APPLICABILITY OF USING REVERSE OSMOSIS MEMBRANE TECHNOLOGY FOR WASTEWATER RECLAMATION IN THE GAZA STRIP

Reverse osmosis (RO) technology shows common popularity in the field of water treatment as an advanced stage to eliminate the residual biogenic elements and dissolved impurities after the traditional treatment processes. This article highlights the applicability of using RO membrane technology as a post-treatment stage to treat the discharged effluent from the Gaza wastewater treatment plant. The designed experimental model reveals optimal removal efficiency between 92 and 100% for a number of physical, chemical and biological pollutants. The RO membrane unit demonstrates significant removal efficiency compared to the sand filter where the RO removal efficiency for BOD, TSS, TDS, Fecal Coliform and NO3 were 100, 97.5, 92, 100 and 100%, respectively. The quality of reclaimed wastewater was idealistic where the contents of BOD, Fecal Coliform and NO3 in the permeate were nil, and the concentrations of TDS and TSS were 20 and 296 ppm, respectively. Practically, the results confirm that the wastewater with the reclaimed quality could be used for agricultural activities with no degree of restriction according to FAO’s guidelines water quality for irrigation. According to the Palestinian Standard (PS), the quality of reclaimed wastewater is high, class (A), and it could be used without restrictions to irrigate many crops and for the purposes of groundwater replenishment. Related to energy estimation and cost analysis, the numerical model and the market analysis study demonstrate the energy of 1.23 kWh and total cost, i.e. fixed and energy costs, of 0.58 USD to produce 1 m3 of reclaimed wastewater using the RO membrane in the Gaza Strip over a projected lifespan of 5 years.


INTRODUCTION
The water scarcity crisis has raised as one of the most pressing problems on the sustainability of life due to the effect of population growth, climate change, water abuse, pollution of water resources, improper management of water and other factors (Mehta, 2007;Karagiannis and Soldatos, 2008;Vörösmarty et al. 2010;Jacobson et al. 2011Ang et al. 2015. The water crisis threatens about four out of every five persons of the human communities and more than two-thirds of the environmental habitats (Jacobson et al. 2010;Vörösmarty et al. 2010). In an effort to alleviate the water shortage, new water sources have been identified and supplied through desalination of seawater or brackish water, and by reclaiming wastewater (Zhang and He, 2013). However, the utilizing of treated wastewater in agriculture and in the replenishment of water resources (i.e., surface and groundwater) is less expensive than using the desalinated water and it is more affordable for most crops (FAO 2006;Zhang and He, 2013;Jaramillo and Restrepo 2017). Moreover, the worth of wastewater comes from being a promising constituent in promoting smart sustainable development especially in arid and semi-arid areas and many counties worldwide. The merit of wastewater as an efficient alternative water source is derived from the fact that about 99% of its ingredients are water and only less than 1% is solid wastes and hence the waste components reduction of wastewater to acceptable levels by means of water recycling could be worthy environmentally and economically to the sustainability of water resource management.
Nowadays, the rapid development in membrane manufacturing technologies, such as micro, ultra, nanofiltration, and reverse osmosis (RO) membrane technologies plays a significant role in enhancing the feasibility of water and wastewater reclamation by adding a quaternary-post-treatment process to remove the remaining pollutants, e.g. pyrogens, color, submicron colloidal matter, bacteria, and viruses, after the tertiary treatment (Metcalf and Eddy, 2013). In comparison to other membrane technologies, the use of RO technology is the most prestigious in the purification of wastewater due to the low energy consumption and the high rate of pollutant and contaminant removal. Edwards and Schubert (1974) and Fang and Chian (1976) reported that the RO membrane technology could eliminate 51% up to 99% of various pesticides. Al Jlil and Sajid (2014) found that the RO water treatment process could remove all the cations and anions from wastewater or seawater especially removing the monovalent ions such as chloride (Cl -) with a removing efficiency of 94.4%. Suzuki and Minami (1991) revealed through several experiments that RO membranes could remove up to 99%, 90% and 99.9% of total dissolved solids (TDS), total organic carbon (TOC) and fecal coliform, respectively. Abid et al. (2012) addressed the dye removal efficiency of more than 97% from industrial wastewater by membrane technologies of RO.
Practically, agriculture shows a huge demand for water as it is considered the biggest consumer of water in volume among all other water sectors. The possibility of using reclaimed wastewater forms an attractive water source to farmers for several reasons such as its reliability, its low or zero cost and its valuable nutrient content, which increases crop production without adding artificial fertilizers (U.S. EPA, 2004;Jiménez, 2005;Qadir et al. 2010). In specific, the typical domestic wastewater is a rice source of the nitrogen, phosphorus, potassium, micronutrients and organic matters that are naturally required for agricultural harvesting (FAO 1992). However, the main challenge that faces the sustainability of any wastewater reclamation activity is compromised by public health assurances. The wastewater is most likely to contain pathogenic organisms similar to those in the original human excreta such wastes: bacteria, viruses, protozoa, and helminth (FAO 1992). Depending on the harsh effects of sunlight and desiccation, almost all excreted pathogens can survive in soil and on crop surfaces for a sufficient length of time ranges between two days and many months which is a sufficient to indicate that public health risk and infections by Diarrhea, Typhoid, Salmonella type and other virus diseases could be conjugated (WHO 1989). Moreover, the accumulation of heavy metals the wastewater demonstrates a major potential risk to consumers and farmers (Jaradat, 2016). Gumbo et al. (2010) found significant public health risks for using wastewater for agricultural irrigation. Jan et al. (2010) detected heavy metals concentrations in a number of tested food crop samples irrigated with wastewater. Generally, the long term using of partially treated wastewater in irrigation can degrade soil and cause public health risks through the transmission of enteric diseases and heavy metal accumulation (Jamil et al. 2010). Obviously, the different water activities and uses in terms of public health and environmental protections are principally governed by specific guidelines and frameworks that are set and adopted by the World Health Organization (WHO) or by the national legislation of countries. In Palestine, the relevant policies, legislation, and regulation related to water quality are set and adopted by the Palestinian Water Authority (PWA) with direct negotiations and discussion with the related ministries such as the ministry of health, ministry of agriculture and the environmental quality affairs (PWL, 2002). Generally, the wastewater treatment passes through a tertiary treatment process which contains an Ultraviolet (UV) disinfection stage to eliminate the pathogenic organisms from the treated wastewater. Unfortunately, the technique of UV is unavailable in most of the regions that suffer from a lack of facilities and energy resources. The wastewater is a valuable source of water to balance the water deficit in the Gaza Strip, therefore, this study demonstrates the applicability of using the RO membrane technology as a post-treatment stage instead of the UV stage to eliminate the remained nutrients and pathogenic organisms from the reclaimed wastewater in order to be efficient in term of quality to be applicable in different water sectors according to specified Palestinian water standards and guidelines. Moreover, not only in the Gaza Strip, the applicability of RO membrane technology as a posttreatment stage could be a potential unique solution for different countries in the Middle East and North Africa which suffer from poor facilities and energy resources to overcome the need to tertiary treatment stage of UV which needs a well-established preparations and facilities.

ENVIRONMENTAL STANDARDS AND REGULATIONS
Investigating the efficiency of any water treatment systems is performed in terms of the produced quality of water and its coinciding with the adopted guidelines and standards.
Water quality, which is as significant and important as quantity, has been overlooked for decades in terms of legislation, investment and public awareness. Water and sanitation infrastructure, water and sanitation policies, good governance and practice, proper legislation, regulations, and standards are key issues to safeguard humans and their surrounding environment. Various legislations and regulations were designed worldwide to regulate water resource protection, water supply, sanitation, protection of the environment, and prevention of pollution. Standards have been adopted for different purposes of water uses and reuse. Monitoring systems have been put in place to ensure responding to these regulations and standards. The availability of adopted water quality standards and regulatory aspects in forms of national or international standards is important to control and restrict the arbitrary use or disposal of wastewater. There are no unified criteria for wastewater reuse in the world and each country proposes its own best practices criteria based on different environmental, economic, social and political conditions. Globally, many organizations and agencies like the World Health Organization (WHO), Food and Agriculture Organization (FAO) and Environmental Protection Agencies (EPAs) suggest guidance standards for wastewater reuse for different purposes. In the 1970s, the WHO started developing guidelines, shown in Table 1, for wastewater reuse in agricultural irrigation to ensure human health and environmental integrity. These guidelines are based on the consensus view that the actual risk associated with irrigation with treated wastewater is much lower than previously thought and that earlier standards and guidelines for effluent quality (WHO, 2006). Traditionally, irrigation water is grouped into various quality classes in order to guide the user to the potential advantages as well as problems associated with its use and to achieve optimum crop production. The water quality classifications are only indicative guidelines and their application has to be adjusted to conditions that prevail in the field. This is so because the conditions of water use in irrigation are very complex and difficult to predict (WHO, 2006). The suitability of water for irrigation greatly depends on the climatic conditions, physical and chemical properties of the soil, the salt tolerance of the crop grown and the management practices. Thus, the classification of water for irrigation will always be general in nature and applicable under average use conditions. Many schemes of classification for irrigation water have been proposed. FAO (1985) classified irrigation water into three groups based on salinity, sodicity, toxicity, and miscellaneous hazards. These general water quality classification guidelines help to identify potential crop production problems associated with the use of conventional water sources. The guidelines are equally applicable to evaluate wastewaters for irrigation purposes in terms of their chemical constituents, such as dissolved salts, relative sodium content and toxic ions. Depending on the sodium ion concentration relative to the concentrations of calcium and magnesium ions (as indicated by SAR) and the total salt concentration, the potential content of sodium ions in the irrigation water affects the infiltration rate and the soil permeability. Obviously, it is clear to indicate that for a given SAR value, an increase in total salt concentration is likely to increase soil permeability and, for a given total salt concentration, an increase in SAR will decrease soil permeability. This illustrates the fact that soil permeability (including infiltration rate and surface crusting) hazards caused by sodium in irrigation water cannot be predicted independently of the dissolved salt content of the irrigation water or that of the surface layer of the soil (FAO 1985). In Palestine, the Palestinian water laws clearly declare the responsibility of the PWA, in consultation with the ministry of health, ministry of agriculture and environmental quality authority, to establish guidelines and framework for monitoring the water-related practices, protecting the water resources and ensuring the safety of Palestinian public health (PWL 2002(PWL , 2014. The Palestinian Standard (PS 2003) imposes a set of mandatory technical instructions for using treated wastewater in agricultural irrigation to maintain the safety of humans, animal health and the crops as well as to sustain environmental constituents. The PS prohibits the usage of treated wastewater for watering livestock and poultry, irrigation of all types of vegetables, groundwater recharge through direct injection and fish farming. Based on the quality of treated wastewater, the PS categorizes the effluents into four classes based on the water quality as shown in Table 2. The PS states that "the irrigation using reclaimed wastewater should be stopped before three weeks of the yield time for the fruit crops and before two weeks for field and feeding crops" to take into consideration the sensitivity of some crops to the properties and the elements of treated wastewater. The PS permits the use of treated wastewater, without any restrictions on the irrigation activities, for harvesting the yields of Cotton, Luffa, Brooms and other crops that are not in direct contact with the public. However, concerning the crops that are in direct contact with the public through touch or eating, the PS imposes intensive preventive restrictions and protective procedures for using the treated wastewater in irrigation. The PS adopts eleven irrigation methods that enable the farming activities from using the reclaimed wastewater for irrigating specific types of crops. Each of these methods is classified as a two-way or a one-way restriction method (PS, 2003). One-way restriction irrigation methods:  A distance ≥ 25 cm above the ground between the dripping point and the crop or fruits.  A distance ≥ 50 cm between the level of sprinklers and the fruits.  A ground plastic cover between the treated wastewater and the fruits.  Crops or fruits with a single crust or an uneaten shell.   Crops or fruits eaten only cooked.  Sand filter detain the wastewater for at least 15 days or water ponds that contain less than 10% of treated wastewater.  Disinfection by chlorine so that the residual chlorine is not less than 0.5 mg/l and the contact time is not less than an hour half or by using any other sterilization method. Two-way restriction:  A distance ≥ 50 cm above the ground between the dripping point and the crop or fruits.
Irrigation methods:  Underground drip irrigation. The PS requires specific standard considerations to irrigate crops by the reclaimed wastewater. The PS restricts the irrigation activities for gardens, playgrounds, and parks to be by a reclaimed wastewater quality of class (A). However, the processes of groundwater replenishment by infiltration and the offshore disposal of about 500 m into the sea should be by wastewater quality of not less than class (C). The standards permit the irrigation of crops of seeds production, dry feeds, forest trees, industrial corps and grains using low quality of treated wastewater. The PS allow using treated wastewater with class quality of (A), or other quality with suitable one-way or two-way restriction irrigation methods to irrigate the crops of artichoke, eaten corns, citrus fruits, uneaten shell fruits, falling leaves trees, tropical crops, grapes, cactus, dates, olives, and flowers (PS, 2003). In the area of the Gaza Strip, four types of crops are cultivated and harvested: (a) field crops: wheat, potatoes, and onions; (b) vegetables: tomatoes, cucumbers, eggplants, squash, green beans, and paprika; (c) fruit trees: citrus fruits, guava, dates, and almonds; and (d) olives (ARIJ, 2015). Generally, drip irrigation remains the domain irrigation system and widely used for agricultural production by farmers in the Gaza Strip (ARIJ, 2015).

WATER ISSUES OF THE STUDY AREA
The Gaza Strip (shown in Fig. 1(a)) is a stretch of area that extends on the coast of the eastern Mediterranean with about 42 km long, between 6 and 12 km wide, and an area of 365 km 2 . The Gaza Strip is recognized as one of the most densely populated areas in the world. The estimations of the Palestinian Central Bureau of Statistics (PCBS) for the year 2018 revealed that the population of the Gaza Strip was more than two million inhabitants (PCBS, 2018). Concerning the current situation of the water sector, the region of the Gaza Strip is characterized by many parties as an area in a continuous humanitarian crisis due to the lack of both quantity and quality of water resources for different uses (PWA, 2011(PWA, , 2013(PWA, , 2014. The underlying coastal aquifer is the only water resource that is exploited in the Gaza Strip for the domestic, industrial and agricultural uses. The status of the coastal aquifer shows permanent deterioration on both quantity and quality. The annual replenishment volume of the aquifer is between 55 and 60 million cubic meters while the Palestinian abstraction from the aquifer in the Gaza Strip is quantified by about 180 million cubic meters per annum. The agricultural sector consumes the largest portion of nearly 50% of the entire pumped groundwater water from the aquifer comparing to the other remaining domestic and industrial sectors that tighter use the second half of the extracted groundwater. Accordingly, the records of water balance show a severe and continuous deficit of about more than 120 million cubic meters per annum (PWA, 2013). In terms of quality, the present rate of deterioration demonstrates that over 95% of the coastal aquifer is contaminated with unacceptably high levels of either nitrate (NO3) or chloride (Cl), posing significant health risks to Gaza's residents (PWA, 2014). The sustainability of coastal aquifer became a crucial and sensitive issue for the Palestinian Water Authority (PWA) and other Palestinian water-related organizations and agencies. Hence, in order to alleviate the huge pressing on the coastal aquifer, the PWA pays more attention to using new alternative and other nonconventional water sources.
i. Purchased water: the water sector in the Gaza Strip is supplied by 10 million cubic meters per year. This quantity is bought from Israeli water utility (Mekorot). In addition, under the negotiation's agreement of Oslo II, there is a commitment to increase the supplied quantity by 10 million cubic meters (PWA, 2014). ii. Desalination of seawater: desalination of seawater became a strategic option for the PWA to overcome the shortage in the water supply. Hence, the PWA provided a strategic plan for water supply along with the short and long terms. Nowadays, short-term low-volume (STLV) seawater desalination plants, the overall capacity of 13 million cubic meters per annum, are built to enhance the quality and quantity supply of water in the Gaza Strip. From a long-term perspective, a plan to construct a regional strategic large-scale seawater desalination was adopted. The first phase of the regional seawater is intended to provide water sector by 55 million cubic meters yearly while the second phase will lift the plant's capacity to 120 million cubic meters per annum (PWA, 2015). iii. Treated wastewater reuse: the reclamation of wastewater draws the attention of PWA as a potential source that could be used to maintain the coastal aquifer sustainable. Exploiting the reclaimed wastewater can achieve the sustainability of groundwater by 50%. Practically, the PWA starts studying the willingness of using reclaimed wastewater for irrigating agriculture (PWA, 2013). On average, the water services suppliers estimate the total amount of produced wastewater in the Gaza Strip by approximately 50 million cubic meters per annum (ARIJ 2015;PCBS, 2018). Currently, there are four small scale and one large scale operating wastewater treatment plants, geographically demonstrated in Fig. 1(b), in the Gaza Strip. The four small scale wastewater treatment plants of Gaza, Wadi Gaza, Khanyunis and Rafah have the capacity to treat partially, and not in an acceptable level of quality, about 38.7 million cubic meters of the collected wastewaters in the Gaza Strip. This quantity which forms a ratio of about 75-80% of the total generated municipal effluents in the Gaza Strip is directly disposed into the marine ecosystem without enough proper treatment and without any mitigation measures. As well, the operating large scale of north Gaza (NGEST) wastewater treatment plant treats about 12.8 million cubic meters of wastewater which is utilized to recover the coastal aquifer through a number of wastewater infiltration basins. The characteristic properties of raw wastewater quality for different governorates in the Gaza Strip, as depicted in Table 3, show that the effluent of wastewater has high organic matter, i.e. BOD5 of 724 mg/l, compared with the average BOD 5 levels in the developing countries which range from 200-300 mg/l (Polprasert, 2007). These high pollutant contents affect adversely the treatment efficiency of the in-operation wastewater treatment plant. The high growth in the population of the Gaza Strip increases the load on the capacity and affects the initial quality design parameters that have been used to plan the current wastewater treatment plants which affect the treatment efficiency. Therefore, the sanitation services in the Gaza Strip are in curial crisis and the existing wastewater treatment plants work under overloaded flow rates and so some wastewater is being treated and the large amount is returned raw and pumped into the sea. Based on the design parameters of BOD, COD, and TSS, illustrated in Table 4, the designed efficiency of the small-scale wastewater treatment plants is about 75% (CMWU, 2012). However, nowadays, and due to over rates of wastewater and the continuous deterioration in the treatment efficiency of the small-scale wastewater treatment plants, the PWA suggested closing these plants and constructing instead three large scale wastewater treatment plants on the eastern border of the Gaza Strip (PWA, 2014). To enhance the status of public health and sanitation services, the PWA plans to replace the current over-loaded wastewater treatment facilities by three new large-scale high-efficiency wastewater treatment plants located as shown in Fig. 1(b) in the North, Central, and South of the Gaza Strip. Recently, as shown in the provided timeline plan in Table 5, the wastewater treatment plant of North Gaza entered the service to replace the old plant at Beit Lahia. Also, the Gaza central and south wastewater treatment plants are under construction and the decision-makers confirm that they will be ready to operate by 2022. The efficiency of the three planned large-scale wastewater treatment plants will be very acceptable, where, the effluent characteristics are between 10 to 20 mg/l for BOD5, 15-20 mg/l for suspended solids, 10 to 15 mg/l for total nitrogen, less than 1 eggs per liter from Helminths and less than 200 CFU per 100 ml for Fecal Coliforms (PWA, 2011). The quality of effluent indicates that these strategic plants were designed to provide treated wastewater of class (A). According to the monitoring records of the PWA, the volume of treated wastewater that was used in 2012 is 1 million cubic meters which represent only 3% of the available partially treated wastewater. The PWA put a strategic plan, shown in Table  6, extends to the year 2032 to enhance the water management system in the Gaza Strip (PWA, 2013). According to this plan, by 2022, in the short-term strategy plan, it is expected to lift the exploited treated wastewater to more than 25% of the available partially treated wastewater. However, by 2032, the expectations of the long-term strategy plan address that about 25 million cubic meters per annum and nearly 75 million cubic meters of treated wastewater are going to be used for agriculture and aquifer recharging, respectively.
In order to alleviate the deterioration in the coastal aquifer, the long-term plan aims to decrease the overall groundwater abstraction in the Gaza Strip from the present rate of 180 million cubic meters per annum to about 70 million cubic meters per annum in 2032. As shown in Table  6, the long-term strategic plan aims to increase the sustainability of the coastal aquifer by using the reclaimed wastewater as a new water source for agricultural purposes. The plan expects that the total anticipated demands of water for agriculture will be reduced as a result of urbanization and population growth, hence, the projected potential irrigated lands will be about 90,000 dunums by 2032.

ATERIALS AND METHODS
This article highlights the performance of RO membrane technology as a post-treatment stage in enhancing the treatment efficiency to improve the quality of wastewater reclamation. The performance of this technology was investigated by setting up an experimental model and by testing and analyzing, in the laboratory, the quality of wastewater before and after the RO treatment. The experimental physical model, shown in Fig. 2, consists of: (1) Collecting tanks: High-density polyethylene (HDPE) feeder, permeate and concentrate tanks each of cubic meter volume were exploited to collect the raw and the reclaimed wastewater before and after the treatment process; (2) Sand filter: a composite of sand and well-graded sizes of gravels range from 2.37 and 9.5 mm was prepared to configure a cylindrical sand filter with a length of 50 cm and   wastewater treatment plant were poured into the feeder tank. The wastewater was passed through the sand filter, which composes of graded-size gravel, the flow of wastewater inside the sand filter is governed by the force of gravity. The general design criteria for the slow sand filter recommend a filtration rate between 0.1 and 0.2 m per hour through the filter layers (Kawamura 2000). However, the filtration treatment through the designed sand filter demonstrated a slow sand filtration, hence, the water head reached about 20 cm above the sand filter layers with a filtration rate of 0.5 liters per minute. The passed wastewater through the sand filter is entered into the three stages of MF/UF membrane unit with 5-and 1-micron pores diameter which was selected as a second pretreatment unit after sand filter to protect the RO device from fouling and to reduce suspended solids. After sufficient pretreatment, the water was pumped by a main pump of 130 psi (8.844 bar) through the RO unit with a flow of the filter was 1.8 liters per minute, the recovery rate was 50% from the fed water. In order to enhance the recovery efficiency of the system, a repass cycle was inserted into the system, where, a small repass pump of 4.23 bar was operated  to repump the concentrate through the RO unit. The operation of the repass cycle was designed to operate when the concentrate in the concentrate tank reaches a volume of 250 liters or more. Finally, the treated water i.e., permeate water, was collected into the permeate tank (5) which is the treatment product that was tested in the lab. For each experimental trial, four samples were taken in every stage of filtration in each of the two experimental trials for testing and analysis purposes. The samples were put in polythene bottles that were pre-washed with acid and distilled water and then were dried. The first sample was taken from the feeder tank and before sand filter, the second was taken after the sand filter, and finally, the last two samples were taken from RO concentrate and permeate tanks, respectively. The four gathered samples were preserved at 4°C in an icebox and brought to the testing department of the laboratory for testing and results declaration. The quality of the treated wastewater in every phase of the treatment process was tested in order to examine the efficiency of each component in the experimental model in terms of Biochemical Oxygen Demand (BOD), Total Suspended Solids (TSS), Total Dissolved Solids (TDS), Hydrogen Ion Concentration (pH), Electrical Conductivity (EC), Fecal Coliform (FC) and Nitrate (NO3-N). The described standard methods for the examination of water and wastewater by the American Public Health Association, American Water Works Association, and the Water Environmental Federation (2017) have been employed to examine the water quality parameters. The content of the organic matters has been tested in terms of the 5-day BOD test according to the procedure of the 5210 B. The described standard method in the procedure of 2540 D and 2520 B were applied to measure the TDS and the EC in the reclaimed wastewater samples, respectively. Furthermore, the acidity of the treated wastewater in term of pH was determined according to the procedure of 4500-H+. Concerning the pathogenic content, the FC concentration was detected according to the procedure of the 9211 B seven-hour fecal coliform test. Else, the concentration of NO3 in the wastewater was determined using the Nitrate Electrode method according to the procedure of 4500-NO3 D Practically, testing the efficiency of membrane technology as a post-treatment in wastewater reclamation from the view of quality is not sufficient to judge on the whole efficiency of this technology. Principally, the main challenge governs the development and the popularity of membrane technology is the challenge of energy. Thus, it is critical to estimate the feasibility of this system from the views of cost and energy consumption. It is difficult to address the cost and energy consumption through the experimental physical model. Hence, with the rapid increase in computer power, it is not surprising to note the highly dependent on numerical simulation as a prediction tool in the predesign and design stages. In this article, the simulation model of Win-flows RO system design and simulation software was used to estimate numerically the cost and energy consumption of wastewater reclamation through the designed experimental model. To ensure better simulation results, the software was run several times and a cartridge-filter-pre-treatment unit was added to the simulation system to provide a better presentation of the reality. The Win-flows has some distinctive features like three pass systems, permeate split and recycle, anti-scalant dosing, energy recovery devices and ability to combine stages that make its ability higher than other similar software. The best presentation Windflows model for the experimental model of this study, shown in Fig. 3, was designed to treat 1,000 m 3 per hour of tertiary treated wastewater with a TDS of 3,800, pH of 7.8 at a temperature of 16°C. After a series of trials and errors, the final design of the model system consisted of Cartridge filter as pretreatment and two stages of RO elements, 90 pressure vessels, and 65 pressure vessels, every vessel contains 7 elements of Duraslick anti-fouling membranes that are designed especially for wastewater treatment. The selected membrane model was DSL RO8040 to maintain the continuity of membrane efficiency for 5 years. In the numerical model, the water is pumped through the system using two pumps. In the first treatment stage, the main pump, which has a pressure of about 18 bar draws the feedwater by a capacity of 505 m 3 per hour and pumps it into the RO membrane system. However, the rejected water from the first stage is mixed with a flow of feedwater and repumped again into the membrane system using a repass pump with a pressure of 4.23 bar and a capacity of 495 m 3 per hour. The numerical model demonstrates that the system gives a recovery ratio of 50% which is acceptable and similar to the recovery ratio of the experimental model.

RESULTS DISCUSSION AND COST ESTIMATION
Reusing of reclaimed wastewater is expected to overcome the deficit in the water management system as a new proper water source. The advance development in membrane technology supports the progress in reusing wastewater; however, the energy challenges could be the main obstacle to make these technologies practical. The applicability of RO membrane technology as a post-treatment unit to recycle wastewater was investigated based on the quality of the reclaimed wastewater and on the energy efficiency of the whole system. The quality of reclaimed wastewater was addressed by the experimental examination for the efficiency of using RO membrane technology in treating wastewater. The experimental trials and laboratory testing were performed several times to ensure better QC/QA and to present precise results and decisions. Table 7 illustrates the average removal efficiencies of impurities and pollutants through the RO experimental model in terms of different physical, chemical and biological parameters for all runs. The quality of collected wastewater from Gaza wastewater treatment shows higher concentrations than designed quality effluent by about 138%, and excess concentrations above the average designed concentrations for three operating wastewater treatment plant in the Gaza Strip by 95%. The tested samples provide practical evidence that the treatment plants in the Gaza Strip operate with an unacceptable quality level. Thus, this article demonstrates efficiency investigation for the applicability of using membrane technology to reclaim wastewater in a region suffers from inefficient wastewater treatment plants. The depicted experimental model results demonstrate that the treated wastewater using RO membrane technology achieves acceptable water quality levels for the agricultural and groundwater replenishment activities. The quality of the treated effluent wastewater through the designed experimental model was examined in terms of the physical, i.e. BOD 5 , TSS and TDS, the biological, i.e. FC and NO 3 , and the acidity or basicity index of pH. In the light of BOD 5 , the whole system shows a high overall efficiency of 100% for organic contaminants rejection, and the efficiency of the sand filter and RO unit were 59% and 100%, respectively. The system units and the whole system reveal lower removal efficiency for TSS and TDS than other parameters. The capacity of the system in eliminating TSS was higher than that of TDS by about 5%. The sand filter contribution in the whole eliminating of TSS and TDS is insignificant and the efficiency of sand filter in removing TDS is almost 0%. The RO unit forms the main removing part by the efficiency of 97.5 and 92% for TSS and TDS rejection, respectively. The contents of FC in the tested wastewater samples were about within 1300 CFU per 100 ml. The ability of sand filters to eliminate the FC from the samples was about 95% and the RO unit shows full ability to remove the remaining FC after the primary treatment by a sand filter. The nitrification process takes a significant place in the experimental system; hence, the sand filter trends to increase the concentration of NO3 in reclaimed water as a result of conversion from NH 4 into NO 3 through the nitrification process. The NO 3 is a rich nutrient to plant growth and it is considered as a valuable element to the agricultural activities. However, the RO unit has a high ability to remove NO 3 and in all the whole system was able to fully remove all NO 3 from the permeate. Related to the pH, the nitrification process trends to covert the state of water into more alkalinity, hence, the results show that the pH of the treated wastewater rises from 7.7 to 8.7 after treatment so for the purposes of agricultural activities the treated wastewater must be adjusted to be within a pH value of 6.5 to 8.4 by adding a point entry injection system to pump an acid solution of acetic acid (white vinegar), citric acid or alum to reduce the basicity of treated water. The energy and cost estimations are two linked tasks, the specific energy consumption (SEC) for the RO system is commonly calculated using over simple analyses depending on the average operation tasks for the specific plant. The results of the built numerical model through the Winflows RO system design and simulation software indicate that the SEC of the whole experimental system is about 1.23 kWh per one cubic meter of permeate. However, the local membrane technology market indicates to set up a membrane-based treatment facility with a capacity of one million cubic meters of permeate per year over a five years lifespan is approximately 6 million United States dollars (USD) as shown in Table 8. Thus, the fixed cost estimation addresses that the loaded fixed costs on the unit volume of treated wastewater are about 0.4 USD per cubic meter of permeate. In the Gaza Strip, the cost of the municipal energy is approximated quantified by 0.15 USD for 1 kWh. Hence, the total cost to reclaim one cubic meter of wastewater is about 0.6 USD. Reviewing the cost and energy consumption estimations for similar RO wastewater treatment plants worldwide could introduce better indication about the expected cost and energy to produce one cubic meter of reclaimed wastewater. Facing all the challenges and constrains relating to using RO in the reclamation of wastewater as new technology, the main two largest membrane-based treatment plants in the world, Sulaibiya wastewater treatment plant in Kuwait and Orange Country wastewater treatment plants in the United States (USA) produce daily hundreds of thousands cubic meters of reclaimed wastewater with stability and continuance. Sulaibiya wastewater treatment plant was opened in 2004; the plant is the biggest membrane-based water treatment plant. Currently, the capacity of SULAIBIYA wastewater treatment plant is about 375,000 m³ per day and it is planned to expand it to reach a capacity of 600,000 m³ per day and it is anticipated to cover 26% of Kuwait's gross water needs, and is expected to reduce the demand from 140 to 25 million cubic meters per annum (Pearce, 2008). The Orange Country plant was opened in 2004, the facility treats 320,000 m 3 daily of wastewater effluent using RO membrane technology. The plant is going to be expanded to a capacity of 590,000 m 3 per day (Water Technology, 2018). Generally, the estimations reveal that the conventional treatment of wastewater by the RO membrane technology, the costs about 85 to 90 cents per cubic meter of permeate (Pearce, 2008;Water technology, 2018). On the other hand, the requirement of energy to purify water depends on the quality of raw water and the applied treatment technique. The energy consumption for the purification of natural sources such as surface water and groundwater using membrane filtration is about 0.1 kWh per cubic meter. However, the energy consumption for treating surface water using both conventional treatment and UF/MF membrane is estimated by about 0.25 to 0.35 kWh per cubic meter. In general, the energy of water treatment could be estimated based on the TDS of the raw water, for TDS of less than 3,000 ppm about kWh of energy is needed to purify one cubic meter however for TDS between 3,000 and 11,000 ppm nearly 1.7 kWh of power is required to treat a cubic meter of water. Regarding, wastewater reclamation the energy requirement to recycle wastewater using dual membrane filtration of MF/UF and RO needs from 0.8 to 1.2 kWh per cubic meter. However, the use of a membrane bioreactor followed by RO requires energy between 1.2 and 1.5 kWh per cubic meter. In the countries of the Mediterranean Sea, the RO seawater desalination technique consumes about 2.3 to 4 kWh per cubic meter of freshwater (Pearce, 2008).