**4. Fixed film processes vs. suspended growth systems**

### **4.1 Suspended sludge reactor**

The Changi Reclaimed Water Plant in Singapore and the Strass Sewage Treatment Plant in Austria are two existing PN/A processes using suspended sludge reactors. In both cases, AnAOB is not affected by oxygen in the anoxic zone, and heterotrophic denitrification makes an important contribution to nitrogen removal at high C/N

influent ratios. One of the most common problems of flocculent sludge is poor settling [29]. Small floc cannot settle and separate well, resulting in increased turbidity of effluent [30].

## **4.2 Biofilm reactor**

Biofilm reactor includes biological drip filter, fixed membrane biofilm reactor, fluidized bed reactor, biological rotary table, and moving bed biofilm reactor. The typical problem with biofilms is mass transfer restriction, in which only the bacteria in the outer layer of the biofilm can promote substrate removal [31]. The biofilm mainly grows on the surface of the filler, and the filler is prone to clogging during the actual operation, which is not conducive to the long-term operation of the process [32]. In addition, the cost of biofilm carriers is also a limitation of their application in large-scale wastewater treatment plants [33].

## **4.3 Granular sludge reactor**

Bacterial aggregation in granular sludge reactors increased biomass concentration [34], as well as biomass retention [35] and tolerance to environmental stress [36]. Granular sludge can fix more AnAOB population under aeration conditions [37, 38]. Compared with floc and biofilm, particles have the advantages of dense structure and do not need to be attached to the surface of the carrier, so they are favored by the engineering field. However, the disadvantage of Anammox granules is that the low cell yield and growth rate of AnAOB lead to long sludge age, and HB grows excessively on autotrophic bacteria under long sludge age conditions.

## **4.4 Mixed reactor**

The mixed reactor has a high concentration of suspended sludge in the liquid phase. The biomass of liquid and solid (biofilm or granular sludge) phases plays an important role in microbial transformation. Based on the theory that AnAOB mainly exists in particles or biofilms, while NOB and HB mainly exist in floc [39]. Compared with the biofilm reactor, AOB, NOB, and HB are mainly suspended in the liquid phase, while AnAOB mainly exists in the biofilm. It has been reported that 60% of aerobic reactions are achieved in the liquid phase, while AnAOB activity occurs almost exclusively in biofilms (>96.5%) [24]. The high suspended sludge concentration in the liquid phase of the mixed reactor significantly reduces the diffusion limit compared with the single particle or biofilm reactor, thus inhibiting the competition of AOB by controlling the low concentration level of DO in the liquid phase. In addition, the sludge age of suspended sludge can be controlled independently of particles or biofilms, which facilitates the washing of HB and NOB, and can tolerate higher influent COD/N [40].

It has been reported that the nitrogen removal capacity of IFAS process co-existing with flocculent sludge and biofilm is three to four times higher than that of MBBR process consisting of biofilm only [37, 41]. Compared with biofilms, particles have the advantage of dense structure and do not need to be attached to the surface of the medium. However, the process of coexisting Anammox particles with continuous flow flocculent sludge has not been reported. If the flocculent sludge with AOB as the main body and Anammox granular sludge is integrated into a continuous flow system, the nitrogen removal effect can be ensured, and the excess AOB and NOB short sludge age bacteria can be eliminated through the flocculent sludge to retain AnAOB, and the NOB can also be washed. Studies have shown that at low concentrations, intermittent anaerobic/aerobic = (15 ~ 20 min)/(5 ~ 15 min) alternating can effectively inhibit NOB [10]. Therefore, continuous flow anaerobic/aerobic alternating operation can inhibit NOB.

In addition to the difficulties of NOB suppression and panning, internal reflux and external reflux are often needed in continuous flow. In this process, the granular sludge of Anammox will inevitably be broken and disintegrated, which will also face the problem of AnAOB loss. If the problem of reflux is not solved, granular sludge can not be used effectively in the reactor with continuous push flow. At present, the methods of retaining granular sludge include membrane screening method and settling selection method, which represent the reactor membrane reactor and granular sludge selector respectively. Compared with membrane reactor, granular sludge selectors can pass flocculent sludge, not easy to clog, and is easy to maintain. If the granular sludge selector based on sedimentation selection is installed in the aerobic zone to retain Anammox particles and ensure the circulating flow of flocculent sludge, the reflux problem can be solved.

To sum up, in order to run the continuous autotrophic nitrogen removal process stably, Anammox granular sludge can be retained in the aerobic zone to participate in nitrite degradation but not reflux according to the anaerobic/aerobic process, so as to ensure the integrity of Anammox particles and effective retention of AnAOB while consuming most nitrite in time. The AOB in the floc is pumped back to alternate anaerobic/aerobic operation for nitrosation reaction. Of course, due to the mixed reactor, a small amount of AnAOB will exist in the floc, and the amount of AnAOB will not be too high because of the short floc sludge age. A small amount of AOB will also adhere to the Anammox particles to form a protective layer to consume dissolved oxygen.
