Cristina Ávila

Emerging organic contaminant removal depending on primary treatment and operational strategy in horizontal subsurface flow constructed wetlands: Influence of redox

This study aimed at assessing the influence of primary treatment (hydrolytic upflow sludge blanket (HUSB) reactor vs. conventional settling) and operational strategy (alternation of saturated/unsaturated phases vs. permanently saturated) on the removal of various emerging organic contaminants (i.e. ibuprofen, diclofenac, acetaminophen, tonalide, oxybenzone, bisphenol A) in horizontal subsurface flow constructed wetlands. For that purpose, a continuous injection experiment was carried out in an experimental treatment plant for 26 days. The plant had 3 treatment lines: a control line (settler-wetland permanently saturated), a batch line (settler-wetland operated with saturate/unsaturated phases) and an anaerobic line (HUSB reactor-wetland permanently saturated). In each line, wetlands had a surface area of 2.95 m(2), a water depth of 25 cm and a granular medium D(60) = 7.3 mm, and were planted with common reed. During the study period the wetlands were operated at a hydraulic and organic load of 25 mm/d and about 4.7 g BOD/m(2)d, respectively. The injection experiment delivered very robust results that show how the occurrence of higher redox potentials within the wetland bed promotes the elimination of conventional quality parameters as well as emerging microcontaminants. Overall, removal efficiencies were always greater for the batch line than for the control and anaerobic lines, and to this respect statistically significantly differences were found for ibuprofen, diclofenac, oxybenzone and bisphenol A. As an example, ibuprofen, whose major removal mechanism has been reported to be biodegradation under aerobic conditions, showed a higher removal in the batch line (85%) than in the control (63%) and anaerobic (52%) lines. Bisphenol A showed also a great dependence on the redox status of the wetlands, finding an 89% removal rate for the batch line, as opposed to the control and anaerobic lines (79 and 65%, respectively). Furthermore, diclofenac showed a greater removal under a higher redox status (70, 48 and 32% in the batch, control and anaerobic lines). Average removal efficiencies of acetaminophen, oxybenzone and tonalide were almost >90% for the 3 treatment lines. The results of this study indicate that the efficiency of horizontal flow constructed wetland systems can be improved by using a batch operation strategy. Furthermore, we tentatively identified 4-hydroxy-diclofenac and carboxy-bisphenol A as intermediate degradation products. The higher abundance of the latter under the batch operation strategy reinforced biodegradation as a relevant bisphenol A removal pathway under higher redox conditions.

Capacity of a horizontal subsurface flow constructed wetland system for the removal of emerging pollutants: an injection experiment.

A continuous injection experiment was implemented in a pilot-scale horizontal subsurface flow constructed wetland system to evaluate the behavior of four pharmaceuticals and personal care products (i.e. ibuprofen, naproxen, diclofenac and tonalide) and a phenolic estrogenic compound (i.e. bisphenol A). The treatment system consisted of an anaerobic reactor as a primary treatment, followed by two 0.65 m² wetlands (B1 and B2) working in parallel and connected to a 1.65 m² wetland (B3) operating in series. Overall removal efficiencies for the selected compounds ranged from 97% to 99%. The response curves of the injected pollutants show that the behavior of these compounds strongly depends on their sorption and biodegradation characteristics. While about 50% of ibuprofen was removed in B1 and B2, 99% was achieved at B3, where the dissolved oxygen concentration was significantly higher (B1-B2=0.5 mg L⁻¹ and B3=5.4 mg L⁻¹). Naproxen and diclofenac were efficiently removed (93%) in B1 and B2, revealing anaerobic degradation as a probable removal mechanism. Moreover, tonalide and bisphenol A were readily removed in the small wetlands (94% and 83%, respectively), where the removal of total suspended solids was 93%. Therefore, given their high hydrophobicity, sorption onto the particulate matter stands for the major removal mechanism. However, the tentative identification of carboxy-bisphenol A as an intermediate degradation product in B3 suggested biodegradation as a relevant bisphenol A removal pathway under aerobic prevailing conditions.

Attenuation of emerging organic contaminants in a hybrid constructed wetland system under different hydraulic loading rates and their associated toxicological effects in wastewater

The capacity of a hybrid constructed wetland (CW) system consisting of two vertical flow (VF) CWs working alternatively (3m(2)), one horizontal flow (HF) CW (2m(2)) and one surface flow (FWS) CW (2m(2)) in series to eliminate 13 emerging organic contaminants (EOCs) under three different hydraulic loading rates (HLRs) (0.06, 0.13 and 0.18 m d(-1) considering the area of the two VF beds) was studied through a continuous injection experiment. General toxicity, dioxin-like activity, antimicrobial activity and estrogenicity were also measured under the highest hydraulic loading rate. The hybrid system was highly efficient on the removal of total injected EOCs (except for antibiotics, 43 ± 32%) at all three HLRs (87 ± 10%). The removal efficiency in the hybrid CW system showed to decrease as the HLR increased for most compounds. The VF wetlands removed most of the injected EOCs more efficiently than the other two CWs, which was attributable to the predominant aerobic degradation pathways of the VF beds (70 ± 21%). General toxicity was reduced up to 90% by the VF beds. Estrogenicity and dioxin-like activity were similarly reduced by the VF and the HF wetlands, whereas antimicrobial activity was mainly removed by the FWS wetland. Bearing this in mind, this injection study has demonstrated that the use of hybrid CW systems is a suitable wastewater technology for removing EOCs and toxicity even at high HLRs.

Emerging organic contaminants in vertical subsurface flow constructed wetlands: Influence of media size, loading frequency and use of active aeration

Four side-by-side pilot-scale vertical flow (VF) constructed wetlands of different designs were evaluated for the removal of eight widely used emerging organic contaminants from municipal wastewater (i.e. ibuprofen, acetaminophen, diclofenac, tonalide, oxybenzone, triclosan, ethinylestradiol, bisphenol A). Three of the systems were free-draining, with one containing a gravel substrate (VGp), while the other two contained sand substrate (VS1p and VS2p). The fourth system had a saturated gravel substrate and active aeration supplied across the bottom of the bed (VAp). All beds were pulse-loaded on an hourly basis, except VS2p, which was pulse-loaded every 2h. Each system had a surface area of 6.2m(2), received a hydraulic loading rate of 95 mm/day and was planted with Phragmites australis. The beds received an organic loading rate of 7-16 gTOC/m(2)d. The sand-based VF (VS1p) performed significantly better (p<0.05) than the gravel-based wetland (VGp) both in the removal of conventional water quality parameters (TSS, TOC, NH4-N) and studied emerging organic contaminants except for diclofenac (85 ± 17% vs. 74 ± 15% average emerging organic contaminant removal for VS1p and VGp, respectively). Although loading frequency (hourly vs. bi-hourly) was not observed to affect the removal efficiency of the cited conventional water quality parameters, significantly lower removal efficiencies were found for tonalide and bisphenol A for the VF wetland that received bi-hourly dosing (VS2p) (higher volume per pulse), probably due to the more reducing conditions observed in that system. However, diclofenac was the only contaminant showing an opposite trend to the rest of the compounds, achieving higher elimination rates in the wetlands that exhibited less-oxidizing conditions (VS2p and VGp). The use of active aeration in the saturated gravel bed (VAp) generally improved the treatment performance compared to the free-draining gravel bed (VGp) and achieved a similar performance to the free-draining sand-based VF wetlands (VS1p).

Enhancement of total nitrogen removal through effluent recirculation and fate of PPCPs in a hybrid constructed wetland system treating urban wastewater

The effect of effluent recirculation on the removal of total nitrogen (TN) and eight pharmaceuticals and personal care products (PPCPs) was evaluated during 9months in an experimental hybrid constructed wetland (CW) system applied in the treatment of urban wastewater. An Imhoff tank was followed by three stages of CWs (two 1.5-m2 vertical subsurface flow (VF) beds alternating feed-rest cycles, a 2-m2 horizontal (HF) and a 2-m2 free water surface (FWS) wetland in series). A fraction of the final effluent was recycled back to the Imhoff tank with a recirculation rate of 50% (hydraulic loading rate=0.37md-1). The systems performance varied throughout the study. In Period I (summer) consistently high load removal efficiencies of TN (89±5%) and a removal rate of 6.6±1.4gTNm-2d-1 were exhibited. In Period II (fall), the poor performance of the FWS during the senescence of macrophytes caused a large increase in organic matter, solids and nutrient concentrations, drastically deteriorating water quality. The determination of PPCPs was conducted during this period. Recalcitrant compounds, namely sulfamethoxazole, carbamazapine, TCEP and sucralose were negligibly removed in all CWs. However, noteworthy was the ≈30% removal of sucralose in the VF wetland. Caffeine (80%) and fluoxetine (27%) showed similar elimination rates in both VF and HF units, whereas trimethoprim and DEET were significantly better removed in the VF than in the HF. The concentration of the four latter compounds showed a severe increase in the FWS, indicating possible desorption from the sediment/biomass during adverse conditions. Harvesting of the aboveground biomass in this unit returned the systems performance back to normality (Period III), achieving 77±7% TN removal despite the winter season, proving effluent recirculation as an effective strategy for TN removal in hybrid CW systems when stringent restrictions are in place.

Nitrogen transforming bacteria within a full-scale partially saturated vertical subsurface flow constructed wetland treating urban wastewater

The aim of this study was to characterize the nitrogen transforming bacterial communities within a partially saturated vertical subsurface flow constructed wetland (VF) treating urban wastewater in southern Brazil. The VF had a surface area of 3144m2, and was divided into four wetland cells, out of which two were operated while the other two rested, alternating cycles of 30days. The nitrifying and denitrifying bacterial communities were characterized in wetland cell 3 (764m2 surface area) over a period of 12months by using the FISH technique. Samples were collected monthly (from Feb 2014 to Feb 2015) from different layers within the vertical profile, during operation and rest periods, comprising a total of 6 sampling campaigns while the cell was in operation and another 6 when the cell was at rest. This wetland cell operated with an average organic loading rate (OLR) of 4gCODm-2d-1 and a hydraulic loading rate of 24.5mmd-1. The rest periods of the wetland cell presented influences on the abundance of ammonia-oxidizing bacteria (AOB) (8% and 3% for feed and rest periods, respectively), and nitrite-oxidizing bacteria (NOB) (5% and 2% for feed and rest periods, respectively). However, there was no influence of the rest periods on the denitrifying bacteria. AOB were only identified in the top layer (AOB β-proteobacteria) in both operational and rest periods. On the other hand, the NOB (Nistrospirae and Nitrospina gracilis) were identified in feed periods just in the top layer and during rest periods just in the intermediate layer. The denitrifying bacteria (Pseudomonas spp. and Thiobacillus denitrificans) were identified from the intermediate layer downwards, and remained stable in both periods. Based on the identified bacterial dynamics, the partially saturated VF wetland operated under low OLR enabled favorable conditions for simultaneous nitrification and denitrification.

Pharmaceuticals and personal care products (PPCPs) in the environment and their removal from wastewater through constructed wetlands

Pharmaceuticals and personal care products are emerging organic contaminants that are continuously introduced into the environment due to their regular and widespread consumption by human beings. Their sources and fate in the aquatic environment are multiple and depend on the particular compound. Conventional wastewater treatment plants constitute the major source of these contaminants in the environment due to their incomplete removal by unspecific treatment methods. While advanced treatment technologies are available to treat these substances, their use is oftentimes not economically justified under the current concept of wastewater treatment. In this context, constructed wetlands represent an appropriate low-cost, energy-efficient, and easy-to-operate technology to treat wastewater, which are able to achieve a very high removal of these substances due to the vast number of biotic and abiotic processes that occur within these units, especially when different wetland types are used.

Unraveling the active microbial populations involved in nitrogen utilization in a vertical subsurface flow constructed wetland treating urban wastewater

The dynamics of the active microbial populations involved in nitrogen transformation in a vertical subsurface flow constructed wetland (VF) treating urban wastewater was assessed. The wetland (1.5m2) operated under average loads of 130gCODm-2d-1 and 17gTNm-2d-1 in Period I, and 80gCODm-2d-1 and 19gTNm-2d-1 in Period II. The hydraulic loading rate (HLR) was 375mmd-1 and C/N ratio was 2 in both periods. Samples for microbial characterization were collected from the filter medium (top and bottom layers) of the wetland, water influent and effluent at the end of Periods I (Jun-Oct) and II (Nov-Jan). The combination of qPCR and high-throughput sequencing (NGS, MiSeq) assessment at DNA and RNA level of 16S rRNA genes and nitrogen-based functional genes (amoA and nosZ-clade I) revealed that nitrification was associated both with ammonia-oxidizing bacteria (AOB) (Nitrosospira) and ammonia-oxidizing archaea (AOA) (Nitrososphaeraceae), and nitrite-oxidizing bacteria (NOB) such as Nitrobacter. Considering the active abundance (based in amoA transcripts), the AOA population revealed to be more stable than AOB in both periods and depths of the wetland, being less affected by the organic loading rate (OLR). Although denitrifying bacteria (nosZ copies and transcripts) were actively detected in all depths, the denitrification process was low (removal of 2gTNm-2d-1 for both periods) concomitant with NOx-N accumulation in the effluent. Overall, AOA, AOB and denitrifying bacteria (nosZ) were observed to be more active in bottom than in top layer at lower OLR (Period II). A proper design of OLR and HLR seems to be crucial to control the activity of microbial biofilms in VF wetlands on the basis of oxygen, organic-carbon and NOx-N forms, to improve their capacity for total nitrogen removal.