Solar energy-based water treatment system applicable to the remote areas: Case of Indonesia

Remote areas usually lack basic clean water services. Considering low population, poor geographical accessibility and lack of electricity, a small-scaled water treatment system capable of producing clean fresh water associated with solar thermal/photovoltaic applications, which is characterized with low capital cost, easy operation and less need of maintenance, is employed in the techno-economic study. Indonesia is one of the countries which owns a lot of water resources in their territories but has moderate coverage in basic water services, and is chosen as a case for demonstration. The price of clean water from this system is pro ﬁ table as compared to that of bottled water, which is an accessible safe water in these areas, but still much higher than that of municipal water.


INTRODUCTION
Fresh water is one of the critical and essential things for human life. Without water, human beings can survive for a few days only (Popkin et al. ; UN ). Only 2.5% of the world's water is fresh water, which is stored in lakes, rivers as well as groundwater and usually cannot be directly used for human life because the conditions of the water are far from the clean water standards (WWAP ). It is well known (Reed ; WHO ) that poor water quality can lead to water-related diseases such as waterborne diseases, malnutrition, diseases related to vector contamination and metal poisoning, which result in 2.2 million people dying globally every year and mostly in developing and underdeveloped countries. Among the waterborne diseases, diarrhoea is one of the major death factors of children less than five years old. It often results from the ingestion of pathogens from faeces that have not been disposed of properly or have been treated with a lack of hygiene (WHO ). It was reported by Patunru () that the odds of getting diarrhoea in a household with unsafe water resources in Indonesia is 12% higher than that with improved water resources. Accordingly, the lack of basic water service, mostly in the developing and underdeveloped countries, has significantly resulted in low life expectancy.
Prevention of diarrhoea can be effectively made through sanitation which requires provision of an improved water supply. Improved water supplies refer to technologies such as piped household water connections, public taps, standpipes, protected dug wells, or rainwater collection (WHO ). Improved water supplies can also be achieved by the current available water treatment technologies (Abdelkareem et al. ).
Throughout the world, 55% of the population lives in urban areas, thus, relatively higher than that in rural areas.
However, the population living in rural areas reached as high as 3.4 billion in 2018 and need improvement of basic water services to achieve the Sustainable Development Goals (UN ). According to a joint survey carried out by WHO & UNICEF (), only 55% of the rural population has access to safely managed water services. In particular, only about 12% of populations living in the Indonesia has already set up a policy regarding water supply. As basic drinking water service is a primary need, it is supposed to be supplied by a governmental unit either at municipality or province scale, but this policy cannot cover the whole rural areas as yet. Private sectors are permitted to be involved in this business through the joint operation with governmental units. The latest amendment was issued in 2016 (Ministry of Home Affairs Republic of Indonesia a) and allows the BOT (i.e., build, operate and transfer) scheme for private sectors. A tariff is set with the limitation of, at most, 4% of the standard municipality minimum income at which the water treatment system is installed (Ministry of Home Affairs Republic of Indonesia b).
The problem in Indonesia is not the lack of water resources. Instead, it lacks safe fresh water in most of the remote areas of the country. A small-scaled water treatment system, which is driven by solar energy and can produce clean fresh water, is suggested to be used in remote areas.
However, in consideration of most residents in the remote areas, who are the interest to be addressed here, the issues of economic incentives including low capital cost, easy operation and less maintenance for the water treatment technologies, must be significantly taken into account in the study. A typical remote village in a rural area of Indonesia with the minimum basic water demand listed in Table 1 will be used in the study for demonstration. A simple economic analysis to estimate both cost investment and payback period is next made in the study. The estimated water price will be compared with the consumer price of bottled water, which is the major safe drinking water in the remote areas of Indonesia.

TECHNO-ECONOMIC ANALYSIS
Before the selection of an available small-scaled water treatment system, the fresh water demand and the energy demand for operating the water treatment system will be evaluated based on typical household population and the type of remote village. Once the available water treatment system and its sizing is determined, an economic analysis, which consists of estimating total investment cost, determining water production cost and estimating payback period, is next conducted to examine the feasibility of the system.

Water consumption of household activities in remote areas
For sustainable living, around 50 litres of fresh water is needed for personal daily activities including drinking,  . This sizing information of households will be used in the following estimate of water demand and economic analysis.

Estimating water demand in remote villages
As seen from Table 1, different amounts of minimum basic water demand are reported. Here, the water demand per capita (w n ) is estimated by the median value in Table 1.
Total water demand (WD T ) is calculated by where h is the household size (¼3.6), and Psv is the household population per sub-village. Two kinds of remote villages are studied here. The remote villages inland usually have access to raw fresh water resources such as rainfall, rivers and lakes. The main water demand of the residents is for drinking and food preparation. In contrast, the fresh water demand of the residents in coastal villages of islets should cover the additional part for sanitization and bathing because such remote villages on islets almost only have access to seawater.

Water treatment technologies
The water treatment processes are mainly classified into two parts in their studies. The first part is a water treatment plant for processing raw feed water for distillation. The second part is the solar thermal field to gather thermal energy for supplying heat for distillation. The last part is the solar PV field to supply electricity as the auxiliary power for operating the whole system. In the remote areas where electrification ratios are normally low, the PV system can be used to supply the required electricity (Abdelkareem et al. ). Figure 1 shows a schematic layout of these solar-driven water treatment systems.
Several kinds of solar collectors, which are responsible for providing thermal energy to the water treatment process, can be used from the available solar thermal technologies, including flat-plate collectors, evacuated-tube collectors, concentrating collectors, salinity-gradient solar ponds, etc.
The auxiliary electrical power for operating the water treatment system can be supplied by integrating PV in the system.

Energy demand
Analysis is done with a small-scaled, stand-alone, solar thermal/PV water treatment system which is equipped with a simple distillation device: the solar still. Although the distillation efficiency of the solar still is low, it is a rather cheap water treatment system that is affordable in the remote areas. The solar condition is taken to be the same as that

Cost estimate
To evaluate accurately the cost for a water treatment system, many factors need to be taken into account, including the plant capacity, required equipment, feed water quality, pretreatment, energy cost, system life cycle, investment amortization, etc. The major cost elements for the investigated water treatment system are cast into the capital and operating costs. The capital cost (C capital ) covers the purchasing cost of major and auxiliary equipments, land and installation charges. Since the present study is addressed at the remote areas, the land can be reasonably assumed as free charge.
Thus, the capital cost is calculated with: The annual total cost (AC total ) is calculated with the total yearly cost of owning (AC capital ) and the annual operating and maintenance cost (AC O&M ) as: The analysis of the capital cost is made with the following conditions and information taken from other research which is recent and generic for the current study of desalination technology or engineering market in Indonesia: (1) The PV-module price is US$5 per watt-peak (Banat & Jwaied ).
(2) The solar still price is US$100 per square metre. (4) The pump price is US$1.054 per watt (Allouhi et al.

).
(5) The installation cost is an estimated 25% of the purchased equipment costs.
(6) Tax is 10% of total cost, as stated by the Indonesian government.
(7) The market penetration, which indicates the proportion of the residents willing to use the clean fresh water generated by the suggested water treatment system, is estimated to be 80%.
(8) Zero land cost, that is, free charge for using user-owned or government-owned land.
Among them, Assumption 7 is made ourselves, while Assumption 8 is reasonable in the remote areas.
The following values of common economic parameters are used in the estimate, based on generic information for the current study of desalination technology or the financial market in Indonesia: (1) The system life cycle, n ¼ 15 year; which is same as the household's solar water heater.
(3) The operating and maintenance costs are estimated to be 20% of the equipment cost.
(4) Zero pre-treatment cost of raw water, that is, directly from nearby water sources.
Supposing that the required capital cost has to be lent by a bank, there will be an interest charge in the estimated capital cost. Amortization factor (a) is calculated with: which implies the a value decreases along with the elapse year and is set to be zero at the end of the amortization period. By taking into account this factor, the total yearly costs of owning, and the annual operating and maintenance costs in Equation (4) are calculated as: It indicates that the salvage value of the system yearly decreases and finally drops to zero at the end of the amortization period.
The water cost (WC) is defined as the ratio of the total annual cost to the annual production of clean fresh water (annual plant capacity): Note the above formulae and the given conditions/parameters are all in a linear relationship. The calculated annual total cost and annual plant capacity are proportional to the water demand. It results in the water cost defined in Equation (9) being independent of the water demand in the present study.

Estimate payback period
The information of payback period is a key factor to justify whether an investment is profitable or not. The payback period (PB) is calculated by: The expected annual net cash flows can be evaluated with the price of substituted clean fresh water such as bottled water and non-potable (municipal) water. However, it is well agreed that the remote areas almost lack safely managed water services. Thus, bottled water is the only accessible clean fresh water which the residents living in remote villages can purchase. Taking the bottled water price as the comparison basis, the payback period of the investigated water treatment system can be evaluated. Two conditions of the governmental policy are considered: with and without tax exemption in the following analysis.

Water demand
Water demand is calculated with Equation (2)

System sizing
Taking into account the requirements for thermal energy and electricity, as previously specified, together with the information in Table 2, the energy demands are presented in Table 3. The sizes and capacities of the investigated water treatment system as schematically shown in Figure 1 are, then, calculated and recorded in Table 3 too. Table 4 summarizes the calculated results of Equations (5)-(9) with the above conditions. It shows that the cost of clean fresh water produced by the employed water treatment system is US$4.45 per m 3 for both the cases of inland remote village and coastal remote village. However, the required investment in the case for the coastal remote village is remarkably higher (a multiple of 3.3) than that for the inland remote village (see Table 4) due to the higher demand for clean fresh water as shown in Table 2.

System cost
Survey of the current price of the refilled bottled water in the remote areas of Indonesia is around US$19 per m 3

(Ministry of Maritime Affairs & Fisheries Republic of
Indonesia ) and US$0.4 per m 3 for municipal water (Makassar ). Evidently, the water cost which is evaluated from the investigated water treatment systems (Table 4) is remarkably higher than that of the municipal water but lower than that of bottled water. It is noted that the price of refilled bottled water would become higher, along with greater remoteness extent, than the average one surveyed by the government due to poorer transportation accessibility.
It implies that the water cost with the suggested water treatment system would become more competitive than the price of bottled water in the significantly remote areas.

Payback period
The distributions of cash flows within the life cycle of the water treatment system (i.e., 15 years) for both the cases of remote villages in either inland or islet and each with and without considering tax exemption are presented in Figure 2.
The payback period calculated from Equation (10) for each case is determined from the interception of the cash-inflows  tax exemption is less attractive to encourage the instalment of such water treatment system in the remote areas. It must be borne in mind that the total investment for the water treatment system in the coastal case is around three-fold of that in the inland case owing to higher water demand.
One may argue that the water demands for bathing and sanitation do not need to be as high as the standards of fresh water for drinking and food preparation. If this concern is taken into account, the water demand for a safely managed service in the coastal remote villages becomes exactly the same as that in the inland remote villages.
As shown in Equation (10), the payback period is inversely proportional to the sum of the expected annual net cash flow, which is directly proportional to the price of bottled water. If the local price of bottled water (sold in the place where the suggested water treatment system was installed) was 10% higher than that adopted in the present study (US $19 per m 3 ) due to higher transportation cost (i.e., higher remoteness extent), the payback period could be shortened by 10%; in other words, it can be reduced from 4.54 years (with the case of 10% tax exemption) to 4.09 years.
The results of Table 4 and Figure 2 were calculated with the given assumptions. If the assumption for the system life cycle, for example, could last from 15 years to 20 years due to the better system quality and maintenance, the water cost would drop from US$4.45 per m 3 to US$3.38 per m 3 . Consequently, the payback period could be shortened from 4.74/4.54 years (shown in Figure 2)   If the consumer price of bottled water was further increased due to the extent of greater remoteness (in other words, poorer transportation accessibility), the suggested smallscaled water treatment system powered by solar energy would be more competitive than the present analysis.
The study reveals that the incentive of 10% tax exemption is less attractive to encourage such investment of water treatment system in the remote areas since the residents mostly live under the poverty line. More attractive incentives such as a subsidy scheme could be considered to promote the suggested idea in the remote areas of Indonesia in the future. However, the suggested idea in the study is limited to the remote areas with a richness of sunshine and not lacking in water sources, no matter what they (sea water or dirty raw fresh water) are.