Nitrogen removal from recycled landfill leachate by ex situ nitrification and in situ denitrification

Denitrification of low C/N landfill leachate in lab-scale landfill simulation bioreactors

Old landfill leachate can be characterized by high ammonia nitrogen concentrations and limited biodegradable carbon availability. A promising and cost-effective option for ammonia nitrogen removal involves ex situ nitrification and in situ denitrification. This study aimed to investigate the denitrification capacity of old MSW in six landfill bioreactors with very low COD/NO₃^--N mass ratios that ranged between 0.12 and 3.99 g/g. In particular, this study is novel in that it tested COD/NO₃^--N mass ratios lower than previous studies. The experiment lasted 83 days. The results showed that denitrification occurred in all bioreactors and even at considerably low concentrations of biodegradable organic matter (BOD₅ ≤ 9 mg O₂/L). In all but one case, when nitrate removal stopped at 55% due to the absence of leachate recirculation, nitrate removal was higher than 95%. The average nitrate removal rates (ANRRs), calculated under significantly different conditions, ranged from 33 to 135 mg NO₃^--N/L/d. The initial COD concentration and COD/NO₃^--N ratio did not appear to affect the ANRRs, which were influenced by the initial nitrate concentration and leachate recirculation. The maximum ANRR (135 mg NO₃^--N/L/d) was measured with the highest initial nitrate concentration (4491 mg NO₃^--N/L) and the lowest COD/NO₃^--N mass ratio (0.12 g COD/g NO₃^--N). The lowest ANRR (33 mg NO₃^--N/L/d) was calculated for a bioreactor with no leachate recirculation. Sulphate production observed in some bioreactors may suggest that, together with the heterotrophic pathway, autotrophic denitrification contributed to the removal of nitrate, especially in bioreactors with low COD/NO₃^--N mass ratio.

Stabilization of solid digestate and nitrogen removal from mature leachate in landfill simulation bioreactors packed with aged refuse

Digestate from biogas plants managing municipal solid waste needs to be stabilized prior to final utilization or disposal. Based on the concept of urban mining, aged refuse from a closed landfill was used to treat landfill leachate, but nitrogen removal by biological denitrification was limited. The aim of this study was to use a digestate layer in bioreactors containing aged refuse to enhance the biological denitrification capacity of the aged refuse, stabilize digestate, and mitigate the ammonia emissions from digestate leaching with leachate recirculation. Six identical landfill columns filled with 0% (R0), 5% (R5), and 15% (R15) of solid digestate above aged refuse (ratios based on Total Solids) were setup and nitrified leachate was periodically fed and recirculated to the columns. The nitrate removal rate in R5 and R15 was 3.4 and 10 times higher relative to the control (no digestate added). A 31.5-35.9% increase of solid digestate biostability was confirmed by tests performed under both aerobic and anaerobic conditions. The results showed that instead of land use, the solid fraction of digestate could be utilized as an inexpensive functional layer embedded in an old landfill site to enhance the denitrification capacity and achieve digestate stabilization with minimal ammonia leaching from digestate.

Highly efficient of nitrogen removal from mature landfill leachate using a combined DN-PN-Anammox process with a dual recycling system

An efficient and stable combined denitrification-partial nitrification-Anammox process with a dual recycling system was used to remove nitrogen from mature landfill leachate. After 155 d of operation, the NO₃^- as the PN-Anammox byproduct was almost treated with biodegradable organic carbon in raw wastewater in a pre-denitrification reactor by external recycling system. When raw landfill leachate with NH₄⁺-N concentration of 1900 mg/L was treated, an integrated reactor with airlift recycling was combined with the PN and Anammox reactions to efficiently remove NH₄⁺ from the inflow. The total nitrogen concentration of effluent stabilized at 20 mg/L and total nitrogen removal efficiency was 99%. The maximum NO₂^- production rate in the aerobic zone was 2.2 kg/(m³·d) and the maximum nitrogen removal rate in the anaerobic zone was 21.4 kg/(m³·d). The most common phyla among the nitrification and the Anammox functional bacteria were Nitrosomonas, Candidatus Kuenenia, and Candidatus Brocadia after landfill leachate treatment.

Two-stage Anaerobic Membrane Bioreactor (AnMBR) system to reduce UV absorbance in landfill leachates

Landfill leachate typically contains UV-quenching organics, which hinder disinfection at POTWs. This study tested a 2-stage submerged AnMBR for the degradation of UV-absorbing compounds in landfill leachate. Leachate was treated in a thermophilic reactor (55 ± 2 °C) followed by a mesophilic AnMBR (37 ± 1 °C), with HRTs of 25 ± 5 days and 40 ± 5 days respectively. Solids were not wasted, in order to promote biomass accumulation. COD, Organic carbon, and UV₂₅₄ absorbance were monitored over 13 months of operation. Known UV-quenching compounds, including humic acids, fulvic acids and hydrophilic matter, were reduced by 55%. Molecular weight distribution analyses revealed that the thermophilic reactor hydrolyzed organic carbon >100 KDa into smaller fractions, which were removed in the AnMBR. The system consistently removed 50% of the total UV absorbance. This promising, new enhanced biological process may provide landfills with a feasible pretreatment alternative to expensive chemical oxidation or RO processes before discharging leachate into sewers.

Use of bioreactor landfill for nitrogen removal to enhance methane production through ex situ simultaneous nitrification-denitrification and in situ denitrification

High concentrations of nitrate-nitrogen (NO₃^--N) derived from ex situ nitrification phase can inhibit methane production during ex situ nitrification and in situ denitrification bioreactor landfill. A combined process comprised of ex situ simultaneous nitrification-denitrification (SND) in an aged refuse bioreactor (ARB) and in situ denitrification in a fresh refuse bioreactor (FRB) was conducted to reduce the negative effect of high concentrationsof NO₃^--N. Ex situ SND can be achieved because NO₃^--N concentration can be reduced and the removal rate of ammonium-nitrogen (NH₄⁺-N) remains largely unchanged when the ventilation rate of ARB-A2 is controlled. The average NO₃^--N concentrations of effluent were 470mg/L in ex situ nitrification ARB-A1 and 186mg/L in ex situ SND ARB-A2. The average NH₄⁺-N removal rates of ARB-A1 and ARB-A2 were 98% and 94%, respectively. Based on the experimental data from week 4 to week 30, it is predicted that NH₄⁺-N concentration in FRB-F1 of the ex situ nitrification and in situ denitrification process would reach 25mg/L after 63weeks, and about 40weeks for the FRB-F2 of ex situ SND and in situ denitrification process . Ex situ SND and in situ denitrification process can improve themethane production of FRB-F2. The lag phase time of methane production for the FRB-F2 was 11weeks. This phase was significantly shorter than the 15-week phases of FRB-F1 in ex situ nitrification and in situ denitrification process. A seven-week stabilizationphase was required to increase methane content from 5% to 50% for FRB-F2. Methane content in FRB-F1 did not reach 50% but reached the 45% peak after 20weeks.

Migration of nitrate, nitrite, and ammonia through the municipal solid waste incinerator bottom ash layer in the simulated landfill

Simulated landfill was operated for 508 days to investigate the effect of municipal solid waste incinerator (MSWI) bottom ash layer on the migration of nitrate, nitrite, and ammonia when it was used as the intermediate layer in the landfill. The result suggested that the MSWI bottom ash layer could capture the nitrate, nitrite, and ammonia from the leachate. The adsorption of the nitrate, nitrite, and ammonia on the MSWI bottom ash layer was saturated at the days 396, 34, and 97, respectively. Afterwards, the nitrogen species were desorbed from the MSWI bottom ash layer. Finally, the adsorption and desorption could reach the equilibrium. The amounts of adsorbed nitrate and nitrite on the MSWI bottom ash layer were 1685.09 and 7.48 mg, respectively, and the amount of the adsorbed and transformed ammonia was 13,773.19 mg, which was much higher than the desorbed. The water leaching test and synthetic precipitation leaching procedure (SPLP) results showed that the leachable nitrate, nitrite, and ammonia in the MSWI bottom ash were greatly increased after the landfill operation, suggesting that the adsorbed nitrogen could be finally leached out. Besides, the results also showed that MSWI bottom ash layer could affect the release of nitrate and ammonia at the initial stage of the landfill. However, it had little effect on the release of nitrite.

Retention and leaching of nitrite by municipal solid waste incinerator bottom ash under the landfill circumstance

The retention and leaching of nitrite by municipal solid waste incinerator (MSWI) bottom ash could affect its migration in the landfill. In this study, the effect of the dosage of MSWI bottom ash as well as the variation of the landfill environmental parameters including pH, anions and organic matter on the nitrite retention and leaching behavior was investigated by batch experiments. The highest removal percentage (73.0%) of nitrite was observed when the dosage of MSWI bottom ash was 10 g L(-1) in 2 mg L(-1) nitrite solution. Further increase of the dosage would retard the retention, as the nitrite leaching from MSWI bottom ash was enhanced. The optimum retention of nitrite was observed when the pH was 5.0, while the leaching of nitrite showed a consistent reduction with the increase of pH. Besides, the presence of Cl(-), SO4(2)(-) and acetic acid could enhance the leaching of nitrite and mitigate the retention process. However, the retention of nitrite was enhanced by PO4(3)(-), which was probably due to the formation of the apatite, an active material for the adsorption of the nitrite. These results suggested that MSWI bottom ash could affect the migration of nitrite in the landfill, which was related to the variation of the landfill circumstance.

N₂O emission from a combined ex-situ nitrification and in-situ denitrification bioreactor landfill

A combined process comprised of ex-situ nitrification in an aged refuse bioreactor (designated as A bioreactor) and in-situ denitrification in a fresh refuse bioreactor (designated as F bioreactor) was constructed for investigating N2O emission during the stabilization of municipal solid waste (MSW). The results showed that N2O concentration in the F bioreactor varied from undetectable to about 130 ppm, while it was much higher in the A bioreactor with the concentration varying from undetectable to about 900 ppm. The greatly differences of continuous monitoring of N2O emission after leachate cross recirculation in each period were primarily attributed to the stabilization degree of MSW. Moreover, the variation of N2O concentration was closely related to the leachate quality in both bioreactors and it was mainly affected by the COD and COD/TN ratio of leachate from the F bioreactor, as well as the DO, ORP, and NO3(-)-N of leachate from the A bioreactor.

Characterization and treatment of organic constituents in landfill leachates that influence the UV disinfection in the publicly owned treatment works (POTWs)

Landfill leachates strongly quench UV light. When discharged to POTWs, leachates can interfere with UV disinfection. To investigate the UV quenching problem of landfill leachates, a variety of landfill leachates with a range of conditions were collected and characterized. The UV blocking component was found to be resistant to biological degradation so they pass through wastewater treatment plants and impact the subsequent UV disinfection system. Leachate samples were fractionated into humic acids (HAs), fulvic Acids (FAs) and hydrophilic (Hpi) fractions to investigate the source of UV absorbing materials. Results show that for all leachates examined, the specific UV254 absorbance (SUVA254) of the three fractions follows: HA>FA>Hpi. However, the overall UV254 absorbance of the Hpi fraction was important because there was more hydrophilic organic matter than humic or fulvic acids. The size distribution was also investigated to provide information about the potential for membrane treatment. It was found that the size distribution of the three fractions follows: HA>FA>Hpi. This indicates that membrane separation following biological treatment is a promising technology for removal of humic substances from landfill leachates. Leachate samples treated in this manner could meet the UV transmittance requirement of the POTWs.

Bioreactor tests preliminary to landfill in situ aeration: a case study

Lab scale tests in bioreactor were carried out in the framework of the characterization studies of a landfill where in situ aeration (possibly followed by landfill mining) had been proposed as part of the novel waste management strategy in a region in northern Italy. The tests were run to monitor the effects produced by aerobic conditions at different temperatures on waste sampled at different depths in the landfill, with focus on the carbon and nitrogen conversion during aeration. Temperatures ranging from 35 to 45°C were chosen, in order to evaluate possible inhibition of biodegradation processes (namely nitrification) at 45°C in the landfill. The results obtained showed positive effects of the aeration on leachate quality and a significant reduction of waste biodegradability. Although a delay of biodegradation processes was observed in the reactor run at 45°C, biodegradation rates increased after 2 months of aeration, providing very low values of the relevant parameters (as in the other aerated reactors) by the end of the study. Mass balances were carried out for TOC and NNH4(+); the findings obtained were encouraging and provided evidence of the effectiveness of carbon and nitrogen conversion processes in the aerated landfill simulation reactors.

Evaluation of on-site biological treatment for landfill leachates and its impact: A size distribution study

A cost effective and widely applied approach for landfill leachate disposal is to discharge it to a municipal wastewater treatment plant (WWTP). The recalcitrant nature of leachate organics and the impact on the downstream WWTPs were comprehensively investigated in this study. Size fractionation by ultrafiltration (UF) and microfiltration (MF) was employed in conjunction with various analyses (TOC, COD, nitrogen species and UV(254) absorbance) on raw and biologically treated landfill leachates to provide insight into biological treatability. Overall, landfill leachate organics showed bio-refractory properties. Less than half of the organic matter, measured as total organic carbon (TOC), could be removed in the biological processes examined. Size distribution data showed that the <1 thousand Daltons (kDa) fraction is dominant in most untreated and treated landfill leachates, indicating difficulties for membrane treatment. Also, most removal occurred for the <1 kDa fraction in the biological processes, while the intermediate size fractions increased slightly. This may be caused by bio-flocculation and/or partial degradation of larger molecular weight fractions. Organic nitrogen was investigated in this study as one of the first explorations for landfill leachates. Organic nitrogen in landfill leachates was more bio-refractory than other organic matter. UV quenching by landfill leachates was also investigated since it interferes with the UV disinfection at WWTPs. The combination of activated carbon and activated sludge (PACT) showed some effectiveness for reducing UV quenching, indicating that carbon adsorption is a potential method for removal of UV quenching substances. Fourier transform Infrared (FT/IR) data showed that aromatic groups are responsible for the UV quenching phenomenon.

A novel technique of semi-aerobic aged refuse biofilter for leachate treatment

We developed a semi-aerobic aged refuse biofilter (SAARB) for leachate treatment and examined its advantages and disadvantages compared to previous aged refuse biofilters (ARBs). To assess its treatment capability, decontamination mechanisms and optimal performance parameters, a single-period experiment and L(9)(3(4)) orthogonal array design experiments were conducted on artificial leachate. The SAARB markedly enhanced the treatment capability and removal efficiency of organic matter and nitrogen pollutants due to the alternating aerobic-anoxic-anaerobic zones in situ. The reduction in chemical oxygen demand (COD), ammonia nitrogen (NH(4)(+)-N) and total nitrogen (TN) exceeded 98%, 94%, and 80%, respectively. After the leachate was distributed onto the SAARB surface, the effluent velocity decreased as a logarithmic function, and there was a concomitant reduction in leachate effluent volume. Based on the capacity for removal of COD, NH(4)(+)-N, and TN, the effective height of aged refuse in a SAARB was enough to be 900mm. An excellent treatment efficiency could be achieved at 20-35°C, with a leachate distribution time of 1h once every period of 2-3 days, hydraulic loading of 11-30L/(m(3)day), and COD loading of 550-1200g/(m(3)day). This new SAARB system demonstrates superior efficacy for biofilter compared to other ARB systems, especially for nitrogen removal from leachate.

Landfill leachate pollutant removal performance of a novel biofilter packed with mixture medium

Landfill leachate pollutants were treated in a biofilter filled with a mixture of aged refuse and slag, and the performance was compared with those reactors filled solely with either medium. Cultural counting method showed that bioreactor filled with slag had the highest amount of nitrifying bacteria, while polymerase chain reaction-denaturing gradient gel electrophoresis method showed that reactor filled with both media had the highest bacterial community diversity. Particle size distributions measurement showed that slag contained less fine particles than aged refuse, which provided better permeability. The reactor containing both media exhibited a high efficiency in removal of pollutants, and a higher resistance to shock loading and low temperature compared with single-medium reactors. It also overcame both the poor permeability of aged refuse filling and the low bacteria diversity of slag. The study shows that a mixture of aged refuse and slag as a new biofilter medium for leachate pollutant removal is technically viable.

Denitrification capacity of bioreactors filled with refuse at different landfill ages

The denitrification capacity of refuse at different landfill ages in bioreactor landfill system was studied. Three reactors filled with 1-year-old refuse (R1), 6-year-old refuse (R6) and 11-year-old refuse (R11), respectively, were operated in the experiment. Nitrate solution (1000 mg NO3(-)-NL(-1)) was added into each reactor. The results showed that the reactors were all able to consume nitrate. However, 1-year-old refuse in R1 had both a higher nitrate reduction rate and concentration of N2. In addition, vertical differences in nitrate removal along the depth of R1 were observed. The bottom-layer refuse and the middle-layer refuse both showed higher efficiency in nitrate depletion than the top layer. Furthermore, N2O accumulation was found in R11 with the concentration up to 19.3% of the released gas. These results suggested that 1-year-old refuse, which was partly degraded, was more suitable to use as denitrification medium.

GISCOD: general integrated solid waste co-digestion model

This paper views waste as a resource and anaerobic digestion (AD) as an established biological process for waste treatment, methane production and energy generation. A powerful simulation tool was developed for the optimization and the assessment of co-digestion of any combination of solid waste streams. Optimization was aimed to determine the optimal ratio between different waste streams and hydraulic retention time by changing the digester feed rates to maximize the biogas production rate. Different model nodes based on the ADM1 were integrated and implemented on the Matlab-Simulink simulation platform. Transformer model nodes were developed to generate detailed input for ADM1, estimating the particulate waste fractions of carbohydrates, proteins, lipids and inerts. Hydrolysis nodes were modeled separately for each waste stream. The fluxes from the hydrolysis nodes were combined and generated a detailed input vector to the ADM1. The integrated model was applied to a co-digestion case study of diluted dairy manure and kitchen wastes. The integrated model demonstrated reliable results in terms of calibration and optimization of this case study. The hydrolysis kinetics were calibrated for each waste fraction, and led to accurate simulation results of the process and prediction of the biogas production. The optimization simulated 200,000 days of virtual experimental time in 8 h and determined the feedstock ratio and retention time to set the digester operation for maximum biogas production rate.

Nitrogen transformation in the hybrid bioreactor landfill

The hybrid bioreactor landfill was promising in solid waste management. In the work, the nitrogen removal and nitrogen transformation in hybrid bioreactor landfill with sequencing of facultative anaerobic and aerobic conditions was explored. The result showed that the combination of facultative anaerobic and aerobic conditions in the hybrid bioreactor landfill was indeed effective in eliminating ammonia both from the leachate and the refuse thoroughly. About 72% of nitrogen was reduced from the landfilled fresh refuse through the operation of 357 days. At the end of the experiment, the concentrations of COD, ammonia, nitrate and TN in the leachate decreased to 399.2 mg l(-1), 20.6 mg N l(-1), 3.7 mg N l(-1) and 25.3 mg N l(-1), respectively.

In situ nitrogen removal in phase-separate bioreactor landfill

The feasibility of in situ nitrogen removal in phase-separate bioreactor landfill was investigated. In the experiment, two sets of bioreactor landfill systems, namely conventional two-phase and in situ nitrogen removal bioreactor landfills, were operated. The in situ nitrogen removal bioreactor landfill (NBL) was comprised of a fresh-refuse filled reactor (NBLF), a methanogenic reactor (NBLM) and a nitrifying reactor (NBLN), while the two-phase bioreactor landfill (BL) used as control was comprised of a fresh-refuse filled reactor (BLF) and a methanogenic reactor (BLM). Furthermore, the methanogenic and nitrifying reactors used aged refuse as bulk agents. The results showed that in situ nitrogen removal was viable by phase-separation in the bioreactor landfill. In total 75.8 and 47.5 g of nitrogen were, respectively, removed from the NBL and the BL throughout the experiment. The methanogenic reactor used the aged refuse as medium was highly effective in removing organic matter from the fresh leachate. Furthermore, the aged refuse was also suitable to use as in situ nitrification medium. The degradation of fresh refuse was accelerated by denitrification in the initial stage (namely the initial hydrolyzing stage) despite being delayed by denitrification in a long-term operation.

In situ nitrogen removal from leachate by bioreactor landfill with limited aeration

The feasibility of simultaneous nitrification and denitrification in a bioreactor landfill with limited aeration was assessed. Three column reactors, simulating bioreactor landfill operations under anaerobic condition (as reference), intermittent forced aeration and enhanced natural aeration were hence established, where aerated columns passed through two phases, i.e., fresh landfill and well-decomposed landfill. The experimental results show that limited aeration decreased nitrogen loadings of leachate distinctly in the fresh landfill. In the well-decomposed landfill, the NH(4)(+)-N of the input leachate could be nitrified completely in the aerated landfill columns. The nitrifying loadings of the column cross section reached 7.9 g N/m(2)d and 16.9 g N/m(2)d in the simulated landfill columns of intermittent forced aeration and enhanced natural aeration, respectively. The denitrification was influenced by oxygen distribution in the landfill column. Intermittent existence of oxygen in the landfill with the intermittent forced aeration was favorable to denitrify the NO(2)(-)-N and NO(3)(-)-N, indicated by the high denitrification efficiency (>99%) under the condition of BOD(5)/TN of more than 5.4 in leachate; locally persistent existence of oxygen in the landfill with enhanced natural aeration could limit the denitrification, indicated by relatively low denitrification efficiency of about 75% even when the BOD(5)/TN in leachate had an average of 7.1.

Characteristics of the bioreactor landfill system using an anaerobic-aerobic process for nitrogen removal

A sequential upflow anaerobic sludge blanket (UASB) and air-lift loop sludge blanket (ALSB) treatment was introduced into leachate recirculation to remove organic matter and ammonia from leachate in a lab-scale bioreactor landfill. The results showed that the sequential anaerobic-aerobic process might remove above 90% of COD and near to 100% of NH4+ -N from leachate under the optimum organic loading rate (OLR). The total COD removal efficiency was over 98% as the OLR increased to 6.8-7.7 g/l d, but the effluent COD concentration increased to 2.9-4.8 g/l in the UASB reactor, which inhibited the activity of nitrifying bacteria in the subsequent ALSB reactor. The NO3- -N concentration in recycled leachate reached 270 mg/l after treatment by the sequential anaerobic-aerobic process, but the landfill reactor could efficiently denitrify the nitrate. After 56 days operation, the leachate TN and NH4+ -N concentrations decreased to less than 200 mg/l in the bioreactor landfill system. The COD concentration was about 200 mg/l with less than 8 mg/l BOD in recycled leachate at the late stage. In addition, it was found that nitrate in recycled leachate had a negative effect on waste decomposition.