A large fraction of the chemical oxygen demand (COD) in municipal wastewater is associated with the particulate matter. The presence of these particles might impose a negative impact on the mass transfer in biofllm systems when they attach to the bulk-biofllm interface. We thus investigated the impact of real wastewater particles by combining optical coherence tomography (OCT) and oxygen microsensor measurements. The deposition of wastewater particles of different size classes onto the biofllm surface was captured in 3D by means of OCT in a lab-scale flume. The thickness of the particle layer was calculated from OCT images. The influence of the particle deposition with respect to oxygen mass transfer into the biofllm was monitored by measuring oxygen profiles before and after particle deposition. By employing OCT, the formation of the particle layer on the biofllm surface was monitored in situ and non-invasively over time. Results from oxygen microsensor measurements concluded 20-70% reduction of the dissolved oxygen flux from the biofllm surface into the biofllm matrix. This decrease in oxygen availability lowered substrate conversion in biofllms exposed to high loadings of organic wastewater particles.