Gas treatment in trickle-bed biofilters: biomass, how much is enough?

The objective of this article is to define and validate a mathematical model that desribes the physical and biological processes occurring in a trickle-bed air biofilter for waste gas treatment. This model considers a two-phase system, quasi-steady-state processes, uniform bacterial population, and one limiting substrate. The variation of the specific surface area with bacterial growth is included in the model, and its effect on the biofilter performance is analyzed. This analysis leads to the conclusion that excessive accumulation of biomass in the reactor has a negative effect on contaminant removal efficiency. To solve this problem, excess biomass is removed via full media fluidization and backwashing of the biofilter. The backwashing technique is also incorporated in the model as a process variable. Experimental data from the biodegradation of toluene in a pilot system with four packed-bed reactors are used to validate the model. Once the model is calibrated with the estimation of the unknown parameters of the system, it is used to simulate the biofilter performance for different operating conditions. Model predictions are found to be in agreement with experimental data. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 583-594, 1997.     

Lipid screening: is it enough to measure total cholesterol concentration?

OBJECTIVES--To determine whether measurement of total cholesterol concentration is sufficient to identify most patients at lipoprotein mediated risk of coronary heart disease without measurement of triglyceride and high density lipoprotein (HDL) cholesterol concentrations. DESIGN--Cross sectional screening programme. SETTING--Six general practices in Oxfordshire. PATIENTS--1901 Men and 2068 women aged 25-59. MAIN OUTCOME MEASURE--Cardiovascular risk as assessed by fasting venous plasma concentrations of total cholesterol, triglyceride, and HDL cholesterol. RESULTS--2931 Patients (74% of those screened) had a total cholesterol concentration of less than 6.5 mmol/l. If the triglyceride concentration had not been measured in these patients isolated hypertriglyceridaemia (greater than or equal to 2.3 mmol/l) would have remained undetected in 185. Among these 185 patients, however, 123 were overweight or obese and only 18 (0.6% of those screened) had an increased risk associated with both a raised triglyceride concentration (greater than or equal to 2.3 mmol/l) and a low HDL cholesterol concentration (less than 0.9 mmol/l). Conversely, in the 790 patients with predominant hypercholesterolaemia (cholesterol concentration greater than or equal to 6.5 mmol/l and triglyceride concentration less than 2.3 mmol/l) measurement of HDL cholesterol concentration showed that 348 (9% of those screened) had only a moderately increased risk with a ratio of total to HDL cholesterol of less than 4.5 and 104 had a low risk with a ratio of less than 3.5. CONCLUSIONS--Fasting triglyceride and HDL cholesterol concentrations identify few patients at increased risk of coronary heart disease if the total cholesterol concentration is less than 6.5 mmol/l. HDL cholesterol and triglyceride concentrations should, however, be measured in patients with a total cholesterol concentration exceeding this value. Total cholesterol concentration alone may overestimate risk in a considerable number of these patients, and measurement of HDL cholesterol concentration allows a more precise estimate of risk. Measurement of the triglyceride concentration is required to characterise the lipoprotein abnormality. A patient should not be started on a drug that lowers lipid concentrations without having had a full lipoprotein assessment including measurement of HDL cholesterol concentration.     

Anesthesia in 1984: how safe is it?

The methods of gathering information to determine the safety of anesthesia and to establish the risk of mortality and morbidity include anecdotal tales, in-hospital audit and peer review, reports to medical protective societies, retrospective studies, reviews of specific problems and prospective studies. All these methods have limitations and, in particular, do not readily differentiate the anesthetic from the surgical contributions. However, it appears that over the past 30 years the risk of death directly attributable to anesthesia has decreased from 1 in 2680 to about 1 in 10 000. The main causes of death are faulty anesthetic techniques due to human error, drug overdose, coexistent disease and failure of immediate postoperative care. Equipment failure, poor preoperative assessment, halothane-associated hepatitis and malignant hyperthermia, although often cited in the literature, are rarely the cause of problems associated with anesthesia.     

Hsp70 and larval thermotolerance in Drosophila melanogaster: how much is enough and when is more too much?

Heat shock proteins (Hsps) and other molecular chaperones perform diverse cellular roles (e.g., inducible thermotolerance) whose functional consequences are concentration dependent. We manipulated Hsp70 concentration quantitatively in intact larvae of Drosophila melanogaster to examine its effect on survival, developmental time and tissue damage after heat shock. Larvae of an extra-copy strain, which has 22 hsp70 copies, produced Hsp70 more rapidly and to higher concentrations than larvae of a control strain, which has the wild-type 10 copies of the gene. Increasing the magnitude and duration of pretreatment increased Hsp70 concentrations, improved tolerance of more severe stress, and reduced delays in development. Pretreatment, however, did not protect against acute tissue damage. For larvae provided a brief or mild intensity pretreatment, faster expression of Hsp70 in the extra-copy strain improved survival to adult and reduced tissue damage 21h after heat shock. Negative effects on survival ensued in extra-copy larvae pretreated most intensely, but their overexpression of Hsp70 did not increase tissue damage. Because rapid expression to yield a low Hsp70 concentration benefits larvae but overexpression harms them, natural selection may balance benefits and costs of high and low expression levels in natural populations.     

Cost effectiveness of incremental programmes for lowering serum cholesterol concentration: is individual intervention worth while?

OBJECTIVE--To evaluate the relative cost effectiveness of various cholesterol lowering programmes. DESIGN--Retrospective analysis. SETTING--Norwegian cholesterol lowering programme in Norwegian male population aged 40-49 (n = 200,000), whose interventions comprise a population based promotion of healthier eating habits, dietary treatment (subjects with serum cholesterol concentration 6.0-7.9 mmol/l), and dietary and drug treatment combined (serum cholesterol concentration greater than or equal to 8.0 mmol/l). MAIN OUTCOME MEASURE--Marginal cost effectiveness ratios--that is, the ratio of net treatment costs (cost of treatment minus savings in treatment costs for coronary heart disease) to life years gained and to quality of life years (QALYs) saved. RESULTS--The cost per life year gained over 20 years of a population based strategy was projected to be 12 pounds. For an individual strategy based on dietary treatment the cost was about 12,400 pounds per life year gained and 111,600 pounds if drugs were added for 50% of the subjects with serum cholesterol concentrations greater than or equal to 8.0 mmol/l. CONCLUSIONS--The results underline the importance of marginal cost effectiveness analyses for incremental programmes of health care. The calculations of QALYs, though speculative, indicate that individual intervention should be implemented cautiously and within more selected groups than currently recommended. Drugs should be reserved for subjects with genetic hypercholesterolaemia or who are otherwise at very high risk of arteriosclerotic disease.     

Concentration of fixed plankton samples via settling: how long is long enough?

Enumerating plankton cells, especially ciliates, usually requiressettling of samples in order to concentrate the cells. Poorlysettled samples could introduce large errors into plankton counts.The time sufficient to settle all ciliates has, however, neverbeen established in the literature. Here, using both theoreticaland empirical studies, we suggest improvements of the currentmethod, which mostly relies on experience to determine settlingtimes. Ciliate density was used to calculate the theoreticalsettling time of fixed ciliates with the Stokes equation. Todetermine ciliate density (g mL^–1), we modified and establisheda density gradient centrifugation method. We found that ciliatedensity was in the range 1.02–1.08 g mL^–1. Additionally,empirical sinking rates were gathered semi-automatically witha digital camera system. The theoretical and experimental settlingtimes were in the same range, though there were differencesfor some species. From this, we recommend working with the empiricalsinking rates that are more reliable: 0.5 and 1.7 mm min^–1for fixed marine (at salinities of 16 and 40, respectively)samples and 2.4 mm min^–1 for fixed freshwater samples.Using these rates potentially saves up to 95% of the time forsettling compared to old, experience derived times. Althoughciliate density was significantly correlated with settling rates,there was no correlation with particle size and shape.