Study On Seawater Pretreatment By Coagulation/Adsorption/Microfiltration Process

Seawater reverse osmosis (SWRO) has been widely applied over the last 20 years for fresh water production. By this means many areas'fresh water shortage has been mitigated. Until recently, in desalination plants seawater was pretreated with a combination of conventional techniques such as coagulation, clarification, sand filtration and cartridge prior to reverse osmosis desalination membranes. Microfiltration (MF) and ultrafiltration (UF) membranes are now considered as an alternative solution for seawater pretreatment in foreign countries. While in China, there are few literatures reported focusing on membrane pretreatment of SWRO, let alone on domestic MF membranes used as SWRO pretreatment.Whereas Bohai Sea has a very poor seawater quality and fluctuates much according to seasons and weather, an integrated process combining coagulation, adsorption and MF was developed for the seawater pretreatment. Operation of such equipment for much long time aimed at finding out treated seawater quality and the membrane performance, then evaluating whether this process could be used as SWRO pretreatment. Subsequently a group of orthogonal experiment was conducted to differentiate factors such as aeration strength, dosage of powdered activated carbon(PAC), cycle of air scouring and duration of air scouring period, and to optimize the integrated pretreatment process, according to their corresponding seawater pretreating capacity.The result shows that after running nearly 410 hours, parameters of pretreated seawater, silt density index (SDI), turbidity, total iron concentration and pH value were absolutely superior to those required by RO feed water and stable, except for the chemical oxygen demand which varied with the quality of the raw seawater.The orthogonal experiment shows that PAC causes membrane specific flux to decline fiercely, and then to downsize its pretreating capacity. Among the factors causing specific flux decline of membrane, PAC dosage is the biggest one, the second is the duration of air scouring period, ratio of air to treated seawater is the third, and cycle of air scouring is the least. In the following conditions of no PAC dose, 20 minutes of air scouring duration, ratio of air to treated seawater being 10 and air scouring every 8 hours, this equipment has the biggest pretreating capacity.