Control Of Mineral Scale Deposition And Metal Corrosion Of Cooling Water Systems Using Secondary-treated Municipal Wastewater

With increasing shortages of freshwater, wastewater is now being recognized as a significant source of water for nonpotable uses. For recirculating cooling water systems, which are very important in a lot of industries, wastewater reusing as cooling water instead of fresh water will not only save a lot of fresh water resource but also reduce pollutant dischargement into the surface water system. Among different types of wastewater, secondary-treated municipal wastewater is of increasing interest, primarily because it holds promise as viable alternative source of cooling water in terms of quantity and proximity to existing and future cooling water exhausting industries (eg. Power plant). The primary challenges with secondary-treated municipal wastewater reuse for cooling arise from its low quality. The secondary-treated municipal wastewater usually contains kinds of cations including Ca+, Mg+, Na+and anions including HCO3-, SO42-, Cl-et al. In recirculating cooling systems, the water constituents become concentrated many times because of the evaporative loss of water. The elevated concentrations and high water temperature can cause severe mineral deposition and metal corrosion problems. In the present study, the effect of inorganic ions and organic maters on the mineral scale deposition was scientificly studied. It is indicated that Ca2+, Mg2+, HCO3-and SO42+showed strong effect on mineral scale deposition on stainless steel specimen. Increasing the ion concentration could make the mineral scale deposition problem much more severe. Insignificant effect of organic matter was detected. Based on the chemical composition of the secondary-treated municipal wastewater, low molecular weight Hydrolyzed polymaleic anhydride (HPMA) was synthesized. Based on the synthesized HPMA, Three component inhibitors including polyacrylic acid (PAA), and l-hydroxyethane-l,l-diphosphonic acid (HEDP) or Amino Trimethylene Phosphonic Acid (ATMP) or2-Phosphonobutane-1,2,4-Tricarboxylic Acid (PBTCA) was chosed to obtain3three-component mixture inhibitor formulations. The composition of formulation was HEDP/HPMA/PAA=10/80/10, ATMP/HPMA/PAA=10/5406/35.4, PBTCA/HPMA/PAA=10/58.2/31.8, respectively. The formulations showed much higher antiscale efficiency than the component at the same dosage.Based on the4times concentrated secondary-treated municipal wastewater, the synthesized HPMA and the three formulations were evaluated according to both the static scale deposition methods and the developed dynamic scale deposition methods. During the static scale deposition method, the synthesized HPMA and the three formulations showed stable inhibition efficiency (72%,80.5%,79.5%and74.3%, respectively) when the inhibitor concentration was in the range of10-30mg/L However, during the developed dynamic scale deposition method, the inhibition efficiency increased significantly with increasing the inhibitor concentration from10mg/L to30mg/L. The dynamic antiscale efficiency of the synthesized HPMA and the three formulations are all above95%, even upto100%.The micromorphology investigation and the crystal phase analysis of the particle precipitates indicated the synthesized HPMA and the three formulations showed strong effect on crystal growth and particle morphology of calcium carbonate during the scaling process in the recirculating cooling water system. The functional group of the Polycarboxylic acid and the organic phosphonate may react with different crystal faces and different direction of crystal growth. The two kinds of inhibitor together showed synergistic inhibition effect on mineral scale depositon.Using secondery-treated municipal wastewater, a dynamic corrosion test installation was set up to simulate the recirculating cooling water system. The three formulations showed excellent corrosion inhibition effect on20#carbon steel, especially formulation2with the efficiency of90%. The electrochemical impedance spectroscopy tests together with the fit results indicated that the inhibitors could adsorbed onto the surface of carbon steel to form a protective film.