Nitrogen removal from pretreated wastewater in surface flow wetlands

The wastewater treatment plant in the town of Oxelösund (12,500 inhabitants), Sweden, has mechanical and chemical treatment for removal of BOD and phosphorus. With the aim to achieve 50% nitrogen removal, a surface flow wetland system of 21 ha was created as a final step during 1993. It consists of 5 cells, where 2+2 are operated in parallel with a final common cell. This allows intermittent filling and emptying, the goal of which is to promote both nitrification and denitrification for a design flow of 6000 m3 day−1. During the first year of operation, August 1993 to July 1994, the wetland removed 720 kg ha−1 of total nitrogen from the load of 1810 kg ha−1. Ammonium-N was the dominant fraction at the inlet as well as at the outlet, 79% and 90% of total nitrogen, respectively. The large fraction of NH4+ at the outlet showed that nitrification was the limiting step. An intensive monitoring effort in May 1994 indicated that neither wastewater toxicity nor oxygen deficiency were likely to limit nitrification. Instead, sub-optimal hydraulic loading conditions; a lack of suitable surfaces for ion exchange of NH4+, as well as for attachment of nitrifiers; and phosphorus deficiency, were considered potentially important factors in limiting nitrification. In addition to nitrogen removal, the wetland system reduced total phosphorus, BOD7 and E. coli (44°C) to very low levels at the outlet.</jats:p

Source apportionment of riverine nitrogen transport based on catchment modelling

Source apportionment of river substance transport, i.e. estimation of how much each source in each subbasin contributes to the river-mouth transport, is a vital step in achieving the most efficient management practices to reduce pollutant loads to the sea. In this study, the spatially lumped (at sub-catchment level), semi-empirical PULSE hydrological model, with a nitrogen routine coupled to it, was used to perform source apportionment of nitrogen transport in the Söderköpingsån river basin (882 km2) in south-eastern Sweden, for the period 1991–93. The river basin was divided into 28 subbasins and the following sources were considered: land leakage from the categories forest, arable and ley/pasture; point sources, and; atmospheric deposition on lake surfaces. The calibrated model yielded an explained variance of 60%, based on comparison of measured and modelled river nitrogen (Total N) concentrations. Eight subbasins, with net contributions to the river-mouth transport exceeding 3 kg ha−1 yr−1, were identified as the most promising candidates for cost efficient nitrogen management. The other 20 subbasins all had net contributions below 3 kg ha−1 yr−1. Arable land contributed 63% of the nitrogen transport at the river mouth and would thus be in focus for management measures. However, point sources (18% contribution to net transport) should also be considered due to their relatively high accessibility for removal measures (high concentrations). E.g., the most downstream subbasin, with the largest wastewater treatment plant in the whole river basin, had a net contribution of 16 kg ha−1 yr−1. This method for source apportionment may provide authorities with quantitative information about where in a river basin, and at which sources, they should focus their attention. However, once this is done, an analysis with higher resolution has to be performed in each of the interesting subbasins, before decisions on actual management measures can be taken.</jats:p

Wastewater treatment wetlands in cold climates

The best prospects for successful wetland treatment should be in the warmer regions of the world, but studies in North America and Scandinavia show that wetland treatment may be feasible also in cooler regions. A review shows that the number of wetlands of different types (free water surface, FWS; horizontal and vertical subsurface flow, SSF), treating different kinds of wastewater, is steadily increasing in most parts of the cold temperate regions of the world. The major wetland engineering concerns in cold climates, which are discussed in this paper, are related to: (1) ice formation, and its implications for hydraulic performance; (2) hydrology and hydraulic issues besides ice formation; and (3) the thermal consequences for biologically or microbiologically mediated treatment processes. Energy- and water-balance calculations, as well as thermal modeling, are useful tools for successful design and operation of treatment wetlands, but the shortage of data makes it necessary to adopt a conservative approach. The treatment processes often appear less temperature sensitive in full-scale wetlands as compared to laboratory incubations. Several possible explanations are discussed in the paper: (1) sedimentation playing a significant role, (2) overdimensioning in relation to some constituents, (3) seasonal adsorption (cation exchange) of ammonium, and (4) temperature adaptation of the microbial community. Experience shows that cold climate wetlands can meet effluent criteria for the most important treatment parameters. To gain wide acceptance, however, we need to become more specific about design and construction, and also about operation, maintenance and cost-effectiveness. These goals require detailed knowledge about processes in full-scale wetlands, including long-term changes and response to maintenance.</jats:p

The consequence of the chemical composition of HPMC in matrix tablets on the release behaviour of model drug substances having different solubility

This study investigates the effect of the chemical heterogeneity of hydroxypropyl methylcellulose (HPMC) on the release of model drug substances from hydrophilic matrix tablets. The hypothesis was that the release of drug substances could be influenced by possible interactions with HPMC batches having different chemical heterogeneity. The cloud point of the most heterogeneous batch was more affected by the model drug substances, methylparaben and butylparaben, and most by butylparaben with the lowest solubility. The different clouding behaviour was explained by the heterogeneously substituted batches being more associative and the more lipophilic butylparaben being able to interact more efficiently with the hydrophobic HPMC transient crosslinks that formed. Interestingly, tablet compositions of the heterogeneously substituted HPMC batches released the more soluble methylparaben at lower rates than butylparaben. The explanation is that the hydrophobic HPMC interactions with butylparaben made the gel of the tablet less hydrated and more fragile and therefore more affected by erosional stresses. In contrast, drug release from compositions consisting of the more homogeneously substituted batches was affected to a minor extent by the drugs and was very robust within the experimental variations. The present study thus reveals that there can be variability in drug release depending on the lipophilicity of the drug and the substituent heterogeneity of the HPMC used

The effect of substitution pattern of HPMC on polymer release from matrix tablets

The purpose of this study was to gain further understanding of how the substituent heterogeneity of hydroxypropyl methylcellulose, HPMC, affects the polymer release from hydrophilic matrix tablets. The hypothesis was that the heterogeneous substituent pattern facilitated hydrophobic interactions that increased the viscosity and therefore affected the release rate to a major extent. Polymer tablets were prepared from three heterogeneously substituted HPMC batches of the same substituent (2208) and viscosity (100 cps) grade. To elucidate the hypothesis, fractions of both the dissolved polymer and the tablet residue were collected from the dissolution bath and further characterised. The extensive characterisation showed that, although the dissolved bath fraction and the tablet residue had a similar average degree of substitution, the residue was more heterogeneously substituted. It was further revealed that the heterogeneous substituent pattern of the tablet residue facilitated the formation of soluble gel-like components already at room temperature, which increased the viscosity. The viscosity increased by 150% at temperatures correlated to the dissolution bath, and it was thus concluded that the gel-like components grew in size with temperature. Finally, much lower release rates were obtained by tablets composed of the residue compared to tablets composed of the bath fraction, which clarified the hypothesis

Accuracy in multiangle light scattering measurements for molar mass and radius estimations. Model calculations and experiments.

Multiangle light scattering (MALS) is a well-established technique used to determine the size of macromolecules and particles. In this study, different extrapolation procedures used in MALS were investigated with regard to accuracy and robustness in the obtained molar mass and rms radius. Three different mathematical transformations of the light scattering function referred to as the Debye, Zimm, and Berry methods for constructing the Debye plot were investigated for two idealized polymer shapes, homogeneous spheres and random coils, with radii from 25 to 250 nm. The effect of the angular interval used for the extrapolation was investigated, as was the robustness of the different transformations toward errors in the measured light scattering intensity at low angles. For an rms radius less than 50 nm, the relative error in molar mass due to extrapolation was less than 1% independent of the method used. For larger radii, the error increased and the extrapolation procedure became more critical. For random coil polymers, the Berry method was superior in terms of accuracy and robustness. For spheres, the Debye method was superior. The Zimm method was inferior to the others. The different extrapolation methods were evaluated and compared on experimental data from a size exclusion chromatography-MALS analysis of an ultrahigh molar mass poly(ethylene oxide) (PEO). The PEO data qualitatively verified the calculations and stressed the importance of optimizing the extrapolation procedure after careful evaluation of the experimental data. A discussion of how to detect erroneous data in an experimental Debye plot is given

Release of theophylline and carbamazepine from matrix tablets - Consequences of HPMC chemical heterogeneity

The release of theophylline and carbamazepine from matrix tablets composed of microcrystalline cellulose, lactose and hydroxypropyl methylcellulose (HPMC) was studied. The aim was to investigate the effect of different substituent heterogeneities of HPMC on the drug release from matrix tablets composed of either 35% or 45% HPMC. The release of the poorly soluble carbamazepine was considerably affected by the HPMC heterogeneity, and the time difference at 80% drug release was more than 12 h between the formulations of different HPMC batches. This was explained by slower polymer erosion of the heterogeneous HPMC and the fact that carbamazepine was mainly released by erosion. In addition, results from magnetic resonance imaging showed that the rate of water transport into the tablets was similar. This explained the comparable results of the release of the sparingly soluble theophylline from the two formulations even though the polymer erosion and the swelling of the tablets were considerably different. Thus, it can be concluded that the drug release was highly affected by the substituent heterogeneity, especially in the case of carbamazepine, which was released mainly by erosion. (C) 2011 Elsevier B.V. All rights reserved

Polysaccharide characterization by flow field-flow fractionation-multiangle light scattering: Initial studies of modified starches

Chemically modified starches are commonly used for various purposes. Depending on the type of derivatization, a chemical degradation of the original polymeric structure may occur, resulting in a change of molar mass. It is therefore always of interest to know the molar mass and possibly the conformation of the derivative. Four commercially available hydroxypropyl and hydroxyethyl modified starches were examined by asymmetrical flow field-flow fractionation combined with multiangle laser light scattering. The weight-average molar mass and the molar mass distribution were determined, with emphasis put on the rapid analysis and studies of the suitable experimental conditions regarding flow rates so that accurate data were obtained. The molar mass distribution determinations showed good reproducibility, and repeatability and were fast. Efforts to obtain conformational information are described