There is a little research related to the interactions between activated sludge flocs and biofilm e. g. migration of the organisms. These interactions are complex, and both relations: between flocs and biofilm, and between heterotrophs and autotrophs in the biofilm should be considered in modeling [15] and operation. Albizuri et al. [15] assumed that these interactions could act with mediation of colloidal components. It is well know fact that there are many grazing species (e. g. Ciliata) which creep on the flocs/biofilm surface or swim near the flocs and biofilm surface. on the other hand there is some number of species existing in deeper layers of biofilm, which probably can not migrate. It is worth to note that the structure of activated sludge flocs is heterogeneous and deep layers of biomass in flocs are anaerobic, what results in different species composition (anaerobic bacteria).

The biological composition of flocs in hybrid bioreactors is similar to typical biological content of activated sludge flocs. Similarly the size of hybrid bioreactor flocs in hybrid reactors is close to typical activated sludge flocs — diameter in the range of 150-500 pm [16].

In hybrid bioreactors various nitrogen removal processes pathways are possible, including autotrophic processes, e. g. anammox [17]. A sufficiently thick layer of biofilm or flocs is needed for complex nitrogen process transformations including denitrification. on the other hand a relatively thin biofilm (due to shearing stress) results in high activity of biomass [18]. Some authors [19,18] found that in sequencing batch biofilm reactors (SBBR) the biofilm is fully penetrated by substrates and electrons acceptors can be released.

Similarly as in case of other attached biomass systems, e. g. trickling filters in MBBR design procedure, surface area loading rate should be the design parameter [5,11]. This approach is based on some typical range of biofilm thickness (in this case surface area can be the indicator of the biomass concentration). From this point of view the size and shape of carriers seem to be less important. The substrate to biomass loading rate is base but not sole design criterion. Important but poorly recognised factors are: access to total biofilm surface area and access to aerobic biofilm surface area. Some authors indicated that carriers of high total surface area thanks to micropores should have some amount of macropores, enabling fluid reach in oxygen contact with deeper inner spaces of carriers, e. g foamed cellulose carriers [20]. The macropores are important for nitrifying biomass, which needs contact with dissolved oxygen. Micropores are often filled completely with biofilm preventing the oxygen penetration. Oxygen access factor is crucial for biofilm thickness, porosity and surface roughness.

The ratio of the suspended to the attached biomass can vary accordingly to many factors and conditions. The amount of attached biomass can reach over 90%. Plattes et al. [21] indicated 93% of biomass in form of biofilm attached to the carrier elements and only 7% of biomass — as suspended in the bulk liquid. Detachment (or sloughing) of biomass is variable in time [5]; probably this phenomenon is similar to sloughing of excess biomass from biofilm growing in others attached biomass systems, e. g. trickling filters. Some authors [22] suggested that in such systems detachment process occurs periodically.

Due to mechanical contact with others carriers and shear stress, the biomass grows mainly on the internal area of carriers, what was reported by several authors [16, 23], excepting carriers having outgrowths on the outside walls surface.

The common forms in typical activated sludge system are aggregated flocs and planktonic free-swimming cells, and bacterial communities are dominated by: Betaproteobacteria, Alphaproteobacteria, Gammaproteobacteia and more less frequent: Bacteroidetes and Firmicutes [24]. Some authors [24] observed in biofilms in MBBR limited bacterial diversity and Firmicutes domination. The research of Biswas and Turner [24] indicated that MBBR communities differ from communities existing in conventional activated sludge reactors. The characteristic feature of MBBR bacteria community was a presence of two distinct communities: suspended biomass with fast-growing aerobic bacteria and biofilm biomass, which was dominated by anaerobic bacteria [24]. In biofilms of WWTP which were studied by these authors the prevailing forms were Clostridia (38% of clones) and sulfate-reducing bacteria (Deltaproteobacteria members). The another forms were less abundant: Desulfobacterales (11-19%), Syntrophobacterales (8-10%), Desulfovibrionales (0.5-1.5%). The other groups were also observed: Bacteroidetes, Synergistes, Planctomycetes, Verrucomicrobia and Acidobacteria.

The suspended biomass observed in two MBBR reactors by Biswas and Turner [24] was consisted mainly of aerobic microorganisms: Alphaproteobacteria (Rhizobiales, Rhodobacterales), Gammaproteobacteria (Pseudomonadales, Aeromonadales), Betaproteobacteria (Burkholderiales, Rhodocyclales). Majority of Firmicutes was represented by Clostridia and one MBBR reactor suspended biomass was reach in Campylobacteraceae (54% of clones).

The differences in microbial composition can appear not only between biofilm and activated sludge in MBBR reactor but also between MBBR bioreactors themselves. Biswas and Turner [24] observed the biomass, both black with sulfurous odour in one MBBR reactor and grayish-brown without obvious odour — in other MBBR reactor. Some authors indicated that in continuous-flow MBBR in which SND process was established, the microbial community structures of biofilm are related to C/N ratios [25]. In MBBRs the volume concentration ratio

of biofilm to the activated sludge flocs cab be even higher: 5-13 [26] than for separated attached biomass and suspended biomass systems. Some important differences between biofilm and flocs features in MBBRs were found by Xiao end Garnczarczyk [26]. They observed 3 — 5 times higher geometric porosity in biofilm than in activated sludge flocs. Biofilm boundary fractal dimension was higher than activated flocs one. These authors observed also some similarities: two different space populations both in biofilm and in flocs were indicated and both attached and suspended biomass shifted some of their structural properties to larger values (thickness, density) with the increased hydraulic loading.