Water Supply Systems for Settlements with Arsenic-Contaminated Groundwater—Making the Right Choice

Abstract

The environmentally and economically sustainable provision of safe drinking water remains one of the most significant public health challenges facing society. It is particularly difficult in small municipalities that lack the human and financial resources to maintain complex and/or expensive water treatment systems, especially in regions where the water is naturally contaminated with toxic substances such as arsenic. This paper analyses two municipalities (Kanjiža and Senta) in northern Serbia with groundwater arsenic concentrations significantly higher than the 10 µg/L maximum allowable concentration. Two different water supply approaches are compared: (1) local supply, where each settlement has their own treatment plant using existing sources; (2) creation of a centralised network, which connects several settlements together with better quality water sources. The analysis considers techno-economical aspects and the quantity and quality of available water, as well as the yields investment and exploitation cost projections for both approaches. The results demonstrate that for small municipalities, a holistic approach to decision making is necessary to find the best solutions for drinking water supply, which in the case of the two municipalities studied is connection to a centralised system supplied from better quality water sources and the creation of connections beyond municipal administrative boundaries.

1. Introduction

Securing an adequate supply of safe drinking water remains one of the most significant public health challenges facing society, especially in rural areas [1,2]. As global living standards improve, the demand for water increases. Meanwhile, the effects of climate change are most visible by their impact on the water cycle. Both flooding and drought are increasing in frequency and severity, with the water balance in many countries under severe pressure [3]. With surface waters subject to diverse sources of pollution, groundwaters are generally viewed as favourable drinking water sources, as they are often of good natural quality, requiring only minimal treatment, and their depth protects them from anthropogenic influences [4]. However, there are situations in which the quality of the groundwater degrades, either from natural contamination by dissolution of the surrounding soil/rocks in the aquifer or from pollution originating from human activities. Globally, arsenic has been identified as one of the most common natural groundwater contaminants [5]. According to the IARC (International Agency for Research on Cancer) classification, inorganic trivalent arsenic is classified as Group I, which means that it is a proven human carcinogen, while pentavalent inorganic arsenic and the organic arsenic species are classified as toxic substances. In addition, a limited number of in vivo and in vitro tests have shown the mutagenic effects of arsenic (i.e., chromosomal aberrations). With this in mind, the control of arsenic contents in drinking water is of particular importance, as chronic exposure of this pollutant leads to an unacceptably high risk of disease.

Sustainable Development Goal 6 specifically mentions, in subgoal 6.4, the need to increase the sustainability of water withdrawals and the supply of freshwater [6]. In order to do so in regions subject to natural groundwater contamination, it is important to take into account the availability of alternative water sources, the concentration of those contaminants, and the local financial capacity to install and maintain the appropriate treatment technology. Groundwaters often contain complex mixtures of contaminants, and frequently even the presence of safe levels of a different chemical can affect the performance water treatment technologies. The correct choice of arsenic removal technology will, therefore, depend on the other water constituents. For example, in the groundwater used for the municipal water supply in the region presented in this paper, natural organic matter (NOM) is also present, whereby total organic carbon (TOC) values as high as 13 mg/L have been detected. This NOM can have a great effect on the behaviour of arsenic during water treatment. The Republic of Serbia is one of the many countries with significant issues because of natural arsenic in the groundwater, especially in the Autonomous Province of Vojvodina, situated in the northern part of the country [7]. Many settlements near the borders with Hungary and Romania are affected by this issue, with groundwaters in this region containing unacceptably high concentrations of arsenic (up to 250 μg/L). Unfortunately, most of the water supply systems in Vojvodina do not have the technology to remove arsenic from groundwater. Instead, local statutes prohibit the human consumption of tap water received from public water utilities, effectively transferring responsibility for the consequences of this issue to the consumers. Figure 1 shows the locations of the affected drinking water wells, which serve approximately 600,000 people (about one-third of the entire population of Vojvodina) with tap water containing more than the 10 µg/L of the maximum allowable concentration of arsenic [8].

Given the status of the tap water in these locations as technical water, the majority of the inhabitants use bottled waters as their main source of drinking water. The use of bottled water places a large financial burden on the local residents; in addition, the water is distributed, for the most part, in single-use plastic water bottles, creating unnecessary plastic waste. According to Schmidt et al. [9], expressed per person per year, Serbia uses approximately 1.6 times more PET bottles than Germany, and only 11% of those bottles are recycled, so the environmental consequences of the substitution of bottled water for tap water should not be underestimated.

Generally, smaller settlements present a greater challenge for drinking water supply: the quality criteria for what constitutes safe water do not change with a decreasing population (although the legally required monitoring frequencies are often less stringent for small settlements [10]), but the available financial and human resources become significantly more limited [11]. Additional care must, therefore, be taken when assessing the feasibility of technically demanding solutions in small settlements. For small systems, the application of expensive and complex treatments to remove arsenic from groundwater is not usually feasible; in addition to the economic costs, the technical expertise required is often beyond the human resources of small settlements. A municipality’s socioeconomic situation can, thus, significantly affect the infrastructure and operations of its water treatment facilities [12]. Research into the link between lower quality drinking water supply and the development of municipalities in Central Europe is a topical issue for multiple reasons. The socioeconomic status of a municipality can influence the quality of piped water supply. Municipalities with lower socioeconomic status may face challenges in maintaining and upgrading water supply infrastructure (e.g., scattered villages or insufficient capacity and quality of water supply). Additionally, lower socioeconomic status may be associated with a lack of awareness or knowledge of safe water practices, leading to further waterborne health risks. At the core of the problem, municipalities with lower levels of economic development may struggle to fund and maintain adequate water treatment facilities, leading to issues with water quality and safety.

Finding technologically, economically, and environmentally viable solutions for water supply in regions with many small settlements and poor water quality resources is thus a significant challenge. The current and future water demands at each settlement must be taken into account, as well as the quantity and quality of the available water resources. In this context, the application of numerical models can be very helpful for long-term planning and to take into account all the factors influencing the long-term supply of safe drinking water [4].

The relative merits of centralisation and decentralisation in the provision of public infrastructure is the topic of much debate. Lubell and Robbins discuss the different drivers of centralisation versus decentralisation and note how different types of public infrastructure may benefit from various levels of governance hierarchy [13].

Duong et al. argue that centralisation is the most cost-effective and also the safest strategy for arsenic-affected groundwater [14]. They note that best practice for drinking water supply requires a multiple barrier approach including not just source selection and protection and adequate treatment but also continuous monitoring of water quality and system maintenance. The last two, in particular, are difficult for decentralised systems to achieve. Duong et al. also state that the digital transformation of the water treatment sector is greatly increasing the monitoring and maintenance capabilities of small decentralised systems. However, they conclude that the increased reliability of centralised water supplies and the economics of scale mean that decentralised systems should only be used as a stop-gap solution to allow low-income communities to better transition to larger scale centralised systems. Stoler et al. also discuss the benefits and disadvantages of decentralisation [15]. They acknowledge that the inherent greater flexibility of decentralisation, with large numbers of small-scale investments, allow decentralisation strategies to react faster to the water supply challenges brought about by climate change. However, despite offering several examples of the successful deployment of decentralised systems, they caution against wide spread decentralisation, noting that the disadvantages in safe supply reliability are largely borne by the most disadvantaged sections of society, potentially placing decentralisation at odds with the UN Sustainable Development Goals.

In contrast, Khalkhali et al. argue in favour of decentralisation, noting that existing centralised systems are facing significant challenges relating to ageing infrastructure and are inherently more vulnerable to natural and man-made threats [16]. Their arguments are supported by Hafeez et al., who also favour decentralisation, especially in rural areas, where the initial capital investments required make centralised systems prohibitively expensive [17].

An interesting pilot study was conducted by Shäfer et al. [18] who demonstrated the use of solar powered membrane technologies in a decentralised water supply system in Namibia. However, the authors themselves note that such high technology systems are especially difficult to maintain in developing countries. In order to form a truly sustainable water supply strategy for a region, the treatment technologies applied need to be well understood and readily available locally.

This paper presents an example of a water supply analysis for two neighbouring municipalities in Vojvodina, where the currently exploited water resources have elevated arsenic concentrations. All available options are evaluated, taking into account the number of inhabitants covered, socioeconomic aspects, quality of the treated water and efficiency of the treatment required, techno-economical aspects of the possible technological solutions, potential availability of arsenic-free water sources in the vicinity, and possibility of agglomerating separate settlements into connected microregional water supply systems.

2. Materials and Methods

2.1. Study Area

The population data relating to the municipalities studied was taken from the 2011 census and recent Statistical Office of the Republic of Serbia data [19,20]. AP Vojvodina, the northern province of the Republic of Serbia, is characterised by a large number of small settlements (Figure 2). In this province, 49.2% of the population live in settlements with less than 10,000 inhabitants.

The majority of the medium-sized and small settlements in Vojvodina have public water supply systems. However, although they have generally solved the water supply problem in terms of water quantity and microbiological safety, they have not solved all issues in terms of chemical water quality. Many of the smaller settlements take water from a local groundwater source, and after chlorine disinfection, pump it directly through the distribution system to the users, without any prior treatment. Figure 3 shows the municipalities where the chemical quality of the water is addressed, to at least some extent. Note that the two settlements chosen for this study, Kanjiža and Senta, are located in an area with no such drinking water treatment plant (WTP). They are featured in the larger scale map inset shown in Figure 1.

2.2. Methodology

After the analysis of the current water requirements and a socioeconomic analysis of the municipalities, the water supply lines for the area were conceptualised based on the best available source of water in relation to the quantity and quality of the water.

In the analysis, two distinct approaches were considered: (1) separate, local supply systems, which implies that each settlement in the municipalities have their own water treatment plant (i.e., local approach); (2) a central supply option for a microregional system, based on a combination of the best available sources in terms of quality and quantity, taking into account only sources realistically close to the distribution system itself (central approach). Fundamentally, the local approach minimises pumping distance but at the expense of multiple WTPs, whereas the central approach would minimise the number of WTPs required but at the expense of significantly longer distribution systems. Note that from a regulators standpoint, an additional advantage of the centralised system is that water quality across the system is considerably simpler to control.

In order to inform the decision making, an estimate was made of the investment and operational costs for both of these approaches. Investment costs were evaluated based on the costs of the equipment required by the system and its supporting infrastructure. For both the local and centralised approaches, this includes, at the very least, a WTP, water towers, pumping basins, pumping stations, booster pumps, and pipelines for settlements that are not connected to the distribution system.

The investment costs of the WTP were estimated based on water quality and a defined treatment technology.

Table 1. Water treatment schemes [21].

Scheme	Technologies Required †	Treated Parameters
B	degasification/aeration, filtration, disinfection	CH4, CO2, NH3, Fe, Mn
C	degasification/aeration, filtration, adsorption, disinfection	CH4, CO2, NH3, Fe, Mn, organic matter < 20 mg KMnO4/L
D	degasification/aeration/oxidation, filtration, adsorption on granulated activated carbon (GAC), disinfection	CH4, CO2, NH3, Fe, Mn, organic matter < 20 mg KMnO4/L, As < 50 µg/L
E	degasification/aeration/oxidation, separation of floccules, GAC adsorption, disinfection	CH4, CO2, NH3, Fe, Mn, As > 50 µg/L
F	degasification/aeration/oxidation, separation of floccules, GAC adsorption, disinfection	CH4, CO2, NH3, Fe, Mn, organic matter < 20 mg KMnO4/L, As > 50 µg/L
G	degasification/aeration/oxidation, filtration, membrane separation, disinfection	CH4, CO2, NH3, Fe, Mn, As, Na, B, electrical conductivity > 1000 µS/cm, organic matter < 20 mg KMnO4/L
H	degasification/aeration/oxidation, separation of floccules, membrane separation, disinfection	CH4, CO2, NH3, Fe, Mn, As, Na, B, electrical conductivity > 1000 µS/cm, organic matter > 20 mg KMnO4/L

^† Technologies defined in the Strategy of water supply and water protection in AP Vojvodina [21].

presents seven possible water treatment schemes, as proposed in the Strategy of water supply and water protection in AP Vojvodina [21] which was co-authored by the current authors. Note that in the Serbian regulation for drinking water quality, the total organic matter is controlled by the oxidisability [10]. In general, the “Treated parameters” column broadly reflects the variations in water quality found in Vojvodina groundwaters, where no issues with other heavy metals, such as nickel or uranium, have been detected.

3. Results and Discussion

3.1. Drinking Water Requirements and Socioeconomic Position of the Investigated Area

Socioeconomic and water supply data from 2021 for the municipalities Kanjiža and Senta are provided in

Table 2. Socioeconomic and water supply data in 2021 [20].

Data Type	Serbia	Vojvodina	Kanjiža	Senta
Area (km2)	88,499	21,614	399	294
Settlements	6158	467	13	5
Inhabitants	6,834,326	1,825,982	23,039	21,120
Population density (per 1 km2)	77.2	84.5	57.7	71.8
Average age	43.5	43.1	44.1	43.8
Number of households	2,487,886	696,157	10,097	9383
Number of employed	2,273,591	596,668	5710	5775
Average salary before taxes and contributions, per employee	65,864	62,494	54,419	59,431
Water supply data
Number of households connected to the water supply network	2,160,943	672,460	10,097	8195
Total captured water, (thousands of m3)	686,664	154,736.7	1259	1245
Delivered drinking water (thousands of m3)	445,409	112,508.3	1167	909
Realisation of investments in new fixed assets in the field of water supply and wastewater management (EUR 1000s *)	114,477	40,961	304	164
Employees in the field of water supply and waste management	35,621	8945	59	103

* Original costs reported in Serbian Dinar, converted at RSD 118 to EUR 1.

. The data provide a comparison between the municipalities of Kanjiža and Senta with the larger region of Vojvodina, as well as the country of Serbia. These statistics show that Kanjiža and Senta are smaller and less densely populated compared to the Vojvodina region as a whole. The municipalities are located in a region that boasts a gross domestic product greater than the national average, whilst Kanjiža and Senta are among the top 20 municipalities in Serbia with a level of development above the national average [22]. However, a more detailed look at the data reveals a less favourable picture. For instance, the average age of the inhabitants in Kanjiža and Senta is slightly higher than the national average, while the average salary before taxes and contributions per employee is slightly lower than both the state and Vojvodina averages.

In terms of water supply, Kanjiža has approximately 10,097 households connected to the water supply network, representing approximately 1.5% of the total number of households connected in the province of Vojvodina. Similarly, Senta has approximately 8195 households connected, accounting for around 1.2% of the total households connected in the province. It is clear from the data in Table 2 that both Kanjiža and Senta capture a relatively small amount of water compared to the provincial and national levels. The same trend can be observed in terms of delivered drinking water, realisation of investments in new fixed assets in the field of water supply and wastewater management, and the number of individuals employed in the field of water supply and waste management. The small size of these municipalities suggest that the expected benefits from the economies of scale achieved by centralisation may be relatively small—the costs per connection in these systems will still be higher than the costs achieved in large urban areas.

In these settlements, groundwater is abstracted almost exclusively from water-bearing layers of basic aquifer complex at depths of 60 m to 220 m, with significant differences in the important hydrogeological parameters. In the upper part, to a depth of approximately 60 m, free aquifers were formed (the first aquifer), and below them are the pressurised artesian aquifers. The quality and quantity of these aquifers encompass a wide range, from the size of the aquifers, capacity, and water restoration method to the water quality, which varies from waters that correspond to the drinking water quality standards to groundwater, which requires highly complex treatment procedures. A summary of the more problematic drinking water quality parameters for the water sources used by the municipalities and settlements in the area covered by this paper is given in

Table 3. Groundwater quality in the major settlements in Kanjiža and Senta.

Parameter	Unit	Municipality		MAC
		Kanjiža	Senta
As	μg/L	49.0	90.2	10
Ammonia	mg N/L	1.08	0.74	* 0.5
Oxidisability	mg KMnO4/L	16.6	20.0	20
TOC	mg C/L	3.23	3.93	NR *
Na	mg/L	127	134	200
Fe	mg/L	0.30	3.29	0.3
Mn	mg/L	0.04	0.04	0.05

* For the waterworks serving up to 5000 EC, the MAC is 1 mg/L. MAC—maximum allowable concentration; NR—not subject to regulation [10].

.

The analysis focused on the determination of arsenic and other parameters that are important for the correct selection of the required drinking water treatment technologies. The obtained results show that the water quality in these two municipalities is generally unsatisfactory. The groundwater quality data, as presented in Table 3, indicate that the amount of arsenic in the main settlements of interest is 5 to 9 times higher than the MAC value given in Serbian regulations [10]. The groundwater source in Senta also contains higher amounts of Fe than the MAC value. In addition, despite the fact that the oxidisability is not higher than the MAC, the high TOC values (above 2 mg C/L) indicate that NOM is present in concentrations higher than recommended for microbial growth prevention [23]. This must be taken into consideration when the treatment and distribution system is planned. In terms of physical and chemical quality, it is necessary to innovate or install water treatment technologies in order to satisfy the Serbian drinking water regulation [10].

Overall, these findings indicate that both Kanjiža and Senta face challenges to secure sufficient resources to invest in water supply and related infrastructure compared to the wider Vojvodina region and the country of Serbia as a whole.

A more detailed breakdown of the water requirements of the settlements analysed is given in

Table 4. Water requirements and current water supply situation in the settlements analysed.

Label	Settlement	Population	Req. Qmax.d. (L/s)	Current Situation		Treatment Scheme 1	Invest. Value of Loc. WTP (EUR)
				Number of Wells	Abstracted Qz (L/s)
Municipality of Kanjiža
K0	Kanjiža	9871	33.4	9	22.0	D	1,550,000
K1	Velebit	277	0.9	1	1.0	B	74,000
K2	Doline	390	1.2	2	0.5	B	110,500
K3	Zimonjić	244	0.8	1	0.5	D	75,500
K4	Male Pijace	1811	5.6	3	2.5	D	395,500
K5	Mali Pesak	94	0.3	1	0.5	D	75,500
K6	Martonoš	1988	6.1	2	2.0	B	307,500
K7	Novo Selo	157	0.5	1	0.5	D	75,500
K8	Orom	1423	4.4	3	2.5	B	307,500
K9	Totovo Selo	618	1.9	2	2.0	B	191,000
K10	Horgoš	5709	18.8	6	10.0	D	1,210,000
K11 *	Trešnjevac	1724	5.3	2	2	B	307,500
K12 *	Adorjan	1037	3.2	1	1.5	E	244,000
Municipality of Senta
S0	Senta	18,704	66.8	7	45	F	3,076,100
S1	Bogaroš	568	1.8	1	1	D	260,900
S2	Gornji Breg	1726	5.5	1	3	C	397,050
S3	Kevi	726	2.3	1	1.5	D	260,900
S4	Tornjoš	1592	5.0	4	2	D	395,650
Municipality of Čoka
S5 *	Čoka	4028	11.8	7	35	G	1,328,000
S6 *	Ostojićevo	2324	6.9	4	4	G	796,500
S7 *	Jazovo	742	2.2	2	2	G	488,000
S8 *	Sanad	1151	3.4	1	2	G	488,000
Municipality of Novi Kneževac
NK	Novi Kneževac	6960	21.4	7	40	D	1,210,000

¹ See Table 1. * Settlements in other municipalities, which are envisioned as connections to the Senta system because of their geographical proximity.

. Note that this table also includes certain settlements from neighbouring municipalities, which because of their geographical proximity to Kanjiža and Senta are sensible candidates for inclusion in the centralised water supply systems proposed in this work.

The municipality of Kanjiža consists of thirteen settlements with a total population of 23,039 inhabitants. The smallest settlement has only 94 inhabitants, and the largest one is the town of Kanjiža with 9871 inhabitants. The area of the municipality is 399 km². All settlements in the municipality have an organised water supply with basic disinfection. The average exploitation of groundwater for the needs of the municipality is approximately 45 L/s.

The municipality of Senta consists of five settlements with a total population of 21,120 inhabitants. The area of the municipality is 294 km². All settlements in the municipality have an organised water supply. The average exploitation of groundwater supply for the needs of the municipality is approximately 55 L/s.

Note that there is an additional issue in Velebit, Zimonjić, Mali Pesak, Novo Selo, Adorjan, Bogaroš, Gornig Breg, Kevi, and Sanad. These communities are all currently supplied from just one well each. This lack of redundancy greatly increases the risk that these settlements will experience extended periods with no water supply, as the single wells may fail, and certainly require downtime for regular maintenance.

Figure 4 shows the water requirements and abstraction data graphically. There is a significant disparity between water requirements and the amount of water currently extracted in Kanjiža, which implies that the installed well capacity there is currently inadequate. On the other hand, taken as a whole, Senta municipality has a surplus of water, although, as detailed below, it is of generally low quality.

3.2. Drinking Water Treatment Options Based on Source Water Quality

In this section of the analysis, we discuss the drinking water treatment technologies required to treat the source waters of each municipality. Focus is given to the water sources currently exploited in the largest settlements, as well as the better quality sources that have been identified as having sufficient water to warrant consideration as the basis of a microregional system.

3.2.1. Treatment Options for the Municipality of Kanjiža Using the Current Source

The largest water consumer in the municipality is the town of Kanjiža, with almost 10,000 inhabitants (40% of the total population in the municipality). Unfortunately, the groundwater source has elevated concentrations of CH₄, CO₂, NH₃, Fe, Mn, organic matter < 20 mg KMnO₄/L, and As < 50 µg/L (see Table 3). As such, this water requires a relatively complex treatment processes (Scheme D in Table 1), which is depicted in Figure 5, so it is advisable to consider the possibility of bringing better quality water from a distant source.

For adequate treatment, this water requires aeration, oxidation (using, for example, chlorine), sand, granulated activated carbon filtration, and a specific adsorber for arsenic removal. The elevated concentrations of arsenic are removed using this scheme in two steps: (i) by addition of chemical oxidant, all As(III) is converted to As(V), increasing the proportion of arsenic removed with the deferrisation process, thus relieving (ii) the arsenic adsorber and extending the replacement of the specific adsorbent used for arsenic, improving the overall economics of the water treatment process.

3.2.2. Treatment Options for the Municipality of Senta Using the Current Source

The town of Senta is, by far, the largest in the municipality, with almost 19,000 inhabitants (approximately 80% of the total population in the municipality). Accordingly, Senta is the largest water consumer, with 82% of the total consumption. However, the groundwater currently used in Senta is of poor quality and requires the even more complex water treatment Scheme F (Table 1, Figure 6) because of the high arsenic content and elevated NOM content. The possible treatment scheme includes (i) oxidation of As(III) into As(V) and partial removal of arsenic during deferrisation; (ii) addition of FeCl₃ coagulant, whereby most of the arsenic is removed by coprecipitation and adsorption at the formed FeOH₃ floccules; and (iii) removal of the remaining arsenic by the adsorbent.

3.2.3. Treatment Options Using a Central Supply from Better Quality Sources

The complex treatment schemes presented in Figure 5 and Figure 6 indicate that it is advisable to, at least, consider the possibility of pumping water of better quality from more distant sources. This is the fundamental idea behind the second centralised approach for solving the water supply issues of these municipalities. From the available data on the groundwater quantity and quality in the vicinity [8], it was concluded that the nearest location with better quality water from Kanjiža is the settlement Velebit, while for Senta it is in Trešnjevac, a settlement also in the municipality of Kanjiža but located just 14 km from the town Senta. In terms of water quantity, the available hydrogeological data suggest both of these sources have adequate groundwater reserves to supply larger centralised systems. Since the major towns, Kanjiža and Senta, both require more water than is currently abstracted, additional wells will be required whichever approach is taken (note that well costs have not been included in this strategic analysis, they are for the next development phase of the decision-making process). The groundwater from both the sites at Velebit and Trešnjevac requires the much simpler treatment Scheme B (Table 1, Figure 7), which includes aeration/degasification, retention, sand filtration, and disinfection. These two options will be further evaluated taking into account techno-economical aspects.

3.3. The Local Supply Approach

The estimation of the costs for the local supply approach is relatively simple. The analysis takes into account the number of facilities required to support local water supply systems for each settlement in the municipalities and includes water treatment plants (WTPs), pumping pools, pumping stations, and water towers (for settlements with greater water consumption) or booster pumps (for smaller settlements). For the operating costs, accurate estimations of energy costs were not included, as they are impacted by the changing dynamics of daily peak water and electricity demands, a level of detail beyond the scope of this work.

The cost of the individual WTPs, which is based on the technological schemes from Table 1 and the amounts of water required, is given in the final column of Table 4. Note that for the analysis of the overall costs below, both the local and centralised supply approaches include not just the WTPs but also the supporting infrastructure required—water towers, pumping pools, pumping stations, and booster pumps. The operational costs include long-term maintenance such as inspection and sanitation.

For the entire municipality of Kanjiža, the total estimated costs of the local supply to all the villages together is shown in

Table 5. Total costs of local supply for the 13 settlements of Kanjiža—each with its own WTP.

Facility	Investments (EUR)	Operating Costs (EUR/Year)
WTP	4,920,000	913,000
Water towers	1,344,000	50,500
Pumping pools	99,000	3500
Pumping stations	220,000	57,000
Booster pumps	29,000	1800
TOTAL	6,612,000	1,055,800

, and the costs per location are broken down in Figure 8. For the municipality of Senta, the total estimated costs of the local supply approach are given in

Table 6. Total costs of local supply for all 5 settlements of Senta—each with its own WTP.

Facility	Investments (EUR)	Operating Costs (EUR/Year)
WTP	4,389,500	1,015,500
Water towers	1,203,500	7200
Pumping pools	84,000	3000
Pumping stations	155,500	52,000
Booster pumps	13,500	500
TOTAL	5,846,000	1,143,000

and Figure 9.

As shown in Table 3, Kanjiža municipality has a population of 23,000, whilst Senta is slightly smaller, with a population of approximately 21,000. The total cost of investments for the local supply approach reflect this, with Kanjiža requiring EUR 6.6 million in investment compared to Senta, which requires EUR 5.8 million. Per capita, Kanjiža is approximately 3.6% more expensive than Senta, which is to be expected, as the population is distributed across twice as many settlements as in Senta, so the local supply approach requires more WTPs and more pumping infrastructure. Meanwhile, despite its smaller population and fewer settlements, the annual operating costs for Senta are actually higher: EUR 54.1/capita/year in Senta and EUR 45.8/capita/year in Kanjiža. This is because of the considerably worse water quality in Senta (almost twice as much As) requiring more complex treatment schemes with higher maintenance costs.

3.4. Central Supply Approach

In order to compare costs between the centralised and local approaches, the scope of the centralised systems must be decided upon and the investment and operational costs estimated based on the assumption that under both approaches safe and good quality drinking water will be provided to all households in the municipalities investigated.

To demonstrate the differences between the local supply and central supply approaches, it is necessary to re-evaluate which settlements are to be connected. Making rational decisions concerning the configuration of a centralised water supply system is considerably more complicated than the local supply approach. After identifying a nearby good quality water source with a sufficient quantity of water to supply an entire municipality, there are two steps that must be performed to complete the analysis.

First, a preliminary analysis must decide which settlements will be included in the central system. Water transmission pipelines for long-distance transport are expensive to build and maintain. The preliminary distribution network analysis should be of sufficient in detail to include or eliminate settlements based on how economical they are to connect to the centralised water source. If the pipelines required to reach a settlement are considerably more expensive than a local WTP, then that settlement should be eliminated from the planned central supply system. The distance among settlements is of course the most significant factor, but the topography of the land and the presence of major rivers and other natural barriers should also be considered.

The final step of the analysis considers each settlement connected to the central system, and examines in detail the drinking water treatment technology, transmission pipelines, pumping stations and pumping infrastructure, and the local distribution system required. It is important to note that some settlements in the two municipalities studied have, therefore, been excluded from the central supply systems modelled based on these connection criteria, and settlements from other municipalities have been added.

3.4.1. Central Supply Approach for Kanjiža

By analysing the quality of the groundwater in the vicinity, it was concluded that the nearest location with higher-quality water is the village of Velebit (K1), located 11.6 km from Kanjiža. The groundwater at the Velebit (K1) site only requires aeration/degasification, retention, and sand filtration prior to disinfection (Scheme B, Table 1). The preliminary analysis is shown in Figure 10, depicting the diameter and distance of the pipeline network required.

This preliminary analysis immediately eliminated the four peripheral settlements in the south-western part of the municipality: Orom (K8), Novo Selo (K7), Doline (K2), and Totovo Selo (K9). Although they are relatively close to one another and not far from Velebit, their good quality water means there is no economic justification for connecting them to the Velebit source: the investment value of the transmission pipeline from Velebit to these four villages (approximately EUR 1,372,000) largely exceeds the investment value of the local water treatment units (about EUR 885,000, including the WTP and local distribution facilities). Therefore, these settlements were left outside of the Kanjiža system, with the possibility of individual water treatment or connection to some of the other microsystems. The villages of Mali Pesak (K5) and Male Pijace (K4) were also eliminated from the common water supply system using a similar principle, best explained by comparing the cost of the transmission pipeline per user connected. Mali Pesak (K5) has a very small population (94 inhabitants), which makes the pipeline cost for the connection to Mali Pesak approximately EUR 3500/user. This compares very unfavourably to the costs otherwise considered in the preliminary analysis. For example, in the case of Martonoš (K6) and Horgoš (K10), the total cost of the pipelines are EUR 115/user and EUR 160/user, respectively, an order of magnitude less. Having, therefore, eliminated Mali Pesak, although Male Pijace is larger (population 1811), it is almost twice as far from the nearest node, and the cost of the pipeline is approximately 30% greater than its local WTP would be, so it was also excluded from the centralised system. The pipeline to Martonoš (K6) and Horgoš (K10) was only 3% higher than the estimated value of their local WTPs, so these villages were included in the centralised supply network, as the benefits of centralisation are assumed to justify this relatively small extra cost.

Trešnjevac (K11) and Adorjan (K12), at the southern part of municipality, were also not considered for connection to the central system in Kanjiža. This is because the water source at Trešnjevac is necessary for the central system of Senta, in which case connecting Adorjan to the Senta system is logical because of its proximity to the path of the transmission pipeline (see Section 3.4.2).

On the other hand, just across the river Tisa, Novi Kneževac lies about 5 km from Kanjiža and has very poor quality water. The analysis determined that the investment value of just the WTP in Novi Kneževac (EUR 1,210,000) was considerably higher than the estimated value of the transmission pipeline (EUR 461,000), and Novi Kneževac was, thus, included for connection to the central system of Kanjiža.

To summarise the preliminary analysis, the central Kanjiža system relies on a water source and WTP in the village of Velebit. The settlements included are: Kanjiža (K0), Velebit (K1), Zimonjić (K3), Martonoš (K6), and Horgoš (K10) in Kanjiža municipality and Novi Kneževac (NK) in the neighbouring municipality. The configuration of the proposed water supply system with all of the basic infrastructure facilities is shown in Figure 11.

According to the proposed solution for a central water supply, water is abstracted and treated onsite in the village of Velebit, and then, via the main pumping station, it is directed to the settlements in the system. The water treatment plant has a capacity of 100 L/s and applies a simple treatment scheme consisting of aeration/degasification, retention, sand filtration, and disinfection. Clean water flows into the pumping pool. Velebit settlement is supplied by a booster pump unit, which draws water directly from the system pumping pool. The main pumping unit of the system displaces water to all settlements in the system. Zimonjić is directly connected to the main pipeline. In Kanjiža, two pipeline branches are directed towards Martonoš and Horgoš (one branch) and the other branch to Novi Kneževac. The settlements Kanjiža, Martonoš, Horgoš, and Novi Kneževac have their own water towers (VT in Figure 11). To increase the pressure and connect to the water towers in these settlements, at node 3, the construction of a pumping station with two blocks is planned: one block to Martonoš and the other to Horgoš. Before entering Novi Kneževac, another pumping station is planned, connected to the water tower in Novi Kneževac.

The transmission network configuration includes water pipelines ranging from DN400 down to DN140 and includes 32.6 km of pipeline among settlements in total. The estimated value of the central supply for the MS Kanjiža system is shown in Table 6 below. The total cost of the central supply system is almost EUR 8.8 million, with annual operational costs of EUR 0.6 million/year. The pipelines represent approximately 54% of the investment costs and 36% of the operating costs.

3.4.2. Central Supply Approach for Senta

None of the settlements in the municipality of Senta have good quality groundwater sources. As mentioned above, the system, therefore, relies on the water source and WTP in Trešnjevac (K11) in Kanjiža municipality and also encompasses nearby Adorjan (K12), which is only 2.9 km from the source. The water quality in Trešnjevac (<10 µg/L As, 0.57 mg/L Fe) is similar to the water in Velebit (<10 µg/L As, 0.74 mg/L Fe) and as such also only requires treatment Scheme B to provide safe drinking water.

In Senta municipality, as the preliminary analysis shown in Figure 12 demonstrates, it is not economically justified to connect the peripheral settlements Kevi (S3) and Tornjoš (S4) with the Senta microsystem. The investment value of the pipeline from Bogaroš to these two settlements (about EUR 700,000 in total) significantly exceeds the investment value of the local water treatment plants (about EUR 460,000). Therefore, these two villages were left outside the Senta system, with the possibility of individual local treatment solutions or connection to some other microsystem.

On the other hand, as shown in Figure 13 below, across the river Tisa from Senta are four settlements in the neighbouring municipality of Čoka, which also have very poor quality water sources: Čoka (S5) (about 5 km from Senta), Ostojićevo (S6) (about 6 km from Čoka) Jazovo (S7) (about 5 km from Ostojić), and Sanad (S8) (about 5 km from Čoka). The preliminary calculations revealed that the investment value of the four plants in these settlements (approximately EUR 2.15 million) is almost double the investment value of the main transmission pipelines required to connect then to Senta (approximately EUR 1.12 million). Therefore, these four villages have been included in the Senta central supply group.

A total of nine settlements are, thus, envisaged for connection to the Senta microsystem: Senta, Gornji Breg, and Bogaroš in Senta municipality; Čoka, Ostojićevo, Jazovo, and Sanad in Čoka municipality; and Trešnjevac and Adorjan in Kanjiža municipality. The configuration of the system with all of the important infrastructure is shown in Figure 13.

Water is abstracted and treated on site in Trešnjevac and directed over the main pumping station to the settlements in the system. The WTP capacity is 125 L/s, and it operates according to the technological Scheme B. Clean water flows into a pumping pool. Trešnjevac is directly connected to the main pipeline of the pumping station. Senta, Gornji Breg, Čoka, Sanad, and Ostojićevo have their own water towers, and the villages Adorjan, Bogaroš, and Jazovo are supplied by booster pump plants. In front of each village, there is a pumping station. The estimated cost of the central supply system for Senta is shown in

Table 7. Estimated value of the central supply for the MS Kanjiža system and for MS Senta.

Facility	MS Kanjiža System		MS Senta System
	Investments (EUR)	Annual Costs (EUR/Year)	Investments (EUR)	Annual Costs (EUR/Year)
WTP	2,078,500	172,000	2,488,400	201,400
Water towers	1,341,700	80,500	1,576,800	94,600
Pumping pools	322,500	11,300	43,000	1510
Pumping stations	37,000	115,600	323,150	128,800
Booster pumps	2150	130	16,950	1000
Pipelines	4,708,100	211,900	5,569,150	250,650
TOTAL	8,759,950	591,430	10,017,450	677,960

.

The main pipeline extends from the main pumping station at the site of the source and WTP Trešnjevac over Gornji Breg and Senta to Čoka. The network among the settlements is 43.9 km long in total and includes DN400 to DN90 pipelines. The total investment cost of the pipelines is EUR 5.6 million, which is approximately 56% of the price of the entire network, which amounts to EUR 10 million.

3.5. Comparison of the Alternative Approaches

In order to allow for a direct comparison between the costs for the local approach and the centralised approach, the following tables compare the local supply costs and the centralised supply costs for all of the settlements included within the centralised systems regardless of which municipality they are in. Kanjiža is presented in

Table 8. Recapitulation—comparison of the approaches for Kanjiža municipality.

Settlement	Population	Q Av.D *	Investments		Operating Costs
			Total	Per Capita	Total	Per Capita	Per m3 of Water
		m3/d	EUR	EUR/p.c.	EUR/Year	EUR/p.c/Year	EUR/m3
Local supply systems for the 6 settlements in the Kanjiža microsystem
Municipality of Kanjiža
Velebit	277	55	91,000	328	7000	25.8	0.36
Zimonić	244	49	91,500	375	18,500	76.8	1.05
Kanjiža	9871	1.984	2,133,500	216	381,500	38.7	0.53
Martonoš	1988	379	486,500	245	43,500	22.0	0.32
Horgoš	5709	1.161	1,560,500	273	288,500	50.5	0.68
Municipality of Novi Kneževac
N. Kneževac	6960	1283	1,625,500	234	293,000	42.1	0.63
TOTAL	25,049	4.911	5,987,500	239	1,032,000	41.2	0.58
Centralised supply system for the 6 settlements in the Kanjiža microsystem
TOTAL	25,049	5773	8,759,950	350	591,480	23.6	0.28

* Average water supplied/day.

, and Senta is presented in

Table 9. Recapitulation—comparison of the approaches for Senta municipality.

Settlement	Population	Q Av.Day	Investments		Operating Costs
			Total	Per Capita	Total	Per Capita	Per m3 of Water
		m3/Day	(EUR)	(EUR/p.c)	(EUR/Year)	(EUR/p.c./Year)	(EUR/m3)
Local supply systems for the 9 settlements in the Senta microsystem
Municipality of Kanjiža
Trešnjevac	1724	327	454,000	263	41,500	24.1	0.35
Adorjan	1037	197	288,000	278	71,000	68.3	0.99
Municipality of Senta
Bogaroš	568	117	285,000	502	56,000	98.8	1.32
Gornji Breg	1726	342	544,500	316	61,000	35.5	0.49
Senta	18,704	3946	4,166,500	223	874,000	46.7	0.61
Municipality of Čoka
Čoka	4028	721	1,598,000	397	230,000	57.1	0.87
Sanad	1151	205	525,000	456	75,000	65.0	1.01
Ostojićevo	2324	419	977,000	420	128,500	55.4	0.84
Jazovo	742	135	517,500	698	72,500	97.6	1.48
Total	32,004	6409	9,354,000	292	1,609,500	51.5	0.69
Centralised supply system for the 9 settlements in the Senta microsystem
Total	32,004	6409	10,017,450	313	678,000	21.2	0.29

.

Note that to enable a fair comparison to the Kanjiža central system, the local supply approach is presented in Table 7 for all settlements envisaged for connection to the centralised system: Kanjiža, Velebit, Zimonjić, Martonoš, and Horgoš in Kanjiža municipality and Novi Kneževac.

For these settlements then, the value of the investments required for the localised approach is EUR 5,987,500, which is approximately 32% less than the investment required for the centralised system, which amounts to EUR 8,759,950. The absolute difference in investment values is about EUR 2,772,450. In contrast, the annual operating costs are approximately 1.75 times smaller in the central supply alternative for Kanjiža (EUR 591,480/year) than in the local supply alternative (EUR 1,032,000/year). The absolute difference in operating costs is about EUR 440,500 in favour of the centralised system every year. This difference is sufficient to cover the difference in investment in fewer than 10 years, depending on the method of financing. As the depreciation period of drinking water infrastructure is not less than 30 years, it can be concluded that, in this case, as a permanent solution, the central supply solution is the more economically favourable approach.

It should be noted that the operating costs for any of the alternatives are not the total production costs of water but only its part. Among the major costs, the following were left out of our analysis: costs of water abstraction, secondary network, and the organisation of operations.

For the entire territory of Kanjiža municipality, the following may be recommended:

• A central microsystem should be constructed for Kanjiža municipality, with water abstraction and a WTP in Velebit.
• The system includes: Kanjiža, Velebit, Zimonjić, Martonoš, Horgoš, and Novi Kneževac
• Drinking water treatment in the settlements Doline, Orom, Novo Selo, Totovo Selo, Male Pijace, and Mali Pesak should be resolved at the local level.

A summary of all costs (investment and operating) for both alternatives for Senta is presented in Table 8. While the amount of investment is somewhat lower (about 7%, or an absolute difference of EUR 663,000) for the individual local water supply alternative, once again there is a very significant difference between the operating costs, which are far lower in the central microregional system (less than half as much, about EUR 678,000/year). The difference in annual operating costs exceeds the difference in the value of investments, such that after just one year, the centralised microregional system will already be more cost effective. Therefore, the centralised solution with the Senta microregional system is highly recommended. Investments amounting to EUR 313 per capita and operating costs of EUR 0.29 per cubic meter of produced water are quite acceptable. It should, however, again be noted that the above operating costs are not the total production cost of water but only a part of it, with the costs of water abstraction, secondary network and the organisation of operations left out of our analysis.

From this comparison, it is clear that from an economic standpoint, the centralised approach to water supply is a significantly more viable solution for water supply in the two municipalities studied. This result is in agreement with the results of Tang et al., who found that decentralised systems for water softening, although demonstrating sufficient treatment efficacy, were 7–10 times more expensive than centralised systems [24]. However, it should be noted that there are several sociopolitical issues that make realisation of the centralised approach harder to achieve:

(1)

The vast majority of drinking water utilities in Serbia are public companies under the direct control of the municipalities. Each municipality is responsible for their own budgets and their own water supply systems. Politically, the level of cross-municipal cooperation required for the more efficient centralised solutions would in practice be hard to achieve.

(2)

Even within the municipalities, local politics will often side against the centralised solution. As discussed by Kaur and Janmaat [11], regulators often face difficulty in convincing smaller municipalities to take action in the face of public health risks from drinking water. The costs of upgrading existing water supply systems is relatively straightforward to demonstrate, but in comparison to the easier path of doing nothing, the health benefits of the investment are hard to quantify. In Kanjiža for example, the economic benefits are not uniformly distributed throughout the municipality—the village of Martonoš is the only settlement which would pay more in operational costs per capita as part of a centralised system as they would with their own local system. Martonoš would, thus, potentially be harder to convince that the public health benefits outweigh the costs. Thus, for the centralised systems to succeed, each settlement would need be included on the municipal water management board, enabling everyone to have a say on how the costs will be distributed and whether water rates would be increased in each settlement to cover those costs.

(3)

For the localised systems approach, where every settlement has their own WTP, the construction could be easily organised into multiple phases—there is no need to build all the separate WTPs at the same time. On the other hand, the centralised systems are harder to divide into a phased construction regime and, therefore, represent a significantly larger initial investment barrier than the local supply approach.

Considering the relatively limited financial resources of Vojvodina, and Serbia as a whole, at the strategic planning stage, it seems clear that the very considerable benefit in operational costs which is obtained with the centralised supply approach means there is no doubt that all three of these drawbacks should be overcome. Once the strategy decision has been made, the next step would be to enter the development phase, where more detailed feasibilities studies would be required to optimise each system. These studies would include a more in-depth analysis of current and future water demands and predict the requirements for additional wells. In this development stage, a final optimisation of the distribution network would be required. With more precise calculations of operating costs, the development stage would be the final opportunity to include additional settlements in a centralised system. As just one example, by investigating methods for maximising energy efficiency, it is possible that situations could be revealed where increasing the capacity of certain water towers would allow for a larger proportion of pumping to occur at off-peak electricity tariffs, reducing operational costs sufficiently to include previously excluded settlements in the system.

In addition to the economic benefits, the centralised approach would help address shortcomings in the supervision and operation of multiple small systems, which would otherwise require significant support from special services and institutions to ensure secure and hygienic water supply. Over time, these systems could be connected in larger units or in a regional system that relies on water sources at a regional level. This approach would enable consumers to be supplied from two or more water sources and, together with the local capacities (microregional and local source), provides the highest level of safety.

Overall, socioeconomic conditions play an important role in determining the appropriateness and effectiveness of water treatment in smaller municipalities. It is essential for policy makers and community leaders to prioritise investment in water treatment infrastructure and to address economic disparities that may contribute to inadequate water treatment in some areas.

4. Conclusions

This paper presents two possible water supply strategies for rural regions that have serious problems with natural arsenic contamination in their drinking water sources. Two municipalities in Vojvodina, the northern region of the Republic of Serbia, were used to demonstrate the two different approaches: in the first approach, each settlement has their own WTP, and in the second, settlements are connected to a centralised (microregional) system. In regions like this, with a large number of small settlements, strategies should focus on finding techno-economically and socioeconomically appropriate solutions to secure safe drinking water supply, both in terms of delivering the required amounts of water and in terms of quality.

Several possibilities should be taken into consideration, such as treatment of the water from the existing source and exploring opportunities to include nearby settlements in microregional water systems with other water sources (connecting several settlements or municipalities in a single unit depending on available water sources). Water treatment technologies must be appropriate for a given quality at each location individually, relatively easy to control, and in case of accident, reconstruction should be prompt. Thus, local or microregional systems have to be designed and implemented in a modern manner, at a high technological level, in order to permanently secure an adequate regional system in the future.

The methodology used in this paper applies a number of cost approximations to evaluate and compare the localised and centralised systems. The approach was kept deliberately simple in order to create a clear argument that may be readily presented to local authorities and policy makers, with the aim of supporting a viable strategy for water supply for an entire region. Once such a strategy is in place, detailed feasibility studies would be required to fully define and specify the actual water supply infrastructure. In the meantime, it is worth noting again the clear long-term economic benefits of a centralised system demonstrated for both municipalities in this paper. Decentralisation has gained increasing popularity as a strategy for public infrastructure development, especially in the field of wastewater treatment. However, when the additional benefits in water quality security brought about by centralisation are weighed against the negatives, centralised water supply systems likely represent the best choice even in rural regions.