The current design is great for a low cost tower and quick to build. But the discs can be used to water more plants. So a tower design with stackable tiers would allow to grow more plants.

Here is the evolution of an individual tower but for building roofs we can still optimise

Farm on a building

The concept is a main reservoir that manages water quality (ph, nutrients, temperature). It is located high above the tower and allows the smaller tanks to be filled by gravity.

Then there is an indeterminate number of ultratowers depending on the space available on a roof or other.

The PCB of the towers only needs to manage the activation of the disks, the water level of their small tank and a valve to fill it and stop the filling.

According to my calculation the cost for a PCB could be around 8~10€ for orders of 100 pieces. 

There would be a single pump that brings the water from the bottom of the towers up into the large reservoir to close the loop.

This farm concept would optimise material costs, running costs and electricity consumption.