The concentrations of phosphorus, nitrogen, and other nutritive elements in lakes, bays and inland seas are increased because domestic waste water, the second-step treated water from sewage treatment facilities, and the industrial waste water from some plants flow continuously into them. As a result, the waters are eutrophicated and the water quality is lowered, which results in unpalatable drinking water, red lakes, and various other problems. In typical cases, phosphorus acts as an accelerating substance for the growth of algae. Furthermore, phosphorus concentration is used as an indicator of the degree of eutrophication.<BR>Either or both of the nutritive elements, i.e., phosphorus and nitrogen, should be removed to prevent this eutrophication. It is difficult to remove nitrogen to a sufficiently low concentration by the ammonia stripping process or the activated sludge process. Therefore, prevention of eutriphication requires the effective removal of phosphorus.<BR>The methods for removing phosphorus include the chemical coagulation process, the adsorption process, the ion exchange process, the crystallization process, and the biological dephosphorization process. The coagulation process, which is most widely used, has problems as a result of the cost of chemical injection and the profuse generation of sludge. In the crystallization process and the biological dephosphorization process, on the other hand, there are many technical problems to be solved.<BR>Though recently an activated carbon adsorption process is often incorporated in the second-or the third-step liquid-waste treatment, there are very few studies on phosphate adsorption to activated carbon. This may be because activated carbon has been reported to be unsuitable for the adsorption of inorganic ions or lower alcohols. However, now that this process is utilized in practice, it is necessary to obtain detailed data on the properties of phosphate adsorption onto activated carbon.<BR>Basic studies on the adsorption treatment of phosphate-containing waste water have been carried out using adsorbents other than activated carbon, such as synthetic aluminium silicate, hydroxyapatite, and ion-exchange resin. These studies have shown that the adsorption of phosphate to such adsorbents proceeds through a chemical adsorption mechanisms.<BR>In the present study, the adsorption equilibrium and the adsorption rate of phosphate to activated carbon are examined to obtain basic data on the adsorption-removal treatment of phosphate-containing waste water by activated carbon. The phosphate concentrations were adjusted within the range of those in the first-or the second-step treated water. The adsorbates used are H₃PO₄, NaH₂PO₄, KH₂PO₄, and H₆P₄O₁₃, which have been reported to be contained in these types of treated water.<BR>In addition, the relationships of the pore size distribution of activated carbon to the amount of adsorbate adsorbed and the intraparticle diffusivity are clarified.