废水悬浮物测定中负值现象的研究

Study on negative value phenomena of suspended solid in waste water

测定清洁的地面水、医疗废水、生活污水的悬浮物浓度时,根据GB 11901-1989《水质悬浮物的测定重量法》中"滤膜准备"的操作方法,在没有蒸馏水润湿、抽滤的情况下,获得滤膜的恒重值,与进一步抽滤后的滤膜恒重值相比,要明显偏大3.0mg～3.8mg,即采用此方法测量悬浮物时,会引入3.0mg～3.8mg的负误差,不仅影响测定结果的准确性,而且对于较清洁水样,还会出现负值现象。分析研究了微孔滤膜的前处理、实验室分析过程,找到了出现负值的原因,提出了改进"滤膜准备"操作步骤的最佳方法,即:滤膜经蒸馏水浸泡24h后,再用蒸馏水抽滤一次,经过反复烘干、冷却、称量,可达到两次恒质量差值≤0.2mg的要求,从而获得滤膜的真实空白值。此方法既解决了清洁水样悬浮物测定的负值现象,又更准确地反映了水样中悬浮物的实际浓度。

According to the operation method of Membrane Preparation in GB 11901-1989 Water Quality Determination by Suspended Solid Weight-Measure Methods,we determine the suspended solid concentration of clean surface water,medical waste water and domestic sewage.Under the circumstance of no distilled water wetting and no pump filtration,the constant weight value of membrane is obtained,and further compared with constant weight value after pump filtration,which is found to be apparently bigger 3.0 mg^3.8 mg than the original value,that means using this method to measure suspended solid will introduce 3.0 mg~ 3.8 mg of negative error,which will not only affects the accuracy of determination results,but also lead to negative phenomenon for the clean water sample.Pretreatment and laboratory analysis of the micro porous membrane are studied and the reason for negative value is found out.Furthermore,the best method to improve the operation is suggested.After membrane is immersed in distilled water for 24 h and suction filtrated by distilled water once again,repeated drying,cooling and weighing,which can achieve the requirements of two constant weight differences less than 0.2 mg,so as to obtain the real blank membrane filter.This method can resolve the negative phenomenon in determination of suspended solid in clean water sample,and reflects more accurately the actual concentration of the suspended solid in water.