Extension of two large wastewater treatment plants in Stockholm using membrane technology

Like many other large cities, Stockholm is facing increased urbanization with densification of infrastructure as a result. At the same time, implementation of the Baltic Sea Action Plan and the EU Water Framework Directive is expected to result in more stringent effluent quality demands. The current situation gives rise to new challenges for the municipal wastewater treatment plants (WWTPs). This paper describes how two of Sweden’s largest municipal water organizations; Stockholm Vatten and Syvab, will face these challenges using ultrafiltration (UF) membrane bioreactor (MBR) technology. The effluent requirements for the rehabilitated plants are expected to be tightened to 6 mg/l and 0.2 mg/l for total nitrogen (TN) and total phosphorus (TP), respectively

Algebraic Reynolds stress modeling of turbulence subject to rapid homogeneous and non-homogeneous compression or expansion

A recently developed explicit algebraic Reynolds stress model (EARSM) by Grigoriev et al. ["A realizable explicit algebraic Reynolds stress model for compressible turbulent flow with significant mean dilatation," Phys. Fluids 25(10), 105112 (2013)] and the related differential Reynolds stress model (DRSM) are used to investigate the influence of homogeneous shear and compression on the evolution of turbulence in the limit of rapid distortion theory (RDT). The DRSM predictions of the turbulence kinetic energy evolution are in reasonable agreement with RDT while the evolution of diagonal components of anisotropy correctly captures the essential features, which is not the case for standard compressible extensions of DRSMs. The EARSM is shown to give a realizable anisotropy tensor and a correct trend of the growth of turbulence kinetic energy K, which saturates at a power law growth versus compression ratio, as well as retaining a normalized strain in the RDT regime. In contrast, an eddy-viscosity model results in a rapid exponential growth of K and excludes both realizability and high magnitude of the strain rate. We illustrate the importance of using a proper algebraic treatment of EARSM in systems with high values of dilatation and vorticity but low shear. A homogeneously compressed and rotating gas cloud with cylindrical symmetry, related to astrophysical flows and swirling supercritical flows, was investigated too. We also outline the extension of DRSM and EARSM to include the effect of non-homogeneous density coupled with "local mean acceleration" which can be important for, e.g., stratified flows or flows with heat release. A fixed-point analysis of direct numerical simulation data of combustion in a wall-jet flow demonstrates that our model gives quantitatively correct predictions of both streamwise and cross-stream components of turbulent density flux as well as their influence on the anisotropies. In summary, we believe that our approach, based on a proper formulation of the rapid pressure-strain correlation and accounting for the coupling with turbulent density flux, can be an important element in CFD tools for compressible flows.QC 20160314. QC 20160704</p

Laboratory-scale assessment of vacuum-degassed activated sludge for improved settling properties

Vacuum degassing of activated sludge was tested at eight different Swedish wastewater treatment plants with laboratory-scale equipment in batch mode in order to evaluate its efficiency on improvement of sludge compaction and settling properties. The results show that the efficiency of the degassing technique is mainly dependent on the initial sludge volume index (SVI) of the target sludge which was found to be related to its process configuration. Facilities with full activated sludge-based nitrogen removal processes, including both nitrification and denitrification, had high SVIs (>300 mL g(-1)) and were strongly affected by vacuum degassing with reduction of SVI up to 30%. Nitrogen removal facilities also including biological phosphorus removal showed better compaction and settling properties with relatively lower SVIs and were affected to a lesser extent by degassing with SVI reduction of 10-20%. Wastewater treatment plants without full biological nitrogen removal, lacking either nitrification or denitrification (or both) processes in the activated sludge had the lowest SVIs observed with almost no effect of vacuum degassing