French vertical-flow constructed wetlands in mountain areas: how do cold temperatures impact performances?

Thermal properties of treatment wetlands operated under freezing conditions

The use of treatment wetlands (TWs) presents particular challenges in regions with sub-zero winter temperatures, due to reduced biological activity and risk of pipe breakage or clogging due to freezing. We studied the vertical temperature distribution in four pilot-scale TWs exposed to winter temperatures in order to determine the impact of operational system parameters and the role of insulation on heat conservation inside the filtering bed. The overall temperature pattern was similar in all wetlands, with a trend of increasing temperature from the surface toward the bottom during the cold season. No freezing was detected in the wetlands despite average daily temperatures as low as -20 °C. Influent water temperature and hydraulic loading had a stronger influence on TW temperatures in winter than air temperature. The vertical distribution of temperatures in TWs is more sensitive to hydraulic loading variation in the percolating operating condition than in the saturated flow with forced aeration configuration. Our results suggest that TW systems can remain operational under cold winter conditions provided the surface is properly insulated by vegetation, mulch and/or snow.

Alpine constructed wetlands: A metagenomic analysis reveals microbial complementary structure

Constructed wetlands (CWs) are used to water treatment worldwide, however their application at high-altitude is poorly studied. In order to survive mountain winters, CWs rely on native flora and associated microbial communities. However, the choice of plant-microbes pairs more suitable for water treatment is challenging in alpine environments. Using a metagenomic approach, we investigated the composition of prokaryotes and fungal communities, through extensive sampling inside a constructed wetland in the SW-Alps. Best performing plant species were searched among those hosting the most diverse and resilient microbial communities and to this goal, we analysed them in the natural environment also. Our results showed that microbial communities were less diverse in the CW than at natural conditions, and they differed from plant to plant, revealing a clear variation in taxonomic composition between forbs and gramineous plants. Carex rostrata, Deschampsia caespitosa and Rumex alpinus hosted bacteria very active in N-cycles. Moreover, fungal and prokaryotic communities associated to R. alpinus (Polygonaceae) turned to be the richest and stable among the studied species. In our opinion, this species should be prioritized in CWs at high elevations, also in consideration of its low maintenance requirements.

In-depth assessment of microbial communities in the full-scale vertical flow treatment wetlands fed with raw domestic wastewater

A multiphase study was proposed to examine microbial communities linked to the nitrogen cycle in the first stage of four full-scale French vertical flow treatment systems. To this end, denaturing gradient gel electrophoresis (DGGE) was performed for structural assessment and quantitative PCR (qPCR) to enumerate the abundance of ammonia-oxidizing (AOB). 16S rRNA sequencing was used to assess the taxonomic profile followed by putative assessment of functional genes. The samples were collected under different conditions, such as operational time (presence/absence of sludge layer on the surface of the filters), season (winter and summer), sampling depth (0, 15 and 30 cm) and operation cycle (rest and feed periods). A structural disparity was noted in the upper layers, whereas higher similarity at 30 cm was observed highlighting the effect of organic matter on bacterial diversity. The 7th rest day was highlighted by an apparent decline in the microbial community abundance. Additionally, qPCR indicated that the largest amount of AOB was found at 30 cm depth and during the feeding period. From the taxonomic profile, Mycobacterium, Acinetobacter, Flavobacterium, and Nitrospira were the most abundant genre found in all systems. The functional prediction results showed predicted genes linked to the denitrification process. The results suggested that operating time and season were responsible for the pattern of the microbial community behavior. This study allowed us to further understand the bacterial dynamics and to advance the idea of design modifications made in the first stage of the classical French system to improve nitrogen removal are promising.

Feeding mode influence on treatment performance of unsaturated and partially saturated vertical flow constructed wetland

This study aimed to evaluate the influence of two different feeding modes on wastewater treatment performance and oxygen consumption rate (OCR) of the unsaturated (UVF wetland) and partially saturated (SVF wetland) vertical flow constructed wetlands operated in parallel under subtropical climate for four years. Each wetland had a superficial area of 7.5 m² and was planted with Typha domingensis. Both units have a filter media depth of 0.75 m, composed by sand (effective diameter of 0.29 mm and uniformity of 4). UVF wetland operated typically unsaturated, while SVF wetland had the bottom part saturated (57% of total depth). Two feeding modes were evaluated for both wetlands. The feeding mode was operated within the limits recommended by the German standard, with a hydraulic loading rate (HLR) of 75 mm d^-1 and specific pulse volume (SPV) of 19 L m^-2 for both wetlands and a specific hydraulic loading rate (SHLR) of 8 and 9 L m^-2 min^-1 for UVF and SVF wetlands, respectively. Meanwhile, the second feeding mode was applied for both wetlands, being an HLR of 103 mm d^-1, 26 L m^-2 of SPV, and 4 L m^-2 min^-1 of SHLR. The load removal efficiency of SVF wetland was higher than the UVF wetland for all parameters and feeding modes. No statistical difference was identified for OCR values between wetlands and feeding mode. The results showed that operating UVF and SVF wetlands with a SHLR around 4 L m^-2 min^-1 and SPV equal of 26 L m^-2 is preferable. This fact could represent a significant reduction in inlet pumping power requirements and also less superficial area requirements.

Use of broken brick to enhance the removal of nutrients in subsurface flow constructed wetlands receiving hospital wastewater

Eight horizontal subsurface flow pilot scale artificial wetlands were constructed to evaluate the effectiveness of broken brick to remove nutrients from hospital wastewater. The average total suspended solids (TSS), 5-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), NH4-N, NO₃-N, and phosphate percent removal efficiency of constructed wetlands were, respectively, 93.2%, 90.4%, 83.7%, 64%, 64.3%, 52.1% and 56.1% in the dry season and 89.7%, 85.8%, 82.9%, 66%, 62.7%, 56.1% and 59.5% in the rainy season. Broken brick bed wetlands provide better removal efficiency of TKN, ammonia, nitrate, and phosphate with an average removal rate of 73%, 71.3%, 79.6% and 77.1% in the dry season and 74.7%, 70.7%, 70.9% and 73.6% in the rainy season, respectively, and it provides better adsorption sites for ammonium, nitrate, and phosphate. Typha with the broken brick bed significantly improved (P < 0.05) the treatment performance of the constructed wetland systems for the removal of ammonia, nitrate, and phosphate. The seasonal variation could not significantly influence the removal of all the pollutants, but better performance of nitrate and phosphate was achieved in a dry season. Use of locally available broken brick as a substrate media can increase the nutrient removal efficiency of wetlands at a cheaper cost when applied in full scale constructed wetlands.

Resilience and reliability of compact vertical-flow treatment wetlands designed for tropical climates

Most of the tropical areas have sanitation problems to contend with. The French system of vertical-flow treatment wetlands (FS-VFTW) fed with raw wastewater could be a good water and sludge management solution. The purpose-adapted tropical design can reduce area requirement to below 1 m²/population equivalents (p.e.). The Taupinière FS-VFTW on Martinique Island was built according to this design, with one stage but with a saturated layer at the bottom of the filter and a simplified trickling filter (TF) added for further treatment to meet the high performances targeted. Unsaturated/saturated vertical-flow filters (US/S FS-VFTW) have shown improved performances on total nitrogen, carbon and suspended solids removal in temperate climates, but the performances in tropical conditions remain unknown. Here, we report on real-world-operation in the French Overseas Territories (FOT), the reliability and performances of this VFCW tropical-design. The system experienced loading conditions ranging from 30% to 165% of nominal carbonaceous biological oxygen demand (BOD₅), as well as tropical rainstorms that brought over 7 times the nominal hydraulic load. Over a period of 3 years, 29 campaigns collected 24-h flow-proportional samples at each treatment stage (raw wastewater, FS-VFTW outlet, TF outlet). When applied loads were close to nominal values, the US/S FS-VFTW itself guarantees 85/90/60/50% removal and 125/25/40/50 mg/L at the outlet for chemical oxygen demand (COD)/total suspended solids (TSS)/total Kjeldahl nitrogen (TKN)/total nitrogen (TN), respectively. By comparison with US/S systems in mainland France, it appears that the warmer tropical-climate temperatures facilitate both nitrification and denitrification kinetics. Performances in overload conditions confirm that the US/S FS-VFTW remains robust and reliable although COD and TKN removal are impacted, especially after strong tropical rain events. By adding a simple compact trickling filter to a US/S FS-VFTW, the treatment system delivers high-level performances (>95% removal for BOD₅, COD, TSS and TKN) at less than 1 m²/p.e.

Using one filter stage of unsaturated/saturated vertical flow filters for nitrogen removal and footprint reduction of constructed wetlands

French vertical flow constructed wetlands (VFCW) treating raw wastewater have been developed successfully over the last 30 years. Nevertheless, the two-stage VFCWs require a total filtration area of 2-2.5 m²/P.E. Therefore, implementing a one-stage system in which treatment performances reach standard requirements is of interest. Biho-Filter^® is one of the solutions developed in France by Epur Nature. Biho-Filter^® is a vertical flow system with an unsaturated layer at the top and a saturated layer at the bottom. The aim of this study was to assess this new configuration and to optimize its design and operating conditions. The hydraulic functioning and pollutant removal efficiency of three different Biho-Filter^® plants commissioned between 2011 and 2012 were studied. Outlet concentrations of the most efficient Biho-Filter^® configuration are 70 mg/L, 15 mg/L, 15 mg/L and 25 mg/L for chemical oxygen demand (COD), 5-day biological oxygen demand (BOD₅), total suspended solids (TSS) and total Kjeldahl nitrogen (TKN), respectively. Up to 60% of total nitrogen is removed. Nitrification efficiency is mainly influenced by the height of the unsaturated zone and the recirculation rate. The optimum recirculation rate was found to be 100%. Denitrification in the saturated zone works at best with an influent COD/NO₃-N ratio at the inflet of this zone larger than 2 and a hydraulic retention time longer than 0.75 days.