MERKEZİ VE YERİNDE ATIKSU GERİ KAZANIMININ KARŞILAŞTIRILMASI: BİR TURİZM BÖLGESİ İÇİN ÖRNEK ÇALIŞMA

Bu calismada, merkezi ve yerinde atiksu geri kazanim alternatiflerinin, su tasarruf potansiyeli ve maliyeti, Antalya’daki bir turizm bolgesi icin karsilastirilmistir. Yerinde atiksu geri kazanimi uygulamasinda, geri kazanilabilecek su miktarinin, olusan atiksu miktari ve otelin sulama yapilan yesil alan buyuklugunun, yatak sayisina orani ile ilgili oldugu belirlenmistir. Incelenen turizm bolgesi icin, kirilma noktasinin 50 m 2 /yatak oldugu tahmin edilmistir. Bu dogrultuda, sulanan yesil alan buyuklugunun yatak sayisina oraninin, bu degerin altinda oldugu otellerde, yerinde atiksu geri kazanimi ekonomik olmayabilir.  Merkezi atiksu geri kazanim uygulanmasi durumunda ise, atiksu kaynagi ve ihtiyac arasinda denge saglanabilecek, incelenen turizm bolgesi icin yerinde aritmaya kiyasla %60 daha fazla su geri kazanilabilecektir. Merkezi atiksu geri kazaniminin ilk yatirim ve isletme maliyetleri de yerinde aritmaya kiyasla oldukca dusuk bulunmus ve geri kazanilacak suyun maliyetinin, sebeke suyunun altinda olacagi tahmin edilmistir. Yerinde geri kazanim icin, sulama suyu olarak kullanilabilecek aritilmis suyun, ortalama birim fiyati 1,29 €/m 3 olarak hesaplanmis, buna karsin, merkezi atiksu geri kazaniminda, aritilmis suyun 0,35 €/m 3 bedel ile otellere satilmasi durumunda, % 20 ic verim orani ve 5 yillik geri odeme suresinin saglanabilecegi belirlenmistir.


INTRODUCTION
Tourism is a water-dependent sector, where the rate of water consumption is especially high when summer tourism on coastal zones are concerned.Irrigation is one of the main water consuming activities especially for those hotels with large irrigated landscapes (Hocaoglu 2017).Physical properties of the hotel, operational features and availability of an environmental management system are other important factors affecting water consumption (Bohdanowicz et al. 2007; Tortella and Tirado, 2011).In near future, water scarcity in the face of increasing water demand due to tourism is likely to drive the sector towards utilizing reclaimed domestic wastewater.
Basically, there are two different wastewater reuse approaches, as decentralized and centralized reuse.The decentralized reuse approach is mainly based on on-site separation of wastewater streams according to the level and type of pollution and possibility for utilization.Centralized wastewater reuse approach is based on end-of-pipe approach, which comprises collection of wastewater in a sewer, treatment in a central treatment plant and distribution of reclaimed water to users.Both approaches have their own advantages and disadvantages, and the most suitable method varies depending on conditions.For example, central wastewater reuse is advantageous in terms of (i) costs, as the costs decrease when the capacity increases (Roefs et al., 2017), (ii) controlled quality of reclaimed water (operated by professionals and analyzed quality) (Chu et al., 2004;Asano 2005).On the other hand, installation of an advanced treatment system and additional pipelines for reclaimed water is needed for central use, which is not only costly, but also investment decision is with the local authority.Instead, in the case of decentralized reuse, the investor can install the reuse plant at any time independent from the decision of the local authority.Furthermore, reuse of reclaimed water inside buildings, for example toilet flushing, is only applicable in grey water reuse, which is a case only in decentralized reuse approach.
In the last decade, decentralized sanitation approach has been well discussed in terms of concept (Wilderer and Schreff 2000; Tchobanoglous et al., 2004 Singh et al. 2015).Selection of the most suitable wastewater reuse method is case specific.The profitability of the decentralized wastewater reuse depends on investment, operation cost and price of service water.On the other hand, profitability of centralized reuse will be additionally affected by the site characteristics, such as topography and distance of the users from each other and the central plant.Wang et al. (2008) discussed the critical distance depending on the relationship between the cost for decentralized reuse system and centralized pipeline construction.They concluded that if the distance from the project site to the nearest access point of the centralized system is shorter than critical distance, then centralized wastewater reuse becomes more feasible.Similarly, Woods et al. (2013) developed a modified Decision Support System and applied to several scenarios and concluded that in case of limited existing capacity, greater elevation differences, and lower discount rates favour decentralized design and construction.Recently, Roefs et al. (2017) analyzed the sanitation systems under urban development uncertainty and concluded that conventional systems perform better than decentralized and hybrid sanitation systems if discounted lifetime costs are considered.
Most of the studies in the literature have been focused on economic issues of the decentralized and centralized reuse and water saving potential has been mostly overlooked.In a recent study by Gonzales and Ajami (2017) a comprehensive socio-hydrologic framework is developed to identify the effect of locally-driven factors such as water use efficiency, stress on existing supplies, and adaptation capacity potential on resilience.Additionally, in another recent study by Hocaoglu (2017) water balance between the available source and demand side is extensively evaluated for decentralized reuse in individual hotels.
In this study, not only costs but also the amount of water saved, which is one of the main issues in terms of sustainability but mostly overlooked in the studies in literature, have been compared for a touristic case study area by considering the balance between the source and demand.For this purpose, first a central wastewater treatment plant in a touristic area was selected, then individual wastewater treatment and reuse was compared with the central wastewater reuse in terms of amount of water to be saved and costs for hotels located in the vicinity of the central wastewater treatment plant.

Case Study Area
The case study area consists of 24 hotels of various sizes and irrigated landscape areas in Antalya, Turkey.The hotels are located on a 3 km coastal line, the area is mostly flat with an elevation difference of maximum 4 m from the treatment plant.The region is very touristic.The weather is mostly dry in summer and partially dry in spring.The size (number of beds) and irrigated landscape area (m 2 ) of each hotel are given in Table 1.The smallest hotel in the study group has 24 beds and the largest has 2,618 beds with irrigated landscape areas ranging from 2,000 m 2 to 250,000 m 2 .There is no specific correlation between the size of hotels and size of the landscape area, the ratio varying between 1 m 2 /bed and 91 m 2 /bed with m 2 /bed on average.There is a central biological wastewater treatment system servicing the area, consisting of coarse and fine screening, sand and oil removal, extended activated sludge reactor and clarifiers.The capacity of the treatment plant is approximately 23,000 m 3 /day of municipal wastewater.

Scenarios
Central and decentralized wastewater reuse for 24 hotels is compared in terms of water saving potential and costs.The schematic illustration of the two scenarios is given in Figure 1.Scenario 1 (Figure 1a), decentralized treatment and reuse alternative, is based on individual onsite collection/treatment and reuse of mixed domestic wastewater for landscape irrigation.Water saving potentials of the hotels are calculated individually.The balance between the source (wastewater generated) and the demand (water needed for landscape irrigation) are analyzed first on a daily basis and then on an annual basis.It is assumed that the total amount of water consumed is treated as wastewater and all treated wastewater is suitable for irrigation.Simply put, if the amount of wastewater is less than the amount of water needed for landscape irrigation, then all domestic wastewater generated will be consumed for irrigation.In this case, maximum reuse potential will depend on wastewater availability and the rest will be drawn from service water.On the other hand, if the amount of wastewater is larger than the amount of water needed for irrigation than the maximum reuse potential will depend on the amount required for the said use.Details of on-site wastewater reuse mass balance calculations are well defined in Hocaoglu, 2017.
Centralized treatment and reuse alternative, Scenario 2 (Figure 1b), is based on the end of pipe approach, which is collection of wastewater in a sewer, treatment in a central treatment plant and distribution of reclaimed water to the 24 hotels in the case study area for irrigation purposes.In this alternative, since the amount of wastewater generated (supply) is larger than the amount of irrigation water required for the landscape (demand), it is assumed that those hotels, which currently use service water for irrigation, will switch to reclaimed water supplied from the central plant.In order for the scenarios to be comparable in terms of costs, use of similar advanced wastewater treatment technology is assumed for both.For this purpose; Scenario 1 includes installation of MBR, then disinfection with UV and as a precaution excess chlorination, while Scenario 2 includes upgrading the available wastewater treatment technology to a membrane system and similar disinfection processes (ultrafiltration followed by available biological treatment, then disinfection with UV and as a precaution excess chlorination).The additional treatment in Scenario 2 is applicable only for the total amount that can be potentially reused by the hotels.The recovery efficiency of the ultrafiltration system is assumed to be 75% on average which is a typical performance value.In order to balance daily variations and attain peak need at day time, treatment system capacity is assumed to be 15% higher than that of average flowrates for both cases.

Water Consumption
Water demand for irrigation is calculated by using the estimated unit water requirement multiplied by the landscape area of each hotel.Then the sum of the daily water demand for the irrigated days is calculated in order to estimate the annual total demand for irrigation.Unit water requirement for irrigation is assumed to be equal in all hotels at 4.5 mm/m 2 -day on average and irrigated days per year are assumed to be 210 (Hocaoglu, 2017).The amount of wastewater generated is estimated by using the number of beds, the average occupancy rate and the estimated daily wastewater generation per guest.The average occupancy rate is assumed to be equal in all hotels as 60% (a modest number for the area), and the average daily wastewater generation per guest is assumed to be 310 L/day-guest (Hocaoglu, 2017).The hotels are servicing as resort hotels and they are usually only servicing for about seven months (210 days) per year.

Cost Assessment
Cost assessment of both scenarios and their comparison is based on service water consumption (for domestic usage plus irrigation), domestic wastewater generation, and WWTP investment and operation costs.Price of water supplied to the hotels is €0.94/m 3 (March 2017 exchange rate applies).The exchange rate was ~4 TL/€.The cost of wastewater collection and treatment is fixed per bed in the area regardless of season and occupation rate; therefore, cost saving as a result of wastewater reduction is not applicable.Additional benefits of resource conservation were not considered.
Investment and operation costs of Scenario 1 for each hotel were estimated using cost curves developed for this study.The investment cost curve was prepared by using price quotations for various treatment capacities obtained from contracting companies operating in the market.The operation cost curve, on the other hand, was created by estimating the energy usage, chemical costs and personnel costs.Estimated energy usage varies between 140 kw/day for 75 m 3 /day treated water and 1,700 kw/day for 1,000 m 3 /day treated water at 0.1 €/kW.Chemical consumption costs are negligible.It is assumed that WWTP operation may require approximately 0.5 person-month and thus the personnel costs are estimated at 500 €/month.
In Scenario 2, estimated investment cost for transporting reclaimed water to the hotels along the 3 km coastal line was approximately 400,000 €, and additional investment cost of advanced treatment was also about 400,000 € which were the average costs obtained from the contracting companies (this is the additional investment costs for reclamation and does not include the cost of the existing biological treatment necessary to comply with discharge standards).Energy usage was based on 750 kwh/day at 0.1 €/kW.Chemical consumption cost was estimated at 35 €/day.Replacement costs were included in the internal rate of return calculations.The cost of membrane replacement was assumed to be one third of the initial cost of membrane module at the end of 10 years which were 400,000 € for Scenario 1 and 250,000 € for Scenario 2.
Net benefit is calculated by subtracting the initial investment and operating expenses from the benefit to be gained from the project.Net Present Value (NPV) is equal to the difference between the present value of the cash inflows and the present value of the cash outflows (Hata!Başvuru kaynağı bulunamadı.).NPV was calculated by discounting the total inflows and outflows over 15 years and discount rate is as assumed to be 10%, as a typical ratio for environmental studies.

∑
Here; NPV: Net Present Value I 0 : initial investment cost B n : net benefit at time t, (benefits -costs) i: discount rate n: time at the end of analysis (in years) Total cost for both scenarios is investment and operational costs; whereas, total benefit is the averted cost of service water which would otherwise be used for irrigation.The net benefit depends on the price of reused water.

Comparison of Water Saving Potentials
Water saving for centralized and decentralized systems was compared in terms of additional service water required for irrigation.In Scenario 1, the decentralized system, landscape is irrigated with treated wastewater.If the amount of water required for irrigation is more than that of treated wastewater, then service water is used to make up for the difference; if not, then excess treated wastewater is discharged.In Scenario 2, the centralized system, wastewater from hotels are collected and treated at a central facility and then treated wastewater is sold as irrigation water.
Water saving potential of the decentralized reuse (Scenario 1) is calculated individually for each hotel depending on the balance between the wastewater generated and water needed for irrigation on a daily bases (Table 2).Total number of beds is 34,924 and the size of the total landscape area is 1,208,600 m 2 .Estimated total water need for irrigation is 2,724 m 3 per day and 571,063 m 3 per year, while the amount of wastewater to be reused is 360,151 m 3 per year.The supply/demand balance is about +515 m 3 /day, which means in total there is still excess wastewater that will not be utilized by the hotels.Meanwhile, in total 1,003 m 3 /day additional service water will still be needed for irrigation.Accordingly, 210,912 m 3 /year of service water is needed for irrigation.
On the other hand, in the case of central wastewater reuse (Scenario 2), a total of 571,063 m 3 per year irrigation water can be replaced with the reclaimed water, due to the fact that supply and demand is balanced in centralized wastewater recovery scenario.For example, the demand of a hotel with a bigger landscaped area but fewer beds that could potentially generate insufficient water for landscape irrigation will be balanced with the production from another hotel having excess wastewater due to a smaller landscaped area, thus lower irrigation need.This is schematically shown for two hotels in Figure 2. As a result, about 60% more water saving may be achieved by the centralized reuse scenario than the decentralized one for the case study area.

Cost Comparison
Investment and operation costs of decentralized reuse for each hotel was estimated by using the equations of the cost curves developed in this study ( ).The treatment technology includes installation of MBR, then disinfection with UV and as a precaution excess chlorination.As seen from the a, unit investment cost is approximately 2,500 €/m 3 for 50 m 3 /day capacity, 1,200 €/m 3 for 100 m 3 /day capacity.Cost decreases to 600 €/m 3 for 1,000 m 3 /day capacity.Although, reduction in unit investment cost is expected when capacity increases, the difference is drastic for small sized treatment plants as in the case of decentralized reuse.Similarly, a significant reduction in operation cost is also observed when capacity increases ( b).Unit operation cost is approximately 0.4 €/m 3 for 200 m 3 /day capacity and decreases to approximately 0.17 €/m 3 for 1,000 m 3 /day capacity.a) b)

Figure 3: Cost curves, a) investment cost, b) operation cost
The costs and benefits for each hotel in Scenario1 in which each hotel constructs and operates its own treatment plant are given in Table 3.Total investment of 4,147,115 € and annual operational cost of 304,386 € is required to treat 2,270 m 3 of wastewater annually.The annual cost of additional service water required where the amount of reclaimed wastewater falls short is 198,942 € and the total cost of landscape irrigation (reused water and service water) is 779,803 € per annum.Distribution of calculated unit price of irrigation water by irrigated landscape per bed is given in Figure 4a.The unit costs fall substantially as the irrigated landscape per bed gets bigger.Figure 4 can also be used to evaluate the profitability of decentralized wastewater reuse depending on the ratio of irrigated area of a hotel.If the unit price of service water is higher than the calculated cost of irrigation water in case of reuse, than reuse may be potentially cost effective.The breakpoint for the case study area is estimated as 50 m 2 /bed.In hotels where the ratio of irrigated area is less than50 m 2 /bed, wastewater reuse is not cost effective unless the water price is very high.In this case study, among the 24 hotels in the area, the unit price of irrigation is less than 3 € for 12 hotels; whereas, the cost for the remaining 12 hotels varies between 6 € to 13 € (Figure 4b).Clearly, an economic analysis of an obviously non-profitable situation with an alternative scenario is meaningless.Therefore, cost -benefit analysis for centralized and decentralized treatment & reuse -is continued with only 12 of the hotels with the unit price of irrigation less than 3 €.a) b)

Figure 4: Cost of irrigation water, a) variation of irrigation cost by ratio of irrigated area, b) histogram of irrigation water cost
Results of the profitability analysis are given in Table 4.In decentralized wastewater reclamation where the hotels install individual wastewater treatment plants, NPV of total investment cost is 2,407,383 €.On the other hand, in central wastewater reclamation, the Net Present Value of investment for reclamation and is 933,884 €.Under these circumstances, the unit price of irrigation water is higher than that of service water for decentralized wastewater reclamation.Individual treatment and reuse is only profitable for Hotel 2 which has a large landscaped area (Table 3).The most important parameter affecting the profitability is the unit price of service water.If the price of service water rises and/or wastewater removal is charged based on the amount of water used rather than a fixed price per bed then profitability rates would change and individual wastewater reuse schemes would find wider use.In centralized wastewater reclamation, advanced treatment is installed at the municipal wastewater treatment plant and the reclaimed water is sold to the hotels for a price.Considering the additional investment and operation costs, as well as, installing ultrafiltration at the outlet of the existing treatment plant, reclaimed wastewater can be provided to the customers at a price of 0.35 €/ m 3 with a payback time of 5 years.Therefore, the centralized system would be profitable.In this case study, the hotels are close to each other (on 3 km coastal line) and to the central wastewater treatment plant, furthermore, the area is mostly flat with an elevation difference of maximum 4 m from the treatment plant, and the irrigation needs are high due to dry climate in summer season.Therefore, a centralized system is preferable to a decentralized system where each hotel builds its own treatment plant.

CONCLUSION
When deciding on the reuse approach, total amount of water saved is a case specific and critical parameter which should be evaluated thoroughly by considering the balance between the amount of available water and need for reclaimed water for each of the users.In this case study, according to the decentralized wastewater reclamation and reuse scheme, excess reclaimed water cannot be used by another hotel that does not generate enough wastewater to meet its irrigation requirements.Therefore, the maximum amount of water saving is limited with either the wastewater generated or the landscape area.As a result, a significantly more water saving, about 60%, may be achieved by the centralized reuse than the decentralized one for the case study area.It should be kept in mind that the total amount of irrigation water needed will depend on climate conditions.Comparison of unit cost of reclaimed water with the unit price of service water is the other critical parameter for selecting the optimum reuse alternative.In this case study area, when all hotels install and operate their own individual wastewater treatment and reuse systems, the cost of reclaimed water is estimated to be 1.29 €/m 3 ; whereas, the unit price of service water is 0.94 €/m 3 .On the other hand, NPV of total investment of additional advanced treatment and operating costs are considerably less when all hotels benefit from a central wastewater treatment and reuse system.In this case, the cost of reused water is 0.29.€/m 3 .A unit price of 0.35 €/ m 3 for reclaimed water leads to almost 22% internal rate of return and a 5-year payback period.Consequently, due to high water saving potential, considerably less investment and operational costs, and the price of reclaimed wastewater being less than the price of service water, centralized wastewater reuse is the more preferable option for the case study area where the area is mostly flat and the users are close to each other.The methodology used in this study may be used and expanded for other touristic areas having similar properties.Furthermore, models and/or programs which can be used to estimate the potential savings and costs will be very helpful for decision makers.

1 :
Schematic illustration of the scenarios, a) Scenario 1: decentralized on-site wastewater reuse, b) Scenario 2: centralized wastewater reuse

Table 4 . Profitability analysis (12 hotels where benefits are relatively higher)
* including membrane replacement after 10 years **N/A: Not applicable as being not profitable