An asymptotic higher-order theory for rectangular beams.

A direct asymptotic integration of the full three-dimensional problem of elasticity is employed to derive a consistent governing equation for a beam with the rectangular cross section. The governing equation is consistent in the sense that it has the same long-wave low-frequency behaviour as the exact solution of the original three-dimensional problem. Performance of the new beam equation is illustrated by comparing its predictions against the results of direct finite-element computations. Limiting behaviours for beams with large (and small) aspect ratios, which can be established using classical plate theories, are recovered from the new governing equation to illustrate its consistency and also to illustrate the importance of using plate theories with the correctly refined boundary conditions. The implications for the correct choice of the shear correction factor in Timoshenko's beam theory are also discussed.

Nitrogen elimination from landfill leachates using an extra carbon source in subsurface flow constructed wetlands.

A three-stage constructed wetland for leachate treatment was monitored on a landfill at a pilot scale. The plant had been designed to achieve at least 75% nitrogen removal. NH4-N input concentration was 240 (median) up to 290 mgl(-1) and COD concentration was 455 to 511 mgl(-1), respectively. A 14 m2 vertical flow sand filter plus a 14 m2 horizontal flow sand filter followed by a 3.3 m2 vertical flow sand filter was chosen. Acetic acid was added to the horizontal flow system for denitrification. The results showed a very stable nitrification rate within the vertical flow system of 94% (median) at NH4-N loading rates of about 10 (median) up to 17 gm(-2)d(-1). Denitrification was mainly dependent on the dosing of acetic acid and could reach a maximum of 98%. One interesting effect was the production of nitrite in the horizontal flow sand filter. This could efficiently be eliminated by the subsequent vertical flow sand filter. The chosen concept proved to be very effective for nitrogen removal. In combination with a final activated carbon filter the COD effluent concentrations could be easily and safely controlled. The design of denitrification reed beds showed a further potential for optimization.

Greywater recycling systems in Germany--results, experiences and guidelines.

Although Germany is not considered a water-poor country, there exist regional differences in water supply and consumption. During the past 15 years, the greywater aspect has been dealt with in Germany with a greater interest and variable success. In addition to an increased environmental awareness, water costs also play an important role in increasing the demand for advanced greywater treatment plants nstalled in buildings. Under favourable conditions, the amortisation costs usually lie between 5 and 7 years. Systems that have been extensively tried and tested and have been shown to be most reliable are those employing an advanced biological treatment followed by an UV disinfection. Systems based on membrane technology are being developed and researched intensively in Germany for municipal wastewater treatment. However, so far they play no role in greywater recycling. Greywater systems operating under low energy and maintenance requirements without the use of chemicals are mostly favoured. In Germany, greywater recycling systems should be registered at the Health Office in order to guarantee that no cross-connections exist with the drinking water network and that pipes are labelled according to regulations. The hygienic requirements for recycled greywater, which is primarily used for toilet flushing, are oriented towards the EU-Guidelines for Bathing Waters. The use of recycled greywater for irrigation purposes is minor. As to the use of recycled water for laundry, the first promising investigation results are now available.

[Microbiological public health aspects in the use of rain water as water reservoirs for toilet flushing, garden irrigation and laundry]. / Mikrobiologisch-hygienische Aspekte bei der Nutzung von Regenwasser als Betriebswasser für Toilettenspülung, Gartenbewässerung und Wäschewaschen.

From a total of 102 rain water cisterns in use for toilet flushing, garden irrigation and laundering washing about 1,600 water samples were collected and subjected to microbiological analysis. The assays included aerobic heterotrophic microorganisms growing at 20 and 37 degrees C, respectively, as well as the identification of Escherichia coli, coliform organisms, faecal streptococci, Pseudomonas aeruginosa, staphylococci, yersiniae, salmonellae, shigellae, legionellae and yeasts. The median of the total number of cells per ml was 1,200 at 20 degrees C and 230 at 37 degrees C, respectively. Approximately 26 E. coli cells and 198 coliform organisms (median values) were found per 100 ml. In the case of cisterns manufactured of plastic the total number of cells was generally found to be lower than in samples collected from concrete or brick-made storage tanks. With the exception of the ubiquitously distributed organism Pseudomonas aeruginosa (found in 11.8% of the samples) and salmonella in only one sample, no other pathogens were detected. More than 95% of all analysed samples met the quality standards for bathing waters as set by the European Community. Provided certain precautions are taken, such as strict separation of mains for drinking water and rain water, as well as correct labelling of pipelines and collection sites, the use of rain water for toilet flushing, garden irrigation and laundry washing presents no unacceptable risk to public health.