Sludge valorisation to obtain high quality water from WWTP

In the face of insufficient water resources and the intensification of extreme events caused by climate change, the generation of non-conventional water sources is an option that should become a priority. Wastewater treated is a water resource that with proper post-treatment can be suitable for maintaining the environmental quality of rivers and wetlands or be used for productive activities such as agricultural uses.

Returning water to the environment in similar conditions to its original state is vital to promote its reuse and to help maintain biodiversity. In this sense, the project Integrating circular economy and biodiversity in sustainable water treatments based on constructed wetlands LIFE RENATURWAT aims to demonstrate that it is possible to obtain high quality water from Waste Water Treatment Plants (WWTP) effluents by combining Nature-Based Solutions (NBS) and industrial wastes.

One of the disruptive issues of this project is exactly the use of a waste generated in the integral water cycle itself, concretely during the production of drinking water, to produce quality water from WWTP. This sludge (DWTS) has inert and non-toxic properties, so usually is disposed in landfills, not taking profit of the economic and environmental benefits derived from its valorisation. Nevertheless, the DWTS has adsorbent capacities due to the coagulant used in the drinking water treatment process.

LIFE RENATURWAT plans to use the DWTS as an active substrate in constructed wetlands (CWs) aimed at upgrading treated urban wastewater. This sludge is dewatered and milled to obtain a grain size similar to sand. The DWTS reinforces the wetland technology so that it can be more efficient and can efficiently remove phosphorus and other pollutants at the same time as generating a habitat in itself.

The solution includes two kinds of CWs operating in series. The first is a vertical subsurface flow constructed wetland with DWTS as a filter medium and the second one is a free water surface constructed wetland. The described system is able to remove phosphorus from wastewater even at very low concentrations, achieving an average total phosphorus concentration in the effluent below than 0.1 mgP/l. This is considerably lower than the legal limit set by Directive 91/271/EEC, UWWTD (1 or 2 mg P/l), as well as the so-called sensitive area 0.6 mg P/l. In this way, a wastewater effluent with a very low phosphorus concentration is obtained, without additional consumption of reagents, addressing one of the main problems faced by WWTP managers, which is the eutrophication of the natural environment and compliance with phosphorus discharge limits. Within the framework of this project, two pilot projects have been implemented, one in the Valencian town of Carrícola, and the other in the Los Monasterios urbanisation (Puçol).