An　empirical　model　for　wastewater　treatment　by　biological　aerated　filters　(BAF)　was　developed　in　the　study,　which　relates　influent　soluble　chemical　oxygen　demand　(sCOD)　to　effluent　sCOD　and　medium　bed　height　in　order　to　simply　engineering　design　and　verify　experimentally　the　suitability　of　this　model　and　ascertain　the　relationship　between　the　constant　values　calculated　with　the　model　and　reactor　performance.　The　theoretical　model　was　based　on　design　of　contact　oxidation　process　and　conventional　biofilter　because　these　fixed-film　processes　show　similar　flow　pattern.　Firstly,　the　two　parallel　BAY　systems　packed　with　lava　medium　and　expanded　clay　medium,　respectively,　were　run　for　actual　domestic　wastewater　at　different　hydraulic　loading　rates　during　the　experiment.　A　great　deal　of　data　was　then　used　to　the　empirical　model,　which　was　based　on　a　first-order　reaction,　to　calculate　the　empirical　values　of　n　(medium　constant)　and　K　(overall　process　constant).　The　experimental　results　show　that　the　treatment　efficiency　of　lava　medium　exceed　to　that　of　expanded　clay　medium,　which　just　conform　to　what　the　model　predict,　the　higher　value　of　K　the　greater　efficiency　of　sCOD　removal,　and　the　lower　value　of　n　the　greater　variation　of　sCOD　removal　indicating　a　poor　resistance　to　shock　loading.　Next,　the　effect　of　suspended　solid　(SS)　on　model　was　estimated　by　simulated　domestic　wastewater　to　verify　its　feasibility.　(c)　2005　Elsevier　Ltd.　All　rights　reserved.