Water treatment using ultrafiltration (UF) membranes is gaining popularity world-wide, but the problem of membrane fouling needs to be addressed. We investigated the characteristics of membrane foulants by conducting two long-term filtration experiments using surface water from the Chitose River. The membrane was made of polyacrylonitrile and had a molecular weight cut-off of 100 kDa. The experiments were conducted in February 2004 Winter operation (Run 1) and October 2005 Summer operation (Run 2), when feed water characteristics were considerably different. Despite this, rates of physically irreversible fouling were similar. By measuring pure water permeability before and after chemical cleaning, we found that acidic or chelate solutions were most effective for cleaning the membrane from Run 1, whereas an alkaline solution was more effective for that from Run 2. Analysis of the chemical solutions that effective worked for canceling the fouling revealed that acidic cleaning in Run 1 extracted iron, carbohydrate and humic acid, while alkaline cleaning in Run 2 extracted carbohydrates. These results indicates that the iron, carbohydrates and humic acid caused the physically irreversible fouling in Run I, and carbohydrates were mainly responsible in Run 2. Based on the findings obtained in this study, it was suggested that the most important foulants and the most effective chemical cleaning agents may differ substantially between seasons. (C) 2014 Elsevier B.V. All rights reserved.