French Reed Bed (FRB) for domestic wastewater treatment

French Reed Beds (FRDs) typically consist of the following main components, as illustrated in Fig.1 below:

• Plastic liner
• Filter layer first stage (from top to bottom): 2 - 6-millimetre (mm) gravel (30 - 80 cm), 5 - 15 mm gravel (10 - 20 cm), 20 - 60 mm gravel (20 - 30 cm)
• Vegetation: typically, Phragmites spp, but also other species can be considered especially for second stage
• Drainage system: with slots that serve as chimneys for aeration.
• Distribution system: can be designed in different ways depending on climate conditions, available material, cost, bed dimensions, type of loading (by pump or by self-priming siphons). The diameter of the pipes depends on the admitted velocity of the wastewater with the recommendation of not going below 110 mm (DOTRO et al. 2017).

Fig. 1: Profile of French VF cells; first stage. Source: DOTRO et al. (2017)

FRBs are typically divided into three (or multiples of three) cells to feed one cell at a time and allow the other two cells to rest and maintain aerobic conditions (see Fig. 2). There are also designs with 2 or 4 cells. The feeding cycle lasts 3 - 4 days, during which the cell is fed in batches, with a break of 1 - 2 hours between one batch to the next, followed by a resting period of 7 days.

Fig. 2: Feeding scheme for the three cells of an FRB 1st stage receiving raw wastewater. Source: DOTRO et al. (2017)

Double-stage systems: in case a FRB is set-up in combination with a second-stage, the raw wastewater is directly fed to the first stage of the FBR system, and filters through the filter layer, before passing over to the second stage. The organic top layer accumulates on the surface of the first stage with a rate of 2 - 3 cm per year, and must be removed after 10 - 15 years, when it is stabilized and can be used as a soil conditioner (RIZZO et al. 2018). Two-stage FRBs guarantee a high level of organics, solids and ammonia removal, as well as a good bacteria reduction. In fact, already the first stage guarantees significant removal of organics, solids, as well as a good nitrification rate. A higher denitrification rate could be obtained with a saturation bottom layer.

FRBs require no other treatment units such as primary tanks or sludge treatment units, as the sludge is separated on the FBR surface. It undergoes a process of dewatering and stabilisation, and its volume is reduced by nearly 90%, after which it can be used in agriculture as soil conditioner.

FBRs generally require less space than other wetland systems (for example: Horizontal Flow Wetland). Specifically, the first stage is designed with 1.0 - 1.3 square metres per person equivalent (m2/PE) for temperate and cold climate and 0.8 - 1.0 m2/PE for hot and tropical climate; the second stage with about 2.0 - 2.5 m2/PE for temperate and cold climate and about 1.5 - 2.0 m2/PE for hot and tropical climate. Since no primary treatment step is necessary operation and management costs are lower than for other wetland types. These advantages, added to the limited energy consumption and easy maintenance, make FBR systems adequate for small communities in low-income communities. The feeding system can be controlled by a Programmable Logic Controller (PLC) or by an operator who switches the feeding twice per week (DOTRO et al. 2017).

Even though the application of FBRs is particularly favourable in tropical and hot climates, as warmer temperatures allow for faster biological activity permitting to reduce the footprint, they can also work efficiently in colder climates if designed and operated properly. The biggest French system in the World for now is operated in Moldova under cold climate conditions. Heavy rainfall (like e.g.: during a monsoon period) must be taken into account for the hydraulic calculations when designing a system, but do not significantly affect the performance of FBRs.

During the commissioning period (about 1 year), good reed growth must be maintained, and weeds must be removed manually.

The routine maintenance can be carried out by on-site trained, non-specialized staff. The system must be checked twice a week, in particular the batch feeding system, and the coarse screening must be cleaned regularly. Filter alternation must be done every 3 - 4 days, weeds removal once a month and reeds harvesting once a year. In tropical climates vegetation maintenance frequency could be higher due to the favourable conditions for plant growth.

The superficial organic layer grows with a rate of 1 - 3 cm/year and must be removed when it reaches a height of 15 - 20 cm to avoid ponding and limitation to the oxygen transfer. After the bed has been in operation for the 10 years, the cell must be stopped for minimum 1 month before removing the sludge. Ideally, this should be done during the dry period before the monsoon, when the climate is ideal to achieve a strong dehydration and stabilisation effect. FBRs can be put back in operation immediately after the sludge removal (DOTRO et al. 2017). In order to avoid intermitting plant operation, sludge removal can happen on each cell in alternation.

As stated above, FRB having a strong potential for application in developing countries - particularly in small (rural) communities. Nevertheless, to the knowledge of the authors, these systems have not found widespread use in India, due to a lack of awareness and local expertise for this technology.

One FBR system was implemented under the EU-Indian cooperation project SWINGS as part of a pilot wastewater treatment system at the Aligarh Muslim University (AMU), Uttar Pradesh. This pilot FBR consists of three cells and is combined with a horizontal flow constructed wetland, the system has a net area of about 82 m2 and was designed to treat 5 m3/day of urban wastewater (CORDIS 2016). In PAVITR, another FBR with an improved design is piloted at AMU to treat minimum 50 m3/day of wastewater; the treated wastewater will be used for irrigation.

French Systems have been used over 20 years, and approximately 1,000 wetlands of this type were already in operation in France more than 15 years ago (MOLLE et.al. 2005); by now, there are several thousand of this type of plant in France.

In Orhei, Moldova, the largest FRB wetland in the world for secondary treatment is in operation since 2015. The plant, followed by a VF second stage wetland, has been designed for an average flow of 2,000 m3/d and occupies an area of 5 hectare (ha).

One of the oldest systems has been operating since about 20 years in Roussillon (France) with a design treatment capacity of 1,250 PE and a total surface of 920 m² (DOTRO et al. 2017). Hydraulic Loading Rate (HLR) is 20 cm/d (50 cm/d in summer). Reduction of Biological Oxygen Demand (BOD) is about 98%, for Chemical Oxygen Demand (COD) 95%, for Total Suspended Solids (TSS) 99% and for Total Kjeldahl Nitrogen (TKN) 95%.

Another example for a successfully running FRB can be found in Castelluccio di Norcia (Italy) where an FRB is combined with a VF wetland, a Free-Water Surface wetland and an infiltration basin. The FBR consists of two beds of 507 m² each, serving up to 1,000 PE and a wastewater flow of up to 150 m³/day. Reduction of BOD is ca. 98.5%, for COD 98.8%, for TSS 95.3% for Total Nitrogen (TN) 80.5%, for Ammonia (N-NH4+) 99.4% and for Total Phosphorus (TP) 93.8%. The total average annual Operation and Maintenance (O&M) costs are 6 - 11 Euro/PE (RIZZO et al. 2018).