The S.An.A.® concept: Semi-aerobic, Anaerobic, Aerated bioreactor landfill

Prediction of oxygen distribution in the waste mass from an aeration well in bioreactor landfills

The key to upgrade the efficiency of aerobic remediation of landfills is to determine the distribution characteristics of oxygen concentration during aerobic ventilation. This study discusses the distribution law of oxygen concentration with time and radial distance based on a single-well aeration test at an old landfill site. The transient analytical solution of the radial oxygen concentration distribution was deduced using the gas continuity equation and approximation of calculus and logarithmic functions. Oxygen concentration data from the field monitoring were compared with the results predicted by the analytical solution. The results indicated that the oxygen concentration initially increased and then decreased with prolonged aeration time. With an increase in radial distance, the oxygen concentration rapidly declined, followed by a gradual decrease. The influence radius of the aeration well increased slightly when the aeration pressure increased from 2 to 20 kPa. The field test data agreed with the analytical solution prediction results, preliminarily verifying the reliability of the oxygen concentration prediction model. Results from this study provide a basis of guidelines for the design, operation and maintenance management of a landfill aerobic restoration project.

Investigation on the microbial community of an accelerating stabilization landfill by aeration engineering

The microbial community of the landfill undergoing aerobic stabilization process by aeration engineering was investigated. The municipal solid wastes (MSWs) were sampled from two aeration well sites with different landfill temperatures (65.5°C and 41.7°C) under higher and lower stabilization level. The physical component, chemical property, and microbial population of MSWs were analyzed and compared. The result showed that the phylum Firmicutes was dominant in the aerobic landfill; and the genus Weissella and Syntrophaceticus were more abundant in high, and low temperature site, respectively. The bacterial distribution showed difference on two temperature sites and four landfill depths, mainly affected by the ammonia-nitrogen and moisture content of MSWs. The ecological profiles of the microorganisms responded the aeration engineering were predicted. The anaerobic hydrolytic and acetogenic microorganisms were decreased in abundance, while the facultative Lactobacillus increased when the landfill under a higher stabilization level. The function abundances of methane oxidation, sulfide oxidation, and aerobic chemoheterotrophy were enriched by aeration engineering, which was the microbial mechanism for accelerating the stabilization process of landfill.

Characterization of anaerobic oxidation of methane and microbial community in landfills with aeration

Landfills are the third largest source of anthropogenic CH₄ emissions. Anaerobic oxidation of methane (AOM) activity and communities of methane-oxidizing bacteria were investigated in three informal landfills in this study, namely, BJ, CH and SZ landfills, among which BJ and CH represent traditional anaerobic landfills, while the SZ landfill was subjected to aeration to accelerate waste stabilization. The AOM rates of the investigated landfilled wastes ranged from 3.66 to 23.91 nmol g^-1 h^-1. Among the three landfills, the AOM rate was highest in the SZ-1-Top sample, which was closest to the aeration pipe. Among the possible electron acceptors for AOM, including NO₃^-, NO₂^-, SO₄^2- and Fe³⁺, the NO₂^--N content was the only variable that was positively correlated with the AOM rate. Compared with α-Proteobacteria methanotrophs, γ-Proteobacteria methanotrophs were more abundant in the landfilled waste, especially Methylobacter, which was detected in nearly all samples. Members of the family Methylomirabilaceae, including Candidatus Methylomirabilis, were also detected in the SZ-1 and SZ-2-Bot samples. The relative abundance of the main methanotrophs in the families Methylomonadaceae, Methylococcaceae, Rokubacteriales and Methylomirabilaceae, the genus Methylocystis and the phylum NC10 were all positive correlations with the contents of NO₂^--N in the landfilled waste samples. Additionally, significantly positive correlations were observed between the AOM rates and the relative abundance of the main methanotrophs except for the family Methylococcaceae. This indicated that aeration could enhance the conversion of nitrogen compounds in the landfilled waste, in which the high contents of NO₂^--N could stimulate the growth of methanotrophs and increase AOM rate. These findings are helpful for understanding the mechanisms of CH₄ oxidation in landfills and for taking effective measures to mitigate CH₄ emissions from landfills.

Conceptualization of Bioreactor Landfill Approach for Sustainable Waste Management in Karachi, Pakistan

Finding a sustainable approach for municipal solid waste (MSW) management is becoming paramount. However, as with many urban areas in developing countries, the approach applied to MSW management in Karachi is neither environmentally sustainable nor suitable for public health. Due to adoption of an inefficient waste management system, society is paying intangible costs such as damage to public health and environment quality. In order to minimize the environmental impacts and health issues associated with waste management practices, a sustainable waste management and disposal strategy is required. The aim of this paper is to present a concept for the development of new bioreactor landfills for sustainable waste management in Karachi. Furthermore, this paper contributes to estimation of methane (CH4) emissions from waste disposal sites by employing the First Order Decay (FOD) Tier 2 model of the Intergovernmental Panel on Climate Change (IPCC) and determining of the biodegradation rate constant (k) value. The design and operational concept of bioreactor landfills is formulated for the study area, including estimation of land requirement, methane production, power generation, and liquid required for recirculation, along with a preliminary sketch of the proposed bioreactor landfill. This study will be helpful for stockholders, policy makers, and researchers in planning, development, and further research for establishment of bioreactor landfill facilities, particularly in the study area as well as more generally in regions with a similar climate and MSW composition.

Intermittent aeration reducing N2O emissions from bioreactor landfills with gas-water joint regulation

Landfills are important emission sources of atmospheric N₂O, especially bioreactor landfills with leachate recirculation. In this study, N₂O emissions were characterized in four bioreactor landfills with different ventilation methods, including intermittent (2-h aeration per 12 h or 4 h/d in continuous) and continuous aeration (20 h/d), in comparison to a traditional landfill without aeration. During the experiment, the N₂O emissions from the landfill reactors with intermittent aeration were 7.48 and 7.15 mg, accounting for only 20.8% and 19.9% of those with continuous aeration, respectively. Continuous aeration was more favorable for the biodegradation of organic matter than intermittent aeration in the landfilled waste and leachate. Intermittent and continuous aeration could both effectively remove total nitrogen (TN) and NH₄⁺-N with removal efficiencies above 64% in the leachate. In the experimental landfill reactors with gas-water joint regulation, the proportion of N₂O-N to TN loss ranged from 0.02% to 0.75%. Luteimonas, Pseudomonas, Thauera, Pusillimonas and Comamonas were the dominant denitrifying bacteria in the landfill reactors. The denitrifying bacterial community in the landfilled waste was closely related to its degree of stabilization and nitrogenous compound concentrations in the landfilled waste and leachate. The NO₃^--N and NO₂^--N concentrations of leachate were the most important environmental factors affecting the succession of nirS-type and nirK-type denitrifying microbial communities in the landfilled waste. These findings indicated that intermittent aeration was an economical and effective way to accelerate the stabilization of landfilled waste and reduce the pollutants in leachate and N₂O emissions during landfill mining and reclamation.

Reduction in VOC emissions by intermittent aeration in bioreactor landfills with gas-water joint regulation

Landfill mining and reclamation is a new strategy for addressing the lack of space available for new landfills and realizing the sustainable development of landfills. A gas-water joint bioreactor landfill is regulated by injecting water and/or recirculating leachate, and a blasting aeration system to optimize waste stabilization. In this study, four landfill reactors were constructed to investigate the effects of ventilation methods, including continuous (20 h d^-1) and intermittent aeration (4 h d^-1 in continuous or 2-h aeration per 12 h, twice a day), on the degradation of organic matter and volatile organic compound (VOC) emissions in comparison with traditional landfills. A total of 62 VOCs were detected in the landfill reactors. Among them, halogenated compounds had the highest abundance (39.8-65.4 %), followed by oxygenated compounds, alkanes and alkenes, and aromatic compounds. Both intermittent and continuous aeration could accelerate the degradation of landfilled waste and increase the volatilization rate of VOCs. Compared with intermittent aeration, the degradation of landfilled waste was more quickly in the landfill reactor with continuous aeration. However, intermittent aeration could create anaerobic-anoxic-aerobic conditions, which were conducive to the growth and metabolism of anaerobic and aerobic microorganisms in landfills and thereby reduced more than 63.4 % of total VOC emissions from the landfill reactor with continuous aeration. Moreover, intermittent aeration could reduce the ventilation rate and decrease the cost of aeration by 80 % relative to continuous aeration. Firmicutes, Bacteroidetes, Proteobacteria and Tenericutes predominated in the landfill reactors. The environmental variables including organic matter and VOCs concentrations had significant influences on microbial community structure in the landfilled waste. These findings indicated that intermittent aeration was an effective way to accelerate the stabilization of landfilled waste and reduce the cost and environmental risks in bioreactor landfills with gas-water joint regulation.

Influence of moisture content and leachate recirculation on oxygen consumption and waste stabilization in post aeration phase of landfill operation

Sustainable completion of municipal solid waste landfills requires post-closure care after a time when utilization of landfill gas produced from biodecomposition of organic waste be not possible/or economically feasible. Research proved that in-situ aeration is a promising approach employed for landfill aftercare. The application of post aeration operation is targeted to achieve accelerated waste stabilization to avoid long term environmental and public health impacts from landfills. In in-situ aeration operation, consumption of supplied oxygen has significant influence on biological stabilization of solid waste placed in the landfills. The consumption of oxygen is regulated by operation parameters of landfill - one of the important is presence of moisture in landfill ecosystem. This research aims to assess the influence of moisture content and leachate recirculation on the oxygen consumption during post aeration phase of landfill operation. The effect of oxygen consumption on the extent of waste stabilization achieved after experiment was also assessed. Three lab-scale landfill simulation reactors (LSRs) were used - in two of three reactors (LSR-1 and LSR-3) operation was carried out in two phases: Anaerobic and post-aeration. One reactor (LSR-2) was operated under anaerobic condition throughout the experiment and used as control. To compare the oxygen consumption, conventional landfill (CLF) conditions without excess water addition and leachate recirculation were simulated in LSR-1 and the bioreactor landfill conditions (BRLF) with excess water injection and leachate recirculation were simulated in LSR-3. In CLF 46.4% of supplied oxygen was consumed during post aeration phase while in BRLF only 0.96% of oxygen consumption was noticed. In result of higher oxygen consumption, biostabilization rate of waste in CLF was 7% higher than BRLF at the end of experiment. This study demonstrated that, in presence of low moisture in landfill ecosystem optimal air distribution can be realized which results in enhanced waste oxidization and stabilization.

Greenhouse gas emissions from semi-aerobic bioreactor landfills with different vent-pipe diameters

The emission patterns of three greenhouse gasses (GHGs), viz. CH₄, CO₂, and N₂O from landfills, were examined on a lab scale. Three simulated semi-aerobic bioreactor landfills (SABL1, SABL2, SABL3), with respective vent-pipe inner diameters (φ) of 25, 50, and 75 mm, were used to investigate their effect on the greenhouse effect (GHE) during the municipal solid waste (MSW) stabilization process. We found that the vent-pipe φ influenced both MSW degradation and GHG emissions, increasing the vent-pipe φ which improved the removal of carbon and nitrogen-based pollutants. The GHG emissions were 364, 356, and 309 kg CO₂ equivalents per ton of MSW from the SABL2, SABL1, and SABL3, respectively, during the operation of 465 days. Of the three GHGs, CH₄ influenced the GHE the most, contributing 72.53%, 79.17%, and 71.42% in SABL1, SABL2, and SABL3, respectively. In the same sequence, CO₂ (14.87%, 14.06%, and 21.9%) and N₂O (12.6%, 6.77%, and 6.69%) were the second and third contributors to the GHE, respectively. Considering the rapidly MSW stabilization and the mitigation of GHG emissions, a vent pipe with φ of 75 mm in the SABL column (φ of 800 mm) was suggested. Moreover, the GHG mitigation in the SABL should be implemented by prioritizing CH₄ collection and oxidation. The results provided a technical guidance for GHG mitigation in MSW management.

A systematic assessment of aeration rate effect on aerobic degradation of municipal solid waste based on leachate chemical oxygen demand removal

Aeration is one mainstream technique to accelerate municipal solid waste (MSW) degradation in landfills. The determination of an appropriate aeration rate is critical to the design and operation of a landfill aeration system. In this study, we analyze 132 waste degradation tests reported in forty one studies in the literature. We use L min^-1 kg^-1 dry organic matter (L min^-1 kg^-1 DOM) as the uniform unit to quantify the aeration rates in all tests. The first order rate coefficient for chemical oxygen demand (COD) removal in leachate (k_COD) is selected as the parameter to characterize MSW degradation process. We further divide aerobic tests into five aerobic groups base on the respective aeration rates, i.e., <0.02, 0.02-0.1, 0.1-0.3, 0.3-1, and >1 L min^-1 kg^-1 DOM. With an increase in the aeration rate, the k_COD increases first and then decreases. The aeration rate between 0.1 and 0.3 L min^-1 kg^-1 DOM has the best enhancement on the k_COD. The k_COD values are not much higher than the anaerobic and semi-aerobic tests when the aeration rates are <0.1 L min^-1 kg^-1 DOM, because such aeration rates may be lower than the actual oxygen consumption rates. An aeration rate >0.3 L min^-1 kg^-1 DOM reduces the k_COD likely due to excess water evaporation and ventilation cooling. Among the analyzed results, the aeration rate is the most related to the k_COD in principal component analysis than the other factors, including liquid recirculation and addition, waste total density, waste degradation level, and waste initial temperature.

Microbial response during treatment of different types of landfill leachate in a semi-aerobic aged refuse biofilter

In this research, for the first time, three kinds of landfill leachate (young (YL), mature (ML) and mixed (MYL) leachate) were treated in a semi-aerobic aged refuse biofilter (SAARB) to compare the effectiveness of, and microbial changes in, this biofilter when treating leachates that have significantly different characteristics. The SAARB achieved stable removal of organic matter from all three leachates and reduced the concentrations of aromatic substances. The best treatment was achieved with YL, followed in order by MYL and ML. The removal of nitrogen from all three leachates by the SAARB was particularly significant. The microbial abundance and diversity in the media of the SAARB changed after treatment of the three leachates, and the order of change from small to large was ML# < MYL# < YL#. The microbial communities were mainly affected by (and negatively correlated to) the relative content of refractory organics in leachate. Proteobacteria was the dominant microorganism. Deinococcus-thermus responded most to the quality of leachate being treated, increasing in relative abundance as the content of refractory organics increased. This was opposite to the response of Chloroflexi. In YL# the dominant species at the genus level was Thauera, and in ML# the dominant species were Truepera and Iodidimonas. The microbial activity and metabolic intensity were enhanced after treatment of the different leachates. The expression of nitrification-related genes was the strongest and the total abundance was the highest when YL was treated. This study promotes the optimization and application of SAARB.

Environmental Sustainability Enhancement of Waste Disposal Sites in Developing Countries through Controlling Greenhouse Gas Emissions

Sustainable management of municipal solid waste is one of the major challenges for authorities in developing countries. Current waste disposal methods in Pakistan and other developing countries are not meeting standards of any proper waste management system opted for in the developed world. This mismanagement of waste is leading to serious environmental problems at local as well as global levels. This study aims to investigate the methane emissions from waste dumpsites in the city of Karachi, Pakistan, and to propose an effective approach to enhance their environmental sustainability. The methane emissions from waste disposal sites were assessed by simulating four different landfill situations during the landfill simulation reactor experiment. The residual methane reduction potential of each waste disposal approach was assessed by a biochemical methane test of waste after the experiment. It is estimated that in the present situation, about 11,500 tons of CO2-eq methane is released annually from waste disposal sites in Karachi. The convectional anaerobic landfill with methane capturing facilities and post-aeration operation was found to be the most environmentally sustainable approach with controlling 65% of residual methane emissions in comparison with the present scenario. For the development of new landfill sites, we recommend the bioreactor landfill approach with methane recovery and post-care (in-situ aeration).

Tracing bacterial and fungal necromass dynamics of municipal sludge in landfill bioreactors using biomarker amino sugars

The dynamics of microbial necromass of municipal solid waste over long-term landfill remain unknown. This study presents the first investigation on the dynamics of bacterial and fungal necromass of municipal sludge in non-aeration versus alternating aeration landfill bioreactors by using amino sugar biomarkers. Results showed that under non-aeration treatment, the decomposition rate of muramic acid derived from bacteria is higher than that of fungal-derived glucosamine. The relative change in glucosamine and muramic acid in the early period of landfills under the alternating aeration treatment is consistent with that under non-aeration treatment. However, with the increase in alternating aeration cycles, bacterial necromass muramic acid exerts a lower decomposition rate than fungal necromass glucosamine. Throughout the entire landfill period, galactosamine is the amino sugar with the slowest decomposition rate under non-aeration mode but the amino sugar with the fastest decomposition rate under alternating aeration mode. The present work fills the knowledge gap of microbial necromass dynamics of municipal solid waste in landfills.

A pilot-scale comparative study of bioreactor landfills for leachate decontamination and municipal solid waste stabilization

Many studies have sought to optimize operation parameters and enhance the treatment capacity of bioreactor landfills (BL) under ideal laboratory conditions. At pilot scale, conclusions drawn from laboratory-scale experiments will be different due to variations in actual landfill composition and changes in environmental conditions. However, comparative pilot-scale studies of traditional anaerobic landfills (AnL) and BLs are rare. In this study, three pilot-scale landfills, including an AnL, anaerobic BL (AnBL) and semi-aerobic BL (SABL), were monitored to examine the difference in performance at different scales and among types of landfills. Settlement amount followed the order SABL (25.45 cm) > AnBL (18.67 cm) > AnL (14.38 cm). Decomposition of organic matter (i.e., volatile fatty acids) was more rapid in SABL than in the other landfills and no hydrolytic acidification period was observed. Therefore, among the three landfills, SABL entered the methanogenic stage in a much shorter time and MSW stabilization was accelerated due to this landfill's unique combination of aerobic-anoxic-anaerobic ambient. In addition, NH₄⁺-N concentration in leachate from the SABL (~19.96 mg/L) was substantially lower than from AnL (338.28 mg/L) and AnBL (233.22 mg/L), and SABL leachate exhibited the least chloride pollution risk. This study provides theoretical support and strong evidence for using SABLs to treat MSW in practical applications.

Lab tests on semi-aerobic landfilling of MSW under varying conditions of water availability and putrescible waste content

Semi-aerobic landfilling is applied increasingly as a sustainable technology worldwide, although frequently controversial results are achieved. The authors suggest that differences in water availability (climate, moisture content, etc.) and putrescible waste content are the key factors involved in controlling performance and efficiencies. The aim of the present study was to investigate the effect of inverse conditions (high/low) of these two factors. Six lab-scale lysimeters were specifically set up to correspond to three different conditions of water availability (wet conditions, dry conditions and artificially controlled watering under dry conditions) and two different waste types (high and low putrescible content). Lysimeters were operated for four months under thermal-insulated conditions and the quality and quantity of emissions monitored regularly. Concentrations of mobile ammonia and total organic carbon (TOC) in landfilled waste were modelled by means of first-order kinetics, and carbon and nitrogen mass balances were calculated. The best performance for the semi-aerobic process was achieved at a water availability of approximately 1.5-2.4 kgH₂O/kgTS using the following two combinations: a) Waste with high putrescible content and no addition of external water due to the presence of sufficient endogenous water in the waste (moisture) to promote biological stabilisation of waste (Respiration index in 4 days, RI₄ = 12.87 mgO₂/gTS, BOD/COD < 0.05); b) Waste with low putrescible content and controlled watering (RI₄ = 12.25 mgO₂/gTS, BOD/COD < 0.04). The study highlighted how semi-aerobic landfilling operations should be carefully adjusted case by case according to waste quality and climate conditions.

Effect of waste compaction density on stabilization of aerobic bioreactor landfills

Landfill stabilization contributes to the safe operation and maintenance of landfills. This study used a simulated aerobic bioreactor landfill to investigate the impact of different compaction densities on its stabilization to provide a basis for optimal parameter selection during landfill design. Samples of municipal solid waste were tested with compaction densities of 450, 500, 550, 600, and 650 kg/m³ during the experiment. The optimum compaction density was obtained by periodically monitoring the temperature of the waste pile, the water quality of leachate, and the composition of the waste. The impacts of waste compaction density on waste pile temperature and leachate were investigated and coupled with the analysis of waste composition to discuss the possible reaction mechanism. Results showed that the most complete waste degradation occurred at 550 kg/m³ compaction density, which was effective at accelerating stabilization of the simulated aerobic bioreactor landfill. Limitations of the experiment are given to lay foundations for further study.

A novel spatiotemporally anaerobic/semi-aerobic bioreactor for domestic solid waste treatment in rural areas

To meet the requirements of domestic solid waste treatment and technological upgrading of bioreactors, a spatiotemporally anaerobic/semi-aerobic bioreactor (STASAB) was designed. The STASAB took full advantages of anaerobic and semi-aerobic bioreactors by the sequential alternation of anaerobic and semi-aerobic operation and by recirculation of mixed leachate from different-stage bioreactors. Results indicated that after the start-up stage, the pH of leachate in the STASAB always remained higher than 6.88, even in the hydrolysis and acidogenesis stage. The maximum total nitrogen concentration in the STASAB was 1461 mg·L^-1, which was merely half that of the sequentially anaerobic/semi-aerobic bioreactor (SASAB) and had no adverse effects on the anaerobic process. Nitrogen removal in the STASAB reached 92.3%-95.5% when operated in the semi-aerobic phase and even reached 49.4% when operated in the anaerobic phase. The peak concentration of chemical oxygen demand was much lower and the anaerobic digestion lag time was much shorter in the STASAB than in the SASAB. The period of rapid biogas production in the STASAB was double that of the SASAB, which resulted in a 70% increase in biogas generation. Moreover, leachate could be exhausted by evaporation in just 3-5 months during the semi-aerobic phase. Therefore, the STASAB can eliminate acidogenic and ammonia inhibition during solid waste treatment, enhance the rate and extent of organic waste decomposition, rapidly initiate methanogenesis, enhance methane generation, and achieve zero leachate discharge (excluding rainfall infiltration). The STASAB is an efficient and feasible technique for treating domestic solid waste in rural areas.

Improving leachate quality and optimizing CH4 and N2O emissions from a pre-aerated semi-aerobic bioreactor landfill using different pre-aeration strategies

Landfill aeration efficiently accelerates municipal solid waste (MSW) stabilization. This method also impacts methane (CH₄) and nitrous oxide (N₂O) emissions during aeration. In this study, the effects of three pre-aeration strategies on leachate quality variations and CH₄ and N₂O emissions from three lab-scale pre-aerated semi-aerobic bioreactor landfills, which were filled with MSW, were investigated: low frequency and high frequency intermittent aeration (LIA and HIA) and continuous micro-aeration (CMA). Experimental results showed that these three strategies effectively reduced organic and N-based pollutants concentration in leachate. Compared with intermittent aeration (IA), CMA increased cumulative CH₄ emissions (9234.3 mg) and resulted in a longer emission period (95 days). HIA generated the least cumulative CH₄ emissions (4297.6 mg) and shortest emission period (65 days) due to organic matter loss during aeration. N₂O emissions were present at low levels in early stages for each bioreactor, and then, increased by 1-3 orders of magnitude in the later stages due to low influent carbon-nitrogen ratio. HIA resulted in maximum cumulative N₂O emissions (2884.6 mg) and experienced a longer emission period (179 days) compared to CMA (2281.6 mg; 151 days). LIA had the longest N₂O emission period (209 days), but had the lowest cumulative N₂O emissions (1486.3 mg). CH₄ and N₂O emissions mainly occurred in the early and later stages of landfill stabilization, respectively. Therefore, the study proposes an optimized pre-aeration strategy for practical landfill aeration management: early CMA may promote rapid organic matter removal and effective CH₄ recovery; and late LIA may reduce N₂O emissions.