Analysis of a ring-laser gyroscope with intracavity phase-conjugate coupling

A ring-laser gyroscope with the opposite modes coupled by an intracavity phase-conjugate interaction is examined analytically and numerically. It is shown that the coupling mitigates cross-saturation-induced mode extinction, allowing a homogeneously broadened medium to be used as the gain for a laser gyroscope. It is also shown that the coupling can induce a bias beat frequency that can potentially be used as an all-optical dither. The phase conjugation is shown to affect the lock-in threshold only indirectly. The results are compared with pertinent experiments from the literature.     

Feasibility of measuring phase relaxation time with subpicosecond pulses

Techniques to determine the phase relaxation time of absorbing dye solutions are evaluated. It is shown that, contrary to a recent paper by Cho et al., transmission of low intensity pulses cannot yield information on the relaxation of broad-band absorbing transitions.     

Validation of a supervisory control system for energy savings in membrane bioreactors

The application of fixed operational protocols and settings for membrane bioreactors (MBR) often leads to suboptimal filtration conditions due to the dynamic nature of mixed liquor characteristics. With regard to process optimization and energy savings, the potential benefits of a dynamic control system, enabling to adapt fouling control actions (ACS outputs) in an automated way to the actual mixed liquor fouling propensity, are thus obvious. In this paper, the pilot-scale validation of such an advanced control system (ACS) is elaborated. A specific on-line fouling measurement method, the MBR-VFM (VITO Fouling Measurement), was used for the evaluation of the mixed liquor’s reversible fouling propensity, which was used as a primary ACS input parameter. A first series of tests with a gradual increase in complexity of the selected input and output parameters indicated the functionality of the ACS and demonstrated a substantial reduction of aeration, however sometimes at the expense of a higher fouling rate. The ACS was further fine-tuned and subsequently tested for a longer period under more dynamic operating conditions. A significant correlation was found between the reversible fouling potential measured by the MBR-VFM and the on-line permeability, indicating that the MBR-VFM is a suitable ACS input. Furthermore, an average 22% reduction in aeration flow to the membranes could be achieved without any obvious negative effect on filtration performance. This indicates that this approach is promising to optimize energy consumption in MBRs.     

Degenerate four-wave mixing of picosecond pulses in the saturable amplification of a dye laser

We demonstrate intracavity degenerate four-wave mixing in the saturable gain medium of a dye laser. The dye laser is a synchronously pumped bidirectional ring laser that produces pulses of 4 ps duration. The dynamics of the interaction provides information on the phase coherence of the pulses. A simple theory of transient degenerate four-wave mixing is presented that accurately describes the measurements.     

Simulating water and nitrogen behaviour in soils cropped with winter wheat

The SWATNIT model [26], predicting water and nitrogen transport in cropped soils, was evaluated on experimental data of winter wheat for different N treatments. The experiments were monitored at three different locations on different soil types in the Netherlands. Crop growth was simulated using the SUCROS model [11] which was integrated in the SWATNIT model. Both water and nitrogen stress were incorporated. Except for the soil hydraulic properties, all model parameters were taken from literature. The model performance was evaluated on its capability to predict soil moisture profiles, nitrate and ammonia profiles, the time course of simulated total dry matter production and LAI; and crop N-uptake. Results for the simulations of the soil moisture profile indicate that the soil hydraulic properties did not reflect the actual physical behaviour of the soil with respect to soil moisture. Good agreement is found between the measured and simulated nitrate and ammonia profiles. The simulation of the nitrate content of the top layer at Bouwing was improved by increasing the NH ₄ ⁺ -N-distribution coefficient thereby improving the simulation of the NH ₄ ⁺ -N-content in this layer. Deviations between simulated and measured nitrate concentrations also occurred in the bottom layers (60–100 cm) of the soil profile. The phreatic ground water might influence the nitrate concentrations in the bottom layers. Concerning crop growth modelling, improvements are needed with respect to the partitioning of total dry matter production over the different plant organs in function of the stress, the calculation of the nitrogen stress and the total nitrogen uptake of the crop through a better estimate of the N-demand of the different plant organs.     

Effect of humic acids on heavy metal removal by zero-valent iron in batch and continuous flow column systems

The effects of Aldrich humic acids (HA) on the removal of Zero-valent iron (ZVI) was investigated in laboratory systems. In batch, the removal rate of Zn and Ni (5 mg l(-1)) was, respectively, 2.8 and 2.4 times lower in the presence of HA (20 mg l(-1)) than in the absence of HA, presumably due to the formation of HA-heavy metal complexes which prevented the removal reactions at the ZVI surface. Chromate removal was not affected. In a column test, two parallel systems were supplemented with a continuous input of simulated groundwater containing a mixture of the heavy metals Zn, Ni and Cr(VI) (5 mg l(-1) each), with or without HA (at 20 mg l(-1)). Initially, the two column systems efficiently (>90%) removed the heavy metals from the simulated groundwater. When the input heavy metal concentration was increased to 8-10 mg l(-1), a significant breakthrough of Ni and Zn, up to 80%, occurred in the column system fed with HA. Chromate and HA did not significantly break through. After 60 weeks, the effect of HA on leaching of the accumulated metals (approx. 2 mg g(-1)) was investigated. No significant leaching was observed. The results of this study suggest that the impact of dissolved organic matter on the efficiency and lifetime of a ZVI barrier for in situ removal of heavy metals should be considered in the design of the barrier.     

Anode and cathode materials characterization for a microbial fuel cell in half cell configuration

Microbial fuel cells (MFCs) are novel bioelectrochemical devices for spontaneous conversion of biomass into electricity through the metabolic activity of the bacteria. Microbial production of electricity may become an important source of bioenergy in future because MFCs offer the possibility of extracting electric current from a wide range of soluble or dissolved complex organic wastes and renewable biomass. However, the materials used in these devices are still not economic and researchers use different materials as cathode and anode in MFCs. This results in variable performance which is difficult to compare. We tested several commercially available materials for their suitability as anode in an acetate fed MFC. Besides, a novel non-platinized activated carbon (AC) based, gas porous air cathode was also tested. Both the anode and cathode were tested in a half cell configuration. Carbon cloth, graphite cloth and dynamically stable anode (DSA) served as ideal anode material with carbon cloth and graphite mesh reaching the open circuit voltage (OCV) of acetate oxidation (-500 mV vs. Ag/AgCl). The effect of increasing concentration of acetate on anode OCV was also investigated and results showed that on increasing the acetate concentration from 10 mM to 40 mM has no adverse impact on the anodic activity towards electrochemical oxidation of acetate. The AC cathode showed stable current (-1.2 mA/cm2) over a period of 100 days.     

Biomass estimations and carbon stock calculations in the oil palm plantations of African derived savannas using IKONOS data

This study developed biomass models to calculate carbon stock levels of the West African oil palms (Elaeis guineensis) using multi-date wet and dry season IKONOS images. Two benchmark areas of the derived savanna eco-regions of Africa were selected for analysis. Allometric equations related above-ground palm biomass to their stem heights. Empirical regression models based on field plot data were established to determine wet and dry biomass (kg m^?2) of oil palm plantations in IKONOS images. The best models were exponential, involving bands 3, 3 and 1, or 3 and 4, and explaining between 63 and 72% of the variability in the data. Model evaluations with independent datasets showed there is 28-36% uncertainty in dry biomass predictions. At the landscape level, multi-date IKONOS data mapped oil palm plantations with an overall accuracy of 88-92%. However, the ability of IKONOS data to differentiate various age groups of oil palms was limited with a high degree of intermixing of classes. The best results were obtained when delineating agro-palm (palms mixed with agriculture and fallows), palm of 1-3 years, and palm of 4-5 years at an overall accuracy of 74.5% using all four IKONOS bands. The results indicate the need for additional spectral bands in the IKONOS sensor. The total carbon per unit area of oil palms was calculated across age groups for the two benchmark areas of West Africa and were 14.75 and 14.94 tonnes ha^?1 (or Mg ha^?1), respectively. The corresponding dry biomass (kg m^?2) were 29.5 and 29.88 tonnes ha^?1 (or Mg ha^?1). The age of the oil palms were between 1 and 5 years across benchmark areas. The mean rate of accumulation of carbon was 2.95 t C ha^?1 year^?1 in benchmark area 1 and 2.99 t C ha^?1 year^?1 in benchmark area 2.