Landfill leachate biological treatment: perspective for the aerobic granular sludge technology

Landfill leachates are high-strength complex mixtures containing dissolved organic matter, ammonia, heavy metals, and sulfur species, among others. The problem of leachate treatment has subsisted for some time, but an efficient and cost-effective universal solution capable of ensuring environmental resources protection has not been found. Aerobic granular sludge (AGS) has been considered a promising technology for biological wastewater treatment in recent years. Granules' layered structure, with an aerobic outer layer and an anaerobic/anoxic core, enables the presence of diverse microbial populations without the need for support media, allowing simultaneous removal of different pollutants in a single unit. Besides, its strong and compact arrangement provides higher tolerance to toxic pollutants and the ability to withstand large load fluctuations. Furthermore, its good that settling properties allow high biomass retention and better sludge separation. Nevertheless, AGS-related research has focused on carbon-nitrogen-phosphorus removal, mainly from sanitary sewage. This review aims to summarize and analyze the main findings and problems reported in the literature regarding AGS application to landfill leachate treatment and identify the knowledge gaps for future applications.

How does the pre-treatment of landfill leachate impact the performance of O3 and O3/UVC processes?

In this study, O₃ and O₃/UVC processes were evaluated for the treatment of landfill leachate after biological nitrification/denitrification, coagulation, or their combinations. The O₃-driven stage efficiency was assessed by the removal of color, organic matter (dissolved organic carbon (DOC) and chemical oxygen demand (COD)), and biodegradability increase (Zahn-Wellens test). Also, fluorescence excitation-emission matrix (EEM) and size exclusion chromatography coupled with OC detector (SEC-OCD) analysis were carried out for each strategy. The bio-nitrified-leachate (L_N) was not efficiently mineralized during the O₃-driven processes since the high nitrites content consumed ozone rapidly. In turn, carbonate/bicarbonate ions impaired the oxidation of the bio-denitrified-leachate (L_D), scavenging hydroxyl radicals (HO^•) and inhibiting the O₃ decomposition. For both bio-leachates, only O₃/UVC significantly enhanced the effluent biodegradability (>70%), but COD legal compliance was not reached. EEM and SEC-OCD results revealed differences in the organic matter composition between the nitrified-coagulated-leachate (L_NC) and denitrified-coagulated-leachate (L_DC). Nonetheless, the amount of DOC and COD removed per gram of ozone was similar for both. Cost estimation indicates the O₃-driven stage as the costliest among the treatment processes, while coagulation substantially reduced the cost of the following ozonation. Thus, the best treatment train strategy comprised L_DC (with methanol addition for denitrification and coagulated with 300 mg Al³⁺/L, without pH adjustment), followed by O₃/UVC (transferred ozone dose of 2.1 g O₃/L and 12.2 kJ_UVC/L) and final biological oxidation, allowed legal compliance for direct discharge (for organic and nitrogen parameters) with an estimated cost of 8.9 €/m³ (O₃/UVC stage counting for 6.9 €/m³).

Development of a treatment train for the remediation of a hazardous industrial waste landfill leachate: A big challenge

This study focuses on the development of a treatment train for a leachate from a hazardous industrial waste landfill (HIWL) previously treated by: (i) catalytic oxidation with hydrogen peroxide (H₂O₂) for sulphide and sulphite conversion into oxidized sulphur species, including sulphate, and (ii) chemical precipitation of sulphate as barite. The complete treatment line counted on four more stages: (iii) 1^st biological oxidation for removal of biodegradable organic compounds and nitrogen species, (iv) coagulation with ferric chloride (coagulant dose of 100 mg Fe L^-1, pH 2.8) for removal of a fraction of recalcitrant organics and suspended solids, (v) photo-Fenton oxidation using ultraviolet A (UVA) radiation (PF-UVA) (pH 2.8, initial total dissolved iron content of 140 mg L^-1, treatment time of ~4 h) for recalcitrant organics degradation and biodegradability improvement, and (vi) 2^nd biological oxidation for removal of the biodegradable organic matter resulting from the PF-UVA process. The use of anodic oxidation or photoelectro-Fenton processes in stage (v) demonstrated to be unfeasible. A chemical oxygen demand (COD) below 1000 mg O₂ L^-1, a common limit imposed by municipal wastewater treatment plants (MWWTPs) to effluents discharged into the municipal sewer, was achieved after a feasible treatment time (~4 h) using the multistep approach. The remediation of the HIWL leachate proved to be a big challenge.

Ozone-driven processes for mature urban landfill leachate treatment: Organic matter degradation, biodegradability enhancement and treatment costs for different reactors configuration

In this work, the application of ozone-driven processes for the treatment of mature landfill leachate was investigated by testing different system setups. As a first approach, ozonation (O₃-only) was tested, using a porous ceramic diffuser combined with a bubble column (BC), and the best operational conditions were established for leachate treatment (initial pH = 9.0; inlet ozone dose = 18 mg O₃/min). Then, a novel photoreactor (FluHelik) was coupled in series with the bubble column, using a diffuser or a Venturi to inject ozone into the fluid stream. The FluHelik/BC-Venturi setup led to the highest efficiency, treating 50% more leachate than BC-alone using the same ozone dose and reaction time (3 h). Following, the oxidation ability of ozone combined with H₂O₂ and/or UVC for leachate treatment was assessed. The highest synergistic effect was obtained for the O₃/UVC process, with pseudo-first-order rate constant for DOC and COD removal, 2.0 and 1.4 times higher than for the O₃-only, respectively. Ozone-driven processes considerably enhanced the leachate biodegradability from 17% to 79% (O₃/H₂O₂), 81% (O₃-only), 85% (O₃/H₂O₂/UVC) and 91% (O₃/UVC), after a 3 h reaction period. With FluHelik/BC-Venturi system, the O₃/UVC process stands out as the most efficient and cost-effective (6.0 €/m³), ensuring an effluent that meets discharge legal limit for COD (150 mg/L) after further biological oxidation.

Removal of bromate from drinking water using a heterogeneous photocatalytic mili-reactor: impact of the reactor material and water matrix

The main goal of this study was to evaluate the removal of bromate from drinking water using a heterogeneous photocatalytic mili-photoreactor, based on NETmix technology. The NETmix mili-reactor consists of a network of channels and chambers imprinted in a back slab made of acrylic (AS) or stainless steel (SSS) sealed, through mechanical compression and o-rings, with an UVA-transparent front borosilicate glass slab (BGS). A plate of UVA-LEDs was placed above the BGS window. TiO₂-P25 thin films were immobilized on the BGS (back-side illumination, BSI) or SSS (front-side illumination, FSI) by using a spray deposition method. The photoreduction rate of a 200 μg L^-1 (1.56 μM) BrO₃^- solution was assessed taking into account the following: (i) catalyst film thickness, (ii) catalyst coated surface and illumination mechanism (BSI or FSI), (iii) solution pH, (iv) type and dose of sacrificial agent (SA), (v) reactor material, and (vi) water matrix. In acidic conditions (pH 3.0) and in the absence of light/catalyst/SA, 28% and 36% of BrO₃^- was reduced into Br^- only by contacting with AS and SSS during 2-h, respectively. This effect prevailed during BSI experiments, but not for FSI ones since back SSS was coated with the photocatalyst. The results obtained have demonstrated that (i) the molar rate of disappearance of bromates was similar to the molar rate of formation of bromides; (ii) higher BrO₃^- reduction efficiencies were reached in the presence of an SA using the FSI at pH 3.0; (iii) formic acid ([BrO₃^-]:[CH₂O₂] molar ratio of 1:3) presented higher performance than humic acids (HA = 1 mg C L^-1) as SA; (iv) high amounts of HA impaired the BrO₃^- photoreduction reaction; (v) SSS coated catalyst surface revealed to be stable for at least 4 consecutive cycles, keeping its photonic efficiency. Under the best operating conditions (FSI, 18 mL of 2% wt. TiO₂-P25 suspension, pH 3.0), the use of freshwater matrices led to (i) equal or higher reaction rates, when compared with a synthetic water in the absence of SA, and (ii) lower reaction rates, when compared with a synthetic water containing formic acid with a [BrO₃^-]:[CH₂O₂] molar ratio of 1:3. Notwithstanding, heterogeneous TiO₂ photocatalysis, using the NETmix mili-reactor can be used to promote the reduction of BrO₃^- into Br^-, attaining concentrations below 10 μg L^-1 (guideline value) after 2-h reaction. Graphical Abstract .

Treatment train for mature landfill leachates: Optimization studies

In the current study, a treatment train strategy for urban mature leachates, comprising biological and physicochemical processes, was tested for full legal compliance. The leachate presents a high organic and nitrogen content (1.1g C/L; 3.6g O₂/L; 2.0gN/L) and low biodegradability (BOD₅/COD=0.05). In the first stage, a sequential batch reactor (SBR), operated in a 24h-cycle mode (15h aeration +8.5h anoxic, with methanol as external carbon source +0.5h settling), was tested for total nitrogen (TN) removal. The maximum daily TN load that could be treated, reaching the legal limit (< 15mgN/L), increased by 50% with the rise in temperature from 20 to 30°C. For the following coagulation stage, the highest dissolved organic carbon (DOC) removal (64%) and lower final turbidity (33 NTU) were obtained with 240mg Fe³⁺/L, at pH3.0. The jar-tests, comparing nitrified (L_NIT.) and nitrified/denitrified (L_N/D.) leachate, stressed the effect of the leachate alkalinity, generated during the denitrification reaction, on process efficiency. For the coagulated L_N/D., with alkalinity of 1.1g CaCO₃/L, the final concentration of sulfate was only slightly below the legal limit (< 2g/L). A photo-Fenton (PF) oxidation process (pH range of 2.8-3.0, 60mg Fe²⁺/L), as third treatment step, promoted a significant enhancement on leachate biodegradability, consuming 75mM of H₂O₂ and 8.9 kJ/L of accumulated UV energy, to achieve an effluent that can be further biologically treated in compliance with the COD discharge limit (150mg O₂/L) into water bodies. Biological continuous mode tests using a conventional activated sludge process, with an hydraulic retention time (HRT) of 12h, allowed to obtain COD and TSS values (107±3 and 50±2mg/L, respectively) below the legal limit.

Intensifying heterogeneous TiO2 photocatalysis for bromate reduction using the NETmix photoreactor

This work focuses on the intensification of BrO₃^- (200 μg L^-1) reduction by TiO₂-assisted heterogeneous photocatalysis, using the NETmix mili-photoreactor illuminated by UVA light-emitting diodes (UVA-LEDs). The mili-photoreactor was assembled in two configurations: i) catalyst deposition on the channels and chambers of a back stainless steel slab (SSS) and ii) catalyst deposition on the front borosilicate glass slab (BGS), allowing the study of front-side (FSI) and back-side (BSI) illumination mechanisms, respectively. The BrO₃^- reduction rate in aqueous solution was assessed as a function of: i) pH; ii) dissolved oxygen (DO); iii) addition of formic acid (CH₂O₂) as a sacrificial agent (SA); iv) photocatalyst film thickness; v) illumination mechanism; vi) irradiation intensity; vii) temperature; and viii) water matrix. Higher BrO₃^- reduction rates were observed using the FSI mechanism and lower pH values. Nitrogen injection (to eliminate DO) did not significantly improve the reaction rate and the addition of CH₂O₂ had a negative effect at pH 6.5. Neither temperature nor irradiance increase showed a considerable improvement on the reduction rate. Moreover, TiO₂ film remains stable for at least 13 consecutive reactions without significant catalyst leaching. The chemically pre-treated fresh water (FW) matrix negatively affected the reaction rate when compared with the synthetic water (SW), under the best operational conditions (SSS: pH = 5.5, 287 mg of TiO₂, 25 °C, SA absence, [DO] = 232-263 μM). This was associated with the presence of both inorganic and organic matter at much higher concentrations than BrO₃^-. Notwithstanding, heterogeneous TiO₂ photocatalysis, using the NETmix mili-photoreactor, was successfully applied to fresh water, achieving [BrO₃^-] < 10 μg L^-1 (guideline value) after 2-hour reaction.

Sulphur compounds removal from an industrial landfill leachate by catalytic oxidation and chemical precipitation: From a hazardous effluent to a value-added product

This study focused on the removal of sulphur compounds from a high-strength leachate of a hazardous industrial waste landfill. Firstly, sulphides (0.5 g L^-1) and sulphites (2.5 g L^-1) were catalytic oxidised at natural pH (8.7). Air or H₂O₂ were applied as oxidants and metals present in the leachate were used as catalysts. Distinct air flow rates and H₂O₂:sulphur molar ratios were tested. Concentrations of sulphide and sulphite lower than 1.0 mg L^-1 (emission limit value - ELV) were obtained after 5-h oxygenation or 1-min peroxidation under the best conditions, i.e. air flow rate of 1 L_air L_leachate^-1 min^-1 and H₂O₂:sulphur stoichiometric ratio. Aeration was considered unsafe since >33 volatile organic compounds (VOCs) and hydrogen sulphide (H₂S) were released to the atmosphere. Thus, only the H₂O₂-oxidised leachate pursued treatment. Sulphates (13 g L^-1) were removed by chemical precipitation as ettringite or barite applying different reactants contents and pH values. Without pH correction, sulphate contents below 2.0 g L^-1 (ELV) were achieved using a [Ca²⁺]:[Al³⁺]:[SO₄^2-] molar ratio of 12:4:3 (2-fold stoichiometry) and a [Ba²⁺]:[SO₄^2-] molar ratio of 1.0:1.0 (1-fold stoichiometry). The analysis of precipitates by X-ray diffraction (XRD) showed a three-phase ettringite (only 67% corresponding to ettringite itself) and single-phase barite. Barite precipitation proved to be more appealing since a value-added product was obtained and, furthermore, less reactants were required. After sulphur compounds removal using H₂O₂-driven catalytic oxidation and chemical precipitation through barite, the leachate was suitable for biological treatment, despite the high salinity, and a high fraction of the organic load (46%) could be biologically oxidised.

Mineralization of humic acids (HAs) by a solar photo-Fenton reaction mediated by ferrioxalate complexes: commercial HAs vs extracted from leachates

The mineralization of bio-recalcitrant humic acids (HAs) by a solar photo-Fenton (SPF) process was investigated in aqueous system, in order to understand its abatement in real high-HA content matrices, such as sanitary landfill leachates. SPF reactions were performed in tubular photoreactors with CPCs at lab-scale (simulated solar light) and pilot-scale (natural sunlight). Considering the experimental conditions selected for this work, the formation of insoluble HA-Fe³⁺ complexes was observed. Thus, to avoid HA precipitation, oxalic acid (Ox) was added, since Fe³⁺-Ox complexes present a higher stability constant. The effect of different process variables on the performance of SPF reaction mediated by ferrioxalate complexes (SPFF) was assessed with excess of H₂O₂ (50-250 mg L^-1), at lab-scale: (i) pH (2.8-4.0); (ii) initial iron concentration (20-60 mg Fe³⁺ L^-1); (iii) iron-oxalate molar ratio (Fe³⁺-Ox of 1:3 and 1:6); (iv) temperature (20-40 °C); (v) UV irradiance (21-58 W_UV m^-2); and (vi) commercial-HA concentration (50-200 mg C L^-1). At the best lab conditions (40 mg Fe³⁺ L^-1, pH 2.8, 30 °C, 1.6 Fe³⁺-Ox molar ratio, 41 W_UV m^-2), commercial HAs' mineralization profile was also compared with HAs extracted from a sanitary landfill leachate, achieving 88 and 91% of dissolved organic carbon removal, respectively, after 3-h irradiation (8.7 kJ_UV L^-1). Both reactions followed the same trend, although a 2.1-fold increase in the reaction rate was observed for the leachate-HA experiment, due to its lower humification degree. At pilot-scale, under natural sunlight, 95% HA mineralization was obtained, consuming 42 mM of H₂O₂ and 5.9 kJ_UV L^-1 of accumulated UV energy. However, a pre-oxidation during 2.8 kJ_UV L^-1 (12 mM H₂O₂) was enough to obtain a biodegradability index of 89%, showing the strong feasibility to couple the SPFF process to a downstream biological oxidation, with low chemicals and energetic demands. Graphical abstract ᅟ.

An innovative multistage treatment system for sanitary landfill leachate depuration: Studies at pilot-scale

In this work, an innovative methodology for the treatment of landfill leachates, after aerobic lagooning, is proposed and adjusted at pilot-scale. This methodology involves an aerobic activated sludge biological pre-oxidation (ASBO), a coagulation/sedimentation step (240mgFe³⁺/L, at pH4.2) and a photo-oxidation through a photo-Fenton (PF) reaction (60mg Fe²⁺, at pH2.8) combining solar and artificial light. The ASBO process applied to a leachate after aerobic lagooning, with high organic and nitrogen content (1.1-1.5gC/L; 0.8-3.0gN/L) and low biodegradability (BOD₅/COD =0.07-0.13), is capable to oxidise 62-99% of the ammonium nitrogen, consuming only the affluent alkalinity (70-100%). The coagulation/sedimentation stage led to the humic acids precipitation, promoting a marked change in leachate colour, from dark-brown to yellowish-brown (related to fulvic acids), accompanied by a reduction of 60%, 58% and 88% on DOC, COD and TSS, respectively. The PF system promoted the degradation of the recalcitrant organic molecules into more easily biodegradable ones. According to Zahn-Wellens biodegradability test, a leachate with 419mg DOC/L after coagulation, would have to be photo-oxidized until DOC <256mg/L, consuming 117mM of H₂O₂ and 10.4kJ/L of accumulated UV energy, to achieve an effluent that can be biologically treated in compliance with the COD discharge limit (150mg O₂/L) into water bodies. The biological process downstream from the photocatalytic system would promote a mineralization >60%. The PF step cost to treat 100m³/day of leachate was 6.41€/m³, combining 1339m² of CPCs with 31 lamps.

Nitrogen Removal from Landfill Leachate by Microalgae

Landfill leachates result from the degradation of solid residues in sanitary landfills, thus presenting a high variability in terms of composition. Normally, these effluents are characterized by high ammoniacal-nitrogen (N-NH₄⁺) concentrations, high chemical oxygen demands and low phosphorus concentrations. The development of effective treatment strategies becomes difficult, posing a serious problem to the environment. Phycoremediation appears to be a suitable alternative for the treatment of landfill leachates. In this study, the potential of Chlorella vulgaris for biomass production and nutrients (mainly nitrogen and phosphorus) removal from different compositions of a landfill leachate was evaluated. Since microalgae also require phosphorus for their growth, different loads of this nutrient were evaluated, giving the following N:P ratios: 12:1, 23:1 and 35:1. The results have shown that C. vulgaris was able to grow in the different leachate compositions assessed. However, microalgal growth was higher in the cultures presenting the lowest N-NH₄⁺ concentration. In terms of nutrients uptake, an effective removal of N-NH₄⁺ and phosphorus was observed in all the experiments, especially in those supplied with phosphorus. Nevertheless, N-NO₃- removal was considered almost negligible. These promising results constitute important findings in the development of a bioremediation technology for the treatment of landfill leachates.

Insights into solar photo-Fenton reaction parameters in the oxidation of a sanitary landfill leachate at lab-scale

This work evaluates the effect of the main photo-Fenton (PF) reaction variables on the treatment of a sanitary landfill leachate collected at the outlet of a leachate treatment plant, which includes aerated lagooning followed by aerated activated sludge and a final coagulation-flocculation step. The PF experiments were performed in a lab-scale compound parabolic collector (CPC) photoreactor using artificial solar radiation. The photocatalytic reaction rate was determined while varying the total dissolved iron concentration (20-100 mg Fe(2+)/L), solution pH (2.0-3.6), operating temperature (10-50 °C), type of acid used for acidification (H2SO4, HCl and H2SO4 + HCl) and UV irradiance (22-68 W/m(2)). This work also tries to elucidate the role of ferric hydroxides, ferric sulphate and ferric chloride species, by taking advantage of ferric speciation diagrams, in the efficiency of the PF reaction when applied to leachate oxidation. The molar fraction of the most photoactive ferric species, FeOH(2+), was linearly correlated with the PF pseudo-first order kinetic constants obtained at different solution pH and temperature values. Ferric ion speciation diagrams also showed that the presence of high amounts of chloride ions negatively affected the PF reaction, due to the decrease of ferric ions solubility and scavenging of hydroxyl radicals for chlorine radical formation. The increment of the PF reaction rates with temperature was mainly associated with the increase of the molar fraction of FeOH(2+). The optimal parameters for the photo-Fenton reaction were: pH = 2.8 (acidification agent: H2SO4); T = 30 °C; [Fe(2+)] = 60 mg/L and UV irradiance = 44 WUV/m(2), achieving 72% mineralization after 25 kJUV/L of accumulated UV energy and 149 mM of H2O2 consumed.

Performance evaluation of different solar advanced oxidation processes applied to the treatment of a real textile dyeing wastewater

The performance of different solar-driven advanced oxidation processes (AOPs), such as TiO2/UV, TiO2/H2O2/UV, and Fe(2+)/H2O2/UV-visible in the treatment of a real textile effluent using a pilot plant with compound parabolic collectors (CPCs), was investigated. The influence of the main photo-Fenton reaction variables such as iron concentration (20-100 mg Fe(2+) L(-1)), pH (2.4-4.5), temperature (10-50 °C), and irradiance (22-68 WUV m(-2)) was evaluated in a lab-scale prototype using artificial solar radiation. The real textile wastewater presented a beige color, with a maximum absorbance peak at 641 nm, alkaline pH (8.1), moderate organic content (dissolved organic carbon (DOC) = 129 mg C L(-1) and chemical oxygen demand (COD) = 496 mg O2 L(-1)), and high conductivity mainly associated to the high concentration of chloride (1.1 g Cl(-) L(-1)), sulfate (0.4 g SO 4 (2 -) L(- 1)), and sodium (1.2 g Na(+) L(-1)) ions. Although all the processes tested contributed to complete decolorization and effective mineralization, the most efficient process was the solar photo-Fenton with an optimum catalyst concentration of 60 mg Fe(2+) L(-1), leading to 70 % mineralization (DOCfinal = 41 mg C L(-1); CODfinal < 150 mg O2 L(-1)) at pH 3.6, requiring a UV energy dose of 3.5 kJUV L(-1) (t 30 W = 22.4 min; [Formula: see text]; [Formula: see text]) and consuming 18.5 mM of H2O2.

Insights into real cotton-textile dyeing wastewater treatment using solar advanced oxidation processes

Different advanced oxidation processes (AOPs) were applied to the treatment of a real cotton-textile dyeing wastewater as a pre-oxidation step to enhance the biodegradability of the recalcitrant compounds, which can be further oxidized using a biological process. Tests were conducted on a lab-scale prototype using artificial solar radiation and at pilot scale with compound parabolic collectors using natural solar radiation. The cotton-textile dyeing wastewater presents a lilac color, with a maximum absorbance peak at 641 nm, alkaline pH (pH = 8.2), moderate organic content (DOC = 152 mg C L(-1), COD = 684 mg O2 L(-1)) and low-moderate biodegradability (40 % after 28 days in Zahn-Wellens test). All the tested processes contributed to an effective decolorization and mineralization, but the most efficient process was the solar-photo-Fenton with an optimum catalyst concentration of 60 mg Fe(2+) L(-1), leading to 98.5% decolorization and 85.5% mineralization after less than 0.1 and 5.8 kJUV L(-1), respectively. In order to achieve a final wastewater with a COD below 250 mg O2 L(-1) (discharge limit into water bodies imposed by the Portuguese Legislation-Portaria no. 423/97 of 25 June 1997), considering the combination of a solar-photo-Fenton reaction with a biological process, the phototreatment energy required is 0.5 kJUV L(-1), consuming 7.5 mM hydrogen peroxide, resulting in 58.4% of mineralization [Formula: see text].

Multistage treatment system for raw leachate from sanitary landfill combining biological nitrification-denitrification/solar photo-Fenton/biological processes, at a scale close to industrial--biodegradability enhancement and evolution profile of trace pollutants

A multistage treatment system, at a scale close to the industrial, was designed for the treatment of a mature raw landfill leachate, including: a) an activated sludge biological oxidation (ASBO), under aerobic and anoxic conditions; b) a solar photo-Fenton process, enhancing the bio-treated leachate biodegradability, with and without sludge removal after acidification; and c) a final polishing step, with further ASBO. The raw leachate was characterized by a high concentration of humic substances (HS) (1211 mg CHS/L), representing 39% of the dissolved organic carbon (DOC) content, and a high nitrogen content, mainly in the form of ammonium nitrogen (>3.8 g NH4(+)-N/L). In the first biological oxidation step, a 95% removal of total nitrogen and a 39% mineralization in terms of DOC were achieved, remaining only the recalcitrant fraction, mainly attributed to HS (57% of DOC). Under aerobic conditions, the highest nitrification rate obtained was 8.2 mg NH4(+)-N/h/g of volatile suspended solids (VSS), and under anoxic conditions, the maximum denitrification rate obtained was 5.8 mg (NO2(-)-N + NO3(-)-N)/h/g VSS, with a C/N consumption ratio of 2.4 mg CH3OH/mg (NO2(-)-N + NO3(-)-N). The precipitation of humic acids (37% of HS) after acidification of the bio-treated leachate corresponds to a 96% DOC abatement. The amount of UV energy and H2O2 consumption during the photo-Fenton reaction was 30% higher in the experiment without sludge removal and, consequently, the reaction velocity was 30% lower. The phototreatment process led to the depletion of HS >80%, of low-molecular-weight carboxylate anions >70% and other organic micropollutants, thus resulting in a total biodegradability increase of >70%. The second biological oxidation allowed to obtain a final treated leachate in compliance with legal discharge limits regarding water bodies (with the exception of sulfate ions), considering the experiment without sludge. Finally, the high efficiency of the overall treatment process was further reinforced by the total removal percentages attained for the identified organic trace contaminants (>90%).

Biodegradability enhancement of a leachate after biological lagooning using a solar driven photo-Fenton reaction, and further combination with an activated sludge biological process, at pre-industrial scale

This work proposes an integrated leachate treatment strategy, combining a solar photo-Fenton reaction, to enhance the biodegradability of the leachate from an aerated lagoon, with an activated sludge process, under aerobic and anoxic conditions, to achieve COD target values and nitrogen content according to the legislation. The efficiency and performance of the photo-Fenton reaction, concerning a sludge removal step after acidification, defining the optimum phototreatment time to reach a biodegradable wastewater that can be further oxidized in a biological reactor and, activation sludge biological process, defining the nitrification and denitrification reaction rates, alkalinity balance and methanol dose necessary as external carbon source, was evaluated in the integrated system at a scale close to industrial. The pre-industrial plant presents a photocatalytic system with 39.52 m(2) of compound parabolic collectors (CPCs) and 2 m(3) recirculation tank and, an activated sludge biological reactor with 3 m(3) capacity. Leachate biodegradability enhancement by means of a solar driven photo-Fenton process was evaluated using direct biodegradability tests, as Zahn-Wellens method, and indirect measure according to average oxidation state (AOS), low molecular carboxylic acids content (fast biodegradable character) and humic substances (recalcitrant character) concentration. Due to high variability of leachate composition, UV absorbance on-line measurement was established as a useful parameter for photo-Fenton reaction control.