On something like this on your photos above,
We in California, have to reclaim all of the water, then filter and send it , by special discharge permit, to the sanitation district for procession ( Sewer Access ). So check with the Sanitation District for the proper discharge procedures in your state. With that said.....
I agree with Chris. High heat is your friend here. Try to increase you heat output by recirculating you water from your tank via you boiler. This will increase the water temp. Do this to 120 -130 degrees. ( Pressure pumps are usually able to handle 140 degrees, HOWEVER, check with the manufacture of the pump you are using for the correct temps)
Once you are at the 120 to 130 degrees mark, take your hose and reattach it to the wand. Turn on you boiler to full. Some boilers temperature settings say 300 degrees (We know that will never happen with a standard pressure washer). However.... being that you have 130 degrees now of inlet water in your tank, you might be able to get 220 to 250 degrees for a little while. ( As much as your water tank can hold) DO NOT ADD COLD WATER TO THE MIX !!! This will cool down you water thus your temp at the wand will decrease as well.
That hotter you can get your 5 to 6 GPM pressurized flow, the better the results.