Abstract
Floating treatment saltmarsh (FTS) is a new concept proposed to name floating treatment wetlands made of estuarine halophytes especially engineered for the control of contamination in brackish and saline waterbodies. The first full-scale FTS was implemented in 2018 to create an anti-contamination barrier for saline aquaculture wastewater treatment in an estuarine tidal lagoon. Results of a two-year investigation validated ‘Phytobatea’ modular technology for floating wetlands implementation and operation. Juncus maritimus crossflow FTS efficiency on main mariculture wastewater constituents' removal under low hydraulic retention time was remarkable, i.e., total phosphorus (86%), total suspended solids (82%), biochemical oxygen demand (78%), total organic carbon (55%), turbidity (53%), Escherichia coli (30%), and dissolved oxygen increased (19%). Key features of the native halophyte Juncus maritimus were determined to ensure 75–100% survival under high water salinities up to 38 g/L. A scientific literature review confirmed strategic sectors' growing interest in Juncus maritimus as raw material, supporting its possible cultivation as an added-value by-product within integrated aquaculture systems. Plants' root systems colonization by crabs, shrimps, and young individuals of the critically endangered European eel (Anguilla anguilla), revealed the role of FTS for biodiversity conservation, and its potential as functional habitat, nursery, and refuge for aquatic fauna species in contaminated waterbodies.
HIGHLIGHTS
Floating treatment saltmarsh (FTS) concept is firstly proposed to define saline FTWs of halophytes.
The initial size of the plants strongly determines J. maritimus survival in saline contaminated waterbodies.
Crossflow FTS are efficient solutions for mariculture wastewater treatment under high salinities and low hydraulic retention times.
The growing interest in halophytes and their by-products supports FTS implementation within integrated aquaculture systems.
FTS can play a strategic role as artificial habitats for biodiversity conservation and enhancement.
INTRODUCTION
Floating treatment wetlands (FTWs) are among the most innovative nature-based solutions for water contamination phytoremediation. In contrast to other wetland technologies, i.e., free water surface, vertical, and horizontal sub-surface flow treatment wetlands (TWs), plants do not necessarily require a substrate to be planted in but are directly installed in the water column through artificial buoyant structures (Pavlineri et al. 2017). Aerial parts of the plants grow above the water surface, and roots and rhizomes develop under water, giving shape to complex three-dimensional rhizo-filters through which water flows and is subjected to physical, chemical, and biological processes. Plants are co-responsible for pollutants remediation in synergism with biofilm microorganisms (Shahid et al. 2018) that grow over the surface of the wetland's submerged components, both organic and artificial (Zhang et al. 2016).
Buoyancy and no requirement for a substrate are the main differential features that make FTWs an especially versatile and resilient technology, enabling TWs implementation in a broad range of natural, semi-natural and anthropogenic waterbodies. FTWs can be even installed where water depth or high-water level and flow fluctuation make it impossible to implement other TWs technologies, as is the case of rivers, lakes, stormwater runoff treatment ponds, and tidal environments, i.e., salt marshes, coastal lagoons, tidal flats, and estuaries (Tanner & Headley 2011; Schwammberger et al. 2017, Karstens et al. 2021; Sharma et al. 2021).
FTWs efficacy has been proved in a broad range of applications, including sewage, stormwater, agriculture, aquaculture, landfill and industrial wastewater treatment, bioenergy crops cultivation, habitat creation for biodiversity conservation, and nature-based solutions for coastal defence (Pavlineri et al. 2017; Headley & Tondera 2019; Kourkoumpas et al. 2019; Ware & Callaway 2019). However, optimal design, performance and management parameters need to be better defined through more full-scale and long-term case studies development (Vymazal et al. 2021), particularly in certain specific fields as is the case of saltwater environments and saline wastewater treatment, on which research is almost non-existent and limited to a few, but promising, lab-scale and small sized pilot systems (Sanicola et al. 2019; Yajun et al. 2019; Calheiros et al. 2020; Karstens et al. 2021).
This paper shows what we consider the first full-scale floating treatment saltmarsh (FTS). It was designed and started in 2018 for mariculture's wastewater treatment in Ria de Tina Menor, an estuary which belongs to the Rías Occidentales y Duna de Oyambre Special Area of Conservation (SAC). FTS was implemented as one of the conservation actions of Convive LIFE, a research project funded by the European Union that addresses the problem of integrating human activities in the conservation objectives of the Natura 2000 Network in the estuaries of the region of Cantabria, in the north of Spain.
The FTS was installed as a floating barrier arranged transversely to the water flow, from side to side of a tidal artificial oblong lagoon receiving the final effluent of a mariculture company's wastewater treatment plant. It was constructed with the novel system for FTWs implementation and management named phytobatea, and it was planted with the native halophyte sea rush Juncus maritimus Lam.
The main aim of the present research was to assess J. maritimus FTS efficacy for saline aquaculture wastewater treatment. High water salinity up to 38 g/L compromised plants survival, and the determination of key features so that the plants could survive became a major challenge and one of the main aims of the project. Secondary goals were to test phytobatea technology in saline wastewater, to assess FTS role as habitat for fauna species of interest, and to undertake a preliminary evaluation of the possibility of incorporating FTSs within integrated aquaculture aystems, based on a literature review of J. maritimus potential as a valuable crop.
The final results of the two-year research demonstrate the feasibility of novel ecotechnologies to tackle environmental issues of major importance such as aquaculture, which according to the Food and Agriculture Organization of the United Nations, accounts for ca. 50% of the world's food fish, and could be the fastest growing food sector. It generates high volumes of wastewater, typically high in suspended solids, organic matter, and nutrients, which may produce a detrimental effect if it was discharged untreated to sensitive ecosystems, i.e., estuaries, compromising their sustainability, and that of the ecosystemic services they provide. In fact, estuaries are among the most productive, but at the same time degraded, ecosystems on the planet: over 60% of European transitional waters do not reach good ecological status (Zal et al. 2018) as a consequence of human overexploitation, transformation, and pollution (Lotze et al. 2006; Kristensen et al. 2021).
MATERIALS AND METHODS
Study area
The origin of the wastewater effluent is a land-based mariculture company which produces phytoplankton, young sea bream and young sea bass following organic procedures. It is settled in a 30 ha saltmarsh in the estuary of Tina Menor, where the river Nansa flows into the Cantabrian Sea. Saline wastewater is produced at an average flow of 119 m3/h, reaching peaks of 176 m3/h. After conventional treatment in a wastewater treatment plant (WWTP) equipped with sludge thickener and total oxidation technology, it is discharged into one end of an artificial tidal lagoon 12 m width and 95 m long, that is hydraulically connected to the open estuary through a 315 mm-diameter drainpipe located at the other end of the lagoon. The drainpipe is at 1.10 m above sea level (masl), whereas average height reached by the tides in Ria de Tina Menor is 2 masl (Fernández-Iglesias & Marquínez 2002). Consequently, water level in the lagoon varies continuously depending on tidal influence and wastewater flow, with an average water depth ranging from 40 to 90 cm in neap and spring tides respectively.
For a better understanding of the hydrology of the system, tidal regime in the lagoon was studied through a bathymetric survey and a tidal influence field assessment undertaken during a spring tide cycle, on 25 May 2017. Water levels inside and outside of the lagoon, at the final discharge point of the drainpipe in the estuary, were monitored hourly. Results indicated that most of the time (9.1 hours) water flowed from the lagoon to the estuary, and that flux was interrupted only during 3.3 hours at high tide. This result corroborated that most of the time tidal influence in the lagoon is limited by drainpipe height and capacity, and predominant water flux occurs from the lagoon to the estuary, while water entering from the estuary at high tide did not mix with that in the lagoon due to the difference of densities. Average hydraulic retention time (HRT) of 6.14 hours in the lagoon and an average water velocity of 14 m/h were estimated when water is flowing along the lagoon. In each tide cycle HRT is increased depending on the time when the water flow is interrupted by the influence of the high tide collapsing the draining pipe.
Floating treatment saltmarsh set up
In February 2018, a 66 m2 FTS was installed within the tidal lagoon, at a distance of 50 m from the WWTP discharging point. A floating modular dock of 1.5 m width and 11 m length was put from side to side of the lagoon as a structural axis for the wetland system, and a series of 11 Phytobatea modules were attached to each side of the dock. The system was anchored to both lagoon banks with swinging systems, permiting FTS vertical movement with water level fluctuation.
Phytobatea technology is a modular system for FTWs implementation and management. Each module consists of a glass fibre reinforced polyester (GFRP) main support beam traversed by a plurality of GFRP rods to which plants are fixed, and an overlaying high density Polyethylene (HDPE) mesh through which plants leaves and stems grow, securing plants' verticality and facilitating biomass periodical harvesting above the water surface, whereas roots and rhizomes grow under water. Bouyancy and stability is provided by sea fishing buoys. Phytobatea design and structural resistance allows the operation of the modules, including their removal from the waterbody if needed. In this case, an ad hoc lifting machine was designed, prototyped and tested for TWs modules operation, as a tilting ramp equipped with a manual winch, which has a 200 kg load capacity, and attached with a textile sling to the dock's cleats.
The FTS infrastructure consisting of the modular dock and the vegetated Phytobateas can be considered as a cross-flow anti contamination barrier 11 m long, 7.5 m width (see Figure 1), with an under water root filter divided into two layers, a densely rooted superficial layer of 9–12 cm depth, and a less dense layer formed by longer roots of 26–36 cm length, so in neap tides, when water depth is around 40 cm, most of the water flux passes through the root systems, whereas in spring tides, when water level rises up to 90 cm, above 50% of water flows underneath the roots system. Considering the 9.1 hours during which water flows through the lagoon to the final discharge point into the estuary, and the 3.3 hours during which water flux is interrupted by the high tides collapsing the draining pipe, an average HRT of 44 minutes was estimated for water passing through the FTS at an average speed of 10.27 m/h.
Plants selection, supply, installation, and maintenance
Following the same principles as in our previous research on TWs development for wastewater treatment in mountain areas (Cicero-Fernandez et al. 2019) plant species selection for this project was undertaken considering the importance of working with native emergent key species, thus contributing to biodiversity conservation (Doody 2010). Therefore, a preliminary study of Ria de Tina Menor saltmarsh vegetation was undertaken. It concluded that the most abundant species of native halophytes in the tidal mudflats of the estuary are Spartina maritima, J. maritimus and Halimione portulacoides.
H. portulacoides (Atriplex portulacoides) is a shrub that frequently grows in association with J. maritimus sharing the same biotopes, and it is a perennial high biomass producer halophyte. However, it was discarded because of its woody structure and lack of aerenchyma, which is considered a disadvantage for wastewater phytoremediation. From the two native emergent helophytes, S. maritima and J. maritimus, the second was finally selected because its root system is deeper than that of the cordgrass, which tends to grow superficially (Castillo et al. 2010), and it is generally accepted that FTWs' efficacy for water contamination remediation mainly depends on a vigorous root system and is related to biofilm development. Consequently, J. maritimus was considered the best species fitting the required features for saline wastewater phytoremediation, noting that it is a perennial halophyte, high biomass producer, which can propagate both sexually and asexually, through rhizomes and stolons. It was the unique native emergent species spontaneously present in the margins of the tidal lagoon where the FTS would be installed, indicating a probably better adaption capacity to the conditions in the site than other species. It is among the most abundant native plant species in the local saltmarshes, and a key species within the Natura 2000 Network priority habitat 1330, Atlantic salt meadows (Glauco-Puccinellietalia maritimae) (Doody 2010), so planting it in the FTS also contributes to local biodiversity conservation through J. maritimus local genes preservation, thanks to the spreading of the seeds of the plants installed in the FTS.
A scientific literature review about the interest in J. maritimus as raw material for different strategic sectors was undertaken, confirming the possibility of J. maritimus FTS implementation not only for wastewater remediation, but also as an interesting crop that could be part of an integrated aquaculture system.
With the preceptive authorisations, 440 Juncus maritimus plants were supplied from a nearby salt meadow. Big mats were taken from the mud using hand tools. During the washing of plants with water in order to eliminate any soil, most of them broke up into smaller mats and rhizomes. Before plantation, rushes were classified according to mat or main rhizome length above or below 10 cm, and leaves were pruned to around 20 cm long. Plants were installed at a density of 6.6 units/m2 (20 units/Phytobatea), in a proportion of 50% of each rhizome size, following a traditional staggered pattern. Months later, unsuccessful plants were replaced with new individuals of varied sizes and ages, supplied from the same area, following the same protocol for preparation and installation except for leaves pruning. No mowing or any other plant maintenance operations were done during the experiment.
Monitoring
FTS was monitored on a regular basis for two years, from March 2018 to March 2020.
Plant survival was assessed every six months over a representative sample of 60 plants (13.6% of the total population) distributed in groups of 10 plants in 6 phytobateas strategically located in the 2nd, 6th, and 10th positions at each side of the floating dock. For vegetative growth evaluation, persistent organs, i.e., mat's length and width, and root systems length, were measured yearly. Green leaves and sprouts were considered the main survival indicators. Time needed for FTS initial installation (i.e., plants attachment to Phytobatea modules and installation in the water), as well as for dead plants replacement (i.e., modules removal, plants replacement and modules return to the water), was recorded to evaluate Phytobatea technology efficiency for TWs' vegetation installation, management, and maintenance.
Water was sampled monthly for 25 consecutive months, always in days with the lowest coefficient tides, therefore preventing tidal influence on and interferences with predominant water quality gradient along the system. Samples were taken in four locations within the lagoon (see Figure 2) according to water flow direction: the first one (A) was in the discharging point of the conventional wastewater treatment plant at one end of the lagoon, the second one (B) just before the FTS, in the centre of the lagoon's section; the third sampling station (C) was established just after the FTS; and the last one (D) at the other end of the lagoon, by the mouth of the drainpipe through which final effluent discharges into Tina Menor estuary. When the Phytobatea modules were removed from the waterbody for maintenance operations, organoleptic water properties beneath the saltmarsh appear to be better than in the rest of the lagoon in terms of transparency and odour, so from August 2019 to March 2020, an additional sampling point (S) was established in the centre of the FTS to evaluate if a substantially better water quality existed within the wetland.
On site analyses were conducted for temperature (T), pH, dissolved oxygen (DO), salinity and turbidity, whereas analyses for conductivity (Cond), biochemical oxygen demand (BOD5), total organic carbon (TOC), total suspended solids (TSS), total nitrogen (TN), total phosphorus (TP), total coliforms (TC) and Escherichia coli (E. coli) analyses were conducted by the Centre for Environmental Research of the Government of Cantabria (CIMA) Certified Laboratories following standard methods.
When fauna was observed in the FTS during maintenance and monitoring works, those species were identified, noted, and photographed, if possible, for a preliminary evaluation of FTS role as habitat.
Data analysis
Hypothesis of the minimum size for Juncus maritimus survival and growth
Plant survival monitoring results suggested that the size of the plants could be the key factor for sea rush individual success. To confirm this hypothesis, a statistical study was done over a representative sample (14%) of plants from the initial plantation to determine if size of persistent parts of the plants (i.e., mat's length and width, and roots length) were related to survival and death rates. Average values of each parameter were calculated for both groups of dead and alive plants respectively. Data were analysed through one-way ANOVA, and one-sided T-Test was applied to determine if there were differences (p-value < 0.05) between the estimated average sizes of dead and alive plants. To discard significative water salinity variation influencing survival rates during the experiment, water conductivity values corresponding to high survival rate period were compared to values corresponding to low survival rate period through the T-Test. Cumulative growth assessment of the plants was conducted on those plants forming part of the monitoring sample that survived during the two-year investigation.
Floating treatment saltmarsh performance
Average concentrations of water parameters were calculated with recorded data from a total of 25 sampling campaigns for A, B, C and D sampling points, and of 7 campaigns for B, C and S sampling points, and removal/increase rates expressed in percentage were also calculated. To discriminate FTS efficacy on wastewater treatment from other processes that could occur in the lagoon (i.e., sedimentation, algae phytoremediation), removal rates from A to B and from C to D were calculated to determine removal rates in the previous and subsequent lagoon sections to the FTS and compared to those obtained from B to C sampling points, representative of the saltmarsh performance through one-sided T-Test at a 95% confidence level, and difference between sampling points was considered as statistically significant if the p-value < 0.05. In addition, water quality average values in B and C (sampling points located just before and after the TWs) were compared with S (additional sampling point located in the centre of the wetland) following the same method, to confirm if better water quality existed in the FTS centre.
RESULTS AND DISCUSSION
Floating treatment saltmarsh terms and classification
As natural wetlands, depending on water salinity TWs can be divided into freshwater and saline, which in turn comprise brackish (salinity 5–30 g/L) and saltwater (salinity > 30 g/L) wetlands (Yang 2019). At the same time, as well as freshwater ones, saline TWs can be designed following the best available technologies, i.e., free water surface, horizontal subsurface flow, vertical subsurface flow, and floating wetland systems, of which the latter is the youngest and most innovative type. This is the main reason of the range of terms used to name it. However, the term Floating Treatment Wetland (FTW) coined by Headley & Tanner (2008) appears to be the most widely accepted to date (Colares et al. 2020; Shen et al. 2021; Vymazal et al. 2021). FTWs are floating systems for water contamination control planted with emergent macrophytes. Therefore, when halophytes and/or salt tolerant shrubs, herbs or grasses are used to create any FTW, to be implemented in saline waterbodies and/or to treat saline wastewater, Saline Floating Treatment Wetlands would be the logical name to define it, as a result of the current definitions for saline constructed wetlands (CWs) and FTWs combination. However, instead of Saline Floating Treatment Wetlands, the authors of the present article consider the term Floating Treatment Saltmarsh (FTS) as the most synthesizing, understandable, and ecologically accurate term to define the novel type of TW which is described and evaluated in the present investigation.
Juncus maritimus survival and growth
Juncus maritimus is documented to be one of the most salt tolerant halophytes of the Juncus genus, capable to survive salinities up 30 g/L (Boscaiu et al. 2011). However, when this investigation was developed, no reference studies were found, neither on survival rates when exposed to higher salinities nor under hydroponic regime, so a trial/error process was undertaken to determine sea rush ability to overcome extreme conditions of the present investigation.
As can be seen in Figure 3, six months after initial plantation a high mortality up to 45% occurred. This was related not only to salt and chemical stress to which plants were exposed, but also to leaves pruning prior to plantation, which could compromise fundamental adaptation mechanisms of J. maritimus to salt stress that depend on photosynthesis, as is the case of osmolytes, i.e., sugars and proline synthesis and accumulation in shoots (Hassan et al. 2016). Consequently, in August 2019 dead plants were replaced with one year old individuals (mat diameter ≤10 cm) with intact leaves, transplanted from a nearby saltmarsh. The first reaction after plantation was the progressive senescence of the leaves, and one year later 78% of those new young plants died, while 91% of larger plants which had survived since the initial plantation were still alive. Survival dependence on plant size under similar environmental conditions has been documented by other authors, as is the case of Lissner & Schierup (1997), who observed a higher mortality of Phragmites australis young plants compared to larger plants when cultivated in saline solutions, and by Calheiros et al. (2020), who in 2018, almost simultaneously to the present project development, were undertaking a research project on saltmarsh halophytes FTS systems feasibility for implementation in port marinas in Portugal, which concluded that small sized J. maritimus individuals could not survive the conditions of the experiment (salinity of 30–34 g/L).
Therefore, for a better understanding of our results, a comparative statistical analysis of plants' key features sizes was conducted between dead and alive plants from the first plantation. Results revealed that survival notably increased, above 75%, for plants whose mat length and width, and roots average and maximum lengths, were equal to or above 22.39, 6.52, 7.82 and 29.3 cm respectively (see Table 1). To verify this hypothesis, a second campaign for dead plants replacement was carried out in August 2019, when 24 plants with sizes above the hypothetic minimum values from which survival rate notably increased were planted. Seven months later, in March 2020, 100% survival was confirmed, suggesting that the hypothesis is correct.
Key features (cm) . | March 2018 plantation . | Aug. 2019 plantation (Survival = 100%) . | Minimum size hypothesis (Survival ≥ 75%) . | |||
---|---|---|---|---|---|---|
Alive Av. . | Dead Av. . | Min. . | Av. . | Max. . | Min . | |
Mat/main rhizome length | 22.39 | 19.57 | 28 | 34.67 | 50 | 22 |
Mat width | 6.52 | 3.72 | 6 | 13.56 | 25 | 7 |
Root system av. length | 7.82 | 6.41 | 12 | 18.56 | 36 | 8 |
Root system max. length | 29.3 | 25 | 12 | 18.56 | 36 | 29 |
Key features (cm) . | March 2018 plantation . | Aug. 2019 plantation (Survival = 100%) . | Minimum size hypothesis (Survival ≥ 75%) . | |||
---|---|---|---|---|---|---|
Alive Av. . | Dead Av. . | Min. . | Av. . | Max. . | Min . | |
Mat/main rhizome length | 22.39 | 19.57 | 28 | 34.67 | 50 | 22 |
Mat width | 6.52 | 3.72 | 6 | 13.56 | 25 | 7 |
Root system av. length | 7.82 | 6.41 | 12 | 18.56 | 36 | 8 |
Root system max. length | 29.3 | 25 | 12 | 18.56 | 36 | 29 |
The first column of data represents average sizes of dead and alive key features of the plants recorded in August 2018, for the group of plants installed in March 2018. The second column represents average minimum (Min.), mean (Av.) and maximum (Max.) values for the main key features of sea rush individuals planted in August 2019 exceeding minimum sizes determined for a survival rate equal or above 75%, represented in the third column.
To determine if water salinity varied during that last period (Sep 19–Mar 20), electrical conductivity results in sampling stations located by the FTS (B and C) were compared with those of the same period obtained in the previous year (Sep 18–Mar 19), when 40% of plants died. Slightly lower salinity was observed in the last period of the study, but T-Test confirmed that no significant differences existed between both periods (Table 2), so we concluded that plants' initial size was the main factor determining J. maritimus survival rate when they were planted bare rooted in contaminated sea water, and that equal or above 75% survival was achieved when the length and width of the mats, and the average length of the roots were above 22, 7 and 8 cm respectively.
Conductivity . | . | B (mS/cm) . | C (mS/cm) . |
---|---|---|---|
September 18–March 19 | Average ± s.d. | 48.40 ± 2.10 | 48.76 ± 1.54 |
Maximum | 50.70 | 51.10 | |
Minimum | 45 | 46.70 | |
September 19–March 20 | Average ± s.d. | 39.74 ± 10.80 | 40.63 ± 10.80 |
Maximum | 49.00 | 48.70 | |
Minimum | 20.40 | 20.80 | |
Averages comparison | P-value (T-Test α < 0.05) | 0.051657113 Non-significant | 0.05594512 Non-significant |
Conductivity . | . | B (mS/cm) . | C (mS/cm) . |
---|---|---|---|
September 18–March 19 | Average ± s.d. | 48.40 ± 2.10 | 48.76 ± 1.54 |
Maximum | 50.70 | 51.10 | |
Minimum | 45 | 46.70 | |
September 19–March 20 | Average ± s.d. | 39.74 ± 10.80 | 40.63 ± 10.80 |
Maximum | 49.00 | 48.70 | |
Minimum | 20.40 | 20.80 | |
Averages comparison | P-value (T-Test α < 0.05) | 0.051657113 Non-significant | 0.05594512 Non-significant |
As Figure 4 shows, during the first year, growth of surviving plants was in general low or non-significant, which according to previous studies (Boscaiu et al. 2011; Hassan et al. (2016) is a known response of J. maritimus to salt stress. In the second year, a remarkable growth of mats and leaves of plants that were still alive was observed, while incipient rhizomes and stolons were firstly detected in the last monitoring campaign, in March 2020, indicating the plants passing from an initial phase focused on mere survival and adaptation, to an establishment and propagation phase starting in the third year after plantation, which is consistent with previous findings relating tolerance to salt stress with plant age and growth stage (Chartzoulakis & Klapaki 2000; Delattre et al. 2021).
FTS efficacy as anti-contamination barrier
Water quality in the tidal lagoon was mainly determined by saltwater aquaculture wastewater effluent. Despite being treated in a wastewater treatment plant prior to dumping in the tidal lagoon, the main parameters were generally above typical values for mariculture wastewater, particularly for suspended solids and nutrients, in comparison with land-based mariculture effluents quality data compiled by De Lange et al. (2013).
When 25 consecutive months' water quality average data from each sampling station were compared, an increasing trend of overall concentrations of constituents was observed from the beginning of the lagoon to the front side of the FTS, where most parameters under study reached their peaks (see Table 3); however, only TC and E. coli concentrations increments were statistically significant along the first 50 m section of the lagoon, and the same was observed in the second 37.5 m unvegetated section, downstream of the FTS, where only salinity was significantly lower. These results demonstrate that no wastewater treatment that might occur because of lagooning, algae growth, dilution, or other processes was taking place in non-vegetated sections of the water body. In contrast, high average removal rates for all parameters were observed from B to C, which is the section of the lagoon corresponding to the J. maritimus floating treatment saltmarsh (see Table 3). Even though the FTS represented 7% of the total surface and 8% of the total length of the lagoon, high average removal rates were performed, with the better results for TP (86%), TSS (82%) and BOD (78%), and lower but also significant efficiencies on TOC (55%), turbidity (53%), and E. coli (30%) removal. In addition, a 19% average increase of dissolved oxygen was also achieved by the FTS.
. | A . | B . | C . | D . | % A-B . | % B-C . | % C-D . |
---|---|---|---|---|---|---|---|
T (°C) | 17.51 ± 3.75 | 17.05 ± 4.44 | 16.73 ± 4.39 | 16.56 ± 4.36 | *2.63 | *1.88 | 1.01 |
pH | 9.36 ± 0.94 | 9.35 ± 0.99 | 9.35 ± 0.98 | 9.35 ± 0.99 | 0.11 | 0.00 | 0.00 |
Cond. (ms/cm) | 47.14 ± 4.46 | 45.01 ± 6.80 | 45.40 ± 6.70 | 45.15 ± 7.34 | *4.52 | −0.87 | 0.55 |
Salinity (g/L) | 33.48 ± 3.89 | 32.16 ± 5.65 | 31.92 ± 5.38 | 31.48 ± 5.72 | 3.94 | 0.75 | *1.38 |
DO (mg/L) | 2.76 ± 1.30 | 2.67 ± 1.56 | 3.18 ± 1.48 | 3.03 ± 1.59 | 3.26 | * − 19.10 | 4.72 |
Turbidity | 22.30 ± 25.65 | 36.10 ± 40.08 | 16.98 ± 15.98 | 14.75 ± 23.31 | −61.88 | *52.96 | 13.13 |
TSS (mg/L) | 175.38 ± 471.33 | <210.12 ± 370.75 | 37.22 ± 43.95 | <54.50 ± 107.31 | −19.81 | *82.29 | −46.43 |
BOD5 (mgO2/L) | <33.08 ± 68.77 | <67.64 ± 138.65 | <15.16 ± 9.35 | <41.36 ± 100.16 | −104.47 | *77.59 | −172.82 |
TOC (mgO2/L) | <13.71 ± 11.19 | <26.73 ± 39.51 | <11.90 ± 6.88 | <13.87 ± 25.35 | −94.97 | *55.48 | −16.55 |
TN (mg/L) | 6.84 ± 5.70 | 7.96 ± 9.20 | 6.17 ± 4.86 | 5.62 ± 6.32 | −16.37 | 22.49 | 8.91 |
TP (mg/L) | 12.15 ± 37.97 | 17.14 ± 36.86 | 2.42 ± 2.26 | <6.86 ± 20.49 | −41.07 | *85.88 | −183.47 |
TC (MPN/100 ml) | >4,193.40 ± 9,078.45 | >8,502.96 ± 11,811.38 | >6,171.36 ± 7,831.60 | >38,488.32 ± 154,054.54 | * − 102.77 | 27.42 | −523.66 |
EC (MPN/100 ml) | <81.76 ± 202.38 | <148.72 ± 250.04 | <104.44 ± 208.19 | <300.44 ± 700.49 | * − 81.9 | *29.77 | −187.67 |
. | A . | B . | C . | D . | % A-B . | % B-C . | % C-D . |
---|---|---|---|---|---|---|---|
T (°C) | 17.51 ± 3.75 | 17.05 ± 4.44 | 16.73 ± 4.39 | 16.56 ± 4.36 | *2.63 | *1.88 | 1.01 |
pH | 9.36 ± 0.94 | 9.35 ± 0.99 | 9.35 ± 0.98 | 9.35 ± 0.99 | 0.11 | 0.00 | 0.00 |
Cond. (ms/cm) | 47.14 ± 4.46 | 45.01 ± 6.80 | 45.40 ± 6.70 | 45.15 ± 7.34 | *4.52 | −0.87 | 0.55 |
Salinity (g/L) | 33.48 ± 3.89 | 32.16 ± 5.65 | 31.92 ± 5.38 | 31.48 ± 5.72 | 3.94 | 0.75 | *1.38 |
DO (mg/L) | 2.76 ± 1.30 | 2.67 ± 1.56 | 3.18 ± 1.48 | 3.03 ± 1.59 | 3.26 | * − 19.10 | 4.72 |
Turbidity | 22.30 ± 25.65 | 36.10 ± 40.08 | 16.98 ± 15.98 | 14.75 ± 23.31 | −61.88 | *52.96 | 13.13 |
TSS (mg/L) | 175.38 ± 471.33 | <210.12 ± 370.75 | 37.22 ± 43.95 | <54.50 ± 107.31 | −19.81 | *82.29 | −46.43 |
BOD5 (mgO2/L) | <33.08 ± 68.77 | <67.64 ± 138.65 | <15.16 ± 9.35 | <41.36 ± 100.16 | −104.47 | *77.59 | −172.82 |
TOC (mgO2/L) | <13.71 ± 11.19 | <26.73 ± 39.51 | <11.90 ± 6.88 | <13.87 ± 25.35 | −94.97 | *55.48 | −16.55 |
TN (mg/L) | 6.84 ± 5.70 | 7.96 ± 9.20 | 6.17 ± 4.86 | 5.62 ± 6.32 | −16.37 | 22.49 | 8.91 |
TP (mg/L) | 12.15 ± 37.97 | 17.14 ± 36.86 | 2.42 ± 2.26 | <6.86 ± 20.49 | −41.07 | *85.88 | −183.47 |
TC (MPN/100 ml) | >4,193.40 ± 9,078.45 | >8,502.96 ± 11,811.38 | >6,171.36 ± 7,831.60 | >38,488.32 ± 154,054.54 | * − 102.77 | 27.42 | −523.66 |
EC (MPN/100 ml) | <81.76 ± 202.38 | <148.72 ± 250.04 | <104.44 ± 208.19 | <300.44 ± 700.49 | * − 81.9 | *29.77 | −187.67 |
Columns on the right side of the table represent the removal or increment (−) rates of the parameters (in percentage) along unvegetated sections (A-B and C-D), and in the floating treatment saltmarsh (B-C). Statistically significant removal rates have been calculated at a 95% confidence level (p-value < 0.05) and are indicated with an asterisk (*). MPN, most probable number.
The overall increment of pollutant concentrations in the anterior zone of the FTS indicated an efficient contamination containment performance of the saltmarsh, clearly visible to the naked eye, suggesting FTS's potential as anti-contamination barriers in shallow waterbodies. This application could be developed as an alternative nature-based solution to silt curtains commonly used for turbidity mitigation, i.e., during dredging operations, the efficacy of which is under discussion since Radermacher et al. (2015) demonstrated that hanging silt curtains are not effective in cross currents because the synthetic cloth of which this technology consists is not permeable to water flux, leading water to pass underneath and around the edges of the curtain creating forced currents and consequently increasing water velocity, turbulence, and thus promoting pollution dispersal. On the contrary, FTWs are permeable rhizofilters that allow water to pass through the system, thus preventing water flux diversion and boosting pollutants removal, as is the case of the FTS implemented in the present investigation, the performance of which was 52.96% average turbidity mitigation. In addition, after water treatment through the FTS, turbidity average value remained virtually steady until the end of the lagoon, indicating that no near-bed or near-shores currents were playing a significant role transporting pollution downstream, and that pollution control was efficiently and unequivocally performed by the J. maritimus FTS. Accordingly, an average efficacy up to 82% for TSS removal was observed, which is in line with previous studies reporting suspended solids removal performance by FTWs, which is known to be achieved mainly by physical settling and rhizofiltration processes such as trapping, coagulation, flocculation, and sedimentation (Tanner & Headley 2011; Pavlineri et al. 2017; Shahid et al. 2018). A very similar removal efficacy (i.e., 85.88%) was monitored for total phosphorus, which according to Tanner & Headley (2011) besides plant uptake, greatly depends on sorption in fine suspended solids.
Similar trends followed by both parameters from the highly variable data before the FTS (B) to the stable concentrations just after (C) can be seen in Table 3. Results up to 5- and 7-fold lower in C were calculated for TSS and TP respectively, and low standard deviations in C highlighted FTW efficacy and resilience against TP and TSS peaks, as well as TP concentration decrease related to TSS removal. Aside from this, exceptionally high TP removal results, far above the 48.75% mean rate determined by Pavlineri et al. (2017), could have been caused by the particularly high TP influent concentration; according to the same authors, these estimations are positively correlated with TP removal rate.
On the other hand, 22.49% TN removal efficiency was below mean rates reported in previous studies for TWs in general, and for FTWs and saline TWs in particular (Vymazal 2007; Liang et al. 2017; Pavlineri et al. 2017), which apart from aerobic and anaerobic zoning for nitrification-denitrification processes and well-developed root systems and associated microorganisms, highlighted the importance of HRT (i.e., days) on nitrogen compounds transformation and removal, which might be the main cause conditioning our results. A similar trend (i.e., 17% average TN removal) was reported by Schwammberger et al. (2017) for a stormwater FTW implemented in a lake intermittently receiving water runoff from an urban area. In addition, water salinity above J. maritimus optimal ecological range might be an additional limiting factor for TN removal, as Yajun et al. (2019) reported for the halophyte Suaeda salsa FTS when implemented in eutrophic brackish water.
Indicators of organic (BOD and TOC) and microbiological pollution (TC and E. coli) reached high average values in D, particularly the pathogen concentrations, which reached their maximum values at the end of the lagoon, indicating faecal contamination entering from the estuary with the tides. This occurred only at certain moments of the year, mainly in July and August, and it was probably originated by local farmers fertilising nearby agricultural fields using liquid bovine slurry, which is afterwards washed by runoff and spilled into the estuarine waterbody as non-point source pollution. However, influent contaminated water did not influence sampling stations B and C, and FTS performances on these parameters' removal could be assessed as can be seen in Table 3. BOD and TOC removal rates agreed with other types of TWs for saline wastewater treatment, suggesting that FTSs might be more efficient than subsurface flow wetlands, as HRT was substantially lower in the present investigation than in the ones compiled by Liang et al. (2017). No previous research was found on coliforms of saline aquaculture wastewater treatment with TWs, so the 27.42% and 29.77% elimination rates for total coliforms and E. coli respectively performed by the J. maritimus FTS are considered as the first demonstration of FTS's potential for mariculture wastewater disinfection, which could be probably easily improved through FTS dimension and/or wastewater HRT increment.
FTS interior water quality and role as aquatic fauna refuge
During the maintenance and monitoring operations undertaken in August 2019, better organoleptic properties were observed in the water beneath the floating saltmarsh, i.e., water transparency and odour. In addition, individuals of the green crab Carcinus maenas, and young individuals of the critically endangered species European eel (Anguilla anguilla) were detected attached to the J. maritimus roots, as well as shrimps (Palaemon sp.), which in addition to colonising root systems were forming schools beneath the wetland.Calheiros et al. (2020) also reported Palaemon sp. and other macrofauna species colonising a small pilot FTS of halophytes made of cork in a port marina in Portugal, while Karstens et al. (2021) coincided on FTS attractiveness for shrimps and particularly for young European eels, which massively colonised a FTS installed in a brackish eutrophicated coastal lagoon in the Baltic Sea.
As Table 4 shows, one of the reasons for aquatic fauna colonisation might be a better water quality observed in the centre of the J. maritimus FTS in comparison with the anterior and posterior edges of the wetland and the rest of the lagoon. This trend was observed for all the parameters under study but for E. coli and DO, which reached better results in C. The rest of the parameters were remarkably lower in S, particularly for TSS, turbidity and TP, which were up to 4-fold, 3-fold, and 2.78-fold lower in the centre of the wetland than in the edges respectively, and differences between B and S for these parameters were statistically significant according to the results of one-sided T-test. Salinity was close to 2 points lower and pH 1 point lower, indicating the formation of a good water quality mass below the FTS, that could play a major role as a refuge for aquatic fauna (see Figure 5). Similar trends were reported in the contaminated estuary of the Yangtze River by Huang et al. (2020), which proved the efficacy of Phragmites australis FTS as larvae fish assemblage structure and demonstrated the relation between the higher fish-larvae density and the better water quality and supporting structure provided by the artificial FTS. This ecological application could be especially interesting in waterbodies subjected to heavy pollution events, as is the case of collapsing coastal lagoons like the Mar Menor, on the Mediterranean coast of Spain (Ruiz et al. 2020).
. | B . | S . | C . | % B-S . | % S-C . |
---|---|---|---|---|---|
T (°C) | 14.45 ± 3.22 | 13.74 ± 3.18 | 14.45 ± 3.60 | 4.91 | −5.17 |
pH | 9.42 ± 1.21 | 8.84 ± 1.09 | 9.36 ± 1.29 | 6.16 | −5.88 |
Cond. (ms/cm) | 40.46 ± 6.80 | 38.93 ± 10.87 | 41.70 ± 10.22 | 3.78 | 7.12 |
Salinity (g/L) | 28.38 ± 5.76 | 26.79 ± 8.72 | 28.63 ± 8.33 | 5.6 | −6.87 |
DO (mg/L) | 3.50 ± 1.57 | 3.69 ± 1.81 | 3.96 ± 1.56 | −5.43 | −7.32 |
Turbidity | 16.43 ± 7.95 | 5.36 ± 3.32 | 10.12 ± 12.68 | *67.38 | −88.81 |
TSS (mg/L) | 73.38 ± 19.65 | 17.49 ± 13.64 | 37.43 ± 67.75 | *76.17 | −114.01 |
BOD5 (mgO2/L) | <24.50 ± 2.56 | <11.29 ± 2.63 | <12.38 ± 24.51 | 53.92 | −9.65 |
TOC (mgO2/L) | <11.25 ± 0.00 | <10.00 ± 0.00 | <12.75 ± 2.82 | 11.11 | −27.5 |
TN (mg/L) | 3.30 ± 1.66 | 2.86 ± 1.78 | 3.54 ± 1.45 | 13.33 | −23.78 |
TP (mg/L) | 3.20 ± 1.81 | 1.15 ± 0.59 | 1.82 ± 1.81 | *64.06 | −58.26 |
TC (MPN/100 ml) | <7,722.50 ± 1,576.46 | 4,468 ± 8,765.35 | <4,876.50 ± 10,397.70 | 42.14 | −9.14 |
EC (MPN/100 ml) | <123.13 ± 17.92 | <222.57 ± 482.61 | <56.25 ± 265.35 | −80.76 | 74.73 |
. | B . | S . | C . | % B-S . | % S-C . |
---|---|---|---|---|---|
T (°C) | 14.45 ± 3.22 | 13.74 ± 3.18 | 14.45 ± 3.60 | 4.91 | −5.17 |
pH | 9.42 ± 1.21 | 8.84 ± 1.09 | 9.36 ± 1.29 | 6.16 | −5.88 |
Cond. (ms/cm) | 40.46 ± 6.80 | 38.93 ± 10.87 | 41.70 ± 10.22 | 3.78 | 7.12 |
Salinity (g/L) | 28.38 ± 5.76 | 26.79 ± 8.72 | 28.63 ± 8.33 | 5.6 | −6.87 |
DO (mg/L) | 3.50 ± 1.57 | 3.69 ± 1.81 | 3.96 ± 1.56 | −5.43 | −7.32 |
Turbidity | 16.43 ± 7.95 | 5.36 ± 3.32 | 10.12 ± 12.68 | *67.38 | −88.81 |
TSS (mg/L) | 73.38 ± 19.65 | 17.49 ± 13.64 | 37.43 ± 67.75 | *76.17 | −114.01 |
BOD5 (mgO2/L) | <24.50 ± 2.56 | <11.29 ± 2.63 | <12.38 ± 24.51 | 53.92 | −9.65 |
TOC (mgO2/L) | <11.25 ± 0.00 | <10.00 ± 0.00 | <12.75 ± 2.82 | 11.11 | −27.5 |
TN (mg/L) | 3.30 ± 1.66 | 2.86 ± 1.78 | 3.54 ± 1.45 | 13.33 | −23.78 |
TP (mg/L) | 3.20 ± 1.81 | 1.15 ± 0.59 | 1.82 ± 1.81 | *64.06 | −58.26 |
TC (MPN/100 ml) | <7,722.50 ± 1,576.46 | 4,468 ± 8,765.35 | <4,876.50 ± 10,397.70 | 42.14 | −9.14 |
EC (MPN/100 ml) | <123.13 ± 17.92 | <222.57 ± 482.61 | <56.25 ± 265.35 | −80.76 | 74.73 |
Percentage removal or increment (−) rates between the edges and the centre of the FTS are represented in the columns on the right, and statistically significant removal rates have been calculated at a 95% confidence level (p-value < 0.05) and are indicated with an asterisk (*). MPN, most probable number.
Juncus maritimus FTS integration in aquaculture systems
Integrated aquaculture (IA) systems are those in which different species of organisms are synergically cultivated in the same water (Boyd et al. 2020), thus increasing the efficiency and sustainability of the process. In this case, in which fish is the main product, vegetation cultivated in the FTS for wastewater treatment could also be produced as a valuable crop, if there are markets interested in J. maritimus.
The review of Juncus genera species potential for phytoremediation made by Syranidou et al. (2016) identified J. maritimus as one of the species of interest for phytoremediation, finding that most of the existing investigations are focused on heavy metal and organic persistent pollutants phytoremediation of contaminated sediments, mainly produced in Portugal (Almeida et al. 2006; Marques et al. 2011; Ribeiro et al. 2013; Montenegro et al. 2016), while publications on J. maritimus implementation for wastewater treatment are almost non-existent, and limited to short-term experiments, such as the one developed by Yahiaoui et al. (2020) for domestic wastewater treatment with vertical flow TWs.
On the other hand, the scientific literature review revealed that there are different sectors interested in J. maritimus as raw material to produce added-value products and services. Several recent examples have been found, such as the potential use of sea rush fibres for building insulation composites manufacture (Saghrouni et al. 2020), or its possible cultivation for producing biomass high in carbohydrates for bioenergy (bioethanol), as an alternative crop to others that are currently compromising human food and fresh water sources (Smichi et al. 2015). Sea rush antifungal properties have also attracted the attention of the agriculture sector, i.e., crude methanolic extracts obtained from J. maritimus rhizomes have been reported by Sahli et al. (2018) as an effective biobased fungicide against Zymoseptoria tritici, the most important pathogen of wheat, which is the most important food crop in the world. There is also remarkable interest in J. maritimus compounds in the pharmaceutical sector. In this context, López-Álvarez et al. (2012) demonstrated the possibility of producing porous silicon carbide scaffolds from J. maritimus for tissue engineering application, i.e., for the fabrication of bio-structures for bone tissue replacement. Sahuc et al. (2019) demonstrated the feasibility to produce new and low-cost direct acting antivirals against hepatitis C virus based on dehydrojuncusol, a natural compound isolated from crude extract of J. maritimus, while Alamer & Algaraawi (2020) demonstrated antifungal activity of methanol extract from phytoconstituents of J. maritimus leaves and stems, to effectively treat dermatophytosis.
Consequently, J. maritimus could be produced in the FTS as a product to supply one or various of the sectors of interest, thus mariculture wastewater would cease to be considered as a waste to be managed but instead as a useful culture medium high in nutrients and organic matter, and hence a valuable by-product, which would mean J. maritimus FTS integration at a level 4 (Boyd et al. 2020) within the aquaculture system.
Phytobatea technology validation in saline tidal environment
After two-years' monitoring, Phytobatea modular devices did not show any signal of deterioration or corrosion and buoyancy was stable. All components remained in perfect condition, and the whole structure did not suffer any apparent damage. This agrees with long-term investigations on GFRP composites' durability in seawater (Idrisi et al. 2021), which determined that immersion for 11 years slightly affected GFRP composites when temperature was 23 °C, whereas deterioration increased with higher water temperatures. Accordingly, since average water temperature in the FTS was 17 °C, no apparent deterioration symptoms in the Phytobatea modules were observed, and water absorption should have not exceeded 3.7% increase in weight, while tensile strength reduction should not exceed 2% after 12 months of exposure. However, mariculture wastewater might influence GFRP component durability in the long-term, and a specific investigation would be of great interest.
Plants grew correctly attached to the fibreglass rods and through the HDPE mesh, maintaining shoots' verticality even after storms, heavy winds, and snow accumulation (see Figure 6). Monitoring of installation process revealed that average time for a FTS implementation, including plant installation in the Phytobateas and module assembly and attachment to the floating dock, was estimated as 5 min/m2, done by two untrained workers.
The lifting machine prototype performed well, making Phytobatea removal from the waterbody feasible, easy, quick (2 minutes per module) and secure (see Figure 7). Phytobatea's removable character was essential when monitoring and maintenance operations needed to be accomplished, such as for dead plants replacement. Neither the modules nor the plants suffered any damage during manipulation. However, manual handling of the device along the dock was identified as the main feature to be improved.
CONCLUSIONS
Results of the first full-scale floating treatment saltmarsh discussed here demonstrate the efficacy of this novel phytotechnology for saline wastewater treatment, as well as its suitability for the implementation of TWs under the harsh and highly variable conditions of contaminated brackish and saline waterbodies. The relatively small size of the FTS (7% of the tidal lagoon total surface), and the low hydraulic retention time (below 1 hour), indicate that the FTS design in the form of a crossflow barrier is highly efficient, particularly on TP, TSS, BOD, TOC, turbidity and E. coli reduction, as well as on wastewater oxygenation. However, it was insufficient for the treatment of nitrogen compounds. Problems with J. maritimus survival and growth during the investigation led to an average density of functional plants of 50% of the initial plantation, so it is considered that these results could be significantly improved if the FTS is assessed at full performance, with all plants alive, which would be desirable in the future for a better understanding of these systems' optimal design and dimensioning. In this regard, J. maritimus limit for direct exposure to highly saline contaminated water was identified as 38 g/L, and its survival rate for salinities of this magnitude was demonstrated to be dependent on the initial sizes of the perennial parts of the plants. This information increases current knowledge on J. maritimus salt tolerance range when planted bare rooted in hydroponic systems, and highlights that, beyond the importance of selecting local, native halophyte plant species for the construction of FTS systems, the size and morphology of the plants could be a critical selection criterion to improve plant survival rates, especially in highly saline environments. This should be considered not only for working with J. maritimus, but also with other halophytes. Phytobatea technology was validated as an efficient, effective, resistant, long lifespan, and manageable technology for FTS implementation and operation, while J. maritimus FTS role as preferential habitat for wild aquatic fauna species revealed the potential of this technology for biodiversity conservation purposes. In addition, there is growing interest from several strategic sectors in J. maritimus as raw material for added-value products and services provision, which boosts the feasibility of incorporating J. maritimus FTSs within integrated aquaculture systems, hence contributing to aquaculture efficiency, environmental behaviour, and sustainability.
ACKNOWLEDGEMENTS
This research was undertaken with the support given by the Centre for Environmental Research of the Autonomous Community Government of Cantabria (CIMA). The authors would also like to acknowledge CIMA laboratory technicians for their work on the water samples analysis, the partners of Convive LIFE Project for financing the floating treatment saltmarsh design and construction, AC Proyectos and Land Lab studios for their contributions in the design of the construction project, Naútica Parayas for their collaboration with the first prototypes design and manufacture of Phytobatea technology, Rubén Caballero for his altruistic contribution producing the artwork for this paper, Eva de la Lama Cicero for her revision of English language, and to the aquaculture company Sonrionansa S.L. for hosting and supporting this project.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.