19. A large sewage pool contains a certain amount of sewage, with sewage continuously flowing in. If 4 sewage treatment units are arranged, it will take 36 days to treat all the sewage in the pool; if 5 sewage treatment units are arranged, it will take 27 days to treat all the sewage in the pool; if 7 sewage treatment units are arranged, $\qquad$ days will be needed to treat all the sewage in the pool.
【Analysis】Assuming each sewage treatment device processes 1 unit of sewage per day, first find the rate of increase of sewage: $(36 \times 4 - 27 \times 5) \div (36-27) = 1$ (unit); then find the initial amount of sewage in the pool: $36 \times 4 - 1 \times 36 = 108$ (units); if 7 sewage treatment devices are arranged, one device can handle the 1 unit of sewage that increases daily, and the remaining (7 - 1 = 6) devices can handle the initial 108 units of sewage, which will take $108 \div 6 = 18$ days; solve accordingly.
【Solution】Solution: $(36 \times 4 - 27 \times 5) \div (36 - 27)$
$$
\begin{array}{l}
= 9 \div 8 \\
= 1 \text{ (unit) } \\
36 \times 4 - 1 \times 36 \\
= 144 - 36 \\
= 108 \text{ (units) } \\
108 \div (7 - 1) \\
= 108 \div 6 \\
= 18 \text{ (days) }
\end{array}
$$

Answer: It will take 18 days to process all the sewage in the pool.
Therefore, the answer is: 18.