Establishing the optimal condition for nutrient recovery from domestic wastewater using the freeze concentration method

Pollution of water sources with excessive nutrient loads, most commonly phosphate and nitrogen, is a major environmental problem faced by many African countries. The water pollution problem is often associated with the rampant discharge of untreated or partially treated domestic (urban) wastewater into the environment. Although domestic wastewater contains nutrients (i.e., nitrogen, phosphorus, and potassium) that can be used in agriculture, its recovery and reuse are still a challenge. Some of the potential methods for recovering nutrients from wastewater are not only costly but also introduce a second pollutant into the waste stream. Potential methods such as struvite precipitation, ion exchange, electrochemical, algae, and freeze concentration have all been researched, but each has its own setbacks and drawbacks (Mavhungu et al. 2021).

Struvite precipitation is a method of recovering nutrients such as phosphorus used in crop production as a fertilizer. However, this technique uses chemicals that add a second pollutant to the environment (Sena et al. 2021).

Algae have been used to produce biofertilizers since they grow very well in wastewater while absorbing nitrogen and phosphorus nutrients which are important for crop production (Huo et al. 2020). However, there are numerous challenges and technical flaws, including the appearance of a second pollutant in wastewater, the high price of algal compounds, algal pollution, low availability of algae, potential cyanobacteria threats to the environment, and high-water consumption as highlighted by Zou et al. (2020).

The ion exchange method is one of the methods that can remove and recover phosphorus to form fertilizer. However, the presence of competing anions such as sulfates in wastewater provides a major bottleneck in limiting the selection of phosphorus (Ownby et al. 2021).

However, freeze concentration is reported as a promising technique for recovering nutrients from wastewater. It is also a clean technology with no secondary pollutant production, no chemical addition, and low equipment erosion. The freezing concentration technique is a physical method where a solution is concentrated by freezing out water content in the form of ice crystals (Samsuri et al. 2015). The freezing method because of its low latent heat of fusion consumes less energy compared to the evaporation method (Moharramzadeh et al. 2021). A saturated liquid phase and a solid crystalline phase are generated as a result of the freezing concentration technique (Lu et al. 2017).

This technology has numerous advantages over other procedures, such as a high rate of recovery, simultaneously recovering of both water and valuable minerals, and the absence of any additional supplemental information (Lu et al. 2017).

The freezing concentration technique has been used in wastewater treatment for solutions where the solubility of the solute is substantially dependent on temperature (Ab Hamid & Jami 2019). Researchers have previously employed the freezing concentration technique to remediate wastewater from the pharmaceutical, chemical, fluoride removal, and chromium(VI) removal industries (Ab Hamid & Jami 2019). However, limited studies have been done on establishing the optimal operating conditions for the freeze concentration method to recover nitrate-nitrogen and phosphate nutrients from domestic wastewater processed in an anaerobic digester.

As a result, the goal of this research was to find the optimal conditions for the freeze concentration method to recover nutrients from domestic wastewater processed in an anaerobic digester. The freezing concentration performance was evaluated using nitrate-nitrogen and phosphate nutrient recovery values. At various coolant temperatures, freezing time, and energy consumption, the performance of freezing concentration was examined. Changes in nutrient recovery were used to investigate the consequences of these various operating conditions. The recovered nutrients will be used as a fertilizer for crop production in agriculture.

Domestic wastewater is a complicated mixture of accumulated chemicals and contaminants in water. Due to varying flow rates induced by water usage and precipitation, the quality and composition of the wastewater also vary on a regular basis.

At the inlet and outlet of each anaerobic biodigester and a gravel filter treatment plant, wastewater samples were collected. Samples were filtered by using Cellulose Nitrate Filter, pore size of 0.45 μmm before being placed in the sample vessel during the experiment. Following the sample filtering, 400 mL of wastewater was added to the sample plastic vessel for the freezing concentration process. Vessels containing the sample solution were placed in the freezer and the operating parameters were adjusted as needed. The temperature of the sample was 25.43 °C ± 0.73 °C at the start of the freezing concentration procedure. The coolant temperature and time tested in this study are shown in Table 1. The sample vessel was removed after the freezing concentration procedure was finished under the specified operating conditions. The volume and concentration of the concentrate (unfrozen liquid), and melt solution (solution from melted ice) were measured and recorded by using standard techniques for the examination of water and wastewater (APHA 2012). The value of nutrient recovery efficiency was used to determine the efficiency of the freezing concentration process (Ab Hamid & Jami 2019).

Using graduated cylinders (500 mL), the volumes of the initial input water sample (collected water samples before freezing method), unfrozen liquid (samples of concentrate formed after freezing method), and melting ice samples (samples of melted ice formed after freezing method) were collected and measured. Standard techniques for the examination of water and wastewater were used to measure nitrate-nitrogen (NO₃-N) and phosphate (⁠⁠) nutrients in liquid and frozen liquid (APHA 2012).

The analysis of nitrate-nitrogen (NO₃-N) concentration was measured by using DR 900 UV-Visible spectrophotometer method number 355 HR. Phosphate (⁠⁠) was measured by using the same spectrophotometer method 490 HR.

The energy used in each freeze concentration stage was calculated taking into account the pre-cooling of the reactors and the experimental time of the freezing process. The power measured with an energy meter (ZHURUI TEC-PR10, CH) was averaged to determine the energy usage.

By examining nutrient recovery, the coolant temperature effect, time and energy consumption on the freezing concentration process were explored. Freeze concentration efficiency in the system was given by measuring variations in the value of nutrient recovery from the wastewater. Ice crystals were seen growing on the vessel once the freezing concentration process was completed under the specified operating conditions.

However, when the freezer temperature was reduced further from −30 to −80 °C, the trend shifted, and the nutrient recovery value declined marginally. This suggests that the process' efficiency has decreased. The discrepancy could be due to a supercooling effect, which occurs when the freezing temperature is at its lowest (Ab Hamid & Jami 2019). The lower nutrient recovery value was caused by the supercooling phenomenon, which reduced the effectiveness of the freezing concentration process. The supercooling effect speeds up the creation of an ice crystal layer, resulting in more solute inclusion on the side of the ice.

Furthermore, depending on the findings of Ab Hamid & Jami (2019) minimum temperatures gave a higher growth rate of ice crystals. It is also important to note that a freezing temperature of −20 °C was chosen as the optimal freezing temperature for this system for both sample points because of its better nutrient recovery value.

Nitrate-nitrogen gave lower nutrient recovery values compared to phosphate. The observed disparities in nutrient recovery of nitrate-nitrogen and phosphate by freezing concentration could be due to the morphology of the ice created, which was a multi-crystalline dendritic ice structure that held more nitrate-nitrogen than phosphate. Also, it could possibly be due to molecular weight and size discrepancies between nitrate-nitrogen and phosphate, nutrients of large molecular weight are more easily recovered compared to nutrients of small molecular weight (Gu 2016).

Since it gives a freezing rate, the freezing temperature is important in controlling the process (Melak et al. 2016). Examination of the effect of freezing temperature is required to give the ideal temperature for the method. The temperature difference between the freezing temperature, the surface temperature, and the temperature of the wastewater causes the transfer of heat between the coolant and sample solutions across the vessel wall surface during the freezing concentration process (Ab Hamid & Jami 2019). The temperature difference between the coolant and wastewater solution is directly related to the heat transfer rate (Samsuri et al. 2015). The heat transfer rate increases as the freezing temperature is lowered. As a result, a lower coolant temperature is better, as this improves the transfer of heat between the coolant and wastewater. A lower temperature is obtained at the surface when there is a lower freezing temperature providing an acceptable initial supercooling environment for ice formation.

In this analysis, 7 h of freezing time at −20 °C coolant temperature was enough to give the highest performance of the process with the highest nutrient recovery value. At this condition, the average values of nutrient recovery of nitrate-nitrogen and phosphate were 1.114 and 4.667, respectively, at the inlet of anaerobic digester 1; 1.325 and 4.975, respectively, at the outlet of anaerobic digester 1; 1.099 and 4.859, respectively, at the inlet of anaerobic digester 2; 1.132 and 4.755, respectively, at the outlet of anaerobic digester 2; and for gravel filter at the outlet the values where 1.111 and 4.861, respectively.

However, at −30, −40, −50, −60, −70, and −80 °C the maximum freezing time was reached at 5.5, 4.5, 4.25, 4, 3.75, and 3.5 h, respectively, because of supercooling effect as described by Ab Hamid & Jami (2019). This result is in agreement with the findings of Moussaoui et al. (2021), Safiei et al. (2019) and Azman et al. (2018).

One of the major problems in several industries is to reduce energy consumption and operational costs (Ghannadzadeh & Sadeqzadeh 2016). For innovative resource recovery techniques to be adopted and established in the agricultural sector, it is essential to evaluate their economic implications. As a result, the present study examined the energy consumption of the entire process as an extra criterion to establish optimal conditions for the freezing concentration process to recover nutrients from domestic wastewater. Heat transfer and energy utilized per unit volume of the wastewater sample processed were used to analyze energy consumption to recover nutrients. However, the estimated energy efficiency is likely to be on the higher side given that freezer geometry was not considered in the calculation. A comparison of energy consumption and coolant temperature is shown in Table 2.

When compared to membrane-based technologies, which are one of the most widely used to concentrate wastewater and recover particular components, the freezing concentration process uses the same amount of energy during operation, if not less. Comparing freeze concentration to heating and evaporation procedures, there is great potential for energy savings (Uald-Lamkaddam et al. 2021). The amount of energy used depends on the technology being used, the feed solution being used, the ambient temperature, the desired recovery rate, and the electricity cost.

In comparison to the energy consumption of the most effective evaporation systems, Pazmiño et al. (2017) claimed energy savings of up to 30% while using a freezing concentration process, treating sucrose solutions, and combining them with the falling film technique. Mtombeni et al. (2013) further claimed that its use of the freezing desalination technique to remove salts from wastewater resulted in the lowest energy consumption (0.39 kWh). This study's findings indicate that employing a freezing concentration system, 1 L of domestic wastewater will require 0.197 kWh energy to recover maximum nutrients. However, one of the most important considerations for separation and concentration technologies is the capital and operating expenses (Uald-Lamkaddam et al. 2021). While other separation techniques like ammonia stripping, thermal treatment, ion exchange, and adsorption may need more energy input and be influenced by factors such as pH and aeration, freezing concentration technology is recognized as an environmentally friendly separation process with simple operation, low energy consumption, and a high rejection rate (Shi et al. 2018).

The possibility of using a freezing concentration method to recover nutrients from domestic wastewater was proven in this study. This method, which is based on the freezing concentration mechanism, has been used in the separation of liquid and solid, e.g., in food and pharmaceutical industries and it has also been effectively used for seawater desalination. Nutrient recovery values were used to assess the effects of freezing temperature, cooling time, and energy consumption on the effectiveness of the freezing concentration process. It's critical to run this process to get the optimal nutrient recovery value. The freezing temperature of −20 °C and freezing time of 7 h gave the highest nutrient recovery value during the process. The amount of energy consumed by the coolant at this particular condition was 0.197 kWh/L. In general, the results show the possibility of the freezing concentration method and give conditions that can be used to recover nutrients from domestic wastewater. However, further analysis is needed on the application of this method in recovering heavy metals, emerging compounds, and other organic compounds from domestic wastewater, which could not be checked.

The authors would like to express our gratitude to The Nelson Mandela African Institution of Science and Technology (NM-AIST), WISE-Futures, and the Higher Education Student Loan Board (HESLB) for their support.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.