It depends on the region of the world. 
In India, the National Commission for Safai Karamcharis (NCSK) estimates a total of 631 people have died while cleaning sewers and septic tanks in the last 10 years.
In the United States, between 2007 and 2018, there has been 2 instances of children falling into a septic tank and dying due to unsecured tank lid.
From 1992 to 2002, there has been 61 worker deaths at sewage treatment plants as per the Census of Fatal Occupational Injuries All Worker Profile, 1999-2002 at
http://www.bls.gov/iif/oshwc/cfoi/cftb0186.pdf.
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