**4. Anaerobic digestion optimization with nitrogen removal: coupled processes**

(**Figure 2**). However, for high COD/N ratios, an effective biomass separation system in the SNAD reactor is essential for both the outflow of suspended biomass and the retention of granules. A bad separation system design can lead to a reactor clogging or a fully biomass

The effect of shear stress on the granular biomass of a REMON continuous reactor system fed with digested poultry manure has been studied [72]. The start‐up was carried out in a contin‐ uously fed granular BCR. The BCR was stabilized with synthetic substrate and then adapted to digested poultry manure until reaching a NLR of 0.4 g N/L d. After adaptation, the applied

physicochemical and molecularly. During the increase of the shear stress, nitrogen removal decreased from 63 to 17%. Relative abundance of aerAOB and anAOB did not show signifi‐ cant differences. However, the specific Anammox and nitrification activities fell 88.54 and 53.10%, respectively (see **Table 3**). In summary, there is an upper limit of the applied agitation power on a granular biomass in a REMON reactor. If this limit is exceeded, a negative effect

The different operation parameters of the process have shown some limitation such as: 0.25–1 g

, 0.13–0.5 g SSV/L, a maximum COD/N ratio of 2.63. With an optimum of 0.7 of CODbiodegradable/N [37], TAN influent concentration of 0.2–0.8 g N/L with HRT of 4–0.4 d, the

on the activities of the biomass and in the reactor performance is shown [72].

. The biomass was characterized

. The removal efficiency of the system is 20–50% of

**NOB [Bacteria/g of biomass]**

**SNA [g N‐ NH4 + /g SSV d]** **SAA [g N2**

**SSV d]**

**/g** 

retention, and the process will collapse [71].

20 Nitrification and Denitrification

NLR assay has been 0.05–1 g TAN/L d−

**EUB [Bacteria/g of biomass]**

**anAOB [Bacteria/g of biomass]**

**Figure 2.** Profile during the adaptation of the PN‐A reactor [71].

**Table 3.** Evaluation of the effects of shear stress in REMON system [72].

**aerAOB [Bacteria/g of biomass]**

8.43 2.86 × 108 1.08 × 108 1.02 × 108 5.18 × 106 0.314 0.113 12.07 2.76 × 108 9.98 × 107 9.13 × 107 7.97 × 106 0.218 0.042 15.72 2.26 × 108 7.02 × 107 8.08 × 107 2.79 × 106 0.036 0.053

nitrite‐oxidizing bacteria; SNA, specific nitrification activity; SAA, specific Anammox activity.

EUB, eubacteria; anAOB, anaerobic ammonia‐oxidizing bacteria; aerAOB, aerobic ammonia‐oxidizing bacteria; NOB,

COD, with a nitrogen removal of 80–95% [71].

SST/L−

**Shear stress (W/**

**m3 )**

power in the BCR was increased from 8.43 to 15.72 W/m<sup>3</sup>

A coupled process prototype at bench scale for the treatment of nitrogen rich wastewaters was developed; the stepwise process has been validated using poultry manure [73]. The first stage comprises the AD of the substrate, where the poultry manure is diluted in order to decrease the ammonium concentration of the substrate to avoid a large inhibition of the methane pro‐ duction. Best results were obtained with three configurations of AD: (1) up flow Anaerobic Sludge Blanket (UASB), (2) thermal pre‐treatment with UASB and (3) two stages anaerobic process with a mixed flow reactor (hydrolytic stage) and a UASB (methanogenic stage). In the first step, the diluted manure is anaerobically digested in one of the aforementioned configu‐ rations. Most of the organic matter (60–95%) is depleted, and the organic nitrogen of proteins is released in the form of ammonia, reaching high concentrations. Biogas is also generated with a high methane percentage (50–75%). A small fraction of the stabilized solid and an effluent with a remnant organic matter measured as COD is obtained at the outlet stream of the AD. In the second step, the ammonia is removed using a REMON reactor. This reactor generates a warm ammonia free effluent. From the outlet stream, a portion is recirculated to the entrance of the AD, and as a consequence, the slurry inlet stream of the anaerobic digester is diluted (see **Figure 3**).

In the REMON reactor, the denitrifying bacteria uses COD as an electron donor and reduces the residual nitrate to gaseous nitrogen (denitrification process) in presence of organic mat‐ ter, allowing a complete nitrogen removal and the elimination of the residual biodegradable organic carbon. The integrated process of aerobic nitrification, anaerobic ammonium oxida‐ tion and facultative denitrifying bacteria with oxygen limited conditions has the potential of a nearly complete conversion of ammonia and organic carbon to nitrogen gas and carbon dioxide, respectively [71].

The economic and technical feasibility of a coupled process of AD and REMON using water reuse and energy savings applied to a full‐scale poultry manure treatment plant was deter‐ mined to comply with the Chilean environmental law of wastewaters disposal. The new pro‐ posed system is more economical than the nitrification‐denitrification orthodox processes and offers 15% less sludge generation. The minimum volume of the AD and REMON reactors did not guarantee the minimum annual cost for the plant; on the contrary, a middle case between the minimum and maximum of an objective function of reactors volumes represents the opti‐ mal operation condition [74]. But the power consumption is 89.76 and 192.99% lower when burning and using the produced methane, respectively, which means a return of energy. The water recycle results in fresh water savings of 70% compared to the case without recycling. Moreover, the operating costs are reduced by 46%.

**Figure 3.** Scheme of coupled processes of anaerobic digester and REMON reactor. (1) anaerobic digester, (2) REMON reactor, (3) overpressure output of anaerobic digester, (4) effluent of anaerobic digester, (5) influent of anaerobic digester, (6) gas recirculation of anaerobic digester, (7) purge of biomass from anaerobic digester, (8) influent of REMON reactor, (9) overpressure output of REMON, (10) effluent of REMON, (11) purge of liquid from REMON, (12) heating water output, (13) heating water input, (14) gas recirculation of REMON, (15) dissolved oxygen [DO] measurement, (16) air make‐up, (17) inlet air flow. XT: DO transmitter, XC: DO controller, CM: control module.
