Progressive freeze concentration for wastewater treatment: dissolved solids removal and waste volume reduction

反渗透和纳滤厂的二级工业废水（例如，炼油厂、纸浆和造纸厂的废水）和膜浓缩物（包括被膜、清洁溶液和反冲洗水阻挡的污染物）中的大多数污染物都是溶解态的，有些还含有有毒化合物。目前，大多数膜厂直接将浓缩物排入地表水或卫生下水道。需要采用多级预处理工艺进行深度处理以去除溶解固体，蒸发或焚烧往往是一些废水残渣/污泥体积减少和处理的唯一选择。与这些先进的处理方案相关的高操作成本和资本成本以及复杂的操作限制了它们的实际应用。冷冻浓缩由于其独特的去除溶解杂质的能力，在不添加任何化学品和预处理的情况下减少废物体积，还能回收纯净水流，在这些领域具有很大的应用潜力。与传统的悬浮结晶法不同，渐进冷冻浓缩法（PFC）需要简化操作系统来从进料水中分离冰晶，从而大大降低了工艺成本。然而，目前还没有对PFC中浓度效率的机理进行系统的研究，PFC工艺处理废水的工程设计资料也很少。本研究计划将进行实验室实验，以：(1)研究选定废水和膜浓缩物的冷冻特性及其对PFC的影响；(2)确定PFC的除杂效率、毒性降低和体积缩小能力；(3)研究机械搅拌、功率超声对除杂和减少废物体积的影响；(4)研究PFC对去除微生物的好处；(5)研究杂质在冰中的分布。该研究结果有望为PFC分离杂质的机理提供新的见解，并为PFC废水处理系统的更好设计提供新的思路。

Most contaminants in secondary industrial effluents (for example, effluents from petroleum refineries and pulp and paper mills) and membrane concentrate (which includes the contaminants blocked by the membranes, cleaning solutions and backwash water) from reverse osmosis and nanofiltration plants are in the dissolved form, some also contain toxic compounds. Currently, most membrane plants directly discharge the concentrate into surface water or sanitary sewers. Advanced treatment with multiple stage pretreatment processes is required to remove dissolved solids and evaporation or incineration is often the only choice for some wastewater residue/sludge volume reduction and disposal. High operating and capital cost and complex operations associated with these advanced treatment options limit their practical applications. Freeze concentration, due to its unique ability to remove dissolved impurities, reduce waste volume without addition of any chemicals and pretreatment, also recover a pure water stream, has great application potentials in these areas. Unlike the conventional suspension crystallization method, progressive freeze concentration (PFC) requires simplified operating system for separation of ice crystals from the feed water, thus can substantially reduce the process cost. However, no systematic investigation has yet been carried out on the mechanism of concentration efficiency in PFC and little information is available for engineering design of PFC processes for wastewater treatment. Laboratory experiments will be undertaken through this research program to: (1) investigate the freezing characteristics of selected wastewaters and membrane concentrate and their influence on PFC, (2) determine the impurity removal efficiency, toxicity reduction, and the capacity of volume reduction by PFC, (3) examine the effect of mechanical agitation, power ultrasound on impurity removal and waste volume reduction, (4) investigate the benefits of PFC on removal of microorganisms, and (5) study impurity distribution in ice. The results of this study are expected to provide new insight into the mechanisms of impurity separation by PFC and better design of the PFC systems for wastewater treatment.