Optimum Retention Time Research Articles

Thermophilic anaerobic acid digestion of biosolids: hydrolysis, acidification, and optimization of retention time of acid digestion

The objective of the study was the optimization of the retention time in a thermophilic acid digestion system. To achieve this optimization, acidification index defined as the ratio between acidification rate and hydrolysis rate was used. First-order reaction model developed by Eastman and Ferguson (1981) was used to describe hydrolysis of particulate matter in sludge. The first-order hydrolysis rate constant was found to be 2.36 d–1, and the initial concentration of degradable particulate COD was found to be 10.4 g/L. A model of acid digestion was proposed that included acidogenic biomass growth and decay. The total acidogenic population was estimated and kinetic parameters of acidogenic population were specific growth rate 0.78 d–1, net specific growth rate 0.25 d–1, decay coefficient 0.52 d–1, and observed yield 0.05 mg biomass / mg COD consumed. Based on the acidification index, the optimum retention time in a thermophilic acid digester was 3–3.5 d. Key words: wastewater sludge, biosolids, anaerobic digestion, hydrolysis, acidification, kinetics.

Restaurant emissions removal by a biofilter with immobilized bacteria.

Pseudomonas sp. ZD8 isolated from contaminated soil was immobilized with platane wood chips to produce packing materials for a novel biofilter system utilized to control restaurant emissions. The effects of operational parameters including retention time, temperature, and inlet gas concentration on the removal efficiency and elimination capacity were evaluated. Criteria necessary for a scale-up design of the biofilter was established. High and satisfactory level of rapeseed oil smoke removal efficiency was maintained during operation and the optimal retention time was found to be 18 s corresponding to smoke removal efficiency greater than 97%. The optimal inlet rapeseed oil smoke loading was 120 mg/(m(3) x h) at the upper end of the linear correlation between inlet loading and elimination capacity.

Optimization of continuous ball mills used for finish-grinding of cement by varying the L/D ratio, ball charge filling ratio, ball size and residence time

During the last decade, semi-finish-grinding plants have been used more and more for the energy efficient grinding of high-quality cement. In 1999, it was found that by decreasing the ball charge filling ratio it was possible to lower the specific energy demand for grinding significantly. It was obvious, too, that the L/D ratio influences the specific energy demand and the mill throughput as well. Therefore, a huge test program was carried with a semi-industrial ball mill, which was operated in closed circuit. The mass-specific surface area of the two feed materials (intermediate product) used were quite typical for industrial semi-finish grinding plants. The values were 2200 and 3000 cm 2/g according to Blaine. The product finenesses were 3000 and 3800 cm 2/g, respectively. The L/D ratio of the ball mill was varied in four steps of 1.75, 2.1, 2.79 and 3.49, and the ball charge filling ratio was varied in three steps of 15%, 20% and 25%. The experiments clearly indicated that the optimal L/D ratio and the optimal ball charge filling ratio are different for each feed fineness. The influence of the ball charge grading on the specific energy demand, characterised by the average ball diameter, was tested by means of a discontinuous laboratory ball mill. The results showed that by using a finer ball grading the specific energy demand could be lowered considerably. The obtained results can be explained well by theoretical considerations regarding the ruling stress intensity and the number of stress events. The stress intensity expressed as the power input per ball is dependent on the ball diameter to the third power and only slightly dependent on the inner diameter of the mill. The number of stresses can be characterised by the average retention time of the ground material inside the mill if the ball charge grading remains unchanged. The optimal retention time depends not only on the feed material and the desired comminution result but also on the ball charge filling ratio and particularly on the L/D ratio. On the basis of the present results and considerations, a specific optimisation of ball mills in semi-finish-grinding plants can be done.

Copper concentrate pressure leaching — plant scale-up from continuous laboratory testing

Phelps Dodge Mining Company (Phelps Dodge) is operating a pressure-leach vessel (PLV) for copper concentrates at the Bagdad Mine facility in Arizona. This single PLV unit was engineered and constructed based on continuous laboratory testing, on reaction chemistry and kinetics and on available pressure-vessel technology. The challenge of designing for optimum process retention time, agitation, oxygen mass transfer, temperature and pressure are reviewed in this paper. The PLV interior lining, agitation equipment and compartment configuration chosen for the PLV are also reviewed.

Anaerobic degradation of p-coumaric acid and pre-ozonated synthetic water containing this compound

Chemical treatment previous to biological treatment of p-coumaric acid may represent a strategy in order to improve its biodegradability. In this work, anaerobic biodegradation of synthetic waters containing this compound and pre-oxidized waters by ozonation was studied. The critical and optimum retention times for the reactor fed with p-coumaric acid were 15.9 and 20 days, respectively. Under the optimal operational conditions removal percentage ranged from 85 to 90%. The bioreactor for ozonated p-coumaric acid was operated at a constant hydraulic retention time (HRT) of 25 days, since lower HRT caused its destabilization and during period of more stability the percentage of removal varied between 71 and 84%. Thus, the use of the ozonation as chemical pre-treatment did not enhance biological degradation of p-coumaric acid. On the other hand, microorganisms able to use p-coumaric acid (or its major intermediates produced from the ozonation treatment), were isolated at maximum tolerated concentrations of these compounds. Since the ozonation as chemical pre-treatment did not improve p-coumaric acid biodegradation, the enrichment of these isolated strains could be the most adequate strategy to enhance the biological degradation of p-coumaric acid.

Separation and identification by gel filtration and high-performance liquid chromatography with UV or electrochemical detection of the disulphides produced from cysteine and glutathione oxidation

Methods for quantification of oxidised and reduced forms of glutathione (GSSG and GSH) and cysteine (CSSC and CSH) and the disulphide glutathione–cysteine (GSSC) resulting from the oxidation of the mixture of CSH and GSH are performed by RP-HPLC with coulometric and UV detection after separation of these compounds by size-exclusion fast protein liquid chromatography. The fractionation of the disulphides (GSSG, GSSC and CSSC) was achieved by size exclusion using a Superdex peptide column coupled with an UV detection at 254 nm. The conditions of separation of these compounds by RP-HPLC were optimised using the response surface methodology. Optimal peak resolution and retention times were obtained on a C 18 YMC ODS AQ column with 20 mM of ammonium phosphate at pH 2.5 and 2% of acetonitrile in the elution phase. In these experimental conditions, CSH, CSSC, GSH and GSSG were eluted within 20 min. Coulometric detection enabled a sensitivity 100 times higher for the disulphides than the UV detection at 220 nm. These methods were applied to follow the consumption of thiols and the disulphide formation by three oxidising systems, sulphydryl oxidase, glutathione dehydroascorbate oxidoreductase and potassium bromate. This study revealed that the relative proportions of the disulphides formed were similar for the three oxidising systems when the reactions are in their state of equilibrium.

Nutrient removal from sewage by an artificial food web system composed of phytoplankton and Daphnia magna

In order to remove nutrients from sewage, ecotechnology with an artificial food web composed of phytoplankton and Daphnia magna was used. To optimise performance of the system, phytoplankton growth, zooplankton growth, and a continuous-flow system were used. For phytoplankton growth, stirring was 6.7 times faster than the settling in growth rate of Scenedesmus. Zooplankton growth was not influenced by phytoplankton succession, and the specific production coefficient of D. magna was 110.4 mg Daphnia dry weight (DW) per mg chlorophyll a (Chl a). Results indicated that removal of nutrients was better in a long hydraulic residence time (HRT) system than in a system with short HRT. The optimum retention time was found to be 3 days for the phytoplankton chamber and 1.5 days for the subsequent D. magna chamber, respectively, with total retention time of the combined chambers being kept at 4.5 days. When a pilot plant was operated under these conditions, the removal rates of total nitrogen (TN) and total phosphorus (TP) were 68 and 56%, respectively. In the material budget of TN, 32% of inputs passed on to effluent, 39% to sludge, 27% to air and 2% to harvested Daphnia. For TP, 44% of inputs passed on to effluent, 51% to sludge and 4% to Daphnia.

The vortex concentrator for suspended solids treatment

The use of vortex concentrators is becoming increasingly popular for suspended solids reduction in combined sewer overflows and stormwater. This study is a laboratory investigation of the use of vortex concentrators to reduce the solids concentration of synthesized stormwater. The synthesized stormwater was made with water and addition of particles; sand, granular activated carbon, and sewer sediments. The vortex concentrator was made of acryl resin 300 mm in diameter. To determine the efficiency for various influent suspended solids (SS) concentrations, tests were performed with different SS concentrations. The samples were taken simultaneously at the influent storage tank and effluent tank, and measured SS concentrations. The range of surface loading rates were 120 to 850 m3/m2/day, and influent SS concentrations were varied from 300 to 5,000 mg/L. To determine the optimum coagulant dosage, jar tests were conducted with coagulants such as PAM and PAC. It was found that optimum coagulant and its dosage were PAM and 2 mg/L. The overall SS removal efficiency of the vortex concentrator for typical stormwater was estimated at about 65%. With an increase of SS concentration, the removal efficiency was increased. Since the SS concentration of stormwater was higher than 1,000 mg/L, the removal efficiency of the vortex concentrator for stormwater could be estimated to be 65-70%. The SS removal efficiency was increased with an increase of retention time, and the optimum retention time was 0.15-1.0 minutes. With an increase of the foul to overflow Q(F)/Q(o), a key parameter for vortex concentrator operation, the removal efficiency was increased. An alternative solution to improve treatment efficiency might be to set a follow-up retention basin. Based on a series of settling tests on the treated overflow water from the vortex concentrator, 5 to 10 minutes hydraulic retention time in a follow-up retention basin would substantially improve the results.

Anaerobic Treatment of Polyethylene Glycol of Different Molecular Weights

Biodegradation of high molecular weight polyethylene glycol has been found difficult under aerobic conditions. The use of a chemical treatment prior to the aerobic one was necessary to remove high molecular weight of polyethylene glycol. Anaerobic treatment of polyethylene glycol may present an alternative to integrated chemical and biological treatment for the elimination of this compound. Anaerobic biodegradation of polyethylene glycol from 200 to 10000 molecular weight was studied. It showed that, unlike aerobic treatment, the size of the molecule up to 10000 does not affect the process efficiency. The critical retention time and the optimum retention time were estimated and ranged from 10 to 14 and from 16 to 19 days, respectively. Under these operational conditions percentage of removal ranged from 85 to 90%. Thus, anaerobic degradation of polyethylene glycol presents removal percentages and hydraulic retention times similar to those of biodegradable compounds tested at the same operational conditions.

PEGylation of microspheres generates a heterogeneous population of particles with differential surface characteristics and biological performance

Surface PEGylation of polystyrene microspheres with methoxy-poly(ethylene glycol)-5000 (mPEG-5000) generated a heterogeneous population of entities that differed in surface characteristics and in vitro biological performance (phagocytosis and complement activation). Surface heterogeneity was determined by hydrophobic interaction chromatography, measurements of particle electrophoretic mobility in a defined field and adlayer thickness of the projected mPEG chains. The particle population separation by hydrophobic interaction chromatography demonstrated a remarkable linear relationship between the particle zeta potential and phagocytosis by J774 A1 macrophage-like cells. Microsphere populations bearing a predominant surface of mPEG molecules as high-density mushroom–brush intermediate and/or brush configuration were most resistant to phagocytosis and activated the human complement system poorly. Conversely, those populations with predominant surface mPEGs in a mushroom regime were potent activators of the complement system and were prone to phagocytosis. Therefore, surface heterogeneity explains why a fraction of intravenously injected ‘long-circulating’ nanoparticles is cleared rapidly by macrophages of the reticuloendothelial system. Hydrophobic interaction chromatography can readily assess the extent of surface heterogeneity of PEGylated particulate drug delivery systems and pre-select particles with optimal retention times in the blood. These observations may also be relevant with respect to successful surface camouflaging of cells, drug depots and implantable devices.

Decolorization and azo dye degradation by anaerobic/aerobic sequential process

This study investigates the anaerobic treatability of Reactive Black 5 in an anaerobic/aerobic sequential process. Laboratory scale upflow anaerobic sludge blanket (UASB) reactor/completely stirred tank reactors (CSTR) were operated at different organic loadings and hydraulic retention times (HRT). The effects of shock organic loading on the chemical oxygen demand (COD), color removal, and methane gas production efficiencies were investigated in UASB reactor. The effects of both sludge retention time (SRT) and food to mass ( F/ M) ratio on the color and COD removal efficiencies were also investigated in aerobic reactor. The reactive dye used in this study contains the groups NN, SO 3, SO and is in use in the some textile industries in Turkey. The studies were carried out in continuous mode and the effluent of the UASB reactor was used as feed for the CSTR reactor. COD removal efficiencies decreased from 56 to 27% with increases in COD loadings from 5 to 25 kg COD m −3 per day in the anaerobic UASB reactor. The color removals were 92 and 87%, respectively, for aforementioned organic loadings. The methane percentages were found to be 76 and 60% at organic loading rates of 2.49 and 14.8 kg COD m −3 per day, respectively, in UASB reactor. COD removal efficiencies of 28, 42, and 90% were obtained at SRTs of 1.7, 5.7 and 11 days in the aerobic CSTR reactor. Optimum sludge retention time was 11 days in aerobic reactor. A total of 67 and 28% COD removal efficiencies were obtained at F/ M ratios varying between 0.05 and 0.17 and between 0.30 and 1.4 kg COD kg −1 MLSS per day. A 90–95% color and 40–60% COD removal efficiencies were obtained depending on the applied organic loadings in the UASB reactor. The remaining COD was removed with a treatment efficiency of 85–90% in the aerobic CSTR reactor.

The use of ultrasound and γ-irradiation as pre-treatments for the anaerobic digestion of waste activated sludge at mesophilic and thermophilic temperatures

The effect of ultrasound and γ-irradiation used as pre-treatments for the anaerobic digestion of waste activated sludge at both mesophilic and thermophilic temperatures was examined. Untreated activated sludge was also subjected to anaerobic digestion at these temperatures as a control. The sonication time was 90 s using a Soniprep 150 (MSE Scientific Instruments) which operated at 23 kHz and had been adjusted to give an output of 47 W and the γ-irradiation dose was 500 krad. The digesters were operated in a semi-continuous mode, being fed with fresh sludge every 24 h at hydraulic retention times (HRT) of 8, 10 and 12 days. Over the 24 h period the differences between the digesters, in terms of volatile solids (VS) reductions and biogas production, were not statistically significant for any particular set of conditions. Thermophilic digestion performed better than mesophilic digestion in terms of biogas production, VS reductions (except at HRT of 8 days) and specific methane yields and the optimum retention time was 10 days, at both temperatures. When gas production over the initial eight hours (probably the hydrolytic stage) was examined, it was found that the gas production rates for pre-treated sludges were higher than those for untreated sludges. This was most pronounced at thermophilic temperatures and a HRT of 10 days. Sonication did not affect the numbers of faecal coliforms in the sludge. However, γ-radiation caused a 3-log reduction and, when coupled with mesophilic digestion, gave a product which contained <100 g −1 TS. Thermophilic anaerobic digestion produced sludges which contained <1 g −1 TS irrespective of any pre-treatment.

Color removal from cotton textile industry wastewater in an activated sludge system with various additives

The low biodegradability of many dyes and textile chemicals indicates that biological treatment is not always successful in the treatment of cotton textile wastewater, in terms of color removal. In this study, a specific organic flocculant (Marwichem DEC), powdered activated carbon (PAC), bentonite, activated clay and commercial synthetic inorganic clay (Macrosorb) were directly added into the activated sludge laboratory pilot plant model. Before dosage, the optimum sludge retention time and hydraulic retention time were determined as 30 days and 1.6 days, respectively. The Monod kinetic constants were determined as Y=0.76 kg MLSS/kg COD, K d=0.026 1/day, K s=113.3 mg/L, k=0.42 1/day and μ max=0.32 kg MLSS/kg COD day. Under these conditions the average COD removal was 94% and color removal was 36%. The addition of these materials did not change COD removal significantly. The most effective materials were found to be DEC and PAC for color removal. While the color removal efficiency for 120 mg/L DEC addition was 78%, it was 65% for 100 mg/L, 77% for 200 mg/L and 86% for 400 mg/L PAC addition. The advantage of DEC compared to PAC was the lower sludge production. Statistical analyses using multiple linear regression indicate that there is no relationship between the effluent color with the influent color and total suspended solids (TSS) for DEC and PAC addition. On the other hand, when only bentonite, activated clay and Macrosorb were added, the effluent color was primarily dependent on the influent color and the TSS concentration had little effect. When the data is examined by using Kruskal–Wallis H and Mann–Whitney U tests and it was found that there was a significant difference between the color data groups.

The design of wetland systems for the treatment of urban run off

The results of studies of the pollution removal performance of constructed wetlands for urban runoff treatment are discussed. The design of urban runoff wetlands including primary treatment components, substrate composition and flow rate regulation are considered. The use of front-end treatment systems such as oil interceptors, spillage containment facilities and wetland forebays or lagoons are recommended to provide an initial pollutant removal capacity as well as attenuating the impact of high discharges. Design criteria which will ensure inlet flow velocities do not exceed 0.7m/s and which will achieve optimum retention times (10-15 hours) for maximum pollutant removal are described. The relative advantages of different soil types are discussed and an overall minimum substrate depth of 0.6m is recommended for sub-surface systems. Two case studies of urban runoff wetlands in the UK which are currently being monitored are described.

The design of wetland systems for the treatment of urban run off

The results of studies of the pollution removal performance of constructed wetlands for urban runoff treatment are discussed. The design of urban runoff wetlands including primary treatment components, substrate composition and flow rate regulation are considered.The use of front-end treatment systems such as oil interceptors, spillage containment facilities and wetland farebays or lagoons are recommended to provide an initial pollutant removal capacity as well as attenuating the impact of high discharges. Design criteria which will ensure inlet flow velocities do not exceed O.7m/s and which will achieve optimum retention times (10–15 hours) for maximum pollutant removal are described. The relative advantages of different soil types are discussed and an overall minimum substrate depth of 0.6m is recommended for sub-surface systems. Two case studies of urban runoff wetlands in the UK which are currently being monitored are described.

Operation optimization of Thiobacillus thioparus CH11 biofilter for hydrogen sulfide removal

Members of the autotrophic species, Thiobacillus thioparus CH11, were isolated from swine wastewater and immobilized with Ca-alginate to produce pellet packing materials for a novel biofilter system that controls hydrogen sulfide emission. The effect of operating parameters, including retention time, temperature, and inlet gas concentration, on the removal efficiency and capacity was evaluated. Criteria necessary for a scale-up design of the biofilter were established and the sulfur balances at various loadings were tabulated. High and satisfactory H 2S removal efficiency levels were maintained during operation and the optimal retention time was found to be 28 s corresponding to a H 2S removal efficiency greater than 98%. The pH drop was insignificant in this biofilter. The optimal inlet S-loading can be noted as 25 g m −3 h −1 that is at the upper end of linear correlation between inlet loading and removal capacity. We suggest that the Thiobacillus thioparus CH11 immobilized with Ca-alginate is a potent method to control hydrogen sulfide emissions.

Resource Use, Plant Defenses, and Optimal Digestion in Ruminants

We studied retention time decisions in mammalian herbivores in relation to food-quality and resource abundance. Under the assumption that foraging (searching and ingestion) and digestion (rumination and fermentation) are mutually exclusive events in ruminants, and that the forager is an energy (nutrient) maximizer, we use the marginal value theorem to predict changes in ingestion rate, retention time and food selectivity. If we assume that foraging time (the time it takes to obtain one gut fill of food) is inversely proportional to resource density, we predict that the optimal retention time increases as foraging time increases; optimal retention time decreases as the digestion rate parameter increases, but is unaffected by maximum digestibility; energy intake rate is differently affectd by the two food quality measures maximum digestibility and digestion rate; there are stronger incentives to be selective of high quality food if the resource is abund than if it is scarce

Influence of Combined Suspended and Fixed-Film Biomass in Anoxic Reactor on Nitrification-Denitrification Process

In single-sludge nitrification-denitrification process biomass is passed periodically through oxic and anoxic reactors. Nitrifying bacteria need oxygen for substrate oxidation. Behavior of these bacteria under anoxic conditions is still not sufficiently known. In this paper influence of retention time in anoxic reactor on nitrifiers has been studied. Optimum retention time for tested biomass was found to be 3 hours. A possible way to set-up this retention time is to increase denitrification rates by addition of support material of fixed-film biomass to anoxic reactor (e.g.polyurethane cubes). Increased denitrification rate results in decrease of necessary volume of anoxic reactor followed by increase of nitrification efficiency. Presence of fixed-film biomass has no influence on clarifier capacity and oxygen concentrations in oxic reactors. Theoretical aspects of this modification were confirmed in laboratory models with synthetic wastewater as substrate.

Digestion Strategies in Nectar- and Fruit-Eating Birds and the Sugar Composition of Plant Rewards

We present a model for the digestion of sugars in avian frugivores and nectarivores (nectar feeders) and use it to analyze the digestive strategies and preferences of birds. The main feature of the model is the assumption that the intestine of birds works as a steady-state chemical reactor (a storage organ coupled to a tubular digestor) where hydrolysis of sucrose and absorption of monosaccharides occurs. We examine two variants of this basic model. In the "hummingbird model," all the uptake of sugars results from carrier-mediated processes. Under such assumptions, the model predicts that energy-maximizing birds feeding on energy-concentrated foods should exhibit longer food-retention times than birds feeding on energy-dilute foods and that birds should exhibit digestibilities of sugars close to 100%. The optimal throughput times and sugar digestibilities predicted by the hummingbird model are independent of the cost of feeding. In the "frugivore model," the uptake of sugars had a substantial passive-diffusion component. This model predicts that when birds feed on hexoses, optimal retention times and digestibilities of sugar decrease as functions of the increased concentration of sugars in food. In the frugivore model, increasing the cost of feeding increases the time food should be retained in the intestine and, hence, the digestibility of sugars. Both models predict that birds should prefer hexoses to sucrose if the rate-limiting step in the absorption of sugars is sucrose hydrolysis. The ratio of maximal hydrolysis of sucrose to maximal uptake of hexoses required for hydrolysis not to be limiting is much higher in the frugivore model than in the hummingbird model. We suggest that the digestive physiology of birds will be a necessary component in an evolutionary explanation of the patterns of sugar distribution in nectars and fruit pulps.

Anaerobic digestion of pesticide-plant wastewater

Experiment on anaerobics digestion using an organic phophorus pesticideplannt wastewater containing a high percentage (62%) of acetate were conducted with chemostat-type digesters maintained at 35°C. The influent COD concentration was 9650 mg/liter. The digesters operated stably at the retention times of 15, 20 and 30 days, with COD removal efficiencies of 87·0, 89·4, and 91·5%, respectively. The biokinetic constants were ν max = 3·37/day ,K s = 4077 mg COD/liter, Y g = 0·148 mg VSS/mg COD, and K d = 0·050/day. Bacilli and sarcinae were the predominant morphological species and the dominance was independent of retention time. The optimal retention time was 15 days.