Deliberate biodegradation in a closed system cooling tower

A series of experiments was made on an experimental cooling tower with a 30 gpm circulation rate and with a feed of make-up water consisting of the mixed and settled effluents from an edible oil plant and a fatty acids distillation plant. Summer conditions were simulated by heating the circulating water to 105 F, thus permitting a 20 F temperature drop through the tower. Almost immediately some aerobic degradation of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) was observed. It was suggested that the biota be fed by the addition of nitrogen and phosphorus while the pH was maintained at neutral. This increased the rate of biodegradation about fourfold, the tower leveling off with 6% blow-down relative to the rate of feed with BODs and CODs substantially equal to, or lower than, their level in this feed. Thus, overall biodegradation of 94% was realized, attributed to: (a) aeration in the tower; (b) high water temperature of 85 F ; (c) feeding of nitrogen; (d) feeding of phosphorus, and (e) pH adjustment. Similar results were obtained in tests on a larger cooling tower used for handling the total effluent water from the barometric system of a single edible oil batch deodorizer. All of the water was recirculated for a period of three months with no blow-down whatever. In this test the COD and BOD leveled off at about 1730 and 360 ppm respectively. The obvious benefits of a biological cooling tower are water conservation and elimination of waste treatment and disposal.