It is very important for those of you who have implemented rainwater harvesting systems with a first flush mechanism to keep the first flush clean!  I visited a site I installed 2 years ago and thought I would do the owner a favor by checking their first flush.  I found the first flush was very full of sediment and hadn't been cleaned recently as best I could tell.  The roof catchment area sits under a few oak trees so it is a site that requires maintenance on a regular basis.  In this case the gutters don't have any gutter guards either so the Leaf Eater is the primary screening mechanism and then the first flush.  The first flush was so full of "gunk" it basically wasn't working any longer.  The "ball" inside the first flush was frozen inside the 4" pipe.  When we checked the tanks they had a lot of sediment sitting in the bottom.  Needless to say we cleaned out the first flush and left it in excellent working order.  So remember, if you have a system with a first flush it is important to clean it out on a regular basis.  Otherwise it will stop serving its purpose and the sediment will be flowing directly to the tanks.  Cleaning them will save you time and money in the long run.  After discussion with the owner he told me, "We had the attack of the oak moths this year along with the nasty caterpillars whose droppings are called frass and is the consistency of fine sand and it was all over the place. That's what clogged the filters. I do clean the small filters regularly and I thought there was a large filter but I couldn't unscrew the cap."  The owner brought up a good point.  The first flush devices have a cap at the bottom with plastic threads.  I have seen where they bind over time and become difficult to remove without a strap wrench.  In this case we actually did remove them with a strap wrench before we cleaned them.  Since this installation I have begun to use a toilet wax ring as a lubricant on the plastic threads.  It helps to resolve the binding problem.  Basically you can buy the toilet bowl wax ring at any hardware store.  You scrape off a portion of the wax and apply it to the plastic threads.  I learned this from a plumber who has helped me in the past.  Therefore part of the problem was not originally installing with the wax to make it easier to spin off the caps.  I have changed my process to always use this when I install now.  I have also updated my maintenance information on this website because over time applying wax will be required again as part of the standard maintenance process.  In regards to the oak moths and caterpillar frass, it is important to clean more often when extraordinary debris situations occur.   It was a good lesson for both the owner and the installer (me)!  In addition the owner is going to install Leaf Solution gutter screens.  This will make a huge difference by keeping debris out of the system.