Sewage - Alarm
VersaLarm a good choice as it is small and not too ugly
$125
http://www.septicsolutions.net/store/alarms_floats.htm
http://www.septicsolutions.com/SepticTankAlarm/HighWaterAlarms.html
http://www.septicsolutions.com/PDFs/VersAlarm7032.pdf
http://www.alderonind.com/ii/7004ii.pdf
http://www.alderonind.com/products/7004.html
You will likely also want to fit a Pump Timer inside the enclosure (in series with the float switch).
In/Outdoor High Water Alarm w/ Pilot Light and Horn for Septic / Sump / Pond & Other Applications
http://www.amazon.com/Outdoor-Alarm-Septic-Sump-Applications/dp/B00GM9FD0A $97 + tax
Superior Pump Universal Sump & Sewage Pump Alarm System
http://www.amazon.com/High-or-Low-water-alarm/dp/B001B4VO36/ref=pd_cp_hi_2#customerReviews $43