Management of the schmutzdecke layer in a slow sand filter to reuse drainage water from a greenhouse

Recycling of the nutrient solution used to irrigate fruit and vegetables in greenhouses can increase water use efficiency and reduce the contamination of local water sources. The nutrient solution cannot be recycled without treatment because of potential contamination of the entire plant system with bacteria, viruses, and/or fungi. Ozonation, pasteurization and chemigation are the typical techniques used, but are often expensive and not always effective[1]. Slow sand filter (SSF) is a technology that is relatively inexpensive to purchase and operate. Data was collected from two SSF to verify treatment efficacy and associated hydraulic characteristics. The SSF was able to consistently produce water with a turbidity less than 1 Nephelometric Turbidity Units (NTU) and with the infiltration capacity to treat 0.27 m³ m^-2 h^-1 of greenhouse effluent; which, equates to treating 3,600 L d-1 of drainage water from a 1,000 m² greenhouse. The recovery rate for the filter was an average of 110 minutes. Ideal growing conditions for bacteria responsible for the treatment of the water in the SSF included warm and consistent water temperature, high nutrient content, and organic loading in the water. The SSF was able to sustain the hydraulic loading rate of 0.27 m³ m^-2 h^-1 for 16 days. At the end of this period the organic layer that was present at the sand/water interface reduced the infiltration rate because of the formation of the Schmutzdecke layer (SL). An air/water jet cleaning system scoured the SL and suspended it so that it could be drained.