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3.4        Low input sewage treatment

Options for waste water treatment are constrained by local factors. A densely populated city, with many industries and limited land availability presents one extreme, while sparsely populated, rural areas represent another. Construction of waste water treatment plant can involve greenhouse gas-intensive activities and high costs, in both the building and operating phases. Alternative solutions can use less energy, release fewer harmful emissions and provide local benefits and control (Water Projects online, 2019) 

A focus on nature-based solutions under the EU’s Biodiversity Strategy (EC, 2020) brings attention to small-scale solutions to sewage treatment. Natural wetland systems are able to transform and/or remove various pollutants through a series of physical, biological, and chemical processes, and therefore improve water quality. These processes are mimicked in so-called “constructed“ wetlands. These treat waste water near its source, without high demands on infrastructure and operational costs, while enabling recovery and use of resources from waste water and increasing green space in rural or peri-urban areas. In a study on Slovenia, the JRC compared various solutions in terms of cost-effectiveness and socioeconomic acceptability. They found that nature-based solutions were a preferred option for areas with a predominance of rural, scattered dwellings and small settlements. The concept was proposed as a mainstream solution for the less urbanized areas of the Lower Danube region (Pistocchi et al, 2020).

Owing to their low costs and low maintenance, constructed wetlands are popular in low-income regions. But they can be also used as a decentralised approach for blocks of buildings, neighbourhoods, commercial facilities, isolated communities and remote areas, etc. Reduction of nutrient concentrations can deliver similar results to tertiary treatment (Cooper et al, 2020).  Studies on micropollutants suggest that such constructed wetlands can be effective at preventing release to water (Gorito et al, 2018) though care must be taken to ensure oxidation conditions in constructed wetlands are appropriate to address organic micropollutants (Reyes Contreras et al, 2019).  

Figure 3‑5 Comparison of needs for land, energy and maintenance between constructed wetlands and conventional urban waste water treatment

 Source: Stefanakis, 2019

Recent technological advances have managed to significantly close the gap with conventional, mechanical technologies in terms of land requirement (e.g. the compact, mobile aerated constructed wetland) (Stefanakis, 2019). Such solutions can be integrated in urban and peri-urban areas for wastewater treatment and urban runoff control and management, following the decentralised approach.  For example, the INNOQUA Horizon 2020 project investigated a modular system for water treatment based on the purifying capacity of biological microorganisms (earthworms, zooplankton and microalgae), developing a technology for decentralised waste water treatment (INNOQUA).

Text Box: Waste water purification in villages in Spain

In Spain, the challenge for administrations to implement waste water purification systems in towns with less than 1 000 inhabitants stood out in the search for balance between the elimination of pollution and the economic sustainability of treatment plants. The cost of operation and maintenance of a conventional treatment plant in municipalities with more than 10,000 inhabitants could be around 10 euros per inhabitant per year, while in a town where fewer than 100 people live it would increase to 345 euros. Use of alternative technologies with lower maintenance costs, adapted to the local situation was expected to significantly reduce this economic gap. Analysis was carried out on the presence of emerging pollutants in the waters of the treatment plants of the Badajo province, such as drugs and herbicides. This highlighted analgesics as being the emerging pollutants with the highest concentration in the water entering the treatment plants, although in 90 percent of the cases, residues were eliminated during the purification process.