Offset filter bed

Inspiration

This design improves on prior designs by providing easy access to the sand medium. This easy access allows operator to observe sand levels and replace as needed. The design also aims to minimize clear-well volume by using multiple filters backwashed out of phase. To minimize clear-well height a siphon in employed to create a pressure gradient during backwash.

Design summary

The plant has four offset filter beds arranged around a common clear-well. The incoming raw water is distributed across the four filters. The diagram shows the design for one filter bed.

1)In down-flow mode, water passes through sand, into the region below the sand (blue arrows.)
2)Pressure forces the filtrate into the clear-well via a pipe for the clean water.
3)The headloss gained as the filter clogs forces the raw water up the siphon tube. Eventually the siphon is filled and water starts the flow out the siphon. This causes a negative pressure that pulls the water from the clear-well, through the sand bed, expanding and cleaning it. This water moves at a flow rate which is about 10x that of the down-flow rate. (white arrows)
4)In backwash mode, the water stored in the clear-well moves through the sand and out the siphon( white arrows)

Discussion

The offset design resolves sand access problems but sealing the filter bed tanks presents new manufacturing challenges. They need to open to replace sand, yet watertight.

The next phase of this design was to simplify it so we could use materials that are easily found in Honduras. We adapted this design to be used with HDPE 55 gallon drums, which are abundant, airtight, machinable and inexpensive.