Improving the reliability of closed chamber methodologies for methane emissions measurement in treatment wetlands.

Non-homogeneous mixing of methane (NHM) within closed chambers was studied under laboratory conditions. The experimental set-up consisted of a PVC vented chamber of 5.3 litres of effective volume fitted with a power-adjustable 12 V fan. NHM within the chamber was studied according to fan position (top vs lateral), fan airflow strength (23 vs 80 cubic feet per minute) and the mixing time before sample withdrawal (5, 10, 15 and 20 minutes). The potential bias of methane flux densities caused by NHM was addressed by monitoring the difference between linearly expected and estimated flux densities of ca. 400, ca. 800 and ca. 1,600 mg CH(4).m(-2) d(-1). Methane within the chamber was under non-homogeneous conditions. Accordingly, methane concentrations at the bottom of the chamber were between 20 to 70% higher than those recorded at the middle or top sections of the chamber, regardless of fan position, fan air-flow strength or time before sample withdrawal. NHM led to notable biases on flux density estimation. Accordingly, flux density estimated from top and middle sampling sections were systematically lower (ca. 50%) than those expected. Flux densities estimated from bottom samples were between 10% higher and 25% lower than expected, regardless of the flux density considered.

Exploring the biocapacitance in M3C-based biosensors for the assessment of microbial activity and organic matter.

Reliable monitoring of microbial and water quality parameters in freshwater ecosystems (either natural or human-made) is of capital importance for improving both the management of water resources and the assessment of microbially-driven bio-geo-chemical processes. In this context, bioelectrochemical systems (BES), such as microbial three-cell electrodes (M3C), are very promising devices for their use as biosensors. However, current experiences on the use of BES-based devices for biosensing purposes are almost exclusively limited to water-saturated environments. This limitation hampers the use of this technology for a wider range of applications where the biosensor may work discontinuously (such as discontinuously saturated ecosystems). Discontinuous operation of M3C-based biosensors creates an electric current peak immediately after the reconnection of the system due to electron accumulation, in a process known as biocapacitance. The present work aimed at quantifying the bioindication potential of biocapacitance for the assessment of key ecosystem parameters such as microbial metabolic activity and biomass, as well as organic matter concentration. Significant linear regression coefficients (R2 > 0.9) were found for all combinations of parameters tested. Moreover, for most of the ecological parameters assessed, an electric charge accumulation of 1-5 min (biocapacitance elapsed time) and discharge of 5 min was enough to get reliable information. In conclusion, we have demonstrated for the first time that biocapacitance in M3C-based biosensors can be used as a proxy parameter for the assessment of microbial activity, microbial biomass and organic matter concentration in a model nature-based ecosystem.

Power assisted MFC-based biosensor for continuous assessment of microbial activity and biomass in freshwater ecosystems.

Microbial activity and biomass are important factors that determine nutrient and carbon fluxes in freshwater ecosystems and, therefore are also related to both water quality and climate change induced stressors. This study aimed at assessing the feasibility of a power assisted Microbial Fuel Cell (MFC)-based biosensors for the continuous monitoring of microbial activity and biomass concentrations in saturated freshwater ecosystems. For this purpose, four lab-scale reactors were constructed and operated for 30 weeks. Reactors were fed with four different organic matter concentrations to promote a suite of microbial activity and biomass conditions. The reactors consisted of 3.8 L PVC vessels filled with 23 extractable gravel- sockets, used for microbial activity and biomass assessment, and 1 MFC granular-graphite socket, for biosensing assessment. Microbial activity was determined by the ATP content and the hydrolytic enzymatic activity, and the biomass content was assessed as the volatile solids attached to the gravel. Very significant linear relationships could be established between the parameters studied and the current density produced by the MFC with a very short detection time: 10 min for the ATP content (R2 = 0.88) and 1 h for the enzymatic activity (R2 = 0.78) and biomass (R2 = 0.74). Moreover, the power assisted MFC-based biosensing tool demonstrated to be functional after a long operation time and under a wide range of organic loading conditions. Overall, the results highlight the feasibility to develop a power assisted MFC-based biosensor for on-line monitoring of the microbial activity and biomass of a given ecosystem (either natural or artificial) even in remote locations.

Saturated constructed wetland-microbial fuel cell system and effect on dissolved oxygen gradient, electricity generation and ammonium removal.

The aim of this work was to assess effect of saturated constructed wetland-Microbial fuel cell system on dissolved oxygen gradient, electricity generation and ammonium removal. Two laboratory-scale systems, one planted with Schoenoplectus californicus (SCW1-MFC) and other without plant (SCW2-MFC), were fed discontinuously with synthetic wastewater over 90 days. Both systems were operated at different organic loading rate (12 and 28 g COD/m2d) and ammonium loading rate (1.6 and 3.0 g NH4+- N/m2 d) under open circuit and close circuit mode. The results indicate that between lower and upper zones of wetlands the average values were in the range of 1.22 ± 0.32 to 1.39 ± 0.27 mg O2/L in SCW1-MFC and 1.28 ± 0.24 to 1.56 ± 0.31 mg O2/L in SCW2-MFC. The effect of operating mode (closed and open circuit) and vegetation on DO was not significant (p > 0.05). Chemical oxygen demand (COD) removal efficiencies, fluctuated between 90 and 95% in the SCW1-MFC and 82 and 94% in the SCW2-MFC system. Regarding NH4+- N, removal efficiencies were above 85% in both systems reaching values maximus 98%. The maximum power density generated was 4 and 10 mW/m2 in SCW1-MFC, while SCW2-MFC recorded the highest values (12 and 22 mW/m2).

Microbial activity enhancement in constructed wetlands operated as bioelectrochemical systems.

Treatment wetlands (TW) operated as bioelectrochemical systems (BES-TW) provide a higher degree of treatment than conventional TW. Yet, the fundamental processes or mechanisms for the envisaged better performance of BES-TW over conventional TW remains poorly understood. This work aimed to determine to which extent microbial activity enhancement could be the reason behind this treatment performance increase. To this purpose, pilot-scale horizontal sub-surface flow BES-TW operated under three different configurations were continuously fed with real urban wastewater. BES-TW were evaluated for COD and ammonia removal efficiency, and two techniques of microbial activity assessment were applied. Configurations, tested in duplicate, were: control TWs without electrodes (C-TW), TWs operated as microbial fuel cells (MFC-TW), and TWs operated as microbial electrolysis cells (MEC-TW). Microbial activity was assessed by measuring the enzymatic activity (EA) (FDA hydrolysis technique) and the aerobic activity (AA) (estimated through respirometry). Results showed that BES-TW outperformed C-TW in terms of both microbial activity enhancement and contaminants removal efficiency, especially in the case of MEC-TW. More precisely, this configuration showed an average improvement of 17%, and 56% in COD removal and EA efficiencies, respectively, compared to C-TW. Regarding AA activity, although MEC-TW seemed to outperform the rest of the configurations, differences were not statistically significant. This work demonstrates that TWs operated as BES increase the overall enzymatic activity of the treatment bed and this, in turn, is the leading cause to a higher degree of treatment performance.

Mechanical resistance properties of gravel used in subsurface flow constructed wetlands: implications for clogging.

Gravel constitutes the filter medium in subsurface flow constructed wetlands (SSF CWs) and its porosity and hydraulic conductivity decrease over time (clogging), limiting the lifespan of the systems. Using gravel of poor quality accelerates clogging in wetlands. In this study, gravel samples from six different wetland systems were compared with regards to their mineral composition and mechanical resistance properties. Results showed that both mineralogy and texture are related to mechanical resistance. Accordingly, gravel with high content of quartz (> 80%) showed a lower percentage of broken particles (0.18-1.03%) than those with lower content of quartz (2.42-4.56% media broken). Although granite is formed by high durability minerals, its non-uniform texture results in a lower resistance to abrasion (ca. 10% less resistance than calcareous gravel). Therefore, it is recommended to use gravels composed mainly of quartz or, when it is not available, limestone gravels (rounded and uniform) are recommended instead. The resistance to abrasion (LAA test) seems to be a good indicator to determine the mechanical properties of gravels used in CWs. It is recommended to use gravels with LAA below 30% in order to avoid a rapid clogging due to gravel crumbling and subsequent mineral solids accumulation.

Organic matter removal and nitrogen transformation by a constructed wetland-microbial fuel cell system with simultaneous bioelectricity generation.

Microbial fuel cells integrated into constructed wetlands have been previously studied. Nevertheless, their application as a suitable treatment for wastewater is still in the developmental stage. In this context, the aim of this study was to evaluate organic matter removal and nitrogen transformation by a microbial fuel cell integrated into a constructed wetland (CWMFC). To accomplish this, three experimental systems were operated under batch-mode conditions over 170 days: i) one was planted with Schoenoplectus californicus (P-CWMFC); ii) another was unplanted (NP-CWMFC); and iii) the third system did not have any electrodes (CW) and was used as a control. Chemical oxygen demand (COD) removal efficiency ranged between 74-87%, 69-81% and 62-72% for the P-CWMFC, NP-CWMFC and CW systems, respectively, with organic loading rates (OLR) ranging from 4.8 to 7.9 g COD/m2 d. NH4+-N removal efficiency exceeded 98%, 90% and 83% for P-CWMFC, NP-CWMFC and CW, respectively. Wastewater treatment performance was improved due to anaerobic oxidation that occurred on the anodes. Organic matter removal was 18% higher in closed-circuit mode than in open-circuit mode in both integrated systems (P-CWMFC and NP-CWMFC), and these differences were significant (p < 0.05). With respect to the performance of microbial fuel cells, the maximum power density (8.6 mW/m2) was achieved at an organic loading rate of 7.9 g COD/m2 d with an internal resistance and coulombic efficiency of 251â€¯Ω and 2.4%, respectively. The results obtained in this work can provide positive impacts on CW development by enhancing anaerobic degradation without forced aeration.

Constructed wetlands operated as bioelectrochemical systems for the removal of organic micropollutants.

The removal of organic micropollutants (OMPs) has been investigated in constructed wetlands (CWs) operated as bioelectrochemical systems (BES). The operation of CWs as BES (CW-BES), either in the form of microbial fuel cells (MFC) or microbial electrolysis cells (MEC), has only been investigated in recent years. The presented experiment used CW meso-scale systems applying a realistic horizontal flow regime and continuous feeding of real urban wastewater spiked with four OMPs (pharmaceuticals), namely carbamazepine (CBZ), diclofenac (DCF), ibuprofen (IBU) and naproxen (NPX). The study evaluated the removal efficiency of conventional CW systems (CW-control) as well as CW systems operated as closed-circuit MFCs (CW-MFCs) and MECs (CW-MECs). Although a few positive trends were identified for the CW-BES compared to the CW-control (higher average CBZ, DCF and NPX removal by 10-17% in CW-MEC and 5% in CW-MFC), these proved to be not statistically significantly different. Mesoscale experiments with real wastewater could thus not confirm earlier positive effects of CW-BES found under strictly controlled laboratory conditions with synthetic wastewaters.

Contaminants removal and bacterial activity enhancement along the flow path of constructed wetland microbial fuel cells.

Microbial fuel cells implemented in constructed wetlands (CW-MFCs), albeit a relatively new technology still under study, have shown to improve treatment efficiency of urban wastewater. So far the vast majority of CW-MFC systems investigated were designed as lab-scale systems working under rather unrealistic hydraulic conditions using synthetic wastewater. The main objective of this work was to quantify CW-MFCs performance operated under different conditions in a more realistic setup using meso-scale systems with horizontal flow fed with real urban wastewater. Operational conditions tested were organic loading rate (4.9 ±â€¯1.6, 6.7 ±â€¯1.4 and 13.6 ±â€¯3.2 g COD/m2·day) and hydraulic regime (continuous vs. intermittent feeding) as well as different electrical connections: CW control (conventional CW without electrodes), open-circuit CW-MFC (external circuit between anode and cathode not connected) and closed-circuit CW-MFC (external circuit connected). Eight horizontal subsurface flow CWs were operated for about four months. Each wetland consisted of a PVC reservoir of 0.193 m2 filled with 4/8 mm granitic riverine gravel (wetted depth 25 cm). All wetlands had intermediate sampling points for gravel and interstitial liquid sampling. The CW-MFCs were designed as three MFCs incorporated one after the other along the flow path of the CWs. Anodes consisted of gravel with an incorporated current collector (stainless steel mesh) and the cathode consisted of a graphite felt layer. Electrodes of closed-circuit CW-MFC systems were connected externally over a 220â€¯Ω resistance. Results showed no significant differences between tested organic loading rates, hydraulic regimes or electrical connections, however, on average, systems operated in closed-circuit CW-MFC mode under continuous flow outperformed the other experimental conditions. Closed-circuit CW-MFC compared to conventional CW control systems showed around 5% and 22% higher COD and ammonium removal, respectively. Correspondingly, overall bacteria activity, as measured by the fluorescein diacetate technique, was higher (4% to 34%) in closed-circuit systems when compared to CW control systems.

Improving domestic wastewater treatment efficiency with constructed wetland microbial fuel cells: Influence of anode material and external resistance.

For the past few years, there has been an increasing interest in the operation of constructed wetlands as microbial fuel cells (CW-MFCs) for both the improvement of wastewater treatment efficiency and the production of energy. However, there is still scarce information on design and operation aspects to maximize CW-MFCs efficiency, especially for the treatment of real domestic wastewater. The aim of this study was to quantify the extent of treatment efficiency improvement carried out by membrane-less MFCs simulating a core of a shallow un-planted horizontal subsurface flow constructed wetland. The influence of the external resistance (50, 220, 402, 604 and 1000Ω) and the anode material (graphite and gravel) on treatment efficiency improvement were addressed. To this purpose, 6 lab-scale membrane-less MFCs were set-up and loaded in batch mode with domestic wastewater for 13weeks. Results showed that 220Ω was the best operation condition for maximising MFCs treatment efficiency, regardless the anode material employed. Gravel-based anode MFCs operated at closed circuit showed ca. 18%, 15%, 31% and 25% lower effluent concentration than unconnected MFCs to the COD, TOC, PO4-3 and NH4+-N, respectively. Main conclusion of the present work is that constructed wetlands operated as MFCs is a promising strategy to improve domestic wastewater treatment efficiency. However, further studies at pilot scale under more realistic conditions (such as planted systems operated under continuous mode) shall be performed to confirm the findings here reported.

Effects of particulate and soluble substrates on microfauna populations and treatment efficiency in activated sludge systems.

To determine the effects of particulate and soluble compounds on microfauna populations and treatment efficiency in activated sludge systems, two experimental wastewater treatment plants were set up and evaluated for a period of five months. The plants were fed with pre-flocculated domestic sewage enriched with starch or glucose as model substrates of particulate and soluble organic matter, respectively. It was observed that the starch-enriched system presented lower abundance of filamentous bacteria that turned into a better sludge sedimentation. Mean sludge volume index (SVI) values for the starch and glucose-enriched systems were 54+/-24 and 885+/-845 mL g(-1), respectively. Although no differences in organic matter removal were detected between the systems, nitrification and denitrification were higher in the starch-enriched system, which is likely to have been the result of its more compact flocs. The mean ammonia-N effluent concentrations for the starch and glucose-enriched systems were 4.7+/-5.7 and 16.2+/-9.7 mg L(-1), respectively, whereas the nitrate-N concentrations were 20.1+/-10.8 and 30.8+/-12.2 mg L(-1), respectively. Concerning microfauna analysis, ciliated protozoa specifically, attached ciliates were the dominant microfauna group in both treatment systems, whereas metazoa, particularly Lecanidae rotifera, were more abundant in the starch-enriched system. Lecanidae rotifera abundances above 400 ind mL(-1) reduced the mean floc area from 60 to 20mm(2) without affecting sludge settleability in the starch-enriched system. Finally, the reduction in floc area caused by metazoa feeding led to effluents of lower nitrogen quality, although no differences in sludge production were detected.

Comparison of microfauna communities in full scale subsurface flow constructed wetlands used as secondary and tertiary treatment.

In order to evaluate the microfauna composition and distribution in two horizontal subsurface flow constructed wetlands used as secondary and tertiary treatment a full-scale wastewater treatment plant was monitored during five months. Results indicate that total microfauna abundance in the wetland treating primary influents is around five times higher than that found in the wetland treating secondary influents. Ciliated protozoa and microflagellates are the most important microfauna groups in both wetlands; microflagellates in terms of abundance and ciliates in terms of biomass. The most abundant ciliate species in the wetland treating primary influents are polysaprobic organisms as Dexiostoma campylum, Trimyema compressum, and to a lesser extend Metopus spp. On the other hand, the most important ciliate species found in the wetland treating secondary influents are mainly aerobic ciliates as Vorticella comvallaria-complex, Aspidisca cicada, Litonotus lamella and some ciliates belonging to the group of the scuticociliates and Hypotrichidae. The sort of the organic matter treated (particulated or dissolved) is at least as important as the amount of it in order to explain microfauna dynamics in constructed wetlands.

Solids accumulation in six full-scale subsurface flow constructed wetlands.

In this study, we evaluated the amount of accumulated solids in six different horizontal subsurface flow constructed wetlands (SSF CWs). We also investigated the relationship between accumulated solids and, on one hand, the wastewater quality and load and, on the other hand, the hydraulic conductivity of the granular medium. Aerobic and anaerobic biodegradability tests were also conducted on the accumulated organic matter. Experiments were carried out on full scale wastewater treatment systems consisting of SSF CWs with stabilisation ponds, which are used for the sanitation of small towns in north-eastern Spain. There were more accumulated solids near the inlet of the SSF CWs (3-57 kg dry matter (DM)/m2) than near the outlet (2-12 kgm DM/m2). Annual solids accumulation rates ranged from 0.7 to 14.3 kg DM/m2 year, and a positive relationship was observed between accumulation rates and loading rates. Most of the accumulated solids had a low level of organic matter (<20%). The results of the aerobic and anaerobic tests indicated that the accumulated organic matter was very recalcitrant and difficult to biodegrade. The hydraulic conductivity values were significantly lower near the inlet zone (0-4 m/d) than in the outlet zone (12-200 m/d). Although hydraulic conductivity tended to decrease with increasing solids accumulation, the relationship was not direct. One major conclusion of this study is that the improvement of primary treatment is necessary to avoid rapid clogging of the granular media due to solids accumulation.

Behavior of selected priority organic pollutants in horizontal subsurface flow constructed wetlands: a preliminary screening.

A discrete injection experiment was carried out in a constructed wetland to evaluate the behavior of selected priority pollutants. A horizontal subsurface flow pilot plant located in the NE of Spain was selected for this study. A total of eight European Priority Pollutants listed in the Water Framework Directive were considered, including a commonly used herbicide (mecoprop). The pollutants encompassed a variety of chemical classes and physicochemical properties. They included organochlorine, organophosphorus, phenols, chloroacetanilides, triazine, phenoxycarboxylic acid and phenylurea pesticides. A time series of composite effluent samples and discrete gravel bed samples from the wetland were analyzed. Response curves for all the pollutants injected from effluent concentrations were obtained and compared with the tracer (clofibric acid). On the basis of an analysis of the samples taken 21 days after the injection, priority pollutants were classified into four groups according to their removal efficiency. These groups were (i) the highly efficiently removed (>90%), namely lindane, pentachlorophenol, endosulfan and pentachlorobenzene; (ii) the efficiently removed (80-90%), namely alachlor and chlorpyriphos; (iii) poorly removed (20%), namely mecoprop and simazine; and (iv) recalcitrant to elimination, namely clofibric acid and diuron. Taking into account the poor accumulation of the injected contaminants in the gravel bed (0-20%), biodegradation and plant uptake are postulated as the most likely elimination pathways for the pollutants.

Life cycle assessment of constructed wetland systems for wastewater treatment coupled with microbial fuel cells.

The aim of this study was to assess the environmental impact of microbial fuel cells (MFCs) implemented in constructed wetlands (CWs). To this aim a life cycle assessment (LCA) was carried out comparing three scenarios: 1) a conventional CW system (without MFC implementation); 2) a CW system coupled with a gravel-based anode MFC, and 3) a CW system coupled with a graphite-based anode MFC. All systems served a population equivalent of 1500 p.e. They were designed to meet the same effluent quality. Since MFCs implemented in CWs improve treatment efficiency, the CWs coupled with MFCs had lower specific area requirement compared to the conventional CW system. The functional unit was 1m3 of wastewater. The LCA was performed with the software SimaPro® 8, using the CML-IA baseline method. The three scenarios considered showed similar environmental performance in all the categories considered, with the exception of Abiotic Depletion Potential. In this impact category, the potential environmental impact of the CW system coupled with a gravel-based anode MFC was around 2 times higher than that generated by the conventional CW system and the CW system coupled with a graphite-based anode MFC. It was attributed to the large amount of less environmentally friendly materials (e.g. metals, graphite) for MFCs implementation, especially in the case of gravel-based anode MFCs. Therefore, the CW system coupled with graphite-based anode MFC appeared as the most environmentally friendly solution which can replace conventional CWs reducing system footprint by up to 20%. An economic assessment showed that this system was around 1.5 times more expensive than the conventional CW system.

Electrochemical characterization of Geobacter lovleyi identifies limitations of microbial fuel cell performance in constructed wetlands.

Power generation in microbial fuel cells implemented in constructed wetlands (CW-MFCs) is low despite the enrichment of anode electricigens most closely related to Geobacter lovleyi. Using the model representative G. lovleyi strain SZ, we show that acetate, but not formate or lactate, can be oxidized efficiently but growth is limited by the high sensitivity of the bacterium to oxygen. Acetate and highly reducing conditions also supported the growth of anode biofilms but only at optimal anode potentials (450 mV vs. standard hydrogen electrode). Still, electrode coverage was poor and current densities, low, consistent with the lack of key c-type cytochromes. The results suggest that the low oxygen tolerance of G. lovleyi and inability to efficiently colonize and form electroactive biofilms on the electrodes while oxidizing the range of electron donors available in constructed wetlands limits MFC performance. The implications of these findings for the optimization of CW-MFCs are discussed. [Int Microbiol 20(2):55-64 (2017)].

Photosynthetic membrane-less microbial fuel cells to enhance microalgal biomass concentration.

The aim of this study was to quantitatively assess the net increase in microalgal biomass concentration induced by photosynthetic microbial fuel cells (PMFC). The experiment was conducted on six lab-scale PMFC constituted by an anodic chamber simulating an anaerobic digester connected to a cathodic chamber consisting of a mixed algae consortia culture. Three PMFC were operated at closed circuit (PMFC(+)) whereas three PMFC were left unconnected as control (PMFC(-)). PMFC(+) produced a higher amount of carbon dioxide as a product of the organic matter oxidation that resulted in 1.5-3 times higher biomass concentration at the cathode compartment when compared to PMFC(-).

Long-term assessment of best cathode position to maximise microbial fuel cell performance in horizontal subsurface flow constructed wetlands.

The cathode of microbial fuel cells (MFCs) implemented in constructed wetlands (CWs) is generally set in close contact with water surface to provide a rich oxygen environment. However, water level variations caused by plants evapotranspiration in CWs might decrease MFC performance by limiting oxygen transfer to the cathode. Main objective of this work was to quantify the effect of water level variation on MFC performance implemented in HSSF CW. For the purpose of this work two MFCs were implemented within a HSSF CW pilot plant fed with primary treated domestic wastewater. Cell voltage (Ecell) and the relative distance between the cathode and the water level were recorded for one year. Results showed that Ecell was greatly influenced by the relative distance between the cathode and the water level, giving an optimal cathode position of about 1 to 2cm above water level. Both water level variation and Ecell were daily and seasonal dependent, showing a pronounced day/night variation during warm periods and showing almost no daily variation during cold periods. Energy production under pronounced daily water level variation was 40% lower (80±56mWh/m(2)·day) than under low water level variation (131±61mWh/m(2)·day). Main conclusion of the present work is that of the performance of MFC implemented in HSSF CW is highly dependent on plants evapotranspiration. Therefore, MFC that are to be implemented in CWs shall be designed to be able to cope with pronounced water level variations.

Microbial fuel cells for clogging assessment in constructed wetlands.

Clogging in HSSF CW may result in a reduction of system's life-span or treatment efficiency. Current available techniques to assess the degree of clogging in HSSF CW are time consuming and cannot be applied on a continuous basis. Main objective of this work was to assess the potential applicability of microbial fuel cells for continuous clogging assessment in HSSF CW. To this aim, two replicates of a membrane-less microbial fuel cell (MFC) were built up and operated under laboratory conditions for five weeks. The MFC anode was gravel-based to simulate the filter media of HSSF CW. MFC were weekly loaded with sludge that had been accumulating for several years in a pilot HSSF CW treating domestic wastewater. Sludge loading ranged from ca. 20kgTS·m(-3)CW·year(-1) at the beginning of the study period up to ca. 250kgTS·m(-3)CW·year(-1) at the end of the study period. Sludge loading applied resulted in sludge accumulated within the MFC equivalent to a clogging degree ranging from 0.2years (ca. 0.5kgTS·m(-3)CW) to ca. 5years (ca. 10kgTS·m(-3)CW). Results showed that the electric charge was negatively correlated to the amount of sludge accumulated (degree of clogging). Electron transference (expressed as electric charge) almost ceased when accumulated sludge within the MFC was equivalent to ca. 5years of clogging (ca. 10kgTS·m(-3)CW). This result suggests that, although longer study periods under more realistic conditions shall be further performed, HSSF CW operated as a MFC has great potential for clogging assessment.

Intermittent aeration to improve wastewater treatment efficiency in pilot-scale constructed wetland.

Forced aeration of horizontal subsurface flow constructed wetlands (HSSF CWs) is nowadays a recognized method to improve treatment efficiency, mainly in terms of ammonium removal. While numerous investigations have been reported testing constant aeration, scarce information can be found about the efficiency of intermittent aeration. This study aims at comparing continuous and intermittent aeration, establishing if there is an optimal regime that will increase treatment efficiency of HSSF CWs whilst minimizing the energy requirement. Full and intermittent aeration were tested in a pilot plant of three HSSF CWs (2.64m(2) each) fed with primary treated wastewater. One unit was fully aerated; one intermittently aerated (i.e. by setting a limit of 0.5mg/L dissolved oxygen within the bed) with the remaining unit not aerated as a control. Results indicated that intermittent aeration was the most successful operating method. Indeed, the coexistence of aerobic and anoxic conditions promoted by the intermittent aeration resulted in the highest COD (66%), ammonium (99%) and total nitrogen (79%) removals. On the other hand, continuous aeration promotes ammonium removal (99%), but resulted in nitrate concentrations in the effluent of up to 27mg/L. This study demonstrates the high potential of the intermittent aeration to increase wastewater treatment efficiency of CWs providing an extreme benefit in terms of the energy consumption.