Corrosiveness Variation And Control Of Reclaimed Water For Recirculating Cooling Systems In Distribution System

A reclaimed water distribution system was used for delivering the secondary effluent of Qisuyan Wastewater Treatment Plant to Zhongtian Steel Plant as the cooling water. Dynamics of water quality and corrosiveness of the reclaimed water were examined during transport in the distribution system, and then the effects of water quality and corrosiveness on reclaimed water were analyzed. Besides, the corrosion control measures were also investigated, such as optimizing wastewater treatment processes and by dosing chemicals. All above studies provided technical support for reclaimed water recycled for the cooling system.Peak values existed for the flow rate of the reclaimed water in the pipeline, indicating variation in the hydraulic retention time (HRT). Corrosion morphology and products of the pipe were analyzed, and the results showed that local corrosion occurred for the carbon steel pipe in conveying process and the corrosion product was mainly of FeOOH. The water quality and corrosiveness were monitored during a year duration for reclaimed wastewater samples before and after transporting. The mean change ratio of water quality for the reclaimed water was-0.6%for Cl-,-2.6%for SO42-,-42.7%for ammonia nitrogen,-14.9%for phosphate phosphorus,3.2%for nitrate nitrogen,28.6%for total iron; average pH had risen by0.2, and corrosiveness of reclaimed water was droped by an average of13.6%.Lab experiments were carried out in annular reactors (ARs). Water quality and corrosiveness of the reclaimed water were affected by hydraulic conditions and increasing HRT in ARs reduced corrosiveness of reclaimed water. At the HRT of180minutes, water quality kept stable, but corrosiveness of the reclaimed water was lowered by15%. Under a certain HRT, corrosiveness of the reclaimed water increased with decreasing flow rates. Corrosiveness of water was constant when the flow rate was above0.9m/s, and compared with the flow rate of0.3m/s, corrosiveness was lowered only by3.9%. From studies on the influence of water quality on the corrosiveness of the reclaimed water, Cl-and SO42-had high impact on the corrosiveness. In the range of6to8, pH affected corrosiveness significantly.The corrosion control experiments were carried out in simulated pipeline systems. Using polymeric aluminum chloride (PAC) and aluminium sulphate as dephosphorization chemicals, the results showed that PAC removed phosphorus better, but the corrosiveness of the reclaimed water after both treatment was similar. Corrosiveness of reclaimed water with chlorine disinfection was7.9%lower than that with ozone disinfection.50%of SO42-and17%of Cl-were removed using lime and aluminum process in the reclaimed water, and thus corrosiveness decreased by10.4%. Based on the above results, the optimized process was using PAC (60mg/L) as the dephosphorization chemical, using sodium hypochlorite (10mg/L available chlorine) as the disinfectant, using Lime and Aluminum process (80mg/L Ca(OH)2+80mg/L NaAlO2) for the removal of SO42-and C1-. Compared with the original process, it decreased the corrosiveness by27.2&for the reclaimed water. By the addition of corrosion inhibitor to the reclaimed water, the optimal dosage was10mg/L; with the pH of6-9and the temperature of20-80℃, the inhibition efficiency could reach as high as95%. The inhibition efficiency of the reclaimed water by adding corrosion inhibitor after water delivered was95%higher than that before the reclaimed water was delivered, so the dosing point of corrosion inhibitor was better at the point after the reclaimed water delivered through the pipeline.