Self-purification of rivers a two-phase model for bacterial biodegradation

A simple two-phase model has been developed, which accounts for up to 95% of the total oxygen consumption during the bacterial phase of river self-purification. The two phases are characterized by a logistic model. The first corresponds roughly to a destruction of of the substrate, and is accompanied by the formation of cellular reserves. The second is the consumption of these stored substances, and begins when the external substrate ceases to be more attractive than the endogenous reserves. The cell yield as well as the stored material proportion appear to be constant, while the kinetic parameter ( ) is strongly S₀ dependent. All this can be studied through oxygen balances.     

Profiles of mean wind speeds and vertical turbulence intensities measured at seashore and two inland sites using Doppler sodars

The authors’ group has been conducting full-scale measurements of wind velocities with Doppler sodars. It is very important to accurately assess the profiles of mean wind speeds and turbulence intensities in relation to terrain roughness. In this study, the profiles were evaluated for all data measured over a long period at a seashore and two inland sites. It is confirmed that for strong winds the profiles can be approximated by a single power law at altitudes between 50 and 340 m. The power law exponents of the mean wind speed profiles are approximately 0.1 for wind from the sea and 0.2-0.3 for wind blown over land. Those of the turbulence intensity profiles are approximately 0 and −0.2 to 0.4, respectively.     

Distribution of selected anthropogenic radionuclides (137Cs, 238Pu, (239,240)Pu and 241Am) in marine sediments with emphasis on the Spitsbergen-Bear Island area

Measurements of anthropogenic radionuclides in marine sediments can provide good estimates of past and present radioactive contamination of the marine environment. The Barents Sea is one of the world's richest ocean areas, and it is therefore of major interest for Norwegian fisheries to document the levels of radioactive contamination of this and adjacent ocean areas. In this study, concentrations of 137Cs, 238Pu, (239,240)Pu and 241Am were measured in marine sediments collected mainly in the Spitsbergen-Bear Island region. Additional samples collected in the deep Norwegian Sea and near the Greenland ice-edge have been included for comparison. The highest radionuclide levels were found near Spitsbergen, reaching over 50% higher than in adjacent areas. Determinations of the mineral content and particle size distribution indicate a relation between the content of clay minerals and radionuclide levels. The present study confirms earlier observations of elevated levels of 137Cs in sediments in the Spitsbergen-Bear Island area and documents elevated levels of 238Pu, (239,240)Pu and 241Am in these sediments. Using an exponential curve fit based on determinations of unsupported 210Pb in sediment cores, sedimentation rates of 0.28, 0.05 and 0.26 cm year(-1) were calculated at locations south of Spitsbergen, near the Greenland ice-edge and in the deep Norwegian Sea, respectively.     

A steady-state model for the rotating biological disc reactor

A steady-state model for substrate removal in a rotating biological disc reactor is presented. The model considers the consumption of substrate by micro-organisms in the biofilm attached to the rotating disc, and mass transfer from the attached liquid film to the biofilm. A mass balance on substrate over the liquid in the trough provides an expression for effluent substrate in terms of the microbial kinetic constants, the geometry of the system and the operating conditions. In order to simplify the solution of the equations in the model, first order kinetics are assumed for the rate of microbial growth and substrate utilization. This simplified model can be solved, for example, by using a programmable calculator. The model predicts that the fractional removal of substrate per stage is strongly dependent on the hydraulic loading rate per unit disc area but independent of feed substrate concentration. It predicts constant removal independent of disc size provided the hydraulic loading per unit area is kept constant. The rotational speed has only a slight predicted effect on the rate of substrate removal. The predictions of the model are compared with published data from the literature for both domestic wastewater and some industrial wastes. Trends observed in the field are predicted qualitatively by the model.     

Betons a hautes performances,a granulats artificiels actifs

High strength concretes, in particular at early ages (more than 40 MPa at 3 days and up to 80 MPa at 7 days), are obtained by replacement of certain granular size ranges of sand or gravel by cement-active artificial aggregates in a given concrete composition elaborated with a local supply of aggregates and cement. The strengthening of the paste-aggregate bond by chemical reactions between the artificial aggregates and the cement also involves an improvement of other concrete performances and not only of mechanical strengths. The cracking ability is decreased, the wear strength is higher, and water permeability is low or even nil for certain compositions. The laboratory results are confirmed by tests on the site and show the possibility of achieving high strength concretes without any particular selection of cement and natural aggregates.     

Concurrent denitrification and oxygen uptake in microbial films

Mixed microbial films were grown on the inner walls of a tubular reactor with recirculation of the reactor contents and continuous flow-through of nutrient solution. The loss of total oxidized nitrogen was correlated to the film population, the nitrite concentration and the dissolved oxygen concentration in the reactor. When film population was greater than 0.5 × 10⁹ cells cm⁻², reactor dissolved oxygen concentration greater than 1 mg l⁻¹ had little effect on the nitrate loss. Nitrate loss declined for film populations greater than 2 × 10⁹ cells cm⁻². Models based on Monod and zero-order microbial kinetics were calibrated using these data and a nonlinear least squares method. There was little difference in the residual errors with these methods.     

Indirect check of aggregate pore effects on concrete

It is very difficult to quantify aggregate pores directly in a way that allows pore/water interaction to be accurately related to hardened concrete properties. This work examines, a number of aggregates through water absorption checks and gives results of tests on some concretes made with them. Three especially relevant concrete properties are examined, initial surface absorption, freeze/thaw behaviour and drying shrinkage; it is shown that aggregate absorption is reflected in these properties.     

Tunnelling projects in Southern Africa

Until some thirty years ago tunnelling in southern Africa for civil engineering purposes had been on a relatively small scale and of a sporadic nature. The first major tunnel to be built in the region was the 82 km long Orange-Fish Tunnel. Since 1970 more than 175 tunnels with an aggregate length of just over 400 km have been built. Much of the first phase of the Lesotho Highlands Water Project, which will have more than 100 km of tunnels, is about to be completed. The last thirty years represent the busiest period of tunnelling by civil engineers that southern Africa has ever known. The paper starts by summarising the early history of tunnelling in the region, and goes onto describe recent and current tunnelling projects. It also gives an indication of the possible demand for tunnels in the future.     

Circular footings resting on geotextile-reinforced sand bed

The note pertains to an experimental study made on circular footings resting on semi-infinite layer of sand reinforced with geotextiles. Using the concept of homogenization of such soils, both analytical and numerical analyses have also been conducted to predict the load-settlement behavior and compared with experimental observations. The study highlights the effect of the footing size, number of reinforcing layers, reinforcement placement pattern and bond length and the relative density of the soil on the load-settlement characteristics of the footings.     

Effect of chlorination practice on soluble organics

As the practice of chlorination for the disinfection of waste effluents is apparently to be extended, this study was initiated to examine the possibility that chlorinated organics could be formed in the disinfection process. A group of pure compounds that had been demonstrated to be susceptible to chlorination were examined under conditions likely to apply under normal disinfection practice.From the results obtained, it appears only organics with ring structures and electron activating substituents, or amino groupings are capable of chlorination. The frequent presence of ammonia or amino groups means that chloramine formation with its asociated toxicity is likely. However, the presence of ammonia or amino groups will retard the reaction with other compounds making chlorination of these unlikely.