# How to avoid negative concentration from numerical solution using FDM scheme?

$$\frac{\partial C}{\partial t} + u \frac{\partial C}{\partial x} + w \frac{\partial C}{\partial x} = D \left(\frac{\partial^2C}{\partial x^2}+\frac{\partial^2C}{\partial y^2}\right)-C \cdot \left(\frac{\partial u}{\partial x}+\frac{\partial w}{\partial z}\right)$$

How to avoid the negative concentration using FDM scheme?