IWA Publishing

Constructed Wetlands for Domestic Wastewater Treatment and its Reuse in Irrigation

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This project focuses on South Europe, the region important for tourism and agricultural production, both activities with high water demand. Mentioned region is already experiencing problems with water scarcity and the situation is expected to worsen according to the international prognosis (Angelakis et al, 1999). Climate change could cause problems with water supply and water management in both urban settlements and rural communities, with difference that latter usually do not have sufficient financial resources to deal with serious problems. Moreover, smaller communities in South Europe are often not treating their wastewater and it can negatively affect the ecosystems. Conventional wastewater treatment plants can be an expensive and resource demanding (Meng et al, 2014) and therefore they are hardly applicable in these cases. Environmentally friendly and cheap solution to the both wastewater treatment and water shortage problems in smaller communities can be constructed wetlands. They are engineered systems that mimic the processes occurring in the natural wetlands and use them to treat different types of wastewater. Moreover, they are environmentally friendly facilities with very little impact to the existing ecosystems. Constructed wetlands can be efficient and so far have obtained good results in wastewater treatment in this region (Masi and Martinuzi, 2007). If they could treat wastewater up to the standards for reuse and their effluent later utilised in different areas (eg. flushing toilets, agriculture), a considerable part of water demand in the smaller communities could be covered by this source. That would provide them with higher independency and resilience to the climate change.

In order to reuse water safely, standards that control this area are necessary (Massoud et al, 2009). Out of fifteen countries in the region of South Europe, four of them (Greece, Italy, Portugal and Spain) have issued standards for wastewater reuse so far. Literature review on the constructed wetlands treating domestic wastewater in these four countries showed that, in general, these systems have troubles meeting the standards for agricultural reuse in the terms of microbiological parameters. Moreover, nitrogen and its compounds were also difficult for removal by certain systems taken into account. Therefore, in order to produce good quality effluent that can be reused, constructed wetlands need to improve their performance further.

Materials and Methods
The experimental part of the research project is currently being done at the University of Bologna where pilot plant is placed (Figure 1). It consists of a septic tank, four vertical flow constructed wetlands (VFCWs) and two horizontal flow constructed wetlands (HFCWs). All VFCWs receive the same water, have the same substrate (gravel and sand) and the same hydraulic retention time, but they differ in their additional characteristics: presence of plants (Phragmites australis) and earthworms (Eisenia fetida):

  • 1st VFCW: substrate only
  • 2nd VFCW: substrate and plants
  • 3rd VFCW: substrate, plants and worms
  • 4th VFCW: substrate and worms

Similarly to the VFCWs, two HFCWs function and have the same characteristics with the exception of one parameter: presence of plants. As a combination of different conditions is necessary for better wastewater treatment (Vymazal, 2013), VFCWs are intended to be aerobic and HFCWs anoxic stages. The real wastewater coming from one of the university buildings is used and treated by the described system. Raw wastewater is pumped to the septic tank that is followed by VFCW and finally HFCW. A range of parameters important for the wastewater reuse is being tested and compared to the different operational characteristics of the systems. Moreover, water balance is monitored through different seasons and atmospheric conditions.

Expected Outcomes
Previous research showed that the plants could have a certain effect on wastewater treatment, but it is still not very clear. As far as worms are concerned, not much research on their use in constructed wetlands is available and it was mostly done in controlled conditions. The existing setting of the pilot plant will allow estimation of the exact effect that plants or earthworms have on the wastewater treatment, and whether their presence improves the operation regarding parameters that are important for wastewater reuse. However, it is not enough to only treat wastewater to the certain level but also to provide enough water volume at the end of the treatment line. As constructed wetlands are natural systems and are open to the environment, they are susceptible to the existing atmospheric conditions as rain, snow, wind, sun or high temperatures. In the warmer parts of the year, evapotranspiration from these systems could be very high that could further produce problems with water quality and quantity that are essential for reuse. Therefore, both pollutants removal and evapotranspiration are the important parameters for optimal hydraulic retention time.

This research aims to improve the efficiency of the constructed wetlands and to expand the knowledge about different processes occurring in these systems (e.g. nitrogen cycle or E. coli removal). Although constructed wetlands are quite well known and used in different parts of the world, in the certain countries of the South Europe their advantages are not being properly used. Before they are introduced as an acceptable wastewater treatment and reuse systems, their ability to mitigate the effects of climate change and water shortage should be further examined and critically reviewed.

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