Comunicación vertical e instrumental

Various novel biological nitrogen removal processes such as short-cut nitrification and denitrification, anaerobic ammonium oxidation (ANAMMOX), completely autotrophic nitrogen removal over nitrite (CANON) process and oxygen-limited autotrophic nitrification-denitrification (Oland) process, bioaugmentation batch enhanced treatment (BABE) and single reactor for high activity ammonia removal over nitrite (SHARON) have been developed exclusively. [78]

Partial Nitrification

Nitrification is a sequential biological oxidation process, which involves two different groups of bacteria. The first step of nitrification is the oxidation of ammonia to nitrite over hydroxylamine (NH2OH), involving the membrane- bound ammonia mono-

oxygenase (AMO) and the hydrox-ylamine oxidoreductase (HAO), and is carried out by ammonia-oxidizing bacteria (AOB); the second group, nitrite-oxidizing bacteria (NOB), further oxidizes nitrite to nitrate. [79]

Under normal conditions, the reaction of ammonia oxidation to nitrite is a velocity-limiting step; in contrast, nitrite is oxidized rapidly to nitrate, so nitrite is seldom accumulated in nitrifying reactors. In partial nitrification process, however, nitrite accumulation is required, and the second step must be restrained so as to accumulate AOB and washout NOB. [80] _{Partial nitrification process is based on} the fact that nitrite is an intermediary compound in both nitrification and denitrification steps: a partial nitrification up to nitrite is performed followed by nitrite denitrification

(Ferhan 1996; Fdz- Polanco et al. 1996), as shown in Figure 2.3. Chung et al., (2007)

showed the benefits of shortcut nitrogen removal by comparing the stoichiometries for O2

and CH2O (representing the organic electron donor) in Equation (2.89) and (2.90)

(conventional BNR) to Equations (2.91) and (2.92) (shortcut BNR). [81]

Conventional BNR:

NH4+ + 2O2 (ammonium and nitrite oxidizers) → NO3- + H2O + 2H+ (2.89)

Shortcut BNR:

NH4+ + 1.5O2 (ammonium oxidizers) → NO2- + H2O + 2H+ (2.91)

NO2- + 0.75CH2OH → 0.5N2 + 0.75HCO3- + 0.75 H+ (2.92)

Partial nitrification to nitrite and nitrite denitrification was reported to be technically feasible and economically favorable, especially when wastewater with high ammonium concentrations or low C/N ratios with high temperature is treated. [79]

Figure 2.3. Shortcut nitrogen removal

Compared to traditional nitrification denitrification via nitrate, the main advantages of

partial nitrification with respect to complete nitrification were reported as followed: [82, 83,

84]

I. 25% lower oxygen consumption in the aerobic stage implies 60% energy savings II. In the anoxic stage the electron donor requirement is lower (up to 40%)

III. Nitrite denitrification rates are 1.5 to 2 times higher than with nitrate; IV. 20% CO2 emission reduction

V. 33∼35% less sludge production in nitrification process and 55% in denitrification process.

Methods to Maintain Partial Nitrification

Researchers have developed many control methods and strategies to achieve partial nitrification. The main objective of these methods and approaches was to accumulate

different dissolved oxygen half-saturation coefficients, and different anti-toxic capacities of AOB and NOB.[79]

Raising temperature cannot only promote the growth rates of AOB, but can also expand the differences of specific growth rates between AOB and NOB.[79] _{From the} aspect of specific growth rate, only at temperatures above 25 °C is it possible for the ammonium oxidizers to effectively out- compete the nitrite oxidizers. [85] _{But the opposite} was the case at temperature below 15 °C.

Based on experiences from full-scale operation, van Kempen et al. (2001) suggested maintaining SRT between 1 day to 2.5 days to washout NOBs while retain AOBs. However, Peng and Zhu (200) and Pollice et al. (2002) reported partial nitrification to nitrite under oxygen limitation, independent of sludge age at SRT of 10, 14 and 40 days. [79, 86, 87]

The dissolved oxygen half-saturation coefficients of AOB and NOB are 0.2–0.4 mg/L and 1.2–1.5 mg/L, respectively. [88] Therefore, low DO concentration is more restrictive for the growth of NOB than AOB, which will result in nitrite accumulation. [79]Garrido et al. (1997) found that both ammonium oxidation rate and nitrite accumulation reached maximum when DO was 1.5 mg/L. Below 0.5 mg/l of DO ammonium was accumulated

and over 1.7 mg/L complete nitrification to nitrate was achieved [89]. On the other hand, it

should be noticed that lower DO will lower nitrification rate and cause filamentous bulking sludge. Considering ammonia oxidation rate and nitrite accumulation, DO

concentration should be maintained about 1.0–1.5 mg/L. [79] Use of intermittent aeration

was in favor of implementation of nitrite accumulation. [86, 87] Autotrophic Nitrification

Autotrophic nitrifying bacteria, such as Nitrosomonas europaea, can use nitrite to

oxidized ammonia with the production of nitrogen gas when dissolved oxygen is not

present. [90] _{However, these bacteria oxidize the ammonia with oxygen as electron}

acceptor when oxygen is present. [66] _{This distinguishes autotrophic nitrification from}

ANOMMOX

The bacteria in the ANOMMOX (ANaerobic AMMonium OXidation) process are

different than the autotrophic nitrifying bacteria. ANOMMOX cannot use oxygen for

ammonia oxidation. [91] _{Under the anaerobic conditions the ammonia oxidation rate by}

Anommox, Equation (30) Table 2.3, was shown to be 6 to 10 times faster than that for

Nitrosomonas europaea. [66, 91] Side Stream Nitrogen Removal

Side streams including the reject streams from the membrane, dewatering process and supernatant liquid from sludge digesters also contain a significant load of nutrients. Estimates of the nitrogen load from this side stream return range between 15% and 30%

of the total nitrogen load on a process. [92] As mentioned before, several relatively new

processes have been developed to remove nitrogen in high-concentration side streams from biosolids processing prior to recycling to the headwork of the publicly owned

treatment works (POTWs); SHARON® (Single reactor system for High activity

Ammonium Removed Over Nitrite), ANAMMOX®, CANON® (Completely Autotrophic

Nitrogen removal Over Nitrite), InNitri® (Inexpensive Nitrification) [93], and

BABE® (Bio-Augmentation Batch Enhanced) [67]. The schematic of the aforementioned

processes are depicted in Figure 2.3. In SHARON® process (known as nitrogen removal

over nitrite) ammonia oxidizing bacteria (AOB) are encouraged and nitrite oxidizing

bacteria (NOB) hindered by operating at higher temperature of 30-35 °C, SRT=HRT of 1-

2 d and lower oxygen concentrations of 1-2 mg/L. The products of SHARON® process

are approximately 50% ammonia and 50% nitrite to be further denitritified by ammonia

as electron donor in ANAMMOX® and CANON® processes or heterotrophic bacteria in

SHARON® _{process. In the ANAMMOX process, also known as fully autotrophic}

nitrogen removal, nitrite and ammonia acts as an electron donor to convert nitrite to

nitrogen gas. Autotrophic ANAMMOX bacteria are very slow growers with µmax of 0.069

1/d, which is significantly lower than nitrifying bacteria with µmax of 0.8 1/d. As a result

very long SRT of 30-50 days are needed to facilitate ANAMMOX process. Moreover nitrite > 40 mg/L and free ammonia > 10 mg/L have inhibitory effects on ANAMMOX.

The temperature for ANAMMOX process should be maintained within 30-35 C.

As depicted in Figure 2.4d, the BABE® process is comprised of a single batch

reactor. Side stream waters high in ammonia content and return activated sludge (RAS) from the main biological treatment process are combined with previously settled sludge

in the batch reactor at average temperature of 25 °C. [94] _{The RAS is used to augment the}

bacteria in the settled sludge. By utilizing a batch reactor, the long residence times necessary to grow both the nitrifying and denitrifying bacteria are possible. There are five

phases to the BABE® process: 1-filling, 2-mixing and aeration, 3-mixing, 4- settling, and

5-settling and decant. [94] The first two steps are done under aerobic conditions. The third

involves mixing without aeration to achieve anoxic conditions. This condition is conducive to denitrification. Steps four and five complete the process.

Figure 2.4. Schematic of innovative biological nitrogen removal processes from side stream waste (a) InNitri Process (b) SHARON (c) SHARON/ANAMMOX (d) BABE (Adapted from USEPA 2008)