To determine the appropriate extent to which Veolia Environnement should utilize hybrid bonds (between 0% and 15% of total adjusted capital) over the next 18 months, we must evaluate the company's financial profile, strategic needs, and the prevailing market conditions for 2022.

**1. Capital Structure and Leverage Optimization (Rating Headroom)**
At the end of 2022, Veolia's equity stood at €14.86 billion and its adjusted net debt was around €19.35 billion, resulting in a Total Adjusted Capital (TAC) of roughly €34.2 billion. The company had approximately €3.49 billion in outstanding deeply subordinated securities (hybrids), representing about 10.2% of its TAC. 
Following the transformational acquisition of Suez—which significantly expanded Veolia's asset base to €73.3 billion and increased its debt—the company's rating headroom remains moderately constrained. However, with the acquisition now complete, its forward-looking acquisition needs over the next 18 months have transitioned to "moderate" as it focuses on integration. Refinancing needs are also moderate, with about €6.5 billion in current financial liabilities against a very healthy cash buffer of €9.0 billion.

**2. Cost of Hybrid Debt vs. Senior Debt**
Market data for 2022 indicates a massive spike in the cost of issuing hybrid bonds. The 5Y and 7Y swap curves jumped into positive territory (around +1.7% to +1.8%), and the sub-senior delta for non-financial investment-grade issuers surged to 2.295% (up from 1.298% in 2021). Consequently, issuing a new hybrid bond would cost substantially more than senior debt (over 2.3% premium). Given Veolia's relatively low average cost of debt (~2.2%), relying heavily on new hybrids would materially or slightly increase their overall cost of debt, rather than having a "marginal impact." 

**3. Recent Hybrid Activity**
During 2022, Veolia assumed €1.62 billion of hybrid debt from Suez but also chose to proactively repay €500 million of its own deeply subordinated securities without issuing new ones to replace them. This repayment demonstrates that Veolia is comfortable letting its hybrid-to-TAC ratio drift downward from the 10-11% range. 

**Conclusion**
While Veolia still benefits from the equity credit and leverage optimization that hybrids provide to preserve its solid investment-grade rating post-Suez, the prohibitively high sub-senior delta and transition toward moderate refinancing/M&A needs suggest that the company should target a somewhat lower, normalized hybrid bucket. Maintaining a hybrid proportion of 7.5% provides the necessary rating flexibility and leverage support while limiting the exposure to the current high cost of subordinated debt.

<answer>7.5%</answer>