Small Scale Plant of Desalination Using Multiple Tray Aerator and Two Different Membranes of Reverse Osmosis Using Low-Pressure Pump

The seawater is an abundant source that still not be utilized as potential raw water source to overcome the deficit of drinking water. This paper aims to design the small-scale desalination plant with the capacity of 2000 gpd for treating the brackish and seawater. There are two split processes, i.e., multi-tray aerator and filtration for reducing the Fe and Mn, while the two different type reverse osmosis membranes were used for desalinating process. Parameters used to verify the performance of small-scale plant are salinity and total dissolved solid for membranes, while the parameters for examining the pre-treatment results are TSS, BOD, pH, Fe, and Mn. The test results will be evaluated under the Regulation of The Minister of Health, Number 32/2017. The influence of pump pressure on flow will also be analysed. The results show that multi-tray aerator needs some improvement to reduce the Mn, and Fe. The two types of membranes could significantly reduce the salinity level up to 91.7%, from 22,900 ppm to 1200 ppm. The pump pressure needs to be replaced into the high-pressure pump to reduce the salinity level significantly.


Introduction
Water is essential and fundamental element for human life.However, only 0.8% of total water in the earth that could be consumed as freshwater [1].In developing country, people living in coastal area and island mostly experience the lack of freshwater.Moreover, the highest freshwater demand is located in the coastal zone and delta estuary [2].The difficulty of having freshwater in coastal community which is surrounding by the brackish and seawater is the irony amid modern era which is supported by advanced technology.Therefore, the abundant of seawater and brackish water near the coastal area should be utilized as raw water source.The characteristic of seawater and brackish water, containing of 1298 (2024) 012008 IOP Publishing doi:10.1088/1755-1315/1298/1/012008 2 high level of salinity, can be desalinated by using the reverse osmosis membrane to get the freshwater [3,4].
Application of the reverse osmosis method as the cutting-edge technology for the water treatment has been investigated by researchers for many years [3,5].One of the researchers studied the application of reverse osmosis membrane using low pressure pump (1.5-3.5 bar) for the brackish water well, located at the Sidoarjo resident area, in East Java [6].Their pre-treatment used two media filters consisting of sand and activated carbon that is contained by FRP.Their result showed that the salinity only reduced by 58%.Moreover, another researcher carried out the combination of biofiltration, membrane ultrafiltration, and reverse osmosis for the treatment of river water, without being influenced by the salinity intrusion [7].Their results show that the system can reduce parameters of TDS, Mn and Fe by 97.8%, 57-85%, 58-84%, respectively.
In coastal area, soil water mostly contains the iron (Fe).To reduce the iron, one of the most popular method that is used in pre-treatment process is multi-tray aerator [8,9].The mechanism of aerator device is to interact between the air and the iron (Fe) as much as possible to.The number of trays is intended to increase the aeration process to reduce the Fe level.In the previous experiments, four trays have the reducing level of Fe at 98.2% [9].On the other hand, the single tray aerator can only reduce the Fe level at 25% [10].Those two experiments demonstrated that the multi-tray aerator is much more effective than compared to the single-tray aerator.
Based on the previous works, the investigation of applying reverse osmosis method using two low pressure pumps, to amplify the pump pressure, in order to reduce the salinity of seawater significantly, is important task.
This paper aims to design the desalination plant for converting sea water or brackish water to freshwater using sequential processes consisting of pre-treatment and core treatment.The pre-treatment stage applies multi tray aerator, whereas the core treatment process, consisting of ultrafiltration membrane (UF), and reverse osmosis (RO) membrane with two types of RO, i.e., ultra-low-pressure membrane (RO ULP) and brackish membrane (RO BW).

Working principle of reverse osmosis
The working principle of reverse osmosis as a technology of desalination is by pressing the seawater using the high-pressure pump, where the pressure of pump must be greater than the pressure of seawater.This condition pushes freshwater through the RO membrane, whereas the residual water, containing the salinity, will be rejected by the RO membrane [11].
Aeration is widely used for treating water containing high iron and manganese [8].Principle process of multiple tray aerator is based on the aeration process, which generates the oxidation process between the water and air.The longer the contact, the increasing the oxygen content.In addition, this process can also reduce the CO2, H2S, CH4, and VOCs which could affect bad taste and flavour of the water.

Materials and installation design
There are two main installations of the desalination equipment, i.e., installation of pre-treatment and core treatment.The scheme diagram of desalination plant design can be shown in Fig. 1.

Figure 1. Scheme diagram of small desalination plant
Moreover, there are two components of pre-treatment that should be assembled, i.e., multi tray aerator and filter media, whereas the core treatment only has component of membranes.The material of multi tray aerator unit made by using galvanized iron frame, consisting of five trays with the total height is 4 m, and the dimension of tray is 0.94 m x 0.74 m x 0.17 m.
The last unit is the core treatment, which consist of three membranes, i.e., UF, RO ULP, and RO BW membranes which is assembled in galvanized irone frame.Figure 2 shows the design of pre-treatment and core treatment of desalination equipment.In this small plant, there are two types of RO membrane, which are implemented in two different situations, i.e., if the water condition is categorized as light brackish, then only the ULPRO membrane is operated.However, if the water condition is categorized as brackish or seawater, then both membranes are operated.
The other vital component of desalination equipment is high pressure pump, which must achieve a certain pressure in order to fulfil the requirement of reverse osmosis process, ranging from 75-350 1298 (2024) 012008 IOP Publishing doi:10.1088/1755-1315/1298/1/0120084 Psi [12,13].In addition, these pressures depend on the salinity content where the higher the salinity, the greater the pressure of pump is required [12].

Methods
The performance of desalination equipment design must be evaluated in real situation.There are two types of raw water sample, i.e., first, the existing of raw water in the river near the coastal area where the salinity level at 5000 ppm.Second, the seawater with the salinity level at 21 ppt.
In this research, tests for raw water samples are divided based on two purposes, the first one is to evaluate the performance of multiple tray aerator using river water sample, and the second one is to examine the performance of RO membrane using seawater sample.Thus, these examinations of both samples are not sequential, where the water sample from pre-treatment does not used as water input for the core treatment.
Figure 3 shows both raw water sample of river water and seawater.Due to always rain falls in Pontianak throughout the year 2022, in order to obtain the seawater salinity, we must take the samples from the ocean, assisting by the fisherman, transporting by small boat and bring it by the water gallon.In this research, for pre-treatment process, there are only seven parameters of water quality tested in the laboratory, namely biological oxygen demand (BOD), total dissolved solid (TDS), total suspended solid (TSS), iron (Fe), and Manganese (Mn).After aeration process, coagulation process is conducted by adding aluminum sulphate (alum) [14].The dosages of alum are determined by jar tests, where the optimal dose is 125 gr for 1 m 3 of water sample, as presented in Fig. 4.

Figure 4. Jar test for determining coagulant dose
The test method for BOD and COD is using the azide winkler and closed reflux method, respectively.The parameter of Fe and Mn are tested using atomic absorption spectrometry (AAS), while the TSS is examined by using gravimetric method.In addition, for the core treatment stage, only two water quality parameters are evaluated, viz.TDS and salinity, which are measured by using conductivity meter.The result of water sample test will be evaluated using the quality standard, according to the regulation of Minister of health, Number 32/2017.
The pressure specification of pump used in this research is 4 bar or 58 Psi, while the pressure specification of membranes ULPRO and BW are 150 Psi and 225 Psi, respectively.Two low pressure of pumps is connected to amplify the pump pressure in order to gain the desalination process.

Result and analysis
Figure 5 shows the 5 water sample bottles, i.e., two inlet bottles before pre-treatment and two outlet bottles after the treatment; and the last bottle (the rightmost bottle) is after adding coagulant to the collecting pan.The outlet bottles show more brighter, indicating pre-treatment process works.However, reducing mineral of Fe and Mn are still not significant.However, after adding the coagulant, the water quality fit the requirements of the Minister of Health regulation.This means that the multi tray aerator need further improvement to reduce parameters TDS, TSS, BOD, and Fe, Mn.This problem also happen to the previous work conducted by researcher where the pre-treatment does not affect several parameters, particularly for the TDS and TSS [7].To further analysis whether the aeration process works or not, it can be measured using dissolved oxygen (DO) parameter and calculated using DO meter.The result of DO parameter can be shown in Figure 6, where the DO increases in increasing time.This reaction proves that the oxidation process occurred, where there is contact between the air and water during the aeration process.As a result, the solubility of oxygen contained in the raw water increases.The process of mixing the oxygen with the water is characterized by an increase in the value of DO in the colleting pan, after aeration process in multi tray aerator.The performance of desalination equipment can be seen from Table 1, demonstrating the result of water quality parameters for river water and seawater sample from the multi tray aerator and RO test, respectively.
From the Table 1, the pH parameter of water samples behaves differently between the adding coagulant and without the adding coagulant.The pH parameter of water sample, where the water samples were added with alum coagulant, decreases from 7.4 to 4.7.This outcome is consistent to the result from the previous study, conducted by another researcher [15].7 shows the comparison of clean water flow and pressure pump between the seawater and brackish conditions.It can be shown that for brackish condition, the pump pressure is lower than that for seawater condition.In contrast, the clean water flow for brackish is higher than that for the seawater condition.It means, the higher the salinity concentration, the higher the pump pressure.As a result, the clean water flow decreased.

Discussion
Theoretically, the multi tray aerator can reduce the Fe and Mn significantly.The multi tray aerator affects the oxidation process, as evidence in decreasing the COD and BOD.Moreover, it can also be proved from the DO parameter analysis result, that the DO increases with increasing time the, implying the aeration process of multi tray aerator is running well.However, the oxidation process is only able to create colloid, not floc as expected.Therefore, to accelerate the formation of large floc, adding alum coagulant is needed.However, alum can result in lower pH due to the chemical process related to salt ions [15].The multi tray aerator probably can be more effective if the settlement time is more than 4 hr, as revealed in the previous paper by another researcher [7].Moreover, the filter component inside the tray of aerator can also be replaced by another filter to be able to improve the decreasing of iron content, such as aquadine filter mat combined with sand and gravel, or combined with silica sand, zeolite, and activated carbon [16].
In the desalination process, the goal of applying multiple membranes is to assure the process of converting seawater to freshwater can work well.As a result, the UF, RO ULP, and BW membrane can effectively reduce the salinity, where the pressure gauge indicates that the total pump pressure can reach 150 Psi.This pressure can be achieved because the experiment test uses the series of two pumps, which is able to amplify the total pressure of pump.
As proven in the experiment test, applying both RO membranes can be more effective in reducing the salinity, compared to applying only 1 RO membrane (See Table 1).This also means that the reverse osmosis process remain works although applying the two low pressure of pump on the reverse osmosis membranes.This happens, because the type of low-pressure membrane of reverse osmosis enabling the low-pressure pump to remain work to obtain desalination process, though must work with heavy loads.
Nevertheless, the flow water rate of clean water for seawater condition is too minor compared to the brackish water condition, where from the flowmeter measurement, it can only reach ~0.5 l/min.It means that the higher the concentration of salinity, the lower the flow can produce.As the comparison to the previous works by other researcher, reducing their salinity concentration by using only one low pressure pump is much more limited, which can only reach at 58% [6,17].Therefore, employing of two series pumps is necessary if a greater reduction in salinity concentration is desired.

Conclusion
Design and construction of small-scale desalination device using multi-tray aerator and reverse osmosis membranes using low-pressure pump has been implemented.The performance of multi-tray aerator can increase oxidation process, implying the aeration process is running well.However, the additional alum coagulant is still needed to accelerate the process of flocculation to meet the quality of water that match with the regulation of Minister of health, Number 32/2017.
The three membranes consisting of UF, RO ULP, and RO BW can reduce the salinity by 91.7%, from 22,900 ppm to 1,200 ppm, though need further improvement.In addition, the discharge of clean water for seawater needs to be increased.To optimally reduce the salinity level and increase the flow rate of clean water, the pump must be replaced that having the pressure around 24 bar or 340 Psi.

Figure 5 .
Figure 5. Water sample bottles before and after the treatment of multi tray aerator.The rightmost bottle is the sample after being added the coagulant.

Figure 6 .
Figure 6.DO parameter test after multi tray aerator process.

Figure 7 .
Figure 7.Comparison of clean water flow and pressure pump between the seawater and brackish conditions.

Table 1 .
Results of water quality samples.