On the design and operation of primary settling tanks in state of the art wastewater treatment and water resources recovery.

In state of the art wastewater treatment, primary settling tanks (PSTs) are considered as an integral part of the biological wastewater and sludge treatment process, as well as of the biogas and electric energy production. Consequently they strongly influence the efficiency of the entire wastewater treatment plant. However, in the last decades the inner physical processes of PSTs, largely determining their efficiency, have been poorly addressed. In common practice PSTs are still solely designed and operated based on the surface overflow rate and the hydraulic retention time (HRT) as a black box. The paper shows the results of a comprehensive investigation programme, including 16 PSTs. Their removal efficiency and inner physical processes (like the settling process of primary sludge), internal flow structures within PSTs and their impact on performance were investigated. The results show that: (1) the removal rates of PSTs are generally often underestimated in current design guidelines, (2) the removal rate of different PSTs shows a strongly fluctuating pattern even in the same range of the HRT, and (3) inlet design of PSTs becomes highly relevant in the removal efficiency at rather high surface overflow rates, above 5 m/h, which is the upper design limit of PSTs for dry weather load.

Analysis of suspended solids transport processes in primary settling tanks.

The paper shows the results of a long-term research comprising FLUENT-based numerical modeling, in situ measurements and laboratory tests to analyze suspended solids (SS) transport processes in primary settling tanks (PSTs). The investigated PST was one of the rectangular horizontal flow PSTs at a large municipal wastewater treatment plant (WWTP) of a capacity of 500,000 population equivalent. Many middle-sized and large WWTPs are equipped with such PSTs. The numerical PST model was calibrated and validated based on the results of comprehensive in situ flow and SS concentration measurements from low (5 m/h) up to quite high surface overflow rates of 9.5 and 13.0 m/h and on settling and other laboratory tests. The calibrated and validated PST model was also successfully used for evaluation of some slight modifications of the inlet geometry (removing lamellas, installing a flocculation 'box', shifting the inlet into a 'bottom-near' or into a 'high' position), which largely affect PST behavior and performance. The investigations provided detailed insight into the flow and SS transport processes within the investigated PST, which strongly contributes to hydrodynamically driven design and upgrading of PSTs.

Urban wastewater development in Central and Eastern Europe.

In the early nineties the region of Central and Eastern Europe (CEE, more than 1 million km² and 100 million inhabitants) went through fundamental political, economic and social changes which eventually led to the European integration process. This positively influenced urban water and wastewater management , which had an unbalanced structure and rather low level of development. The paper outlines first the 1990 situation (water supply, sewerage and wastewater treatment (WWT)) and the infrastructure development of the last two decades, on the basis of a comprehensive data collection for six countries (Bulgaria, the Czech Republic, Hungary, Poland, Romania and Slovakia). Austria serves as a reference basis. Alterations of some of the drivers such as GDP (Gross Domestic Product), water tariff, investment funding and legislation are studied in detail. Then, the paper focuses on WWT by analyzing data of 20 large plants. Influent and effluent quality is evaluated. Technology indicators are estimated and assessed. They include plant removal rates and violation ratios assuming the application of the Urban Wastewater Directive, primary clarifier removal rates, actual anoxic volume and sludge age in comparison with the recommendations of the ATV guideline, criteria of secondary settling tanks and energy consumption. Finally, nutrient removal rates and upgrading options are outlined.