I got this achievement in Feb 2020, so this guide will be how I got it at that time. The actual requirements may not change, but some of the steps I took may change as the game develops.
Requirement: Fill the Tanker truck to capacity (10,000 L) with sewage.
Please note I did not empty the truck until it was full, which took 4 full septic tanks and part of a 5th.
How to get the Tanker:
*** Spoiler - click to reveal ***At the time of this writing, the tanker spawns at the waste processing plant in Peräjärvi. However, you cannot drive it until your Uncle Kesseli gives you the keys as part of the story progression. The first time he spawns, he gives you the keys to his van. This is triggered by installing either the drivers seat, engine block, or both trail arms into the Satsuma. For me, this was on the first Sunday of the game. The second time he spawns is random, and he is outside his place drinking beers. Sit and talk with him and he'll give you the keys to the tanker. This action permanently unlocks the tanker and van for your use. (For me, he spawned for this event on the second Wednesday of the game.) Now, go to the waste plant, grab the truck, and starting sucking!
Emptying Septic Tanks:
*** Spoiler - click to reveal ***Now that you have the tanker, you'll start getting calls for septic jobs, which will be noted on your map. Drive to one of these houses and you'll see a concrete circle on the ground with a square lid on it.
1. Back the tanker up to the tank
2. Put the truck in neutral, engage the parking brake, and leave the engine running
3. Get out of the truck and grab the hose on the passenger side of the truck. This will connect it to the pump automatically.
4. Grab the lid of the septic tank and move it aside. Do not drop it into the tank!
5. Grab the loose end of the hose and feed the hose into the tank
6. Go into the truck and turn on the hydraulic pump, then turn up the hand throttle (the higher the faster the pump works)
7. Once the tank is empty, the homeowner will start talking to you and hold out some money. To save fuel, turn off the engine and reduce the hand throttle right when the homeowner starts talking.
8. Pull the hose out of the tank, grab the loose end and hover it over the side of the truck that you originally pulled it from. Clicking should put the hose back
9. Grab the money and on to the next one!