Before starting the design of the system, a few samples from the outflow discharging into the lake were taken and analyzed. Information was also collected on the number of people living in the settlement and discharging wastewater into the ditches that fed the outflow.

According to the results of the investigation an estimation of the wastewater to be treated in the constructed wetland has been provided and used for the sizing of the system.

After the field survey and preliminary analysis, the detailed design was delivered in spring 2019.
The general lay-out of the proposed treatment wetland can be summarized as follows:

• Preliminary treatment and polluted water diversion: a manual screen in envisaged to block coarse solids along the existing outflow and a small weir, about 20 cm high that feeds a pipe to extract part of the flow (maximum around 10 l/s) and directs it to a pumping station that diverts the flow into the CW.
• Constructed Wetland: the CW is composed by one single stage: Free Water Surface System (FWS): fully vegetated with emergent plants; total area of the bottom 4350 m², total area of the water surface 5480 m².
• Gravity discharge into the lake.

The chosen solution achieves a high degree of biodegradation of organic load, suspended solids removal and pathogen reduction and presents very limited Operation and Maintenance (O&M) requirements and costs. The estimated removal efficiencies allows the effluent to fully achieves the proposed standard limits (compliant with the EU wastewater Directive 91/271) related to the urban domestic discharge in surface water.

After the bidding procedure that took place during the summer of 2019, the construction company was selected in October 2019, and in November 2019 the company visited the construction site for the first time. During the winter of 2019/2020 a few minor changes to the design were provided by the consultant to bypass the problems encountered by the construction company. In the spring of 2020, the main basin of the future constructed wetland was excavated.

The works proceeded slowly, due to the restrictions linked to the COVID 19 epidemic, however in the summer of 2020 the impermeable liner was correctly lied down and the gravel and stones sections put in place. In October 2020 the new constructed wetland was built and the vegetation planted.

To be fully developed it will take probably two years, so the wetland will be fully vegetated in the summer of 2022. Refer to Image 1 of what the wetland will look like in the future in the following pictures a graphic simulation is provided.

On January 22, 2021 the inlet and the outlet of the wetland were sampled. The results of the analysis are reported in the following table.

Considering that the system is not yet colonized by aquatic vegetation, the results are encouraging. The removal of the suspended solids is presently lower than expected but still over 50%. One of the reason why the sediment removal is lower than expected may depend on the feeding system that send to the system more than the exptected 260 m³ per day. Since the CW is not fed by the envisaged pumps but by gravity, it’s more difficult accurately control the inflow volume.

The system appears very effective in removing organic matter (BOD and COD) showing a removal rate higher than expected and near to 100%. However the most surprising result concerns the removal rate of ammonium (85%). The constructed wetland object of the present report is a free flow (FW) system, a typology that generally shows a lower removal rate for ammonium. Such an high removal rate could be transitory and due to the diffuse presence in the system of aerobic conditions. With the growth of vegetation, however, and with the increase of the content of organic matter, anaerobic sites will establish in the system, reducing the availability of aerobic sites for nitrification (the oxidation of ammonium).

The excess of aerobic condition is the reason why the content of nitrate (NO₃) increases inside the system. The degradation of organic matter and the oxidation of ammonium generates nitrates. To remove the nitrate from the system by denitrification (the reaction transforming NO₃ in N₂), anaerobic sites and availability of organic matter are needed. Such conditions are presently lacking and therefore nitrates could not be removed from the system and we find them in the outlet flow.

Phosphate concentration in the inflow is lower than estimated, therefore the removal rate could not be expected to exceed 20/30%. Phosphates in CW are removed by uptake by the vegetation and by absorption on the plants surface. In the next months, with the expected vegetation, we will certainly observe an increase in the phosphate removal activity.

The restoration project will demonstrate the effectiveness of the Nature Based Solutions, such as constructed wetlands, in reducing water pollution. CW technology is still poorly known in Azerbaijan and the implemented “demo” project will be an opportunity to test the affordability of a “nature based” technology that could be easily replicated in the country.

The most important drawback of CW technology is in fact that, being an extensive technology, it requires larger areas compared to “grey” treatment technologies, such as activated sludge wastewater treatment plants. The population density in Azerbaijan, however, is much lower than in most European countries, and the lack of available land is not a constraint, particularly in the rural context and in isolated villages. Restoration of wetland habitats throughout the country by replicating the demo project in other location in Azerbaijan is therefore recommended.

The most interesting feature of the CW for wastewater treatment is their very low operation and management (O&M) costs. In chapter Ошибка! Источник ссылки не найден. an estimate of O&M costs is provided that show that – even envisaging some energy costs due to the use of the pumping station that was installed but presently is not in operation, being the wetland fed by gravity – the cost is lower than 1 US cent per treated m³. The present “demo” project deals with diluted wastewater, thus a
very simple treatment scheme was applied (a single free water basin): that’s why the O&M costs are so low.

For more complex CW, however, O&M costs are still very competitive compared to conventional treatment systems. A recent paper showed that a complex CW (ad “hybrid” vertical subsurface flow “French system” + free water flow) show O&M costs 1 order of magnitude lower than activated sludge treatment plant (Rizzo et Al.2018).

REFERENCES

https://kura-river.org

• Langergraber G., Dotro G., Nivala J., Rizzo A. Otto R. Stein O.R. 2019. Wetland Technology. Practical Information on the Design and Application of Treatment Wetlands. IWA Publishing ISBN: 9781789060171 (eBook)
• Rizzo A., Bresciani R., Martinuzzi N., Masi F. 2018. French Reed Bed as a Solution to Minimize the Operational and Maintenance Costs of Wastewater Treatment from a Small Settlement: An Italian Example. Water 2018, 10, 156; doi:10.3390/w10020156