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Abeer Al-HJOO

Department Of Environmental Engineering, Damascus University, Damascus, Syria.

eng.abeer.alhjoo92@gmail.com

Abstract

Integrated Fixed Film Activated Sludge (IFAS) can be one of the most promising methods to increase the efficiency of carbon compounds catabolism in conventional activated sludge systems when the design period of the treatment plant expires, and the area of the plant is limited, or if the expansion of the plant requires large financial costs.

(IFAS) bases on combining the features of both suspended and attached growth of biomass, and it involves adding media to the aeration tanks in order to provide a larger area for bacterial growth, whereby suspended media of a suitable material or a fixed surface is used as carriers of active microorganisms on its surface.

This research includes a mechanism for improving the efficiency of aeration tanks in activated sludge wastewater treatment plants by adding Foam Concrete as a media, which can be manufactured locally as an alternative to imported media, and has not been used globally, and according to experimental results of the study, which included adding Foam Concrete to the aeration tank in an experimental treatment plant with two filling percentages (10% and 20%), the removal percentage of (BOD5) reached about 95%, while the removal of (COD) was about 94%, and (SS) removal about 96%.

 Introduction

Wastewater treatment plants are essential infrastructure facilities all over the world. The importance of studying these facilities increased with the large population expansion and the development of industries, which led to a multiplicity of the types of waste resulting from them in terms of composition, quantity and toxicity level. Initially, treatment plants were only based on mechanical treatment to purify water of undissolved pollutants before being discharged into the watercourses [1].
However, with the development of legislation and laws that have become more strict regarding the issue of protecting surface and groundwater resources, wastewater treatment techniques have developed and interest in the issue of secondary (biological) treatment has begun in order to purify water of organic pollutants. This stage later developed to include water content of nitrogen and phosphorous, and Then the tertiary treatment appeared, represented by the final filtration processes [1].
The design life of wastewater treatment plants is about (25-30) years, but at the end of this period the plants become unable to achieve the required yield from it, as a result of the increase in the organic load, and then, certain measures must be taken to maintain the yield of the plant, including [1]:
-    Engineering measures: which mean the expansion of the treatment plant units (building new treatment units), but this procedure requires the availability of sufficient space and making modifications to the design of the plant, which may lead to a high cost.
-    Technical measures: where the objective of these measures is to increase the effective biomass and thus maintain the yield without the need to build new units for the plant.
Based on this, the concept of treatment with the (Integrated Fixed Film Activated Sludge - IFAS) method appeared, which is a relatively new concept that appeared in the early seventies of the last century, and it is based on adding media made of certain materials to the aeration basin, where these media form biological carriers on which bacteria grow and destroy compounds in the aeration tanks in wastewater plants [1].
IFAS has witnessed a great development since its inception, it has been applied in many sewage treatment plants around the world in order to develop and raise their efficiency, and the materials used as media in this method have varied.

 Biological treatment with activated sludge method

The beginnings of the traditional activated sludge (AS) method go back to the early nineteenth century, when some researchers suggested that wastewater aeration should take place in tanks and thus oxidize organic matter. Later, wastewater aeration was studied, and it was noted that sewage aeration played an important role in improving water quality. [1,2].
AS method depends on activation of bacteria in wastewater by pumping the oxygen needed for their growth and mixing the contents of the tank to increase the seam between organic substances and bacteria, through this process of reproduction and growth, the degraded organic matter needed by the bacteria is demolished and the water is cleared. And the ventilation process secures the oxygen needed for bacteria to multiply, and the tank content is moved so that deposition processes do not occur [1,2].
Bacteria proliferation clears water from degraded organic material and converts it into non-decadent sedimentary material in the sludge tank [1].
The need to preserve an adequate amount of bacteria in the aeration tank requires returning a portion of the non-dissolved substances deposited in the final deposition basin to the ventilation basin of. This part is called returned sludge. The remaining part of the sludge, which may contain a large proportion of organic substances, is subject to subsequent treatment (sludge treatment) [1].
There are many factors that affect treatment with activated sludge method, the most important of which are [2]

 Integrated Fixed Film Activated Sludge (IFAS):

The traditional activated sludge method has been developed with some modifications, and this is now being applied in newly established treatment plants or that reaching the end of their design life and the processing yield is decreasing. One of these modifications is Integrated Fixed Film Activated Sludge (IFAS), which bases on combining the features of both suspended and attached growth of biomass, and it involves adding media to the aeration tanks in order to provide a larger area for bacterial growth [3].
The application of IFAS system ensures a high volume density of biomass thanks to its growth on the added media, so the average residence time of the sludge will increase, this will be reflected in the amount of sludge discharged towards the secondary deposition tank which will in turn decrease, thereby decreasing the loading on the secondary deposition basins. Conversely, carbon vehicle demolition and nitrogen removal efficiency increases, and sludge retention time increases, enhancing the nitrous process compared to simple suspended growth systems [4].
Many media are used in IFAS system which can be (plastics, sponges…), so designers are paying great attention to studying the properties of these materials so as to determine which of these materials meets the desired purpose and the required efficiency [3]].

 Factors affecting IFAS:

There are a range of factors that need to be taken into account when adopting the IFAS method as an option to raise the efficiency of the wastewater treatment plant among the most important [3,4]:
Fine Screening: the wastewater be screened to capture materials such as hair and plastics, which may bind the media, plug the retention sieves, or become entrapped in the aeration basin as floating material.
Media Containment: The free-floating media are contained in the aeration basins by sieves, The effect of the sieves must be evaluated because it contributes to head loss in the system.
Thickness of Biomass: Estimation and control of the biomass thickness on the carrier elements is critical to proper operation of the IFAS process. Accurate modeling of the process for design is dependent upon reasonable estimates of the biomass thickness on the media. Since the thickness can be controlled to some extent by the patterns and intensity of mixing imparted by the diffused aeration system, it should be considered during design of the aeration system.
Aeration System Design: some additional concerns arise when considering the installation of aeration for an IFAS system. it must subject the plastic elements to sufficiently vigorous contact and abrasion to control the depth of biological growth.

 Types of media used in IFAS:

Fixed media:
Include two types (Ringlace, Bioweb). Ringlace media are flexible ropes, where sludge stick on these ropes, and in such a way as to ensure that they are well-ventilated [5]. This technology was developed in Japan for the first time, where spongy cubes of polyurethane (8cm3) were used as chain-hung media on ropes placed in the aeration basin, and the results showed that the nitrification rate reached (0.33mg/hr/cube) at a temperature (15C°) when using these media at (10-20%) of basin volume [6].
While Bioweb media are high surface area net-like fabric, which is designed to be installed directly in aeration basins in order to enhance the stability and growth of biomass [7], By securing a protected surface that allows bacteria to persist, reproduce and increase their concentration And these nets consist of small rings from which hair threads emerge, It secures the space needed for biomass growth, and because these media are so coherent because of their knitting mechanism, they are protected from degradation and do not go out of place when placed in ventilation basins and operated [7].
Moving media:
There are 3 main types of moving media which are:
Linpor: Suspended media of high porous plastic foam cubes that act as moving bearing materials, are used with high quality surfaces so as to ensure the growth and accumulation of activated sludge bacteria, these cubes occupy between 10% to 30% of the size of the aeration tank, and allow the overall biomass concentrations to be maintained significantly within the basin [8].
Captor: which consists of semi-cubic forms manufactured from polyurethane foam with approximately 97% porosity, and experimental studies have shown that the use of these media contributed to an increased rate of BOD removal, and more stable operation [9].
(MBBR) Moving-Bed Biofilm Reactor: In this method, the whole size of the pelvis is utilized for the growth of biofilm as in the activated sludge method, but unlike the activated sludge method, MBBR does not need a sludge-returning line [10]. Since there is no sludge-returning line, excess biomass is eliminated and this is considered as one of the most important features compared to the activated sludge method. Studies have shown that the ideal concentration of biomass in MBBR basin relative to the size of the basin is about (3-4 Kg MLSS/m3) and is equal to the same value in the activated sludge method [10].

 Pilot study:
The pilot study was conducted at the wastewater treatment plant north of Homs Governorate in Syria, spanning an area of 240,000 m2. The average population served by the plant is approximately 700,000.
Table 1 shows the plant's operational parameters in 2021 according to the data obtained from the plant:
Table 1: Operational Parameters

 Materials

A pilot plant was designed and put in Homs Wastewater Treatment, in order to conduct the pilot study, and this pilot plant consisted of:
-    Mechanically treated wastewater collection tank with a capacity of 50 liters.
-    Aeration tank, which is a cylindrical shape with a diameter of 20 cm and height of 33 cm, the effective size of the basin is 10 liters, the inflow is 2 L/hr, and hydraulic residence time is 5 hours.
-    Secondary deposition tank, which is also 30 cm high cylindrical shape, 20 cm in diameter, contains a 15 cm sludge assembly cone.
-    Water treatment tank, a basin where treated water is assembled.
Foam Concrete was used as a media, and the samples were prepared in a cylindrical form with a diameter of 2.5 cm and a height of 2.5 cm. These samples were prepared of a mixture consists of sulfate-resistant Portland cement, sand, water and foam-generating material used to create holes within the samples.
The volume weight of samples was 1.7 gr/cm3. The qualitative surface value was 600 m2/m3.
Water and biomass specifications are listed in table 2:

Table 2: Water and biomass specifications

 Results

Three series of experiments were conducted
1.    First series (without foam concrete).
2.    Second series (filling rate of foam concrete 10%).
3.    Third series (filling rate of foam concrete 20%).
The results of the series were as shown in tables (3-4-5):

Table 3: First series results (without foam concrete)

 Table 4: Second series (filling rate of foam concrete 10%).

 Discussion

-    At the end of the third series of experiments, the largest value of organic loading is 3.04g (BOD)/l/d, at a 20% filling ratio.
-    The experimental plant's yield without biological carriers (the first series of experiments) was greater than 80% when the organic load ranged within (0.91 - 1.1) g. BOD/l/d, in order to maintain a greater yield than 80%, biological holders of foam concrete have been used, and through the two series of experiments conducted with filling rates (10% and 20%) it has been noted that:
In the second series of experiments (10% filling ratio), the range of increased organic load coming to the plant was between (1.1-2.35) g. BOD/l/d, hence the 10% filling ratio is needed to increase organic loading by 113.7%, compared to the case of the first series.
In the third series of experiments (20% filling ratio), the range of increased organic load coming into the plant was between (2.35-3.04) g. BOD/l/d, so 20% is the ratio needed to increase organic loading by 176.4%, compared to the case of the first series.
-    When the filling ratio is fixed, and changing the organic load coming to the plant, an increase in the plant's yield was observed, as the removal ratio of (BOD5 - COD - SS) increased, accompanied by an increase in biomass concentration (suspended and attached) in the ventilation basin and a decrease in the value of F/M, but as organic loading continued to increase, the plant's yield gradually decreased, and the value of F/M increased, as the increase in biomass concentration became less than the increase in organic load coming into the plant.
-    IFAS provides effective and proven wastewater treatment technology, and a suitable system for modifying and developing existing wastewater treatment plants whose design life has expired in order to increase their efficiency and yield.

 Recommendations

-    Conducting other studies using foam concrete in other forms (cubic, parallel rectangular, pronounced) and other dimensions so as to increase the specific surface and compare it with the results of this study.
-    Studying the impact of the use of foam concrete as biological carriers in treatment plants on the removal of both nitrogen and phosphorus and using different filling ratios.
-    Studying the impact of foam concrete on the yield of treatment plants with other filling ratios (25%, 30%, 60%...), and compare the results with the results reached in other studies using plastic media.
-    Design a computer model to predict the respective removal ratios (BOD5 - COD - SS) using foam concrete, and compare the results achieved through this model with the results obtained experimentally in this study.

 

References:

[1]: Faraj, MHD. Ali (2004). (Wastewater and Liquid Waste Treatment). Cairo. Egypt.
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[7]: Hubbell, S B. Pehrson, R. Schuler, A. (2006). Eight Years of Successful Cold Weather Nitrification with Integrated Fixed-Film/Activated Sludge. USA. Available on: https://www.researchgate.net/publication/233504755_Eight_Years_of_Successful_Cold_Weather_Nitrification_with_Integrated_Fixed-FilmActivated_Sludge
[8]: Gilligan، T.P.، and Morper، M. 1999. A unique process for upgrading conventional activated sludge systems for nitrogen removal. Paper presented at NE WEA، October 1999.
[9]: Golla, P.S., Reddy, M.P., Simms, M.K., and Laken, T.J. 1994. Three years of full-scale captor process operation at Moundsville WWTP. Water Sci. Technol. 29: 175–181.
[10]: Ødegaard, H. (1999). The Moving Bed Biofilm Reactor. Norway.