The aim of this special edition ?Technological trends with the small-scale domestic johkasou
                                                        (johkasou :On-site waste water treatment system)?
Fumihiko Jiku

Ryukoku University

A johkasou has been installed as equipment to dispose sewage from a flush toilet of each house in areas with no sewage systems. The processing methods employed in earlier johkasous were to degrade sewage in a septic tank without electricity and to conduct water-spray on filter bed media.
Subsequently, the activated sludge process which increases aerating microorganisms and the contact aeration process to dispose sewage have been developed. There are many small-scale johkasous for domestic use. However, there are also large scale disposal facilities at residential complexes, which are used by thousands of people. Furthermore, because of the recent increase in water pollution of bodies of water, the disposal of not only excreta disposal but also miscellaneous drainage (except human waste), which occupies 2/3 of the load of domestic effluent, has become important. In 2000, the use of the tandoku-shori johkasou, which treats only human waste, was banned and the use of domestic wastewater treatment systems to dispose of human waste and domestic effluent was allowed. Johkasous were positioned as temporary treatment before the sewage systems were laid. Subsequently, johkasous were used as equipment for water treatment in the environment, similarly to sewage systems.
The contact aeration process is currently used as a treatment technology for domestic wastewater treatment systems. This is a kind of biofilm method, where membranes of mirobes are absorbed to moving beds made of plastic. Organic matter is then biodegraded by the microbes. This contact aeration process has been developed as a technology for johkasous. Also, as a technology designed to remove nitrogen, the aeration contact filter process has been developed recently. This is an innovative method that fills contact filtration media in the sedimentation tank, and then expels disposed, denitrified water.
Based on the disposal technology of similar johkasous, the technologies of various jouhkasous have recently been developed: the membrane separation process, a phosphorus and nitrogen removal method, and a disposer model. Moreover, the development of small scale johkasous has also been advanced. In this series, recent technological advances in domestic small scale johkasous (Gappei-shori johkasou) are described.
The moving bed biofilm method and biofilm filtration process are employed as new processing methods for compact type johkasous. These processing methods can increase bio-retention by enlarging the surface area of media, and resulting treatment efficiency is high. However, because retention volume is large, sludge must be transferred to the storage tank by backwashing regularly.  
Also, as a general johkasou challenge, when a large amount of effluent wastewater from a bath is discharged at a time, low quality wastewater, which is not sufficiently disposed, may also be discharged. However, many compact type johkasous have a structure that allows the whole anaerobic filter chamber to respond to flow equalization, thus remediating the bio shock load problem.
In johkasous using membranes, an innovative processing method is employed which overcomes shortfalls with the activated sludge process. In the activated sludge process, when sludge which was not sufficiently deposited is generated, it has been known that the water quality of disposed water degenerates. In johkasous using membranes, because water is obtained by transporting sludge across the membrane from the activated sludge aeration tank without using the sedimentation tank, pure disposed water is always obtained, regardless of sludge property. This indicates that technical improvements were made concerning membrane performance, cleaning procedure, and costs, and are now put into practice.
Nitrogen and phosphorus removal johkasous are used to remove nitrogen and phosphorus in order to prevent lakes and areas around inner bays from eutrophication. Regarding nitrogen disposal, nitrogen is removed by circulating disposed water in the anaerobic filter chamber. Furthermore, regarding phosphorus, a new method which removes phosphorous by transforming it into phosphorus iron has been developed. Nitrogen can be discharged into the atmosphere as nitrogen gas, while phosphorus can not be released to the atmosphere, thus delaying the development of phosphorus removal technology.
The disposer type johkasou is a type of johkasou which disposes food waste from kitchens. Influent load is significantly larger than in past johkasous. Highly efficient processing methods such as biofilm filtration tanks are employed in the disposer type johkasou. This model is designed to solve problems such as blockage of foreign materials at the influent area and calculate feasible volume for sludge load.
The johkasous before Gappei-shori johkasou were used as supplemental equipment of sewage systems. Because gappei-shori johkasous with high performance can perform as effectively as a sewage system and play a role in environmental preservation, there are some areas where the johkasous can be used instead of sewage systems.
Sewage systems are necessary as basic facilities of an urban region, however, when sewage systems are laid, water disappears from above-ground, and moisture disappears. This destroys habitat for aquatic organisms. In addition, it is possible that this effect of water not contacting with the soil is also increasing the levels of COD, as in the case of Lake Biwa.
Solving these problems is crucial, along with increasing the use and availability of sewage systems.  
For this purpose, johkasous are used as on-site equipment dispersed in an area. Because water can be returned to local waterways and small rivers, maintaining a high quality water environment is possible. In addition, an effect of water improvement is that a slight amount of contaminated source is also anticipated.
From a financial viewpoint, an enormous expense is necessary because sewage needs to lay culverts underground. These expenses are covered by taxes. It is thought that the expense of sewage systems built by loan flotation could become a significant financial burden for small cities, towns and villages in the future. On the other hand, because johkasous are largely paid for by the individual, administrative financial costs can be reduced.
Given that domestic gappei-shori johkasous were installed with funds used for sewage construction of Shiga Prefecture, a trial calculation done indicated that the cost would exceed the available funds of present Shiga Prefecture. The sewage coverage rate of Shiga Prefecture is presently around 80%, but there is still 20% that is not yet covered. These areas are so-called "neighboring areas", and construction expenses are extremely high. If a problem of home drainage processing is solved by a domestic gappei-shori johkasou, agricultural drainage measures and initial rainwater measures, which are necessary for anticontamination of Lake Biwa can be undertaken. Thus, this example shows that johkasous can be cost effective in certain situations.

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1 Domestic wastewater treatment facility(Small-scale Gappei-shori johkasou)