Interface transmissivity of conventional and multicomponent GCLs for three permeants

A laboratory investigation of the interface transmissivity is reported for five different geosynthetic clay liners (GCLs) and a range of different geomembranes (GMBs) for a range of stresses from 10 to 150?kPa. The GCLs were prehydrated under normal stress before permeation. The GCLs examined comprised three multicomponent (a smooth coated, a smooth laminated, and textured coated) and two conventional (one with granular and one with powdered sodium bentonite) GCLs. The effect of a 4?mm circular defect in the coating of a multicomponent GCL directly below the 10?mm diameter hole in the GMB is investigated. The effect of GMB stiffness and texture is examined. Additionally, the effect of hydration and permeation of smooth coated GCL with highly saline solution and synthetic landfill leachate (SL3) is presented. It is shown that the 2-week interface transmissivity (θ_2-week) can be one to two orders of magnitude higher than steady-state interface transmissivity (θ _steady-state) at low stresses (10?kPa–50?kPa), whereas at high stresses (150?kPa) the variation is substantially less. For a smooth coated GCL hydrated and permeated with reverse osmosis (RO) water, GMB stiffness and texture has a limited effect on interface transmissivity when the coating is placed in contact with GMB at normal stresses of 10?kPa–150?kPa, whereas coating indentations result in much high interface transmissivity when placed in contact with GMB. GCL prehydration and permeation with highly saline solutions leads to higher interface transmissivity compared to RO water. With a 4.0?mm defect in the coating, the interface transmissivity between the coating and woven geotextile is higher than that between the coating and GMB for the stress levels and GCL examined.     

Self-healing of circular and slit defects in GCLs upon hydration from silty sand under applied stress

The self-healing of a GCL with artificial defects (circular holes and rectangular slits, both with and without the carrier geotextile preserved below the holes) upon hydration on a Godfrey silty sand (GSS) subgrade with w_fdn = 5, 10 and 16% under 2–100 kPa is examined. Circular holes with the carrier geotextile missing below holes with diameters up to 25.4 mm self-healed on the w_fdn = 5% and 10% GSS but not on 16% GSS, while none self-healed when carrier geotextile was preserved below the holes. When DI water was introduced to the surface under 100 kPa, circular holes with diameter up to 38.1 mm self-healed. Neither the single 15 mm-wide slit nor double 15 mm-wide parallel slits with 20 mm-wide strip of undamaged GCL between them resting on w_fdn = 10% GSS under 20 kPa fully self-healed. The introduction of simulated synthetic landfill leachate (SSL) to the GCL surface under 70 kPa did not result in self-healing. Post-hydration k tests found that GCL without a carrier geotextile below a hole up to 25.4 mm in diameter would not have a significant adverse effect on the hydraulic conductivity compared with an intact GCL provided the permeant was tap water rather than SSL.     

嘉兴垃圾填埋场渗滤液的土柱淋滤实验研究

通过室内淋滤实验分析垃圾渗滤液-水-岩相互作用过程。实验发现Cl-的保守性是相对的,其中在粉砂土中Cl-存在吸附解吸现象,平衡浓度为4446mg/L时,分配系数Kd=0.18L/kg,而在壤土(亚粘土)中Cl-保持保守。SO42-和F-积极参与水岩相互作用,粉砂土和壤土对SO42-和F-均有较强的吸附作用。阴离子交换作用的结果使大量的NO 3-从粉砂土中解吸出来。     

Water quality and pollution loading of a river segment affected by landfill leachate and domestic waste

The Marikina River in the Philippines has been polluted by Payatas landfill leachate, and domestic and agricultural waste. This study monitored the water quality at five stations on the river and two stations on two creeks that discharge to the river to determine the effects of Payatas landfill and to estimate pollution loading. The dissolved oxygen (DO), chemical oxygen demand (COD) and other water quality parameters were compared with the Philippines Standards for river water classification. It was found that Payatas leachate has a significant influence on the DO and COD levels as well as other water quality parameters. Per capita pollution loading for Quezon City was found to be lower than for Europe and Japan. The effect of leachate is more significant during the dry season. It is recommended that a leachate collection system be established to prevent leachate form entering Payatas creek, and that the Patayas landfill be replaced with a modern landfill site, conforming to current best practice at another location.

List of Abbreviation: BOD= Biological Oxygen Demand COD = Chemical Oxygen Demand DO = Dissolved Oxygen EC = Electrical Conductivity M1, M2, M3, M4, M5 = monitoring stations TDS = Total Dissolved Solids TSS = Total Suspended Solids     

Treatment of landfill leachate by combined aged-refuse bioreactor and electro-oxidation

Two-stage aged-refuse bioreactor (ARB) was applied to treat landfill leachate in Shanghai Waste Laogang Disposal Plant. The removal efficiencies of chemical oxygen demand (COD), biological oxygen demand (BOD), total organic carbon (TOC), total nitrogen (TN) and ammonia nitrogen (NH(3)-N) of landfill leachate treated by the two-stage bioreactor system were 98.5%, 99.9%, 98.0%, 64.2% and 99.9%, respectively. The COD and BOD in the second stage effluent were 239 and 7 mg l(-1), respectively. Thus three types of electrolysis were employed to further treat the second effluent, undivided electrolysis (UDE), divided electrolysis (DE) with Ti/PbO(2) cathode and DE with gas diffusion cathode. All electrolysis processes possessed good color removal effect, while the DE with gas diffusion cathode had the best TOC removal effect. The optimum electrolysis time of leachate was 30 min. The TOC removal efficiencies were 51.4% and 39.7% in anolyte and catholyte, respectively, after 30 min electrolysis at 5 V. In addition, the DE with gas diffusion cathode showed the least energy consumption of 9.8 k Whm(-3) at 30 min. The organic pollutants in the leachate were analyzed through a gas chromatography coupled with mass spectrometry (GC-MS) system. Through the two-stage ARB, the species and concentrations of organic pollutants in landfill leachate reduced greatly. Several chlorinated organic compounds were detected in the effluent after the UDE and the anolyte of the DE. In addition, the concentration of absorbable organic halogens (AOX) increased greatly during the electrolysis. Hence, careful consideration should be given in the application of electro-oxidation into the treatment of chloride-containing wastewater.     

Effect of leachate composition on the long-term performance of a HDPE geomembrane

Aging of the same geomembrane immersed in the same four synthetic municipal solid waste leachates tested by Rowe et al. (2008) for 2.65 years is continued for an additional 6.5 years (i.e., providing data over more than 9 years of aging). The additional data is used to (a) update the initial estimates for antioxidant depletion stage, and (b) investigate the effect of leachate constituents on the timing and magnitude of the changes in the physical properties of the geomembrane. While all the examined leachates had surfactant and trace metals in common, reduced Leachate 1 (full leachate) had salts and volatile fatty acids (VFAs), Leachate 2 was not reduced and had neither salts nor VFAs, reduced Leachate 3 had salts, reduced Leachate 4 had VFAs, and Leachate 5 was similar to Leachate 2 except that it was reduced. Incubation in all leachates gave very similar predictions of the antioxidant depletion stage, however, the salts in Leachates 1 and 3 are shown to have the largest effect on the geomembrane mechanical properties, especially stress-crack resistance, resulting in a shorter time to nominal failure than for leachates without salts. Furthermore, reduced Leachates 1 and 3 showed faster degradation in stress-crack resistance than Leachate 2 which was not reduced and without salts. Arrhenius modeling is used to extrapolate the time to nominal failure (i.e., a reduction in stress-crack resistance to 150 h) at a range of temperatures.     

Optimizing the treatment of landfill leachate by conventional Fenton and photo-Fenton processes

Landfill, a matured and economically appealing technology, is the ultimate approach for the management of municipal solid wastes. However, the inevitable generation of leachate from landfill requires further treatment. Among the various leachate treatment technologies available, advanced oxidation processes (AOPs) are among powerful methods to deal with the refractory organic constituents, and the Fenton reagent has evolved as one promising AOPs for the treatment of leachates. Particularly, the combination of UV-radiation with Fenton's reagent has been reported to be a method that allows both the photo-regeneration of Fe²⁺ and photo-decarboxylation of ferric carboxylates. In this study, Fenton and photo-Fenton processes were fine tuned for the treatment of leachates from the Colmenar Viejo (Madrid, Spain) Landfill. Results showed that it is possible to define a set of conditions under which the same COD and TOC removals (≈ 70%) could be achieved with both the conventional and photo-Fenton processes. But Fenton process generated an important quantity of iron sludge, which will require further disposal, when performed under optimal COD removal conditions. Furthermore conventional Fenton process was able to achieve slightly over an 80% COD removal from a “young” leachate, while for “old” and ”mixed” leachates was close to a 70%. The main advantage showed by the photo-assisted Fenton treatment of landfill leachate was that it consumed 32 times less iron and produced 25 times less sludge volume yielding the same COD removal results than a conventional Fenton treatment.     

Effect of water chemistry on the hydro-mechanical behaviour of compacted mixtures of claystone and Na/Ca-bentonites for deep geological repositories

In the French deep geological disposal for radioactive wastes, compacted bentonite/claystone mixtures have been considered as possible sealing materials. After emplacement in place, such mixtures are hydrated by the site solution as well as the cement solution produced by the degradation of concrete. In this study, the effects of synthetic site solution and cement solution on the hydro-mechanical behaviour of compacted mixtures of claystone and two types of bentonites (MX80 Na-bentonite and Sardinia Ca-bentonite) were investigated by carrying out a series of swelling pressure, hydraulic conductivity and mercury intrusion porosimetry (MIP) tests. It was found that for the MX80 bentonite/claystone mixture hydrated with synthetic site solution, the swelling capacity was reduced compared to the case with deionised water owing to the transformation of Na-montmorillonite to multi-cation dominant montmorillonite by cation exchanges. For the Sardinia bentonite/claystone mixture, the similar increasing rate of swelling pressure was observed during the crystalline swelling process for different solutions, suggesting insignificant cation exchanges. Additionally, the cations in the synthetic site solution could reduce the thickness of diffuse double layer and the osmotic swelling for both MX80 bentonite/claystone and Sardinia bentonite/claystone mixtures. The large-pore volume increased consequently and enhanced water flow. In the cement solution, the hydroxide could also dissolve the montmorillonite, reducing the swelling pressure, and increase the large-pore volume, facilitating the water flow. Furthermore, the decrease of swelling pressure and the increase of hydraulic conductivity were more significant in the case of low dry density because of more intensive interaction between montmorillonite and hydroxide due to the high permeability.     

Inhibition and recovery in a fixed microbial film leachate treatment system subject to shock loading of copper and zinc

The impacts of shock loadings of copper and zinc (up to 50 mg l(-1)) on the treatment efficiency of a mesoscale-fixed microbial film landfill leachate treatment system were investigated. Treatment inhibition and recovery were monitored in sequence over two 36 h experimental runs. The fate of added metals was also investigated. Copper, and to a lesser extent zinc, added to the treatment systems accumulated on the biofilm media. Increasing copper inputs (>10 mg l(-1)) progressively inhibited biological treatment of ammoniacal-nitrogen and carbon; this inhibition persisted into the recovery phase for nitrogen but not for carbon. Only the highest input of zinc affected media metal contents and carbon treatment rates; the latter inhibitory effect did not persist into the recovery phase. A small proportion of the metals accumulated on the biofilm media during the inhibition phase was released into the bulk leachate during the recovery experiment. These findings suggest a need to manage metal inputs into leachate treatment systems in order to ensure their continued efficacy.     

Complete treatment of olive pomace leachate by coagulation, activated-carbon adsorption and electrochemical oxidation

A battery scheme comprising sequential alum coagulation, activated-carbon adsorption and electrochemical oxidation over boron-doped diamond electrodes to mineralize a leachate from olive pomace processing is demonstrated. The effect of coagulant and adsorbent concentration on treatment efficiency was assessed in the range 0.1-50 mM Al(3+) and 2.5-50 g/L activated-carbon and optimal conditions were established. Coagulation at 7.5mM Al(3+) resulted in substantial solids and color removal (i.e. 80% and 93%, respectively). This was accompanied by only 30% chemical oxygen demand (COD) reduction (initial COD was about 3,500 mg/L). The latter increased to 80% though when coagulation was coupled with adsorption at 25 g/L activated carbon. Electrochemical oxidation of the original effluent for 360 min led to 63% and 82% COD reduction at 10 and 20A current intensity, respectively. When this process was tested as a polishing stage following coagulation and adsorption, overall COD removal reached values of 92% and 97%, respectively. The final effluent was also colorless and solids free. However, the treated effluent still exhibited ecotoxicity possibly due to the formation of ecotoxic oxidation products.     

Pilot treatment of olive pomace leachate by vertical-flow constructed wetland and electrochemical oxidation: An efficient hybrid process

A hybrid process comprising biological degradation in a vertical-flow constructed wetland (CW) and electrochemical oxidation over boron-doped diamond electrodes to decolorize, mineralize and detoxify a leachate from olive pomace processing (OPL) was investigated. Two alternative treatment schemes were compared: According to the first treatment scheme, OPL was treated by electrochemical oxidation followed by treatment in a constructed wetland pilot unit (CW-A). The second scheme comprised of treatment in a constructed wetland followed by electrochemical treatment (CW-B). The constructed wetlands units were planted with Phragmites australis (reeds) and were fed intermittently at organic loadings between 5 and 15 g COD m⁻² d⁻¹ and a residence time of 3 d. Electrochemical oxidation (EO) was performed for 360 min at 20 A.Treatment of OPL in the wetland at 15 g COD m⁻² d⁻¹ led to mean COD and color reduction of 86% and 77%, respectively; the wetland effluent with a COD of about 800 mg L⁻¹ was polished electrochemically for 360 min after which the overall COD and color removal of the combined process (i.e. CW-B/EO) was around 95%, while the final effluent was not toxic against the marine bacteria Vibrio fischeri.Electrochemical oxidation of the original OPL at COD values between 6250 and 14 100 mg L⁻¹ led to moderate COD and color reduction (i.e. less than 40%) through zero order kinetics. When this was coupled to constructed wetland post-treatment (i.e. EO/CW-A), the overall COD and color removal was 81% and 58%, respectively. The decreased efficiency may be assigned to the increased toxicity of the electrochemically treated effluent which was only partially removed in the natural treatment system.     

Treating landfill leachate using passive aeration trickling filters; effects of leachate characteristics and temperature on rates and process dynamics

Biological ammoniacal-nitrogen (NH₄⁺-N) and organic carbon (TOC) treatment was investigated in replicated mesoscale attached microbial film trickling filters, treating strong and weak strength landfill leachates in batch mode at temperatures of 3, 10, 15 and 30 °C. Comparing leachates, rates of NH₄⁺-N reduction (0.126-0.159 g m^− 2 d^− 1) were predominantly unaffected by leachate characteristics; there were significant differences in TOC rates (0.072-0.194 g m^− 2 d^− 1) but no trend relating to leachate strength. Rates of total oxidised nitrogen (TON) accumulation (0.012-0.144 g m^− 2 d^− 1) were slower for strong leachates. Comparing temperatures, treatment rates varied between 0.029-0.319 g NH₄⁺-N m^− 2 d^− 1 and 0.033-0.251 g C m^− 2 d^− 1 generally increasing with rising temperatures; rates at 3 °C were 9 and 13% of those at 30 °C for NH₄⁺-N and TOC respectively. For the weak leachates (NH₄⁺-N < 140 mg l^− 1) complete oxidation of NH₄⁺-N was achieved. For the strong leachates (NH₄⁺-N 883-1150 mg l^− 1) a biphasic treatment response resulted in NH₄⁺-N removal efficiencies of between 68 and 88% and for one leachate no direct transformation of NH₄⁺-N to TON in bulk leachate. The temporal decoupling of NH₄⁺-N oxidation and TON accumulation in this leachate could not be fully explained by denitrification, volatilisation or anammox, suggesting temporary storage of N within the treatment system. This study demonstrates that passive aeration trickling filters can treat well-buffered high NH₄⁺-N strength landfill leachates under a range of temperatures and that leachate strength has no effect on initial NH₄⁺-N treatment rates. Whether this approach is a practicable option depends on a range of site specific factors.     

Impact of solution chemistry on viral removal by a single-walled carbon nanotube filter

This study investigates the effectiveness of a single-walled carbon nanotube (SWNT) filter for removal of viruses from water. MS2 bacteriophage viral removal was examined over a range of environmentally relevant solution chemistries, spanning various ionic strengths, monovalent and divalent salts, pH, and natural organic matter (NOM) concentrations. Viral removal by the SWNT filter was governed by physicochemical (depth) filtration. The removal of viruses increased at higher ionic strengths (NaCl) due to suppression of repulsive electrostatic interactions between viruses and SWNTs. Addition of divalent salts, however, had varying impacts. While CaCl₂ increased virus removal, likely due to complexation of calcium ions to viral surfaces, addition of MgCl₂ reduced viral removal by the SWNT filter. Solution pH also had significant impact on viral removal as the interactions between viral particles and SWNTs changed from attractive below the virus isoelectric point (about pH 3.9) to repulsive at higher pH. Suwannee River NOM was shown to be detrimental to filter viral removal. Reduction of viral removal by NOM was attributed to adsorption of NOM macromolecules to viruses and SWNTs, thereby resulting in steric repulsive forces. Modifications of the filter to incorporate thicker SWNT layers mitigate the negative impacts of NOM on filter performance. This study has shown that while it is possible to attain high levels of viral removal over a broad range of solution chemistries, the extent of viral removal will be highly dependent on the specific solution chemistry of the treated water.     

The effects of reduction of the deposited waste on short‐term landfill leachate composition of a landfill: a case study in Norway

Municipal solid waste landfill leachate was sampled over from a landfill receiving varying amounts of municipal solid waste. The investigation aimed to provide information on expected leachate changes in the short term, either after closure of an active landfill, or after a strong decline in the amount of waste deposited at smaller landfills. It was found that during a two year period following a sudden decline in the amount of waste deposited, the levels of various chemical and physical parameters all dropped sharply. The reasons for the decline in discharge levels are thought to be aerobic decomposition taking place in the municipal solid waste just after landfilling, and thus the decline in the impact of this process when there were less fresh waste masses available, and formation of preferential flow paths for the leachate as the municipal solid waste stabilized in the landfill.     

Removal of refractory organics and nitrogen from landfill leachate by the microorganism-attached activated carbon fluidized bed process

The microorganism-attached activated carbon fluidized bed (MAACFB) process was applied to treat a real landfill leachate containing refractory organics and a high concentration of ammonium nitrogen. The MAACFB process consisted of two fluidized bed reactors in series: anaerobic and aerobic.The MAACFB process was found to be effective in treating the landfill leachate. The MAACFB process removed about 60 and 70% of refractory organics and nitrogen, respectively, from the landfill leachate simultaneously and steadily over more than 700 days of the operation period. A mass balance of the organics around the MAACFB process revealed that most of the removed organics may be biodegraded.     

Size effect on aperture and permeability of a fracture as estimated in large synthetic fractures

Synthetic fractures of from 0.2 to 12.8 m in size were created on a computer by a new spectral method to reproduce the ratio of the power spectral density of the initial aperture (the aperture when the surfaces are in contact at a single point) to that of the surface height determined for a tensile fracture of 1 m. First, the size effect on the standard deviation of the initial aperture was analyzed for fractures with and without shearing. Next, by taking aperture data at constant intervals to establish a flow area, water flow was simulated for fractures during both normal closure and closure after shearing, by solving Reynolds equation to determine the hydraulic aperture. When the fracture is closed without shearing and has the same mean aperture, the effect of the fracture size on the hydraulic aperture disappears if the fracture is larger than about 0.2 m, since beyond this size the standard deviation of the initial aperture is almost independent of the fracture size. When the fracture is closed after shearing, the hydraulic conductivity shows remarkable anisotropy, which becomes more significant with both shear displacement and closure. However, the relation between the hydraulic aperture normalized by the mean aperture and the mean aperture normalized by the standard deviation of the initial aperture is almost independent of both the fracture size and shear displacement when the shear displacement is less than about 3.1% of the fracture size, at which point the standard deviation of the initial aperture of the sheared fracture is almost independent of the fracture size.     

Effect of dissolved organic carbon on the mobility of cadmium, nickel and zinc in leachate polluted groundwater

The ability of dissolved organic carbon (DOC) from landfill leachate polluted groundwater to form complexes with the heavy metals cadmium (Cd), nickel (Ni) and zinc (Zn) was investigated. The DOC samples originated from the leachate pollution plume at Vejen Landfill, Denmark and were studied in the original matrix with a minimum of manipulation. The experiments were performed as batch sorption experiments and the metal distribution between the aquifer material and the solution (K_d) was determined in the leachate polluted groundwater samples and in reference solutions of synthetic inorganic leachate. The difference in distribution coefficients was a direct indication of complex formation between DOC and heavy metals. The results showed, that DOC from landfill leachate polluted groundwater has the ability to form complexes with Cd, Ni and Zn, and the distribution coefficients were a factor of 2–6 lower in the presence of DOC. Based on the distribution coefficients, the relative migration velocities of the heavy metals were estimated. The migration velocity of the metals was increased by the presence of DOC but did not exceed 1.2% of the water migration velocity, indicating that the effect of DOC on the mobility of Cd, Ni and Zn may have only minor environmental importance. Conditional complex formation constants (log K_c) were estimated from the K_d-values. The constants for 1:1 complexes increased slightly in the following order Zn < Cd < Ni. However, increasing the metal concentration into the mg l⁻¹ level decreased the constants by about one order of magnitude, showing the importance of determining the constants at environmentally relevant metal concentrations.     

Detection of enteric viruses in municipal sewage sludge by a combination of the enzymatic virus elution method and RT-PCR

Pathogenic enteric viruses can be retained in municipal sewage sludge as has been reported by many researchers. Although the RT-PCR technique has been extensively employed for the virus detection from various environmental samples, the application of RT-PCR to the detection of viruses in sewage sludge has the difficulty because of inhibitory substances to the gene amplification. However, a combination of the enzymatic virus elution (EVE) method with RT-PCR made it possible to effectively detect viruses in sewage sludge. The enzymatic breakdown of sludge flocs in the EVE method enhanced the virus elution from poliovirus 1 (PV1)-inoculated sewage sludge, and the detection of PV1 was performed by RT-PCR without any inhibitions. On the contrary, the application of RT-PCR to the viral assay in the US EPA method using the 10% beef extract solution was not practical because of inhibitions to the viral gene amplification. The combination of the EVE method using lysozyme (polysaccharide-degrading enzyme), papain (protease), and chymotrypsin (protease) with RT-PCR resulted in a virus recovery efficiency of 31%, but a synergistic effect of these enzymes on the virus recovery efficiency was not observed. The EVE method using lysozyme or papain could be a promising procedure for the virus elution from sewage sludge in detecting these viruses with RT-PCR.     

Control of aerosol dispersion and removal in a room using synthetic jet actuators

Synthetic jet actuators, a common actuator for flow control applications, were used to control particle dispersion in a closed chamber. The presented results demonstrate the application of this technique for controlling indoor air quality in confined spaces. This technique was demonstrated in a dedicated 0.23 m³ closed chamber that included a simulated ventilation system. The flow field in the room was determined using particle image velocimetry (PIV). Arrays of synthetic jets located on either side of an aerosol inlet (intake) provided the flow control. The effectiveness of the synthetic jet-based flow control was demonstrated for controlling airflow and aerosol dispersion and removal in the closed chamber. The use of such devices in smart buildings is expected to improve indoor air quality while minimizing the energy burden on heating, ventilating, and air conditioning (HVAC) systems.     

Estimation of the mass-balance of selected metals in four sanitary landfills in Western Norway, with emphasis on the heavy metal content of the deposited waste and the leachate

A worst-case simulation of the mass-balance for metals in the waste deposited during 1 year and the levels of cadmium (Cd), lead (Pb), mercury (Hg), chromium (Cr) and iron (Fe) in the leachate was calculated for four sanitary landfills in Western Norway. Estimates of the levels of metal content in mixed municipal solid waste (MSW) were found by using recent literature values calculated in a mass-balance study at a Norwegian waste incinerator plant. Leachate from the landfills were sampled and analyzed monthly during 1 year, and from these measurements the total annual discharge of the selected metals through the leachate was determined. The levels of the measured heavy metals in the leachate were low. For Cd less than 0.06%, for Pb less than 0.01% and for Hg less than 0.02% of the estimated year's deposited mass of metals were leached from the landfills during the year of investigation. The high retention of these metals are most likely due to sulfide precipitation, but also due to the immobile condition of the metals in their original deposited solid state (plastics, ceramics, etc.). The percentage of Cr leached was relatively higher, but less than 1.0% per year. The mass balance of Fe suggests that this element is more mobile under the prevailing conditions. The percentage of Fe leached varied and was estimated to be between 1.9% and 18%. The present study clearly supports the theory that MSW only to a small extent will lead to discharge of metals if deposited at well-constructed sanitary landfills with top layers.