Abbreviations: Moving-bed biofilm reactor (MBBR), Chemical oxygen demand (COD), COD removal (CODr), Total dissolved solids (TDS), Hydraulic retention time experiment (HRT)
Water security is one of the major concerns nowadays, as it is predicted that half Earth inhabitants will be facing water shortages by the year 2030 . Water resources pollution in addition to population and industrial growth, had increased the need to find suitable and effective ways for water treatment . Both organic and inorganic wastewater pollutants are present in either suspended or dissolved state, and while the suspended pollutants are easily separated by sedimentation, dissolved compounds are difficult to remove; however, there is a large portion of biodegradable organic substances such as carbohydrates and alcohols which are a nutritional source of bacteria and microalgae .
The rapid evolution in chemistry and biology has opened the door for biological wastewater treatment using microalgae and bacteria with high efficiency in nutrient removal (Phosphorus and Nitrogen), reduction of chemical oxygen demand (COD) and biological oxygen demand (BOD) [5-12]. Biological wastewater treatment is based upon a simple methodology; however, there are many complications in treatment condition management such as pH levels  and toxic shock due to the presence of various chemical compounds in treated water .
Moving-bed biofilm reactor (MBBR) is an alternative and efficient way to treat different types of wastewater under different conditions . Due to its effectiveness and simplicity, this technology has grown from 400 fully operating MBBR reactor to more than 1200 in recent years . This method operates similarly to the integrated fixed-film activated sludge (IFAS) method  with the addition of free movement of the bacterial mass carrying medium. More specifically, this technique relies on the growth of a biological membrane adhered to suspended substances in moving conditions within the reactor. The advantages of MBBR outweigh those of activated sludge through better oxygen transport, lower hydraulic residual time, greater organic loading rate, higher nitrification rate and more surface area of biomass .
Many studies were conducted to optimize MBBR startup and operational parameters . This study aims to evaluate the operational parameters of an experimental MBBR system and to optimize its startup and retention times by measuring deferent water biochemical indicators.
The reactor was a glass tank with a capacity of 25 L followed by a 5 L settler tank. The system contained two pumps: an air pump that provides the reactor with air through the jets at the bottom of the tank and a second water pump that circulates water within the reactor. Cylindrical shape plastic carrier centers (11*7mm) were used in the reactor (30% of the reactor volume) as an adherence surface for the formation of a biofilm. The reactor was equipped as well with valves and plastic pipes for water pumping and discharging (Fig. 1).
Activated sludge and wastewater were obtained from a fully functional local water treatment facility. Wastewater had the specifications shown in (Table 1).
Chemical oxygen demand (COD), total dissolved solids (TDS), in addition to NO3-, and PO4-3 concentration were continuously monitored considering APHA A. WPCF (2005) standard water examination methods . COD was determined using COD Set-Up MD 200 (Lovibond – Germany). The pH and TDS were measured using Jenway 3540 pH and conductivity meter (Jenway - UK). NO3- and PO4-3 concentrations were determined at 230 nm wavelength absorbance using Hach chemistry detection kits (Hach – USA) with OPTIZEN POP UV-Vis spectrophotometers (Mecasys – Korea) and according to manufacturer instructions.
COD and TSD for the wastewater within the reactor were monitored every 3 days throughout a startup enclosed wastewater circulation period. After the startup phase (42 days) and based on the monitored biochemical indices, the reactor was considered started. MBBR efficiency was then evaluated for different hydraulic retention time (HRT) periods (3, 4, 5, 6, 7, and 8 h). The water used in retention time experiment had the same specifications shown in (Table 1). The reactor efficiency was also evaluated with different feeding concentrations for the studied indices (Table 2) with a chosen water retention time of 6 h.
The following formulas were used to calculate removal efficiency indices:
COD and TDS removal efficiency means were compared using Fisher’s Least Significant Difference (LSD) test at (p=0.05) using Minitab 19.2 software.
Graphs were plotted, and the linear regression relationship for each index before and after treatment for different initial loadings was calculated using GraphPad Prism 7 software.
Water quality was monitored throughout the startup phase. Results illustrated that COD and TDS values decreased with increasing startup and reached a value of 110 and 608 mg/l respectively after 42 days of initiation (Table 3) and (fig. 2 A) with COD and TDS removal efficiency of 94.4% and 44.73% respectively (fig. 2 B). The gradual improvement in treatment efficiency throughout startup time is predictable due to the formation of larger biomass over time. The steady COD after 42 days of constant water influent refers to the full development of biofilm on the carrier and suggests reaching the stable working phase .
Reactor efficiency was evaluated for six HRT times (3, 4, 5, 6, 7, and 8) h. The concentration of pollutants was decreased with the increase of hydraulic residue time and 8 h retention time registered the lowest COD value of 25 mg/l which is similar to the results of  and in correspondence with  results who reported an increase in effluent COD levels by decreasing HRT. Furthermore, It can be noticed that the Syrian (Class a) standard for treated wastewater for agricultural irrigation uses  was reached after 6 h retention time period with 70, 660, 17.87, and 4.79 mg/l for COD, TDS, NO3- and PO4-3 concentrations respectively (Table 4) and (Fig. 3 A and B). However, CODr values illustrated that by increasing HRT from 3 to 8 h, the removed COD weight on daily basis will decrease gradually from 10.88 to 5.78 Kg COD/m3.day, respectively and this decrease was significant with each additional HRT after 4 h. These results can be justified due to the fact that most of COD load was removed with 4 h HRT treatment (94.1% COD removal) which rendered the longer HRT treatments useless in terms of COD removal (Table 4) and (Fig. 3 C). The short HRT treatments were reported to disrupt the bacterial growth which thereafter decreases the efficiency of the system ; therefore, HRT treatments of more than 5 h were recommended .
Table 4. MBBR system monitoring for different hydraulic retention time (HRT) periods and different biochemical indices
COD removal efficiency increased from 69.74% to 98.7% by increasing HRT treatment from 3 to 8 h. Furthermore, TDS removal efficiency increased from 9.9% to 48.82% for 3 and 8 HRT respectively showing a significant increase until 7 h treatment; however, with 96.4% COD removal efficiency, there was no significant difference between 6 h treatment and longer treatments (Table 4) and (Fig. 3 D). Therefore, and based on previous results that recommended 6 h treatment for domestic wastewater MBBR plants, 6 h treatment was chosen as the most efficient HRT to reach the Syrian (Class a) irrigation water quality.
The effect of initial COD, TDS, NO3-, and PO4-3 on MBBR treatment efficiency was studied. Treated water COD, TDS, NO3-, and PO4-3 increased by increasing their initial inputs which means that there was a decrease in removal efficiency (Tables 5 and 6). This decrease is attributed to the lack of biomass required to sustain the removal of the high organic load. The decrease in removal efficiency was previously reported by increasing influent COD in sequencing batch reactors (SBRs) , up-flow anaerobic sludge blanket digestion (UASB) reactors , anaerobic baffled reactor (ABR) , and in MBBR . Furthermore, denitrification and the treatment efficiency of membrane biofilm reactor (MBfR) were drastically decreased when nitrate influent was greater than 120 mg/l .
Table 5. MBBR system monitoring under constant hydraulic retention time (HRT) of 6 h and different CODa and TDSb influent concentrations
Table 6. MBBR system monitoring under constant hydraulic retention time (HRT) of 6 h and different NO3- and PO4-3 influent concentrations
Linear regression relationship between the initial COD, TDS, NO3-, and PO4-3 concentrations of raw water and treated water were:
These equations, to calculate the maximum wastewater concentration in order to reach (Class a) irrigation water quality according to the Syrian standard  with a COD, TDS, NO3-, and PO4-3 of 75, 1500, 20, and 15 mg/l respectively and a hydraulic retention time of 6 h. Results showed that initial COD, TDS, NO3-, and PO4-3 in wastewater should be below 1997.4, 2122.86, 55.04, and 20.96 mg/l respectively in order to achieve the quality of (Class a) irrigation water based on the Syrian standards with the chosen HRT of 6 h. These results indicate that our system can be classified among the highly efficient laboratory-scale systems in terms of COD removal  especially for a single phased aerobic reactor. The dramatic decrease in NO3- and PO4-3 removal efficiency by increasing their input levels (Table 6) might be partially alleviated by increasing HRT as seen in (Table 4); however, the increased levels of these pollutants might endanger the whole system with the prospect of a toxic shock . Previously, it was reported that high Nitrogen and Phosphorus input in MBBR systems can be removed by adding an additional anaerobic phase with certain operating protocol ; therefore, the single phased aerobic system reviewed in the current experiment is only recommended for wastewater treatment with low Nitrate and Phosphate levels.
The experiment showed that the most efficient HRT for the designed MBBR system was 6 h. Regression analysis showed that the maximum influent feed for COD, TDS, NO3-, and PO4-3 in wastewater should be below 1997.4, 2122.86, 55.04, and 20.96 mg/l respectively in order to achieve the quality of the Syrian (Class a) irrigation water under 6 h HRT treatment. Furthermore, the single phased aerobic MBBR system can be recommended as a simple method of wastewater treatment with high COD and low Phosphorus and Nitrogen inputs.
More studies are required to investigate effect different organic and inorganic loads on treatment efficiency, and to evaluate different starter media and carrier materials under Syrian conditions and water standards.