COD’s Level Reduction of Tofu Industrial Wastewater by Using Activated Coal Fly Ash as Adsorbent

Tofu wastewater contained a high concentration of COD and will be dangerous if directly discharged into the environment without any pre-processing. In this research, tofu wastewater is processed using an adsorption process to determine the amount of COD reduction in tofu wastewater using coal fly ash. To maximize the adsorption process by enlarging the fly ash’s pores and removing the impurities, fly ash is activated first by using 2 M of H2SO4 100 mL. Fly ash was tested with XRD (X-Ray Diffraction) to know its crystal structure and tested with XRF (X-Ray Fluorescence) to determine the difference in the elemental composition of fly ash before and after the activation process. The research on the effect of time on the adsorption process was carried out with time variations of 15, 30, 45, 60, 75, 90, 105, and 120 minutes. The results showed that the intensity of silica in the fly ash after the activation process was more significant than before the activation process. The optimum time for fly ash to adsorb tofu wastewater was 105 minutes with eficiency of 79,71%.


Introduction
Tofu is one of the most popular foods in Indonesia.This statement is proven by data from the Central Agency on Statistic (Badan Pusat Statistic), that the consumption of tofu in Indonesia in 2021 reached 0.158 kg per capita.This figure is 3.27% higher than the consumption of tofu in 2020.In the process of making tofu, around 18-30 liters of water is needed in every 1 kg of soybean raw material for the soaking, washing, and pressing processes so that almost 95% of the water will be wastewater [1].Tofu wastewater which is quite large will cause damage to the aquatic ecosystem if it's discharged into rivers without treatment because it contains high levels of COD (Chemical Oxygen Demand) [2], which ranges from 1300-9000 mg/L as in research conducted by Rahayu [3], Prasetyo [4], and Kartika [5].This high COD level causes the dissolved oxygen content in water to become low or even depleted [6].Therefore, it is deemed necessary to carry out treatment before the wastewater is discharged directly into the river by reducing the COD level 2 to less than 300 mg/L by applicable quality standards.One way that can be done is by the adsorption process.The adsorption process can be carried out by utilizing several materials, one of which is coal fly ash in research conducted by Rani Devi [7], Revi Afrianta [8], and Abdul Karim [9].Fly ash from coal burning is known to be effective as an adsorption medium to reduce water pollutants [10].But generally, fly ash is activated before it's finally used.Activation of fly ash in this research used H2SO4 because it aims to induce a dealumination process, namely the process of releasing Al (Aluminum) from fly ash to increase Si (Silica) ion levels and remove other impurities (containing alkaline/alkaline earth elements) from the structural lattice that can cover the active side of the adsorbent to increase its adsorption ability [11].This research examined the ability of fly ash to reduce the COD content of tofu wastewater by varying the adsorption contact time.Adsorption contact time is one of the important factors that can affect the adsorption process because the longer the contact time, the molecular adsorption force of a solute will increase [12].

Tofu Wastewater
The tofu wastewater used in this research was taken from the tofu industry in Gunung Sulah Village, Way Halim District, Bandar Lampung.The collection of wastewater refers to SNI 6989.59:2008 by taking the sampling method using a long-stemmed dipper and then storing itin a closed container made of 5 L plastic.

Coal Fly Ash
Fly ash was taken from PT. Bukit Asam Tbk Tarahan and refers to SNI 6863.3:2002.The fly ash was then sieved using a 230 mesh sieve and dried using an oven to remove the water content.After that, the fly ash was dissolved with a solution of H2SO4 and stirred for 5 hours at a temperature of 90 o C and a rotational speed of 2 mod.After 5 hours, it was filtered and rinsed with distilled water until a neutral pH was obtained.Finally, the active fly ash is dried using an oven than a characteristic test is carried out.

Characteristic Test and COD Measurement
The characteristic test conducted in this research consisted of 2 types, namely XRD (X-Ray Diffraction) and XRF (X-Ray Fluorescence).Characterization of fly ash with XRD and XRF was carried out before and after activation of the fly ash.The COD measurement process was carried out using the spectrophotometric method by adding reagents for the Hach system.

Adsorption of Tofu Wastewater with Activated Fly Ash
The adsorption process of tofu wastewater begins by incorporating 5 gr of activated fly ash into 500 mL of tofu wastewater and then stirring using a jar test at a speed of 200 rpm and a stirring time of 15 minutes.10 mL of the solution was stirred and then filtered through filter paper.The same procedure was carried out with various contact times: 30, 45, 60, 75, 90, 105, and 120 minutes.Coal fly ash has a chemical composition that is not always the same, even though it's producedfrom the same source [13].The XRF test in this research was carried out before and after the activation process, so it displays differences in mass percent as shown in the following table (Table 1).

Characterization of Fly Ash Adsorbents Using
The XRF results in Table 1 showed that the activated fly ash has a higher silica content than other metal oxides.In this research, activation of fly ash using 2M of H2SO4 proved to be quite good in increasing the silica content by 17%.This happens because activation with H2SO4 in fly ash replaces the impurities in the fly ash with H + ions which can cause fly ash to have an active group that easily releases protons, namely the Bronsted acid group [14].From these results, it's expected that the activation process goes well because silica can strengthen the adsorption power of the adsorbent [15].

Characterization of Fly Ash Adsorbents Using XRF
As with XRF, XRD testing is carried out before and after the activation process on coal fly ash.The difference in peak intensity of the fly ash composition can be seen in the Based on the graph obtained from the coal fly ash XRD test, the presence of quartz (SiO2) in the fly ash before and after activation forms several main peaks which are shown in table 2 below.The results of the characterization of coal fly ash with XRD in Table 2 show that the highest SiO2 diffraction peak is at an angle of 2θ : 26.6 o .The diffraction peaks of the XRD results showsimilarities to the peaks of the database on JCPDS No. 46-1045, the phase formed from silica is the mineral quartz (SiO2).The quartz phase that is owned by SiO2 is due to the stability range that is owned by quartz (quartz) is a temperature of less than 870 o C, while for tridymite with temperatures of 870 -1470 o C, and cristobalite with temperatures of 1470 -1723 o C [16].
If the XRD fly ash results before and after activation are compared, it will be seen that there is a difference in SiO2 intensity.The SiO2 phase after activation has a higher intensity at an angle of 2θ : 26.6 o , which is 2827.98 cps compared to the SiO2 phase before activation which only has an intensity of 1839.99 cps.From this, it can be seen that the presence of acid activation and high temperatures can increase the relative intensity (I), which means an increase in product crystallinity.The presence of chemical activation in fly ash can cause an increase in the product or crystals produced [17]

COD Adsorption of Tofu Wastewater Based on Time Variation
The samples analyzed came from leftover tofu production from Gunung Sulah Village, Way Halim, Bandar Lampung.The tofu production process in Gunung Sulah Village still uses simple technology and produces tofu wastewater that has a cloudy color, as shown in Figure 1.
Tofu Wastewater Adsorption Process.The adsorption process can occur optimally because of the addition of an activator to the adsorbent before use.Activation of the adsorbent causes an exchange of cations in the adsorbent with H + from H2SO4 as an activator so that H + which has replaced the cations from the adsorbent can bind with organic substances from tofu wastewater.The estimated reaction mechanism for the adsorption of coal fly ash in tofu wastewater is depicted in Figure 2 [18].At the initial measurement (0 minutes) the COD level reached 1,324 mg/L.The COD levels examined in this research were much lower than the studies conducted by Fibria Kaswinarni [19] or Febrian Sayow [20], with the COD levels of tofu waste reaching more than 6000 mg/L.This proves that the COD level of tofu wastewater will be different in every place.Based on the research that has been done, the results obtained regarding the effect of activated fly ash adsorbent and time variations in reducing COD levels in tofu industrial wastewater.The resultsof decreasing COD levels can be seen in Figure 3 below.

Figure 3. Graph Results of Reducing COD Levels of Tofu Wastewater Based on TimeVariations
Figure 3 showed that the COD adsorption process of tofu wastewater was not maximized at 15 minutes, but absorption continued to increase until 105 minutes.At 105 minutes, the COD level of tofu wastewater reached an average of 269 mg/L and after that, the fly ash no longer absorbs a lot of organic matter.This condition is because the fly ash has reached a saturation point where almost the entire surface of the adsorbent is covered by the existing adsorbate particles [8].The adsorption kinetics of the COD content of tofu wastewater by coal fly ash adsorbent can be determined by plotting.A first-order linear curve was made by plotting lnCe (final concentration) with variations in contact time (minutes), and a second-order curve was made by plotting between 1/Ca and variations in contact time (minutes).The adsorption kinetics curves for both models are shown in Figure 4 and Figure 5.

6
Based on Figure 4 and Figure 5, it can be seen that the R 2 value for the pseudo-first order is higher by 0.8256 compared to the pseudo-second order with R 2 = 0.0174.The coefficient valueof the first-order relationship (R 2 ) is closer to number 1 than the second-order.This shows thatpseudo-order modeling makes the adsorption data more representative.The pseudo-first orderequation assumes that adsorption involves a physical process (physisorption) caused by weak van der Waals bonds between the surface of the adsorbent and the adsorbed molecule.The physisorption mechanism forms bonds between metal ions in solution.The two bonds are weakbecause they are only held together by van der Waals forces.This allows the adsorbate to movefreely until finally, a layered adsorption process occurs [23].Table 3 summarizes the calculation of the resulting adsorption kinetics model parameters, namely R 2 (coefficient of determination) and K (reaction rate constant).The K value is one of the adsorption kinetics parameters to determine whether the adsorption process is fast or slow.The lower the K value, the faster the adsorption process takes place [23].

Conclusions
In this research, silica in coal fly ash has an amorphous phase structure with SiO2 diffraction peaks at an angle of 2θ : 26.6 o with higher silica intensity after going through the activation process, and the optimum time to reduce COD levels in tofu wastewater using 5 grams of fly ash was activated in the 105 minutes with a reduction efficiency of 79.71% and COD levels to269 mg/L.

3. 4 .
COD Adsorption KineticsAdsorption kinetics plays an important role in determining the adsorption pollutant content inthe design of industrial adsorption processes[21].Adsorption kinetics is the rate of adsorbent at a certain time.The adsorption kinetics of a substance can be determined by measuring changes in the concentration of the adsorbed substance and then analyzing and graphing the value of k (slope)[22].The kinetic model that will be reviewed for the adsorption reaction in this research is the Pseudo-First Order and Pseudo-Second Order models.The two adsorptionkinetics models were calculated to determine the adsorption ability of the adsorbent on the adsorbate.The model that fits the research results is the kinetic model with the highest R 2 value[23].

Figure 4 .
Figure 4. Pseudo-First Order COD Adsorption Kinetic Curve of Tofu Wastewater Using CoalFly Ash

Figure 5 .
Figure 5. Pseudo-Second Order COD Adsorption Kinetic Curve of Tofu Wastewater UsingCoal Fly Ash

Table 1 .
XRF Chemical Composition of Coal Fly Ash PT Bukit Asam Tbk Tarahan

Table 2 .
Coal Fly Ash SiO2 Peaks Data

Table 3 .
COD Adsorption Kinetic Parameters