Recirculation wash water sand filter

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Sand filters play a key role in drip irrigated crops as the sand bed within the filter removes all floating particles and thereby prevents drippers from clogging. As time passes, the sand bed starts silting and the filters have to be back washed. In case nutrient solutions pass through the sand filters, the wash water contains important nutrient concentrations. The wash water of a soilless tomato crop contains on average a concentration of 360 mg nitrate (NO3) per litre. In general, this wash water is discharged to the surface water, leading to serious nutrient pollution in the surrounding water bodies. For example, a Flemish greenhouse with a 3 ha tomato crop, produces 1 648 m³ of wash water per year (2 sand filters of 48” and 1 sand filter of 36”). On a daily basis, a maximum of 8 m³ wash water is produced. In 2012, the grower started to recirculate the wash water. He programmed the filters to be rinsed on a daily basis during the night. Firstly, the wash water is pumped to a sedimentation pit of 20 m³ where it stands for at least 20 hours allowing the floating particles to sediment.  After this phase, the water is pumped to the drain water tank where it is recirculated again in the water system. The pump is installed at 1m above the bottom of the storage. In this way, suction of the sediment is prevented. After 20 hours, the water is pumped to the drain. Only some hours later, the tank is filled again with a new batch of wash water. An alarm system is recommended, in order to alert in the case the pump fails. After installing the system, described above, the grower’s annual cost for fertilizers dropped with € 1 500. The annual cost for the installation costs him around € 450 per year.

Els Berckmoes

Proefstation Voor De Groenteteelt (PSKW), Belgium
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