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Abstract: In the recent years, there has been an increasing interest in the development of hybrid reactors, especially in the up-grading of existing activated sludge plants that are no longer able to comply with concentration limits established by regulatory agencies. In such systems the biomass grows both as suspended flocs and as biofilm. In this way, it is possible to obtain a higher biomass concentration in the reactor, but without any significant increase of the load to the final clarifier. The paper presents the setting-up of a dynamic mathematical model aimed at quantitatively describing the biokinetic processes occurring in a hybrid moving bed biofilm reactor (HMBBR), and, more in general, in integrated fixed-film activated sludge (IFAS) processes, as well as to compare the simulation results with measured data from a HMBBR pilot plant built at the Norwegian University of Science and Technology in Trondheim (Norway). Particularly, the pilot plant consisted of three aerobic tanks in series; the first and third aerobic reactors were pure suspended biomass systems, while the second aerobic reactor was filled with the AnoxKaldnes™ K1 carriers for biofilm development. The mathematical model consists of two connected models for the simulation of both suspended biomass and biofilm. Biochemical conversions are evaluated according to the well known matrix notation used in the Activated Sludge Model No. 1 (ASM1) for both attached and suspended biomass and, in addition to biochemical conversion, the model contains the simulation of particulate detachment from the biofilm into the bulk liquid. The results showed an overall good agreement between measured and simulated data, for both biofilm and suspended biomass, with a good reproduction of dynamic processes in the hybrid moving bed pilot plant, and they are encouraging for further developments. [Copyright &y& Elsevier]

Graphical abstract Highlights • The effect of a co-substrate addition in TPH removal was studied in a MBR. • A novel hydrocarbons extraction method from complex mixture was developed. • A lower co-substrate supply enabled approximately 98% of biological TPH removal. • Bacteria able to grow on hydrocarbons as primary substrate were selected. • The membrane fouling increased under lower co-substrate supply. Abstract The paper reports the main results of an experiment carried out on a membrane bioreactor (MBR) plant designed for the treatment of shipboard slops. With a view of a co-treatment process of the slop with other wastewaters, sodium acetate, as external co-substrate, was supplied (high dosage – Period 1, low dosage – Period 2) to evaluate its effects on hydrocarbons removal. The MBR pilot plant enabled approximately 99% of total petroleum hydrocarbon (TPH) removal during the entire experiment, confirming the robustness of the MBR technology for the treatment of slops. The chromatography/mass spectrometry analysis showed that the removal efficiency for each alkane was close to the value observed for total mixture removal (>99%) and the hydrocarbons removal was mostly due to the microorganism-mediated biodegradation. The biological contribution to TPH removal increased from approximately 85% to 98% when the co-substrate was decreased. Biomass kinetics revealed that a lower co-substrate dosage enhanced the growth of bacterial groups able to use hydrocarbons as primary substrate. A clear predominance of Microthrix Parvicella under low co-substrate dosage was observed. However, the lower co-substrate addition caused a significant worsening in the physical properties of the activated sludge, which resulted enriched in soluble exopolymers (>70%), more hydrophobic (>90%) and with small and dispersed flocs (<30 μm). Consequently, the membrane permeability reduced because of the irreversible fouling increase. [ABSTRACT FROM AUTHOR]