Greenhouse gas emissions from home composting of organic household waste

The emission of greenhouse gases (GHGs) is a potential environmental disadvantage of home composting. Because of a lack of reliable GHG emission data, a comprehensive experimental home composting system was set up. The system consisted of six composting units, and a static flux chamber method was used to measure and quantify the GHG emissions for one year composting of organic household waste (OHW). The average OHW input in the six composting units was 2.6–3.5 kg week^?1 and the temperature inside the composting units was in all cases only a few degrees (2–10 °C) higher than the ambient temperature. The emissions of methane (CH₄) and nitrous oxide (N₂O) were quantified as 0.4–4.2 kg CH₄ Mg^?1 input wet waste (ww) and 0.30–0.55 kg N₂O Mg^?1 ww, depending on the mixing frequency. This corresponds to emission factors (EFs) (including only CH₄ and N₂O emissions) of 100–239 kg CO₂-eq. Mg^?1 ww. Composting units exposed to weekly mixing had the highest EFs, whereas the units with no mixing during the entire year had the lowest emissions. In addition to the higher emission from the frequently mixed units, there was also an instant release of CH₄ during mixing which was estimated to 8–12% of the total CH₄ emissions. Experiments with higher loads of OHW (up to 20 kg every fortnight) entailed a higher emission and significantly increased overall EFs (in kg substance per Mg^?1 ww). However, the temperature development did not change significantly. The GHG emissions (in kg CO₂-eq. Mg^?1 ww) from home composting of OHW were found to be in the same order of magnitude as for centralised composting plants.     

Greenhouse gas emissions from landfill leachate treatment plants: A comparison of young and aged landfill

With limited assessment, leachate treatment of a specified landfill is considered to be a significant source of greenhouse gas (GHG) emissions. In our study, the cumulative GHG emitted from the storage ponds and process configurations that manage fresh or aged landfill leachate were investigated. Our results showed that strong CH₄ emissions were observed from the fresh leachate storage pond, with the fluxes values (2219–26,489 mg C m^?2 h^?1) extremely higher than those of N₂O (0.028–0.41 mg N m^?2 h^?1). In contrast, the emission values for both CH₄ and N₂O were low for the aged leachate tank. N₂O emissions became dominant once the leachate entered the treatment plants of both systems, accounting for 8–12% of the removal of N-species gases. Per capita, the N₂O emission based on both leachate treatment systems was estimated to be 7.99 g N₂O–N capita^?1 yr^?1. An increase of 80% in N₂O emissions was observed when the bioreactor pH decreased by approximately 1 pH unit. The vast majority of carbon was removed in the form of CO₂, with a small portion as CH₄ (<0.3%) during both treatment processes. The cumulative GHG emissions for fresh leachate storage ponds, fresh leachate treatment system and aged leachate treatment system were 19.10, 10.62 and 3.63 Gg CO₂ eq yr^?1, respectively, for a total that could be transformed to 9.09 kg CO₂ eq capita^?1 yr^?1.     

Greenhouse gas emissions from MSW incineration in China: Impacts of waste characteristics and energy recovery

Determination of the amount of greenhouse gas (GHG) emitted during municipal solid waste incineration (MSWI) is complex because both contributions and savings of GHGs exist in the process. To identify the critical factors influencing GHG emissions from MSWI in China, a GHG accounting model was established and applied to six Chinese cities located in different regions. The results showed that MSWI in most of the cities was the source of GHGs, with emissions of 25–207 kg CO₂-eq t^?1 rw. Within all process stages, the emission of fossil CO₂ from the combustion of MSW was the main contributor (111–254 kg CO₂-eq t^?1 rw), while the substitution of electricity reduced the GHG emissions by 150–247 kg CO₂-eq t^?1 rw. By affecting the fossil carbon content and the lower heating value of the waste, the contents of plastic and food waste in the MSW were the critical factors influencing GHG emissions of MSWI. Decreasing food waste content in MSW by half will significantly reduce the GHG emissions from MSWI, and such a reduction will convert MSWI in Urumqi and Tianjin from GHG sources to GHG sinks. Comparison of the GHG emissions in the six Chinese cities with those in European countries revealed that higher energy recovery efficiency in Europe induced much greater reductions in GHG emissions. Recovering the excess heat after generation of electricity would be a good measure to convert MSWI in all the six cities evaluated herein into sinks of GHGs.     

Spatial variability of nitrous oxide and methane emissions from an MBT landfill in operation: Strong N2O hotspots at the working face

Mechanical biological treatment (MBT) is an effective technique, which removes organic carbon from municipal solid waste (MSW) prior to deposition. Thereby, methane (CH₄) production in the landfill is strongly mitigated. However, direct measurements of greenhouse gas emissions from full-scale MBT landfills have not been conducted so far. Thus, CH₄ and nitrous oxide (N₂O) emissions from a German MBT landfill in operation as well as their concentrations in the landfill gas (LFG) were measured. High N₂O emissions of 20–200 g CO₂ eq. m^?2 h^?1 magnitude (up to 428 mg N m^?2 h^?1) were observed within 20 m of the working face. CH₄ emissions were highest at the landfill zone located at a distance of 30–40 m from the working face, where they reached about 10 g CO₂ eq. m^?2 h^?1. The MBT material in this area has been deposited several weeks earlier. Maximum LFG concentration for N₂O was 24.000 ppmv in material below the emission hotspot. At a depth of 50 cm from the landfill surface a strong negative correlation between N₂O and CH₄ concentrations was observed. From this and from the distribution pattern of extractable ammonium, nitrite, and nitrate it has been concluded that strong N₂O production is associated with nitrification activity and the occurrence of nitrite and nitrate, which is initiated by oxygen input during waste deposition. Therefore, CH₄ mitigation measures, which often employ aeration, could result in a net increase of GHG emissions due to increased N₂O emissions, especially at MBT landfills.     

GHG emission factors developed for the collection,transport and landfilling of municipal waste in South African municipalities

Greenhouse gas (GHG) emission factors are used with increased frequency for the accounting and reporting of GHG from waste management. However, these factors have been calculated for developed countries of the Northern Hemisphere and are lacking for developing countries. This paper shows how such factors have been developed for the collection, transport and landfilling of municipal waste in South Africa. As such it presents a model on how international results and methodology can be adapted and used to calculate country-specific GHG emission factors from waste. For the collection and transport of municipal waste in South Africa, the average diesel consumption is around 5 dm³ (litres) per tonne of wet waste and the associated GHG emissions are about 15 kg CO₂ equivalents (CO₂ e). Depending on the type of landfill, the GHG emissions from the landfilling of waste have been calculated to range from ?145 to 1016 kg CO₂ e per tonne of wet waste, when taking into account carbon storage, and from 441 to 2532 kg CO₂ e per tonne of wet waste, when carbon storage is left out. The highest emission factor per unit of wet waste is for landfill sites without landfill gas collection and these are the dominant waste disposal facilities in South Africa. However, cash strapped municipalities in Africa and the developing world will not be able to significantly upgrade these sites and reduce their GHG burdens if there is no equivalent replacement of the Clean Development Mechanism (CDM) resulting from the Kyoto agreement. Other low cost avenues need to be investigated to suit local conditions, in particular landfill covers which enhance methane oxidation.     

Effects of compost biocovers on gas flow and methane oxidation in a landfill cover

Previous publications described the performance of biocovers constructed with a compost layer placed on select areas of a landfill surface characterized by high emissions from March 2004 to April 2005. The biocovers reduced CH₄ emissions 10-fold by hydration of underlying clay soils, thus reducing the overall amount of CH₄ entering them from below, and by oxidation of a greater portion of that CH₄. This paper examines in detail the field observations made on a control cell and a biocover cell from January 1, 2005 to December 31, 2005. Field observations were coupled to a numerical model to contrast the transport and attenuation of CH₄ emissions from these two cells. The model partitioned the biocover’s attenuation of CH₄ emission into blockage of landfill gas flow from the underlying waste and from biological oxidation of CH₄. Model inputs were daily water content and temperature collected at different depths using thermocouples and calibrated TDR probes. Simulations of CH₄ transport through the two soil columns depicted lower CH₄ emissions from the biocover relative to the control. Simulated CH₄ emissions averaged 0.0 g m^?2 d^?1 in the biocover and 10.25 g m^?2 d^?1 in the control, while measured values averaged 0.04 g m^?2 d^?1 in the biocover and 14 g m^?2 d^?1 in the control. The simulated influx of CH₄ into the biocover (2.7 g m^?2 d^?1) was lower than the simulated value passing into the control cell (29.4 g m^?2 d^?1), confirming that lower emissions from the biocover were caused by blockage of the gas stream. The simulated average rate of biological oxidation predicted by the model was 19.2 g m^?2 d^?1 for the control cell as compared to 2.7 g m^?2 d^?1 biocover. Even though its V_max was significantly greater, the biocover oxidized less CH₄ than the control cell because less CH₄ was supplied to it.     

Anaerobic digestion of autoclaved and untreated food waste

Anaerobic digestion of autoclaved (160 °C, 6.2 bar) and untreated source segregated food waste (FW) was compared over 473 days in semi-continuously fed mesophilic reactors with trace elements supplementation, at organic loading rates (OLRs) of 2, 3, 4 and 6 kg volatile solids (VS)/m³ d. Methane yields at all OLR were 5–10% higher for untreated FW (maximum 0.483 ± 0.013 m³ CH₄/kg VS at 3 kg VS/m³ d) than autoclaved FW (maximum 0.439 ± 0.020 m³ CH₄/kg VS at 4 kg VS/m³ d). The residual methane potential of both digestates at all OLRs was less than 0.110 m³ CH₄/kg VS, indicating efficient methanation in all cases. Use of acclimated inoculum allowed very rapid increases in OLR. Reactors fed on autoclaved FW showed lower ammonium and hydrogen sulphide concentrations, probably due to reduced protein hydrolysis as a result of formation of Maillard compounds. In the current study this reduced biodegradability appears to outweigh any benefit due to thermal hydrolysis of ligno-cellulosic components.     

Quantification of methane emissions from full-scale open windrow composting of biowaste using an inverse dispersion technique

An inverse dispersion technique in conjunction with Open-Path Tunable-Diode-Laser-Spectroscopy (OP-TDLS) and meteorological measurements was applied to characterise methane (CH₄) emissions from an Austrian open-windrow composting plant treating source-separated biowaste. Within the measurement campaigns from July to September 2012 different operating conditions (e.g. before, during and after turning and/or sieving events) were considered to reflect the plant-specific process efficiency. In addition, the tracer technique using acetylene (C₂H₂) was applied during the measurement campaigns as a comparison to the dispersion model. Plant-specific methane emissions varied between 1.7 and 14.3 g CH₄/m³d (1.3–10.7 kg CH₄/h) under real-life management assuming a rotting volume of 18,000 m³. In addition, emission measurements indicated that the turning frequency of the open windrows appears to be a crucial factor controlling CH₄ emissions when composting biowaste. The lowest CH₄ emission was measured at a passive state of the windrows without any turning event (“standstill” and “sieving of matured compost”). Not surprisingly, higher CH₄ emissions occurred during turning events, which can be mainly attributed to the instant release of trapped CH₄. Besides the operation mode, the meteorological conditions (e.g. wind speed, atmospheric stability) may be further factors that likely affect the release of CH₄ emissions at an open windrow system. However, the maximum daily CH₄ emissions of 1 m³ rotting material of the composting plant are only 0.7–6.5% of the potential daily methane emissions released from 1 m³ of mechanically–biologically treated (MBT) waste being landfilled according to the required limit values given in the Austrian landfill ordinance.     

Nitrogen evolution during the co-combustion of hydrothermally treated municipal solid waste and coal in a bubbling fluidized bed

Nitrogen evolution was studied during the co-combustion of hydrothermally treated municipal solid wastes (HT MSW) and coal in a bubbling fluidized bed (BFB). HT MSW blending ratios as 10%, 20% and 30% (wt.%) were selected and tested at 700, 800, 900 °C. Emissions of NO and N₂O from blends were measured and compared with the results of mono-combustion trials. Moreover, concentrations of precursors like NH₃ and HCN were also quantified. The results are summarized as follows: NO emissions were predominant in all the cases, which rose with increasing temperature. The blending of HT MSW contributed to the NO reduction. N₂O emissions decreased with temperature rising and the blending of HT MSW also presented positive effects. At 30% HT MSW addition, both NO and N₂O emissions showed the lowest values (391.85 ppm and 55.33 ppm, respectively at 900 °C). For the precursors, more HCN was detected than NH₃ and both played important roles on the gas side nitrogen evolution.     

Long-term thermophilic mono-digestion of rendering wastes and co-digestion with potato pulp

In this study, mono-digestion of rendering wastes and co-digestion of rendering wastes with potato pulp were studied for the first time in continuous stirred tank reactor (CSTR) experiments at 55 °C. Rendering wastes have high protein and lipid contents and are considered good substrates for methane production. However, accumulation of digestion intermediate products viz., volatile fatty acids (VFAs), long chain fatty acids (LCFAs) and ammonia nitrogen (NH₄-N and/or free NH₃) can cause process imbalance during the digestion. Mono-digestion of rendering wastes at an organic loading rate (OLR) of 1.5 kg volatile solids (VS)/m³ d and hydraulic retention time (HRT) of 50 d was unstable and resulted in methane yields of 450 dm³/kg VS_fed. On the other hand, co-digestion of rendering wastes with potato pulp (60% wet weight, WW) at the same OLR and HRT improved the process stability and increased methane yields (500–680 dm³/kg VS_fed). Thus, it can be concluded that co-digestion of rendering wastes with potato pulp could improve the process stability and methane yields from these difficult to treat industrial waste materials.     

Impact of nitrate-enhanced leachate recirculation on gaseous releases from a landfill bioreactor cell

This study evaluates the impact of nitrate injection on a full scale landfill bioreactor through the monitoring of gaseous releases and particularly N₂O emissions. During several weeks, we monitored gas concentrations in the landfill gas collection system as well as surface gas releases with a series of seven static chambers. These devices were directly connected to a gas chromatograph coupled to a flame ionisation detector and an electron capture detector (GC-FID/ECD) placed directly on the field. Measurements were performed before, during and after recirculation of raw leachate and nitrate-enhanced leachate. Raw leachate recirculation did not have a significant effect on the biogas concentrations (CO₂, CH₄ and N₂O) in the gas extraction network. However, nitrate-enhanced leachate recirculation induced a marked increase of the N₂O concentrations in the gas collected from the recirculation trench (100-fold increase from 0.2 ppm to 23 ppm). In the common gas collection system however, this N₂O increase was no more detectable because of dilution by gas coming from other cells or ambient air intrusion. Surface releases through the temporary cover were characterized by a large spatial and temporal variability. One automated chamber gave limited standard errors over each experimental period for N₂O releases: 8.1 ± 0.16 mg m^?2 d^?1 (n = 384), 4.2 ± 0.14 mg m^?2 d^?1 (n = 132) and 1.9 ± 0.10 mg m^?2 d^?1 (n = 49), during, after raw leachate and nitrate-enhanced leachate recirculation, respectively. No clear correlation between N₂O gaseous surface releases and recirculation events were evidenced. Estimated N₂O fluxes remained in the lower range of what is reported in the literature for landfill covers, even after nitrate injection.     

Greenhouse gases emissions accounting for typical sewage sludge digestion with energy utilization and residue land application in China

About 20 million tonnes of sludge (with 80% moisture content) is discharged by the sewage treatment plants per year in China, which, if not treated properly, can be a significant source of greenhouse gases (GHGs) emissions. Anaerobic digestion is a conventional sewage sludge treatment method and will continue to be one of the main technologies in the following years. This research has taken into consideration GHGs emissions from typical processes of sludge thickening + anaerobic digestion + dewatering + residue land application in China. Fossil CO₂, biogenic CO₂, CH_4, and avoided CO₂ as the main objects is discussed respectively. The results show that the total CO₂-eq is about 1133 kg/t DM (including the biogenic CO₂), while the net CO₂-eq is about 372 kg/t DM (excluding the biogenic CO₂). An anaerobic digestion unit as the main GHGs emission source occupies more than 91% CO₂-eq of the whole process. The use of biogas is important for achieving carbon dioxide emission reductions, which could reach about 24% of the total CO₂-eq reduction.     

Single-phase and two-phase anaerobic digestion of fruit and vegetable waste: Comparison of start-up,reactor stability and process performance

Single-phase and two-phase digestion of fruit and vegetable waste were studied to compare reactor start-up, reactor stability and performance (methane yield, volatile solids reduction and energy yield). The single-phase reactor (SPR) was a conventional reactor operated at a low loading rate (maximum of 3.5 kg VS/m³ d), while the two-phase system consisted of an acidification reactor (TPAR) and a methanogenic reactor (TPMR). The TPAR was inoculated with methanogenic sludge similar to the SPR, but was operated with step-wise increase in the loading rate and with total recirculation of reactor solids to convert it into acidification sludge. Before each feeding, part of the sludge from TPAR was centrifuged, the centrifuge liquid (solubilized products) was fed to the TPMR and centrifuged solids were recycled back to the reactor. Single-phase digestion produced a methane yield of 0.45 m³ CH₄/kg VS fed and VS removal of 83%. The TPAR shifted to acidification mode at an OLR of 10.0 kg VS/m³ d and then achieved stable performance at 7.0 kg VS/m³ d and pH 5.5–6.2, with very high substrate solubilization rate and a methane yield of 0.30 m³ CH₄/kg COD fed. The two-phase process was capable of high VS reduction, but material and energy balance showed that the single-phase process was superior in terms of volumetric methane production and energy yield by 33%. The lower energy yield of the two-phase system was due to the loss of energy during hydrolysis in the TPAR and the deficit in methane production in the TPMR attributed to COD loss due to biomass synthesis and adsorption of hard COD onto the flocs. These results including the complicated operational procedure of the two-phase process and the economic factors suggested that the single-phase process could be the preferred system for FVW.     

Tracer method to measure landfill gas emissions from leachate collection systems

This paper describes a method developed for quantification of gas emissions from the leachate collection system at landfills and present emission data measured at two Danish landfills with no landfill gas collection systems in place: Fakse landfill and AV Miljø. Landfill top covers are often designed to prevent infiltration of water and thus are made from low permeable materials. At such sites a large part of the gas will often emit through other pathways such as the leachate collection system. These point releases of gaseous constituents from these locations cannot be measured using traditional flux chambers, which are often used to measure gas emissions from landfills. Comparing tracer measurements of methane (CH₄) emissions from leachate systems at Fakse landfill and AV Miljø to measurements of total CH₄ emissions, it was found that approximately 47% (351 kg CH₄ d^?1) and 27% (211 kg CH₄ d^?1), respectively, of the CH₄ emitting from the sites occurred from the leachate collection systems. Emission rates observed from individual leachate collection wells at the two landfills ranged from 0.1 to 76 kg CH₄ d^?1. A strong influence on emission rates caused by rise and fall in atmospheric pressure was observed when continuously measuring emission from a leachate well over a week. Emission of CH₄ was one to two orders of magnitude higher during periods of decreasing pressure compared to periods of increasing pressure.     

Quantifying methane oxidation in a landfill-cover soil by gas push–pull tests

Methane (CH₄) oxidation by aerobic methanotrophs in landfill-cover soils decreases emissions of landfill-produced CH₄ to the atmosphere. To quantify in situ rates of CH₄ oxidation we performed five gas push–pull tests (GPPTs) at each of two locations in the cover soil of the Lindenstock landfill (Liestal, Switzerland) over a 4 week period. GPPTs consist of the injection of a gas mixture containing CH₄, O₂ and noble gas tracers followed by extraction from the same location. Quantification of first-order rate constants was based upon comparison of breakthrough curves of CH₄ with either Ar or CH₄ itself from a subsequent inactive GPPT containing acetylene as an inhibitor of CH₄ oxidation. The maximum calculated first-order rate constant was 24.8 ± 0.8 h^?1 at location 1 and 18.9 ± 0.6 h^?1 at location 2. In general, location 2 had higher background CH₄ concentrations in vertical profile samples than location 1. High background CH₄ concentrations in the cover soil during some experiments adversely affected GPPT breakthrough curves and data interpretation. Real-time PCR verified the presence of a large population of methanotrophs at the two GPPT locations and comparison of stable carbon isotope fractionation of CH₄ in an active GPPT and a subsequent inactive GPPT confirmed that microbial activity was responsible for the CH₄ oxidation. The GPPT was shown to be a useful tool to reproducibly estimate in situ rates of CH₄ oxidation in a landfill-cover soil when background CH₄ concentrations were low.     

Changes in cadmium mobility during composting and after soil application

The effect of twelve weeks of composting on the mobility and bioavailability of cadmium in six composts containing sewage sludge, wood chips and grass was studied, along with the cadmium immobilization capacity of compost. Two different soils were used and Cd accumulation measured in above-ground oat biomass (Avena sativa L.). Increasing pH appears to be an important cause of the observed decreases in available cadmium through the composting process. A pot experiment was performed with two different amounts of compost (9.6 and 28.8 g per kg of soil) added into Fluvisol with total Cd 0.255 mg kg^?1, and contaminated Cambisol with total Cd 6.16 mg kg^?1. Decrease of extractable Cd (0.01 mol l^?1 CaCl₂) was found in both soils after compost application. The higher amount of compost immobilized an exchangeable portion of Cd (0.11 mol l^?1 CH₃COOH extractable) in contaminated Cambisol unlike in light Fluvisol. The addition of a low amount of compost decreased the content of Cd in associated above-ground oat biomass grown in both soils, while a high amount of compost decreased the Cd content in oats only in the Cambisol.     

Potential of anaerobic digestion for material recovery and energy production in waste biomass from a poultry slaughterhouse

This study was carried out to assess the material and energy recovery by organic solid wastes generated from a poultry slaughterhouse. In a poultry slaughterhouse involving the slaughtering of 100,000 heads per day, poultry manure & feather from the mooring stage, blood from the bleeding stage, intestine residue from the evisceration stage, and sludge cake from the wastewater treatment plant were discharged at a unit of 0.24, 4.6, 22.8, and 2.2 Mg day^?1, consecutively. The amount of nitrogen obtained from the poultry slaughterhouse was 22.36 kg 1000 head^?1, phosphate and potash were 0.194 kg 1000 head^?1 and 0.459 kg 1000 head^?1, respectively. As regards nitrogen recovery, the bleeding and evisceration stages accounted for 28.0% and 65.8% of the total amount of recovered nitrogen. Energy recovered from the poultry slaughterhouse was 35.4 N m³ 1000 head^?1 as CH₄. Moreover, evisceration and wastewater treatment stage occupied 88.1% and 7.2% of the total recovered CH₄ amount, respectively.     

Evaluating the biochemical methane potential (BMP) of low-organic waste at Danish landfills

The biochemical methane potential (BMP) is an essential parameter when using first order decay (FOD) landfill gas (LFG) generation models to estimate methane (CH₄) generation from landfills. Different categories of waste (mixed, shredder and sludge waste) with a low-organic content and temporarily stored combustible waste were sampled from four Danish landfills. The waste was characterized in terms of physical characteristics (TS, VS, TC and TOC) and the BMP was analyzed in batch tests. The experiment was set up in triplicate, including blank and control tests. Waste samples were incubated at 55 °C for more than 60 days, with continuous monitoring of the cumulative CH₄ generation. Results showed that samples of mixed waste and shredder waste had similar BMP results, which was in the range of 5.4–9.1 kg CH₄/ton waste (wet weight) on average. As a calculated consequence, their degradable organic carbon content (DOCC) was in the range of 0.44–0.70% of total weight (wet waste). Numeric values of both parameters were much lower than values of traditional municipal solid waste (MSW), as well as default numeric values in current FOD models. The sludge waste and temporarily stored combustible waste showed BMP values of 51.8–69.6 and 106.6–117.3 kg CH₄/ton waste on average, respectively, and DOCC values of 3.84–5.12% and 7.96–8.74% of total weight. The same category of waste from different Danish landfills did not show significant variation. This research studied the BMP of Danish low-organic waste for the first time, which is important and valuable for using current FOD LFG generation models to estimate realistic CH₄ emissions from modern landfills receiving low-organic waste.     

Design of top covers supporting aerobic in situ stabilization of old landfills – An experimental simulation in lysimeters

Landfill aeration by means of low pressure air injection is a promising tool to reduce long term emissions from organic waste fractions through accelerated biological stabilization. Top covers that enhance methane oxidation could provide a simple and economic way to mitigate residual greenhouse gas emissions from in situ aerated landfills, and may replace off-gas extraction and treatment, particularly at smaller and older sites. In this respect the installation of a landfill cover system adjusted to the forced-aerated landfill body is of great significance. Investigations into large scale lysimeters (2 × 2 × 3 m) under field conditions have been carried out using different top covers including compost materials and natural soils as a surrogate to gas extraction during active low pressure aeration. In the present study, the emission behaviour as well as the water balance performance of the lysimeters has been investigated, both prior to and during the first months of in situ aeration. Results reveal that mature sewage sludge compost (SSC) placed in one lysimeter exhibits in principle optimal ambient conditions for methanotrophic bacteria to enhance methane oxidation. Under laboratory conditions the mature compost mitigated CH₄ loadings up to 300 l CH₄/m² d. In addition, the compost material provided high air permeability even at 100% water holding capacity (WHC). In contrast, the more cohesive, mineral soil cover was expected to cause a notably uniform distribution of the injected air within the waste layer. Laboratory results also revealed sufficient air permeability of the soil materials (TS-F and SS-Z) placed in lysimeter C. However, at higher compaction density SS-Z became impermeable at 100% WHC.Methane emissions from the reference lysimeter with the smaller substrate cover (12–52 g CH₄/m² d) were significantly higher than fluxes from the other lysimeters (0–19 g CH₄/m² d) during in situ aeration. Regarding water balance, lysimeters covered with compost and compost-sand mixture, showed the lowest leachate rate (18–26% of the precipitation) due to the high water holding capacity and more favourable plant growth conditions compared to the lysimeters with mineral, more cohesive, soil covers (27–45% of the precipitation).On the basis of these results, the authors suggest a layered top cover system using both compost material as well as mineral soil in order to support active low-pressure aeration. Conventional soil materials with lower permeability may be used on top of the landfill body for a more uniform aeration of the waste due to an increased resistance to vertical gas flow. A compost cover may be built on top of the soil cover underlain by a gas distribution layer to improve methane oxidation rates and minimise water infiltration. By planting vegetation with a high transpiration rate, the leachate amount emanating from the landfill could be further minimised. The suggested design may be particularly suitable in combination with intermittent in situ aeration, in the later stage of an aeration measure, or at very small sites and shallow deposits. The top cover system could further regulate water infiltration into the landfill and mitigate residual CH₄ emissions, even beyond the time of active aeration.     

Co-treatment of fruit and vegetable waste in sludge digesters. An analysis of the relationship among bio-methane generation,process stability and digestate phytotoxicity

The co-digestion of a variable amount of fruit and vegetable waste in a waste mixed sludge digester was investigated using a pilot scale apparatus. The organic loading rate (OLR) was increased from 1.46 kg VS/m³ day to 2.8 kg VS/m³ day. The hydraulic retention time was reduced from 14 days to about 10 days. Specific bio-methane production increased from about 90 NL/kg VS to the maximum value of about 430 NL/kg VS when OLR was increased from 1.46 kg VS/m³ day to 2.1 kg VS/m³ day. A higher OLR caused an excessive reduction in the hydraulic retention time, enhancing microorganism wash out. Process stability evaluated by the total volatile fatty acids concentration (mg/l) to the alkalinity buffer capacity (eq. mg/l CaCO₃) ratio (i.e. FOS/TAC) criterion was <0.1 indicating high stability for OLR <2.46 kg VS/m³ day. For higher OLR, FOS/TAC increased rapidly. Residual phytotoxicty of the digestate evaluated by the germination index (GI) (%) was quite constant for OLR < 2.46 kg VS/m³ day, which is lower than the 60% limit, indicating an acceptable toxicity level for crops. For OLR > 2.46 kg VS/m³ day, GI decreased rapidly. This corresponding trend between FOS/TAC and GI was further investigated by the definition of the GI ratio (GIR) parameter. Comparison between GIR and FOS/TAC suggests that GI could be a suitable criterion for evaluating process stability.