Fixed-bed column sorption kinetic rates on the removal of both biochemical oxygen demand (BOD₅) and chemical oxygen demand (COD) in domestic greywater by using palm kernel activated carbon

Increasing concerns regarding the impact of untreated greywater on the environment have prompted the need for stricter environmental regulations governing the discharge of both industrial and domestic wastewater. Greywater, which can differ in both volume and quality, is influenced by various factors. It has been noted that the composition of greywater is influenced by elements such as cooking and cleaning methods, personal hygiene habits, biodegradability, frequency of water usage prior to disposal, and sanitary practices (Oteng-Peprah et al. 2018). Over the past few decades, growing recognition of the ecological consequences associated with organic loads has resulted in heightened requests for the treatment of greywater before it is released into natural bodies of water (Kurniawan et al. 2021). Of particular interest is the concentration of BOD₅ and COD in greywater discharged into the environment. High levels of BOD₅ and COD in water can lead to oxygen depletion, altered ecosystem balance, nutrient imbalance and impaired water quality (Ademiluyi et al. 2009; Alewi et al. 2021). These can lead to pollution and may have detrimental effect on aquatic life and water quality. The Ghana Environmental Protection Agency (Ghana. 2000) has set the limit for the concentration of the discharge of BOD₅ and COD at 50 mg L⁻¹ and 150 mg L⁻¹ respectively. However, wastewater discharged into the environment are mostly above this regulatory limit. To meet the discharge guidelines, greywater from households and industrial settings will require an appreciable level of treatment. Chemical, biological, and physical methods are the conventional approaches employed to treat wastewater containing organic substances. Among the various physico-chemical processes utilized in wastewater treatment, adsorption techniques have emerged as the most extensively employed and effective means (Patel et al. 2021). One of the most efficient and simple methods for removal of BOD₅ and COD from greywater is adsorption which is much preferred due to its ease of use and wide-scale application. This technique is quite popular due to its simplicity as well as the availability of a wide range of adsorbents. Activated carbon remains one of the most widely used range of adsorbents. It remains one of the most effective and widely used adsorbents because of its excellent adsorption ability (Mohammad-Khah & Ansari 2009). It has been prepared from many substrates over the years, however, commercial grade activated carbon comes with a higher price tag as compared to those prepared with low-cost materials (Bulut et al. 2007; Errais et al. 2011; Sartape et al. 2013; Nayl et al. 2017). Batch adsorption experimental designs are the most commonly utilized tests by many researchers in assessing the suitability of potential adsorbents because of its simplicity and low cost (Crini et al. 2008). Data obtained from batch studies are usually used to construct isotherms which are considered a critical step in the design and optimization of the adsorption process (Hamdaoui & Naffrechoux 2007; Behnamfard & Salarirad 2009). Many studies have reported removal of BOD₅ and COD in batch systems using various adsorbents (Bansode et al. 2004; Devi et al. 2008; Nayl et al. 2017) but little has been known on the removal of BOD₅ and COD in a fixed-bed column application. Of particular interest is the batch study by Oteng-Peprah et al. (2019) which reported BOD₅ and COD removal from domestic greywater using activated carbon prepared from palm kernel shell. It has been reported that the data obtained under batch conditions are generally not applicable in most systems or do not mimic what happens in the real world (Patel & Vashi 2012). Therefore, in order to validate the data obtained in this batch study there is the need to perform column studies to provide data for direct application to large scale use. There exist many studies that have assessed batch and column studies of many contaminants in wastewater. However, little has been known on column study on greywater targeting the continuous removal of BOD and COD. The aim of this study is therefore to investigate the adsorption capacity of the Palm Kernel Activated Carbon (PKAC) in a fixed-bed column with regards to BOD₅ and COD removal. The PKAC investigated in this study has previously been shown to be effective in removing BOD₅ and COD from greywater in a previous study under batch conditions (Oteng-Peprah et al. 2019). The breakthrough curves are analyzed using Adams–Bohart, Thomas, Yoon–Nelson and the BDST models to determine the characteristic parameters of the column useful for process design.

To design a column adsorption process, it is important to predict the breakthrough curve or concentration-like profile and adsorption capacity of the adsorbent for the selected adsorbate under a given set of operating conditions. It is also necessary to determine the maximum sorption column capacity which is a significant parameter for any sorption system. For analysis of the experimental breakthrough curves under different conditions, these common models were used, thus, the Thomas, Yoon–Nelson, Adams–Bohart models and the bed-depth service time (BDST) model.

A plot of ln(C_o/C_t – 1) against t gives a straight line from which the values of k_Th and q_o are determined from the intercept and the slope respectively.

A plot of ln(C_o/C_t – 1) against t gives a straight line from which the model constants k_AB and N_o can be determined from the slope and intercept.

A linear plot of ln(C_t/C₀-C_t) against time t gives a straight line where K_YN and τ are determined from the intercept and slope.

Greywater for this experiment was obtained from domestic sources within University of Cape Coast community, Ghana.

The samples were stored, refrigerated and transported to the laboratory in an ice chest after the field parameters were analysed onsite. The greywater was filtered to remove large and floating objects and their initial concentrations determined. The pH was determined using a Horiba U-50 multiparameter water quality meter; the BOD₅ was measured using the Lovibond BD 606 system, and the concentration of chemical oxygen demand (COD) was determined using the closed reflux colorimetric method as stated in (APHA) 5220D. To ensure quality assurance, blank sample was prepared using 50 mL of distilled water and treating it with the same reagents for both BOD₅ and COD analysis.

The palm kernel shell was obtained from a local palm oil mill in Cape Coast Ghana. The material is cleaned several times with distilled water to eliminate any dirt and water-soluble impurities. The cleaned shells were then airdried in a laboratory drier for 8 hours and then further oven dried at 110 °C in a Heratherm gravity convection laboratory oven for 24 hours in order to remove any surface moisture. The dried samples were then size reduced to desired sizes (2–6 mm) using a laboratory crusher and sieves. The activated carbons were prepared by physical activation with steam at 800 °C and steam flowrate at 120 mL hr⁻¹. After the activation process, the samples were taken out and washed with distilled water to remove any residual ash that might be on the carbons.

The density of the materials was determined by using an ultra-pycnometer 1000 after it had been dried at 105 °C in an oven for 24 hours. The surface area Brunauer–Emmet–Teller S_BET was determined using a Micromeritics tristar 3000 system.

The fixed-bed column experiments were carried out to assess the performance of palm kernel activated carbon in removing BOD₅ and COD from greywater. This was conducted in a 25 mm inner diameter and 37 cm length Perspex glass column. Palm kernel activated carbon of average particle size of 2 mm was loaded into the Perspex glass column. Glass beads were placed at the bottom of the column to provide support for the adsorbents and placed at the top to prevent the adsorbents from being pulled with the overflow. All experiments were conducted at room temperature with a pH of 7.3 and the direction of flow was from top to bottom using a peristaltic pump. The effluents were collected at regular intervals and the BOD₅ and COD concentrations determined. Flowrate and bed-depth were varied in these experiments. Flowrates of 5, 10 and 15 mL min⁻¹ were used to analyze the effect of flowrate variations while bed-depth of 10, 15 and 20 cm were used to analyze the effect of different bed-depth on the column performance. Before the column experiment, deionized water was circulated through the column in order to wet the entire adsorbate and remove any dirt residue on the surface of the carbon.

The apparent density of palm kernel activated carbon (PKAC) was determined to be 0.46 g cm⁻³. The recommended range of densities for activated carbon is within 0.4–0.5 g cm⁻³ (TIGG 2019) which implies that PKAC falls within the recommended densities for commercial activated carbon. The BET surface area obtained for PKAC was 620 m² g⁻¹. The recommended range of surface area of commercial activated carbon has been estimated to be 500–1,500 m²/g. this shows that the surface area for PKAC falls within the recommended surface area for commercial grade activated carbon. A similar study by Hidayu & Muda (2016) on characterization of activated carbon using palm kernel shell obtained surface area of 584 m² g⁻¹. The slight variations in the surface area recorded for this study may be due to the activation method that was used. A higher BET generally suggests higher adsorption capacity because the carbon has wider surface to allow for adsorption under different conditions.

The model was applied to the experimental data under conditions of constant influent concentration, different bed-depth and different flowrates. The linearized form of the model was used to determine the Thomas rate constant (k_TH) and the bed capacity (q_o) of the Thomas equation for both BOD₅ and COD. The model parameters which were determined have been presented in Table 1. From the table, it can be seen that k_Th for BOD₅ and COD decreased with increasing bed-depth at constant flowrate, but increased with increasing flowrate at constant bed-depth. On the other hand, q_o for BOD₅ and COD increased with increase in bed-depth at constant flowrate, whereas an increase in flowrate at constant bed-depth saw a corresponding decrease in q_o. Other studies have reported similar findings in fixed-bed column (Chowdhury et al. 2013; Lin et al. 2013; Nguyen et al. 2015). A maximum bed capacity of 34.4 mg g⁻¹ and 56.93 mg g⁻¹ were obtained for BOD₅ and COD respectively at flowrate of 5 mL min⁻¹ and a bed-depth of 20 cm. This maximum bed capacity was obtained at the lowest flowrate and a highest bed-depth. The maximum bed capacity occurring at this point can be due to the fact that at lower flowrates and maximum height, the adsorption capacity is high due to sufficient residence time which allows for effective utilization of the vacant sites before equilibrium is reached as suggested by Tan et al. (2008). A good fit (R² > 0.98) was obtained for both BOD₅ and COD as shown in Table 1. Comparing the k_TH between BOD₅ and COD, it can be seen that BOD₅ has a higher k_TH which is an indication of good column adsorption for BOD₅ as compared with COD. Comparing the q_o, it can also be seen that the q_o for COD is higher than BOD₅ indicating a higher uptake capacity of the Palm kernel activated carbon to reduce higher proportions of COD than BOD₅.

The experimental data were fitted to the Yoon–Nelson model. The linear form of the Yoon–Nelson model was used to determine the mass transfer coefficient (K_YN) and the time for 50% breakthrough (τ) and is presented in Table 2. Generally, as the flowrate increases at constant bed-depth, it can be seen that there is a corresponding increase in K_YN for BOD₅ and COD. Greater mass transfer coefficient (K_YN) implies narrower mass transfer zone, greater transfer coefficient between the phases and therefore lower mass transfer resistance. This implies adsorption of BOD₅ and COD is easier with higher values of K_YN. There is also a corresponding decrease in the time taken to reach 50% breakthrough as flowrate is increased. Brion-Roby et al. (2018) have reported similar changes in the Yoon–Nelson model parameters in a similar study. The changes observed can be attributed to reduced residence time associated with high flowrates. An increase in bed-depth leads to a corresponding reduction in the mass transfer coefficient but an increase in the time taken for 50% breakthrough to be achieved. This could be due to more vacant sites for adsorption to take place and hence a longer time to reach 50% of the breakthrough period. A comparatively good fit (R² > 0.89) was obtained for both BOD₅ and COD as shown in Table 2 which suggests that the Yoon–Nelson model fitted the experimental model for BOD₅ and COD in the sorption experiment.

The linear form of the Adams–Bohart model was applied to the experimental data by plotting C_i/C_o against t for both BOD₅ and COD. This model is based on surface reaction theory and assumes that equilibrium is not instantaneous therefore the adsorption rate is proportional to both the residual capacity of the adsorbent and the adsorbate concentration (Lopez-Cervantes et al. 2018). The values of maximum adsorption capacity (N_o) and the kinetic constant (K_AB) of the model have been calculated and presented in Table 3. Comparatively, the values for BOD₅ are higher than COD for all the studied conditions. The values of K_AB and N_o increase with an increase in flowrate for both BOD₅ and COD. Additionally, as the bed-depth is increased, the value of K_AB decreases while N_o increases. Similar findings have been obtained in a fixed bed column study (Brion-Roby et al. 2018; Mondal et al. 2018; Xavier et al. 2018). The values of the R² > 0.78 recorded are lower as compared to the other models studied and thus suggests that the Adams–Bohart model would not be the best model to be used to predict the experimental data in the range of conditions used. This may be due to the fact that the Adams–Bohart model is applied in regions of low concentration and large discrepancies can be found between the experimental and predicted curve if applied outside the concentration range (Karimi et al. 2012).

The removal of BOD₅ and COD from domestic greywater by sorption using PKAC was investigated in a fixed bed column under conditions of fixed inlet concentration, different inlet flowrate and different bed-depths. Based on the experimental results, PKAC has shown to be a very efficient adsorbent in the removal of BOD₅ and COD under the studied conditions. The sorption of BOD₅ and COD was strongly dependent on flowrate and bed-depth. An increase in bed-depth resulted in increased uptake capacity while an increase in flowrate resulted in a reduced uptake capacity for both BOD₅ and COD. A maximum uptake capacity of 35 mg g⁻¹ and 54 mg g⁻¹ was achieved for BOD₅ and COD respectively based on the Thomas model. The experimental data fitted well the Thomas, Yoon–Nelson and BDST models. The fit with Adams–Bohart model was comparatively low as compared to the other models used in this study. The Yoon–Nelson provided a better description of the experimental kinetic data in comparison to the Thomas model. This study has established the potential of using a free, locally and abundantly available waste material for the preparation of activated carbon in reducing BOD₅ and COD in domestic greywater.

The authors acknowledge funding from the Netherlands government under the NUFFIC project NICHE 194-01.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.