First off, I don't know specifics yet at this point, I have only received the call from a relative. I worked for years as a plumber, but my experience with septic tanks is only basic.
1. Septic tank was full and backing up. This system was installed 3 years ago on new ground, all new construction. Waste hauler came and pumped out the tank
2. The system has an effluent filter and a sand filter of some sort on the outlet side(which I know nothing about at all). Apparently, they attempted to run a hose on the outlet side into the filter and water wouldn't pass through the sand, which if it was full of debris, I can understand; the downstream part of the line is what is puzzling me
3. Past the sand filter is a distribution box for the lateral lines. Supposedly the line into the box was completely dry and had dust standing in it. If the sand filter was stopped up, I can even see this, but the distribution box and the lateral lines were all full of black sludge. I don't know what would cause this at all. The septic company is blaming laundry detergent, but I can't accept that. 3- 4" lateral perforated lateral lines can't fill with soap in a 3 year time span. It is a physical impossibility. not even if all the soap in the household were poured in, and the lines then capped off should they be full

The homeowner and general contractor told me that the trenches were filled with 2 feet of sand, then 12" of gravel before the lines were laid. The lines were laid, filled with gravel 6" all around, and then a barrier from above between the soil and gravel. I think he said it was felt, but I don't know for sure.

It is now being excavated and replaced in the same manner. I would like to know if anyone has any idea about what may have happened, and suggestions to prevent it in the future. I don't know if the laterals had any kind of fabric to prevent material from coming back inside. I am heading to his house tonight or tomorrow to take a look, anything I should be checking?

its not just the laundry soaps and softeners. Its the cooking grease and undissolved foods.
Did the septic people consider inducing air into the septic field at the ends to force any jel/gel and greases.. backwards towards the tank so that it can be removed? Also, did the family who was using this septic ever use a septic system before moving into that house.
Best Wishes

One of them grew up with a septic, but grease and oil shouldn't be a problem in a lateral field. Grease doesn't pass the tank. grease and oil both float on water and wouldn't make it to a submerged outlet pipe. I think someone used a blow bag on a line somewhere, but I think it was from the yard clean out by the house toward the tank to move any 'leftovers' out to the tank

Not a plumber OR a septic tank installer

"-----but grease and oil shouldn't be a problem in a lateral field. Grease doesn't pass the tank. Grease and oil both float on water and wouldn't make it to a submerged outlet pipe." -----

The usual continuous gravity flow system septic tank outlet pipe is at the designed operating high water level of the tank, not submerged -- when tank is operating as designed (not overfilled due to stoppage). Of course there should be a tee installed to prevent effluent from flowing directly out the outlet pipe. Modern systems also have filters at that point which may / may not be installed and maintained. It is my understanding that there is a possibility that grease could exit the main tank to the distribution box to the drainfield. I don't know of course but I would be surprised that a system only 3 years old could fail that fast. When / if top of main tank is opened the first thing I would check would be if the outlet tee / filter is properly installed.

Grease interceptors and septic tanks are designed in a very similar fashion. The outlet from a septic tank is below the water line, with a piece of pipe on a 90 degree bend to prevent the crust of the top from being drawn into the outlet. Unlike a grease interceptor(which may only contain a baffle) this pipe is positioned somewhere in the middle between the water line and the bottom of the tank to prevent solids from blocking it as well. I haven't seen it yet, but if it has a tee fitting instead of a 90, the line on the outgoing side of the tank would have to be stopped up already to cause the water level to rise. As I stated initially, my concern is why this happened in the first place, and what can be done to prevent a reoccurrence.

Before a drainfield is condemned, I would do everything possible to determine that there is nothing prior to the drainfield that is the problem; from the tank to the field. Has the outlet pipe been tested to eliminate crushing as the cause?

I didn't get to visit the site as planned, but talked to the homeowner again; he told me incorrectly about the placement of the sand filter. It is actually at the end of the laterals, not between the tank and the distribution box. I have a feeling that the 'black sludge' was simply just solid waste, most likely due to buildup and simply needed pumped out, and the line cleaned. As he only called me after the laterals were removed, the damage is done now. In the future I am going to recommend that they limit antibacterial cleaners down the drain, and treat with ridX or a similar product. Thank you all for your responses.