Volatile Solids Degradation Research Articles

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 CO 2 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 °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.

Improving acidogenic performance in anaerobic degradation of solid organic waste using a rotational drum fermentation system

The effects of leachate from a methanogenic process on acidogenic performance of a solid recycle (SR) process by a rotational drum fermentation (RDF) system were evaluated under mesophilic condition and a hydraulic retention time (HRT) of 20 days. Two SR process configurations, SR1 and SR2, were evaluated, using fresh soybean meal or Okara as substrates. An apparent first-order hydrolysis rate constant of 5.0 × 10 −3/d for SR1 at pH values of 4.4 and 14.4 × 10 −3/d for SR2 at pH of 5.0, were obtained. The apparent volatile solids (VS) degradation ratio ranged from 9.6% to 19.4% and total volatile acid (as acetic acid) from 10.8 to 14.9 g/L. Occupying ratios for ionized volatile acid (VA) increased from 30.6% to 63.4% after recycling the leachate to process. However, occupying ratios of acetic acid decreased from 93.3% to 42.0% whereas propionic acid and butyric acid ratios increased in SR2. Integrating the VA production with the hydrolysis rate constants, it is clear that the recirculation of leachate considerably enhanced acidogenic performance of solid recycle process.

Pretreatment of robusta coffee hulls and co-digestion with cow-dung for enhanced anaerobic digestion

The effect of alkali pre-treatment on anaerobic digestion of Robusta coffee hulls with a lignin content of 29% was studied in batch culture at 55˚C. In order to investigate the effect of cow dung in minimising the effect of coffee-based inhibitory compounds, digestion experiments were performed with blends of pre-treated and non-treated coffee hulls and cow dung with 0,5,10,20,30,50,75 and 100% coffee hulls at a total volatile solids concentration of 1.8g/l fermentation volume. Methane production increased with increase in coffee hulls concentration up to 30% for both substrates. The lowest production was observed at 100% non-treated coffee hulls. Methane production was significantly higher (13-164%) for the pre-treated substrates compared to the non treated one. The highest increase was obtained at 100% coffee hulls. Degradation of volatile solids decreased with increase in coffee hulls concentration for both substrates. The extent of degradation of the pre-treated substrate (59-94%) was significantly higher than that obtained for the non-treated substrate (30-91%). The lowest extent of degradation was measured for the non-treated hulls at 100%. It is possible that there was an inhibition of methanogenesis by coffee based compounds with increase in the coffee hulls concentration in the digestion mixture. Tanz. J. Sci Vol. 28(2) 2002: 1-10

Algorithms for calculating methane and nitrous oxide emissions from manure management

Biogenic emissions of methane (CH4) and nitrous oxide (N2O) from animal manure are stimulated by the degradation of volatile solids (VS) which serves as an energy source and a sink for atmospheric oxygen. Algorithms are presented which link carbon and nitrogen turnover in a dynamic prediction of CH4 and N2O emissions during handling and use of liquid manure (slurry). A sub-model for CH4 emissions during storage relates CH4 emissions to VS, temperature and storage time, and estimates the reduction in VS. A second sub-model estimates N2O emissions from field-applied slurry as a function of VS, slurry N and soil water potential, but emissions are estimated using emission factors. The model indicated that daily flushing of slurry from cattle houses would reduce total annual CH4 + N2O emissions by 35% (CO2 eq.), and that cooling of pig slurry in-house would reduce total annual CH4 + N2O emissions by 21% (CO2 eq.). Anaerobic digestion of slurry and organic waste produces CH4 at the expense of VS. Accordingly, the model predicted a 90% reduction of CH4 emissions from outside stores with digested slurry, and a >50% reduction of N2O emissions after spring application of digested as opposed to untreated slurry. The sensitivity of the model towards storage temperature and soil water potential was examined. This study indicates that simple algorithms to account for ambient climatic conditions may significantly improve the prediction of CH4 and N2O emissions from animal manure.

Ultrasonic waste activated sludge disintegration for improving anaerobic stabilization

The pretreatment of waste activated sludge by ultrasonic disintegration was studied in order to improve the anaerobic sludge stabilization. The ultrasound frequency was varied within a range from 41 to 3217kHz. The impact of different ultrasound intensities and treatment times was examined. Sludge disintegration was most significant at low frequencies. Low-frequency ultrasound creates large cavitation bubbles which upon collapse initiate powerful jet streams exerting strong shear forces in the liquid. The decreasing sludge disintegration efficiency observed at higher frequencies was attributed to smaller cavitation bubbles which do not allow the initiation of such strong shear forces. Short sonication times resulted in sludge floc deagglomeration without the destruction of bacteria cells. Longer sonication brought about the break-up of cell walls, the sludge solids were disintegrated and dissolved organic compounds were released. The anaerobic digestion of waste activated sludge following ultrasonic pretreatment causing microbial cell lysis was significantly improved. There was an increase in the volatile solids degradation as well as an increase in the biogas production. The increase in digestion efficiency was proportional to the degree of sludge disintegration. To a lesser degree the deagglomeration of sludge flocs also augmented the anaerobic volatile solids degradation.

Simultaneous sewage sludge digestion and metal leaching: effect of aeration

The effect of oxygen concentration on simultaneous sewage sludge digestion and metal leaching (SSDML) process was studied in 30 l reactors. Sewage sludge from Black Lake (Québec) wastewater treatment plant was used. Concentrations of 1, 2, 3, 5 and 7 mg/l of oxygen were maintained successively in each of the reactor experiments. The increase in the DO concentration from 2 to 7 mg/l in the sludge resulted in an increase of the oxidation-reduction potential (ORP) and an increase in the acidification rate and degradation of the total volatile solids (VS) in the sludge. The pH reduction was directly related to the dissolved oxygen (DO) concentration in the sludge. This marked decrease of pH resulted in significant solubilization of metals (Cd: 17.32–47.03%; Cr: 9.12–18.00%; Cu: 41.53–80.72%; Ni: 26.87–56.75%; Pb: 9.12–17.60%; Zn: 48.01–92.98%). There was also a reduction of VS up to ≈50%. This method allowed the sludge to retain its fertilizing properties intact. Nonetheless, after 14 days of treatment, the pH value, the percentage removal of the VS and the percentage solubilization of metal obtained were practically the same for tests involving 2, 3, 5 and 7 mg/l DO. After the first 4 days, the air required in each reactor in order to maintain the same initial DO concentration was less. In the cases of tests using 3 and 1 mg/l DO, the system was operated with aeration rates of 0.2 vvm (volume of air/vol of sludge in reactor/min) to 0.03 vvm and 0.18–0.01 vvm, respectively.

Kinetic analysis of forced aeration composting- II. Application of multilayer analysis for the prediction of biological degradation

Extensive pilot-scale composting tests for organic solid waste were conducted under different modes of aeration, to investigate vertical temperature distributions and their effect on biological degradation at different heights of the composting mass. The modes of aeration applied were upflow, downflow, alternate upflow/downflow, and internal air recirculation. Temperatures at different heights of the composting mass were continuously monitored. Results show that (i) significant variation in temperature and biodegradable volatile solids (BVS) degradation occur in the composting mass along the vertical direction when unidirectional aeration is applied; (ii) application of alternate upflow/downflow aeration or periodic mixing greatly reduces vertical gradients in temperature and biodegradable volatile solids (BVS) degradation; and (iii) the rate and extent of degradation in different layers of the composting mass can be quantitatively predicted by applying a combination of multilayer analysis and a previously established temperature-dependent first-order reaction model.

Effects of disintegration on anaerobic degradation of sewage excess sludge in downflow stationary fixed film digesters

The effects of mechanical disintegration on anaerobic digestibility of sewage excess sludge in downflow stationary fixed film (DSFF) digesters were investigated on laboratory scale. Mechanical pretreatment using a high pressure homogenizer led to significantly enhanced concentrations of soluble proteins and carbohydrates in the feed sludge. Using DSFF digesters with two different tubular plastic media as support material it was shown that a stable digestion process could be achieved at hydraulic retention times (HRT) down to 5 days. Compared to conventional digesters at 10 d and 15 d HRT respectively, the degradation of volatile solids was enhanced up to 25%, also resulting in a higher specific biogas production. Further investigations on degradation of soluble proteins and carbohydrates showed that a slowly degradable fraction of carbohydrates was released via disintegration. Using the distribution of chain length and the concentrations of volatile fatty acids as process parameters, the dependability on the HRT and the degree of disintegration (the release of soluble COD) predominated the effects of specific surface area of the support media.

Fatty Acids Profile during Anaerobic Digestion of Night Soil-Effect of temperature, Calcium Carbonate and Selectively-enriched Inoculum

Anaerobic biodegradation of night soil was carried out at 5-30 degree centigrade with 1.8-10.62 per cent volatile solids (VS). Biogas production increased with the temperature and VS up to 6.2 per cent. Further increase in VS caused higher Volatile fatty acids (VFA) accumulation resulting in decreased gas production. Acetate and propionate accounted for 62-83 per cent of total VFA. Butyrate to isobutyrate ratio increased with VS. Calcium Carbonate promoted VS degradation, biogas production and VFA degradation. The increased methanogenic and decreased sulphate-reducing bacteria caused proportional changes in CH4 and H2S gases. Enrichment with H2 oxidising methanogenic consortium is beneficial by enhancing VFA utilisation by two to three fold.

Anaerobic degradation of inedible crop residues produced in a Controlled Ecological Life Support System

An anaerobic reactor seeded with organisms from an anaerobic lagoon was used to study the degradation of inedible crop residues from potato and wheat crops grown in a closed environment. Conversion of this biomass into other products was also evaluated. Degradation of wheat volatile solids was about 25% where that of potato was about 50%. The main product of the anaerobic fermentation of both crops was acetic acid with smaller quantities of propionate and butyrate produced. Nitrate, known to be high in concentration in inedible potato and wheat biomass grown hydroponically, was converted to ammonia in the anaerobic reactor. Both volatile fatty acid and ammonia production may have implications in a crop production system.

Three-phase anaerobic digestion of organic wastes

A new process has been developed for efficient anaerobic digestion of the organic fraction of municipal solid waste. The BTA-process comprises pretreatment of incoming waste followed by a three-phase anaerobic biology consisting of acidification, solids hydrolysis and methanization. One main fraction of organic wastes is cellulose causing a diauxie effect during anaerobic digestion. Acidification of the pretreated waste occurs spontaneously, and is not limited by the level of anaerobic microorganisms in the waste. Solid retention time and pH in the hydrolysis reactor have an important influence on volatile solids degradation. Favourable conditions are pH 6.7 and solid retention times of 50 to 60 hours. Cellulose to lignin ratios in the products indicate an extensive degradation of cellulose.

Rate of Disappearance of Actaplanin and Nutrient Changes in Dairy Cattle Manure Stored as Slurry or with Bedding

An 84-day trial was conducted to determine the effect of actaplanin on volatile solids degradation and rate of disappearance when directly added to dairy cattle manure stored as a slurry or with bedding. Actaplanin was added to the manure slurry and bedding storage containers until the concentration was 20 ppm, similar to levels found in manure excreted from lactating dairy cows, and was compared to manure with no actaplanin (control). There were triplicate storage containers for each sampling time period of 0, 2, 4, 8, and 12 weeks. Actaplanin added to dairy cattle manure stored as a slurry or as a solid with bedding did not affect volatile solids degradation or the composition of the manures after 84 days of storage. The level of residual actaplanin in the manure was higher in slurry than in the solid manure after the 84-day experiment. The estimated half-life of actaplanin in slurry and solid manure was 11.8 and 11.6 days, respectively.

Aerobic Sludge Stabilization—Factors Affecting Kinetics

The effects of temperature, pH, sludge age in the prior activated sludge process, initial sludge solids concentration, and sludge type on the rate of solids degradation during aerobic sludge stabilization are discussed. Primary sludge (PS), waste activated sludge (WAS), and mixed primary and waste activated sludges (PS/WAS) are stabilized. The results indicate that: (1) First‐order kinetics satisfactorily describe biodegradable volatile solids (BVS) degradation for WAS and mixed PS/WAS, but not for PS; (2) the BVS decay coefficient b increases with increasing temperatures between 10 to 30 °C with a thermal coefficient of 1.097; (3) b values decrease as the sludge age in the prior activated sludge process increases; (4) b values are higher at reactor pH values ⩾6.5; and (5) b values tend to decrease as the initial VS concentration of the sludge increases. The WAS has an initial BVS and refractory volatile solids (RVS) content of 44–67% and 33–56%, respectively; the mixed PS/WAS has a content of 33–40% and ...

Effect of Chemical Pretreatment on the Aerobic, Thermophilic Digestion of Piggery Wastes

ABSTRACT LABORATORY scale vessels were used to determine the effects of chemical pretreatment on the aerobic, thermophilic digestion of dilute piggery waste (= 22 TS g/L1). The wastes were untreated or mixed with either sodium hydroxide (NaOH 10 percent W/W), ammonium chloride (NH4C1 10 percent W/W) or sulphuric acid (H2S04 5 mL 1 M/100 g dry matter). Treated wastes were stored at ambient temperature for up to 2 days prior to digestion at 55 C for an average retention time of 2 days. Aerobic, thermophilic digestion alone or in combination with chemical pretreatment resulted in significant (P < 0.05) reductions in the levels of volatile solids (VS) and biochemical oxygen demand (BOD5). Although NaOH pretreatment increased the mean BOD5 of the influent by 34 percent the BOD5's of the digested effluents were not significantly different. The addition of alkali resulted in significantly (P < 0.05) lower levels of VS compared with the untreated effluent whereas H2S04 pretreatment resulted in less effective VS degradation than in the control digestion. NH4C1 pretreatment did not have a significant effect on the removal of organic pollutants. The neutral detergent fibre fraction in the wastes was reduced from 55 percent dry matter to approximately 31 percent of dry matter as a result of NaOH pretreatment; the level after digestion was 24 percent. Aerobic, thermophilic digestion of untreated wastes reduced the levels of fecal coliforms (FC) from a mean of 2.5 x 106 organisms/mL to 2.4 x lOVmL. NaOH pretreatment enhanced bacterial removal by reducing the numbers in the influent to 3.3 x lOVmL while maintaining the same reduction (< lOVmL) during thermophilic digestion. The other treatments had no effect on FC numbers in the influent and resulted in an increased number of organisms in the effluent.

Nutrient Distribution and Retention during Winter Storage of Animal Wastes in Two-Stage Simulated Systems as Affected by Aeration and Buffering

Six two-stage animal waste storage simulators were designed, constructed, and used to measure the distribution and retention of nutrients during winter storage of liquid animal wastes from a dairy herd. Effects of surface aeration and pH buffering plus aeration on nutrient distribution and retention were determined.Nutrients retained during simulated untreated (control) storage were total solids 99%, volatile solids 97%, total nitrogen 89%, and ash 100%. Aeration increased degradation of volatile solids but did not reduce total nitrogen retention appreciably in unbuffered systems. Aeration increased transfer of soluble nutrients to the low-solid stage. Buffering of waste at a pH from 6 to 7 with phosphoric acid effectively retained nitrogen in a system in which mild aeration was applied to the high-solid stage. From 30 to 42% of the total nitrogen was recovered in the second low-solid stages demonstrating the significance of the liquid phase as a valuable fertilizer resource for irrigation. Appreciable synthesis of protein by microflora apparently occurred through utilization of ammonia as a nitrogen source in the nonbuffered system.Odor intensity indexes of liquids in the surface layer were reduced by aeration of that layer. Oxidation-reduction potentials appear to be a reliable indication of degree of malodor production.

Effect of Temperature and Retention Time on Methane Production from Beef Cattle Waste

The effect of temperature and retention time on the rate of methane production from waste of beef cattle fed a finishing diet was investigated by using continuously mixed 3-liter working volume anaerobic fermentors. The temperatures ranged from 30 to 65 degrees C with 5 degrees C increments between fermentors. The fermentors were fed once per day with 6% volatile solids (organic matter). Retention time for each temperature was varied from 18 to 2.5 days. After 3-volume turnovers, samples were obtained on 4 consecutive days. The highest methane production rate (liters/liter of fermentor per day) and methane yield at that rate (liters/gram of volatile solids) were 1.27 and 0.19 at 9 days and 30 degrees C, 1.60 and 0.16 at 6 days and 35 degrees C, 2.28 and 0.23 at 6 days and 40 degrees C, 2.42 and 0.24 at 6 days and 45 degrees C, 2.83 and 0.14 at 3 days and 50 degrees C, 2.75 and 0.14 at 3 days and 55 degrees C, 3.18 and 0.14 at 2.5 days and 60 degrees C, and 1.69 and 0.17 at 6 days and 65 degrees C. Volatile solids degradation at these retention times and temperatures was between 46 and 54%. The concentrations of volatile acids in the 30 to 55 degrees C fermentors were generally below 2,000 mg/liter, with the exception of the 3-day retention time. The 60 and 65 degrees C fermentors were usually above this level for all retention times. These studies indicate potential rates of methane production from the fermentation of untreated waste of beef cattle fed high-grain finishing diets. This information should serve as preliminary guidelines for various kinetic analyses and aid in economic evaluations of the potential feasibility of fermenting beef cattle waste to methane.