Every biological treatment process in wastewater engineering can be understood as a redox reaction where some reduced substrate donates electrons and some oxidized species accepts them, and matching this makes the pattern easy to check quickly. Nitrification is an aerobic autotrophic process: the bacteria gain energy by stripping electrons off ammonium (the donor, since ammonium is in a reduced nitrogen state) and handing them to dissolved oxygen (the acceptor), which is exactly what option A describes. Denitrification flips the acceptor: under low-oxygen (anoxic) conditions, heterotrophic bacteria instead use nitrate as a substitute final electron acceptor, oxidizing organic carbon (the donor) in the process and releasing nitrogen gas, matching option B precisely. Sulfate reduction is where the trap lies: option C claims oxygen is the electron donor for this process, but sulfate-reducing bacteria are obligate anaerobes for whom oxygen is toxic and never appears as a donor; the real donor is organic matter or hydrogen, with sulfate itself as the acceptor, so option C is wrong. Methanogenesis, the final step of anaerobic digestion, has hydrogenotrophic archaea using hydrogen gas as the donor and carbon dioxide as the acceptor to produce methane, matching option D exactly. Filtering out only the true statements leaves A, B and D as the correctly matched pairs.
\[\boxed{\text{Correct options: A, B, D}}\]