Just about all brewery wastewater systems will need a lift station, aka pump station. The sewer in the street is usually a few feet below grade. The wastewater pipe leaving your brewery will also usually be a few feet below grade. Maybe you could gravity flow the whole way, but if you need to add a tank somewhere, you will need to either lift water up in to the tank- or lift water out of the tank if it’s underground.
All below ground brewery wastewater tanks should be concrete or stainless steel. A concrete tank should be coated on all sides, especially the roof. The area above water line is actually more corrosive than the parts under water- due to hydrogen sulfide gas and water vapor creating sulfuric acid. I’d love to say underground plastic tanks work, but the water will be too hot leaving your brewery, averaging about 120F or so. There will also be spills and other events leading to water that can be close to 200F in that tank. Most underground plastic tanks are rated for no higher than 120F. A collapsed wastewater tank because of an overflowing hot liquor tank would be a major bummer.
My personal preference is for a smallish underground lift station with an above ground EQ stainless or plastic tank. Above ground tanks are a lot easier and safer to work on, and in most cases you can keep virtually all of your equipment out of the corrosive environment in that tank. This leads to longer equipment life and less headaches.
Please contact me for more details about brewery lift station design. Remember, don’t reinvent the wheel. Go with what works so you don’t have headaches due to improper design.