We aim to establish a zero-emission-type phosphorus removal and recovery system using a Mg-Al-Cl form of hydrotalcite (HT), which is one of the inorganic ion exchanger compounds having a layered structure. In previous papers, it was confirmed that HT is a practical adsorbent having high selectivity and a large adsorbing capacity for phosphate ions. Phosphate ions adsorbed onto HT can be efficiently desorbed, thereby regenerating HT adsorption capacity, using a “two-step regeneration system”, which uses a NaOH-NaCl mixed aqueous solution as the desorbing solution and a MgCl₂ aqueous solution as the regenerating solution. Moreover, phosphate ions can be recovered as magnesium ammonium phosphate (MAP) from the desorbing solution in the system. In this paper, the conditions, such as optimum pH and the mixing molar ratios of PO₄^3-, Mg²⁺ and NH₄⁺ ions, for recovering phosphate ions in high yields as MAP from the desorbing solution were investigated. In addition, the effects the number of times the desorbing and regenerating solutions were used on phosphate recovery ratio and what regeneration ratio were studied. As a result, more than 99% of the phosphate ions were recovered as MAP under the conditions in which the mixing molar ratios of PO₄^3-, Mg²⁺ and NH₄⁺ ions were 1:3:5 at an initial pH of 12 (convergence pH 8.7). The desorbing and regenerating solutions can be used repeatedly. When using the desorbing and regenerating solutions five times, the phosphate recovery ratio was higher than 99%. Further more, the phosphate adsorption capacity of HT was maintained at 80% of the initial value. These results suggest that the establishment of the zero-emission-type system that removes and recovers phosphate from wastewater using HT progressed greatly.

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Historically the water quality of Lake Inawashiro has been kept very good in terms of COD and other variables. The reason for this is due to the very low concentration of phosphorus in the lake for example, 0.003mg · l^-1. Additionary, the primary production of phytoplankton is controlled at very low level. The authors studied the mechanism by which floc removes phosphorus that is produced in the acidic Nagase River. The major results obtained from this study are as follows : (1) The floc produced in the Nagase River coagulate and then settle as phosphorus sediment. (2) The floc has a particle size distribution that is effective for coagulation and then for phosphorus sedimentation. (3) The main components of the floc are Fe, Ca and Al. (4) When floc was added to the river water, the concentration of phosphorus decreased and the AGP^M also decreased.

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The high water quality of Lake Inawashiro is due to the low concentration of phosphorus, for example, 0.003mg · l^-1. In acidic Nagase River, a floc is produced which coagulates and forms as phosphorus sediment in Lake Inawashiro. We studied the mechanism by which the floc is produced. The major findings are as follows : (1) There are two processes involved in floc production in the Nagase River. (1) The first is the dissolution of Fe, Si, Ca, Al and other conpounds into the acidic water in the Iou River. (2) The second is the neutralization of the water by dilution, producing floc. (2) Iou River water was neutralized with an NaOH solution, then floc was precipitated. The resulting floc has almost identical chemical elements and particle size distribution as floc found at the mouth of the Nagase River. (3) Nine conglomerate samples from the Iou River were soaked in an aqueous solution of H₂SO₄, and the levels of chemical elements were measured. Then, these solutions were neutralized with a NaOH solution, artificially producing floc. This floc has almost the same composition of chemical elements and particle size distribution as floc found at the mouth of the Nagase River.

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Riparian wetlands, which form the transition zone between hillslopes and streams, are important landscape units controlling water chemistry in catchments. Concerning the forest-related nonpoint source pollution in Japan, however, there have been few studies in which the roles of riparian wetlands in catchment water chemistry were clarified. We examined the changes in riparian stream nitrate (NO₃-N) and dissolved organic carbon (DOC) concentrations in seven riparian wetlands during nonstorm periods in a forested headwater catchment. In addition, we discussed the contribution of surface soil waters to the changes in NO₃-N and DOC concentrations. The stream NO₃-N concentrations at the outlets of riparian wetlands were lower than those at the inlets; in contrast the stream DOC concentrations at the outlets were higher than those at the inlets. Among the riparian streams, the maximum changes in concentration per unit flow distance on a riparian wetland were 8- and 12-fold as high as the minimum changes in NO₃-N and DOC concentrations, respectively. We conclude that for NO₃-N, the differences in concentration change between riparian streams can result from the spatial variability of water permeability of surface soil; for DOC, the differences can be due to the spatial variability of both water permeability and soil water DOC concentration.

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Data on the water quality of about 500 effluent samples collected from septic tank (johkasou) systems being used at individual residences in Gunma prefecture were examined to determine the treatment performance of each system. The pollutant loads discharged from the johkasou systems were calculated using the water quality data. In addition, more effective means of wastewater management in an unsewered area where most of town's wastewater was mainly treated by tandoku-shori johkasou systems were discussed on the basis of the calculated pollutant load factor. The results of the water quality analysis showed that the treatment performance of the advanced gappei-shori johkasou (type 1-3) system was the highest of all the johkasou systems. Changing the tandoku-shori johkasou system to the type 1-3 system is recommended to prevent the eutrophication of a lake in the unsewered area.

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