Wastewater treatment to recover nutrients
The water line of Water2REturn system consists of a complex wastewater treatment system. Water2REturn is addressed to recover nutrients and other added-value product from the wastewater generated in slaughterhouses, which has a high content of organic matter and nutrients. For this reason, the first stage of Water2Return system, i.e., the water line, focuses on organic matter removal (through the oxidation of the organic matter and conversion to CO2 and water) and nutrients recovery (especially nitrogen and phosphorus).
Bioazul is the partner that has implemented an aerobic biological process for that purpose, being materialised in the construction of a Sequencing Batch Reactor (SBR). This bioreactor, which contains activated sludge, operates with a double objective: reducing the content of organic matter below the Urban Wastewater Treatment Directive (91/271/EEC) limits and oxidising the nitrogen compounds to nitrate (NO3). The operational cycles of the SBR are set according to the characteristics of the influent and the quality requirements imposed on the effluent, being possible to establish phases with appropriate environmental conditions to facilitate the assimilation or elimination mechanisms by microorganisms. In this case, only nitrification (and not denitrification) is promoted to enhance nutrients removal and further recovery. This NO3-enriched effluent is then conducted for a tertiary treatment to sanitise and adapt the treated water to be used for those applications requiring the maximum water pureness (reclaimed water), e.g. crops irrigation. In this case, it consists of a membrane module (Microfiltration (µF) system + Ultrafiltration (UF) system + Reverse Osmosis (RO) system) designed for the recovery of the nitrate and other nutrients and the production of reclaimed water. This membrane technology works by the application of mechanical pressure to counteract the natural osmotic pressure, so that water can flow from the area with higher salt concentration to that of less concentration to obtain filtered water. With the appropriated mesh size, the organic and inorganic micro-contaminants contained in the water can be separated, together with the dissolved ions, and E. coli and viruses can be removed. The resulting concentrate will be rich in nutrients, and it will constitute the first secondary raw material to further manufacture an organic fertiliser.
The water line is therefore integrated by the following units:
- Primary treatment: Rotatory Screen (RS) + Dissolved Air Flotation (DAF). The RS is a separation technology for large flows in small areas, considerably saving space this way. Fats are removed in the DAF and sent to the sludge line (following treatment line of the Water2Return system).
- Buffer tank for a primary sedimentation. The water fraction is homogenised in this buffer tank prior to the secondary treatment in the SBR.
- Secondary treatment: SBR especially designed for organic matter removal and nitrification. The excess sludge produced is sent as well to the sludge line (following treatment line of the Water2Return system).
- Tertiary treatment: membrane module (µF + UF + RO). The treated water fraction (around 48 m3/day) goes through this membrane module for disinfection and soluble nutrients recovery.
Once all units were strictly placed as planned, including all needed pumps, pipes and valves, and the water line came to life. The start-up phase was also initiated; in this sense, the SBR was inoculated with activated sludge brought from a nearby slaughterhouse’s wastewater plant to have it ready once the real wastewater from Matadero del Sur can be used, as this start-up process may take more than a month.
Nevertheless, this process has been stopped due to the appearance of COVID-19, what definitively is slowing down the whole Water2Return implementation process. It is expected to continue the work in the site in the next few weeks, so that start-up process can be finished not only for the water line, but for the others as well, Matadero del Sur wastewater can be used and the circle is truly closed.