ESD Grounding Straps with No Chassis Ground
With ESD the current discharge pathway from the ESD event needs to be controlled. On connections to the board TVS diodes could be used to shunt the current away from the board. 





Source: https://www.sunrom.com/p/tvs-diode-12v-smbj12ca

The best way to handle ESD with a DC connection is to have another ground or chassis ground or shield ground that can take the current from any of the inputs and shunt it to mains ground (since it appears this device can be plugged in). It is not ideal to have ESD shunted on the negative (or ground) side of the DC connection because the inductance of the wire nH's to mH's can cause common mode voltages to make the entire ground voltage rise and cause problems for electronics. 

So make sure you have TVS diodes (or diodes) on connections to the outside world from your PCB and especially on connections to a microprocessor. Another thing to try is put a conductor on the switches/pump-inlets that runs to mains ground.