BACKGROUND: This study aimed to investigate the synthesis of zeolites from paper sludge ash (PSA) with added diatomite to remove both NH4+ and PO43- for water purification. The PSA had low Si and significant Ca contents. Four types of diatomite: white (T-W) and brown (T-B) from deposits of marine origin in Takanosu, and white (S-W) and gray (S-G) from lacustrine deposits in Shonai, were added to NaOH solution to increase the Si content and thereby synthesize zeolites with high cation exchange capacity (CEC). RESULTS: The order of the amounts of Si extracted from the diatomite to the alkali solution was S-W > T-W = T-B > S-G, which correlates with the amorphous SiO2 content of diatomite. The original ash without addition of diatomite yielded hydroxysodalite with CEC of about 1.0mmol g(-1). For all samples, the addition of diatomite to the solution yielded zeolite-P with a higher CEC, but the addition of excess Si inhibited the synthesis of zeolite-P, and the CEC of the product was low. A product with high CEC including zeolite-P was obtained in a solution with around 500 mmol L-1 of Si concentration, and had the ability to remove both NH4+ and PO43-. CONCLUSION: Diatomite has the potential for used as an additive for the synthesis of high CEC zeolite from PSA. The product with zeolite-P exhibited relatively high CEC, capacity for NH4+ uptake, and the ability to remove PO43- by precipitation, which is preferable for water purification applications. (c) 2008 Society of Chemical Industry.