1. Introduction
The responsibility for providing drinking water, sewage and sanitation services, in accordance with Article 115 of the Political Constitution of the United Mexican States, corresponds to the municipalities, for which each municipality possesses entities that provide the above water services. For this reason, drinking water rates are set differently in each municipality, depending on what is established by the legislation of each federal entity. In some, the rates are approved by the local congress of the entity, while in others they are approved by the governing body or directive congress of the drinking water operating agency of the municipality or locality, or the state water commission [
1].
In this work, the costs generated from the extraction of water from the wells operated by the drinking water board of the municipality of Villagrán, Guanajuato (JUMAPAV) were determined, and the characteristics of each well were considered for evaluation and analysis. Unit costs were determined through the unit price analysis methodology proposed by the Regulations of the Law of Public Works and Services Related Thereto [
2]. The electromechanical efficiency of the wells was also calculated and the influence of its value on energy costs and the volume extracted was evaluated. Finally, the price of the agency’s fixed rate for 10 m
3 of water was compared with the price obtained from the proposed analysis.
2. Materials and Methods
This study was carried out in JUMAPAV, in the municipality of Villagrán, Guanajuato. The INEGI [
3] reports that the total population of the municipality of Villagrán is 65,791 and the number of private homes is 21,920, of which 16,201 are inhabited and 16,125 have piped water in their housing area. JUMAPAV has 7 deep wells dedicated to the extraction of groundwater for drinking water use.
The total costs of the wells were calculated, dividing them into fixed and variable costs according to article 195 of the RLOPSRM. For the fixed costs, the establishment and equipment costs were determined; the former were estimated with the Catalog of Direct Costs of Drilling Wells for Drinking Water [
4], updating them to the year 2021. From these, the fixed costs for depreciation, investment and maintenance were calculated, and for the latter they were quoted with a private company. The equation to determine depreciation costs is as follows:
where
D is the hourly cost for depreciation of the drilling and well equipment,
Vm is the value of the drilling (P) or equipment (E) of the well,
Vr is the salvage value of the P or E of the well and
Ve is the economic life of the drilling or well.
The cost per investment is obtained from the following equation:
where
Im is the hourly cost per investment,
Vm and
Vr are defined in Equation (1),
Hea is the effective hours of operation per year and
i is the annual interest rate.
The maintenance cost considers a value of 4% of the cost of drilling. Therefore, the fixed cost is integrated as follows:
where CF: annual fixed cost, D: annual depreciation cost, I: annual investment cost and CM: maintenance cost.
The variable costs are a function of the operating time of the well; the costs were considered for electrical energy, oil, labor and maintenance:
where CV: annual variable cost, CE: electric energy cost, CO: operating cost, CM: maintenance cost and CA: oil consumption cost.
For the cost of electrical energy, it is necessary to determine the electromechanical efficiency of each well, which is the total efficiency of the pumping equipment considering energy consumption and pumping expenses. This was calculated in accordance with NOM-006-ENER-2015, with the electrical and hydraulic variables; the power consumed by the engine and the power used were determined using the following formulas:
where
fp: power factor,
n: 3 for three-phase motors, 745: conversion factor to HP,
Im: average intensity measured in amperes and
Tm: average voltage measured in Volts.
where specific weight of water, 1000 kg/m
3,
Q: flow rate in m
3/s,
CDT: total dynamic load and 76: conversion factor to HP.
The power used is less than the power consumed due to losses caused by hydraulic friction in the column and discharge, among others.
The calculation of the total cost is the sum of the fixed costs and the variable costs:
where CT: total annual extraction cost, CF: fixed cost and CV: annual variable cost.
To determine the unit extraction cost, and to obtain a cost per m
3 and hourly extraction cost:
where
CAS: cost of groundwater per m
3,
CT: total cost of exploitation and
Ve exploitation volume, m
3/year.
where
Ch: hourly cost of groundwater extraction, EUR/h,
CT: total cost of exploitation and
TO: annual operation time.
3. Results
The drilling and equipment costs were estimated with the Catalog of Direct Costs of Drilling Wells for Drinking Water [
4], updating them according to annual inflation until 2021; see
Table 1.
To obtain the fixed cost, Equations (1)–(3) were applied.
Table 2 presents the costs for depreciation, investment and maintenance and the fixed cost for three wells in order to illustrate them.
In the variable costs, for the consumption of electrical energy, the hours that the equipment operates in a year and the power it consumes were considered. The annual operating time was 8 h a day for 365 days a year, resulting in 2920 h. Equations (5)–(7) were applied, and
Table 3 shows the results for three wells and the annual energy consumed (AEC) in kW hours. For oil consumption, only the “Salida a Juventino” well has vertical turbine-type equipment; its cost for annual oil consumption was EUR 228.95. For the maintenance cost, 10% of the cost of drilling each well was considered. The cost of labor was calculated considering a salary of 15.92 EUR/day, resulting in an annual cost of EUR 727.87.
Total and unit costs: The total annual cost is the sum of the fixed and variable costs (Equation (8)), the unit cost for the extraction of one cubic meter of water was obtained by applying Equation (9) and the cost for one hour of extraction was obtained by applying Equation (10); see
Table 4.
The JUMAPAV operating body charges a rate according to use: domestic, commercial, industrial, mixed or special. If the sum of the total costs of the wells is taken as a direct cost (EUR 246,311.90) and the indirect costs are added to this (the cost of administration of the organization), reaching a total of EUR 1,383,668.42 (year 2021), these are divided by the total annual volume multiplied by a driving efficiency of 70%, and a cost per 10 m3 of 11.94 EUR/m3 is obtained.
5. Conclusions
The power consumption of the well pumping equipment directly influences the electromechanical efficiency, so low values generate higher variable costs. In the “DIF” and “Salida a Juventino” wells with efficiencies of 32.27% and 33.24%, respectively, it is recommended that rehabilitation or replacement of the pumping equipment be carried out. In the total costs, when relating them to the volume extracted, it is observed that the greater the expense, the lower the price per cubic meter generated, since the greatest impact of the costs occurs mainly in the fixed costs and when comparing the operation time with the costs. It was observed that the price was higher when the efficiency in the wells is low. The cost of the annual domestic rate (2021) of 10 m3 of the operating organization was EUR 121.02; the estimate in this proposal is EUR 11.94, and if considering direct plus indirect cost, there is a lack of financing and utility costs; therefore, it is recommended to review the methodology used for the cost of obtaining rates from the operating organization since the rate for this work represents 9.8% of what is charged.