We have a few underground tanks with only the manholes visible from the outside. All tanks are cubic-shaped. How can we accurately measure the dimensions (depth,length and width) of the tanks? Are there any types of equipment to do that? The tanks store wastewater and it won't be possible for anyone to enter it through the manholes.
Depth can be measured by dipstick whether partially full or not.
A range of ultrasonic "tape measures" in a Google image search.
Distance from the manhole to the walls should be measurable by laser or ultrasonic "measuring tape" provided there is some air space at the top of the tank.
The problem with most handheld measurement devices is that you need someone to enter the manhole, for example to hold the laser distance meter against one wall to measure the distance to the next. To get an accurate measurement of the conditions above the water level, contact a surveying company to do a LIDAR scan. Explan the situation to them, the will need a way (possibly they need to build a jig to do so) to lower the scanner down the manhole and keep it stationary at one place. Talk to the surveying company if you need a 2D or 3D scan. A skilled CAD operator should be able to turn the cloud of points gerneated by the scan into a 3D-Model or drawing.
The measure the depth beneath the wastewater surface, you will need a dipstick. Any measurement device will have to be lowered below the liquid surface, so make it low cost => Dipstick.
Be aware that wastewater channels are Ex-hazard zones an you will likely not get an ATEX approved LIDAR device (if such a thing even exists). Do a risk assessment, consider bringing a gas sensor and maybe equipment to ventilate the tank before measuring.