## 3 Results and Discussion
Proteobacteria are commonly known to have the ability to survive in a wide range of environmental conditions, either in a clean [39] or disturbed environment in the presence of pollutants [40]. In a previous study, Proteobacteria were found to be dominant in the rainwater despite different sampling times [41]. The predominance of Proteobacteria in the rainwater was also recorded despite the low availability of nutrients and the shift in temperature [42]. Bacteroidetes were also reported to be among the dominant phylum in the rainwater [43] which were able to survive with high and low nutrient contents [44]. Likewise, Proteobacteria were also recorded as the dominant bacterial community found in the POME final discharge [45]. The composition of Bacteroidetes was also noticeable in the POME final discharge as reported in the previous studies [46]. It was noted that the compositions of Proteobacteria and Bacteroidetes in the samples did not change much before and after the implementation of zero-emission of POME final discharge. Even so, the implementation of zero-emission of POME final discharge could restore the original community found in the clean water. This is based on the observation of Actinobacteria that were detected in the rainwater (13.6%) but not in the POME final  
Figure 5: Relative abundance of the bacterial phyla before and after the implementation of zero-emission of POME final discharge. Rainwater was first introduced as a control on day 1, followed with the addition of POME final discharge from day 2 to day 7. The addition of POME final discharge was stopped and the rainwater was reintroduced into the system from day 8 to day 15 to mimic the zero-emission concept. Notes: AP-archaeal phylum; BP-bacterial phylum; FD-final discharge; RW-rainwater. discharge, but their composition was slightly higher once the rainwater was re-introduced into the system from day 10 onwards. Actinobacteria are some of the most abundant phyla that are usually affiliated with freshwater [47] and able to flourish in various freshwater habitats, including the rainwater [48]. They are known to play a key role in nutrient and energy cyclings as they contribute to glucose assimilation and heterotrophic nitrification in aquatic habitats [49] and are able to survive in environments with low levels of organic carbon [48]. Since it was previously reported that the POME bacterial indicators, Alcaligenaceae and Chromatiaceae, from the Proteobacteria phylum originated from POME final discharge [13] and were not detected elsewhere including the clean river and other non-POME polluted rivers [15], both of them were used as representative bacteria to further analyze the implementation effect of the zero-emission system towards the composition of the bacterial community. A heatmap was constructed to further assess the shift in the bacterial community at the order and family levels within the Proteobacteria phylum before and after the implementation of zero-emission of POME final discharge, after cutting-off its relative abundance at \(\leq\)5% (Figure 6). In the rainwater sample, the most detected bacterial communities were dominated by the Alpha-proteobacteria, including Caulobacteraceae (4.3%), Hyphomicrobiaceae (1.9%), Rhodospirillaceae (3.8%), Acetobacteraceae (5.6%), and Sphingomonadaceae (8.1%), as well as the Comamonadaceae (8.5%) from the Beta-proteobacteria class. Except for Comamonadaceae, these bacteria were also found to be unique in the rainwater and were not detected in the POME final discharge. The Alpha-proteobacteria are normally found in the rainwater due to their ability to utilize recalcitrant organic compounds such as humic substances and to form segregation with each other [50]. Meanwhile, Beta-proteobacteria have the ability to withstand low nutrient content in the rainwater [51].