Residual currents and fluxes through the mouth of Vassova coastal lagoon

An intensive sampling program of physical and chemical parameters at the mouth of Vassova lagoon (Northern Greece) during 4 separate tidal cycles is described. The study aims at understanding the tidal circulation and estimating the instantaneous and residual fluxes of water, salt and nutrients through the entrance canal of this micro-tidal lagoon. Results showed that tidal flood exceeded in duration tidal ebb, under spring and neap tidal conditions. Ebb tidal currents were recorded higher than flood currents, especially under neap tidal conditions. Unsteady flow characterized the temporal variation of longitudinal and lateral velocity, inducing a rightward deflection on flood or ebb flow. The intra-tidal variability of dissolved inorganic nitrogen showed seasonal dependence, with higher values during September, October and early March, and lower during the late March period. Residual current and flux analysis into a Eulerian and a mass transport Stokes drift mechanism illustrated that advective water and dissolved parameters (i.e., salt and nitrates, phosphates and chlorophyll-·) fluxes were an order of magnitude higher than tidal pumping effects. Water and dissolved constituents moved into the lagoon under neap tidal conditions and out of the lagoon during spring tidal conditions. Calculated flushing times ranged from 5 to 14 days, with neap tidal conditions and nearly zero freshwater discharge producing the longer flushing time. Lower water flushing effects were generated under spring tides and increased precipitation

Predicting pollutant removal in constructed wetlands using artificial neural networks (ANNs)

Growth in urban population, urbanisation, and economic development has increased the demand for water, especially in water-scarce regions. Therefore, sustainable approaches to water management are needed to cope with the effects of the urbanisation on the water environment. This study aimed to design novel configurations of tidal-flow vertical subsurface flow constructed wetlands (VFCWs) for treating urban stormwater. A series of laboratory experiments were conducted with semi-synthetic influent stormwater to examine the effects of the design and operation variables on the performance of the VFCWs and to identify optimal design and operational strategies, as well as maintenance requirements. The results show that the VFCWs can significantly reduce pollutants in urban stormwater, and that pollutant removal was related to specific VFCW designs. Models based on the artificial neural network (ANN) method were built using inputs derived from data exploratory techniques, such as analysis of variance (ANOVA) and principal component analysis (PCA). It was found that PCA reduced the dimensionality of input variables obtained from different experimental design conditions. The results show a satisfactory generalisation for predicting nitrogen and phosphorus removal with fewer variable inputs, indicating that monitoring costs and time can be reduced

Kinetics of nitrogen removal processes in constructed wetlands

The aim of this paper is to present a state-of-the-art review of the kinetics of nitrogen removal in constructed wetlands. Biological processes of nitrogen removal from wastewater can be described using equations and kinetic models. Hence, these kinetic models which have been developed and evaluated allow for predicting the removal of nitrogen in treatment wetlands. One of the most important, first order removal model, which is still applied, was analysed and its rate coefficients and factors were compared. This study also demonstrates the validity of Monod and multiple Monod kinetics, commonly seen today. Finally, a computational example of the reaction kinetics of nitrogen removal was also included in the study

Removal of the pesticide tebuconazole in constructed wetlands: design comparison, influencing factors and modelling

Constructed wetlands (CWs) are a promising technology to treat pesticide contaminated water, but its implementation is impeded by lack of data to optimize designs and operating factors. Unsaturated and saturated CW designs were used to compare the removal of triazole pesticide, tebuconazole, in unplanted mesocosms and mesocosms planted with five different plant species: Typha latifolia, Phragmites australis, Iris pseudacorus, Juncus effusus and Berula erecta. Tebuconazole removal efficiencies were significantly higher in unsaturated CWs than saturated CWs, showing for the first time the potential of unsaturated CWs to treat tebuconazole contaminated water. An artificial neural network model was demonstrated to provide more accurate predictions of tebuconazole removal than the traditional linear regression model. Also, tebuconazole removal could be fitted an area-based first order kinetics model in both CW designs. The removal rate constants were consistently higher in unsaturated CWs (range of 2.6–10.9 cm d−1) than in saturated CWs (range of 1.7–7.9 cm d−1) and higher in planted CWs (range of 3.1–10.9 cm d−1) than in unplanted CWs (range of 1.7–2.6 cm d−1) for both designs. The low levels of sorption of tebuconazole to the substrate (0.7–2.1%) and plant phytoaccumulation (2.5–12.1%) indicate that the major removal pathways were biodegradation and metabolization inside the plants after plant uptake. The main factors influencing tebuconazole removal in the studied systems were system design, hydraulic loading rate and plant presence. Moreover, tebuconazole removal was positively correlated to dissolved oxygen and all nutrients removal

Wetlands for wastewater treatment and subsequent recycling of treated effluent : a review

Due to water scarcity challenges around the world, it is essential to think about non-conventional water resources to address the increased demand in clean freshwater. Environmental and public health problems may result from insufficient provision of sanitation and wastewater disposal facilities. Because of this, wastewater treatment and recycling methods will be vital to provide sufficient freshwater in the coming decades, since water resources are limited and more than 70% of water are consumed for irrigation purposes. Therefore, the application of treated wastewater for agricultural irrigation has much potential, especially when incorporating the reuse of nutrients like nitrogen and phosphorous, which are essential for plant production. Among the current treatment technologies applied in urban wastewater reuse for irrigation, wetlands were concluded to be the one of the most suitable ones in terms of pollutant removal and have advantages due to both low maintenance costs and required energy. Wetland behavior and efficiency concerning wastewater treatment is mainly linked to macrophyte composition, substrate, hydrology, surface loading rate, influent feeding mode, microorganism availability, and temperature. Constructed wetlands are very effective in removing organics and suspended solids, whereas the removal of nitrogen is relatively low, but could be improved by using a combination of various types of constructed wetlands meeting the irrigation reuse standards. The removal of phosphorus is usually low, unless special media with high sorption capacity are used. Pathogen removal from wetland effluent to meet irrigation reuse standards is a challenge unless supplementary lagoons or hybrid wetland systems are used

Optimization of design parameters of horizontal subsurface flow constructed wetlands using pilot-scale units

Για την εξέταση της επίδρασης της θερμοκρασίας, του υδραυλικού χρόνου παραμονής, του είδους φύτευσης και του μεγέθους και είδους του πληρωτικού υλικού στην απόδοση εγκαταστάσεων τεχνητών υγροβιοτόπων υπόγειας ροής για την επεξεργασία υγρών αποβλήτων, λειτούργησαν συνεχώς από τον Ιανουάριο του 2004 έως των Ιανουάριο του 2006, σε παράλληλα πειράματα, πέντε πιλοτικές μονάδες διαστάσεων 3 m σε μήκος και 0.75 m σε πλάτος. Τρεις πιλοτικές μονάδες περιείχαν μέσο χαλίκι από λατομείο. Οι άλλες δύο περιείχαν η μια λεπτό χαλίκι και η άλλη κροκάλες από ποτάμιες αποθέσεις. Οι τρεις πιλοτικές μονάδες, που περιείχαν μέσο χαλίκι, ήταν η μία φυτεμένη με καλάμι, η άλλη με ψαθί και η τρίτη ήταν αφύτευτη. Οι άλλες δύο πιλοτικές μονάδες ήταν φυτεμένες με καλάμι. Οι συνδυασμοί είδους φύτευσης και πληρωτικού υλικού ήταν κατάλληλοι ώστε να γίνει σύγκριση της επίδρασης του είδους φύτευσης και του πληρωτικού υλικού στην λειτουργία του συστήματος. Στις πιλοτικές μονάδες διοχετευόταν συνθετικό υγρό απόβλητο. Κατά την διάρκεια λειτουργίας εφαρμόστηκαν τέσσερις υδραυλικοί χρόνοι παραμονής (6, 8, 14 και 20 ημέρες), ενώ οι θερμοκρασίες του υγρού αποβλήτου κυμάνθηκαν από περίπου 2 έως 26oC. Η απόδοση σε αφαίρεση ρύπων των τεχνητών υγροβιότοπων ήταν ιδιαίτερα ικανοποιητική, αφού έφθασε κατά μέσο όρο το 89, 65 και 60% για BOD, TKN και ορθό-φωσφορικά, αντίστοιχα. Όλες οι αποδόσεις αφαίρεσης ρύπων επηρεάστηκαν από την θερμοκρασία. Φαίνεται ότι ο χρόνος παραμονής των 8 ημερών ήταν αρκετός για μια αποδεκτή αφαίρεση οργανικής ύλης, TKN και ορθο-φωσφορικών για θερμοκρασίες άνω των 15oC. Επιπλέον, βάσει στατιστικών ελέγχων το ψαθί και το λεπτόκοκκο υλικό ποτάμιας προέλευσης έδειξαν καλύτερες αποδόσεις στην αφαίρεση TKN και ορθο-φωσφορικών. Με βάση τα πειραματικά δεδομένα αναπτύχθηκαν και παρουσιάζονται επίσης τρία μοντέλα χρήσιμα για τον σχεδιασμό τεχνητών υγροβιότοπων οριζόντιας υπόγειας ροής. Τα μοντέλα αυτά αναπτύχθηκαν με την βοήθεια τεχνητών νευρωνικών δικτύων. Για την κατάλληλη επιλογή των παραμέτρων εισόδου των νευρωνικών δικτύων, πραγματοποιήθηκε αρχικά ανάλυση σημαντικών παραμέτρων (PCA). Από τα αποτελέσματα της PCA και των νευρωνικών δικτύων παρατηρήθηκε ότι οι παράγοντες, που επηρεάζουν κυρίως την απόδοση αφαίρεσης BOD, TN και φωσφόρου ήταν το πορώδες, η θερμοκρασία του υγρού αποβλήτου και ο υδραυλικός χρόνος παραμονής. Συνολικά εξετάστηκαν δύο νευρωνικά δίκτυα για κάθε ρύπο. Το πρώτο περιελάμβανε τρεις μεταβλητές εισόδου επιλεγμένες από την PCA, και το δεύτερο, εκτός από τις αρχικές τρεις, περιελάμβανε και μετεωρολογικά δεδομένα (βαρομετρική πίεση, ύψος βροχής, ένταση ανέμου, ένταση ηλιακής ακτινοβολίας και σχετική υγρασία). Το πρώτο νευρωνικό δίκτυο προέβλεψε αρκετά ικανοποιητικά τις πειραματικές τιμές και το δεύτερο έκανε ακόμα καλύτερες προβλέψεις. Από τα αποτελέσματα των νευρωνικών δικτύων παρήχθησαν τρεις υπερβολικές εξισώσεις, οι οποίες θυμίζουν εξισώσεις Monod και συνδυάζουν κινητική μηδενική και πρώτης τάξης, για την πρόβλεψη αφαίρεσης BOD, TN και φωσφόρου. Τα αποτελέσματα των νευρωνικών δικτύων και των εξισώσεων σχεδιασμού πιστοποιήθηκαν με την χρήση δεδομένων άλλων εργασιών από την διεθνή βιβλιογραφία και έδειξαν ικανοποιητική σύγκριση. Η απόδοση αφαίρεσης του COD βρέθηκε να έχει ισχυρή συσχέτιση με την απόδοση του BOD, και με βάση την εξίσωση σχεδιασμού του BOD προέκυψε αντίστοιχη εξίσωση για την αφαίρεση του COD. Το ίδιο συνέβη και για την απόδοση αφαίρεση της αμμωνίας σε σχέση με την απόδοση αφαίρεσης του TN και του ολικού φωσφόρου σε σχέση με την απόδοση των ορθο-φωσφορικών.In order to investigate the effect of temperature, hydraulic residence time (HRT), vegetation type, and porous media material and grain size on the performance of horizontal subsurface flow (HSF) constructed wetlands treating municipal wastewater, 5 pilot-scale units of dimensions 3 m in length and 0.75 m in width were operated continuously from January 2004 until January 2006 in parallel experiments. Three units contained medium gravel obtained from a quarry. The other two contained one fine gravel and one cobbles, both obtained from a river bed. The three units with medium gravel were planted one with common reeds and one with cattails, and one was kept unplanted. The other two units were planted with common reeds. Planting and porous media combinations were appropriate for comparison of the effect of vegetation and media type on the function of the system. Synthetic wastewater was introduced in the units. During the operation period four HRTs (i.e., 6, 8, 14 and 20 days) were used, while wastewater temperatures varied from about 2 to 26oC. The removal performance of the constructed wetland units was very good, since it reached on the average 89, 65 and 60% for BOD, TKN and ortho-phosphate (OP), respectively. All pollutant removal efficiencies showed dependence on temperature. It seems that the 8-day HRT was adequate for acceptable removal of organic matter, TKN and OP for temperatures above 15oC. Furthermore, based on statistical testing, cattails, finer media and media from a river showed higher removal efficiencies of TKN and OP. Three models are also presented, which can be used in the design of a horizontal subsurface flow (HSF) constructed wetland. These models were developed based on the experimental data from five pilot-scale CW units, used in conjunction with artificial neural networks. For the proper selection of the components entering the neural network, a principal component analysis (PCA) was performed first. From the PCA and model results it seems that the factors effecting BOD, TN and phosphorus removal are porous media porosity, wastewater temperature and hydraulic residence time. Two artificial neural networks for each pollutant (ANNs) were examined: the first, included only the three factors selected from the PCA, and the second included, in addition, meteorological data (i.e., barometric pressure, rainfall depth, wind speed, intensity of solar radiation and relative humidity). The first ANN can predict the removal rather satisfactorily and the second one examined resulted in even better predictions. From the predictions of the ANNs three hyperbolic design equations, which are Monod like equation and combine zero nad first order kinetics, were produced to predict BOD, TN and phosphorus removal. The results of the ANNs and of the model design equations were validated against available data from the literature, and showed a rather satisfactory correlation. COD removal was found to be strongly correlated to BOD removal. An equation for COD removal prediction was also produced. The same was done for ammonia with correlation to TN removal, and total phosphorus with correlation to ortho-phosphate removal