NOVELTY - A filter uses activated granular coal obtained as follows. Low-grade coal unsuitable for use as fuel, is oxidation-treated and ash content is removed. The granular coal has a fine porous structure and an average particle diameter of 5 mm or more. The ratio of silicon and carbon in component element is regulated to 1:7-1:13, preferably 1:10.    USE - Filter medium is used in water-purification facilities for purifying eutrophic sewage caused by nitrogen and phosphorus, e.g. seawater (both claimed).    ADVANTAGE - The filter medium is prepared using economical low-grade coal, and efficiently removes offensive odor and heavy metals.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) aeration treatment system required for water-purification facility of eutrophic sewage, consisting of contact-type aeration structure by the treated-water fall using elevation difference;    (2) sludge-disposal method, which involves mixing sludge containing heavy metal and having a moisture content of 80% or more, with fine powder of solid, such as activated coal containing element having ion-exchange capability in non-dehydrated state, detoxifying heavy metal, injecting microorganisms suitable for decomposing sludge, and decomposing; and    (3) recycle material, which is a solid containing element(s) having a property which enables ion-exchange coupling with heavy metal and having an average particle diameter of 5 mm or less, preferably 1.5 mm or less, in fine-powder form. The ion-exchange material(s) is injected into sludge in required amount computed from the total amount of sludge, and a correlation with concentration of heavy metal. Heavy metal (50% or more, preferably 95% or more) is coupled with the ion-exchange material. The contents are mixed such that the heavy-metal concentration is below legal standard value. A preset moisture-absorption material and a water-retaining material are injected and mixed, with a moldability of grade which can maintain a defined shape. The seed of plant(s) is mixed with the microorganisms and molded into preset shape. The material is put in a container as greening material.