Zn(II)　and　dialkyldimethyl　ammonium　compounds　(DADMAC)　are　common　heavy　metal　and　disinfectant　contaminants　in　wastewater　treatment　plants.　The　response　of　sulfur　autotrophic　denitrification　(SAD)　system　under　single　and　joint　stress　with　different　concentrations　of　Zn(II)　(1–25　mg/L)　and　DADMAC　(0.2–10　mg/L)　was　evaluated　using　sequencing　batch　fixed-bed　biofilm　reactors　in　this　study.　The　joint　stress　of　high　concentrations　of　Zn(II)　and　DADMAC　had　stronger　inhibitory　effects　on　microbial　activity　and　metabolic　function　genes　compared　to　the　single　stress,　thus　significantly　inhibiting　the　performance　of　the　SAD　system.　Furthermore,　Zn(II)　stress　contributed　to　the　proliferation　of　resistance　genes　(RGs)　in　water　(w-RGs),　whereas　stress　of　DADMAC　was　able　to　induce　the　proliferation　of　intracellular　RGs　(si-RGs)　and　extracellular　RGs　(se-RGs)　in　sludge.　And　under　the　joint　stress　of　Zn(II)　and　DADAMC,　the　abundances　of　si/se-RGs　were　much　higher　than　that　under　single　stress,　with　a　higher　risk　of　RGs　transmission.　The　α-Helix/(β-sheet　+　random　coil)　in　tightly　bound　extracellular　polymeric　substances　correlated　positively　with　si-tnpA-04　and　se-tnpA-04.　Horizontal　gene　transfer　via　mobile　genetic　elements　was　the　main　mode　of　RGs　transmission.　This　work　assessed　the　risk　of　DADAMC　and　Zn(II)　in　the　SAD　system　and　established　new　insights　into　the　transmission　of　three-fraction　RGs　under　DADMAC　and　Zn(II)　stress.　©　2023　Elsevier　B.V.