The situation in the treatment of wastewaters from small sources in Czechoslovakia has been discussed and two types of manufactured rotating biological contactors have been described. The evaluation of RBCs' operation showed the main disadvantages of the contactors with conventional discs, viz. the low 0C and low mixing effect. In a newly designed RBC, the discs or packets of discs were replaced by a cage packed with a random medium. The cage was equipped with tubular aeration and mixing elements. The long-term tests with a pilot-plant and a full-scale unit using synthetic as well as municipal wastewaters proved the ability of the packed-cage RBC to achieve a low effluent BOD with such organic loadings when the effluent from the conventional RBCs already deteriorated. Besides the BOD removal the 0C of the packed-cage RBCs was tested to verify the possibility of the combined cultivation of suspended and fixed-film biomass. On the basis of results presented here, a new package wastewater treatment plant for about 500 PE will be designed.

Damage of construction works due to flooding in 1997 and 2002 in the Czech Republic initiated investigations of structural failures and reassessment of available data for discharge extremes. In this study, hydrological data for 166 annual maximum discharges of the Vltava River in Prague since 1827 are analysed using various statistical methods. Moment characteristics of the measurements -the mean, standard deviation and skewness -are estimated and the enhancing effect of an exceptional observation in 2002 is detected. The annual maxima are described by two-or three-parameter lognormal distributions and the extreme value distributions of the type I and II. Standard statistical Kolmogorov and chi-square tests are applied to assess goodness of fit of the theoretical models. It appears that a two-parameter lognormal distribution may be the most suitable theoretical model. Assuming this distribution, extreme discharges corresponding to characteristic and design values are estimated. It is shown that the partial safety factor estimated from the measurements significantly differs from the recommended value of 1.5. The discharge in 2002 corresponds to an exceptionally long return period. It is concluded that statistical methods provide a valuable background for evaluation and prediction of discharges. However, the presented analysis should be further improved to include non-statistical aspects that influence discharges such as the effects of water management and deforestation.