Recycling of laundry wastewater using ceramic nanofiltration (ReWaMem)

Around 3600 companies in Germany are part of the German Textile Cleaning Association¹. Due to the variety of laundry items treated every day, such as textiles from hospitals, nursing and retirement homes, hotels and catering services, work and professional clothing, electricity, water and heat consumption requirements fluctuate strongly². Different laundry items require different water qualities for cleaning, and wastewaters with varying degrees of pollution are generated. The specific water consumption is therefore in a range of 5.0 to 13.7 liters per kilogram of textiles².

The aim of the project ReWaMem was to reduce the volume of freshwater needed in textile laundries through systematic wastewater treatment within an individual process chain. To exemplify this, a new partial treatment line was implemented for the recycling of mat and towel wastewater.

The first step was to develop new ceramic membrane supports (tubular and rotating disks [RD]) and membrane layers as well as treatment methods based on advanced oxidation processes (AOP) for the wastewater and concentrates generated from membrane filtration.

In the area of tubular  elements, starting from a membrane area (A_M) of approx. 1.3 m² per element, various supports up to approx. 6 m² were developed. Various active layers providing micro-, ultra- and nanofiltration (NF) were synthesized on these supports. Qualitatively equivalent NF membranes were realized up to geometries with an A_M of approx. 2.9 m². These prototypes should help to reduce the specific membrane costs (€/m² A_M) in the short term. In the area of rotating disks, elements with an outer diameter (D_A) of up to 374 mm were developed together with the project partners. Various active layers were also synthesized on these geometries. The focus of the membrane synthesis was on RD with a D_A of 152 mm and 312 mm respectively. The latter were reliably coated up to nanofiltration. In addition to good retention, the RD also show very good mechanical properties. A breaking strength of over 40 N/mm² was demonstrated in the 3-point bending test. Rotating disks with NF coating were not previously available on the market. These can be operated with higher solids contents than tubular disks, and such systems may also require less space with lower energy consumption.

Test series revealed retention rates of up to 94 % based on the Chemical Oxygen Demand (COD) of the feed solution. The COD indicates the pollutant content in the feed solution. The operating mode and the volume concentration factor (VCF) remain crucial. The pilot plant at a laundry business achieved a retention rate of 68 % based on the COD of  the initial feed with a VCF set at 5 (which means 80 % of the initial volume is recycled). This means that cleaned towel wastewater can currently be used to pre-wash mats, translating into a saving of 1.52 €/m³ of treated mat wastewater, or 3.23 €/m³ when treating and reusing towel  wastewater. However, the economic evaluation is not yet complete.

For customers, Fraunhofer IKTS offers the application-specific development and design of membrane systems with both tubular elements and rotating disks.