K. L. Pinder

Effective diffusivities of volatile fatty acids in methanogenic biofilms

Abstract In a methanogenic biofilm reactor, fatty acids diffused in the symbiotic biofilms and were degraded to methane and carbon dioxide. The diffusion of fatty acids was retarded due to the presence of gelatinous biomass and the formation of tiny gas bubbles within the biofilms. The effective diffusivities of fatty acids (acetate, propionate and butyrate at pH 7.1) were measured ‘in situ’ in active methanogenic biofilms after the fatty acid concentration profiles approached the steady state in the biofilms. By solving reaction-diffusion models for each acid after the three boundary conditions had been experimentally determined with a specially designed diffusion-cell, the effective diffusivities of acetate, propionate and butyrate in methanogenic biofilms at 35°C were found to be 0.54,0.60 and 0.36 × 10 5 cm 2 / s , respectively. The diffusion coefficients in biofilms were 33% of those in water solution at infinite dilution. Compared with the diffusion in water, the effective diffusivities in biofilms were reduced by about 54% due to the presence of solid biomass, and a further retardation in diffusion was attributed to the formation of tiny gas bubbles (CH 4 and CO 2 ) in the active methanogenic biofilms.

Utilization of volatile fatty acids in methanogenic biofilms

Utilization kinetics of volatile fatty acids (acetic, propionic and butyric acids) in mesophilic (35°C) methane-producing biofilms were investigated in a continuous flow reactor. The mass transfer resistances occurring inside and outside biofilms were tested and minimized during the experiments. By monitoring the biofilm growth in terms of carbon content (mgC), the utilization rate of each fatty acid was measured per unit biomass of a balanced symbiotic biofilm containing all concerned bacterial groups. The experiments were designed and conducted to avoid bacterial population shift in the mixed consortium. Thereby, the relations between utilization rates and concentrations of the volatile fatty acids were governed and explained by enzyme-catalyzed substrate catabolisms. The acetate utilization rate fits a Monod model quite well while consumption rates of propionic and butyric acids deviated from the two-parameter model to some degree. It was found that high propionate concentrations promoted propionate utilization but inhibited butyrate consumption. Three-parameter models were discussed to fit the experimental data better. Judged by the Monod model parameters, acetate was utilized at the fastest rate with a maximum rate of 0·098 mg HAc mg C−1 h−1 followed by butyrate (apparent maximum utilization rate of 0·0334 mg HBu mg C−1 h−1) and propionate (apparent maximum utilization rate of 0·021 mg HPr mg C−1 h−1). Butyrate utilization had the lowest apparent half velocity concentration (20 mg HBu liter−1) followed by propionate utilization (99 mg HPr liter−1) while acetate utilization had the highest half velocity concentration (160 mg HAc liter−1).

Pulmonary microvascular exchange: An analog computer simulation

An analog computer simulation of human pulmonary microvascular exchange was programmed and tested. This model of blood-to-lymph transport is based on a compartmental analysis of the lungs and has the capacity to describe fluid volumes, plasma protein content, flows and pressures for both normal and perturbed conditions. Normal conditions were predicted and shown to be in agreement with the literature. The trends in the response of the lungs to changes in microvascular pressure is shown also to agree with the literature both for steady state and transient predictions. The results of a sensitivity analysis, which demonstrates the response of the lungs to a variety of perturbations, is also reported. The simulation predicts reasonable results for normal, transient, and perturbed conditions in human lungs.

An analog computer simulation showing the effect of volume exclusion on capillary fluid exchange

Abstract The analog computer simulation of capillary fluid exchange by C. A. Wiederhielm (1979, Microvasc. Res. 18, 48–82) based on human skin and muscle has been modified to incorporate recent concepts in volume exclusion. The effect on parameters descriptive of the system for different degrees of volume exclusion is shown. Of the normal 7.0 liters of interstitial fluid, exclusion of plasma protein from 2.8 liters resulted in computed normal values of parameters consistent with the previous simulation. Using this constant value of excluded volume, perturbations characteristic of different forms of edema were individually programmed and the responses for both steady state and transients were recorded. The use of a constant value of excluded volume is consistent with recent experimental observations of the exclusion of albumin in tissues, requires fewer assumptions in the model upon which the simulation is based, simplifies the program, and predicts values of parameters for both normal and several perturbed conditions in good agreement with other studies.

Analog simulation of the human microvascular exchange system

An analog model of human microvascular exchange of fluids and plasma proteins has been programmed and tested. The results agree well with clinical observations but less well with indirectly calculated terms. Examples show the model used to predict the dynamic response of the microvascular exchange system to body changes. These changes include a simulated heart attack and an allergic reaction.

Fast Fourier transform analysis of dynamic data: sine wave stress - strain analysis of biological tissue

We propose an improvement to the dynamic oscillation method using as an example the measurement of the tensile stiffness of the rabbit tracheal mucosal membrane. A sine wave oscillation technique was used to study the tissue mechanical properties. A mathematical model was developed using fast Fourier analysis. After mathematically eliminating the machine response, this analysis reveals the tissue frequency response over a wide range of frequencies. This study addresses the advantages of using Fourier analysis to interpret dynamic properties of biological tissue and provides a complete description of how to obtain the pure tissue response.

Digital simulation of pulmonary microvascular exchange

A previously published analog computer simulation of blood to lymph fluid and protein transport in the human lung [Microvascular Research 27, 51-70 (1984)] has been converted to a digital program. Comparisons have been made between the predictions of the two programs for both transient and steady state responses to perturbations. The advantages of each program are discussed. The FORTRAN simulation, including the input and output files are explained. Copies of the program will be made for anyone who wishes to use it.

Alveolar flooding: a computer simulation

A validated computer simulation of pulmonary microvascular exchange (J.L. Bert and K.L. Pinder, Microvasc. Res., 27 (1984) 51-70) has been extended to include exchange with the air space (alveoli). Equations which hypothetically describe characteristics associated with this additional compartment and the exchange of both fluid and plasma proteins between the lung tissue and the alveolar space are presented. These are incorporated into the simulation which has been used to predict the behavior of the pulmonary microvascular exchange system including alveolar flooding. The predicted trends associated with alveolar flooding are reasonable. However, due to the lack of specific experimental or clinical findings, the simulation remains essentially unvalidated. The effect on alveolar flooding and interstitial edema of the parameters associated with the additional relationships is presented and discussed. Primarily, results for perturbations in circulatory pressure are presented. Additionally, changes in permeability characteristics are shown and discussed.

Build-up of symbiotic methanogenic biofilms on solid supports

Support surfaces can have selective action which determines the relative quantities of acetogens (propionate and butyrate degraders) and methanogens (acetate degraders) immobilized in a symbiotic biofilm. The preference of the bacteria for hydrophilic substrata as their immobilization supports is in the order; butyrate degraders, acetate degraders followed by propionate degraders.

Direct digital control of a chemical reactor

This paper presents a senior-level experiment to familiarize the student with the use of Direct Digital Control (DDC) of a continuous process. The z-transform technique is applied to the control of a simple stirred-tank reactor.