Residual COD Research Articles

Evaporation–condensation of olive mill wastewater: Evaluation of condensate treatability through SBR and constructed Wetlands

In this study, a novel strategy for olive mill wastewater (OMW) treatment, which combines an evaporation–condensation step followed by the biological treatment of the condensate, was evaluated. Different solutions were evaluated as biological step, through a experimental tests, by operating for three weeks: two bench scale sequencing batch reactor (SBR), with and without media for attached growth, and two pilot scale constructed wetlands, a vertical (VF) and a horizontal (HF) submerged flow beds. When treating an influent OMW with a COD concentration in the range from 130 to 150gL−1, the evaporation/condensation process allowed to achieve up to 98.7% of COD removal (2.4gCODL−1 in average in the condensate); moreover, the condensate showed a large biodegradable fraction, allowing the further treatment of the residual COD by simple biological reactors like SBRs or constructed wetlands (CWs). The CWs, either in HF or VF configuration, showed more stable and barely higher performances in comparison with the SBR, especially during the first week of operation and allowed to obtain an effluent with a pH close to neutrality without alkalinity dosing. The overall COD removal was about 99.8% and less than two weeks were needed for the start-up of the biological system, which is of pivotal importance given the olive mill process cycle. The evaporation process also generated a concentrate containing a high concentration of valuable chemicals, including polyphenols.

The Law of the Residual COD in Distilled Spirit Wastewater after Two Stages of Biochemical Treatment

Based on the tests of the high concentration organic wastewater from the sampling of Maotai flavor liquor enterprises in Chishui River Basin after two stages of biochemical treatment, this study found out the residue COD concentration range and investigated the rules of its formation. Under anaerobic conditions,the inert COD (SI) were measured.The experimental results showed that: the anaerobic retention time was 48 hours and the COD concentration of the effluent after aerobic biological treatment was about 300~350mg/L, the residual BOD5accounted for 1~2%, the difficult to degrade in raw water COD accounted for 28~33% and SMP accounted for 65~73%. In conclusion, the research findings can provide a scientific justification for the depth treatment process of liquor wastewater to reach the standard.

Evaluation of anaerobic co-digestion of spent brewery grains and an azo dye

Anaerobic batch biodegradation of spent brewery grains (SBG) was investigated in the presence of co-substrates and a monoazo dye (Acid Orange 7 – AO7) under mesophilic and thermophilic regimes. The highest values for the yield coefficient of biogas (STP) on substrate (Ybs) were obtained under mesophilic conditions (0.381–0.516 Lbiogas/g CODremoved and 0.147 to 0.475 Lbiogas/g CODremoved for mesophilic and thermophilic regimes, respectively). A stimulation of the degradation of SBG associated with microbial growth was observed in the presence of co-substrates (glucose and acetate). Supplemented co-substrates also lowered the residual COD leading to an increase in the COD removal efficiency, particularly under thermophilic regime (from 41% to 70%). Although biogas yield (Ybs) indicates a decrease in the presence of the dye, suggesting that it has inhibitory effects, the overall COD removal was not significantly altered. An increase of colour removal was observed when the temperature of the operation was increased (87 ± 2% and 93 ± 1% for mesophilic and thermophilic reactors, respectively), which could be explained by both faster adsorption and biotic reductive cleavage of azo dye bond mechanisms. These results indicate that raw SBG is more prone to biodegradation under an anaerobic mesophilic regime; hence its bio-energetic valorisation is possible.

Reuse of ferric sludge as an iron source for the Fenton-based process in wastewater treatment

Abstract A mature municipal landfill leachate (S1), a wood soaking basin effluent from a plywood manufacturing plant (S2), and a leachate collected from a semicoke landfill area of an oil-shale thermal treatment plant (S3) were subjected to the Fenton/Fenton-based treatment (H 2 O 2 /Fe 2+ , H 2 O 2 /Fe 3+ and H 2 O 2 /sludge systems). The results of ferric coagulation trials indicated a high efficacy in S2; thus, only pre-coagulated samples (cS2) were further treated by the H 2 O 2 /iron systems. Irrespective of reaction duration, S1 was more recalcitrant to oxidation than cS2 and S3. The optimal COD/H 2 O 2 weight ratio for cS2 and S3 was 1:1 with respective 21(35) and 36(45)% residual COD(DOC). In the case of S1, the COD/H 2 O 2 w/w of 1:3.45 proved the most efficient with 30 and 35% residual COD and DOC, respectively. The results also indicated an increase in BOD 7 /COD ratio and a reduction in the acute toxicity to Daphnia magna of the studied wastewater samples after the Fenton/Fenton-based treatment. The application of the Fenton-based process catalysed with iron-containing sludge exhibited efficacies similar to classical Fenton treatment over four reuse cycles. The reuse of ferric sludge without any regeneration as an iron source in the Fenton-based process was displayed as a feasible solution to minimise the production of hazardous ferric waste and reduce the overall cost of the treatment process.

Utilization of micro-electrolysis, up-flow anaerobic sludge bed, anoxic/oxic-activated sludge process, and biological aerated filter in penicillin G wastewater treatment

System which included micro-electrolysis (ME), up-flow anaerobic sludge bed (UASB), anoxic/oxic (A/O) activated sludge process and biological aerated filter (BAF) were investigated in penicillin G-processing wastewater treatment. The main quality of the wastewater was shown as follows: chemical oxygen demand (CODcr) of 1,4507–1,5280 mg L−1, ammonium nitrogen (NH3–N) of 598–826 mg L−1, suspended solid of 1,850–2,190 mg L−1, and pH of 3.5–5.2. Moreover, penicillin G residue in the wastewater was 130–150 mg L−1. ME was utilized as the pretreatment method, UASB and A/O process were designed for the secondary treatment, and BAF was used as the advanced treatment. The results revealed that biodegradability of the wastewater could be effectively improved by ME, and most of CODcr and NH3–N in the wastewater could be removed by UASB and A/O process, and the residual COD and NH3–N could be removed by BAF. The average COD and NH3–N in final effluent was about 275.3 and 19.8 mg L−1, respectively, and the treated wastewater quality reached the requirement of the national discharge standards (wastewater quality standards for discharging into municipal sewer pipelines, China, COD ≤ 300 mg L−1, NH3–N ≤ 25 mg L−1 in C standard). Especially, about 80% of penicillin G residue in the wastewater could be removed or converted by ME reactor, and the wastewater could be effectively deposed in this system.

Recovery of residual soluble protein by two-step precipitation process with concomitant COD reduction from the yeast-cultivated cheese whey

The present study was conducted to recover the residual soluble protein after cultivation of yeast (K. marxianus) in cheese whey. Cheese whey continuous fermentation with cell recycle system was carried out at 40°C and pH 3.5. The yeast biomass was separated from the fermented broth by centrifugation and residual soluble protein from fermented whey supernatant was precipitated by heat treatment (at 100°C, pH 4.5 and 10min incubation). The maximum soluble protein recovery up to 53% was achieved at pH 4.5 with 54% residual COD removal. However, gravity sedimentable precipitates were obtained at pH 3.5 with 47% protein recovery. Therefore, the reactor (scale up) study was conducted at pH 3.5 with agitation, which resulted in 68% of residual soluble protein recovery and simultaneously residual COD removal of 62%. Further precipitation/coagulation of soluble protein was also evaluated using carboxymethylcellulose (CMC) and then two precipitation (thermal followed by CMC precipitation) processes were combined to increase the protein precipitation, which finally reached up to 81% of total soluble protein recovery from the supernatant. This optimized process could be applied to recover the residual protein left after fermentation of cheese whey without centrifugation.

Wet air oxidation pretreatment of biomethanated distillery effluent: Mapping pretreatment efficiency in terms color, toxicity reduction and biogas generation

The effluents from molasses-based distilleries after biomethanation are beset with problems of intensified dark brown color, high residual COD, low biodegradability index (BOD/COD ratio <0.2) and toxicity issues for possible land application as a potential fertilizer. Wet air oxidation (WAO) pretreatment of biomethanated distillery effluent resulted in substantial enhancement in the biodegradability index (BI) (up to 0.8). WAO pretreated effluent on anaerobic digestion indicated favorable biogas generation with methane content up to 64% along with concomitant COD reduction up to 54.75%. The HPLC analysis indicated that the pretreatment facilitated degradation of major color containing compounds-namely melanoidins, up to 97.8%. The pretreated effluent with enhanced biodegradability along with substantially reduced color also indicated positive effect on seed germination (up to 100%), implying toxicity reduction of the effluent post WAO pretreatment.

Coagulation–flocculation treatment of municipal wastewater based on anionized nanocelluloses

The synthetic polymers normally used in the coagulation–flocculation treatment of waste water are in most cases derived from oil-based, non-renewable raw materials. Consequently, there has been a growing interest in replacing them with more sustainable natural bio-based alternatives. However, derivatives of cellulose, which is the most abundant biopolymer on earth, are still scarce. In this work dicarboxylic acid nanocellulose (DCC) flocculants were produced by nanofibrillation of periodate and chlorite-oxidized celluloses with a homogenizer. The flocculation performance levels of five such anionic nanocelluloses with variable charge densities were examined in the coagulation–flocculation treatment of municipal waste water and the results compared with the performance of a commercial coagulant and a synthetic polymeric flocculant. In addition, the aldehyde and carboxyl content, charge density (CD), size and stability of each DCC were determined in an aqueous solution. The results showed that all the DCCs synthesized were able to flocculate waste water very efficiently, and the dosages required were similar to that of the commercial reference flocculant. The combined coagulation–flocculation treatment of municipal wastewater resulted in a lower residual turbidity and COD in a settled suspension, with considerably reduced total chemical consumption relative to coagulation with ferric sulphite alone. A high charge density and high nanofibril content of the DCC flocculant gave the best flocculation performance. The DCCs showed high stability in aqueous suspensions over a long period of time and good performance within the chosen pH range.

Modeling organics biodegradation and ammonia nitrification by entrapped mixed microbial cell carriers

Entrapped mixed microbial cell (EMMC) process offers good capability to remove organics and nitrogen compounds from wastewater in a single aerobic chamber. This research modeled quantitatively the hydraulic characteristics and biochemical process of immobilized activated sludge process (ASP) for the removal of COD and NH4+-N, providing insights to mass and oxygen transfer limitation in EMMC spherical carriers. Based on the conceptual kinetic model and previous experimental results, hydraulic and reaction rate constants were determined for both COD degradation and NH4+-N nitrification with the EMMC carrier. The dissolved oxygen (DO) distribution profile along the radius of EMMC carriers was also simulated. The depletion of DO in the EMMC carrier was very rapid resulting from COD removal and ammonia nitrification given the mass transport condition of DO. The anoxic/anaerobic zone developed in the EMMC carrier within 1 cm from its external surface in contact with the bulk water phase. Beyond this anoxic/anaerobic boundary, denitrification of nitrate occurred utilizing the residual COD. The efficiency of organics biodegradation and nitrification was not influenced by the thickness or diameter of the EMMC carriers. EMMC carriers of 1 cm in thickness supported removal of organics by biodegradation and nitrogen compounds via nitrification and denitrification processes. The EMMC carrier enabled combined nitrification and denitrification in the aerobic chamber, which signified the enhancement of a traditional ASP to an anoxic/oxic (AO) or anaerobic/anoxic/oxic reactor system via the EMMC carrier in an aeration tank.

Degradation of a model azo dye in submerged anaerobic membrane bioreactor (SAMBR) operated with powdered activated carbon (PAC)

This work investigated the anaerobic degradation of the model azo dye Remazol Yellow Gold RNL in an upflow anaerobic sludge blanket reactor (UASB) and two submerged anaerobic membrane (SAMBR) bioreactors, one of which (SAMBR-1) was operated with powdered activated carbon (PAC) in its interior. The reactors were operated at 35 °C with a hydraulic retention time of 24 h in three operational phases, aimed to assess the effect of external sources of carbon (glucose) or redox mediator (yeast extract) on the removal or color and organic matter. The results showed that removal efficiencies of COD (73–94%) and color (90–94%) were higher for SAMBR-1 when compared to SAMBR-2 (operated without PAC) and UASB reactors. In addition, the presence of PAC in SAMBR-1 increased reactor stability, thereby leading to a lower accumulation of volatile fatty acids (VFA). The microfiltration membrane was responsible for an additional removal of ∼50% of soluble residual COD in the form of VFA, thus improving permeate quality. On its turn, PAC exhibited the ability to adsorb byproducts (aromatic amines) of azo dye degradation as well as to act as source of immobilized redox mediator (quinone groups on its surface), thereby enhancing color removal.

Pre-treatment of industrial landfill leachate by Fenton’s oxidation

Biological treatment (aerobic and anaerobic) of industrial landfill leachate is limited by the presence of toxic contaminants (e.g., heavy metals) and recalcitrant (biopersistent) organics (e.g., polyphenols, pharmaceuticals, cosmetics, etc.,), hindering viable conditions for biomass proliferation in biological reactors, with difficulties in meeting concentration limits imposed by applied regulations. Fenton’s oxidation by the use of Fe2+-H2O2-H+ mixture may be used as a pre-treatment of industrial landfill leachate for preliminary abatement of the organic load and to improve biodegradability (BOD/COD&gt;0.4) to favour biological oxidation in conventional wastewater treatment plants. Leachate from Grottaglie (S.E. Italy) non-hazardous landfill (pH 8.6; COD=11.000 mg l-1; BOD5=2.400 mg l-1; NH4-N=2.900 mg l-1; conductivity=60.000 μS cm-1) was laboratory tested in different operative conditions, i.e., initial pH, Fe2+/H2O2 ratio, concentrations and reaction time. The oxidation reaction was monitored by equilibrium pH and residual COD and BOD5 concentrations. Best operative conditions were obtained at pH 3, Fe2+=700 mg l-1, H2O2=9,900 mg l-1 (H2O2/Fe2+ratio~13w/w), reaction time=2h. Following the oxidation reaction, solution pH was neutralized by the addition of Ca(OH)2 or NaOH (120 meq l-1) for further abatement of target parameters by precipitation/sorption. Preliminary technical/economical evaluation of possible process schemes is also given in the paper.

Performance of a lab-scale bio-electrochemical assisted septic tank for the anaerobic treatment of black water

Septic tanks are used for the removal of organic particulates in wastewaters by physical accumulation instead of through the biological production of biogas. Improved biogas production in septic tanks is crucial to increase the potential of this system for both energy generation and organic matter removal. In this study, the effect on the biogas production and biogas quality of coupling a 20 L lab-scale septic tank with a microbial electrolysis cell (MEC) was investigated and compared with a standard septic tank. Both reactors were operated at a volumetric organic loading rate of 0.5gCOD/Ld and a hydraulic retention time between 20 and 40 days using black water as an input under mesophilic conditions for a period of 3 months. The MEC-septic tank was operated at an applied voltage of 2.0±0.1V and the current experienced ranged from 40 mA (0.9A/m(2) projected electrode area) to 180 mA (5A/m(2) projected electrode area). The COD removal was of the order of 85% and the concentration of residual COD was not different between both reactors. Yet, the total phosphorous in the output was on average 39% lower in the MEC-septic tank. Moreover, the biogas production rate in the MEC-septic tank was a factor of 5 higher than in the control reactor and the H2S concentration in the biogas was a factor of 2.5 lower. The extra electricity supplied to the MEC-septic tank was recovered as extra biogas produced. Overall, it appears that the combination of MEC and a septic tank offers perspectives in terms of lower discharge of phosphorus and H2S, nutrient recuperation and a more reliable supply of biogas.

Evaluación de la tuna (Opuntia cochenillifera) para la remoción del color en agua potable

&lt;p&gt;El estudio evaluó la eficiencia del cactus de la tuna para remover el color en agua de río y agua artificial para el consumo humano. Se comparó su desem- peño con el coagulante sulfato de aluminio y un floculante catiónico. El agua fluvial se caracterizó por presentar alto color, baja turbidez, alcalinidad y dureza y alto porcentaje de oxígeno disuelto.&lt;/p&gt; &lt;p&gt;Las hojas de tuna no mostraron actividad como coagulante único al ser evaluadas en agua de río. Se observó una eficiencia de remoción de color del 94% en agua artificial, cuando se aplicó como tratamiento único (45 mg/l), poco superior al tra- tamiento convencional (20 mg/l Al&lt;span&gt;2&lt;/span&gt;(SO&lt;span&gt;4&lt;/span&gt;)&lt;span&gt;3 &lt;/span&gt;y 2 mg/l del floculante catiónico) con una remoción del 89%.&lt;/p&gt; &lt;p&gt;Sin embargo, el agua tratada con tuna dejó un valor de DQO residual del doble (21 mg/l) con relación al tratamiento convencional. En el tratamiento de agua artificial, la tuna como floculante mostró una remoción de color del 92% y una DQO residual de 31,5 mg/l aplicada a una dosis de 22,5 mg/l y 7 mg/l de Al&lt;span&gt;2&lt;/span&gt;(SO&lt;span&gt;4&lt;/span&gt;)&lt;span&gt;3&lt;/span&gt;.&lt;/p&gt; &lt;p&gt;La tuna fue poco efectiva para remover el color generado por sustancias húmicas en aguas de baja turbidez y alcalinidad, pero su aplicabilidad como coagulante/floculante en aguas de alta turbidez y alcalinidad es promisoria.&lt;/p&gt; &lt;p&gt;Se recomienda extraer los principios activos de productos naturales para reducir la carga orgánica del agua tratada y evitar la formación de productos secundarios de desinfección.&lt;/p&gt;

Modeling biodegradation characteristics of grey water in membrane bioreactor

The study evaluated the fate of grey water in a membrane bioreactor (MBR). It mainly involved the evaluation of system performance based on modeling biodegradation characteristics of grey water. The investigation was conducted on grey water using a pilot-scale MBR operated at a sludge age of 60 days. The system secured full removal of biodegradable organics with only 10mg/L residual COD in the effluent, suitable for reuse. Model calibration of cyclic dissolved oxygen profiles fitted the experimental data and defined all related model coefficients with a lower maximum growth rate and lower hydrolysis rates compared with domestic sewage. Model simulation using process kinetics confirmed full removal of available biodegradable COD and generation of soluble microbial products. Significantly, higher soluble COD levels in the bulk liquid compared with the process effluent could be explained by means of cake filtration in the membrane, capturing larger fractions of soluble residual COD. Consequently, entrapment and partial biodegradation of soluble residual organics ensured the very low levels of effluent COD.

Combined industrial and domestic wastewater treatment by periodic allocating water hybrid hydrolysis acidification reactor followed by SBR

A pilot-scale hybrid hydrolysis acidification reactor (HHAR) with periodic water allocation mode operation followed by sequencing batch reactor (SBR) in anoxic and aerobic metabolic function was evaluated for the treatment of low-biodegradable combined industrial and domestic wastewater. The HHAR combines the advantages of both the UASB reactor and AF, omitting the three-phase separator. Furthermore, it has lower average up-flow velocity (0.38–0.92m/h) and higher periodic up-flow velocity (6m/h), which made the reactor keep higher MLSS concentration (more than 10,000mg/L) and sludge-bed is in periodic “expansion-sedimentation-expansion” state. When HRT less than 10h, the B/C variation was positive and reached the maximum value of 0.07 at 8h. SBR with a total cycle period of 4.5h was applied as the post-treatment process to remove residual COD, NH3-N and TN. At steady stage, the pilot-scale SBR effluent COD, NH3-N and TN concentration was 65, 0.75 and 17.71mg/L, corresponding in this case to full-scale SBR plant effluent was 93, 16.4 and 34mg/L. Comparison results indicated that the application of HHAR–SBR system to treat combined industrial and domestic wastewater can improve effluent quality significantly.

Role of electrode materials for the anodic oxidation of a real landfill leachate – Comparison between Ti–Ru–Sn ternary oxide, PbO2 and boron-doped diamond anode

In this paper the electrocatalytic properties of Ti–Ru–Sn ternary oxide (TiRuSnO2), PbO2 and boron-doped diamond (BDD) anodes have been compared for the electrochemical oxidation of a real landfill leachate from an old municipal solid waste landfill (average values of COD 780mgdm−3 and NH4+-N266mgdm-3). The experiments have been performed using an undivided flow cell equipped with a stainless steel cathode, under constant current of 2A and flow-rate of 420dm3h−1. The performance of the electrodes has been compared measuring the time evolution of aromatic compounds, COD, ammonium, colour removal, current efficiency and energy consumption. The experimental results indicated that after 8h of electrolyses TiRuSnO2 anode yields only 35% COD, 52% colour and 65% ammonium removal. Using PbO2 ammonium and colour were completely removed but a residual COD (i.e. 115mgdm−3) was present.On the contrary BDD enables complete COD, colour and ammonium removal due to the electrogeneration of hydroxyl radicals from water discharge and active chlorine from chloride ions oxidation. BDD also exhibits greater current efficiency along with a significantly lower energy cost than other electrodes. These results indicated that the electrochemical oxidation with BDD anode is an effective process for the treatment of landfill leachate.

Response of horizontal sub-surface flow constructed wetlands to sudden organic load changes

The aim of this study is to evaluate the buffering capacity of constructed wetlands by analyzing the response to sudden organic load changes. Nine horizontal flow experimental beds were divided into three equal groups and monitored in a three-phase experiment. Influent COD mass loads during Phase I were 11.4, 5.3 and 0g/m2/day for Groups A–C, respectively. During Phase II there was a rise in COD mass load and Phase III restored initial conditions. Group A showed a reduction in COD removal efficiencies with mass load increase despite more intense microbiological activity. In Group B planted beds there was a reduction of the mass load removal, which could have been due to a biofilm disturbance caused by an invasive millipede species. When mass load was lowered, removal efficiencies improved. Group C showed residual effluent COD during Phase I but provided adequate removal efficiencies (75–84%) when supplied with 1.7g/m2/day during Phase II. The results of this study suggest that the buffering capacity of constructed wetlands may not be enough to maintain COD removal efficiencies during a sudden organic load increase.

Removal of readily biodegradable substrate in super fast membrane bioreactor

The study investigated performance characteristics of super fast membrane bioreactor (MBR) in removing readily biodegradable substrate. MBR experiments were adjusted to extremely high rate operation at steady-state, with all combinations of sludge retention times between 0.5–2.0d and hydraulic retention times between 0.5–2.0h. Substrate feeding was adjusted to 200mg/L and involved a synthetic mixture representing the readily biodegradable COD fraction in domestic sewage. Effluent COD always remained below 25mg/L with an average value of 15mg/L under all tested operating conditions. Respirometric analyses indicated complete biodegradation of available substrate so that the effluent COD was essentially composed of soluble microbial products. High rate MBR operation also limited substrate storage below 10mg/L and generation of proteins and carbohydrates down to 1.5–4.5mg COD/L. Significant differences in the residual soluble COD levels between permeate and reactor volume showed that effective COD removal relied on biodegradation as well as on membrane filtration, which entrapped soluble microbial products with relatively larger particle sizes. Parallel 16s rRNA experiments based on PCR-DGGE methodology revealed significant changes in the composition of the microbial community as a function of the sludge retention time, an observation with possible implications on different biodegradation kinetics depending on the characteristics of the microbial community.

Mitigating ammonia inhibition of thermophilic anaerobic treatment of digested piggery wastewater: Use of pH reduction, zeolite, biomass and humic acid

High free ammonia released during anaerobic digestion of livestock wastes is widely known to inhibit methanogenic microorganisms and result in low methane production. This was encountered during our earlier thermophilic semi-continuously fed continuously-stirred tank reactor (CSTR) treatment of piggery wastewater. This study explored chemical and biological means to mitigate ammonia inhibition on thermophilic anaerobic treatment of piggery wastewater with the aim to increase organic volatile carbon reduction and methane production. A series of thermophilic anaerobic batch experiments were conducted on the digested piggery effluent to investigate the effects of pH reduction (pH 8.3 to 7.5, 7.0 and 6.5) and additions of biomass (10% v/v and 19% v/v anaerobic digested piggery biomass and aerobic-anaerobic digested municipal biomass), natural zeolite (10, 15 and 20 g/L) and humic acid (1, 5 and 10 g/L) on methane production at 55 °C for 9–11 days. Reduction of the wastewater pH from its initial pH of 8.3 to 6.5 produced the greatest stimulation of methane production (3.4 fold) coupled with reductions in free ammonia (38 fold) and total volatile fatty acids (58% TVFA), particularly acetate and propionate. Addition of 10–20 g/L zeolite to piggery wastewater with and without pH reduction to 6.5 further enhanced total VFA reduction and methane production over their respective controls, with 20 g/L zeolite producing the highest enhancement effect despite the ammonia-nitrogen concentrations of the treated wastewaters remaining high. Without pH reduction, zeolite concentration up to 20 g/L was required to achieve comparable methane enhancement as the pH-reduced wastewater at pH 6.5. Although biomass (10% v/v piggery and municipal wastes) and low humic acid (1 and 5 g/L) additions enhanced total VFA reduction and methane production, they elevated the residual effluent total COD concentrations over the control wastewaters (pH-unadjusted and pH-reduced) unlike zeolite treatment. The outcomes from these batch experiments support the use of pH reduction to 6.5 and zeolite treatment (10–20 g/L) as effective strategies to mitigate ammonia inhibition of the thermophilic anaerobic treatment of piggery wastewater.

Degradation of reactive dyes in a photocatalytic circulating‐bed biofilm reactor

Decolorization and mineralization of reactive dyes by intimately coupled TiO₂-photocatalysis and biodegradation (ICPB) on a novel TiO₂-coated biofilm carrier were investigated in a photocatalytic circulating-bed biofilm reactor (PCBBR). Two typical reactive dyes--Reactive Black 5 (RB5) and Reactive Yellow 86 (RY86)--showed similar first-order kinetics when being photocatalytically decolorized at low pH (~4-5) in batch experiments. Photocatalytic decolorization was inhibited at neutral pH in the presence of phosphate or carbonate buffer, presumably due to electrostatic repulsion from negatively charged surface sites on TiO₂, radical scavenging by phosphate or carbonate, or both. Therefore, continuous PCBBR experiments were carried out at a low pH (~4.5) to maintain high photocatalytic efficiency. In the PCBBR, photocatalysis alone with TiO₂-coated carriers could remove target compound RB5 and COD by 97% and 47%, respectively. Addition of biofilm inside macroporous carriers maintained a similar RB5 removal efficiency, but COD removal increased to 65%, which is evidence of ICPB despite the low pH. ICPB was further proven by finding microorganisms inside carriers at the end of the PCBBR experiments. A proposed ICPB pathway for RB5 suggests that a major intermediate, a naphthol derivative, was responsible for most of the residual COD, while most of the nitrogen in the azo-bonds (-N=N-) was oxidized to N₂.