Just watched your Part 5a video and I believe that the sinmy mess you have there is called "Microbiologically influenced corrosion" or MIC.
The fire protection industry have been battling MIC for a long time. Essentially this MIC will eventually - over time "eat" through our steel sprinkler piping.
Back in the day, it wasn't that immediate of a problem because we piped our fire protection systems with schedule 40 pipe. In current time we changed
over to schedule 10 and sometime schedule 5 pipe (depending how the engineer designed the sysyem); now the MIC has less material to "eat" through
and our industry as a whole has been dealing with this problem.
In the video you rinsed and shook out of that tank a snot-like glob, that the MIC, and as you well know, it'll clog up your water blocks over time.
What I would do to remedy the problem (this is just MY opinion):
1) Replace the steel tank with a polyethlene tank that have the same fitting layout - Wnen you kill-off the MIC the unprotected steel will still rust and discolor the coolant and clear tubing.
The poly sprayer tanks you see mounted on the backs of four-wheelers for garden spraying comes to mind, and they have a huge fill port that can let you clean out the insides too.
2) Use chlorine or hydrogen peroxide (biocides) to kill the MIC.
3) Use some detergent ( a surfactant) to get rid of the oily film from inside of the piping.
4) treat your new coolant with biocide or maybe use "sacrificial" silver (I have NO experience with using silver and cannot vouch for it).
Just my 2 cents worth, good luck with your project.
Ps. Here's an excerpt from page 321 - 322 of the 2008 NFPA Water-Based Fire Protection Handbook:
Microbiologically influenced corrosion (MIC): Corrosion initiated or accelerated by the presence and activities of microorganisms, including bacteria and fungi. Colonies (also called bio-films and slimes) are formed in the surface of pipes among a variety of types of microbes. Microbes deposit iron, manganese, and various salts into the pipe surfaces, forming nodules, tubercles, and carbuncles. The formation of these deposits can cause obstruction to flow and dislodge, causing blockage (plugging) of system piping, valves, and sprinklers. MIC can be found in many types of piping systems, not just water-based fire protection systems. MIC is thought to be responsible for 10 to 30 percent of corrosion in all piping systems in the United States. Generally, two types of bacteria are responsible for MIC: sulfate-reducing bacteria (SRB) and acid-producing bacteria (APB). SRB are anaerobic (not requiring oxygen) and are typically found in carbon steel systems. APB are also anaerobic but can survive in aerated or anaerobic environments. APB will produce acids that stimulate SRB growth. MIC can survive in a variety of environments from 0 to 100 percent oxygen-saturated water and water with a pH from I to 10. MIC can be recognized by the presence of a gray or black mud-like slime (typical of anaerobic bacteria) or a brown or rusty color (typical of aerobic bacteria). Once established, MIC will create nodules on the inner surface of pipe. Left untreated, this will result in pitting of the pipe wall and eventual pinhole leaks (although pinhole leaks can be caused by other factors). Nodules can also increase in size, causing obstruction to waterflow. Exhibits D.8, D.9, and D.10 illustrate pipe with nodules from MIC. If the presence of MIC is suspected, water samples should be taken and analyzed before treatment is initiated. In cases of extreme pitting, the affected pipe should be replaced. In addition, the water supplying the system should be treated with a biocide, such as chlorine, to prevent recurrence of the problem. Exhibits D.11, D.12, and D.13 illustrate pipe with moderate to severe pitting and copper pipe with pitting and a pinhole leak. Where pitting is not extensive, flushing the entire system with a chemical cleaning agent to remove nodules is recommended. After flushing, the system should be refilled with treated water to prevent recurrence of MIC.
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