The use of the “night soil treatment facility” is limited for collected night soil and septic tank sludge. The progress of populations using different night soil treatment systems since 1989 is shown in Fig. 1. The membrane separation technology for the night soil treatment plant was introduced in 1986. Today, the technology is commonly applied to night soil treatment and has been improved and widespread. In this paper, the application of the membrane technology to night soil treatment is outlined.
2.Membrane separation with high-loading nitrogen removal (two-stage membrane system)
2.1 Typical flow
The typical flow of membrane separation with high-loading nitrogen removal developed for night soil treatment is shown in Fig. 2. It consists of six unit processes, such as pretreatment facility, biological denitrification by membrane bioreactor (MBR), advanced treatment by membrane coagulation, activated carbon treatment, and disinfection. The two membrane units for MBR and coagulation are the heart of the system. A solid-liquid separation is performed using the membranes, thus called the two-stage membrane system. This system is currently ranked as the main technology of night soil treatment.
2.2 Types and functions of separation membrane applied to night soil treatment
The five classification of membrane separation modules employed for night soil treatment are tubular membrane module, flat sheet with circulation of liquid, rotating flat discs, submerged flat sheets, and hollow fibers. The operating condition of each type is shown in Table 1. Among them, the tubular membrane, rotating flat discs, and submerged flat sheets are specifically installed in a night soil treatment facility. The structures of these three modules are shown in Fig. 3. The outline of each module is described briefly below.
(1) Tubular Module
As the structure of a tubular membrane module is shown Fig. 3(a), 18 elements are set in one module. All the elements are connected in series by the worked groove in the flange at the end of an element. The inside diameter of an element is 14mm. The inner area is 2m3 per module. Membrane material is made from PolyAcryloNitrile (PAN) and element material is NORYL. The processes of biological treatment and coagulation use the same material. The molecular weight cut-off of the membrane unit for MBR is 20 000. The average pressure is 200-300 kPa. The flux is approximately 1m3/m2.d. On the other hand, the molecular weight cut-off of the membrane unit for coagulation is 40 000. The average pressure is approximately 250 kPa with the flux approximately 2m3/m2.d. For cleansing the units, sodium hypochlorite and citrate are often used at least once a month.
(2) Rotating flat discs membrane
The schematic diagram of a rotating flat discs membrane is shown in Fig. 3(b). Membrane discs with organic membrane (polysulfone series, molecular weight cut-off 750 000) attached on both sides of a rotating disc are installed at the hollow rotation stem. It is called “occlusion type”, in which a membrane disc of a next rotation stem is set out at the spacer between a membrane disc and a membrane disc of one rotation stem. Raw water is separated by the membrane surface. The treated water is discharged through the hollow rotation stem. Filtration pressure is given by the suction pump. In the membrane unit for MBR, the sludge concentration is 6 000-23 000mg/L, which is the wide range of MLSS concentration. The filtration pressure is 10 kPa and the flux is approximately 1m3/m2.d. The unit is cleaned using oxalic acid around twice a month. Similarly, the membrane unit for coagulation has a filtration pressure of 10 kPa and the flux is approximately 1m3/m2.d with a wide sludge concentration range of 6 000-23 000mg/L. The unit is cleaned with oxalic acid around twice a month, as well.
(3) Submerged flat sheets membrane
As the structure of the submerged flat sheets membrane is shown in Fig. 3(c), the sheet-like membrane of polyethylene resin, which is a non-woven fabric-based high-intensity resin, is installed on the ABS resin filter plate. Microfiltration membrane of the average pore size 0.4μm is used. For bio-sludge, the filtration pressure is 10-30 kPa and the flux is approximately 1m3/m2.d. The unit is cleaned using sodium hypochlorite twice or three times a month. On the other hand, in the membrane unit for coagulation, the filtration pressure is 10-30 kPa and the flux is approximately 1m3/m2.d also. Oxalic acid is used for washing the unit twice or three times a month. Since the greatest characteristic of the submerged flat sheets membrane is the still standing filtration at low operation pressure, energy consumption is low. Recently, adoption of this type of membrane has been increasing.
2.3 Operating performance of two-stage membrane system using tubular membrane
Water quality as a result of the two-stage membrane system using a tubular membrane is shown in Table 2. It is natural that SS removal by the bio-system membrane separation device should be 100%. However, due to the total effect between biotreatment and membrane separation, the following removal rates were achieved; BOD (99.5%), COD (approximately 95%), T-N (approximately 99%), and T-P (approximately 60%). For the membrane unit for coagulation, the removal rates of COD (approximately 70%), T-N (approximately 99%), and color (80%) were obtained.
3. Membrane separation technology for septic tank sludge (one-stage membrane system)
There are some specific flows on the membrane separation technology responding to septic tank sludge. The typical one is shown in Fig. 4. Characteristically, the treatment equipment gets compacted by integrating coagulation and sludge dehydration processes into the pretreatment process. Concurrently, the costs of construction and maintenance can be reduced using the one-stage membrane system. The operating performance of the one-stage membrane system for the septic tank sludge is shown in Table 3. As shown in Table 2, in case the rate of septic sludge is high (more than 60%), the water treatment quality of the one-stage membrane system is equivalent to the two-stage membrane system.