舟山近海海域养殖水体悬浮物沉降特性试验研究

了解海水中悬浮物颗粒的静沉降特性,对于海水工厂化养殖的水源处理具有重要指导意义。该研究以舟山长峙岛海域为例,通过2019年7-9月进行的静沉降试验,分析总结水体中悬浮物浓度和粒径分布随时间的变化规律,采用斯托克斯沉速公式和重复深度吸管法计算悬浮物颗粒的沉降速率。结果表明:1)在悬浮物静沉降特性试验中,沉降装置直径的小幅度变化对沉降结果的影响基本可以忽略;2)舟山近海海域表层海水悬浮物的粒径分布属于粉砂范畴,很难通过静沉降的方式完全除去;3)沉降初期,以大颗粒悬浮物沉降为主,沉降速率大;沉降中后期,以小颗粒悬浮物絮凝沉降为主,沉降速率小;4)通过重复深度吸管法计算得到舟山近海海域悬浮物沉降速率范围为0.001~0.01cm/s,并通过非线性拟合,得到悬浮物平均沉降速率与悬浮物浓度之间的Logistic曲线关系。研究结果可以为舟山近海海域工厂化水产养殖用水悬浮物去除提供数据支撑。

Experimental study on settling characteristics of suspended solids in seawater of nearshore in Zhoushan of China

The industrial aquaculture has been widely applied all around the word for its obvious benefits of eco-friendly,water-saving and good welfare conditions due to the excellent use of technology to manage aquaculture environments effectively.Water quality management,mainly composed of sedimentation,filtration,biological purification,oxygenation,temperature regulation and sterilization,is of vital importance to the aquaculture.Generally,sedimentation can be applied to deal with the water before the influence into the aquaculture tank and that after the effluent from the aquaculture tank.Most of the previous studies focused on the settling characteristics of suspended solid in the waste water from the aquaculture tank.However,few attentions are paid to that in the source of water,resulting in limited knowledge on the determination of the appropriate time required for solid sedimentation.In this study,settling characteristics and size distribution of the SS(suspended solid)in the nearshore seawater of Changzhi island,Zhoushan have been investigated by carrying out four hydrostatic settling experiments.In addition,settling apparatus with different diameters have been developed to examine the effect of apparatus diameter on the settling characteristics.Stokes equation and Mclaughlin method have been applied to calculate the settling velocity in the fourth hydrostatic settling experiments.The results show that the accumulating particle size D90(the accumulating particle volume reaches to 90%)of suspended solids in the seawater of Changzhi island is around 45.800μm,indicating that the suspended solids are mainly composed of silts.The mass concentration of suspended solids increases sharply with time in the first 22 min due to the quick settlement of large particles,and recoveries to the initial level after 40 min.And it continues to decreases due to the flocculating settling of small particles and reaches the level less than 40%of that in the initial samples 6-9 h later.During the final stage,it decreases slightly,stabilizing at the level of 50-60 mg/L after 12 h.Thus,the settling characteristics demonstrates that it is difficult to completely remove the suspended solids in seawater by increasing settling time and other water management strategy also should be employed.The comparisons of settling characteristics among three purpose-designed settling apparatus show that the effect of apparatus diameter can be ignored in this study.As for the settling velocity,it reaches the maximum value of near 0.01 cm/s in the early stage because of the similar reason that the large particles settle first as in the concentration analysis.In the later stage,the settling velocity decreases rapidly and becomes less than 0.001 cm/s after 6 h.Moreover,the settling velocity and concentration has been fitted in a Logistic model and the fitting equation has been obtained.The results can enrich the understanding of the settling characteristics in the source water for industrial aquaculture in Zhoushan island and provide scientific guidance on how to set up appropriate setting time for source water in settling tank.