The possibilities of recycling wash water at Vidinge

Sammanfattning: Water is a resource of importance and with drier weather around the world, including Swe-den, there is a risk of too low levels of water to satisfy for instance households, animals and industries in the future. Fresh-cut fruit and vegetable industries are water intense and almost all process water is used in the washing process. To decrease the water consumption recircu-lation of wastewater has been proposed. Investigations in possibilities of recirculation in one of the largest processing and packaging industries of salad in Scandinavia, Vidinge, has been done. The washing lines at Vidinge consist of several steps, including baths and sprinklers, and water is the only disinfectant used at the salad. The water flows in opposite direction to the salad to decrease both water consumption and risk of cross-contamination. Pathogens in the ready-to-eat food is a severe risk to human health and it is of importance to continuously decrease the risk of cross-contamination. Therefore, a disinfection method for removal of microorganisms before the water is recirculated back into the process is required. Today tap water from the municipal water supply is used as process water at Vidinge and in the future a recirculation system of water with drinking quality is required to be used at the last step in the washing line. However, in initial washing steps only treated water is required. Today Vidinge uses a biological treatment applied during summertime for reduction of im-purities in the wastewater. Some of the wastewater is after being treated and disinfected by UV-C reused for irrigation on nearby fields. These treatment methods have shown good po-tential. However, as wastewater is produced during the whole year a more sustainable treat-ment system is required. Measurements of the wastewater have shown too high concentrations of pesticides, chemical characteristics as nutrients, BOD and COD, physical characteristics as turbidity and TS and microorganisms for the water to be recirculated. In a future treatment plant biodegradation is proposed as a secondary treatment step for nutrient removal and reduction of BOD, COD and TOC after the mechanical filtration already applied at Vidinge. For reduction of metals, TSS/TS and chlorinated compounds different filtration techniques as membrane filtration and sand filtration have been investigated. With membrane filtration with low pore sizes (as RO) removal of pesticides and chlorinated compounds are achieved. Adsorption methods (as GAC) have also been reported to be effective. Disinfection methods as UV-C, US (ultrasound) and AOP (advanced oxidation processes as an energy source (UV) in combination with an oxidant and/or a catalyst) have been investi-gated in several studies to remove microorganisms in wash water from salad. Both UV-C and AOP have shown great potential to be used in industries. Treatment of UV-C is also a rela-tively cheap method. In conclusion, a multifunctional WWTP is required for the water to be sufficiently treated for recirculation and the most important step is disinfection of microorganisms. However, trial in pilot scale of the wastewater at Vidinge and a more accurate costal estimation are required before application.