You can see on this slide the tremendous progress we have made as an organization over the past two decades, reducing our total recordable incident rate, or TRIR, by 74% since 2002.
As part of our continuous improvement approach to safety, we relaunched our highly successful Safety Starts with Me: Live It 3-6-5 program this year.
Adjusted EBITDA in Q1 was $129.5 million, which included $5.4 million in benefits from government assistance programs.
Our margin grew by an impressive 130 basis points to 16%, reflecting the benefits of our cost controls, along with productivity and pricing initiatives.
We also generated strong adjusted free cash flow of more than $62 million.
Adjusted EBITDA was down 4% from a year ago, mostly due to the lower revenue.
Even so, our margins were up 70 basis points and this was driven by a combination of pricing, cost savings, revenue mix and $4.5 million in government assistance.
Revenue from our COVID-19 decon work totaled $28 million in Q1, which was higher than we anticipated.
We've now completed well over 17,000 total COVID responses since the program began a little over a year ago.
In Q1, our incineration utilization dipped to 80%, entirely due to the lost days in February at both our Deer Park and El Dorado incinerator as a result of the storm.
At the same time, we continue to gather a lot of high-value waste streams and record number of drums, which pushed our average price up 8% from a year ago.
The adverse weather and strong volumes drove our deferred revenue to the highest level in our history at $83.2 million, providing us with a terrific backlog heading into Q2.
Landfill volumes were down 29% due to less projects, while average pricing rose 24%, which helped to offset a portion of that volume decline.
The pace of parts washer services picked up nicely in the quarter, growing 6% from Q4 to 235,000 services.
On a sequential basis, SKSS revenue increased 19% from Q4, fueled by stronger demand and higher pricing.
Adjusted EBITDA in this segment climbed 31% to go along with a 480 basis point margin improvement, which reflects the widening of our rerefining spread.
Government programs accounted for $800,000 of contribution in Q1 in this segment.
Waste oil collections were modest at 47 million gallons.
Given our Q1 results, we now expect $30 million to $40 million in COVID-related revenues for the full year 2021.
As expected, revenue declined 6% year-over-year, but was up sharply in March versus the same month last year.
Adjusted EBITDA grew 3% to $129.5 million.
Our cost reduction programs, productivity initiatives and mix of revenue, combined with $5.4 million of government program assistance, resulted in 130 basis points improvement in gross margin.
If you back out the government monies, our EBITDA margin improved a healthy 60 basis points.
Depreciation and amortization in Q1 declined to $72.2 million, in line with our expectations.
For 2021, we continue to anticipate depreciation and amortization in the range of $280 million to $290 million.
Income from operations increased by 12%, reflecting our cost controls and revenue mix in the quarter.
Cash and short-term marketable securities at March 31 were $570.7 million, flat with year-end and up more than $76 million from the same quarter a year ago.
Our debt was $1.56 billion at quarter end, with leverage on a net debt basis at 1.8 times.
Our weighted average cost of debt is 4.2% with no debt maturities until 2024.
Cash from operations in Q1 was extremely strong at $103 million.
capex, net of disposals, was down year-over-year at $40.7 million, which was slightly below our expectation, mainly due to timing of some projects.
The result of these two items, along with some other smaller factors, was a record Q1 adjusted free cash flow of $62.3 million.
For 2021, we continue to expect net capex in the range of $185 million to $205 million, which is higher than 2020.
During the quarter, we bought back 300,000 shares at a total cost of $26.5 million.
Of our $600 million authorization, we now just -- we now have just over $180 million remaining.
We now expect adjusted EBITDA in the range of $560 million to $600 million with a midpoint of $580 million.
Looking at guidance from a quarterly perspective, we expect Q2 adjusted EBITDA to be 15% to 20% above prior year levels based on the positive momentum we are experiencing in our business and the pandemic-related slowdown in Q2 a year ago.
At the midpoint of that range, our adjusted EBITDA would be 6% higher than Q2 of 2019.
However, these benefits will not offset the decline in high-margin decontamination work, and more significantly, the large contribution from government assistance programs in 2020 that totaled $35.6 million in this segment.
For Safety-Kleen Sustainability Solutions, we anticipate adjusted EBITDA to increase in the range of 50% to 60% from 2020.
These improvements in the SKSS segment in 2021 will more than offset the $3.7 million in government assistance that SKSS received in 2020.
For 2021, our adjusted EBITDA guidance now assumes receiving a total of $7 million to $9 million of government program assistance, primarily from Canada.
Based on our current EBITDA guidance and working capital assumptions, we now expect 2021 adjusted free cash flow in the range of $230 million to $270 million or a midpoint of $250 million.
Second, we expect to improve our adjusted EBITDA margins between 30 to 50 basis points annually, excluding extraordinary items such as government assistance programs.
Third, we are targeting more than $300 million of adjusted free cash flow by 2025.

Summary:
As expected, revenue declined 6% year-over-year, but was up sharply in March versus the same month last year.
We now expect adjusted EBITDA in the range of $560 million to $600 million with a midpoint of $580 million.
Looking at guidance from a quarterly perspective, we expect Q2 adjusted EBITDA to be 15% to 20% above prior year levels based on the positive momentum we are experiencing in our business and the pandemic-related slowdown in Q2 a year ago.
Based on our current EBITDA guidance and working capital assumptions, we now expect 2021 adjusted free cash flow in the range of $230 million to $270 million or a midpoint of $250 million.