Evaluation of the efficiency of a gray water treatment system based on aeration and filtration

Gray water is a great resource for replacing fresh water to be used in standardized usages. The use of treated gray water reduces water consumption and the entry of pollutants into the environment. However, if left untreated, it can be dangerous. The present study examines the efficiency of a gray water treatment system consisting of primary filter, aeration, secondary filter and ultraviolet disinfection unit. After examining the characteristics of gray water, the efficiency of this system was analyzed to remove the pH, TSS, BOD, COD, ABS and total coliform parameters. Then, the gray water treated through this system was compared with the environmental standard of Iran. The pH of the treated gray water was 7.5–7.6. The efficiency of this system for removing the BOD and COD parameters was 98–100 and 76–100%, respectively. This system had an efficiency of 96–97% to eliminate the ABS parameter. Also, this system was able to eliminate total coliform with 100% efficiency. Results showed that in the three series of experiments performed on this system, according to the Iranian standard, the treated wastewater is suitable for irrigation and agricultural uses. However, this system could not be licensed for the COD parameter regarding the discharge to surface water and absorbent wells.


GRAPHICAL ABSTRACT INTRODUCTION
Today, with the increasing trend of industrialization and urbanization, the use of water resources has become much higher than before, which can be considered as a serious obstacle to sustainable development (Shamabadi et al. ; da Silva et al. ). This trend has widened the gap between supply and demand in the use of water resources, so that one of every six people in the world does not have access to potable water (Ghaitidak & Yadav ; da Silva et al. ). This issue is more important in Iran where the rate of rainfall is low and the weather is rather warm. In Iran, unlike some other countries, drinking water is used for all household purposes. One way to deal with the shortage of water is to make use of alternative sources such as gray water (Alfiya et  and groundwater (Abdel-Kader ). Therefore, the use of treated gray water reduces the volume of water consumed in homes and has been validated as a sustainable alternative source (Zita et al. ). Birjand is one of the cities in Iran that, due to low rainfall, is facing difficulties in supplying water. Rainfall in this city is low and irregular. Therefore, using treated gray water in this area would be a more stable source compared to rainwater usage (Leong et al. ). There are many villas in Birjand, which is why a large amount of water is spent on irrigating the green space of the house and cleaning the yard. Therefore, the use of treated gray water can significantly reduce potable water consumption (Al-Hamaiedeh & Bino ). So far, many systems with different technologies have been used.
Generally, all systems are based on physical, chemical and biological processes or a combination of them. Each system has its advantages and disadvantages. For example, the rotating biological contactor (RBC) system is able to remove biodegradable materials and is odorless, but has high power consumption and a high probability of breakage of the shaft connected to the discs (Metcalf et al. ). A constructed wetland costs very little to operate and maintain, and is practically the cheapest way to treat gray water. However, it requires a large space that makes it unsuitable for urban areas and the other problem is that a large amount of water is lost due to evaporation. Although the reverse osmosis (RO) system is able to remove a high percentage of pollutants, it has high energy consumption (Prajapati et al. ). Membrane processes are of high quality in system output but require regular washing and membrane sedimentation is extremely high (Ding et al.

).
Due to the high contamination of this wastewater after examining the characteristics of gray water, Halalsheh et al. () proposed the up flow anaerobic sludge blanket reactor (UASB) system. The proposed system was a low-cost one with flexible operation and maintenance. The treated gray water was suitable for irrigating fruit trees according to Jordanian standards. Kim et al. () designed and reviewed a system consisting of an anaerobic tank, anoxic tank, oxic tank and membrane filter for the treatment of gray water. The system was successful in removing chemical oxygen demand (COD), turbidity, color, suspended solids, and total coliform. In this system, ozone was used in the disinfection unit. Li et al. () found that the membrane bioreactor (MBR) system is suitable for high contamination and the RBC and sequence batch reactor (SBR) systems are more appropriate for moderate contamination. They also found that anaerobic processes are not suitable for the treatment of gray water due to poor efficiency in the removal of organic matter and surfactants. Oh et al. () identified a physical filtration system with a disinfection unit for low-pollution treatment of gray water.
Gray water contains many compounds which are practically impossible to measure (Leal et al. ). Therefore, parameters related to organic matter, solids and microbiology are usually more significant (Pidou et al. ).
Standards vary depending on the application and place of use of the treated wastewater; also, regional standards may differ. Normally, standards are less stringent for neighborhoods where there is less contact with humans (Pidou Countries that do not have specific guidelines for assessing the quality of treated wastewater can use WHO or EQS criteria (Eriksson & Donner ). For irrigation purposes, the Iranian standard allows pH ¼ 6-8.5, BOD ¼ 100 mg/L, COD ¼ 200 mg/L, TSS ¼ 100 mg/L, ABS ¼ 0.5 mg/L and total coliform ¼ 1,000 (in 100 mL) (Kashafi ).
In this study, after examining the specifications of gray water for the above parameters, a gray water treatment system was designed at domestic scale according to Iranian standards and the design and operational aspects of it were described. Whereas in the systems previously studied only one filtration unit was used after the aeration tank, this gray water treatment system consists of a membrane filter, aerobic tank and membrane filter, and an ultraviolet (UV) lamp was used in the disinfection unit. Finally, the performance of this system in eliminating the above parameters was examined and compared with Iranian standards.

Sampling
Birjand is located in the South Khorasan province in Iran with a longitude of 59.12 and a latitude of 32.52 , and an altitude of 1,491 meters above the sea level. With an average rainfall of 168.5 mm and an average temperature of 16.5 C, this city is one of those that has always faced water supply problems. Therefore, the use of treated wastewater in this city is most important. Birjand University, located in Birjand, uses drinking water for all its uses, even irrigating green spaces.
Due to the problems in Birjand with supplying water and the lack of sufficient land area for the construction of a central treatment plant, this research was carried out to investigate and evaluate the efficiency of the system under study.
Dormitories are one of the main sources of gray water production at Birjand University. Separation of gray water from black water has been done only in the three dormitories of Towhid, Sadaf and Sarv. In addition, in these three dormitories, the hand basin sewers are separated from the bath sewers.
Therefore, sampling of these three sites was taken as follows: • Site 1: wastewater from the hand basins in Sadaf women's dormitory of Birjand University, which included six hand basins on three floors, collecting wastewater from a total number of 18 hand basins.
• Site 2: wastewater from the hand basins in Sarv men's dormitory of Birjand University, which included six hand basins on two floors, collecting sewage from a total number of 12 hand basins.
• Site 3: wastewater from the baths in Towhid women's dormitory of Birjand University, which included four baths on three floors, collecting sewage from a total number of 12 baths.
The quality of gray water can vary greatly depending on the day and even the time. Therefore, for accurate characterization and reliable results, sampling was repeated on two different days and three different times. Sampling was carried out at 9a.m., 2p.m. and 9p.m. It was repeated on Tuesday and Friday. Finally, 18 samples were tested for BOD and COD values. Table 1 shows the number of each sample along with the location, time and day of sampling. According to Figure 1, gray water from the hand basins of women's dormitories has the highest rate of contamination with an average BOD of 93.3 mg/L and an average COD of 217/3 mg/L. Therefore, using this gray water is the best option for evaluating the performance of the designed system.

Analytical methods
According to Iranian standards, samples should be stored in a plastic container at 4 C and tested within 6 hours (Kashafi ). tometer. The TSS test was performed using the standard 2540D gravimetric method. Also, total coliform experiment was performed using the standard method of ISIRI42079221B and the most probable estimation method with a Behdad incubator.

Experimental pilot plant set-up
In order to set up a gray water treatment system, in addition to the cost of purchasing a treatment system, the cost of separating gray water from black water and the storage and distribution of treated wastewater in a pipe, separated from drinking water, should also be considered (Abdel-Kader ). Therefore, designing a simple, effective system which is cost-effective and easy to use is very important (Santos et al. ). In the first step, the sources generating wastewater and its pollutants must be identified. In the next step, the place in which the treated wastewater is consumed should be examined, also standards and restrictions should be controlled. In order to choose a suitable treatment system, consideration of types of treatment systems and formance is needed to treat gray water using chemical processes, which makes it more difficult to be used at domestic scale. The concern about using a domestic wastewater treatment plant is smell production, which can be suppressed by using an aerobic biological process. On the other hand, the deployment of an anaerobe biological process requires long-term storage, which allows a larger storage tank to be included in the design and more space to be allocated to the treatment system. Due to the presence of many contaminants in the gray water, the use of the physical treatment process alone causes the filters installed in the system to lose their efficiency quickly and the check period becomes shorter.
According to the above, a combination of aerobic biological and physical treatment processes were used in the design of the treatment system. The aeration unit is the most time-consuming treatment unit. To reduce this time, a physical treatment unit is used which decreases the contaminants in the gray water. Therefore, less time is required to inject air into the wastewater. Finally, a filter was installed to remove finer contaminants and microorganisms that entered the wastewater during the aeration stage.
Gray water treatment systems based on aeration and filtration include lint filters, a primary filter, collecting tank, secondary filter, disinfection units and reuse tanks. In fact, the collecting tank simultaneously functions as

RESULTS AND DISCUSSION
Quality of the gray water The designed system with the volume of 120 L was tested three times. In all three series of experiments the raw gray water had a different contamination amount. Table 2 shows the amount of each parameter for raw gray water in all three series of experiments.

Treatment efficiency
According to Table 2, the amount of pollution in test B is more than test A and test C is more than test B. GW1, GW2, GW3, GW4, GW5 and TGW1 in Figure 2 are sampling points. Sampling was performed for all three experiments (A, B, and C). Table 3 shows the measured parameters for tests A, B and C.
As it is clear in Table 3, raw gray water is slightly alkaline due to the use of detergents. The presence of a collecting tank prevents pH shock to a large extent.
During the experiments, all samples were within the standard range. Only in test C, the GW1 and GW2 samples had a high alkalinity with pH ¼ 8.3, which eventually reached 7.6 in the GW5 sample. This is because, in the two stages of filtration and aeration, the pollutants that increase the alkalinity of gray water are destroyed and removed.
Suspended materials in wastewater disrupt agricultural activities. The presence of these particles causes sedimentation in irrigation systems. Also, the presence of large amounts of TSS, even if there are no health risks, causes a negative mentality in consumers and their unacceptability.
Therefore, according to the ultimate goal of this system, which is the treatment of gray water for irrigation and agricultural consumption, the TSS parameter is very important.
In the TSS test, the efficiency of test A was 100% as TSS GW1-¼ 34 mg/L and TSS GW5 ¼ 0 mg/L. In test B, TSS GW1 ¼ 51.5 mg/L which was purified with 98% efficiency, also for the GW5 sample, it reached 1 mg/L. In test C of the TSS tests, it was decreased from 68 to 2.5 mg/L with the   Figure 5 shows the TSS removal efficiency in tests A, B and C. The presence of a lint filter and two filters that can remove particles up to 100 and 1 μm, respectively, resulted in the optimal efficiency of this system in reducing TSS. The 100% reduction in Series A tests is probably due to the new filters. However, in two series of B and C tests, the real efficiency of the system to remove TSS parameter was obtained. to these values, it was found that the gray water of the basin has more COD than the bathroom. This could be due to the use of more detergents than water. Also, the gray water of the women's basin is more polluted than the men's basin. This is because women use cosmetics and more detergents than men. In the COD test, the COD GW1 reduced from 83.4 mg/L to COD GW2 ¼ 0 mg/L in the first series of experiments (test A). In the B and C series of experiments, the GW1 samples reduced from 250, 529 to 60.5 and 77.8 mg/L, respectively. The efficiency of the system in tests B and C is 75.8 and 85.3%, respectively. Figure 7 shows the efficiency of COD removal for all three test series.
The ABS test measures alkyl benzene sulfonate. Alkyl benzene sulfonate exists in soap and has high cleansing power. Nowadays linear alkyl benzene sulfonate (LAS) is applied. However, detergents with ABS are still in use in Iran. In addition to detergents, ABS also exists in cosmetics.
Few studies have investigated and measured the ABS parameter in gray water. The ABS level of raw gray water in all three series of experiments was within the allowable range of the Iranian standard. According to Table 3, the    Figure 8 shows the efficiency of removing ABS for three series of tests by the designed system.
If the better performance of UV disinfection systems is to be considered, the parameters that prevent light reaching the sewer must be removed before reaching the disinfection unit. Utilizing a UV-C lamp in the disinfection unit was quite effective. The disinfection properties of the UV lamp expire as soon as it is turned off. For this reason, a pump is installed in the storage tank to regularly pump the treated gray water to the disinfection unit in order to prevent the growth of microorganisms and maintain the microbial quality of the treated wastewater. The efficiency of the disinfection system in this study was 100% in all three series of experiments, and all coliforms were destroyed, and finally, no coliform was observed in the treated gray water.

Reuse potential
To investigate the reuse potential of gray water treated through this system, the measured parameters were compared with the Iranian environmental standard. Iran's environmental standard has set different permissible limits for various purposes. Table 4 shows a comparison of the permitted environmental standard in Iran for reusing wastewater with gray water treated through the designed system.
According to Table 4, the gray water treated by this system failed to meet the Iranian environmental standard for the COD parameter in the B and C series of tests for discharging in surface waters and absorbent wells, but the treated gray water is quite suitable for agricultural and irrigation purposes. The inability of this system to reach the permissible standard value for the COD parameter is due to the lack of a chemical treatment unit. Discharging the treated gray water in absorbent wells or surface water is not reasonable unless it is not possible to use gray water for other purposes.
In fact, the aim of this study was to refine the raw gray water to use for irrigation and agriculture, which was achieved.
In 2006, the World Health Organization introduced a standard for microbial parameters. This guideline outlines microbiological requirements without considering the other physical and chemical parameters (WHO ).

CONCLUSIONS
Reusing treated gray water, in addition to reducing freshwater consumption, also lessens the volume of wastewater entering the environment. Most gray water treatment systems require an operator in addition to the high initial construction cost. On the other hand, the volume of the produced treated wastewater is in excess of the domestic consumption. Also, smaller systems that are used on a domestic scale may not be as efficient as removing contaminants of the gray water. Therefore, it is important to design an effective system that is commensurate with the amount of domestic consumption. Sampling the raw gray water from three different production sources in 18 shifts, it was found that the gray water from women's hand basins had a higher pollution load than men's basins and women's bathrooms. Thus, in the next experiments designed to evaluate the performance of the treatment system, this wastewater was used. This study, after examining the characteristics of gray water produced at Birjand University in Birjand, showed that using an aeration process including two filters and a UV disinfection unit can be an effective system in treating gray water. This system worked perfectly well, especially regarding removing the total coliform parameter.
Based on the obtained results, the efficiency of the system designed for the TSS, BOD, COD, ABS and total coliform in the critical state was 96.3, 97.6, 758, 96 and 100% respectively.
For different uses, the Iranian standard and WHO has set a different allowable range for water pollutant parameters. This study examined the parameters of pH, TSS,