A kinetic model for mixed phototrophic biofilms is introduced, which focuses on the interations between photoautotrophic, heterotrophic, and chemoautotrophic (nitrifying) functional microbial groups. Biofilm-specific phenomena are taken into account, such as extra-cellular polymeric substances (EPS) production by phototrophs as well as gradients of substrates and light in the biofilm. Acid-base equilibria, in particular carbon speciation, are explicitly accounted for, allowing for the determination of pH profiles accounted for, allowing for the determination of pH profiles across the biofilm. Further to previous models reported in literature, the PHOBIA model combines a number of kinetic mechanisms specific to phototrophic microbial communities, such as internal polyglucose storage under dynmaic light conditions, phototrophic growth in the darkness using internally stored reserves, photoadaptation and photoinhibition, preference for ammonia over nitrate as N-source and the ability to utilize bicarbonate as a carbon source in the absence of CO2. The sensitivity of the PHOBIA model to a number of key parameters is analyzed. An example on the potential use of phototrophic biofilms in wastewater polishing is discussed, where their performance is compared with conventional algal ponds. The PHOBIA model is presented in a manner that is compatible with other reference models in the area of water treatment. Its current version forms a theoretical base which is readily extendable once further experimental observations become available.