Demo case: Steel industry
CELSEA
Main challenges of the demo case: The area around Barcelona (where the demo site is located) faces water availability constraints, with seasonal consumption limits set by authorities. This affects plant production capacity. The production process generates a total discharge of 400 km³/year, treated in the site’s wastewater treatment plant (WWTP) using a simple physical-chemical process to meet discharge limits. While the WWTP complies with these limits, removing oil and grease, and managing salinity levels remains challenging. Future limits are also expected to be more restrictive, and the authorized water concession may decrease. Currently, 20-25% of treated wastewater (80-100 km³/year) is reused in a specific production step. CELSA aims to increase water reuse to over 90% (>350 km³/year) to reduce freshwater intake, emphasizing on higher quality of the water to be used for other demanding production steps.
Approach to overcome the challenges: The proposed treatment process for the current WWTP effluent at this site involves two stages based on membrane technologies. Such processes offer an outstanding range of possibilities for wastewater treatment, coupled with a promising sustainable approach to meet the increasingly strict industrial requirements for the consumption and discharge of natural resources and chemical compounds.The first stage combines ultrafiltration (UF) and nanofiltration (NF) in series, with ceramic membranes preferred to prevent oil and grease fouling. This stage, demonstrated on-site at a 1 m³/h scale, removes oil/grease, suspended solids (SS), and hardness. It also allows the currently discharged water to be returned to the cooling cascade system at a sufficient quality for steel cooling processes. In the second stage, demonstrated at lab scale (litres/h), the reclaimed water from stage one undergoes further polishing while increasing water recovery through reverse osmosis (RO). The RO brine, primarily composed of sodium chloride (NaCl), is concentrated using membrane distillation (MD) with excess waste heat from CELSA. This concentrated brine is then used to produce HCl and NaOH via bipolar electrodialysis (BPED) for membrane and plant cleaning operations. Without a zero liquid discharge (ZLD) scheme like the one proposed here, CELSA will be unable to increase water recovery due to the already restrictive salinity discharge limits (6 mS/cm).
