Financial comparison of sewage treatment and disposal systems in rural areas

: Rural communities suffer from the lack of sanitary sewage, which ends up compromising the quality of life of these populations. Studies show that conventional sanitation technologies, due to their high costs, end up excluding rural populations. Among the options for the treatment and final disposal of sewage in rural areas we can mention septic tanks with sump and biodigester tanks with filtering gardens.


INTRODUCTION
In rural communities, investments in infrastructure, sanitary sewage, solid waste collection and water supply are minimal and these services are mostly very precarious, directly influencing the quality of life of the residents (KOLLING NETO; et al 2018).
For Guimarães et al. (2001), sanitation associates systems consisting of a physical infrastructure and an educational, legal, and institutional structure, which covers the following services: water supply to the populations, collection, treatment of sanitary sewage, and control of vectors such as insects, rodents, mollusks, among others.The lack of basic sanitation is pointed out by academia as one of the current factors that are strongly associated with public health problems and environmental pollution.
In rural areas of municipalities, household waste is commonly disposed of in dry pits, which consist of irregular excavations of uncertain size in the ground.Raw sewage is received in the dry pits without prior treatment and ends up seeping into the ground, making it quite likely that the groundwater and soil where these pits are installed will be contaminated at some point.In addition, dry pits are sometimes located near water catchment wells, which can also be contaminated with pathogens and unwanted organic matter.This situation puts the population at risk, which can be contaminated by waterborne diseases such as hepatitis, cholera, salmonellosis and others.(SILVEIRA E SOUZA et.al, 2021, p.02).
As stated by Silveira e Souza et.al (2021), without basic sanitation infrastructure and exposed to health problems that can be caused by poor management of household facilities and effluents, isolated communities place themselves in a very vulnerable position with regard to human health.Studies indicate that an improvement in the sanitation services provided to the population will considerably reduce the incidence of waterborne diseases, but on the other hand, conventional sanitation technologies (because they are expensive), in research already conducted have been excluding rural populations (JOÃO, SOUSA, and SILVA, 2015).So, for rural areas, where there is no basic infrastructure necessary for the treatment of domestic sewage, it is necessary to have knowledge of the elements available and avoid such contamination that interfere with quality of life, and among the options for the treatment and final disposal of sewage we can RIMA, v.5, n.1, 2023, e212.mention septic tank with drain and biodigester tank with filtering garden.Considering that, in general, rural communities have a low purchasing power, this article aims to make a financial comparison of two individual sewage treatment systems: septic tank and floor drain, and biodigester septic tank and filtering garden.

Septic tank and floor drain
The septic tank is the device responsible for the primary treatment of domestic sewage.Its main purpose is to receive and store the sewage for a certain period of time.They are commonly used when there is no option of connecting to the municipal sewage system, usually in rural areas (BRK, 2020).
They may be square or circular in shape, made of concrete or masonry, the latter being the most commonly used material.According to Dacach (1979), they have a reinforced concrete bottom and roof slab, and the walls can be made of reinforced concrete or brick masonry internally lined with cement and sand mortar in a 1:3 ratio.One advantage this item offers is its low maintenance cost.Because it is made of an extremely resistant material, such as concrete, it does not require constant maintenance.
The drain is a vertical unit for the purification and final disposal of effluents from septic tanks, built in cylindrical shape, and must ensure a minimum distance of 1.50 m between its bottom and the maximum level of the aquifer (ARAGÃO, 2020).The walls are commonly built in perforated concrete shackles (also known as zimbras), but can also be made with bricks with dry joints -to allow percolationor perforated ceramic blocks laid in a radial way.It must have a reinforced concrete cover, with a 60 cm inspection cover.

METHODOLOGY
The methodology consisted of the sizing of the septic tank and the drain, based on the considerations adopted (residence, number of people, characteristic of the soil where it will be located), followed by obtaining the costs of the elements: septic tank, drain, filtering garden and biodigester tank, in order to estimate the costs of each element (SINAPI -Sistema Nacional de Pesquisa de Custos e Índices da Construção Civil) and make a comparison for each set of elements.

Septic tank and floor drain dimensions
For this work we will use a hypothetical residence with 5 people built on sandy soil.For the septic tank and the drain the dimensions according to Table 01, below, will be adopted.In the case of the biodigester septic tank, three fiberglass tanks with 1000 liters each will be used; and for the construction of the filtering garden, an area of 10 m² will be adopted (2 m² per resident) as indicated by ASPROESTE (2018).

Quantity of materials
For the financial analysis, it will be necessary to determine the amount of materials for the execution of each device.In the case of the septic tank and the drain, these quantities will be calculated based on the dimensions presented in Table 01.In the case of the biodigester septic tank and the filtering garden, the quantities of materials indicated by ASPROESTE (2018) will be adopted for the case of a residence with 5 people.Finally, after the quantities are determined, the cost of these devices will be estimated based on the prices indicated in SINAPI -Sistema Nacional de Pesquisa de Custos e Índices da Construção Civil.

Septic tank cost estimate
A quantitative survey of the services that make up the execution of the cast-in-place septic tank was performed, which are: excavation volume, execution of the lean concrete ballast, execution of the reinforced concrete cover, masonry wall raising, and finally, the wall covering.The results are presented in Table 02.

Estimated cost of the drain
A quantitative survey of the services that comprise the execution of the drain was performed, which are: excavation volume, execution of the masonry wall, gravel layer and execution of the reinforced concrete cover.The results are presented in Table 03.

Estimate of the cost of the biodigester septic tank
A quantitative survey of the materials and services that make up the execution of the biodigester tank was carried out, which are basically the fiberglass water tanks, hydraulic materials (pipes, fittings, valves and records), various materials (glue, paint, sandpaper, etc.).To avoid a very long table, the costs of these materials were added up and inserted in the table as two items.The results are presented in Table 04.

Estimated cost of the filtering garden
A quantitative survey of the materials and services that make up the filtering garden was carried out.The results are presented in Table 5.It is worth noting that this table does not include the purchase of water plants (Taboa, papyrus, yam, etc.).The results show that the septic tank and drain system had a total cost of R$ 2580.22.The most expensive item in both devices is the masonry wall.This cost can be reduced by substituting alternative materials, such as used tires.The system's biodigester septic tank and filtering garden showed a total cost of R$ 8,682.93 being the most expensive item, the fiberglass water tanks.

CONCLUSIONS
However, it can be stated that: (a) the septic tank and drain system presents a lower cost of execution, on the other hand, for the effluent forwarded to the system is not given any utility; b) The biodigester tank and filtering garden system has a higher cost, but allows the reuse of this effluent as biofertilizer; c) The values found for the execution of the systems are relatively high for low-income populations, and an alternative is the implantation of governmental programs that deliver these devices to these populations.

FIGURE 03 :
FIGURE 03: Schematic of filter garden assembly.

TABLE 02 :
Cost to build a septic tank for a residence with 5 people.

TABLE 03 :
Cost of executing a drain for a residence with 5 people.

TABLE 04 :
Cost of executing a drain for a residence with 5 people.

TABLE 05 :
Cost of a filter garden for a residence with 5 people.