Introduction

The rapidly growing population has generated greater demand and consequently a greater impact on natural resources, a main one being water. Since ancient times rivers, lakes, and seas have collected waste produced by human activity; these indiscriminate discharges, along with the over-extraction and the low capacity of treatment of the resource, have generated a greater increase in water pollution.

According to data collected by the World Health Organization (WHO) in ²⁰¹⁴, 2.5 billion people lack access to improved sanitation. In developing countries more than 80% of wastewater is discharged without treatment. According to the World Bank (2014), between 70 and 75% of wastewater in Latin America returns to water bodies without any treatment, creating a serious ecological and public health problem. This figure is particularly troubling when we consider that 80% of the population lives in cities, many of them in settlements near contaminated sources.

Although for several decades now following international regulations and meeting national needs and local contexts, governments and agencies responsible for water management have placed the process of wastewater treatment as a high priority and central strategy to improve the quality of life, protect public health, and progress towards sustainable development. These goals have not been achieved. In Mexico, sewer system coverage in 2012 reached 90.5%. The waste treated flow during the last decade has increased, at national level, to almost 100%, from 64.5 m³/s in 2004 to 105.9 m³/s in 2013, totaling 2 287 municipal plants. However, untreated waters still account for 49.8% of the total (^{Conagua, 2014}) which shows the huge lagging behind that remains in the country.

According to ^{Lahera (2010)} water treatment is an unfinished process due mainly to four aspects: the population, process technology, the economy, and the normative policy. The population aspect with its consequential urbanization, pressures the demand of water, and in other words, the untreated waste stream generates an inadequate disposal problem.

The process technology is very important in regards to the treatment techniques used and their degree of complexity. According to National Water Commission (^{Conagua, 2014}), 57.3% of the total treatment plants at the national level use activated sludge treatment. According to ^{Lahera (2010)} this method makes intensive use of chemicals and energy. The method generates emissions of air pollutants (ammonia) and large amounts of toxic sludge whose final disposal is not guaranteed despite the existence of NOM- 004 of biosolids.

Regarding the economic aspect, it has been found that the water treatment infrastructure available does not cover the needs. In Mexico the infrastructure only collects 50.2% of the total wastewater (^{Conagua, 2014}) and it is non-efficient in its operation. There is an overuse and underuse of resources, that is, some treatment plants operate with deficiencies and treat a flow greater than their installed capacity, while others have an installed capacity greater than the flow of sewage they capture (the total installed national capacity is 152.17 m³/s, although the treated flow is only 105.93 m³/s (^{Conagua, 2014}: 105) this non-effective planning implies that the costs are higher. Despite regulations that encourage inter-municipal groupings to save costs, most plants are allocated to municipalities individually. Further, the plants operation and maintenance are municipal (the construction of plants is responsibility of the state and federal government (such as “Agua Potable, Alcantarillado y Saneamiento en Zonas Urbanas (APAZU);” “Programa para la Construcción y Rehabilitación de Sistemas de Agua Potable y Saneamiento en Zonas Rurales (Prossapys);” and, “Programa de Tratamiento de Aguas Residuales (Protar)”). The operation and maintenance costs are municipal responsibility) responsibility, many of them do not have the necessary solvency nor prioritize the management of their plants.

The treatment of water does not have ecological recognition, nor factors the benefits that could be obtained by the generation of revenue from the use and sale of treated water in non-potable uses (^{Lahera, 2010}). Additionally, a large percentage of the population is not in a position to assume cost increases for the treatment (which requires of tariff strategies and a review of inequity in services).

In the normative policy aspect, political profitability is favored. Thus, when facing insufficient budgetary resources, priority is given to providing potable water and building the drainage network in detriment of sanitation measures, since the former represent greater electoral capital and the expansion of the network of influence over the communities (^{Carabias, Landa, Collado, & Martínez, 2005}; ^{Islas & Sainz, 2007}; ^{Aguilar, 2010}). In the normative policy area, factors such as the legislation and sanctions can also influence it.

It is necessary to clarify that, although the sanitation of wastewater involves the assurance of different interrelated phases (collection, transportation, treatment, and adequate disposal to the receiving bodies), this study specifically focuses on the analysis of the water treatment because it lags the most. The objective of this work is to provide elements (mainly qualitative, since the information on the treatment plants in the entity is not uniform) for the evaluation of water plants operation and treatment, and the search of alternatives in the management of wastewater in the state of Zacatecas. This work is divided into four sections, the first outlines the methodology used to complete this work. The second makes an initial diagnosis that indicates the operation of the treatment plants in the state according to the information collected. The third provides an analysis that incorporates the types of treatment used in the state and their relationship with the location. Finally, the fourth section conclusions, analyzes the limitations that prevail in treatment plants and is grouped in the four axes proposed by ^{Lahera (2010)} but applied to the specific Zacatecas state case.

Methodology

This study is based on the Master´s thesis of ^{Rivera-Salinas (October, 2011)} in which she makes an inventory of the water treatment plants in the state of Zacatecas. In her thesis, the author geo references the state's treatment plants, characterizes them according to their type of treatment, indicates their regulatory compliance, builds a photographic archive, and identifies and proposes rehabilitation measures for the plants that require it. Further, the thesis was the input for subsequent inventories of the state plants: Inventario nacional de plantas municipales de potabilización y de tratamiento de aguas residuales en operación ^{Conagua (2011)}, Diagnóstico de plantas de tratamiento de agua residual (^{PTAR, 2014}), and Inventario de la Secretaría de Agua y Medio Ambiente (^{SAMA, 2017}).

From the review of these inventories, an annual analysis of the treatment plants was conducted and the treatment plants were classified according to their degree of operation in four types: plants operating satisfactorily, plants operating, plants operating with deficiencies and plants out of operation (see Figure 2). The plants that operate satisfactorily are those that comply with NOM-001 and are close to compliance with NOM-003. These plants present acceptable conditions regarding cleaning and maintenance. Plants operating are those that work acceptably but not at their optimum levels. The plants with deficiencies are characterized by major difficulties such as lack of maintenance and wastewater holdback, installations require the building of basic areas and some areas are flooded. In terms of administration, the plants have financial difficulties and some plants have issues incorporating users to their network. Finally, the plants out of operation as their name indicates do not operate at all and the vast majority of their infrastructure is in deplorable conditions.

Figure 2 Treatment plants according to level of annual operation. Source: Developed by authors with data from ^{SAMA, 2017}. 

A second classification of treatment plants was done according to their types of treatment, based on the classification of ^{Conagua (2007)}, which divides them into primary, secondary and tertiary treatments. In the primary treatment, the suspended solids are physically removed and the biochemical oxygen demand is reduced in a certain percentage (sedimentation and flotation processes in lagoons). In the secondary treatment, colloidal and dissolved organic materials are removed by natural and chemical processes such as biological filter system, anaerobic reactors, stabilization ponds and activated sludge. Finally, tertiary treatment, removes dissolved materials such as gases, natural and synthetic organic substances, ions, bacteria, and viruses (via ion exchange, reverse osmosis, advanced oxidation, electrode injection, etc.).

Water treatment in Zacatecas

The state of Zacatecas has a 74 502 km² extension distributed between 58 municipalities that own 4 672 cities and towns as well as 34 available water reserves. The total population in 2013 was 1.55 million and it is expected to be 1.73 million by 2030 (^{INEGI, 2014}). The state has an interesting demographic fact: there is a contrast in between the few urban zones in development and a big number of rural towns with a small or decreasing population. These demographic issues could explain the lack of service provisions. Overall, the state drinking water coverage is 94.31% and sewer system coverage is 89.07%. When separating urban areas from rural areas, the urban areas’ figures are at 98.38% and 97.6%. Rural areas’ figures are at 88.42% and 76.69% and this indicates a lower coverage (^{Conagua, 2014}).

According to ^{INEGI (2011)}, the state of Zacatecas was one of the top five states in the country with the most lags in sewer water connections in 1990 (see table 1), just preceded by the states of Campeche, Guerrero, Nayarit and Yucatan. Zacatecas reached a rate of 85.3% of the total houses connected by 2015. Currently, the state generates 4545.4 lps of sewer water (1.98% of country total, 21st place nationwide) and collects 4117.80 lps (19th place nationwide). This means that 427.6 lps of sewer water are spilled on non-authorized sites generating pollution (^{INEGI, 2014}).

Table 1 Progress of housing sewer water connected in Zacatecas compared with nationwide data 1990-2010. 

Year	Zacatecas (%)	Nationwide (%)
1990	46.3	62
2000	68.2	75
2005	84.4	85
2010	89.0	89.1
2015	85.3	92.8

Source: Developed by authors with data from ^{INEGI, 2011} and ^{INEGI, 2015}.

Although the state provision of sewerage to housing has improved, progress in water treatment has not been as fortunate. According to ^{Semarnat (2013)}, the volume of wastewater produced in 2013 was 44.59 hm³ (3.77 hm³ industrial source, and 40.82 hm³ municipal origin), about 70% (31.44 hm³) of this wastewater is not treated. In the state of Zacatecas, the installed capacity of water treatment in municipal plants is 1194.6 lps, this represents only 0.85% in a nationwide comparison (^{INEGI, 2014}). Although the treated flow differs from the capacity, ie, if 4 117.8 lps are collected and only 1049 are treated. The installed capacity (25.47%) is well below its requirements and is located in the 26th place at the nationwide level (see table 2). It is also relevant to point out that the wastewater processed by the industry infrastructure is incipient with just only 1.1 % of the nation. This locates the state in last place nationwide in industrial treated effluent.

Table 2 State comparison regarding the national advance of wastewater and municipal and industrial treatment plants. 

Concept	Nationwide	State	State percentage (%)	Place
Wastewater generated (lps)	229 734.5	4 545.5	1.98	21
Wastewater collected (lps)	210 169.4	4 117.8	1.96	19
Municipal treatment plants in operation	2 342	73	3.11	11
Installed capacity (lps)	140 142.1	1 194.6	0.85	29
Treated flow (lps)	99 750.2	1 049	1.05	26
Treatment coverage (%)	47.5	25.5	25.5	26
Industrial treatment plants in operation	2 530	15	0.59	26
Installed capacity (lps)	74 934	157.3	0.21	30
Flow rate in operation (lps)	60 532	48	0.80	31
Participation with respect to waste water generated (% )	26.3	1.1	1.1	31

Source: ^{INEGI, 2014}.

According to the most recent information on treatment plants, the Inventario SAMA (^{SAMA Inventory, 2017}) has registered 69 plants in operation, 20 plants out of operation, and six under construction. Of the total plants built in the state, 31 are located in urban areas, 61 in rural areas, and three are privately run (the private plants are Bernardez, Zona Militar, and Minera Proaño in Fresnillo). The plants are distributed throughout 58 municipalities, Fresnillo is the municipality with the highest number of treatment plants (6), followed by Pinos and Tabasco with four plants each, Genaro Codina, Sain Alto, Villa García and Villanueva with three plants each (see figure 1).

Figure 1 Distribution of municipal treatment plants in Zacatecas. Source: ^{PTAR, 2014}. 

According to the classification elaborated and mentioned in the methodology, it has been found that from the 96 plants built in the state from 2004 to present day, 34% of the plants in the state operate satisfactorily, 19% operate acceptably but not at their optimal level and present limitations that need to be improved. These two groups together add to 53% of the state total, meaning that more than half of the state's treatment plants are currently in operation.

In contrast, plants operating with deficiencies represent 13% of the state total. According to ^{SAMA (2017)}, these 12 plants are in the way of causing inventory loss due to abandonment of local authorities and the lack of interest in their operation and maintenance. Plants out of operation are 21%. Finally, from 2014 to 2016, seven plants are in the certification process and six plants are under construction, these represent 14% of the total. According to ^{SAMA (2017)}, if all the currently built up infrastructure is accounted for, the state should have the capacity to treat 81% of the state's wastewater (it is important to point out that between the years 2014 and 2016, 6 plants are in the building process (two of them in the receiving delivery process which will then follow the stabilization process) and seven plants in the stabilization process, which means the plants are working but not yet at their optimum level. They currently work in search of their adequate sludge level and their optimum hydraulic retention time. This is a testing period that continues until the processed water reaches the projected water quality that complies with the established regulations).

From the annual operation review (see figure 2) it can be seen that there is no pattern of constant growth or maintenance, indicating there is no long-term planning to reverse the water treatment problem in the state. In regards to regulations compliance, the information obtained does not allow an analysis of the plants in their entirety. However, it is observed that the plants in operation comply (only in general) with NOM-001 that establishes the maximum permissible limits of contaminants in wastewater discharges, and some of them comply with NOM-003, which focuses on water reuse by establishing allowable limits for reusing water in public services. No information was available about compliance with NOM-002 and NOM-004. The first refers to sewer water discharges and the second refers to sewer water and biosolids (^{Semarnat, 2014}).

Despite the lack of an annual pattern, the demographic factor, the urbanization in particular explains the installation and the operation of the plants. When the data from the plants in operation is analyzed, it is found that in the group of plants that operate satisfactorily (a total of 32 plants), 66% of them are located in urban areas. The remaining 34% are found in towns with less than 2 500 persons. In the group of plants that operate without complying with regulations, 33% are located in urban areas while the remaining 67% are located in rural areas. In the group of plants that operate with deficiencies, 20 plants total, we find that 83% serve rural populations. The group of plants that are out of operation fully accentuates this trend and we see how a 95% of the non-operating plants are located in rural areas.

Conclusions

Over the years, population and urban growth have been the basis for the allocation of wastewater treatment plants in the state of Zacatecas. This is not only visible in the establishment of plants with greater treatment capacity but also regarding maintenance and operation. In recent years, the advance in infrastructure has been considerable with the construction of large plants in urban areas. However, there are significant remaining challenges that can be grouped in four categories that jointly enable or inhibit the performance and efficiency of treatment plants:

The first category could be classified as divided into two economically important aspects. One aspect refers to a tendency to municipal administrative-financial cuts or changes; this is particularly frequent in small municipalities because they are in unfavorable conditions to assume the operating costs. Costs such as electricity, availability of resources for related construction, rehabilitation, maintenance of infrastructure, chemical reagents supply for operation, personnel training, etc. The second aspect refers to the difficulty of cost recovery by adding the cost of sanitation services to the water services fees, either due to the financial incapacity of the communities or the low political profitability of such measures (in this regard, the Comisión Estatal de Agua Potable y Alcantarillado (CEAPA, State Water and Sewage Commission) states that the federal government allocates 0.50 Mexican pesos to the municipalities for each cubic meter of treated wastewater and this should be invested in the same water treatment plant. However, this amount is rarely claimed by the municipalities. It is necessary to enforce the existing regulations to have the economic resources for an adequate wastewater plant operation. Further, CEAPA states that municipalities use the plants seasonally and do not provide a regular service, between 15 and 20% of the plants release untreated water to the entities’ lake vessels, becoming eligible for an economic sanctions and generating additional expenses to the city’s council (^{Regalado & Alonzo, 2012}).

The second category refers to the fact that the constraints overlap economic and political normative factors at the municipal level. The water treatment is a responsibility that falls on municipalities; however, such delegation of powers in most cases is not accompanied by technical and financial capacities, limiting progress. At the state level, greater attention is being paid to build large technologically advanced plants, prioritizing more specialized plants, and transporting water to them. One example of this is the Osiris plant in the conurbation zone of Zacatecas-Guadalupe that increased the state's treatment capacity to 23.19%. There is also an existing lack of efficiency within the staff recruitment, either over or under-qualified to operate and maintain the plants, high costs for electricity consumption and waste disposal, etc.

The third category refers to the failure in the decentralization of public policies at different government levels. The urban areas, high population centers are favored over small municipalities with low population density. Although urban areas challenges are more relevant because they serve more people (if a comparison is made concerning the pollution generated in the urban areas and in the rural areas, it is evident that the greatest pollution concentration is located in urban areas. This justifies, to a certain extent, the attention given to them. However, one of the problems generated by this urban concentration implies that rural inhabitants who often depend on communal public services and do not have sewer system services are exposed to higher pollution levels.), it should not be forgotten that sanitation in small communities entails higher costs and allocated budgets should reflect this, according to ^{Pombo (2004)}, rural-to-urban cost ratio could exceed values of 15 to 1.

The fourth category is of a technical nature and is related to the operation of the plants. The most recurrent problem according to ^{PTAR data (2014)} is the maintenance of the plants, this derives from the materials' corrosion which needs to be integrated into an urgent planning. Although in newer plants other deficiencies have been found, such as the low incorporation of the population into the network and low budgets (as municipal officials recognize) for operational payments that indicate the lack of municipal solvency.

Together, these problems and their interrelations show the state’s inability to fully provide wastewater treatment services, denoting a non-efficient decentralization of functions where discourses are always advancing and realities stay behind. As a proposal, we suggest medium and long-term wastewater plant planning, where the data from the plants is presented in a periodical, public and accessible information system (currently, there are methodological limitations that do not allow elaborating detailed analysis because the information collected from each plant varies continually over time and these variations are not made available accordingly. Emphasis is also placed on the inadequate training of the staff working on them and the lack of consideration of the effective lifetime of each plant.). This would allow the understanding of the performance of the PTAR in light of the normative regulations that monitor its operation.