Disinfection for decentralized wastewater reuse in rural areas through wetlands and solar driven onsite chlorination.

Increasing water scarcity is of growing concern in Europe, especially in Mediterranean countries along coastlines. Wastewater reuse reduces water stress, but often requires the absence of pathogen indicators and the application of chlorine to assure residual disinfection. However, the effluent qualities of typical Wastewater Treatment Plants (WWTP) show immense chlorine demands. This makes the supply, handling and dosing of typical WWTP effluent challenging, especially in rural regions. In the work presented here, a vertical flow constructed wetland (VFCW) was combined with a small-scale solar-driven Onsite Chlorine Generation system (OCG) to further improve effluent qualities for different WWTPs and to produce chlorine stock solution directly at the site. To test different operational conditions the VFCW received WWTP effluent from a) an Activated Sludge and b) a High-Rate Algae Pond system. The VFCW reduced TSS by 92%, COD by 80%, and NH4 by over 99% and the chlorine demand by 85%. The log-unit reduction of the VFCW/OCG system reached ≥5.1 for total coliforms and ≥4.6 for E. Coli. During VFCW passage the already high electrical conductivity further increased to beyond permissible reuse limits due to high evapotranspiration (ET) rates of the planted vegetation Arundo donax. Unique aspects of this setup were that neither chemicals nor external electricity were required to run the system. The elevated chloride concentration of the treated WW (819 ± 132 mg/L) proved sufficient for the production of chlorine stock solution. However, the solar-driven OCG system tested here consumed considerably more electricity compared to other OCGs available on the market. Nevertheless, the system presented here may be considered an efficient disinfection alternative for decentralized WW reuse applications at remote sites with both limited access to grid electricity and strict requirements for pathogen indicators.

Under-sampling and compressed sensing of 3D spatially-resolved displacement propagators in porous media using APGSTE-RARE MRI.

A method for under-sampling and compressed sensing of 3D spatially-resolved propagators is presented and demonstrated for flow in a packed bed and a heterogeneous carbonate rock. By sampling only 12.5% of q,k-space, the experimental acquisition time was reduced by almost an order of magnitude. In particular, for both systems studied, a 3D image was acquired at 1 mm isotropic spatial resolution such that 134,400 local propagators were obtained. Data were acquired in ~1 h and ~11 h for the packed bed and rock, respectively. It is shown that spatial resolution and under-sampling using this implementation retains the quantitative nature of the propagator measurement, and differences between implementation of this measurement in two and three dimensions are identified. The potential for 3D spatially-resolved propagators to provide new insights into transport processes in porous media by characterisation of the statistical moments of the propagators is discussed.

Fast spatially-resolved T2 measurements with constant-gradient CPMG.

Speed of acquisition is paramount for the application of magnetic resonance to flow experiments through porous rocks. One popular method for imaging core floods is the spatially resolved T2 experiment which can separate fluids either by their viscosity contrast or by doping one fluid with a relaxation agent. Existing techniques for spatial-T2 may suffer from long acquisition times and eddy currents due to the pulsing of magnetic field gradients. Here, we propose a constant gradient method for 1d spatially-resolved T2 which embraces the speed of frequency encoding techniques and avoids eddy currents by the absence of any gradient ramps during the radio frequency (r.f.) pulse train. We provide the operating envelope for this kind of experiment, which is restricted due to the slice selectivity of the r.f. pulses in the presence of the magnetic field gradient. Additionally, we show that the effects of self-diffusion and the mixing of T1 and T2 contributions are manageable. As an illustration, we have applied this technique to an enhanced oil recovery experiment. The two fluid phases were tracked without any doping and with a time resolution of 40 s. In this case, the increased time resolution allowed us to observe dynamic flow phenomena such as fluid fingering and the calculation of the velocity of the fluid displacement fronts.

Magnetic-resonance pore imaging of nonsymmetric microscopic pore shapes.

Imaging of the microstructure of porous media such as biological tissue or porous solids is of high interest in health science and technology, engineering and material science. Magnetic resonance pore imaging (MRPI) is a recent technique based on nuclear magnetic resonance (NMR) which allows us to acquire images of the average pore shape in a given sample. Here we provide details on the experimental design, challenges, and requirements of MRPI, including its calibration procedures. Utilizing a laser-machined phantom sample, we present images of microscopic pores with a hemiequilateral triangular shape even in the presence of NMR relaxation effects at the pore walls. We therefore show that MRPI is applicable to porous samples without a priori knowledge about their pore shape and symmetry. Furthermore, we introduce "MRPI mapping," which combines MRPI with conventional magnetic resonance imaging (MRI). This enables one to resolve microscopic pore sizes and shapes spatially, thus expanding the application of MRPI to samples with heterogeneous distributions of pores.

Symmetry of the gradient profile as second experimental dimension in the short-time expansion of the apparent diffusion coefficient as measured with NMR diffusometry.

The time-dependent apparent diffusion coefficient as measured by pulsed gradient NMR can be used to estimate parameters of porous structures including the surface-to-volume ratio and the mean curvature of pores. In this work, the short-time diffusion limit and in particular the influence of the temporal profile of diffusion gradients on the expansion as proposed by Mitra et al. (1993) is investigated. It is shown that flow-compensated waveforms, i.e. those whose first moment is zero, are blind to the term linear in observation time, which is the term that is proportional to mean curvature and surface permeability. A gradient waveform that smoothly interpolates between flow-compensated and bipolar waveform is proposed and the degree of flow-compensation is used as a second experimental dimension. This two-dimensional ansatz is shown to yield an improved precision when characterizing the confining domain. This technique is demonstrated with simulations and in experiments performed with cylindrical capillaries of 100 µm radius.