The flocculation and coagulation are frequently used in drinking water and wastewater treatment where flocculent systems for water purification are designed as large agitated tanks. The flocculent price represents about a half of operating water treatment costs. For this reason, the knowledge of flocculation mechanics is desirable. The optical methods are used to determine the crystal growth. There are basically two approaches to optical measurements of particle growth. The first one was initially developed for investigation of the crystal growth. In the paper, the results of the development of the non-invasive method based on the laser knife visualisation, digital camera recording and an image analysis have been presented.