Biodegradable Volatile Solids Research Articles

Determination of hydraulic retention time using a Piecewise linear regression analysis in the anaerobic digestion process

ABSTRACTHydraulic retention time (HRT) determination in the anaerobic digestion process is an important factor for reactor design. This study aims to investigate the optimal HRT determination method for beef manure and sawdust mixture in the anaerobic digestion (AD). The Piecewise linear regression (PLR) analysis method was applied to compare the HRT with MGTP95 determined by the Modified Gompertz model analysis, cumulative methane yield (Cum.CH4), cumulative biomass removal (Cum.BMR) and biodegraded volatile solids (PLTBVS). Lab-scale (6 cases of lab-scale in the anaerobic digestion) experiments were performed and analyzed by Modified Gompertz model and Piecewise linear regression analysis. The parameters selected for Piecewise linear regression analysis: Cumulative methane yield (PLTCUM.CH4), cumulative biomass removal (PLTCUM.BMR), biodegradable volatile solids (PLTBVS) and time to produce 95% of methane yield potential from Modified Gompertz model (MGTP95). The HRT determined from PLTCum.CH4 varied in between 26.4~34.9 days, and which were 0.4~3.9 days, 8.3~16.0 days, and 9.4~22.2 days shorter than PLTCum.BMR, MGTP95, and PLTBVS, respectively. The daily maximum methane yield rate in the HRT from PLTCum.CH4 showing the highest daily methane yields rate which was less than 2.1%. Therefore, reactor utilization rate and organic loading rate for PLTCum. BMR, MGTP95, and PLTBVS were 94.4~99.4%, 78.0~84.4%, and 70.9~83.9%, respectively, lower than PLTCum.CH4. Conclusively, the HRT from PLTCum.CH4 is the most appropriate method to maximize the organic loading rate relative to other determination methods.

Spent coffee ground as a new bulking agent for accelerated biodrying of dewatered sludge

The feasibility of using spent coffee ground (SCG) as a new bulking agent for biodrying of dewatered sludge (DS) was investigated in comparison with two other frequently-used bulking agents, air-dried sludge (AS) and sawdust (SD). Results showed that the moisture contents (MC) of 16-day DS biodrying with AS (Trial A), SCG (Trial B) and SD (Trial C) decreased from 70.14 wt%, 68.25 wt% and 71.63 wt% to 59.12 wt%, 41.35 wt% and 57.69 wt%, respectively. In case of Trial B, the MC rapidly decreased to 46.16 wt% with the highest water removal (70.87%) within 8 days because of the longest high-temperature period (5.8 days). Further studies indicated that the abundant biodegradable volatile solids (BVS) and high dissolved organic matter (DOM) contents in SCG were the main driving forces for water removal. According to pyrosequencing data, Firmicutes, most of which were recognized as thermophiles, was rapidly enriched on Day 8 and became the dominant phylum in Trial B. Four thermophilic genera, Bacillus, Ureibacillus, Geobacillus and Thermobifida, which can produce thermostable hydrolytic extracellular enzymes, were the most abundant in Trial B, indicating that these thermophilic bacteria evolved during the long high-temperature period enhanced the biodegradation of BVS in SCG. The 8-day biodried product of Trial B was demonstrated to be an excellent solid fuel with low heating value (LHV) of 9284 kJ kg−1, which was 2.1 and 1.8 times those of biodried products with AS and SD, respectively. Thus SCG was found to be an excellent bulking agent accelerating DS biodrying and producing a solid fuel with a high calorific value.

Predicting manure volatile solid output of lactating dairy cows

Organic matter (OM) in livestock manure consisting of biodegradable and nonbiodegradable fractions is known as volatile solids (VS). According to the Intergovernmental Panel on Climate Change (IPCC) Tier 2 guidelines, methane produced by stored manure is determined based on VS. However, only biodegradable OM generates methane production. Therefore, estimates of biodegradable VS (dVS; dVS = VS - lignin) would yield better estimates of methane emissions from manure. The objective of the study was to develop mathematical models for estimating VS and dVS outputs of lactating dairy cows. Dry matter intake, dietary nutrient contents, milk yield and composition, body weight, and days in milk were used as potential predictor variables. Multicollinearity, model simplicity, and random study effects were taken into account during model development that used 857 VS and dVS measurements made on individual cows (kg/cow per day) from 43 metabolic trials conducted at the USDA Energy and Metabolism laboratory in Beltsville, Maryland. The new models and the IPCC Tier 2 model were evaluated with an independent data set including 209 VS and dVS measurements (kg/cow per day) from 2 metabolic trials conducted at the University of California, Davis. Organic matter intake (kg/d) and dietary crude protein and neutral detergent fiber contents (% of dry matter) were significantly associated with VS. A new model including these variables fitted best to data. When evaluated with independent data, the new model had a root mean squared prediction error as a percentage of average observed value (RMSPE) of 12.5%. Mean and slope biases were negligible at <1% of total prediction bias. When energy digestibility of the diet was assumed to be 67%, the IPCC Tier 2 model had a RMSPE of 13.7% and a notable mean bias for VS to be overpredicted by 0.4 kg/cow per day. A separate model including OM intake as well as dietary crude protein and neutral detergent fiber contents as predictor variables fitted best to dVS data and performed well on independent data (RMSPE = 12.7%). The Cornell Net Carbohydrate and Protein System model relying on fat-corrected milk yield and body weight more successfully predicted dry matter intake (DMI; RMSPE = 14.1%) than the simplified (RMSPE = 16.9%) and comprehensive (RMSPE = 23.4%) models to predict DMI in IPCC Tier 2 methodology. New models and the IPCC Tier 2 model using DMI from the Cornell Net Carbohydrate and Protein System model predicted VS (RMSPE = 17.7-19.4%) and dVS (RMSPE = 20%) well with small systematic bias (<10% of total bias). The present study offers empirical models that can accurately predict VS and dVS of dairy cows using routinely available data in dairy farms and thereby assist in efficiently determining methane emissions from stored manure.

Pilot scale high solids anaerobic digestion of steam autoclaved municipal solid waste (MSW) pulp

Steam autoclaving is an efficient method for the separation and near complete recovery of organics from MSW. The material produced by the autoclave contains a high concentration of solubilized food waste absorbed onto a lignocellulosic matrix. Reported here is the operation of a 1500 gal (5677 L) high solids anaerobic digester to digest this feedstock. Total solids (TS) reductions were high, 56%, and volatile solids (VS) and biodegradable volatile solids (BVS) reductions were 63 and 79%, respectively. Gas yields were also high, producing 248 L CH4/kg VS fed or 393 L CH4/kg VS destroyed at a methane content of 60%. Unique design elements such as hydraulic conveyance of material, in situ classification, and in-place buffering to maintain pH stability were tested and confirmed. The digestate passed all criteria for land application of biosolids in the US, but exceeded the EU limits for Cu, Ni, and Zn.

Evaluation of Mixing Effects on Solid-state Anaerobic Digestion Performance of Dairy Manure and Sawdust Bedding Mixtures

The influence of mixing on biogas production and organic material removal performance of solid state anaerobic digestion of dairy manure and sawdust bedding mixtures was evaluated using 22 L volume lab-scale digesters. After 45 days of anaerobic digestion at 37°C, cumulative methane yield of unmixed test unit (73.1 N·mL/g-VS) was almost 1.3 times of that of mixed one (56.3 N·mL/g-VS). The biodegradable volatile solids removal rate of unmixed test unit was 67%, which was almost 28% greater than mixed one. Our results reveal that unmixed condition is better than mixed one in terms of biogas production and organic material reduction.

하수슬러지 케이크 열수분해 탈리액의 혐기성 분해 특성

This study was performed to evaluate the feasibility of anaerobic pretreatment for the leachate solubilized from thermal hydrolysis of sewage sludge cake. Overall process for the treatment of sludge cake consists of thermal hydrolysis, crystallization of magnesium, ammonium, and phosphate (MAP) for the leachate and anaerobic digestion of supernatant from MAP crystallization. The experimental evidence showed that the optimum ratio of Mg : P for the struvite crystallization of leachate solubilized from thermal hydrolysis of sludge cake was 1.5 to 1.0 as weight basis at the pH of 9.5. With this operational condition, the removal efficiencies of ammonia nitrogen and phosphorous achieved 50% and 97%, respectively. The mesophilic batch test showed that the ultimate biodegradability of the supernatant from MAP crystallization reached 63% at S/I ratio of 0.5. The readily biodegradable fraction of 90% (S1) of the MAP supernatant BVS (Biodegradable Volatile Solids, S0) degraded with k1 of 0.207 day for the initial 17 days where as the rest slowly biodegradable fraction (S2) of 10% of BVS degraded with k2 of 0.02 day for the rest of the operational period. Semi-Continuously Fed and Mixed Reactor (SCFMR) was chosen as one of the best candidates to treat the MAP supernatant because of its total solids content over 6%. Maximum average biogas production rates reached 0.45 v/v-d and TVS removal efficiency of 37~41% was achieved at an hydraulic retention time (HRT) of 20 days and its corresponding organic loading rate (OLR) of 1.43 g VS/L-d.

Importance of Initial Moisture Content and Bulking Agent for Biodrying Sewage Sludge

For the final disposal and reutilization of organic wastes, reduction of moisture content (MC) is essential because water contained in the organic waste generates leachate, lowers energy content, and indirectly causes odor problems. MC also directly determines the drying cost. In this study, MC of sewage sludge was effectively decreased by applying a biodrying process in which the microbial metabolic heat evaporated the water. By controlling the initial MC through air drying and its subsequent biodrying, it was found that 50–70 wt% was the optimal initial MC range for the sludge biodrying process. In this range, 33.7–47.1% of the initial water was removed by consuming 12.3–21.2% of the volatile solids (VS) initially contained in the sludge during 10 days of biodrying. When treating the same amount of raw sludge, air-dried and biodried sludge showed better performance as bulking agents than rubber and sawdust. About 55.1% of the initial water was removed from the mixture with air-dried sludge by consuming 23.8% of the initial VS and 38.3% of the initial water was lost from the sludge mixture with biodried sludge by consuming 14.0% of the initial VS during 12.1 days of biodrying. When rubber and sawdust were used as the bulking agents, only 18.2 and 16.5% of the initial water was removed, respectively. It was thought that the easily biodegradable VS contained in the air-dried sludge produced more heat and consequently removed more water. Although air-dried sludge showed better performance than biodried sludge, the use of biodried sludge as a bulking agent was thought to be more practical than air-dried sludge because biodried sludge can be used as bulking agent for the next round of biodrying in a repeated operation of the biodrying reactor. After biodrying, the lower heating value of the biodried sludge (51.5 wt% of MC) was 14.644 MJ kg−1, which was much higher than that of the original wet sludge.

Optimization of methane output for an anaerobic waste digester

In response to the need for renewable energy resources, the replacement of fuel gas with methane produced from the anaerobic digestion of sewerage, agricultural and municipal solid wastes is considered. The utilization of methane for power generation offsets the energy requirements of the digester facility. We discuss the optimization of methane output for a model digester. The model uses Monod based kinetics of methane fermentation and does not include spatial effects. The model assumes that the solid waste acts as a substrate for acid forming bacteria which produce volatile fatty acids, which is converted to methane by a second type of bacteria. It is found that the initial concentrations of the two bacteria and biodegradable volatile solids that maximize the total methane output are independent of the temperature. However, the optimal hydraulic residence time and initial concentration of volatile fatty acids are temperature dependent. This suggests that flow rates should be adjusted, depending on the temperature, to maximize methane output. References D. T. Hill, Simplified Monod Kinetics of Methane Fermentation of Animal Wastes, Agricultural Wastes, 5:1–16 (1983). doi:10.1016/0141-4607(83)90009-4 D. T. Hill, E. W. Tollner and R. D. Holmberg, The Kinetics of Inhibition in Methane Fermentation of Swine Manure, Agricultural Wastes, 5:105–123 (1983). doi:10.1016/0141-4607(83)90089-6 D. T. Hill, Design Parameters and Operating Characteristics of Animal Waste Anaerobic Digestion Systems–-Swine and Poultry, Agricultural Wastes, 5:157–178 (1983). doi:10.1016/0141-4607(83)90081-1 Y. R. Chen and A. G. Hashimoto, The Kinetics of Methane Fermentation, Biotechnology and Bioengineering Symposium, 8:269–282 (1978). A. Husain, Mathematical Models of the Kinetics of Anaerobic Digestion, Biomass and Bioenergy, 14:561–571 (1998). doi:10.1016/S0961-9534(97)10047-2 D. J. Batstone, J. Keller, I. Angelidaki, S. V. Kalyuzzhni, S. G. Pavlostathis, A. Rozzi, W. T. M. Sanders, H. Siegrist and V. A Vavilin, Anaerobic Digestion Model No. 1 (ADM1), Water Science and Technology, 45:65–73 (2002). http://www.iwaponline.com/wst/04510/wst045100065.htm O. Levenspiel, Chemical Reaction Engineering, 3rd Ed., Wiley, New York (1999). J. F. Andrews, A Mathematical Model for the Continuous Culture of Microorganisms Utilizing Inhibitory Substrates, Biotechnology and Bioengineering, 10:707–723 (1968). doi:10.1002/bit.260100602 K. J. Beers, Numerical Methods for Chemical Engineering: Applications in Matlab, Cambridge, New York, USA (2007).

Novel bioevaporation process for the zero-discharge treatment of highly concentrated organic wastewater

A novel process termed as bioevaporation was established to completely evaporate wastewater by metabolic heat released from the aerobic microbial degradation of the organic matters contained in the highly concentrated organic wastewater itself. By adding the glucose solution and ground food waste (FW) into the biodried sludge bed, the activity of the microorganisms in the biodried sludge was stimulated and the water in the glucose solution and FW was evaporated. As the biodegradable volatile solids (BVS) concentration in wastewater increased, more heat was produced and the water removal ratio increased. When the volatile solids (VS) concentrations of both glucose and ground FW were 120 g L−1, 101.7% and 104.3% of the added water was removed, respectively, by completely consuming the glucose and FW BVS. Therefore, the complete removal of water and biodegradable organic contents was achieved simultaneously in the bioevaporation process, which accomplished zero-discharge treatment of highly concentrated organic wastewater.

Energy Balance of a Biodrying Process for Organic Wastes of High Moisture Content: A Review

Biodrying is a technology that makes use of bioenergy from organic waste with high water content to remove moisture and improve the utilization value and treatability of the waste. The essential feature of biodrying is the utilization of thermal energy, generated by aeration degradation of organic matter in waste, to evaporate water, thus achieving self-drying. The effectiveness of biodrying depends on the energy balance of the system, in which simultaneous heat and mass transfer are driven by biogenerated heat. We have focused on improving drying efficiency of these wastes and in this article we review the energy balance characteristics of the biodrying process. This encompasses the framework of biodrying energy balance analysis methodology, modeling expressions for energy release and utilization, and identification of the main model parameters and the main factors that affect the energy balance of the process. Finally, this review investigates possible ways to improve the efficiency of energy utilization. The heat source of the biodrying process can be calculated by combining a first-order biodegradation kinetics model of organic components with combustion heat (H c ) of biodegradable volatile solids (BVS). Heat consumption is mainly described by the enthalpy difference of the ventilation air flow through the system. The biodegradation rate and ventilation mode—which includes the ventilation rate, ventilation time, and ventilation distribution—all influence the energy balance of the biodrying system. There are some possible alternatives that can be considered in order to optimize the energy balance and improve the drying efficiency, such asoptimizing organic biodegradation, improving material physical structures, utilizing sensible heat in exhaust gas in biodrying, and utilization of external low-grade heat sources, such as solar and industrial waste heat, to assist drying.

Dapting Dynamic Mathematical Models to a Pilot Anaerobic Digestion Reactor

A dynamic model has been adapted to a pilot anaerobic reactor fed diary manure. Both steady-state data from online sensors and laboratory analysis and dynamic operational data from online sensors are used in the model adaptation. The model is based on material balances, and comprises four state variables, namely biodegradable volatile solids, volatile fatty acids, acid generating microbes (acidogens), and methane generating microbes (methanogens). The model can predict the methane gas ow produced in the reactor. The model may be used for optimal reactor design and operation, state-estimation and control. Also, a dynamic model for the reactor temperature based on energy balance of the liquid in the reactor is adapted. This model may be used for optimization and control when energy and economy are taken into account.

Biodegradation potential of bulking agents used in sludge bio-drying and their contribution to bio-generated heat

Straw and sawdust are commonly used bulking agents in sludge composting or bio-drying. It is important to determine if they contribute to the biodegradable volatile solids pool. A sludge bio-drying process was performed in this study using straw, sawdust and their combination as the bulking agents. The results revealed that straw has substantial biodegradation potential in the aerobic process and sawdust has poor capacity to be degraded. The temperature profile and bio-drying efficiency were highest in the trial that straw was added, as indicated by a moisture removal ratio and VS loss ratio of 62.3 and 31.0%, respectively. In separate aerobic incubation tests, straw obtained the highest oxygen uptake rate (OUR) of 2.14 and 4.75 mg O 2 g −1VS h −1 at 35 °C and 50 °C, respectively, while the highest OUR values of sludge were 12.1 and 5.68 mg O 2 g −1VS h −1 at 35 °C and 50 °C and those of sawdust were 0.286 and 0.332 mg O 2 g −1VS h −1, respectively. The distribution of biochemical fractions revealed that soluble fractions in hot water and hot neutral detergent were the main substrates directly attacked by microorganisms, which accounted for the initial OUR peak. The cellulose-like fraction in straw was transformed to soluble fractions, resulting in an increased duration of aerobic respiration. Based on the potential VS degradation rate, no bio-generated heat was contributed by sawdust, while that contribution by straw was about 41.7% and the ratio of sludge/straw was 5:1 (w/w, wet basis).

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.

Simulation of substrate degradation in composting of sewage sludge

To simulate the substrate degradation kinetics of the composting process, this paper develops a mathematical model with a first-order reaction assumption and heat/mass balance equations. A pilot-scale composting test with a mixture of sewage sludge and wheat straw was conducted in an insulated reactor. The BVS (biodegradable volatile solids) degradation process, matrix mass, MC (moisture content), DM (dry matter) and VS (volatile solid) were simulated numerically by the model and experimental data. The numerical simulation offered a method for simulating k (the first-order rate constant) and estimating k 20 (the first-order rate constant at 20 °C). After comparison with experimental values, the relative error of the simulation value of the mass of the compost at maturity was 0.22%, MC 2.9%, DM 4.9% and VS 5.2%, which mean that the simulation is a good fit. The k of sewage sludge was simulated, and k 20, k 20s (first-order rate coefficient of slow fraction of BVS at 20 °C) of the sewage sludge were estimated as 0.082 and 0.015 d −1, respectively.

Leaching-bed reactor for producing stabilised plant growing media from dairy manure

Recovering dairy manure fibre for use as a higher value plant growing media offers an economically attractive manure management alternative. Market realisation can occur only if physical properties are altered to be more in line with the peat moss industry standard as, and pathogens along with indices for pathogen growth, such as nutrients and readily biodegradable volatile solids, are removed. However, available technologies for reducing pathogen counts, such as composting, require considerable reaction time and energy inputs. In this study a two-step leaching-bed system was developed to process raw fibrous dairy solids to produce a fibre product with higher quality that could readily be used as plant growing media. The effects of leaching temperature and flow rate were studied at 100 litre reactor scale. The leaching process was also optimised to reduce water usage and improve process efficiency. The optimal operating conditions obtained include (1) a leaching temperature of 50 °C at a leaching rate of 4 l h −1 per kg dry solids using a 50/50 ratio of recycled and fresh water; (2) first step washing time of 1 h with recycled water, and second step washing time of 3 h with fresh water. Under the optimal conditions, indicator coliform bacteria were reduced by 1.6 log units, and the physical properties of the fibre product were positively altered to be similar to those of peat moss. Subsequent growth trials demonstrated the potential of this process and the performance of the developed product for use as a plant-growing medium.

Preventive control of odor emissions through manipulation of operational parameters during the active phase of composting

Better understanding of the effects of key operational parameters or environmental factors on odor emission is of critical importance for minimizing the generation of composting odors. A series of laboratory experiments was conducted to examine the effects of various operating conditions on odor emissions. The results revealed that airflow rates that were too high or too low could result in higher total odor emissions. An optimal flowrate for odor control would be approximately 0.6 L/min.kg dry matter with intermittent aeration and a duty cycle of 33%. Temperature setpoint at 60°C appeared to be a turning point for odor emission. Below this point, odor emissions increased with increasing temperature setpoint; conversely, odor emissions decreased with increasing temperature setpoint above this point. With regard to the composting material properties, odor emissions were greatly affected by the initial moisture content of feedstock. Both peak odor concentration and emission rate generally increased with higher initial moisture content. Odor emission was significant only at moisture levels higher than 65%. An initial moisture level below 45% is not recommended due to concern with the resulting lower degree of biodegradation. Biodegradable volatile solids content (BVS) of feedstock had pronounced effect on odor emissions. Peak odor concentration and emission rate increased dramatically as BVS increased from 45% to 65%, thus, total odor emission increased exponentially with BVS.

Mesophilic and Thermophilic Anaerobic Digestion of Municipal Sludge and Fat, Oil, and Grease

The anaerobic biodegradability of municipal primary sludge, thickened waste activated sludge (TWAS), and fat, oil, and grease (FOG) was assessed using semi-continuous-feed, laboratory-scale anaerobic digesters and compared with the ultimate degradability obtained from 120-day batch digestion at 35 degrees C. In run 1, combined primary sludge and TWAS (40/60%, volatile solids [VS] basis) were fed to digesters operated at mesophilic (35 degrees C) and thermophilic (52 degrees C) temperatures at loading rates of 0.99 and 1.46 g-VS/L x d for primary sludge and TWAS, respectively, and a hydraulic retention time (HRT) of 12 days. The volatile solids destruction values were 25.3 and 30.7% (69 and 83% biodegradable volatile solids destruction) at 35 degrees C and 52 degrees C, respectively. The methane (CH4) yields were 159 and 197 mL at the standard temperature and pressure (STP) conditions of 0 degree C and 1 atm/g-VS added or 632 and 642 mL @ STP/g-VS destroyed at 35 degrees C and 52 degrees C, respectively. In run 2, a mix of primary sludge, TWAS, and FOG (21/31/48%, volatile solids basis) was fed to an acid digester operated at a 1-day HRT, at 35 degrees C, and a loading rate of 52.5 g-VS/L x d. The acid-reactor effluent was fed to two parallel methane-phase reactors operated at an HRT of 12 days and maintained at 35 degrees C and 52 degrees C, respectively. After an initial period of 20 days with near-zero gas production in the acid reactor, biogas production increased and stabilized to approximately 2 mL CH4 @ STP/g-VS added, corresponding to a volatile solids destruction of 0.4%. The acid-phase reactor achieved a 43% decrease in nonsaturated fat and a 16, 26, and 20% increase of soluble COD, volatile fatty acids, and ammonia, respectively. The methane-phase volatile solids destruction values in run 2 were 45 and 51% (85 and 97% biodegradable volatile solids destruction) at 35 degrees C and 52 degrees C, respectively. The methane yields for the methane-phase reactors were 473 and 551 mL @ STP/g-VS added, which is approximately 3 times larger compared with run 1, or 1040 and 1083 mL @ STP/g-VS destroyed, at 35 degrees C and 52 degrees C, respectively. The results indicate that, when co-digesting municipal sludge and FOG, a large FOG organic load fraction could have a profound effect on the methane gas yield.

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.

DETERMINATION OF BIO-OXIDATION ENERGY RELEASED BY THERMOPHILES IN SECONDARY AND MIXED BIO-SOLIDS

Reaction constants and heat release during thermophilic aerobic digestion of typical secondary and mixed sludges were determined. Maximum reaction constant, determined by measuring the amounts of biodegradable volatile suspended solids (bVSS) removed, occurred in the temperature range of 55-60 oC. Measurement of heat releases were carried out, at the same range, by the aid of a temperature programmable logic controller (PLC). An amount equal to 16.2 KJ/g-cell was measured for secondary sludge digestion. The maximum amount of kinetic constant for secondary sludge (0.45 d -1 ) was observed at temperature of 55 oC. Addition of primary sludge to secondary sludges decreased kinetic constants due to stimulate of biomass growth. Cell growth in mixed sludges provided an extra heat equal to 6.7 KJ/g-cell generated. However, it decreased the overall rate of heat release compared to the rate in the secondary sludge digestion.

Windrow composting of municipal biosolids in a cold climate

This work investigated the technical feasibility of composting digested municipal biosolids using outdoor windrows in an extremely cold climate. Common belief suggested this was not possible because of low ambient temperatures and low biodegradable volatile solids content of the biosolids. Four trials were conducted: (i) batch started in late fall; (ii) batch started in winter; (iii) batch mixed and stored over winter, then started in spring; and (iv) batch mixed with leaves to improve heat generation. All recipes used 2.5 parts wood chips to 1 part biosolids (by volume). All four trials easily met the thermophilic temperatures required by the pathogen reduction criteria. Trial 2 (batch started in winter), however, did not achieve the thermophilic period until the following summer. The results suggest provisions be made for storage of material over winter. This means a 32% increase in area and a doubling of the system-turning capacity are needed. Practical observations included significant heat loss during turning and mixing, snowfall impact on oxygen supply and pile moisture content, wind direction impact on pile temperature profiles, and the impact of large frozen zones that may affect pathogen inactivation efficacy. Key words: municipal sludge, biosolids, windrow composting, pathogens, low temperature, winter.