Monitoring of an indoor pilot plant for osmosis rejection and greywater reuse to flush toilets in a hotel

Toilet flushing with wastewater was monitored in a tourist hotel. The wastewater was a mixture of a rejection flow from an osmosis unit for low quality water purification and treated greywater from bathrooms. Greywater treatment included chlorination with hypochlorite and sand filtration. The osmosis rejection flow improved considerably the quality parameters of the treated greywater. The average reused flow was 50 m3/day which corresponded to 26.7 m3 greywater/day and 23.3 m3 osmosis waste rejection/day. The cost of reusing was estimated as €1.14 for each m3 of reused wastewater. Acceptance from customers was clearly satisfactory. Reused flows were characterised in terms of pH, suspended solids, turbidity, total organic carbon, total nitrogen and conductivity. The presence of 1 mg l−1 residual chlorine at the outlet was adopted as working criterion. In such conditions wastewater was always negative in total coliforms, and the required hypochlorite dosage was of the order of 45 mg Cl2 l−1. Therefore, it can be stated that flushing toilets with low quality water is a useful strategy for saving water (around 20% of total consumption), and that drinking water quality is not required for this purpose. Advantages derived from mixing greywater with osmosis rejection are pointed out.