QUESTION: The Mishnah and Gemara discuss the laws of an Ukah (pit) and a Biv (pipe) with regard to water disposal, and they describe the ways in which one may discard waste water on Shabbos. How do these laws apply today with regard to pouring water on Shabbos into the drain of a sink, or using any other means of waste water disposal on Shabbos (such as flushing a toilet)?
ANSWER: The Mishnah differentiates between two different means of water disposal: (a) pouring water into one's Chatzer and letting it overflow into Reshus ha'Rabim (via the Ukah), and (b) pouring water into a pipe (Biv) which brings the water directly to Reshus ha'Rabim.
(a) THE UKAH. If the area of a Chatzer is 16 square Amos or larger, whether it is rectangular (long and narrow) or square in shape, one is permitted to pour out water into the Chatzer, because the Chatzer is large enough that the water will be absorbed into the ground before it flows into Reshus ha'Rabim. If the Chatzer is smaller than 16 square Amos and thus cannot absorb the full two Se'ah of water that a person tends to use every day, a pit must be dug to accommodate the remaining water.
This pit is necessary only during the summer season, when people prevent their Chatzeros from getting wet and muddy. A person does not want the waste water to stay in his Chatzer and make it muddy. He is pleased that the water flows out into Reshus ha'Rabim. Therefore, one may not pour water into the Chatzer without a pit to receive it. In the winter, however, one may pour into the Chatzer as much water as he wants, regardless of how small the Chatzer is, even without a pit to receive it (like the ruling of Abaye). He does not care whether the water stays in the Chatzer or flows out, because the Chatzer is already muddy from the winter rains.
(b) THE BIV. Rebbi Eliezer ben Yakov says that one may pour water directly into a Biv (pipe) as long as it is at least four Amos long. The Chachamim disagree and say that one may not pour water directly into a Biv, because the water will flow into Reshus ha'Rabim with such force that people will think that he is standing right next to Reshus ha'Rabim and pouring it directly into Reshus ha'Rabim (which, of course, is forbidden). The Chachamim maintain that one may pour the water only near the Biv but not directly into it.
RASHI (88a, DH Lo Yishpoch) writes that the Biv also must be broad enough to absorb two Se'ah of water (i.e., four Amos long by four Amos wide, and not just four Amos long with a narrow width.)
HALACHAH: The RAMBAM rules stringently, in accordance with the Chachamim, that one may pour water near the Biv but not into it. Others, however, rule leniently, and the MISHNAH BERURAH (OC 357:21) is lenient as well and permits pouring water directly into the Biv.
However, the SHULCHAN ARUCH (OC 357:2) rules (based on Rashi) that one may pour water into a pipe that goes into Reshus ha'Rabim only when the pipe is made from an absorbent material. According to his ruling, one should not pour water into a metal or plastic pipe (such as the ones used in modern residential drainage systems) that lead to Reshus ha'Rabim, regardless of whether he pours it directly into the pipe or only near the pipe. However, the Shulchan Aruch (OC 357:3) adds that it is forbidden only when the outlet of the water opens to Reshus ha'Rabim or to a large public street (Karmelis) in the city. If it opens into a valley, sea, or any other Karmelis outside the city, one is permitted to pour water into that pipe. Since most modern sewage systems lead to a Karmelis outside of the city, one is permitted to pour water down the drain (even directly into the drain) on Shabbos.