User Response – Based Sustainable Solutions to Traffic Congestion Problem using Public Transport: The Case of Uttara, Dhaka

Abstract

The complete nitrifi cation process, i.e. complete oxidation of ammonia to nitrate (COMAMMOX), by only one microorganism was experimentally confi rmed only two years ago. That discovery is now considered a real breakthrough in the nitrogen cycle in the environment and it opens new questions regarding the nitrogen metabolism by microorganisms. Moreover, it also brings opportunities to revise the approach to nitrogen management in wastewater treatment systems employing the novel nitrogen removal processes, such as deammonifi cation or shortened nitrifi cation-denitrifi cation. The comammox bacteria may signifi cantly disturb nitrite production in partial nitrifi cation, which is the critical step for the successful operation of both novel processes. The crucial role in identifi cation of "comammox" bacteria is attributed to the latest, advanced molecular techniques (metagenomics and metatranscryptomics). Nitrifi cation is an important step in the global nitrogen cycle and plays an essential role in many engineered systems, including wastewater treatment plants (WWTPs). Conventionally, nitrifi cation has been considered a two-step process catabolized by different groups of microorganisms, i.e. the fi rst step (nitritation)-by ammonia oxidizing bacteria (AOB) or ammonia oxidizing archaea (AOA), and second step (nitratation)-by nitrite oxidizing bacteria (NOB). However, this functional separation has been proven to be energetically less advantageous. A postulate assuming the presence of a single organism with both lower growth rates and higher growth yields than the canonical AOB, capable of performing both nitrifi cation steps, was presented more than 10 years ago by Costa et al. [1]. Those authors modelled the trade-off between the growth rate (favored by short metabolic pathways) and growth yield (favoured by longer pathways). Based on model calculations, they demonstrated that the existence of complete nitrifi ers should be favored when the microorganisms grow slowly in clonal colonies (e.g. biofi lms that cover surfaces in many natural and engineered systems).The authors concluded that in chemostats and other well-mixed systems, the faster-growing incomplete ammonia oxidizer would outcompete the complete oxidizers. In contrast, in biofi lms and other microbial aggregates with low substrate diffusion gradients and low mixing of biomass (clonal clusters), a higher yield of biomass per amount of substrate consumed (which is equivalent to a more economical use of resources) would benefi t only the neighborhood. Therefore, under a broad range of favourable conditions in biofi lms, more economical (but slower-growing organisms) would have a higher fi tness than resource-wasting, fast-growing competitors. The hypothesis of Costa et al. [1], was supported by the very recent discoveries of bacteria capable of performing comammox (complete ammonia oxidation) in an aquaculture system (van Kessel et al. 2015) and deep subsurface pipe [2], and subsequently in a bioactive fi lter at a drinking water treatment plant (Pinto et al. 2015) and a WWTP [3]. In the latter case, Chao et al. (2016) [3], have emphasized that the biofi lm in aerobic reactors is exposed to the dissolved oxygen (DO) concentration gradient, which may induce the growth of comammox bacteria. Earlier cross-section studies of biomass distribution in nitrifying biofi lm systems (Okabe et al. 1999) found the highest NOB abundance in deeper zones with less DO availability. Moreover, the study of Okabe et al. (1999) showed that these NOB clusters were dominated by Nitrospira sp., whereas the faster growing NOB species Nitrobacter sp. were almost absent. Therefore, Nitrospira sp. implicitly adapt better to DO limited conditions in comparison with Nitrobacter sp. This may be related to the presence of "comammox Nitrospira" in deeper biofi lm layers, which have been recently shown to thrive in substrateinfl ux, DO limited zones [2]. Modelling studies of a one-dimensional stratifi ed biofi lm have revealed that oxygen penetrates to the depth of 20 μm (Mahendran et al. 2012). Moreover, micro-scale studies have demonstrated that there is a correlation between the DO profi le and spatial