Abstract
Water is an asset that is becoming scarce and its reuse is an alternative to preserve it. Social and environmental feasibility motivate the adoption of reuse, but the financial factor can be limited to its use. The objective of this study was to evaluate the economic and financial feasibility for the investment in a domestic wastewater treatment system in which the constructed wetlands could be used aiming at the reuse for irrigation. Instruments such as the minimum attractiveness rate, discounted payback, the internal rate of return and the profitability index were used. The treatment system composed of an equalization tank, a septic tank, horizontal subsurface flow constructed wetland, subsurface flow constructed wetland, and a maturation pond comprising 52 m², demonstrating that it is able to meet the criteria required for reuse. The organic load affluent to the system was projected to 220 mg L-1 at a flow rate of 600 L day-1 of sewage, obtaining removal efficiency above 98% for all parameters, except nitrogen and phosphorus. In the analysis of economic and financial feasibility, it was found that the internal rate of return found for the cash flow considered was 98% and the profitability index according to the present value was 3.28. It was found that the total employed would be recovered in up to two years. With the use of the system, 180 m³ of water would be saved annually, reducing the pressure on the public supply system, in addition to the sewage treatment and the scenic beauty provided by wetlands. Therefore, the system is not limited only to economic gains, but also to social and environmental gains.
Highlights
In recent years, water scarcity in Brazil has been constantly presented in the media
Based on data provided by CEPEL, gardens irrigation would not require major renovation in the hydro-sanitary installation and would imply a reduction in the consumption of 15m3 month-1 of water supply, which would result in savings of 180m3 year-1
The system was structured in such a way that the equalization tank sized according to EMBRAPA (2015) with 0.6 m3 of useful volume, functioned as an inlet flow regulator, while the septic tank, sized according to NBR 7229 (ABNT, 1993), of cylindrical shape with three chambers of 1.8 m3 of useful volume, had the main function of reducing the amount of total suspended solids (TSS) of the horizontal subsurface flow constructed wetland (HSFCW)
Summary
Water scarcity in Brazil has been constantly presented in the media. For industries, reusing their effluents after appropriate treatment as far as possible is a more attractive option, with lower installation and operating costs than those associated with capturing and water treatment from springs or purchasing water offered by sanitation companies, both for drinking. Water reuse programs in the industry can reduce the consumption of this resource by up to 80% (RIBEIRO, 2012). According to Ribeiro (2012), the growth of reuse in Brazil is still very small concerning its potential. The non-potable water reuse is allowed as a rationalization practice, and the guidelines for that are established in Resolution n. 54 (CNRH, 2005), there is a need to invest in treatment, whether through biological, physical-chemical, or mixed systems. The system of constructed wetlands (CW) should be noted, which is able to mix all these characteristics and can become a natural solution for most of the difficulties found
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