Physical modelling of the composting environment: a review. Part 1: Reactor systems

In this paper, laboratory- and pilot-scale reactors used for investigation of the composting process are described and their characteristics and application reviewed. Reactor types were categorised by the present authors as fixed-temperature, self-heating, controlled temperature difference and controlled heat flux, depending upon the means of management of heat flux through vessel walls. The review indicated that fixed-temperature reactors have significant applications in studying reaction rates and other phenomena, but may self-heat to higher temperatures during the process. Self-heating laboratory-scale reactors, although inexpensive and uncomplicated, were shown to typically suffer from disproportionately large losses through the walls, even with substantial insulation present. At pilot scale, however, even moderately insulated self-heating reactors are able to reproduce wall losses similar to those reported for full-scale systems, and a simple technique for estimation of insulation requirements for self-heating reactors is presented. In contrast, controlled temperature difference and controlled heat flux laboratory reactors can provide spatial temperature differentials similar to those in full-scale systems, and can simulate full-scale wall losses. Surface area to volume ratios, a significant factor in terms of heat loss through vessel walls, were estimated by the present authors at 5.0-88.0m(2)/m(3) for experimental composting reactors and 0.4-3.8m(2)/m(3) for full-scale systems. Non-thermodynamic factors such as compression, sidewall airflow effects, channelling and mixing may affect simulation performance and are discussed. Further work to investigate wall effects in composting reactors, to obtain more data on horizontal temperature profiles and rates of biological heat production, to incorporate compressive effects into experimental reactors and to investigate experimental systems employing natural ventilation is suggested.     

Laboratory studies of the remediation of polycyclic aromatic hydrocarbon contaminated soil by in-vessel composting

The biodegradation of 16 polycyclic aromatic hydrocarbons (PAHs), listed as priority pollutants by the USEPA, present in a coal-tar-contaminated soil from a former manufactured gas plant site was investigated using laboratory-scale in-vessel composting reactors to determine the suitability of this approach as a bioremediation technology. Preliminary investigations were conducted over 16 weeks to determine the optimum soil composting temperature (38, 55 and 70 degrees C). Three tests were performed; firstly, soil was composted with green-waste, with a moisture content of 60%. Secondly, microbial activity was HgCl2-inhibited in the soil green-waste mixture with a moisture content of 60%, to evaluate abiotic losses, while in the third experiment only soil was incubated at the three different temperatures. PAHs and microbial populations were monitored. PAHs were lost from all treatments with 38 degrees C being the optimum temperature for both PAH removal and microbial activity. Calculated activation energy values (E(a)) for total PAHs suggested that the main loss mechanism in the soil-green waste reactors was biological, whereas in the soil reactors it was chemical. Total PAH losses in the soil-green waste composting mixtures were by pseudo-first order kinetics at 38 degrees C (k = 0.013 day(-1), R2 = 0.95), 55 degrees C (k = 0.010 day(-1), R2 = 0.76) and at 70 degrees C (k = 0.009 day(-1), R2 = 0.73).     

Bioconversion of industrial wastes: paint sludge from automotive manufacturing

The compostability of water-based paint sludge originating from the automotive industry was investigated. Six reactors were operated. Wastewater treatment sludge from the same industry was used as additional substrate, and corncob was used as a bulking agent. The level of paint sludge within the compost mixtures varied between 55 and 85%. All reactors yielded a temperature increase up to thermophilic phase levels (>?40 °C) for a minimum of 5 days, and organic matter and C/N losses were observed. BTEX concentrations decreased during composting. Nickel and tin levels in the final product exceeded the legal compost limits. The calorific value of the compost mixtures increased from 9532 to 18774 kJ/kg at the end of the composting process. It was seen that the process applied in this study can be utilized as a biodrying step before the usage of paint sludge at cement kilns as additional fuel.     

Feasibility study of a passive aeration reactor equipped with vertical pipes for compost stabilization of cow manure.

Pilot-scale composting was carried out with cow manure to evaluate the performances of two passive aeration systems: a conventional passive aeration system equipped with horizontal pipes and an unusual passive aeration method based on air delivery by means of vertical pipes. The effects of both types of passive aeration apparatus were investigated in order to determine the degree of composting rate by continuously monitoring temperature, moisture content, organic matter, electrical conductivity, pH and C/N ratio in the piles. Temperatures in the range of thermophily (55-65 degrees C) were reached in all runs within 1-2 days then lasting for about 1 week, a span long enough for pathogen abatement. Results suggest that passive aeration carried out by vertical pipes is more effective for air delivery into compost piles than conventional passive aeration of air adduction with horizontal pipes. The variation in the number of vertical pipes was revealed to be an important parameter for the control of composting rate and temperature. Composting rates estimated from the heat balance equation were substantially in agreement with those computed through the conversion ratio of total organic matter decrement. The conversion ratios and composting rates obtained in this study using passive aeration with vertical pipes were well aligned with those found using forced air delivery systems.     

Mathematical modelling of the composting process: a review

In this paper mathematical models of the composting process are examined and their performance evaluated. Mathematical models of the composting process have been derived from both energy and mass balance considerations, with solutions typically derived in time, and in some cases, spatially. Both lumped and distributed parameter models have been reported, with lumped parameter models presently predominating in the literature. Biological energy production functions within the models included first-order, Monod-type or empirical expressions, and these have predicted volatile solids degradation, oxygen consumption or carbon dioxide production, with heat generation derived using heat quotient factors. Rate coefficient correction functions for temperature, moisture, oxygen and/or free air space have been incorporated in a number of the first-order and Monod-type expressions. The most successful models in predicting temperature profiles were those which incorporated either empirical kinetic expressions for volatile solids degradation or CO2 production, or which utilised a first-order model for volatile solids degradation, with empirical corrections for temperature and moisture variations. Models incorporating Monod-type kinetic expressions were less successful. No models were able to predict maximum, average and peak temperatures to within criteria of 5, 2 and 2 degrees C, respectively, or to predict the times to reach peak temperatures to within 8 h. Limitations included the modelling of forced aeration systems only and the generation of temperature validation data for relatively short time periods in relation to those used in full-scale composting practice. Moisture and solids profiles were well predicted by two models, but oxygen and carbon dioxide profiles were generally poorly modelled. Further research to obtain more extensive substrate degradation data, develop improved first-order biological heat production models, investigate mechanistically-based moisture correction factors, explore the role of moisture tension, investigate model performance over thermophilic composting time periods, provide more information on model sensitivity and incorporate natural ventilation aeration expressions into composting process models, is suggested.     

Utilization of solar energy in sewage sludge composting: Fertilizer effect and application

Three reactors, ordinary, greenhouse, and solar, were constructed and tested to compare their impacts on the composting of municipal sewage sludge. Greenhouse and solar reactors were designed to evaluate the use of solar energy in sludge composting, including their effects on temperature and compost quality. After 40 days of composting, it was found that the solar reactor could provide more stable heat for the composting process. The average temperature of the solar reactor was higher than that of the other two systems, and only the solar reactor could maintain the temperature above 55 °C for more than 3 days. Composting with the solar reactor resulted in 31.3% decrease in the total organic carbon, increased the germination index to 91%, decreased the total nitrogen loss, and produced a good effect on pot experiments.     

Predicting biodegradable volatile solids degradation profiles in the composting process

This paper presents a new method for the prediction of the pattern of biodegradable volatile solids (BVS) degradation in the composting process. The procedure is based on a re-arrangement of the heat balance around a composting system to numerically solve for the rate of BVS carbon (BVS-C) disappearance. Input data for the model was obtained from composting experiments conducted in a laboratory-scale, constant temperature difference (CTD) reactor simulating a section of an aerated static pile, and using a simulated feedstock comprising ostrich feed, shredded paper, finished compost and woodchips. These experiments also provided validation data in the form of exit gas CO(2) carbon (CO(2)-C) profiles. The model successfully predicted the generic shape of experimental substrate degradation profiles obtained from CO(2) measurements, but under the conditions and assumptions of the experiment, the profiles were quantitatively different, giving an over-estimate of BVS-C. Both measured CO(2)-C and predicted BVS-C profiles were moderately to well fitted by a single exponential function, with replicated rate coefficient values of 0.08 and 0.09 d(-1), and 0.06 and 0.07 d(-1), respectively. In order to explore the underlying shape of the profiles, measured and predicted data at varying temperature were corrected to a constant temperature of 40 degrees C, using the temperature correction function of Rosso et al. [Rosso, L., Lobry, J.R., and Flandrois, J.P., 1993. An unexpected correlation between cardinal temperatures of microbial growth highlighted by a new model. Journal of Theoretical Biology, 162, 447-463], with cardinal temperatures of 5, 59 and 85 degrees C. Multi-phase profiles were generated for both the measured CO(2)-C and the predicted BVS-C data in this case. However, when alternative cardinal temperatures of 5, 55 and 80 degrees C, or 5, 50 and 80 degrees C, were used, the predicted profiles assumed an exponential shape, and excellent fits were obtained using a double exponential function. These findings support the argument that a substrate degradation curve generated under laboratory conditions at 40 degrees C, would, given correct cardinal temperatures, generate a correct substrate degradation profile under varying temperature conditions and that this in turn would enable an accurate and precise prediction of the temperature profile, using a heat and mass balance approach. In order to realise this prospect, it is proposed that further work to obtain experimental data under completely mixed conditions, more accurately estimate the overall heat transfer coefficient and obtain correct values for the cardinal temperatures used in the temperature correction function, is required.     

Prediction of temperature and thermal inertia effect in the maturation stage and stockpiling of a large composting mass

A macroscopic non-steady state energy balance was developed and solved for a composting pile of source-selected organic fraction of municipal solid waste during the maturation stage (13,500 kg of compost). Simulated temperature profiles correlated well with temperature experimental data (ranging from 50 to 70 degrees C) obtained during the maturation process for more than 50 days at full scale. Thermal inertia effect usually found in composting plants and associated to the stockpiling of large composting masses could be predicted by means of this simplified energy balance, which takes into account terms of convective, conductive and radiation heat dissipation. Heat losses in a large composting mass are not significant due to the similar temperatures found at the surroundings and at the surface of the pile (ranging from 15 to 40 degrees C). In contrast, thermophilic temperature in the core of the pile was maintained during the whole maturation process. Heat generation was estimated with the static respiration index, a parameter that is typically used to monitor the biological activity and stability of composting processes. In this study, the static respiration index is presented as a parameter to estimate the metabolic heat that can be generated according to the biodegradable organic matter content of a compost sample, which can be useful in predicting the temperature of the composting process.     

Composting clam processing wastes in a laboratory- and pilot-scale in-vessel system

Waste materials from the clam processing industry (offal, shells) have several special characteristics such as a high salinity level, a high nitrogen content, and a low C/N ratio. The traditional disposal of clam waste through landfilling is facing the challenges of limited land available, increasing tipping fees, and strict environmental and regulatory scrutiny. The aim of this work is to investigate the performance of in-vessel composting as an alternative for landfill application of these materials. Experiments were performed in both laboratory-scale (5L) and pilot-scale (120L) reactors, with woodchips as the bulking agent. In the laboratory-scale composting test, the clam waste and woodchips were mixed in ratios from 1:0.5 to 1:3 (w/w, wet weight). The high ratios resulted in a better temperature performance, a higher electrical conductivity, and a higher ash content than the low-ratio composting. The C/N ratio of the composts was in the range of 9:1-18:1. In the pilot-scale composting test, a 1:1 ratio of clam waste to woodchips was used. The temperature profile during the composting process met the US Environmental Protection Agency sanitary requirement. The final cured compost had a C/N ratio of 14.6, with an ash content of 167.0+/-14.1g/kg dry matter. In addition to the major nutrients (carbon, nitrogen, calcium, magnesium, phosphorus, potassium, sulfur, and sodium), the compost also contained trace amounts of zinc, manganese, copper, and boron, indicating that the material can be used as a good resource for plant nutrients.     

Composting in small laboratory pilots: performance and reproducibility

Small-scale reactors (<10 l) have been employed in composting research, but few attempts have assessed the performance of composting considering the transformations of organic matter. Moreover, composting at small scales is often performed by imposing a fixed temperature, thus creating artificial conditions, and the reproducibility of composting has rarely been reported. The objectives of this study are to design an innovative small-scale composting device safeguarding self-heating to drive the composting process and to assess the performance and reproducibility of composting in small-scale pilots. The experimental setup included six 4-l reactors used for composting a mixture of sewage sludge and green wastes. The performance of the process was assessed by monitoring the temperature, O₂ consumption and CO₂ emissions, and characterising the biochemical evolution of organic matter. A good reproducibility was found for the six replicates with coefficients of variation for all parameters generally lower than 19%. An intense self-heating ensured the existence of a spontaneous thermophilic phase in all reactors. The average loss of total organic matter (TOM) was 46% of the initial content. Compared to the initial mixture, the hot water soluble fraction decreased by 62%, the hemicellulose-like fraction by 68%, the cellulose-like fraction by 50% and the lignin-like fractions by 12% in the final compost. The TOM losses, compost stabilisation and evolution of the biochemical fractions were similar to observed in large reactors or on-site experiments, excluding the lignin degradation, which was less important than in full-scale systems. The reproducibility of the process and the quality of the final compost make it possible to propose the use of this experimental device for research requiring a mass reduction of the initial composted waste mixtures.     

Fuzzy control for the process of heat removal during the composting of agricultural waste

This paper shows the concept and preliminary test of the composting process control with fuzzy logic. The temperature in the compost heap during the process is greater than 80 °C. Because wastes of agricultural origin do not require hygienization, part of the heat can by retrieved, which lowers the temperature in the heap to 55 °C (this is also the optimum temperature for the composting process). The heat retrieved from a compost heap can be used in another place. Therefore, a composting system was developed in which the aeration rate and heat removal rate can be adjusted. There are two goals of this system: maximising the amount of the heat retrieved from the heap and minimising the duration of the composting process. In the preliminary experiments freshly mown grass with 50 % cabbage was used as the compost material. The results show that the fuzzy logic control system functions correctly with respect to calculations. During the study, a median 90 MJ of heat was produced by the bioreactor; the thermophilic phase was shortened to 12 days. There was also a reduction in the emission of ammonia.     

Effect of the mixing ratio on the composting of OFMSW digestate: assessment of compost quality

This study presents the results obtained in compostability tests of organic fraction of municipal solid waste (OFMSW) digestate. The final aim was to obtain mature compost without phytotoxic effects. For the evaluation of the composting process, a novel parameter describing the performance of the composting process, the relative heat generation standardized with the initial volatile solid content (RHGVS₀), was defined and evaluated at laboratory-scale. From these laboratory-scale test, the optimum operational conditions were obtained, a mixing ratio (v/v) of 1:1:0 (bulking agent:digestate:co-substrate) and with 15% of mature compost as inoculum. Subsequently, these optimum operational conditions were applied in the active phase of the composting pilot-scale reactor. The active composting stage took 7 days, subsequently a curing phase of 60 days was carried out at ambient conditions. After 30 days of curing, the mature compost showed a specific oxygen uptake rate (SOUR) of 0.14 mg O₂/g VS·h, a germination index (GI) of 99.63% and a low volatile fatty acids (VFA) concentration (41.3 AcH mg/kg_dm), being indicative of the good compost stability and maturity of the compost. The very good quality of the final compost obtained indicated that the RHGVS₀ accurately describes the performance of the composting process.

     

Characterization and modelling of the heat transfers in a pilot-scale reactor during composting under forced aeration

The paper focused on the modelling of the heat transfers during composting in a pilot-scale reactor under forced aeration. The model took into account the heat production and the transfers by evaporation, convection between material and gas crossing the material, conduction and surface convection between gas and material in bottom and upper parts of the reactor. The model was adjusted thanks to the measurements practised during fifteen composting experiments in which five organic wastes were, each, composted under three constant aeration rates. Heat production was considered proportional to oxygen consumption rate and the enthalpy per mole oxygen consumed was assumed constant. The convective heat transfer coefficients were determined on basis of the continuous measurements of the temperatures of both the lid and the bottom part of the reactor. The model allowed a satisfying prediction of the temperature of the composting material. In most cases, the mean absolute discard between the experimental and the simulated temperatures was inferior to 2.5°C and the peaks of temperature occurred with less than 8h delay. For the half of the experiments the temperature discard between the simulated peak and the experimental one was inferior to 5°C. On basis of the calculation of a stoichiometric production of water through oxidation of the biodegradable organic matter, the simulation of water going out from material as vapour also allowed a rather satisfying prediction of the mass of water in final mixture. The influence of the aeration rate on every type of heat loss was characterized. Finally, the model was used to evaluate the impacts on material temperature caused by the change of the insulation thickness, the ambient temperature, take the lid away, the increase or the decrease of the mass of waste to compost.     

Effects of air flow directions on composting process temperature profile

In this study, chicken manure mixed with carnation wastes was composted by using three different air flow directions: R1-sucking (downward), R2-blowing (upward) and R3-mixed. The aim was to find out the most appropriate air flow direction type for composting to provide more homogenous temperature distribution in the reactors. The efficiency of each aeration method was evaluated by monitoring the evolution of parameters such as temperature, moisture content, CO(2) and O(2) ratio in the material and dry material losses. Aeration of the reactors was managed by radial fans. The results showed that R3 resulted in a more homogenous temperature distribution and high dry material loss throughout the composting process. The most heterogeneous temperature distribution and the lowest dry material loss were obtained in R2.     

Food waste treatment in a community center

For urban community composting centers, the proper selection and use of bulking agent is a key element in not only the cost but also the quality of the finished compost. Besides wood chips (WC) widely used as BA, readily usable cereal residue pellets (CRP) can provide biodegradable carbon and sufficient free air space (FAS) to produce stabilizing temperatures. The objective of the present project was to test at a community center, the effectiveness of CRP in composting food waste (FW). Two recipes were used (CRP with and without WC) to measure: FAS; temperature regimes, and; losses in mass, water, carbon and nitrogen. Both recipes were composted during three consecutive years using a 2 m³ commercial in-vessel composter operated in downtown Montreal (Canada). For all recipes, FAS exceeded 30% for moisture content below 60%, despite yearly variations in FW and BA physical properties. When properly managed by the center operator, both FW and CRP compost mixtures with and without WC developed within 3 days thermophilic temperatures exceeding 50 °C. The loss of total mass, water, carbon and nitrogen was quite variable for both recipes, ranging from 36% to 54%, 42% to 55%, 48% to 65%, and 4% to 55%, respectively. The highest loss in dry mass, water and C was obtained with FW and CRP without WC aerated to maintain mesophilic rather than thermophilic conditions. Although variable, lower nitrogen losses were obtained with CRP and WC as BA, compared to CRP alone, as also observed during previous laboratory trials. Therefore and as BA, CRP can be used alone but nitrogen losses will be minimized by adding WC. Compost stabilization depends on operator vigilance in terms of aeration. The measured fresh compost density of 530-600 kg/m³ indicates that the 2 m³ in-vessel composter can treat 6.5 tons of FW/year if operated during 7 months.     

Influence of aeration rate on nitrogen dynamics during composting

The paper aimed to study the influence of aeration rate on nitrogen dynamics during composting of wastewater sludge with wood chips. Wastewater sludge was sampled at a pig slaughterhouse 24h before each composting experiment, and mixtures were made at the same mass ratio. Six composting experiments were performed in a lab reactor (300 L) under forced aeration. Aeration flow was constant throughout the experiment and aeration rates applied ranged between 1.69 and 16.63 L/h/kg DM of mixture. Material temperature and oxygen consumption were monitored continuously. Nitrogen losses in leachates as organic and total ammoniacal nitrogen, nitrite and nitrate, and losses in exhaust gases as ammonia were measured daily. Concentrations of total carbon and nitrogen i.e., organic nitrogen, total ammoniacal nitrogen, and nitrite and nitrate were measured in the initial substrates and in the composted materials. The results showed that organic nitrogen, which was released as NH4+/NH3 by ammonification, was closely correlated to the ratio of carbon removed from the material to TC/N(org) of the initial substrates. The increase of aeration was responsible for the increase in ammonia emissions and for the decrease in nitrogen losses through leaching. At high aeration rates, losses of nitrogen in leachates and as ammonia in exhaust gases accounted for 90-99% of the nitrogen removed from the material. At low aeration rates, those accounted for 47-85% of the nitrogen removed from the material. The highest concentrations of total ammoniacal nitrogen in composts occurred at the lowest aeration rate. Due to the correlation of ammonification with biodegradation and to the measurements of losses in leachates and in exhaust gases, the pool NH4+/NH3 in the composting material was calculated as a function of time. The nitrification rate was found to be proportional to the mean content of NH4+/NH3 in the material, i.e., initial NH4+/NH3 plus NH4+/NH3 released by ammonification minus losses in leachates and in exhaust gases. The aeration rate was shown to be a main parameter affecting nitrogen dynamics during composting since it controlled the ammonification, the ammonia emission and the nitrification processes.     

Influence of dairy manure addition on the biological and thermal kinetics of composting of greenhouse tomato plant residues

A laboratory-scale bioreactor was used to investigate the influence of dairy manure addition (as an inoculum and a carbon source) on the biological and thermal kinetics of the composting process of tomato plant residues-wood shavings mixture. Urea was added (as a nitrogen source) to correct the initial C:N ratio to 30:1 and the initial moisture content was also adjusted to 60%. The result of this study indicated that manure addition to the tomato residues-wood shavings mixture is a good source of macro and micronutrients required for supporting the composting microorganisms. Manure addition increased the rate of temperature increase and the duration of maximum temperature and reduced the lag and the peak time, all of which resulted in a significant reduction in the retention time. However, thermophilic temperature (> or = 40 degrees Celsius) was only achieved with 30%, 40% and 50% manure addition for 3, 7 and 9h. Total carbon reductions were in the range of 9.4-10.8% and TKN reductions were in the range of 3.4-6.0%. Neither the nitrogen nor the moisture content were limiting factors as the C:N ratio remained in the range of 26:1 to 28:1 and the moisture content remained within the optimum range of 58-61%. The maximum temperature of each mixture correlated with the reduction of total carbon, but carbon availability was a limiting factor in these experiments. In order to attain and sustain a thermophilic phase during the composting process, the addition of a readily available carbon source to the tomato should be investigated and carbon type (carbohydrates, proteins and fats) should be taken into account.     

Effect of substrate feeding frequencies on the methane production and microbial communities of laboratory-scale anaerobic digestion reactors

Even though full-scale digesters have been designed based on laboratory-scale tests, the substrate feeding modes of laboratory-scale tests might be different from those of full-scale digesters. The effect of substrate feeding frequencies on the performance and microbial community of laboratory-scale anaerobic digestion reactors was investigated. Feeding frequencies of twice a day, once a day, and every two days were tested in three 2-L reactors with an organic loading rate of 0.5 g-glucose/L/day under mesophilic condition. According to the results of this study, all the reactors showed similar methane production rates and SCOD removal efficiencies after sufficient time of acclimation, but frequently feeding promoted more stable digestion. Although there was no significant difference in microbial diversities from pyrosequencing analyses, the changes of archaeal community composition were observed. The decrease in feeding frequency appeared to cause shifts from acetoclastic methanogens affiliated with Methanosaeta to H₂-utilizing methanogens. The increase of Methanosaeta at a frequently feeding might contribute to the stability of reactor operation. Since this study uses glucose as the substrate, there is still possibility of different results for more complex substrates, such as sludge, food waste, etc.     

Evaluation of maturity parameters and heavy metal contents in composts made from animal manure

The objective of this study was to evaluate a variety of maturity indices and heavy metal concentrations in the composting process. A full-scale study was performed on animal manures (dairy:beef:swine = 50%:30%:20%, w/w) mixed with sawdust using a self-screwed turner over a time period of 63 days. Several chemical compost characteristics and germination indices were monitored on samples collected weekly at a prototype composting facility. NH(4)-N concentrations decreased during the composting time, on the contrary NO(3)-N concentrations increased sharply after 30 days. As composting proceeded, the humic acids content increased from 1.45% to 2.70% of the organic carbon content, whereas the fulvic acids content decreased gradually. Temperature in the compost pile and concentration of NH(3) averaged 32.4 degrees C and 12.9 ppm at the initial composting phase and 69.5 degrees C and 47.3 ppm at the active bio-oxidation composting phase, respectively. Seed germination and germination index (GI) increased during the composting process, and the compost samples at the end of composting phase had GI values greater than 120. Among the composting processes, there were significant differences (p<0.05) in concentration of Zn, Cu and Pb in the compost. In conclusion, the suggested maturity parameter values and metal concentrations of composted animal manures can provide better understanding of maturation process of animal manure compost.     

The influence of operational conditions in sequencing batch reactors on removal of nitrogen and organics from municipal landfill leachate.

The removal of nitrogen and organics from municipal landfill leachate in sequencing batch reactors (SBR) was investigated in the present study. The influence of hydraulic retention time (HRT), sludge age, manner of leachate dosage (short filling period of SBR and filling during the reaction period), and operational conditions with and without a mixing phase in the SBR cycle was explored. Four series were performed. In each series, the HRT used in the four SBRs was 12, 6, 3 and 2 days, respectively. Series 1 and 2 were characterized by a short leachate filling period, whereas series 3 and 4 were characterized by filling during the 4 h duration of the reaction in the SBR cycle. In series 1-3 SBR reactors worked with mixing and aeration phases, whereas in series 4 they worked only with an aeration phase. The effectiveness of the removal of organics increased with the extension of the HRT of leachate, particularly under operational conditions with the mixing and aeration phases in the SBR cycle. At 12 days HRT, the SBRs with the mixing and aeration phases in the cycle (series 1-3) showed better results than those with only an aeration phase (series 4). However, at 2 days HRT the operational conditions in SBR reactors with leachate filling over the reaction period (series 3 and 4) were more suitable. The highest efficiency of ammonium removal was obtained in series 1 with a short leachate filling period. In this series, at an HRT of 3-12 days, the ammonium concentration in the effluent did not exceed 1 mg NNH4 L(-1). Nitrogen removal proceeded mainly in the aeration phase as a result of ammonium losses and, to a lesser extent, dissimilative nitrate reduction over the mixing phase. The highest percentage of nitrogen removal as a result of ammonium losses was observed in series with a short filling period and long sludge age (series 1) and also in series without a mixing phase and filling over the aeration phase (series 4), whereas the highest nitrogen consumption for biomass production occurred in series 3 with filling during the reaction period and mixing phase of the cycle.