We describe the transient-state, multiple-species biofilm model (TSMSBM), which is
a novel synthesis of key modeling features needed to describe multiple-species biofilms
that experience time-varying conditions, particularly including periodic detachment
by backwashing. The TSMSBM includes six features that are essential for describing
multiple-species biofilms that undergo changes over time: (1) four biomass types:
heterotrophs, ammonia oxidizers, nitrite oxidizers, and inert biomass; (2) seven chemical
species: input biodegradable organic material (BOM), NH4(+)-N, NO2(-)-N, NO3(-)-N,
utilization-associated products, biomass-associated products, and dissolved oxygen;
(3) eight reactions that describe the rates of consumption or production of the different
species, as well as the stoichiometric linkages among the rates; (4) reaction with
diffusion of all the soluble species in the biofilm; (5) growth, decay, detachment,
and flux of each biomass type by location in the biofilm; and (6) constant or periodic
detachment of biofilm, both of which allow for protection of biomass deep inside the
biofilm. The last two features of the TSMSBM provide novel additions to biofilm modeling,
and the synthesis of all features is a unique advancement. A series of examples illustrates
insights that the TSMSBM can provide about the transient development of multiple-species
biofilms; the roles of soluble microbial products and detachment in controlling the
distribution of biomass types and process performance; and how backwashing affects
the biofilm in drinking-water biofiltration.