Phosphorus Recovery from Night Soil and Johkasou Sludge

Abstract

In Japan, sanitation for 26% of the population is covered by decentralized treatment facilities called Johkasou and night soil treatment plants (NSTPs). The former is installed to treat black water from small communities or individual households. Johkasou is a general term for compact on-site wastewater treatment unit and/or facility and is applicable to a population of several to several thousands, depending on the installation condition. The latter is installed to treat mainly night soil (human feces) coming from 6% out of the 26% population that uses decentralized treatment facilities. Since the sludge extracted from Johkasou is also treated in NSTPs, they play a key role in the Japanese sanitation system. As part of a social sustainability policy, the “Plan of Sludge Resource Recycling Treatment Center” (SRRTC) was enacted as a bylaw in 1997. It demands that NSTPs be furnished with facilities for resource and/or energy recovery from organic wastes including night soil and Johkasou sludge. Facilities implemented by this plan are categorized as sludge resource recycling treatment centers. This chapter describes the first SRRTC project in which a chemical precipitation process was applied to the recovery of phosphorus as calcium phosphates from night soil and Johkasou sludge.

Appendix: Computational Fluid Dynamics of the Crystallizer

Computational fluid analysis was carried out to visualize the water movement in the continuously stirred crystallizer.

1.1 Conditions

The conditions for the computer simulation are shown in Table 18.6, and the modeling of crystallizer is shown in Fig. 18.9b.

Table 18.6 Analysis conditions

Fig. 18.9

(a) Simulation model of the crystallizer and (b) simulated velocity distribution at 35 Hz (41.7 rpm)

1.2 Software

The software, SCRYU/Tetra (Software Cradle Co., Ltd.), was used for the fluid movement analysis. The steady-state simulation was carried out using a Reynolds average-type turbulence model RANS (Reynolds-averaged Navier-Stokes) simulation.

1.3 Result of Simulation

The simulation results of the velocity distribution in the crystallizer are shown in Fig. 18.9b. The particle settling velocity calculated by the Allen’s law is shown in Table 18.7.

Table 18.7 Particle settling velocity calculated by Allen law

In the crystallization zone, the vertical gyrate movement of 0.0–0.08 m/s was observed over the upper paddle and streamline directions were a mix of upward and downward. The synthesis velocity at the bottom of crystallization zone was more than 0.1 m/s, and the streamline direction was roughly from the center to the periphery. This means small particles less than 300 μm could be suspended in the upper paddle zone, while the possibility of particle accumulation on the bottom of crystallizer was small. There was some potentiality of accumulation of particles more than 600 μm on the bottom of the crystallizer. In the settling zone, the streamline direction was a mix of upward and downward. The upward stream of 0.01 m/s or higher was observed near the surface. This means small potentiality of washout of particles less than 100 μm.

Figure 18.10 shows the distribution of suspended solid (SS) in the crystallization zone and settling zone (Ministry of the Environment 1973). The concentration gradient of SS was clear in the settling zone. The SS concentration was only 4.4 mg/L at the surface layer of the settling zone. On the other hand, the vertical change in the concentration of SS in the crystallization zone was relatively small. These results suggested that a gravitational particle separation was possible while avoiding the influence of stirred mixture in the crystallizer.

Fig. 18.10

SS distribution of crystallizer at 35 Hz (41.7 rpm)