The achievement rate of the environmental standard for water quality concerning nitrogen and phosphorus in lakes of Japan is low and nutrient enrichment is worsening. Nutrient enrichment brings not only deterioration of the ecosystem but also great damage in use of water for agriculture, fisheries, landscaping, scenic values, and water utilities. Remediation can be very difficult.
In Japanese domestic wastewater disposal systems, sewage systems are used in urban areas, whereas, in rural areas which have low population density, there is not a full sewage system, and devices named Johkasous are mostly used.
To prevent nutrient enrichment in public water areas, the removal of nutrients such as nitrogen and phosphorus as well as organic matter in domestic wastewater is needed. In 2002, a small-scale johkasou with advanced treatment processes (hereinafter referred to as "CRX") which can greatly reduce nitrogen and phosphorus was developed for practical use. In this paper, its structural function and properties are introduced.
2. Structure and function of small-scale johkasou with advanced treatment processes (CRX series joukasou)
A flow diagram of the CRX series johkasou is shown in Fig. 1. Schematic illustration of the CRX series johkasou (NUD 7) is shown in Fig. 2. Structure of the CRX series johkasou and functional explanation of each chamber is shown in Fig. 3. Additionally, the mechanism of nitrogen and phosphorus removal is also described below.
In the moving bed biofilm and biofilm filtration chamber shown in Fig. 3, a hollow, cylindrical media, which was developed and produced by Fujiclean Company, is adopted. Because this media (See Fig. 4) has a specific surface area more than 10 times as large as a conventional media, it can enhance the concentrations of attached microorganisms and promote degradation of organic substances such as BOD and nitrification of ammoniac nitrogen (in the aerobic chamber of the upper portion). Concurrently, in the filtration chamber of the lower portion, this media is used as a filter media.
Next, the mechanism of nitrogen removal will be described. Ammoniac nitrogen in sewage is transformed into oxidized nitrogen (NO2-N and NO3-N) under aerobic conditions in the aerobic chamber by activated nitrifying bacteria. Subsequently, nitrification liquid containing oxidized nitrogen is transferred into the first room of the anaerobic filter chamber by an airlift pump. In the anaerobic filter chamber the BOD in the inflowing sewage is used as a hydrogen donator. Oxidized nitrogen are denitrified by denitrifying bacteria and discharged into the air as harmless nitrogen gas.
The phosphorus removal mechanism employed for the CRX series johkasou is the iron electrolytic method. When two iron plates surged in the moving bed biofilm and biofilm filtration chamber are applied to a direct current, bivalent ferric iron (Fe2+) will be generated from the iron plate, which works as an anode. This bivalent ferric iron will react with oxygen (O2) dissolved in the water and be transformed to trivalent ferric iron (See Fig. 5 and Fig. 6). The trivalent ferric iron reacts with a phosphate ion (PO４３?) in the water and becomes insoluble phosphate iron (FePO４). Phosphorous compounds supplemented in the filtration chamber are transferred into the first room of the anaerobic filter chamber and stored by daily back washing and sludge transfer. Then, the stored phosphorous compounds outflow periodically as sludge. The “CRX series johkasou” was designed for not only periodical current dipole inversion but a long stable phosphorus removal with less maintenance in order not to form nonconductor or biofilm by the media blasting on the iron plates.
3. Properties of the CRX series johkasou
The main properties of the small-scale johkasou with advanced treatment processes (CRX series johkasou) are described below.
1) Performance; the quality of final effluent from the small-scale johkasou with advanced treatment processes is as follows: BOD≦10mg/L, T-N≦10mg/L, T-P≦1mg/L (Performance rating scale valued by The Building Center of Japan). High performance is expected.
2) Economic efficiency; sludge treatment process per unit quantity of water costs 1/2 less than a sewage system (effluent quality; BOD≦15mg/L) with a recent standard performance.
3) Immediate effectiveness; stabilization of quality and shortening of the work period are expected because the majority of small/medium-scale johkasous are manufactured in factories. The prompt improvement of water quality is expected because a johkasou for single-family housing is easily installed in a few days and can be instantly used.
4) Recirculation; no installation of a pipe line, which collects discharged water, or a relay pump is necessary because the treatment process is performed near the source of sewage drainage. Moreover, the treated water is discharged as pure treated water. Thus, sound water recirculation and the improvement of water quality are expected, and it becomes possible to prevent water sources from depletion.