Research On The Feasibility Of Regenerated Water Reused For Recirculating Colling System In Corn Deep Processing Enterprises

Corn processing industry is enormously water-consuming with water used for the recirculating cooling system reaching nearly 30% of the total water consumption. If the produced wastewater can be treated to satisfy the cooling system supplement water standards and reused, it will not only save the amount of fresh water needed, but also reduce emissions of pollutants, which would have both environmental and economical benefits. This study is part of the "11th five-year plan" of national water project "water quality improvement, ecological restoration techniques and demonstration research" This study is also based on the regenerated water reuse project of COFCO biochemical energy (Gongzhuling) Co., LTD,.Based on the investigation and analysis about status of recirculating cooling system, the study combines the technologies of rotating hanging pieces with scanning electron microscope, and investigates the influence of significant corrosion factors on the corrosion of carbon steel and stainless steel hanging pieces. Exploration on the relevance of water quality and corrosion degrees is performed. Regenerated water corrosion test is carried out on pilot-scale deep treatment facilities and the results are analyzed compared to influence in current systems due to the corrosion of supplement water. Discussing on the feasibility of the using such supplementing water in the recirculating cooling systems. According to the field investigation and monitoring analysis, the water quality in the recirculating cooling system fluctuated obviously. The recirculating cooling system has risk of being corrodederoded by Cl- because the instantaneous concentration of Cl- can reach 1140mg/L in dextrin production workshop.The impacts of ammonia nitrogen, organic matter, chloride ion, sulfate ion on carbon steel and stainless steel are observed to be significant and the corrosion rate increases with the concentration. For stainless steel, when the concentrations of NH3-N, CODCr, Cl- and SO42- are larger than 20mg/L,50mg/L,500mg/L and 200mg/L respectively, the requirements in《Code for design of industrial recirculating cooling water treatment》(GB50050-2007) could not be satisfied. It is also found that the corrosion types of choride and sulfate ions are similar. For carbon steel, when the concentrations of NH3-N, BOD5 and CODCr are greater than 5mg/L,20mg/L and 25mg/L respectively, the requirements could not be satisfied, while at the same time the impact of chloride ions and sulfate ions on carbon steel is more significant compared to that on the stainless steel. With the concentration levels of chloride ions and sulfate ions are at 50mg/L and 30mg/L, the corrosion rate is observed to exceed the standard values by a factor of two times.Studies on the corrosion of materials used in the systems caused by supplement water reveals that the navy copper, with the corrosion rate 0.006mm/a, has less corrosion tolerance than all kinds of stainless steel of which the tolerances are measured to be less than 0.005mm/a. The tolerability of each material against the local groundwater can be identified to be stainless steel 316>stainless steel> 304 stainless steel 321>navy brass.The treated water from deep treatment pilot scale facilities are mixed with currently used supplemental water with a mixing ratio of 10%-50%, and the dosage of corrosion inhibitor are varied as well. The mixtures with different mixing ratios and dosages are compared with the supplemental water that is currently in use based on the corrosion of the systems and the observation on the corrosion of tested hanging plates with SEM is involved as well. The results show that it is not feasible for the water from BAF and MBR to be used as water supplement of circulation cooling system directly. The chloride ion is the main factor causing corrosion, so it is necessary to further reduce chloride ion concentration before recycle.The chloride ion concentration will be reduced by reverse osmosis technology to less than 0.5 u g/L, and the quantities of freshwater and corrosion inhibitors, as well as the expenses of waste water will be reduced if the water from reverse osmosis can be used for the water supplement of recirculating cooling system directly. The operation cost of cyclic cooling system in COFCO biochemical energy (Gongzhuling) Co., LTD,. is 2.38yuan/t. The operation cost of reverse osmosis technology as deep treatment technology of the cyclic cooling system is 2.05yuan/t if the operation cost of investment depreciation is not taken into account.