−3) at much lower production cost for the former (0.3% cost compared to Pt-C). Thus, the present work established that microalgae derived activated biochar can act as a low-cost cathode catalyst for scaling-up applications of MFC.

Phycoremediation of treated palm oil mill effluent (TPOME) using Nannochloropsis sp. cells immobilized in the biological sodium alginate beads: Effect of POME concentration

The use of freely suspended cells of microalgae culture to treat wastewater is of current global interest because of their effective photosynthetic uptake of pollutants, carbon dioxide sequestration, and biomass production for desirable high value-products. Biomass immobilization is a promising option to overcome the harvesting problem that is encountered when using free-cells upon completion of the wastewater treatment process. In this study, Nannochloropsis sp. cells were immobilized in sodium alginate beads to eliminate the harvesting limitation. The microalgal beads were further cultivated in treated palm oil mill effluent (TPOME) for removal of chemical oxygen demand (COD). The effect of POME concentration on COD removal and microalgal cells growth was investigated, respectively. It was found that the maximum biomass concentration of 1.23 g/L and COD removal of 55% from 10% POME were achieved after 9 days. An increment of POME concentration did not cause any improvement to the treatment efficiency due to the inhibitory effect of high initial COD of POME on the biomass concentration and was further responsible for low COD removal. The immobilized cells showed a systematic growth, demonstrating that the beads are biocompatible as immobilization carrier. In conclusion, the immobilized microalgal cells could be a viable alternative technology system for POME treatment as well as biomass production.

Bio-electrochemical power generation in petrochemical wastewater fed microbial fuel cell

The petrochemical wastewater (PCW) from acrylic acid plants possesses a very high chemical oxygen demand (COD) due to the presence of acrylic acid along with other organic acids. The treatment of PCW by conventional aerobic and anaerobic methods is energy intensive. Therefore, the treatment of PCW with concurrent power generation by employing microbial fuel cell (MFC) could be a potential alternative to solve the energy and environmental issues. This study demonstrates the potentiality of PCW from acrylic acid plant with an initial COD of 45,000 mg L−1 generating maximum power density of 850 mW m−2 at a current density of 1500 mA m−2 using acclimatized anaerobic sludge (AS) as biocatalyst. The predominant microbes present in acclimatized AS were identified using Biolog GEN III analysis, which include the electrogenic genera namely Pseudomonas spp. and Bacillus spp. along with methanogenic archea Methanobacterium spp. The mechanism of electron transfer was elucidated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) which clearly demonstrated the natural metabolite-based electron transfer across the electrode/biofilm/solution interface. The abundance of the electron shuttle metabolites was increased with the microbial growth in the bulk solution as well as in the biofilm leading to a high power generation. The COD removal efficiency and the coulombic efficiency (CE) were found to be 40% and 21%, respectively after 11 days of operation using initial COD of 45,000 mg L−1. The low COD removal efficiency could drastically be increased to 82% when the initial COD of PCW was 5000 mg L−1 generating a power density of 150 mW m−2. The current work proves the feasibility of the MFC for the treatment of acrylic acid plant PCW using acclimatized anaerobic sludge (AS) as a biocatalyst.

Biodegradation of high saline petrochemical wastewater by novel isolated halotolerant bacterial strains using integrated powder activated carbon/activated sludge bioreactor

The halotolerant bacteria including Pseudomonas stutzeri strain Ahv‐S12, Bacillus vietnamensis strain Ahv‐Q1 and Brevibacterium sediminis strain Ahv‐Z20 were used as inoculums in an integrated powder activated carbon/activated sludge bioreactor. The influence of organic loading rates (OLRs) and total dissolved solids (TDS) concentrations on chemical oxygen demand (COD) removal efficiency was evaluated. Furthermore, biokinetic coefficient was evaluated under TDS variation. Results showed that the COD removal value of 68.32% was observed in OLR of 0.4 kg COD m−3 day−1 and TDS concentrations of 20,000 mg L−1. Higher OLR and TDS concentration led to lower COD removal efficiency. On the other hand, the biodegradation function of isolated salt tolerant strains was much more affected by salinity rather than OLR. Results of biokinetic coefficient determination showed that yield value (Y), kd, K, and μmax were 0.199–0.274 mgVSS mg COD−1, 0.016–0.038 day−1, 0.48–0.922 day−1 and 0.097–0.244 day−1, respectively. © 2018 American Institute of Chemical Engineers Environ Prog, 38:e13088, 2019

Electrochemical Oxidation of Organic Pollutants Powered by a Silicon-Based Solar Cell.

Currently available (photo-)electrochemical technologies for water treatment establish a trade-off between low-pollutant concentration and costs. This paper aims at decoupling these two variables by designing a photo-oxidation device using earth abundant materials and an electronic-free approach. The proposed device combines a graphite/graphite electrochemical system with a silicon-based solar cell that provides the necessary electrical power. First, the optimum operational voltage for the graphite/graphite electrochemical system was found to be around 1.6 V. That corresponded closely to the voltage produced by an a-Si:H/a-Si:H tandem solar cell of approximately 1.35 V. This configuration was shown to provide the best pollutant degradation in relation to the device area, removing 70% of the initial concentration of phenol and 90% of the methylene blue after 4 h of treatment. The chemical oxygen demand (COD) removal of these two contaminants after 4 h of treatment was also promising, 55 and 30%, respectively. Moreover, connecting several solar cells in series led to higher pollutant degradation but lower COD removal, suggesting that the degradation of the intermediate components is a limiting factor. This is expected to be due to the higher currents achieved by the series-connected configuration, which would favor other reactions such as polymerization over the degradation of intermediate species.

Influence of PVP surfactant on the morphology and properties of ZnO micro/nanoflowers for dye mixtures and textile wastewater degradation

Flower-like ZnO micro/nanostructures were successfully prepared using a polyvinylpyrrolidone (PVP)-assisted co-precipitation route and characterized by X-ray diffraction (XRD), field emission-scanning electron microscopy (FESEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), fourier transform infrared (FTIR) and photoluminescence (PL) spectroscopy. Smaller sizes of ZnO micro/nanoflowers were synthesized with different amounts of PVP as surfactant. Possible morphology evolution and formation mechanism of the flower-like micro/nanostructures were postulated. Concentrations of PVP in the reaction mixtures served as key role for morphology-controlled synthesis of ZnO as PVP can assist the oriented crystal growth during the synthesis. The as-synthesized ZnO samples prepared at 12.5 mg/mL PVP displayed higher photoactivity for the simultaneous degradation of methyl orange and methyl green mixtures. The photocatalytic tests also indicated that process parameters such as the initial dye concentration and solution pH exerted their individual influences on the dye mixtures degradation. Under determined experimental conditions, 94.1% chemical oxygen demand (COD) removal of dye mixtures was attained after 240 min of reaction. The ZnO photocatalytic degradation of real textile wastewater was then evaluated and a lower COD removal efficiency (∼75%) was observed at similar treatment duration. Other real textile wastewater quality parameters of biochemical oxygen demand (BOD5), turbidity, total suspended solid (TSS), colour, ammoniacal nitrogen (NH3-N) and pH were also investigated.

Biological leachate treatment using anaerobic/aerobic process: suspended growth-activated sludge versus aerobic granular sludge

Landfill leachate treatment was investigated using two anaerobic/aerobic sequencing batch reactors inoculated with suspended growth-activated sludge (ASBR) and aerobic granular sludge (GSBR). The total ammonium nitrogen (TAN) concentration in the GSBR influent was as high as 1200 mg/L with an average TAN removal efficiency of 99.7%. However, the ASBR treatment did not show a consistent performance in TAN removal. The TAN removal efficiency decreased with increasing ammonium concentration in the influent. Aerobic granular sludge was found to be more resistant to free ammonia (FA). In the GSBR, nitrification was partially inhibited at FA concentration from 48 to 57 mg/L, which was two times more than the FA concentration that inhibited nitrification in the ASBR. Low chemical oxygen demand removal efficiencies were obtained in both reactors, which was associated with the refractory organic content of the leachate used in this study. This resulted in poor phosphorous removal in both treatments. The results prove that aerobic granular sludge is a robust method as compared to suspended-activated sludge to treat leachate containing high levels of TAN and FA.

UV/H2O2/TiO2/Zeolite Hybrid System for Treatment of Molasses Wastewater

Wastewater from molasses processing contains a large amount of coloured substances that give a recalcitrant dark brown colour and high organic load to the effluent. Photocatalytic decolourization of molasses wastewater was performed using titanium dioxide catalyst coated on the surface of South African natural zeolite using the solid-solid dispersion method. Addition of hydrogen peroxide as an oxidant was investigated and 30W UV-Clamp was used as source of irradiation. The Chemical Oxygen Demand (COD) of the wastewater treated was varied from 20 g/L to 1 g/L. Batch experiments were conducted in a thermostatic shaker fitted with the UV lamp. The effects of pH, catalyst loading, oxidant dosage and irradiation time on the COD reduction and decolourization of the Molasses Waste Water (MWW) were investigated in this study. The highest colour removal of more than 90% was achieved at pH = 4 and oxidant dosage of 1.47 mM, while low COD removal (< 20%) was observed during photodegradation. A H 2O2/UV/TiO 2 system achieved higher colour removal of 97% compared to a UV/TiO 2 system which achieved 44% while H 2O2 /UV system achieved 34% colour removal. The rate of decolourization was found to fitpseudo - first order reaction kinetics with the highest rate constant value of 1.36 x 10 -2 min -1 .

Dissolved oxygen and idle time effects on sewage bioremediation

This paper explores the effects of dissolved oxygen (DO) concentrations and idle time on the performance of aerated-sludge portable sewage systems for rapid deployment during natural calamities or warfare. Using synthetic sewage, three different reactors were operated at DO levels of 2–6 mg/l to determine the optimum DO for reactor performance, chemical oxygen demand (COD) and total organic content (TOC) removal, pH, conductivity and biomass concentrations. Low DO values result in very low COD removal efficiency, while DO levels over 6 mg/l are not significantly more efficient. The oxygen utilisation rate and specific oxygen utilisation rate were in the ranges 0·87–1·08 mg O2/h and 0·44–0·895 mg O2/g MLSS (mixed liquor suspended solids) per hour respectively for the three reactors. The highest achieved specific removal rate was 14 mg/g MLSS per hour and 6 mg/g MLSS per hour for COD and TOC respectively. Investigation of idle time (no-aeration) revealed that an idle time of less than 24 h results in low removal efficiency (COD and/or TOC) but the system recovers in the next 24 h; however, an idle period of 48 h or greater results in very poor efficiency and may cause permanent failure of the bioremediation process. The results of the present study can be applied to avoid system failure in a sewage treatment system as a result of, say, intermittent or sudden power shutdown.

Integrated UV photodegradation and anaerobic digestion of textile dye for efficient biogas production using zeolite

• UV/TiO 2 pre-treatment improved biodegradability of high COD (5200 mg/l) methylene blue. • Integrated AOP and anaerobic digestion (AD) achieved high COD, BOD and colour reductions. • UV pre-treatment improved biogas generation by 2.7-fold and enhanced methane yield. • AOP-AD system is more efficient than AD-AOP system treatment technique. The efficiency of an integrated ultra-violet (UV) photocatalysis and anaerobic digestion (AD) system in the degradation of methylene blue (MB) dye was studied in up-flow fixed bed reactors. Zeolite was applied as support material for micro-organism and photocatalyst in the bioreactor and photoreactor, respectively. It was established that the application of either UV photodegradation or AD process as a single treatment method was not efficient in colour removal and chemical oxygen demand (COD) reduction. The AD process as a stand-alone process could only remove 32% of the colour and 57% COD while photodegradation as a stand-alone process had a good performance on colour removal (70%) but low COD removal efficiency (54%). However, integration of the two processes led to high COD, biochemical oxygen demand (BOD) and colour reduction efficiencies of above 75%. Results show that UV pre-treatment of MB before the AD step improved the biodegradability of the MB dye by 3-fold, this resulted in a 2.7-fold increase in biogas production as compared to a non-UV pre-treated MB.

Ozone pretreatment of olive mill wastewaters (OMW) and its effect on OMW biochemical methane potential (BMP)

The possibility of coupling a physicochemical pretreatment (ozonation) with a biological treatment (anaerobic digestion) was investigated for the case of olive mill wastewaters (OMW). Batch ozonation experiments were performed in a glass bubble reactor. The parameters which were tested included the ozone concentration in the inlet gas stream, the reactor temperature and the composition of the liquid medium in terms of raw or fractionated OMW used. In the sequel, ozone-pretreated OMW samples were tested for their biochemical methane potential (BMP) under mesophilic conditions and these results were compared to the BMP of untreated OMW. The ozonation process alone resulted in a 57-76% decrease of total phenols and a 5-18% decrease of total carbohydrates contained in OMW, depending on the experimental conditions. Nevertheless, the ozone-pretreated OMW exhibited lower chemical oxygen demand removal and methane production during BMP testing compared to the untreated OMW.