The company, which owns Oakleaf Grounds Services, said the awards recognised the hard work of staff.
"We have clear business plans and invest in training but this is far from a paper-gathering exercise," said Oakleaf head Keith Burgess, whose organisation tackles grounds maintenance for schools, colleges and other housing associations.
"Carrying out services safely and reducing impact on the environment can lead to cost savings and an improved corporate image."
CHG director of asset management Ian Hancock said the firm's diverse operations made the audit a complex process. The standards demonstrate a commitment to the environment and to safety for staff and customers.