Reduction of nitrates in a photocatalytic membrane reactor in the presence of organic acids.

The present study explores the capability of the Photocatalytic Membrane Reactor (PMR) technology to remove nitrates from drinking water sources in the presence of organic electron donors. A systematic investigation was performed in a laboratory-pilot PMR, employing a hybrid TiO2/UV-A catalysis-ultrafiltration process, using formic acid as the most favorable hole scavenger for nitrate reduction. The naturally occurring humic acid as well as the cheaper and harmless acetic acid were also tested as sacrificial electron donors. The performance of the PMR system was evaluated in respect of nitrate and Total Organic Carbon (TOC) percentage removal. The results demonstrate the superiority of formic acid regarding nitrate reduction, followed by acetic acid and humic acid, as well as the negligible effect of nitrates on organic mineralization. Formic acid was further used to assess the effects of molar organic/nitrate ratio, catalyst dosage and power of irradiation per unit volume (PR) on nitrate reduction. With the present laboratory-pilot, the near optimum nitrate removal was ~60 % and the TOC reduction ~85 %; selectivity to dinitrogen was in the range 65-90 %. Key issues for improving this synergistic process and related R&D directions are discussed.

Assessing the efficiency of a pilot-scale GDE/BDD electrochemical system in removing phenol from high salinity waters.

Enhanced mineralization of phenol in brines with high chloride content was investigated by employing an electrochemical advanced oxidation treatment that couples anodic oxidation, electrochlorination and electro-Fenton in a single process. Experimental work was carried out in a pilot scale unit with an undivided plate-and-frame cell equipped with a boron-doped diamond anode and a carbon-PTFE gas diffusion electrode as cathode, in batch recirculation mode. The effects of operating conditions on phenol degradation, including current density, air flow rate, water feed flow rate, Fe2+ dosage and pH as well as of the water matrix, were evaluated. Applied current exhibited the greatest effect on phenol degradation/mineralization efficiency. Complete degradation of phenol (of initial concentration 50 mg L-1) was achieved under the near-optimum operating conditions (40 mA cm-2, pH 7, 0.4 m3 h-1 water circulation rate) within 30 min. Both air flow rate and Fe2+ dosage did not show a measurable impact on phenol removal. However, increasing the chloride content of water significantly improved the efficiency of treatment due to the enhanced indirect oxidation by the electrogenerated chlorine. Several trihalomethane intermediates (chloroform, bromodichloromethane) and chlorinated/brominated phenol byproducts forming during treatment, were eliminated after 60 min of processing time.

Sustainability assessment of tertiary wastewater treatment technologies: a multi-criteria analysis.

The multi-criteria analysis gives the opportunity to researchers, designers and decision-makers to examine decision options in a multi-dimensional fashion. On this basis, four tertiary wastewater treatment (WWT) technologies were assessed regarding their sustainability performance in producing recycled wastewater, considering a 'triple bottom line' approach (i.e. economic, environmental, and social). These are powdered activated carbon adsorption coupled with ultrafiltration membrane separation (PAC-UF), reverse osmosis, ozone/ultraviolet-light oxidation and heterogeneous photo-catalysis coupled with low-pressure membrane separation (photocatalytic membrane reactor, PMR). The participatory method called simple multi-attribute rating technique exploiting ranks was employed for assigning weights to selected sustainability indicators. This sustainability assessment approach resulted in the development of a composite index as a final metric, for each WWT technology evaluated. The PAC-UF technology appears to be the most appropriate technology, attaining the highest composite value regarding the sustainability performance. A scenario analysis confirmed the results of the original scenario in five out of seven cases. In parallel, the PMR was highlighted as the technology with the least variability in its performance. Nevertheless, additional actions and approaches are proposed to strengthen the objectivity of the final results.

On modeling incipient crystallization of sparingly soluble salts in frontal membrane filtration.

Modeling incipient crystallization ("scaling") in desalination membrane modules is a very difficult task due to several complications arising from the interplay of physico-chemical solution conditions (leading to supersaturation) with the flow field and related transport processes, including solid phase generation phenomena and membrane surface geometrical changes caused by the developing discrete particles. Although eventually all these aspects must be included in a comprehensive process model, it is fruitful to isolate and tackle them separately, thereby improving our understanding and developing techniques which will facilitate the ensuing synthesis of an integrated modeling framework. The focus in this work is on solid phase generation phenomena accounting for the membrane surface geometrical changes. A mean field model is developed that includes bulk and surface particle nucleation and growth processes. The relative importance of the two types of processes is analyzed. It is shown that, if thick concentration boundary layers exist around surface particles, the mean field theory--although not strictly valid--can be approximately used to estimate the transport coefficients, in conjunction with a unit cell problem for transport processes around a single surface particle. The unit cell problem is formulated and typical results for the flow and concentration field therein are presented as well as the corresponding mass transfer coefficients.

Impact of European legislation on marketed pesticides--a view from the standpoint of health impact assessment studies.

The very significant impact of European legislation (Directive 91/414/EEC) on the authorization of plant protection products is reviewed herein, which has resulted in withdrawal of 704 active substances (AS) out of 889 assessed so far. The list of currently approved 276 AS includes 194 AS "existing" in the market before 1993 and 82 "new" AS introduced during the last 15 years. Results of toxicity characterization of the approved AS are also summarized, utilizing several well-known databases. Although significant data gaps exist for a rather large part of the approved AS, it is found that 84 AS are positive for at least one health effect (after chronic and/or acute exposure) including carcinogenicity, reproductive and neuro-developmental disorders, as well as endocrine disruption. The toxicity characterization results of this study are compared to those of recent assessments by other organizations (KemI, the Swedish Chemicals Agency, and the Pesticide Safety Directorate of the UK), where interpretation and use is made of AS "cut-off" criteria foreseen in new EU legislation. These studies report a comparatively smaller AS number with positive toxicity characterization. The possibility of some additional AS withdrawal in the near future, combined with the rather small rate of new AS introduction (approx. 5 per year) suggest that the list of approved AS over the next 10-15 years may not change very drastically. Consideration of the above trends is necessary and instructive in evaluating results of existing health impact assessment (HIA) studies, as well as in planning new ones. Due to the very drastic change in the number and type of marketed AS, that took place within the past 8-9years, it is suggested that new HIA studies (based on epidemiological data after year 2000) should focus on a rather short time frame and, therefore, on appropriate cohort groups, e.g. young children. For the same reason, results of epidemiological studies of the past (involving banned AS) should be carefully interpreted and used with caution.

A study of the collisional fragmentation problem using the Gamma distribution approximation.

The nonlinear fragmentation population balance formulation has been elevated in recent years from a prototype for studying nonlinear integro-differential equations to a vehicle for analyzing and understanding several physicochemical processes of technological interest. The so-called pure collisional fragmentation, which is the particular mode of nonlinear fragmentation induced by collisions between particles, is studied here. It is shown that the corresponding population balance equation admits large time asymptotic (self-similarity) solutions for homogeneous fragmentation and collision functions (kernels). The self-similar solutions are given in closed form for some simple kernels. Based on the shape of the self-similar solutions the method of moments with Gamma distribution approximation is employed for transient solution (from initial state to establishment of the asymptotic shape) of the collisional fragmentation equation. These solutions are presented for several sets of parameters and their behavior is discussed rather extensively. The present study is similar to the one has already been performed for the case of the much simpler linear fragmentation equation [G. Madras, B.J. McCoy, AIChE J. 44 (1998) 647].

On the self-similar solution of fragmentation equation: Numerical evaluation with implications for the inverse problem.

It is well known that the fragmentation equation admits self-similar solutions for evolving particle-size distributions (PSD); i.e., if the shape of PSD is independent of time after an initial transient period. Although an analytical derivations of the self-similar PSD cases have been studied extensively, results for cases requiring numerical solutions are rare. The aim of the present work is to fill this gap for the case of homogeneous breakage functions. The known analytical and approximate solutions for the self-similar PSD are reviewed and a general algorithm for the numerical solution is proposed. Results for a broad range of breakage functions (kernel and rate) are presented. Further, the work is focused on the sensitivity of the relation between self-similar PSD and breakage kernel and its influence on the inverse breakage problem, i.e., that of estimating the breakage kernel from experimental self-similar PSDs. Useful suggestions are made for tackling the inverse problem.

Incipient CdS thin film formation.

The quality of a final thin film is essentially determined by the processes taking place at incipient CdS deposition, which in turn are strongly influenced by the physicochemical properties of the substrate and liquid in contact. SEM pictures of deposits formed through steady flow of a supersaturated (with respect to CdS) solution suggest that initially nuclei are continuously generated on the substrate and grow as discrete "surface" particles. With time, these particles tend to "coalesce" with neighboring ones, while new nuclei keep forming and growing, leading to the formation of a coherent film. There is evidence that similar growth patterns prevail in CdS deposition via the chemical bath deposition (CBD) process. Based on experimental observations, a simple model is developed, which is capable of predicting macroscopically determined film characteristics such as the temporal thickness evolution including the "induction period." Two cases of the growth pattern are examined theoretically; one based on instantaneous surface nucleation (due to its simplicity) and another with a constant surface nucleation rate, which appears to be closer to experimental observations.