Water reuse potential for irrigation in Brazilian hydrographic regions

The present paper carried out an evaluation of the reuse potential of the Wastewater Treatment Plants (WWTPs) ef ﬂ uents for irrigation in the 12 Brazilian Hydrographic Regions (BHRs). For this purpose, initially, the WWTPs were categorized and the ef ﬂ uent ﬂ ow rate was estimated. Category 1 represents secondary ef ﬂ uent with an ef ﬁ ciency of organic matter removal greater than 80%; Category 2 represents ef ﬂ uent that underwent some disinfection step; and ef ﬂ uents that perform less than the other categories were called ‘ Uncategorized ’ . After that, the irrigation water demands for each BHRs were compiled, and ﬁ nally, the production of water for reutilization was compared with the demand for irrigation. Thus, it was observed that all the sewage ﬂ ow rates generated in Brazil classi ﬁ ed in Categories 1 and 2 represent 9% of the total irrigation water demand in the country (1,078.71 m 3 /s) and it stands out that only 7% of the ﬂ ows treated in Brazil undergo a tertiary treatment step.


INTRODUCTION
Although Brazil has large water reserves, about 12%-16% of the total amount available in the world (Ramos ), its water availability is not evenly distributed throughout its territory. Approximately 260 thousand m 3 /s of water flows through Brazilian territory and of this amount, 80% is in the Amazon region, where there is the smallest portion of the population and the lowest water use demand (ANA ).
This context, which can become even more complex in a scenario of climate change and increased water use, leads to water stress in some regions of Brazil, like currently in the Southeast (ANA ). In the Semi-Arid Region, which covers the states of the northeast region (Alagoas, Bahia, Ceará, Paraíba, Pernambuco, Piauí, Rio Grande do Norte, and Sergipe), in addition to the north of the Minas Gerais state, the water scarcity is historic. Thus, this region has one of the lowest socioeconomic development rates in the country. The surface water availability in Brazilian territory is presented in Figure 1, with emphasis on the Semi-Arid region.  Table 1.
The BHRs with the most critical water levels are Atlântico Nordeste Oriental, located in the Semiarid Region; and Atlântico Sul, which has extremely high water demand for irrigation. In addition, BHRs Atlântico Leste and São Francisco have high water demands in comparison with their water availability (ANA ).
Throughout the world, due to water scarcity and the increase in water use conflicts, water conservation and reuse have gained prominence as water resource management tools (Angelakis et al. ). In addition, population growth, climate instability, and increased demand for food  The water reuse for irrigation purposes was already adopted in the world since the prehistoric period to the current, considering different aims and perspectives throughout these years (Mays et  for approximately 70% of total freshwater in the world, and this value is even higher in many developing countries (Peng et al. ). In Brazil, according to ANA (), irrigation demands about 52% of water withdrawals, followed by urban supply, the processing industry and animal supply.
Thus, the main destination of reclaimed water in the world is irrigated agriculture (Angelakis et al. ). However, it is important to highlight that this reclaimed water comes mainly from wastewater treatment plants (WWTPs).
It is estimated that more than 10% of the world's population consumes agricultural products produced through wastewater irrigation ( Jeong et al. ).
Regions with high rates of wastewater treatment have, theoretically, a greater potential for generating water for reutilization. This is the case in Israel, which treats 97% of the wastewater generated and reuses 80% of it in irrigation, supplying 40% of the demand for this purpose (Marin et al. ). In Brazil, only 42.6% of the wastewater generated is treated (ANA ), reducing its potential for reuse.
In general, water reuse in agriculture becomes an effective alternative source of water for production of different crops and also the supply of nutrients in the practice of fertigation (Maryam & Buyukgungor ). However, negative aspects such as the accumulation of substances that hinder plant growth, the potential damage to the soil through the transformation of its physical-chemical characteristics, and contamination by microorganisms must be evaluated (Xu et al. ). According to Maryam & Buyukgungor (), primary effluent is not recommended for reuse in agriculture;

MATERIAL AND METHODS
The study was developed in three stages (Figure 3), as of the consolidation of public data presented in national documents (Table 2)  (iii) treatment technologies adopted in the WWTP; (iv) operational flow rate.
Subsequently, study categories were defined in relation to the performance of organic matter removal and pathogenic organisms' removal/inactivation. This categorization aimed to classify effluents in two situations: • Category 1 -Secondary WWTP with organic matter removal efficiency greater than 80%: To produce effluent available to reuse in most of crops, it would be needed only to include a disinfection tertiary step.
• Category 2 -Tertiary WWTP with some disinfection technology (mostly maturation ponds): Effluent can be distributed for direct reuse for the irrigation of most crops.
The WWTPs with only primary level or only UASB (upflow anaerobic sludge blanket) reactor and organic matter removal efficiency below 80% were not categorized.
They were considered for this study as 'uncategorized' because these facilities require high investments to adapt the effluent to reuse, since they would still need a secondary stage or a polishing prior to disinfection.
It is important to discuss the issue of the type of crop to be irrigated and, therefore, the quality of the water required for this purpose. In this sense, this article does not consider a discussion in relation to water quality standards, but rather evaluates possibilities for reuse depending on demand and supply in different qualities. These qualities are represented here according to the categories defined for the study and previously mentioned. Regions. Furthermore, the WWTP flow rates were divided into the two categories described in the methodology.  It contemplates the definition of methods, the construction of a database beyond the production, storage, and availability of estimates consumptive water uses for all Brazilian municipalities ANA () In this discussion, the importance of increasing wastewater treatment coverage rates is highlighted, not only in the sense of compliance with the national guidelines for effluent discharges, but also in relation to the production of higher volumes of water for reutilization. This action could be the target of areas with lower rates of socioeconomic development (Maryam & Buyukgungor ).

Stage 2irrigation water demand
The Stage 3water reuse potential In general, in all BHRs, the effluent flow rates of Category 1 and/or Category 2 are significantly lower than the water demand for irrigation. However, it is noteworthy that for BHRs Atlântico Sudeste, Paraguai and Paraná, the effluent flows of Category 1 represent approximately 24, 29, and 30% of the water demand of irrigation, respectively.
In relation to Category 2, none of effluent flow rates reaches  In order to facilitate understanding and include new questions for discussion, in Figure 5, the following data are At BHR Paraná, the total flow corresponding to the sum of Categories 1 and 2 is 63.38 m 3 /s and represents 33% of the water demand for irrigation. If 'uncategorized' WWTPs were to be added to this total, with their due interventions to adjust the effluent, this value would be 44%. It should be noted that this is a considerable percentual and, therefore, the governance of this region should add this issue to the planning of water resources. Still, it is important to highlight that, because it is the most developed region in the country. The supply of an alternative source of water for irrigation could not only lead to economic growth but also the reduction of conflicts over the use of water (Nölting & Mann ).
At BHR Paraguai, the uncategorized effluent (representing 12% of the water demand for irrigation) added to the effluents of Categories 1 and 2 represent 44%, similarly to HR Paraná.
The water demand for irrigation in this BHR is only 4.46 m 3 /s and in BHR Paraná is 189.97 m 3 /s. Thus, there is a need to adapt these WWTPs (uncategorized), not only to comply with the current legislation on effluent discharges, but also to structure reuse planning in emergency situations.
The BHR Atlântico Sudeste also deserves to be highlighted for its high potential for the adoption of water reuse in irrigation. The flow rates of Categories 1 and 2 combined represent 25% of water demand for irrigation. When the 'uncategorized' flow rate is added, this ratio increases to 35%. Again, the discussion supports the need for more attention to WWTPs and their performance. Regarding the sanitary deficit faced in Brazil and the imminent need to apply the practice of water reuse to minimize the drought impacts that will be aggravated by climate changes and water use growth, the need to achieve universal sanitation is evident. This is not only to minimize the pollution of water bodies, but also to provide, in quantitative terms, effluents treated for the practice of water reuse.
In the Brazilian Semi-Arid region, the driest area in the country, 470 municipalities have intermittent or ephemeral water bodies. Thus, in addition to the need to remove BOD (Biochemical Oxygen Demand), it is important to take into account the practice of water reuse and/or prioritize treatment processes that result in high removal and inactivation of pathogenic microorganisms (ANA ).  However, in most of them, the potential is low, due to the low attendance rates in relation to sanitation.
In situations where the installed potential is high, it is necessary to consider the operational levels of the WWTPs, to guarantee an adequate quality for reuse. In cases of low installed potential, it is necessary to assess the capacity to implement complementary units for such a practice, in addition to advances in service rates, with a view to universalization and water security.

The Brazilian Hydrographic Regions Parnaíba and São
Francisco depend heavily on irrigated agriculture. Thus, water scarcity can curb socioeconomic growth of these regions, bringing water for reutilization with an important planning factor. In these two cases, the intermittent rainfall accompanied by climate change and population growth aggravate the situation. Furthermore, the sanitation services coverage is low and must be assessed for adequate water resources and sanitation management.
Only 7% of the treated wastewater flow rate in Brazil goes through a tertiary stage of disinfection, showing a fragility in relation to the quality of the effluent both for discharging and for the application of the practice of water reuse.
It is reinforced that the present work took into consideration only the amount of effluent generated in relation to the water demand for irrigation in each Brazilian Hydrographic Region. Thus, it is concluded that for the effective adoption of the practice of water reuse in planning, it is necessary to take into account the locations of both WWTPs, irrigated fields and transport, and storage logistics, in addition to the different levels of quality required for each type of culture. Still, in this work, an estimate with punctual flows was adopted. However, these flow rates that represent demands can occur on a seasonal basis in irrigation.