Treatment of cooling tower blowdown water: The effect of biodispersant on the ultrafiltration membrane

Taking into account the growing population, water scarcity is a relevant problem which needs to be addressed. Wastewater needs to be assured by quality, ecosystems protected and the wastewater reuse in industry should be a more common approach that would also assure the reduction of fresh water intake. Dow Chemicals decided to reduce the fresh water consumption by reusing the wastewater streams. Evides built a pilot plant in Terneuzen that will treat 3 water sources: rainwater, wastewater treatment plant effluent and cooling tower blowdown (CTBD) water. The biggest stream and the most difficult to treat is the CTBD water due to its high salinity and composition resulting after the evaporation process. Since the performance of the pilot was not very efficient and the ultrafiltration (UF) membrane suffered from rapid fouling during operation with CTBD, a solution to reduce the fouling was researched. First, the coagulation/flocculation step was evaluated. It is known that iron chloride used as coagulant will decrease the pH of solution, which will result in smaller and more difficult to settle flocs. For the improvement of the coagulation step a base was added together with the coagulant in order to keep the pH in the sweep coagulation zone. Since the focus of the research was on improving the UF operation, more attention was put into what could be the cause of it. It is known that in the cooling tower many chemicals are added to prevent corrosion, scaling or microbial growth. Of all chemicals, the biodispersant was the unknown solution which prevents microorganisms attaching to surfaces. The assumption made was that the biodispersant will form a layer on the membrane’s surfaces which causes the pressure increase after continuous operation. The nature of the biodispersant was investigated through surface tension and electrical-conductivity measurements and it was concluded that the biodispersant is a non-ionic surfactant. The critical micelle concentration was established to be 0.03 mg/l. CMC is the concentration above which the biodispersant is not only present as molecules, but it start forming micelles. The higher the concentration, the higher the number of micelles will be. When the concentration is equal with the CMC the solution is saturated with surfactant molecules which form a layer on the surfaces. Furthermore, the operation of the UF was observed in experiments with different biodispersant concentrations. Solutions of demiwater and Schie canal water with and without biodispersant were filtrated in order to observe the fouling behavior. It was indeed seen that the biodispersant is causing the fouling of the UF membrane and the backwashes did not help restore the initial membrane resistance. Experiments with powdered activated carbon (PAC) and clay were also performed to see if the adsorption of biodispersant was possible, but the results were not promising. For clay the adsorption area was too small and not much was adsorbed. For PAC, besides the biodispersant there are present other organics that will compete for adsorption. Because the surfactant concentration in the blowdown water was much higher than the CMC concentration (assumed 4 mg/l) experiment with solutions with surfactant concentration closer to the CMC were performed. It was observed that at a concentration of 0.1 mg/l biodispersant the fouling was much lower than the fouling increase recorded with solutions at 4 mg/l biodispersant and the backwashes were more efficient. At a biodispersant concentration of 0.1 mg/l the system is supposed to still be saturated. Therefore, it is assumed that the microbial growth should not be increased and the system’s behavior will not be affected by the decrease of biodispersant dosage. This statement should be further researched, but it a decrease in biodispersant dosage would not only result in a stable efficient operation of the pilot, but also lower operational cost, since less surfactant would be added in the cooling tower.