Introduction

Colombia is among the four main coffee producers worldwide [1]. Global consumption is close to 40 liters of water per kilogram of traditional dry parchment coffee processing, which involves pulping, washing, and transportation stages, known as wet processing. This results in wastewater with physicochemical characteristics that are harmful to the environment, as the water possesses low hydrogen potential (pH), complex chemical compositions due to with organic matter, and dissolved solids that correspond to pollutant levels 60 to 240 times higher than domestic wastewater. The electrical conductivity is higher than 1900 µS/cm, dissolved oxygen is less than 3.1 mg/L, and chemical oxygen demand (COD) is greater than 1500 mg/L [2, 3], with pH values between 4 and 4.05 [4]. When wastewater is discharged without prior treatment, surface and groundwater sources become contaminated by agro-toxic residues, as evidenced by research conducted in Manhuaçu, Minas Gerais, Brazil [5, 6], in Coipa, Bolivia [7], and in the Chopim River and its tributaries, Palmas municipality, Brazil, where the contamination of surface sediments was investigated [8].

Research is underway on treatments to remove pollutant loads and stabilize the pH of wastewater, which indicates the level of water alkalinity or acidity [9]. For Colombia, this corresponds to a minimum pH value of 5 according to current regulations for agro-industrial wastewater produced in coffee processing (for both traditional and ecological processes) [10]. Various technologies are being employed, including electrocoagulation [11], artificial wetlands with macrophytes [7], anaerobic digestion with manure for methane production [12, 13], and anaerobic treatment [14].

Many methods are used to increase pH, including phytoremediation with macrophytes [15] and oxygenation lagoons [16]. Quicklime is currently used for pH adjustments. It results from the calcination of limestone at high temperatures, ranging between 650-900 °C [17], and is mainly composed of calcium carbonate and other alkalinizing agents, such as sodium hydroxide, sodium carbonate, and sodium bicarbonate [18, 19]. Water with a high carbonate content has an alkaline pH [20]. Therefore, chicken eggshell, composed mostly of calcium carbonate, can be used to alkalize coffee processing wastewater. As eggs are a widely consumed household product, they could be produced in coffee plantations as a productive diversification option [24]. The calcium in the chicken's diet influences the weight and thickness of the eggshell, as well as storage time [25, 26, 27], producing more calcium carbonate per egg.

Coffee solid waste is being researched for potential applications, such as activated carbon production with phosphoric acid [28], alcohol from pulp [29], and mucilage [30]. Coffee wastewater treated with leachate from coffee pulp inoculated with efficient microorganisms is used as a bioinput for vegetable production [31]. This adds value to these residues and contributes to the mitigation of negative environmental impacts.

At the Research Laboratory of the Universidad Cooperativa de Colombia, Ibagué-Espinal campus, chicken eggshells were prepared by cleaning, crushing, and maceration steps. The shells were used to treat the acidic pH of coffee processing wastewater obtained from a coffee farm in the municipality of Líbano, Tolima, Colombia, with a pH of 4.05. The material was passed through a series of sieves with American Society for Testing and Materials (ASTM) classification numbers 30 to 140, which contained openings from 0.600 to 0.106 mm, respectively. Through this process, the most efficient size of the process was determined, as well as the necessary concentration to increase the pH to 5, which is the minimum value established by current regulations for this type of water before being discharged into the environment [9]. Therefore, chicken eggshells would be useful as a simple, accessible, and effective alternative to alkalize acidic residual water. As a result, this solid domestic waste can be utilized rather than discarded in landfills.

Materials and Methods

The research was conducted through experimental tests to evaluate the use of eggshell for increasing the pH of acidic wastewater from the wet processing of coffee. The preparation steps involved washing the eggshell to remove adhered material and removing the testaceous membrane inside the shell, which does not contain calcium carbonate [32] and is not useful for the purpose of this research. The eggshell was then crushed using a 600-watt electric blender at 10,000 rpm for approximately 20 seconds and further ground in a mortar. The sample was dried in an oven at 120 °C for a maximum of three hours to eliminate microorganisms and remove moisture.

Before the trial process, the eggshells without the testaceous membrane were left at room temperature (average 24 °C) for eight days to check for decomposition and unpleasant odor. This was performed to permit further storage and use in coffee farms to increase the pH of wastewater.

The crushed material was sieved through stainless steel wire mesh sieves in the ASTM series from No. 30 to No. 140, with openings ranging from 0.600 mm to 0.106 mm, respectively. The retained samples were separately stored in closed glass containers to prevent contamination, as depicted in Figure 1.

The eggshells were applied to wastewater through manual mixing. Specifically, 200-milliliter samples of raw water with varying concentrations of eggshell were weighed using a precision balance and the residual pH for each concentration and particle size obtained in the sieves were evaluated. The goal was to achieve a pH greater than 5 to establish trend lines for the results between the eggshell concentration and pH. The pH measurement of wastewater was conducted using a pH meter for wastewater with an electrode and integrated temperature sensor, with hundredth-degree precision.

Results

After eight days of storage at room temperature, the clean eggshell without the testaceous membrane showed no odor, physical changes, or decomposition. Therefore, the shell does not require special preservation processes for use, but contact with humid environments should be avoided. Laboratory tests were performed using this material, and the pH in the wastewater from wet coffee processing was evaluated using a volume of 200 ml for each sieve size in the ASTM series from No. 30 to No. 140. The concentration of applied eggshell was changed until a pH value greater than 6 was obtained, as indicated in Table 1.

An increase in wastewater pH was observed when crushed eggshell was applied, as shown in Figure 2. A pH range of 5 was established as the minimum requirement for coffee processing wastewater, as per Colombian regulations [10]. Concentrations ranged from 1 to 10 mg/L eggshell, depending on the sieve size selected.

Lower concentrations of eggshell were needed for sieves No. 100 (0.150 mm), No. 120 (0.125 mm), and No. 140 (0.106 mm), which attained similar pH results, as shown in Figure 2. It can be inferred that smaller particle sizes dissolve better in water. Sizes up to 0.150 mm (No. 100 sieve) are suitable for increasing the pH of wastewater from wet coffee processing. Figure 3 describes the trend line for pH values obtained with the No. 100 sieve, with a polynomial curve and an R² correlation coefficient of 1, indicating a perfect linear fit. Equation 1 is as follows:

where

C: Concentration of eggshell (mg/L)

In wastewater generated by coffee wet processing at a coffee farm in Líbano, Tolima, the concentration of eggshell needed to increase the pH from 4.05 to 5 (the minimum value established by Colombian regulations for disposal in water bodies) is 1.001 mg/L based on the equation obtained for the No. 100 sieve, as indicated in Figure 4. The same equation was applied to achieve a neutral pH of 7 in wastewater, and 6.23 mg/L eggshell was needed.

The pH obtained for eggshell concentrations varying every 0.5 mg/L, according to the equation obtained for the No. 100 sieve, is described in Table 2 and ranged from 4.05 to 6.56.

Discussion of Results

Wastewater from coffee processing requires a neutral pH value of 7 to eliminate pollution during treatment processes, especially secondary treatment, and to minimize odors [33]. This can be achieved using chicken eggshells, and a concentration of 6.23 mg/L is theoretically needed according to the trend equation obtained for SIEVE No. 100. Notably, in this research, the experimental results approached a pH close to 6.30. If a pH of 6.5 is considered, which corresponds to the minimum pH needed for drinking water according to current regulations in Colombia [34], 6.18 mg/L eggshell material from SIEVE No. 100 is needed based on the obtained equation. The standard deviation was low, indicating that most data points clustered around the mean.

The Colombian regulation that governs discharge limits defines a minimum pH of 5 for coffee processing wastewater [10]. This value is low and subject to revision, especially considering that the same regulation sets a pH of 6 for wastewater used in fruit and vegetable processing [10]. A pH of 5 may also affect the treatment process, particularly nitrogen removal [35]. This pH range (between 5.65 and 4.70) is slightly acidic [36], lower than the minimum value for wastewater from the coffee processing industry established in the Mexican standard, which is 6 [37].

The research involved neutralizing the acidic pH of wastewater from wet coffee processing using calcium carbonate. This provides the possibility of investigating other parameters, such as alkalinity and total hardness, which are not regulated in current standards [10]. This type of wastewater has total hardness concentrations below 40 mg/L [38], whereas up to 300 mg/L is permissible for drinking water [34].

Conclusions

The obtained results lead to the conclusion that chicken eggshells are a viable option to increase the pH of wastewater from wet coffee processing. This solution can be applied in coffee-growing regions of countries in which wastewater is discharged into water bodies without any treatment, leading to negative consequences by decreasing pH.

Chicken eggshells are a domestic solid waste with limited use and are often deposited in open dumps, landfills, and dumpsites, wasting its potential as a raw material. As the bioceramic structure of eggshells contains organic and inorganic components, the percent of primarily calcium carbonate exceeds 95% in the form of calcite; thus, eggshells can be useful for various anthropogenic activities.

Eggshell crushing sizes below 0.15 mm achieve better dissolution in wastewater and, consequently, greater increases in pH. In addition to wastewater from coffee processing, this utility extends to other types of agro-industrial, domestic, and industrial wastewater in general. Therefore, further research is necessary. Additionally, eggshells from other oviparous animals, such as ostriches and quails, can be considered for their potential to increase wastewater pH.

Referencias

Notes

* Research article.

Author notes

^a Correspondence author: oscar.ospina@campusucc.edu.co

Additional information

How to cite this article: Ospina Zúñiga, O. E., Aldana Giraldo, A., Rojas García, O. E., “Increasing the pH of coffee wet mill wastewater using eggshells.” Ing. Univ. vol. 27, 2023. https://doi.org/10.11144/Javeriana.iued27.ipcw

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