In reverse osmosis, the hydraulic pressure and osmotic pressure at the feed side of the membrane are $P_1$ and $\pi_1$, respectively. The corresponding values are $P_2$ and $\pi_2$ at the permeate side. The membrane, feed, and permeate are at the same temperature. For equilibrium to prevail, the general criterion that should be satisfied is
1. $\pi_1 = \pi_2$
2. $P_1 = P_2$
3. $P_1+\pi_1 = P_2+\pi_2$
4. $P_1 – \pi_1 = P_2-\pi_2$