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Potassium recovery from bamboo chip fuel residue after combustion was evaluated. The extraction conditions, which include the kinds of extract and the solid (weight)-liquid (volume) ratio, as well as the recovery method, which includes precipitation due to the difference in temperature after concentration by heating, were investigated.The condition for highly efficient potassium extraction was 1.0 mol L −1 HCl solution with a solid-liquid ratio of 1 : 10 and a potassium recovery ratio of 85.4 wt% from the extract. However, the most suitable extraction method, based on the reagent cost, was 0.01 mol L −1 HCl solution, which allowed a potassium extraction ratio of 65.5 wt%, using pulverized combustion residue.The amount of water-soluble potassium (W-K2O) was 53.7 wt% in the recovered matter, with the chemical form of KCl. Therefore, the potassium content of the recovered matter met the requirement for potassium chloride in fertilizers based on the Fertilizers Regulation Act of Japan. In addition, the amounts of harmful elements, such as arsenic, cadmium, chrome, lead and nickel, were within the permissible range in incinerated sludge fertilizer, based on the same law. Thus, it was concluded that the recovered matter was utilizable as a potassium fertilizer.

Sand capping using granulated coal ash was carried out for the entire surface of the Hosoioki borrow pit in Lake Nakaumi. The water quality in the borrow pit was measured, and the release rates of nutrients and hydrogen sulfide were determined by the benthic chamber method to estimate the effect of sand capping. NH 4 -N and PO 4 -P concentrations of overlying water at the center were reduced after the sand capping; however, the nutrient concentration still changed greatly in the summer. H 2 S concentrations differed locally, but a declining tendency of the concentration was confirmed in comparison with before sand capping. The mean release rates of NH 4 -N, PO 4 -P and H 2 S were 267 mgN m -2 day -1 , 35.0 mgP m -2 day -1 and 227 mgS m -2 day -1 , respectively. These rates were reduced by approximately 14%, 22% and 88%, respectively, compared with before sand capping. Since nutrients and H 2 S were contained in the pore water of sediment, their existence is the cause of decline in the release-suppressing effect. The nutrient-release-suppressing effect was low. On the other hand, the H 2 S-release-suppressing effect was high and continued for around 2 years under the condition of sedimentation on the granulated coal ash.