6. Intangible Assets

The following tables set forth the costs, accumulated amortization and net book value of the Company’s separately identifiable intangible assets as of June 30, 2021, and September 30, 2020, and estimated future amortization expense.

The trade names are amortized on a straight – line basis over their respective estimated useful lives of between five and ten years. Intangible assets that represent customer relationships are amortized on the basis of estimated future undiscounted cash flows or using the straight – line basis over estimated remaining useful lives of five to ten years. Non-compete agreements are amortized based on a straight-line basis over the term of the respective non-compete agreements, which are typically five years in duration.

The amortization expense for intangible assets was $1,015 and $3,074, and $1,125 and $3,921 for three and nine-month periods ended June 30, 2021 and 2020, respectively.