Biofiltration Tanks
Sometimes they are large flat pools like this, with soil or mulch.
From http://www.likusta.com/en/waste-gas-treatment/biofilter0.html
Sometimes they are large flat pools like this with water.
From https://www.wrallp.com/our-work/northeast-water-pollution-control-plant-new-gravity-thickenerfacilities
Sometimes they are covered tanks like this.
From http://www.envirogen.com/pages/applications/odor-control-and-voc/
Sometimes they are cylindrical tanks like this.
From http://www.envirogen.com/pages/markets/food-beverage/
Sometimes they are stacked in series like this.
From http://www.biorem.biz/resources/