For the record, blowers compress air to pressures intermediate between fans and compressors. They're used at wastewater treatment works to aerate activated sludge plants and aerobic digesters (the former are not shown on the hmi - presumably on another page to the 'left'?). The aerobic bacteria that eat the waste would otherwise use up the available dissolved oxygen very quickly and die. The process would then 'go septic' and be populated by anaerobic bacteria producing methane and hydrogen sulfide and various other gases that are foul-smelling, corrosive and/or explosive.
ie it's unlikely the blowers are there to 'disperse accumulated gas'.
I suspect the plant in question would be this one - http://g.co/maps/ybhvb
I'm actually pretty terrified about attacks on water infrastructure SCADA. Petrochem and Chemical plants are much more susceptible to going bang but equally have vastly more money thrown at them. For the most part water infrastructure is, once built, minimally funded until something finally breaks.