H. Kainz

Influence of secondary settling tank performance on suspended solids mass balance in activated sludge systems

Secondary settling is the final step of the activated sludge-based biological waste water treatment. Secondary settling tanks (SSTs) are therefore an essential unit of producing a clear effluent. A further important function of SSTs is the sufficient thickening to achieve highly concentrated return sludge and biomass within the biological reactor. In addition, the storage of activated sludge is also needed in case of peak flow events (Ekama et al., 1997). Due to the importance of a high SST performance the problem has long been investigated (Larsen, 1977; Krebs, 1991; Takács et al., 1991; Ekama et al., 1997; Freimann, 1999; Patziger et al., 2005; Bürger et al., 2011), however, a lot of questions are still to solve regarding e.g. the geometrical features (inflow, outflow) and operations (return sludge control, scraper mechanism, allowable maximum values of surface overflow rates). In our study we focused on SSTs under dynamic load considering both the overall unsteady behaviour and the features around the peaks, investigating the effect of various sludge return strategies as well as the inlet geometry on SST performance. The main research tool was a FLUENT-based novel mass transport model consisting of two modules, a 2D axisymmetric SST model and a mixed reactor model of the biological reactor (BR). The model was calibrated and verified against detailed measurements of flow and concentration patterns, sludge settling, accompanied with continuous on-line measurement of in- and outflow as well as returned flow rates of total suspended solids (TSS) and water. As to the inlet arrangement a reasonable modification of the geometry could result in the suppression of the large scale flow structures of the sludge-water interface thus providing a significant improvement in the SST performance. Furthermore, a critical value of the overflow rate (q(crit)) was found at which a pronounced large scale circulation pattern develops in the vertical plane, the density current in such a way hitting the outer wall of the SST, turning then to the vertical direction accompanied with significant flow velocities. This phenomenon strengthens with the hydraulic load and can entrain part of the sludge thus resulting in unfavourable turbid effluent. As a representative case study an operating circular SST most commonly used in practice was investigated. Focusing on the sludge return strategies, it was found that up to a threshold peak flow rate the most efficient way is to keep the return sludge flow rate constant, at 0.4Q(MAX). However, once the inflow rate exceeds the threshold value the return sludge flow rate should be slowly increased up to 0.6Q(MAX), performed in a delayed manner, about 20-30 min after the threshold value is exceeded. For preserving the methodology outlined in the present paper, other types of SSTs, however, need further individual investigations.

Analysing sludge balance in activated sludge systems with a novel mass transport model.

In activated sludge systems the mechanically treated wastewater is biologically cleaned by biomass (activated sludge). The basic requirement of an efficient biological wastewater treatment is to have as a high biomass concentration in the biological reactor (BR) as possible. The activated sludge balance in activated sludge systems is controlled by the settling, thickening, scraper mechanism in the secondary settling tank (SST) and sludge returning. These processes aim at keeping maximum sludge mass in the BR and minimum sludge mass in the SST even in peak flow events (storm water flow). It can be, however, only reached by a high SST performance. The main physical processes and boundary conditions such as inhomogeneous turbulent flow, geometrical features of the SST, wastewater treatment plant (WWTP) load, return sludge flow, sludge volume index etc. all influence settling thickening and sludge returning. In the paper a novel mass transport model of an activated sludge system is presented which involves a 2-dimensional SST model coupled with a mixed reactor model of the biological reactor. It makes possible to investigate different sludge returning strategies and their influence on the sludge balance of the investigated activated sludge system, furthermore, the processes determining the flow and concentration patterns in the SST. The paper gives an overview on the first promising model results of a prevailing peak flow event investigation at the WWTP of Graz.

Messung und Modellierung von physikalischen Prozessen in Nachklärbecken

KurzfassungBei der Dimensionierung von Nachklärbecken (NKB) nach den aktuellen Bemessungsvorschriften werden die beckeninternen Prozesse und die Betriebsweise kaum berücksichtigt, auch wenn diese großen Einfluss auf eine wirksame Abscheidung besitzen. Die Strömungs- und lónzentrationsverteilungen eines NKB können anhand vonIN SITU, hochauflösenden Geschwindigkeits- und Konzentrations messungen detailliert beobachtet werden. Die gewonnenen Messdaten liefern die Grundlage zur Kalibrierung und Validierung eines numerischen Nachklärbecknmodells. Mit diesem Modell kann die Geometrie und die Betriebsweise des NKB optimiert werden. Mit einer guten Einlaufgeometrie und angepassten Rücklaufschlammförderung kann die Beckenleistung wesentlich gesteigert werden. Dieser Beitrag behandelt Ultraschall Doppler Geschwindigkeitsmessungen und optische Konzentrationsmessungen in einem NKB auf der Kläranlage Graz. Die Untersuchungen zeigeninteressante Details im Hinblick auf die Strömungsvorgänge und Turbulenzen sowie die Veränderungen der Schlammoberfläche und den Feststofftransport im untersuchten NKB. Mit Hilfe eines numerischen Simulationsmodells erfolgte im zweiten Schritt die Abbildung und Analyse der Systemverhältnisse.SummaryIn the design of Secondary Settling Tanks (SST) the detailed inner hydrodynamic behaviour and the operation parameters are still often poorly addressed. Coupled fine scale flow and concentration measurement in operating tanks can give more insight into the interconnected flow and concentration pattern, furthermore, provide validation data for advanced numerical models. The latter can serve then as design tool to optimize the performance by finding proper tank geometry, sludge recycling ratio etc. This paper presents the first results of such an investigation of one of the SSTs at the Waste Water Treatment Plant of Graz. Acoustic Doppler velocity and optical turbidity measurements gave both overall information and novel details on the wavy character of the sludge-water interface. FLUENT-based axisymmetric flow and coupled activated sludge transport modelling could then reasonably reproduce measured distributions and patterns, promising for future modelling in order to reach enhanced performance.

Adaption of the main waste water treatment plant in Vienna to meet Austrian emission regulations

The modification of the main waste water treatment plant in Vienna will take place in accordance with the minimum efficiences laid down in the emission regulations as issued in 1991 by the Austrian Federal Office for Agriculture and Forestry. To meet these figures it is necessary to adapt the plant by 2001. The studies on several variants and the evaluation process showed a 2-step technology with partial by-passing of the 1st step to be the optimal solution. For this flexible system a new aeration tank volume of only 210,000 m 3 is sufficient. Test-runs with a semi-commercial plant confirmed the correctness of all calculations. Possibilities for further modifications have been considered, e.g. dimensioning of all relevant hydraulic installations up to 24 m 3 /sec, final purification by sand or flocculant filtration and spare areas for measures after 2015.

Haltung, Konsequenz, Leidenschaft und Energie

Univ.-Prof. Dipl.-Ing. Dr. techn. Dr. h.c. Harald Kainz Michael Muhrs Vitae ist nicht nur eng mit der Technischen Universitat Graz verknupft, fur mehr als vier Jahrzehnte wurde Osterreichs alteste technische universitare Institution gleichermasen von dessen Arbeit mitgeformt und gepragt. Eine an Konstanz und Leistungsbereitschaft nur schwer zu uberbietende universitare Karriere fuhrte uber die vielfaltigsten Positionen und Funktionen. In den Fuhrungsgremien unserer Universitat, als Senatsvorsitzender und Vizerektor konnte Michael Muhr all diese Erfahrungen bestmoglich einsetzen. Mit enormer Energie durchlief Michael Muhr samtliche universitare Herausforderungen, feierte Erfolge, ohne dabei soziale Fahigkeiten zu vernachlassigen. Deutliche Haltungen, absolute Leistungsbereitschaft bei gleichzeitig wertschatzendem Umgang mit allen ihn umgebenden Menschen, das Verstandnis fur Sorgen und Herausforderungen anderer und klare Worte gegenuber auftretenden Ungerechtigkeiten: Eigenschaften, welche, neben hochster wissenschaftlicher Kompetenz, zu Michael Muhrs elementarsten Qualitaten zahlen. Michael ist daruber hinaus ein begnadeter, von seinen Studierenden zutiefst geschatzter Lehrer, international hochst reputierter und dekorierter Wissenschafter, politisch denkender, das klare Wort sprechender rastloser Arbeiter. Das Institut fur Hochspannungstechnik ebenso wie die gleichnamige Versuchsanstalt verfugen heute uber Auftrage aus aller Welt; die europaische E-Wirtschaft kooperiert aufgrund einmaliger Laborbedingungen intensiv mit der TU Graz. Dies macht wissenschaftliche Qualitat und wirtschaftliche Bedeutung der von Michael Muhr und dessen Team geleisteten Arbeit einmal mehr deutlich, Leistungen, fur welche Dir, lieber Michael, namens der gesamten Universitat zu danken ist. Wer Michael Muhr kennt, weis, dessen Tatendrang ist nicht an Ehrungen oder Jubilaen, sondern ausschlieslich entlang wissenschaftlicher Neugierde und sozialem Interesse ausgerichtet. Der Mensch und Wissenschafter Michael Muhr verfolgt Ziele mit endlos erscheinender Energie, Fokus und Leidenschaft. Namens der gesamten TU Graz wunsche ich Dir, lieber Michael, viel Freude bei der Wahrnehmung neuer Herausforderungen und bitte Dich, Deiner TU Graz weiterhin verbunden zu bleiben!