Composting kinetics in full-scale mechanical–biological treatment plants

This study focuses on the investigation of the kinetics of municipal solid waste composting in three full-scale mechanical–biological treatment (MBT) plants. The aims were to test a kinetic model based on volatile solids (VS) content change for describing the composting process in MBT plants, and to identify the model parameters that affected the estimation of the reaction rate constant most. To achieve this, VS content and several environmental conditions, namely temperature, moisture content, oxygen concentration and total bulk density were monitored throughout the composting process. Experimental data was fitted with a first-order kinetic model, and a rate constant (k) characteristic of composting under optimum environmental conditions was obtained. The kinetic model satisfactorily described the experimental data for the three MBT plants. k values ranged from 0.043 ± 0.002 d^?1 to 0.082 ± 0.011 d^?1. Sensitivity analysis showed that the model parameters that most affected the estimation of k were the initial biodegradable volatile solids content, the maximum temperature for biodegradation and the optimum moisture content. In conclusion, we show for the first time that full-scale MBT plants can be successfully modelled with a composting kinetic model.     

Impact of different particle size distributions on anaerobic digestion of the organic fraction of municipal solid waste

Particle size may significantly affect the speed and stability of anaerobic digestion, and matching the choice of particle size reduction equipment to digester type can thus determine the success or failure of the process. In the current research the organic fraction of municipal solid waste was processed using a combination of a shear shredder, rotary cutter and wet macerator to produce streams with different particle size distributions. The pre-processed waste was used in trials in semi-continuous ‘wet’ and ‘dry’ digesters at organic loading rate (OLR) up to 6 kg volatile solids (VS) m^?3 day^?1. The results indicated that while difference in the particle size distribution did not change the specific biogas yield, the digester performance was affected. In the ‘dry’ digesters the finer particle size led to acidification and ultimately to process failure at the highest OLR. In ‘wet’ digestion a fine particle size led to severe foaming and the process could not be operated above 5 kg VS m^?3 day^?1. Although the trial was not designed as a direct comparison between ‘wet’ and ‘dry’ digestion, the specific biogas yield of the ‘dry’ digesters was 90% of that produced by ‘wet’ digesters fed on the same waste at the same OLR.     

Medical waste generation in selected clinical facilities in Taiwan

This study investigated the type and amount of medical waste generated from small clinical facilities in Taiwan. We sampled 200 small medical establishments, with few or no patient beds, to survey the wastes generated and disposed. The surveyed medical facilities consisted of four groups including private clinics, medical laboratories, blood centers and public clinics. Private clinics providing surgical, dental, obstetrical, and dialysis services were included in this survey because they may generate higher amounts of infectious waste than other specialties. The overall mean general waste production rate was 3.97 kg/bed/day (or 0.075 kg/patient/day) at all the surveyed facilities, higher than that obtained from larger hospitals in Taiwan, which ranged from 2.41 to 3.26 kg/bed/day. The highest amount of infectious wastes generated among the four groups of facilities were from blood centers (3.14 kg/bed/day), followed by private clinics, medical laboratories and public clinics (1.91, 1.07, and 0.053 kg/bed/day, respectively). The overall average was 2.08 kg/bed/day. This study suggests that the waste generated at small medical facilities ranged widely.     

Lactic acid production with undefined mixed culture fermentation of potato peel waste

Potato peel waste (PPW) as zero value byproduct generated from food processing plant contains a large quantity of starch, non-starch polysaccharide, lignin, protein, and lipid. PPW as one promising carbon source can be managed and utilized to value added bioproducts through a simple fermentation process using undefined mixed cultures inoculated from wastewater treatment plant sludge. A series of non-pH controlled batch fermentations under different conditions such as pretreatment process, enzymatic hydrolysis, temperature, and solids loading were studied. Lactic acid (LA) was the major product, followed by acetic acid (AA) and ethanol under fermentation conditions without the presence of added hydrolytic enzymes. The maximum yields of LA, AA, and ethanol were respectively, 0.22 g g^?1, 0.06 g g^?1, and 0.05 g g^?1. The highest LA concentration of 14.7 g L^?1 was obtained from a bioreactor with initial solids loading of 60 g L^?1 at 35 °C.     

Batchwise mesophilic anaerobic co-digestion of secondary sludge from pulp and paper industry and municipal sewage sludge

Residues from forest-industry wastewater-treatment systems are treated as waste at many pulp and paper mills. These organic substances have previously been shown to have potential for production of large quantities of biogas. There is concern, however, that the process would require expensive equipment because of the slow degradation of these substances. Pure non-fibrous sludge from forest industry showed lower specific methane production during mesophilic digestion for 19 days, 53 ± 26 Nml/g of volatile solids as compared to municipal sewage sludge, 84 ± 24 Nml/g of volatile solids. This paper explores the possibility of using anaerobic co-digestion with municipal sewage sludge to enhance the potential of methane production from secondary sludge from a pulp and paper mill. It was seen in a batch anaerobic-digestion operation of 19 days that the specific methane production remained largely the same for municipal sewage sludge when up to 50% of the volatile solids were replaced with forest-industry secondary sludge. It was also shown that the solid residue from anaerobic digestion of the forest-industry sludge should be of suitable quality to use for improving soil quality on lands that are not used for food production.     

Cylinder cyclone (LARCODEMS) density media separation of plastic wastes

Cylindrical cyclone media separators using a suspended calcite separation media simulating industrial scale operations are demonstrated to effectively separate a wide variety of forms and a greater range of particle sizes of plastics by density than presently recycled. Purities of plastic products and recoveries obtained from mixed plastic wastes are comparable to those reported for established separations. Products of ≈100% purity with recoveries of >99% were obtained for high density fractions and >98% purities and recoveries for the low density fractions. Cyclonic centrifugal forces and/or the fine particle size of the separation media appear to minimize hydroscopic and particle shape effects.A mathematical model is proposed for defining plastic waste feed rates and treatable particle size ranges for the LARCODEMS media separator.Waste plastic separations yielded Ecart probable (Ep) values ?0.024 for a water only separation media. The Ep for 1.1 g cm^?3 separation medias was <0.032 with minimal to no variation in values for 1–8 mm particle sizes. Variation in the quality of separations is shown to be minimal with <72 μm, <45 μm and <2 μm media particle sizes. Media density offset created varied according to particle size.     

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.     

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.     

Treatment of the biodegradable fraction of used disposable diapers by co-digestion with waste activated sludge

The results presented in this paper are part of a project aimed at designing an original solution for the treatment of used disposable diapers permitting the recycling of materials and the recovery of energy. Diapers must be collected separately at source and transported to an industrial facility to undergo special treatment which makes it possible to separate plastics and to recover a biodegradable fraction (BFD) made up mainly of cellulose. The methane yield of BFD was measured and found to be 280 ml CH₄/g VS_fed on average. 150 kg of dry BFD can be retrieved from the treatment of one ton of used disposable diapers, representing an energy potential of about 400 kW h of total energy or 130 kW h of electricity. As the treatment process for used diapers requires very high volumes of water, the setting up of the diaper treatment facility at a wastewater treatment plant already equipped with an anaerobic digester offers the advantages of optimizing water use as well as its further treatment and, also, the anaerobic digestion of BFD. The lab-scale experiments in a SBR showed that BFD co-digestion with sewage sludge (38% BFD and 62% waste activated sludge on volatile solids basis) was feasible. However, special attention should be paid to problems that might arise from the addition of BFD to a digester treating WAS such as insufficient mixing or floating particles leading to the accumulation of untreated solids in the digester.     

Optimization of thermo-chemical pretreatment and enzymatic hydrolysis of kitchen wastes

The use of abundant waste materials with high carbohydrate content may contribute substantially to reduction of biofuels production cost. The present study aimed at optimizing the combined effect of thermo-chemical pretreatment and enzymatic hydrolysis of kitchen wastes (KW) for maximizing the production of fermentable soluble sugars. To this end, acid pretreatment of KW samples was performed with hydrochloric acid (0–3% HCl) at 30–100 °C for 0–120 min treatment time. Alternatively, alkaline pretreatment of KW samples was performed with potassium hydroxide solution (0–11%) at constant temperature and time (0 °C and 20 min, respectively). KOH pretreatment at such conditions targets to degrade the resistant starch of KW samples. Both acid and alkaline pretreatments were followed by addition of variable levels of enzyme dosage (0–3.6% v/v α-amylase and 0–3.2% v/v amyloglucosidase-AMG) at constant pH, temperature and time (pH = 5, T = 50 °C and t = 30 min, respectively). Based on our results, glucose concentration increased by ～300% after pretreatment with either acid or KOH in combination with enzymatic hydrolysis (2% HCl, 85 °C, 80 min, 0.1% α-amylase, AMG, and 1% KOH, 0 °C, 20 min, 1.1% α-amylase, 0.4% AMG) compared to raw (untreated) KW. Estimating the different Y_G yields at KW loading of 5%, an increase of 192% and 121% for total soluble monosugars and total soluble sugars, respectively, was succeeded compared to untreated KW. The effect of solids loading on the obtained sugar yields using the optimum conditions for thermo-chemical pretreatment followed by enzymatic hydrolysis was also tested resulting to 27.5% increase of the soluble glucose yield when half of the solids loading (2.5%) was used. A decrease of total soluble sugars yield by 32.2% was observed when solely acid hydrolysis at optimum conditions from our previous study was applied at 30% solids loading.     

High efficiency chlorine removal from polyvinyl chloride (PVC) pyrolysis with a gas–liquid fluidized bed reactor

In this research a gas–liquid fluidized bed reactor was developed for removing chlorine (Cl) from polyvinyl chloride (PVC) to favor its pyrolysis treatment. In order to efficiently remove Cl within a limited time before extensive generation of hydrocarbon products, the gas–liquid fluidized bed reactor was running at 280–320 °C, where hot N₂ was used as fluidizing gas to fluidize the molten polymer, letting the molten polymer contact well with N₂ to release Cl in form of HCl. Experimental results showed that dechlorination efficiency is mainly temperature dependent and 300 °C is a proper reaction temperature for efficient dechlorination within a limited time duration and for prevention of extensive pyrolysis; under this temperature 99.5% of Cl removal efficiency can be obtained within reaction time around 1 min after melting is completed as the flow rate of N₂ gas was set around 0.47–0.85 Nm³ kg^?1 for the molten PVC. Larger N₂ flow rate and additives in PVC would enhance HCl release but did not change the final dechlorination efficiency; and excessive N₂ flow rate should be avoided for prevention of polymer entrainment. HCl is emitted from PVC granules or scraps at the mean time they started to melt and the melting stage should be taken into consideration when design the gas–liquid fluidized bed reactor for dechlorination.     

Evaluation of batch anaerobic co-digestion of palm pressed fiber and cattle manure under mesophilic conditions

Palm pressed fiber (PPF) and cattle manure (CM) are the waste which can be managed properly by anaerobic co-digestion. The biogas production in co-digested PPF and CM at three volatile solids (VS) ratios of 3:1, 1:1, and 1:3 was investigated in a series of batch experiments at an organic loading rate of 30.0 g VS/L under mesophilic (37 ± 1 °C) conditions. The highest daily biogas yield of PPF and CM only, was 90.0 mL/g VS_added at day 12 and 23.4 mL/g VS_added at day 7. For co-digestion of PPF/CM at mixing ratios of 3:1, 1:1 and 1:3, there were 93.6 mL/g VS_added at day 11, 86.8 and 26.4 mL/g VS_added at day 8. VS removal rate for PPF, CM, and co-digestion at mixing ratio of 3:1, 1:1, and 1:3 were 91.1%, 86.0% and 71.0%, respectively. The anaerobic digestion of PPF and CM and their co-digestion systems were stable in operation with low range of volatile fatty acids (VFA)/TIC (total inorganic carbon) of (0.035–0.091). The main volatile fatty acids were propionic, and iso-butyric acids for PPF, iso-butyric and n-butyric acids for CM. The VFAs and ammonium inhibition were not occurred. The modified Gompertz model can be used to perform a better prediction with a lower difference between the measured and predicted biogas yields. A VS ratio of 3:1 is recommended for practice.     

Impact of alkaline-hydrolyzed biosolids (Lystek) addition on the anaerobic digestibility of TWAS in lab – And full-scale anaerobic digesters

The effect of different Lystek biosolids doses on the anaerobic digestability of thickened waste activated sludge (TWAS) was evaluated in a lab- and full-scale anaerobic digester. The overall findings of this study emphasize the beneficial impact of Lystek addition to the lab- and full-scale anaerobic digesters in terms of enhanced biogas production and increased volatile suspended solids reduction (VSSR) efficiency. Lystek added at 4% by volume to TWAS increased the methane yield from 0.22 to 0.26 L CH₄/g VSS_added at an solids retention time (SRT) of 10 days, and from 0.27 to 0.29 L CH₄/g VSS_added at an SRT of 15 days. Furthermore, the VSSRs of 37% and 47% were observed for the TWAS, and the TWAS with 4% Lystek, while at an SRT of 15 days, the observed VSSR were 49% and 58%, respectively. The lab-scale study showed that the influence of Lystek addition on methane yield and solids destruction efficiencies was more pronounced at the shorter SRT, 20% enhancement (SRT of 10 d) vs. 9% enhancement (SRT of 15 d) for methane yield, and 27% (SRT of 10 d) vs. 22% (SRT of 15 d) for VSS destruction efficiency improvement. Furthermore, addition of 4% of Lystek to the feed of the full-scale anaerobic digester at St. Marys wastewater treatment plant (WWTP) resulted in a 50% increase in the average specific methanogenic activity and 23% increase in methane yield of the biochemical methane potential tests after eight months. The results showed that Lystek degradation kinetics were 40% faster than the TWAS, as reflected by first order kinetic coefficients of 0.053 d^?1 and 0.073 d^?1 for TWAS and Lystek at an SRT of 10 days.     

Case study of landfill leachate recirculation using small-diameter vertical wells

A case study of landfill liquids addition using small diameter (5 cm) vertical wells is reported. More than 25,000 m³ of leachate was added via 134 vertical wells installed 3 m, 12 m, and 18 m deep over five years in a landfill in Florida, US. Liquids addition performance (flow rate per unit screen length per unit liquid head) ranged from 5.6 × 10^?8 to 3.6 × 10^?6 m³ s^?1 per m screen length per m liquid head. The estimated radial hydraulic conductivity ranged from 3.5 × 10^?6 to 4.2 × 10^?4 m s^?1. The extent of lateral moisture movement ranged from 8 to 10 m based on the responses of moisture sensors installed around vertical well clusters, and surface seeps were found to limit the achievable liquids addition rates, despite the use of concrete collars under a pressurized liquids addition scenario. The average moisture content before (51 samples) and after (272 samples) the recirculation experiments were 23% (wet weight basis) and 45% (wet weight basis), respectively, and biochemical methane potential measurements of excavated waste indicated significant (p < 0.025) decomposition.     

Pyrolysis of municipal plastic wastes: Influence of raw material composition

The objective of this work is the study of pyrolysis as a feedstock recycling process, for valorizing the rejected streams that come from industrial plants, where packing and packaging wastes are classified and separated for their subsequent mechanical recycling. Four real samples collected from an industrial plant at four different times of the year, have been pyrolysed under nitrogen in a 3.5 dm³ autoclave at 500 °C for 30 min. Pyrolysis liquids are a complex mixture of organic compounds containing valuable chemicals as styrene, ethyl-benzene, toluene, etc. Pyrolysis solids are composed of the inorganic material contained in the raw materials, as well as of some char formed in the pyrolysis process, and pyrolysis gases are mainly composed of hydrocarbons together with some CO and CO₂, and have very high gross calorific values (GCV).It has been proved by the authors that the composition of the raw material (paper, film, and metals contents) plays a significant role in the characteristics of pyrolysis products. High paper content yields water in the pyrolysis liquids, and CO and CO₂ in the gases, high PE film content gives rise to high viscosity liquids, and high metals content yields more aromatics in the liquid products, which may be attributed to the metals catalytic effect.     

Effect of temperature on VFA’s and biogas production in anaerobic solubilization of food waste

The effectiveness of methane fermentation treatment used in food waste processing is currently limited by solubilization and acidogenesis. In efforts to improve the treatment process, this study examined the effects of temperature on solubilization and acidogenesis. The solubilization rate of food waste, which was based on suspended solid removal, was 47.5%, 62.2%, 70.0%, 72.7%, 56.1% and 45.9% at 15 °C, 25 °C, 35 °C, 45 °C, 55 °C and 65 °C, respectively. Solubilization rate was accelerated from the middle to late experimental periods under mesophilic (35 °C and 45 °C) conditions. In contrast, overall solubilization rate was significantly lower under thermophilic (55 °C and 65 °C) conditions than under mesophilic conditions, although solubilization occurred rapidly in the early experimental period. The production of biogas was high under mesophilic conditions of 35 °C and 45 °C, at 64.7 and 62.7 mL/g-VS, respectively, while it was scarce under thermophilic conditions. Solubilization of food waste was accelerated under both mesophilic and thermophilic conditions; however, solubilization rate was observed to be particularly high under mesophilic conditions, and a shortening of the hydraulic retention time is expected under thermophilic conditions.     

Evaluation of laboratory-scale in-vessel co-composting of tobacco and apple waste

Efficient composting process requires set of adequate parameters among which physical–chemical properties of the composting substrate play the key-role. Combining different types of biodegradable solid waste it is possible to obtain a substrate eligible to microorganisms in the composting process. In this work the composting of apple and tobacco solid waste mixture (1:7, dry weight) was explored. The aim of the work was to investigate an efficiency of biodegradation of the given mixture and to characterize incurred raw compost. Composting was conducted in 24 L thermally insulated column reactor at airflow rate of 1.1 L min^?1. During 22 days several parameters were closely monitored: temperature and mass of the substrate, volatile solids content, C/N ratio and pH-value of the mixture and oxygen consumption. The composting of the apple and tobacco waste resulted with high degradation of the volatile solids (53.1%). During the experiment 1.76 kg of oxygen was consumed and the C/N ratio of the product was 11.6. The obtained temperature curve was almost a “mirror image” of the oxygen concentration curve while the peak values of the temperature were occurred 9.5 h after the peak oxygen consumption.     

Valorisation of used cooking oil sludge by codigestion with swine manure

The addition of lipid wastes to the digestion of swine manure was studied as a means of increasing biogas production. Lipid waste was obtained from a biodiesel plant where used cooking oil is the feedstock. Digestion of this co-substrate was proposed as a way of valorising residual streams from the process of biodiesel production and to integrate the digestion process into the biorefinery concept.Batch digestion tests were performed at different co-digesting proportions obtaining as a result an increase in biogas production with the increase in the amount of co-substrate added to the mixture. Semi-continuous digestion was studied at a 7% (w/w) mass fraction of total solids. Co-digestion was successful at a hydraulic retention time (HRT) of 50 d but a decrease to 30 d resulted in a decrease in specific gas production and accumulation of volatile and long chain fatty acids. The CH₄ yield obtained was 326 ± 46 l/kg VS_feed at an HRT of 50 d, while this value was reduced to 274 ± 43 l/kg VS_feed when evaluated at an HRT of 30 d. However these values were higher than the one obtained under batch conditions (266 ± 40 l/kg VS_feed), thus indicating the need of acclimation to the co-substrate. Despite of operating at low organic loading rate (OLR), measurements from respirometry assays of digestate samples (at an HRT of 50 d) suggested that the effluent could not be directly applied to the soil as fertiliser and might have a negative effect over soil or crops.     

Mesophilic and thermophilic anaerobic biodegradability of water hyacinth pre-treated at 80 °C

Water hyacinth (Eichornia crassipes) is a fast growing aquatic plant which causes environmental problems in continental water bodies. Harvesting and handling this plant becomes an issue, and focus has been put on the research of treatment alternatives. Amongst others, energy production through biomethanation has been proposed. The aim of this study was to assess the anaerobic biodegradability of water hyacinth under mesophilic and thermophilic conditions. The effect of a thermal sludge pre-treatment at 80 °C was also evaluated. To this end, anaerobic biodegradability tests were carried out at 35 °C and 55 °C, with raw and pre-treated water hyacinth. According to the results, the thermal pre-treatment enhanced the solubilisation of water hyacinth (i.e. increase in the soluble to total chemical oxygen demand (COD)) from 4% to 12% after 30 min. However, no significant effect was observed on the methane yields (150–190 L CH₄/kg volatile solids). Initial methane production rates for thermophilic treatments were two fold those of mesophilic ones (6–6.5 L vs. 3–3.5 L CH₄/kg COD·day). Thus, higher methane production rates might be expected from thermophilic reactors working at short retention times. The study of longer low temperature pre-treatments or pre-treatments at elevated temperatures coupled to thermophilic reactors should be considered in the future.     

Two-year performance by evapotranspiration covers for municipal solid waste landfills in northwest Ohio

Evapotranspiration (ET) covers have gained interest as an alternative to conventional covers for the closure of municipal solid waste (MSW) landfills because they are less costly to construct and are expected to have a longer service life. Whereas ET covers have gained acceptance in arid and semi-arid regions (defined by a precipitation (P) to potential evapotranspiration (PET) ratio less than 0.75) by meeting performance standards (e.g. rate of percolation), it remains unclear whether they are suitable for humid regions (P:PET greater than 0.75). The goal of this project is to extend their application to northwest Ohio (P:PET equals 1.29) by designing covers that produce a rate of percolation less than 32 cm yr^?1, the maximum acceptable rate by the Ohio Environmental Protection Agency (OEPA). Test ET covers were constructed in drainage lysimeters (1.52 m diameter, 1.52 m depth) using dredged sediment amended with organic material and consisted of immature (I, plants seeded onto soil) or mature (M, plants transferred from a restored tall-grass prairie) plant mixtures. The water balance for the ET covers was monitored from June 2009 to June 2011, which included measured precipitation and percolation, and estimated soil water storage and evapotranspiration. Precipitation was applied at a rate of 94 cm yr^?1 in the first year and at rate of 69 cm yr^?1 in the second year. During the first year, covers with the M plant mixture produced noticeably less percolation (4 cm) than covers with the I plant mixture (17 cm). However, during the second year, covers with the M plant mixture produced considerably more percolation (10 cm) than covers with the I plant mixture (3 cm). This is likely due to a decrease in the aboveground biomass for the M plant mixture from year 1 (1008 g m^?2) to year 2 (794 g m^?2) and an increase for the I plant mixture from year 1 (644 g m^?2) to year 2 (1314 g m^?2). Over the 2-year period, the mean annual rates of percolation for the covers with the M and I plant mixtures were 7 and 8 cm yr^?1, which are below the OEPA standard. The results suggest the application of ET covers be extended to northwest Ohio and other humid regions.