The White Lumps in Odaiba and Rainy-Day Sewers

At Odaiba Seaside Park in Minato Ward, Tokyo, reports have surfaced since the late 1990s regarding a problem with whitish lumps washing up on the sandy beach. Although these are called “oil balls,” the Tokyo Metropolitan Government identified the primary cause not as oil spilled at sea, but as grease and oil discharged into the sewer system from households and other sources. The original article explains that substances derived from oil and soap form these clumps, which are washed out after it rains. The key to this mechanism is the “combined sewer system.” In many areas of Tokyo, domestic wastewater and stormwater are transported through the same pipes. Normally, the wastewater can be sent to treatment facilities, but when heavy rain increases the flow, some of it is discharged into rivers or the ocean. Clumps of grease adhering to the inside of the pipes can also be dislodged by stormwater and washed out. The oil clumps washing up on Odaiba provided a visible demonstration of the connection between oil poured down kitchen drains and how the sewer system functions on rainy days. Subsequently, the Tokyo Metropolitan Government installed devices at stormwater outfalls to prevent debris and oil clumps from flowing out, and also built facilities to temporarily store contaminated sewage at the onset of rainfall. At the same time, the government is urging households and restaurants not to pour oil down the drain. Wiping off oil from dishes and pots before washing them is also one measure to reduce pipe blockages and runoff into waterways.