Backwash Expansion Tool

AWWA guidelines recommend 20% - 30% expansion of the filter bed when backwashing conventional filters.  Here is a tool I made to measure the expansion.  

Basically, you have a weight at the bottom of some 3/8 threaded rod and secure a stack of the discs with nuts and washers on the threaded rod where the expansion should be.  Our filters are 72" deep, so I set our discs from 13" to 24" above the bottom of the weight (13/72=18% to 24/72=33%).  

Our weight is an 8" flange cover, but I included a base and plug that can be filled with sand and held together with threaded rod, nuts, and washers.  If you use this, put thread locker on the bottom nut so it does not fall off.

You will also need a threaded rod coupling and eye bolt for the top (see the pictures)

The crane is made from 2" pvc pipe and a cheap trailer winch.  It clamps onto the 2" railing with modified tees that have a little less than half of the side cut off so they can pop onto the railing.  Really look at the pictures if you want to make your own.  The tee at the end has a pulley hanging from the bolt that the rope goes through.  You will also notice a black mark on the rope against the measurements.  This is for noting the level of the media before the backwash.  How you secure the tool really depends on your filters.  You can even use a bar clamp on the edge of the filter and tie the rope off at the media level before the backwash.

Use:

1. Before the backwash, lower the tool down until it rests on the filter media

2. Remove the tool before the air scour or surface wash (we only want the expansion of the high-flow rate)

3. Lower the tool back in to the same depth as the media was about halfway through the high-flow step

4. Pull the tool out after the high flow step and note the highest disc that is filled with media

5. Divide the expansion number by the total depth of the filter (14" disc on our 72" filters is about 19%, 14/72=0.19444=19.44%)

As for printing, nothing needs supports.  The thicker you print the base, the better.  I haven't printed it yet nor do I plan to, but it should work since it is similar to funnels I have printed in the past.  Mock assemble it first with the threaded rod, nuts, and washers with some caulking in the top hole of the base to hold the rod in place, then later fill it with some filter sand with the rod in place upside down, then cap it.  tap the sides as you fill it to settle the sand in as much as possible.  Seal the plug and threading with more caulking and add some thread locker to the bottom nut.

The base is also exactly 6" tall for when you go to add the expansion discs.  I recommend centering your discs at 25% above your static filter level (1.25x filter media depth in inches) and adding as many as needed on either side to get down bast 20% and above 30% (1.2x and 1.3x).  I marked the discs with sharpie which held up for 5 years on the modified PVC caps we used to use for this.