Laboratory tests were conducted to investigate the performance of the "Niimi Process" or capillary siphon trench, a soil-based wastewater disposal system common in Japan. The system is similar to conventional septic tank leaching field trenches except that the trenches are filled with capillary sand instead of gravel and the lower quarter of the trench is lined with an impermeable membrane. The capillary sand and impermeable membrane are used to induce the unsaturated flow of wastewater through the upper soil profile, or zone of aeration, where maximum biological activity occurs. According to Japanese scientists, this results in improved organics and nutrient removal.

The Niimi Process was evaluated by constructing a laboratory scale Niimi Process trench and a conventional soil absorption trench. Synthetic wastewater characteristic of domestic sewage was then applied to the two trenches in two 14-week experiments. No statistically significant differences were found between the operation of the two systems for the soils and synthetic wastes, except for NO3-N and Total-N removal during the second experiment where the Niimi trench was slightly more effective. Both reactors removed 99 percent of the applied COD, 95 to 00 percent of the applied phosphorous, and 18 to 31 percent of the total nitrogen. The hypothesis that the Niimi trench was superior to the conventional trench was not demonstrated.