Darcy's law for the laminar flow of Newtonian fluids through porous media has been modified to a more general form which will describe the flow through porous media of fluids whose flow behavior can be characterized by the Herschel-Bulkley model. The model covers the flow of homogeneous fluids with a yield value and a power law flow behavior. Experiments in packed beds of sand were carried out with solutions of paraffin wax in two oils and with a crude oil from the Peace River area of Canada. The model fitted the data well.A sensitivity analysis of the fitting parameters showed that the model fit was very sensitive to errors in the flow behavior index, n, of the Herschel-Bulkley model. A comparison of the "n" values calculated from viscometer measurements and from flow measurements agreed well.A more general Reynolds number for flow through porous media, which includes a fluid yield value, was developed. The data were fitted to a Kozeny-Carman type equation using this Reynolds number. The constant in the Kozeny-Carman equation was determined for the two packed beds studied using Newtonian oils.The data could all be represented, within the experimental error, by the relationship f* = 15O/Re*. Since the mean volume to surface diameter of the packing was determined by the measurement of its permeability to a Newtonian oil, assuming C" = 150, the new definition of the Reynolds number allows the direct use of the Kozeny-Carman equation with Herschel-Bulkley type fluids. On a adapt6 la loi de Darcy pour I'Ccoulement laminaire de fluides newtoniens B travers des milieux poreux afin de rendre plus gentral et de dCcrire I'Ccoulement 21 travers des milieux poreux, de fluides obkissant au modtle Herschel-Bulkley . Ce modtle dCcrit I'ecoulement de fluides homogtnes comportant un seuil d'Ccoulement ayant un comportement de type loi de puissance.Des expCriences dans des lits 9 garnissage de sable ont CtC mentes avec des solutions de cire de paraffine dans deux huiles et avec du Ntrole brut provenant de la rkgion de Peace River au Canada. Le modtle reprksente bien ces donntes.Une analyse de sensibilitk des paramttres du modtle montre que le calage du modtle est trts sensible aux erreurs sur I'indice du comportement de I'tcoulernent, n, du modtle Herschel-Bulkley. Une comparaison des valeurs de "n" calcultes a partir des mesures au viscombtre et des mesures d'Ccoulement rCv8le un bon accord.On a dCfini un nouveau nombre de Reynolds pour I'Ccoulement k travers un milieu poreux, qui inclut la notion de seuil d'Ccoulement des fluides. Les donnCes ont CtC reprCsentCes par une Cquation de type Kozeny-Carman en utilisant ce nombre de Reynolds. La constante dans I'Cquation de Kozeny-Carman a CtC dCterminke pour les deux lits a garnissage CtudiCs avec des huiles newtoniennes.Toutes les donnCes ont pu &re reprtsenttes, a I'erreur exptrimentale prbs, par la relation F* = 150/Re*. bisque le diamttre moyen (volume sur surface) du garnissage a it6 dCtermint par la mesure de sa permCabilitC B une huile newtonienne, en supposant C " = 150, la ...

The anaerobic digestion of cheese whey was studied in a UASB reactor. The profiles of the reactor, i.e., the distributions of the substrate concentration and pH under different operating conditions were developed. From the concentrations of substrates measured at various levels above the bottom of the reactor, two reaction stages, namely acidogenesis and methanogenesis, were distinguished. The instability caused by high influent concentration was interpreted as the accumulation of VFAs in the acidogenic stage beyond the assimilative capacity of the methanogenic stage. A range of stable operating conditions was predicted from the results of the profile measurements. The optimal influent concentration was found to be between 25 and 30 g COD/L at an HRT of 5 days for system stability. Other options fro stability control were discussed.

Cheese whey is the main component of waste streams from cheese manufacturing plants. Whey is a high biochemical oxygen demand (BOD) effluent that must be reduced before the streams are sent to the sewer. It is proposed in this article that the production of methane by anaerobic fermentation would be the best use of this stream, especially for small plants. Single-stage fermentation of lactose, the main component of whey, results in a very low pH and a stalled process. Two-phase fermentation will eliminate this problem. The acidogenic stage of fermentation has been studied at pH of between 4 and 6.5. The nature of the main products of the reaction have been found to be pH dependent. Below a pH of 4.5 a gas (CO(2) and H(2)) is produced along with ethanol, acetate, and butyrate. Above a pH of 4.5 no gas was produced, and the liquid products included less ethanol and butyrate and more acetate. A separate study on the conditions for gas formation showed that if the pH dropped for a short time below 4.5 gases were formed at all subsequent pH. This would indicate a change in population distribution due to the period at a low pH. By assuming that the desired products from the acidogenic stage were butyrate, acetate, and no gases, the optimum pH range was found to be between 6.0 and 6.5.

Airway responsiveness is exaggerated in infancy and declines with maturation. These age-related differences (R.S. Tepper, T. Du, A. Styhler, M. Ludwig, and J.G. Martin. Am. J. Respir. Crit. Care Med. 151: 836-840, 1995; R.S. Tepper, S.J. Gunst, C.M. Doerschuk, Y. Shen, and W. Bray. J. Appl. Physiol. 78: 505-512, 1995; R.S. Tepper, J. Stevens, and H. Eigen. Am. J. Respir. Crit. Care Med. 149: 678-681, 1994) could be due to changes in the smooth muscle, the lung, and/or the airway wall. Folding of the mucosal membrane can provide an elastic load (R.K. Lambert, J. Appl. Physiol. 71: 666-673, 1991), which impedes smooth muscle shortening. We hypothesized that increased stiffness of the mucosal membrane occurs during aging, causing an increased mechanical load on airway smooth muscle and a decrease in airway responsiveness. Forty female New Zealand White rabbits between 0.75 and 35 mo of age were studied. Rectangular mucosal membrane strips oriented both longitudinally and circumferentially to the long axis of the trachea were dissected, and the stress-strain relationships of each strip were tested. The results showed that the membrane was stiffer in the longitudinal than in the circumferential direction of the airway. However, there was no significant change with age in either orientation. We conclude that the mechanical properties of the airway mucosal membrane did not change during maturation and were not likely to influence age-related changes in airway responsiveness.

Anerobic biofilms with dominantly acidogenic bacteria were grown in fixed-bed recycle reactors. The influence of calcium concentration in the culture medium on biofilm mass accumulation, immobilized calcium concentration, and biofilm-specific activity was investigated. The results indicate that the biofilm mass accumulation was increased by the presence of calcium in the growth medium when calcium concentration was not higher than 120 mg/L. Calcium accumulated in the biofilms increased in proportion to the calcium level in the feed. The biofilms for an increased input calcium concentration showed a trend of decrease in specific activity. The biofilms with a thickness of less than 0.5 mm had the highest specific activity. The optimum calcium concentration for substrate consumption by the biofilms was 100 to 120 mg/L. The biofilms transferred from higher calcium medium to lower calcium medium were more susceptible to sloughing from their support surfaces, which indicates calcium's role in the stability of the biofilm structure. 0 1995 John Wiley & Sons, Inc.