Small-scale wastewater treatment plants (SWTPs) are widely used as decentralized wastewater treatment systems in sparsely populated areas of Japan. Iron electrolysis, an electrocoagulation technology, is installed in these SWTPs for phosphorus removal. Phosphorus can be removed via the formation of an insoluble compound containing phosphate and iron, such as FePO; however, it was necessary to determine the conditions under which phosphorus can be effectively and stably removed in actual SWTPs. According to previous studies using iron compounds, improved phosphorus removal was obtained by Ca addition. It is therefore thought that calcium addition may also be effective in improving the phosphorus removal during iron electrolysis in SWTPs. It is also important to determine the chemical state of iron to understand the phosphorus removal mechanism during iron electrolysis. In this study, laboratory-scale batch experiments with the iron electrolysis method were conducted to investigate the effect of phosphorus removal using treated wastewater from actual SWTPs without or with Ca addition. The results indicated that the addition of Ca improved the phosphorus removal performance. Furthermore, phosphorus removal was inhibited in the presence of high dissolved organic carbon (DOC). The X-ray absorption fine structure measurements of the produced particulates in the experiments showed no substantial change in the chemical state of iron without or with Ca addition. The statistical analyses revealed the range of improving or inhibiting effects on phosphorus removal due to the Ca and DOC. Thus, the results of this study provided useful information pertaining to the influence of coexisting substances on phosphorus removal and the chemical state of iron in the produced particulates.
Small-scale wastewater treatment plants (SWTPs), called Johkasou, are widely used as decentralized and individual wastewater treatment systems in sparsely populated areas in Japan. Even in SWTPs, nutrients should be removed to control eutrophication. An iron electrolysis method is effective to remove phosphorus chemically in SWTPs. However, it is necessary to determine the precise conditions under which phosphorus can be effectively and stably removed in full scale SWTPs for a long period. Therefore, long-term phosphorus removal from SWTPs was investigated and optimum operational conditions for phosphorus removal by iron electrolysis were analyzed in this study. Efficient phosphorus removal can be achieved for a long time by adjusting the amount of iron against the actual population equivalent. The change of the recirculation ratio had no negative effect on overall phosphorus removal. Phosphorus release to the bulk phase was prevented by the accumulated iron, which was supplied by iron electrolysis, resulting in stable phosphorus removal. The effect of environmental load reduction due to phosphorus removal by iron electrolysis was greater than the cost of power consumption for iron electrolysis.
Phosphorus removal and collection from domestic wastewater will make a great contribution to conservation of water environment and saving phosphorus as a valuable resource. Iron electrolytic method, one of the practicalized phosphorus removal technologies, brings about increment in electrical consumption. In this study, energy-saving operation was examined on a small-scale anaerobic-oxic domestic wastewater treatment "Johkaso" system with iron electrolytic method. The obtained results were as follows. The increment in electrical consumption by introduction of the iron electrolytic system could be offset by turning off the blower for aeration in the oxic tank during the hours when there might be almost no wastewater inflow; 4 hours per day. Besides, further development for reduction of electrical consumption would be achieved by control of operation of the phosphorus removal device and the number of working blowers. From the point of wastewater treatment performance, there was little observed bad influence to the suspension of aeration in the oxic tank on the removal of phosphorus and BOD. In contrast, there observed slight decrease in both the nitrification activity and the nitrogen removal efficiency especially in winter. Further it may be required for the development of such as shortening of acclimation of the sludge in the treatment system.
In this study, the relationship between outflow of settled sludge and water velocity in a primary treatment tank of on-site wastewater systems �johkasou� was examined. In the experiment, settled sludge outflowed easily when the height of settled sludge increased to a certain value. Water velocity�simulated by computational fluid dynamics �CFD� �agreed well with the velocity measured by an acoustic Doppler velocimeter �ADV�. The water velocity on the surface of settled sludge was relatively fast when the sludge was outflowed, and affected on outflow�the total outflow quantity�of the settled sludge. To prevent outflow of settled sludge, the distance from the lower end of the inlet baffle to the surface of the settled sludge must be kept greater than 30 cm. Therefore, the results suggest that the sludge would be stored until its thickness increases to the difference between the water depth and 70 cm.
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