Removal of Suspended Solids Using Pumice Stone in Integrated Fixed Film Activated Sludge Process

Wastewater treatment plants operators prefer to make adjustments because they are more cost effective, to use the existing tank instead of building new ones. In this case an imported materials would be used as bio-loads to increase biomass and thus maintain efficiency as the next organic loading increases.In the present study, a local substance "pumice stone" was used as a biological carrier in the aeration tank, and the experiments were carried out in five stages: without biological carriers, filling ratio of 4%,10%,20%, and25% with pumice stone, the maximum organic loading at each stage (1.1884, 1.2144, 1.9432, 2.7768, 3.3141)g BOD /l.d respectively.Other experiments were carried out to determine the best filling ratio, the SS removal ratio was (67.57%, 69.5%,79.44%,89.61%,and 99.2%) when the filling ratio with pumice stone was (0, 4, 10, 20, and 25)% respectively, at organic loading 2 ± 0.0528 g BOD /l.d, so the best filling ratio of pumice stone was 25% .


Introduction:
Sewage treatment plants are designed for a period of (25-30) years, and at the end of it, engineering procedures must be taken to preserve the plant's output due to the increased organic loading coming to the plant. These procedures are: Either the horizontal expansion of the treatment plant units (i.e. building new treatment units) and this procedure requires a high cost, or technical procedure to increase the biomass without adding new units. At the present time, it is preferable for the sewage treatment plants to make modifications because they are more cost-effective, as they use the existing tank instead of building new tanks.
The combined modification of the conventional activated sludge process uses suspended or fixed media of a suitable material, which acts as carriers for the growth of active biomass on its surface, and this is called the IFAS system (1).
An integrated fixed-film activated sludge (IFAS) system is one of the most popular modified activated sludge processes which increases the microbial population and accelerates the biodegradation of organic compounds by adding a fixed media to a suspended growth basin (1). This process is actually an integration process which includes the suspended and attached growth and provides the advantages of both attached and suspended growth systems (2).
The IFAS process has many advantages in comparison with conventional processes of the activated sludge. This system provides more resistance against organic and hydraulic shock load; besides, it has more flexibility and higher efficacy than other activated sludge processes (3,4) The IFAS system is a good option to upgrade the Activated Sludge System especially in case of facing scarcity of land and provides higher removal efficiency of COD and nutrients relative to conventional activated sludge. It also possesses a lower retention time, higher hydraulic load, and less tank volume (5,6). In the present study, a local substance "pumice stone" with a surface specificity of 224 m 2 / m 3 was used as a fixed biological carrier in the aeration tank over 253 days, with different filling ratio of 0%, 4%, 10%, 20%, and 25% when the organic loading changed from 0.92±0.0384 to 3.4 ±0.0706 gBOD / l.d .
While in a previous study, moving plastic holders of type PVC with a surface specificity of 350 m2/ m3 was used with a filling ratio of 25% with increasing the organic loading inside the experimental plant over 105 days, the best removal ratio of SS was 85.24 ± 3.21% when the organic loading was 0.44 gCOD / l.d .. (7) In another study ,Moving Bed Biofilm Reactors (MBBR) was used to rehabilitate a sewage plant in Kiththal city, India, samples at inlet and outlet during the period from January 2014 to April 2014 were collected, the SS removal ratio was 83.11% .. (8) Six full-scale IFAS were surveyed to quantify Trace organic contaminants and estrogenic activity removal, the type of media varies for each IFAS, it is shown in Table 1, all of them performed well in terms of the removal of TSS (91-99%). (9).

Materials and Methods:
The experiments were conducted at the sewage plant in Homs, Syria, over 253 days.
The operation was started on July 1, 2018 using aerated sludge from wastewater treatment plant in Dweir, Homs, Syria which is a plant that works with conventional activated sludge technology, to accelerate the formation and growth of biomass. The wastewater used to accomplish this research was brought from the outlet of the first sedimentation tank.
The characteristics of the wastewater and sludge which were taken from Homs wastewater treatment plant, are shown in Table(2): The pumice stone is considered chemically inert due to its high content of Silicon dioxide, SiO 2 =75%. The black basalt pumice stone with a specific weight = 1.2 gr /cm 3 , was used in this study.
The pumice stone was prepared in a cylindrical shape with dimensions D = 2.5 cm, H = 1 cm, and the specific surface measurement was 224 m 2 /m 3 . The pumice stone was fixed on a metal holder.
The percentage of filling of fixed biological carriers is calculated by dividing the size of the outer frame of the holder of the biological carriers are fixed on it, by the effective tank size ... (10) Effective aeration tank volume = 10 liters Single holder size = 122.7185 cm 3 = 0.123 liters The OLR (Organic Loading Rate) of the experimental plant was changed by mixing the water from the sedimentation with a percentage of the water entering the Homs wastewater treatment plant.

-Pilot plant components:
The experimental tank was manufactured according to studies conducted by many researchers… (11,12) Fig (1) shows the components of the experimental plant used in the experiments, which consist of the following: 1-50 liters wastewater collection tank, from which the water was transferred to the aeration tank with a flow of 2 liters / hour " Q AT = 2 l/h". 2-Aeration tank, cylindrical in shape D = 20 cm, H = 33 cm, where the effective tank volume is 10 liters "V AT = 10 L", it means the Hydraulic Retention Time (HRT) in aeration tank was 5 hours: "HRT= V AT / Q AT ".
The tank was equipped with a compressed air system located at the bottom of the tank, with dissolved oxygen value more than 2 mg/l. Water enters the tank by flowing from the collecting tank to the secondary sedimentation tank 3-Secondary sedimentation tank, cylindrical with dimensions D = 20 cm, H = 30 cm, which has in it a sludge collection funnel with height H=15cm. The treated wastewater collected on the surface is transferred to the treated water tank 4-The sludge return pump: to transfer returned activated sludge "RAS" from the bottom of the secondary sedimentation tank to the aeration tank, tank with a flow of 2 liters / hour " Q RAS = 2 l/h", which means the recycle ratio was 100%. 5-Treated water tank: The treated water is collected in this tank.  The experiments were carried out in stages: without biological carriers, and by adding pumice stone with different filling ratios, all stages are shown in Table (3),the pumice stone filling ratio increased when the SS removal decreased to less than 83%.

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The organic loading increased over periods starting from the beginning of the experimental plant. SS was measured for five samples per week for the wastewater coming from the secondary sedimentation in the experimental plant and the removal percentage was calculated. MLSS was also measured in the experimental aeration tank.
The number 15 means 15 days from the start of operation, and the period (15-22) indicates that the organic loading in this period was almost constant and the yield also. As for the subsequent periods, the loading increased and the yield decreased and became lower than 83% .   Fig (3), (4), (5), (6), and (7) show the effect of the change in the organic loading rate on removal SS when using pumice stone with a filling ratio 0, 4, 10, 20, 25%, respectively. The maximum organic loading rate was (1.1884, 1.2144, 1.9432, 2.7768, 3.3141)g BOD /l.d, at each stage.      The yield decreases due to the increase of organic loading rate with slightly increase of biomass. (6) Second: Experiments were carried out to determine the best pumice stone filling ratio at the same OLR. The SS removal ratio was (67.57%, 69.5%,79.44%,89.61%,and 99.2%) when the filling ratio with pumice stone was (0, 4, 10, 20, and 25)% respectively, at organic loading 2 ± 0.0528 g BOD /l.d. Figure(8) shows the removal of SS with different pumice stone filling ratio, when OLR was 2 ±0.0528 g BOD /l.d.

Fig 8. Removal of SS with different pumice stone filling ratio
In a previous study, the best removal ratio of SS was 85.24 ± 3.21% when the organic loading was 0.44 gCOD/ l.d. (8) In another study the removal of TSS was (91-99%). (9).Comparing the results of different studies with those of the present study revealed that the IFAS system has had high efficiency in TSS removal which might be due to the fixed media of the biological reactor.
Indeed, hybrid systems like IFAS create the conditions of attached and suspended biological growth, and thereby the sludge sedimentation capability improves, so suspended solids along with the sludge are deposited faster with better conditions.(1)