Challenges of Recycling Cooling Tower Blow Down in a Power Plant
Cooling tower owners face many challenges with the application of membrane filtration processes in zero liquid discharge (ZLD) wastewater recycle systems for cooling tower blow down (CTBD) recovery and water reuse. Cooling tower blow down wastewater is composed of scaling minerals, soluble salts, organic tower treatment compounds, biological matter, and solids that have been scrubbed from the air as the tower circulates water. Reverse osmosis technology is chosen because of its ability to remove those soluble salts. The most important factor in water recycling and water reuse with Reverse Osmosis (RO) is the pretreatment process that protects the desalination membranes against RO fouling, RO scaling, the causes for reduced RO life. The use of low-pressure cross flow Microfiltration (MF) prior to Reverse Osmosis has been used to remove these problematic contaminants found in CTBD wastewater. Polymeric membranes are also subject to fouling by various organic, mineral, and chemicals that must be mitigated by the pre-treatment chemistry and the proper selection of membranes. Use of Tubular membranes with an ability to operate at high flux at the same time filter out the RO fouling materials is a critical part of the water recycle process. The selection of Tubular membranes is based upon their ability to operate in the presence of high concentrations of suspended solids (TSS) and because their materials of construction (PVDF) allow for membrane cleaning with strong cleaning chemicals that would destroy RO membranes. The selection of microporous membranes (0.1 micron size) provides separation of contaminants in particle form that assures the hardness removal, scaling control and organic fouling prevention. The primary goal of this membrane pretreatment process is to produce a filtrate that can allow to RO membranes to operate a very high recovery up to 95%.
These advancements made in membrane fouling minimization technique results in substantial reduction in both the capital (CAPEX) and operating (OPEX) cost of microfiltration membrane systems used for protection of RO desalinization systems.
This paper provides a review of field operating data for pretreatment chemistries applied in ZLD water recycle systems to achieve membrane fouling reduction and flux rate enhancement for the tubular membrane pretreatment process. The case study is presented for pretreatment chemistries developed for cooling water blow-down water reuse from a power plant.