In a dialysis machine, wastes are removed from blood plasma by the process of

The answer is diffusion. Since the patient’s blood is high is waste products such as urea, the waste products diffuse into the dialyzing solution used in the dialysis machine. To remove excess water and sodium ions from the blood, however, the dialysis machine utilizes ultrafiltration process. This is achieved by lowering the hydrostatic pressure of the dialyzing solution against that of the patient's blood plasma.

I think it is by the process of diffusion. Blood contains particles of many different size and types, including cells, proteins, dissolved ions, and organic waste products. Some of these particles, such as proteins like hemoglobin, are essential for the body. Others such as urea must be removed from the blood or they will accumulate and interfere with normal metabolic process. The dialysis machine does the work of cleaning the blood by replicating the work of the kidney.