Magnesium-Modified Biochar for Removing Phosphorus from Aquaculture Facilities: A Case Study in Idaho, USA

Abstract

This study aims to improve water quality and reduce eutrophication downstream when receiving water from aquaculture facilities that can support the aquaculture industry and increase fish production capacity. The primary objective is to investigate a novel approach, using magnesium (Mg)-modified biochar water treatment systems from pinewood, to remove the main eutrophication agents (i.e., phosphorous and nitrogen) from the effluents of aquaculture facilities in Magic Valley, Idaho. The downstream water contains approximately 0.14 mg/L of phosphorous and 2.25 mg/L of nitrogen. The results show that the initial P₂O₅ concentration between Mg-modified and non-modified biochar is comparable. After exposure to aquaculture production water, the modified biochar is shown to have a significant increase in phosphorous and nitrogen adsorption. Non-modified biochar started with noticeably higher concentration levels of nitrogen than modified biochar. Over time in the treatment water, the modified biochar showed a significant increase in nitrogen concentration. Mg bonded to the modified biochar is shown to decrease drastically after exposure to the effluent. This could be due to the insufficient bonding of the magnesium to biomass feedstocks during pre-processing and biochar production. The amount of biochar near the end of experimentation is almost comparable to the non-modified char. We concluded that the proposed approach, using a Mg-modified biochar water treatment system, could sequester more nitrogen and phosphorous over time.

1. Introduction

Idaho is considered the most prominent commercial producer of rainbow trout in the United States [1]. Products of Idaho’s aquaculture industry also include cold-water facilities that raise trout, sturgeon, salmon, and steelhead, as well as warm-water facilities that raise tropical fish, tilapia, and catfish [1]. Approximately 70% of the 115 permitted aquaculture facilities in Idaho operate in the Magic Valley. Effluent from these facilities is discharged directly to the Snake River or its tributaries [1]. The effluent contains solid and liquid pollutants, such as fish feces, parasites, uneaten fish food, algae, pathogens, drugs, and excess nutrients, which detrimentally affect downstream surface and groundwater, posing potential environmental problems [1].

Currently, all the fish farms operate under a discharge permit issued by the State of Idaho, Department of Water Resources, including 49 in operation and 12 non-producing. A permit issued by the Federal Government to the State of Idaho identifies a limit to the pollutants that can be locally discharged from the Hagerman area into the Snake River. The individual farm permits identify discharge limits through a series of regularly monitored water quality parameters based upon the requirements established by the federal permit. This monitoring has determined that phosphorous is the chief pollutant discharged in farm effluents and has the greatest impact on freshwater aquatic ecosystems [2]. The recommended limit for total phosphorous in streams was set at 0.05 mg/L, and larger flowing waters at 0.1 mg/L [3]. These limits are identified in the federal permit, which was last renewed in 2019 but is subject to change for future renewals. If the federal permit does alter the area’s phosphorous limit, it will most likely be lowered, as phosphorus is a chief eutrophication element. Current monitoring shows that aquaculture effluent struggles to meet the requirements, with some reports showing phosphorus levels are 35% higher than allowed. The same monitoring also revealed that 65% of the farms had exceeded their permit limits for phosphorus at some point in the last five years. Due to these challenges, fish farm operators are trying several methods to meet compliance. These methods include limiting production, diverting effluents to agriculture irrigation for part of the year, obtaining permission to use the permit rights from producers who are not currently operating, and developing bio-filtration ponds.

Mismanagement of these pollutants results in an increase in pH levels, phosphorus, nitrogen, and sediment. Other water conditions (e.g., temperature, oxygen supply, and nutrient levels) can also be negatively impacted, resulting in harm to aquatic organisms. Therefore, sustainable water treatment and aquaculture waste management practices are vital to the longevity and quality of ground and surface water in Idaho. The motivation behind this study lies in improving water quality and sustainability benefits across Idaho’s aquaculture industry by developing and improving a biochar-based water treatment system. The main objective of this study is to explore a magnesium (Mg)-modified biochar water treatment approach for reducing excess phosphorus and nitrogen levels that can subsequently reduce the overall eutrophication of the Snake River.

Previous studies have examined modified biochar contaminant adsorption capacities and effects on water quality [4,5,6,7].

Table 1. Similarities and dissimilarities of the prior conducted studies with this project.

Authors	Biochar Modification	Eutrophic Target	Biomass Type	Ref.
Cai et al. (2017)	Fe-oxide	${\mathrm{PO}}_4^{3-}$	Water hyacinth	[8]
Ngatia et al. (2017)	-	${\mathrm{PO}}_4^{3-}$	Switchgrass, kudzu, and Chinese tallow	[9]
Yin et al. (2018)	Al and Mg	${\mathrm{NH}}_4^{+},{\mathrm{NO}}_3^{-},{\mathrm{PO}}_4^{3-}$	Soybean straw	[10]
Novais et al. (2018)	Al	${\mathrm{PO}}_4^{3-}$	Poultry manure and sugarcane straw	[11]
Yin et al. (2019)	Mg		Poplar chips	[12]
Liu et al. (2019)	Fe-Mn-Ce	$\mathrm{As}\left(\mathrm{III}\right)$	-	[13]
Zhu et al. (2019)	-	${\mathrm{NH}}_4^{+},$	Phyllostachys pubescens	[14]
Ren et al. (2021)	Fe	${\mathrm{PO}}_4^{3-}$	Reed straw	[15]
Yang et al. (2021)	Fe and Mg	${\mathrm{PO}}_4^{3-}$	Sawdust and sediment	[16]
Cheng et al. (2022)	-	${\mathrm{NH}}_4^{+},{\mathrm{NO}}_3^{-}$	-	[17]
Konczak and Huber (2022)	-	${\mathrm{NH}}_4^{+},{\mathrm{PO}}_4^{3-}$	Sewage sludge	[18]
Bare et al. (2023)	-	${\mathrm{NH}}_4^{+},{\mathrm{NO}}_3^{-},{\mathrm{PO}}_4^{3-}$	Pine	[19]
Bare et al. (2023)	-	${\mathrm{NH}}_4^{+},{\mathrm{NO}}_3^{-},{\mathrm{PO}}_4^{3-}$	Pine	[20]
This project *	Mg	${\mathrm{NH}}_4^{+},{\mathrm{NO}}_3^{-},{\mathrm{PO}}_4^{3-}$	Pine	-

* Results from the proposed approach in this study.

summarizes the key parameters from these studies, four of these studies have considered Mg-modified biochar, of which three different biomass feedstocks were tested (e.g., soybean straw, poplar chips, and sawdust/sediment). All these studies, however, examined contaminant adsorption in controlled lab settings with prepared eutrophic solutions. This study aims to develop Mg-modified biochar and test adsorption capacity as a part of field experiments at fish farms in Hagerman, Idaho. We have tested non-modified biochar and examined its potential in prior studies. Other goals are to expand on the preceding research, compare modified and non-modified biochar, and create a more efficient pathway for fish farm eutrophication reduction.

2. Materials and Methods

This study aims to improve water quality through a reduction in pollutants through the application of modified biochar and quantitatively determine the characteristics that support sustainable water treatment, employing emerging technologies and biomass-based products. In this study, we measured the nitrogen, phosphorous, pH, and temperature of the water using various sensors and testers in aquaculture facilities located near the Snake River in the Hagerman and Buhl, southern Idaho area, USA. Samples are taken before and after biochar applications to determine impact. Each experiment is repeated three times to promote repeatability and identify critical factors.

The Mg-modified biochar samples were produced using a built-in-house, custom ball-bearing mixer and slow pyrolysis reactor. Magnesium chloride was mixed with the pinewood and dried prior to undergoing pyrolysis. The Mg–pine mixing ratio was 1:1, 100 g of pinewood mixed with 100 g of magnesium chloride hexahydrate (MgCl₂·6H₂O). Magnesium chloride hexahydrate is a technical-grade chemical used for a variety of different applications. The primary active component (magnesium chloride) is a natural mineral produced by the evaporation of seawater. Because it is an exothermic reaction with water, it is commonly used for the low-temperature de-icing of highways, parking lots, and walkways. Biochars were produced from pinewood, which was shown previously to have desirable pollutant adsorption capabilities [19]. The pyrolysis process was performed under inert conditions, purging the reactor with nitrogen under a flow rate of 0.5 L/min. The reactor was heated using internal heating cartridges to 450 °C and ran for 1 h and 30 min. The pressure was considered to be ambient.

This study explored Mg-modified biochar production through biomass conversion in the presence of magnesium chloride to create an efficient water pollution mitigation and abatement candidate. The collected data can generate knowledge to assess the feasibility of this water treatment pathway moving forward.

Table 2. Detailed experimental tasks in this study.

Day	Experimental Tasks
0	Collect 3 untreated water samples and check water pH, temp, and Magic Valley Lab for N, P, and C (3 times). Set up the water filter bags with 500 g biochar—lay them flat for better absorption. Duration: 1, 3, 5, and 7 days (24 water filter bags).
1	Collect 3 modified biochar bags and 3 non-modified biochar bags.
3	Collect 3 modified biochar bags and 3 non-modified biochar bags.
5	Collect 3 modified biochar bags and 3 non-modified biochar bags.
7	Collect 3 modified biochar bags and 3 non-modified biochar bags.

shows the detailed experimental tasks in this study. The required materials for the experiments are as follows:

• A total of 24 water filter bags (remove particle size down to 100 mesh);
• A total of 12 non-modified biochar bags and 12 Mg-modified biochar bags (each has 500 g of biochar);
• In total, 48 ft. of 2-inch flexible hose (2 ft per tank);
• pH meter and thermometer;
• A total of 25 zip ties and 25 Ziploc bags for collecting the samples;
• A total of 3 sample bottles for collecting water samples.

2.1. Techno-Economic Assessment

In this study, a life cycle costing model was developed to assess the feasibility of a proposed portable pyrolysis unit for water nutrient adsorption from fish farms. Both the capital (fixed) and operational (variable and labor) costs were incorporated into the total cost estimation, encompassing activities, such as collection, pretreatment (grinding and dewatering), magnesium chloride extraction/production, portable refining, storage, transportation, and water treatment. The total cost, as defined by Equation (1), includes the cost components for pine collection (C1), grinding (C2), drying (C3), magnesium production (C4), conversion (C5), storage (C6), distribution (C7), and water treatment (C8). Detailed nomenclature for these parameters is provided in the Supplementary Materials.

$$\mathrm{Min} \mathrm{Z}={\sum }_{i=1}^n{C}_i=C_1+\dots +C_8$$

(1)

$$\begin{gathered} C_1={\displaystyle {\displaystyle \sum }_a}{\displaystyle {\displaystyle \sum }_b}{\displaystyle {\displaystyle \sum }_t}{C}_{C-col}\times Z_{abt}+\left(C_{V-col}\right)\times {\displaystyle \frac{P{N}_{abt}}{U_{col}}} \\ {C}_2={\displaystyle {\displaystyle \sum }_a}{\displaystyle {\displaystyle \sum }_b}{\displaystyle {\displaystyle \sum }_t}(C_{C-gr}+C_{V-gr})\times {\displaystyle \frac{P{N}_{abt}}{U_{gr}}} \\ {C}_3={\displaystyle {\displaystyle \sum }_a}{\displaystyle {\displaystyle \sum }_b}{\displaystyle {\displaystyle \sum }_t}\left(C_{C-dr}+C_{V-dr}\right)\times {\displaystyle \frac{P{N}_{abt}}{U_{dr}}} \\ {C}_4={\displaystyle {\displaystyle \sum }_a}{\displaystyle {\displaystyle \sum }_b}{\displaystyle {\displaystyle \sum }_t}\left(C_{C-mg}+C_{V-mg}\right)\times {\displaystyle \frac{MG{N}_{abt}}{U_{mg}}} \\ {C}_5={\displaystyle {\displaystyle \sum }_a}{\displaystyle {\displaystyle \sum }_b}{\displaystyle {\displaystyle \sum }_t}\left(C_{C-pyr}+C_{V-pyr}\right)\times {\displaystyle \frac{P{N}_{abt}}{U_{pyr}}} \\ {C}_6={\displaystyle {\displaystyle \sum }_b}{\displaystyle {\displaystyle \sum }_c}{\displaystyle {\displaystyle \sum }_t}\left(C_{C-bs}+C_{V-bs}\right)\times {\displaystyle \frac{B_{bct}}{U_{bs}}}+{\displaystyle {\displaystyle \sum }_b}{\displaystyle {\displaystyle \sum }_c}{\displaystyle {\displaystyle \sum }_t}\left(C_{C-os}+C_{V-os}\right)\times {\displaystyle \frac{O_{bct}}{U_{os}}} \\ {C}_7={\displaystyle {\displaystyle \sum }_c}{\displaystyle {\displaystyle \sum }_e}{\displaystyle {\displaystyle \sum }_t}\left(C_{C-di}+C_{V-di}\right)\times {\displaystyle \frac{B_{cet}}{U_d}} \\ {C}_8={\displaystyle {\displaystyle \sum }_c}{\displaystyle {\displaystyle \sum }_e}{\displaystyle {\displaystyle \sum }_t}\left(C_{C-wt}+C_{V-wt}\right)\times {\displaystyle \frac{B_{cet}}{U_{wt}}} \end{gathered}$$

2.2. Environmental Impact Assessment

A life cycle assessment (LCA) study was performed using the open-source OpenLCA v2.1.2 software in conjunction with data from previous studies to evaluate the environmental impacts of the proposed water treatment method [19,21,22]. The assessment was structured into four distinct phases: (1) definition of the goal and scope, (2) life cycle inventory analysis, (3) life cycle impact assessment, and (4) interpretation.

Goal and scope definition: The objective of this study is to assess the environmental impacts of producing modified biochars for nutrient adsorption in fish farms. The LCA study evaluates both the global warming potential (GWP) and eutrophication effects throughout the conversion of residues into biomaterials for treating downstream waters. GWP is quantified using greenhouse gas (GHG) emission factors expressed in kg CO₂ equivalents, with 28 kg CO₂ eq./kg CH₄ and 265 kg CO₂ eq./kg N₂O as provided by the Intergovernmental Panel on Climate Change (IPCC) for a 100-year time horizon. Eutrophication is measured in PO₄ equivalents based on factors of 3.07 kg CO₂ eq./kg P and 0.24 kg CO₂ eq./kg N₂O. The study’s scope covers upstream and midstream processes, including residue collection, on-site pre-processing (size reduction and drying), conversion into biomaterials, utilization of intermediate products (pyrolysis oil and gas) for process energy (heat and electricity), and distribution. The system boundary is cradle-to-gate, with one kilogram of Mg-modified biochar for phosphorus removal as the functional unit.

Life cycle inventory: Parameter data for both inputs and outputs were sourced from prior studies and the AGRIBALYSE and OpenLCA v2.1.2 databases [19,23,24]. The machinery required for collecting pine residues includes a forwarder and a loader, and key inputs in this stage are the pine residues and diesel fuel, while diesel combustion emissions constitute the primary output. For magnesium chloride production, the process examined involves brine processing and the use of hydrochloric acid. GHG emission factor for the upstream stage of the product system accounts for both the collection and hauling of forest residues and the production of magnesium chloride. After collection, the residues are processed using an on-site grinding machine to generate suitably sized pine particles, followed by dewatering in a rotary dryer. During this pretreatment stage, pine residues and diesel fuel (for the grinding and drying operations) are consumed, producing pretreated residues alongside emissions from fuel combustion and water vapor from drying. Notably, enhancing residue quality for the pyrolysis process requires additional energy input, thereby increasing emissions and the overall GWP. Subsequently, a portable pyrolysis unit converts the pretreated residues into biomaterials. This process employs nitrogen as an inert carrier gas, utilizes energy from the combustion of pyrolysis oil and gas, and may require supplementary diesel fuel. The inputs for the pyrolysis process include the pretreated residues, nitrogen, energy derived from byproduct combustion, and cooling water for condensing the pyrolysis oil, while the outputs comprise the target biomaterials and all associated process emissions. Emissions from the combustion of pyrolysis byproducts are treated as biogenic GHGs. Finally, the produced biomaterials are loaded and transported by truck to fish farms for water treatment, with diesel fuel consumption and environmental impacts primarily influenced by the distance between the collection site and the fish farms. An LCA study was performed using data from a previous case study in Hagerman, ID, and the process was modeled as a production system in OpenLCA. This comprehensive assessment of pathway impacts and emissions offers valuable insight into the biomaterial-based water treatment approach.

Life cycle impacts assessment: The impact assessment was conducted using the CML-IA baseline method within OpenLCA. GWP100 was calculated based on Equations (2)–(9) and encompasses both upstream and midstream processes. These processes include residue collection, on-site pre-processing (i.e., size reduction and drying), conversion of residues into biomaterials, on-site reuse of intermediate products (pyrolysis oil and gas) for process energy (heat and electricity), and biomaterial distribution. Upstream emission factors and GWP for biomaterial production are calculated using Equations (2) and (3). Midstream emission factors and GWP for biomaterial production are calculated using Equations (4) and (5). Emission factors and GWP of biomaterial transportation are calculated using Equations (6) and (7). Emission factors and eutrophication potential of modified biomaterial production and application are calculated using Equations (8) and (9).

US_EF = ER_CO2 × US_EFCO2 + ER_CH4 × US_EFCH4 + ER_N2O × US_EFN2O

(2)

US_GWP = M_P × US_EF

(3)

MS_EF = ER_CO2 × MS_EFCO2 + ER_CH4 × MS_EFCH4 + ER_N2O × MS_EFN2O

(4)

MS_GWP = M_bc × MS_EF

(5)

TR_EF = ER_CO2 × TR_EFCO2 + ER_CH4 × TR_EFCH4 + ER_N2O × TR_EFN20

(6)

TR_GWP = M_bc × TR_EF × D

(7)

EP_EF = EP_P × EP_EFP + EP_N2O × EP_EFN2O

(8)

E_P = M_bc × EP_EF

(9)

Interpretation: The interpretation step, which integrates the findings from the life cycle inventory and the impact assessment, is detailed in Section 4.

3. Case Study

Hagerman is located within the Snake River Canyon, approximately 40 miles west of Twin Falls, Idaho. It is the home of over 60 fish farms, which produce up to 75% (around 41 million pounds per year) of the nation’s commercial rainbow trout depending upon how many farms are operating each year. The desirability of operating in this location depends upon two factors: (1) a sustainable freshwater supply and (2) a temperature range favorable to the rearing of rainbow trout (54–65 °F) [25]. These conditions are met by the outflow of the Snake Plain aquifer, also known as “Thousand Springs”, which serves as the water source for the majority of the farms. These farms have a significant financial impact on the community and the entire Southern Idaho region, currently estimated at more than 150 million dollars annually (Figure 1).

4. Results and Discussion

The study investigated the potential for the Mg-modified biochar to remove excess nutrients (e.g., phosphorus- and nitrogen-based compounds) efficiently and economically from the effluent water of fish farms. The detailed analytical techniques for laboratory and field sample analyses are provided in an earlier published study by Bare et al. (2023) [19].

Table 3. Field experiment results of water samples.

Parameter	Sample			Method
	1	2	3
Total organic carbon (mg/L)	1.15	1.22	1.08	SM 5310B
Total P (mg/L)	0.09	0.09	0.09	EPA 36.1
Nitrate/N (mg/L)	2.58	2.61	2.59	EPA 300
Nitrite/N (mg/L)	<0.40	<0.40	<0.40	EPA 300

presents the water sample results.

The initial phosphorus concentrations of the Mg-modified and non-modified biochar are comparable. Both show a linear increase in phosphorus adsorption through day 5. A noticeable deviation between the two is observed by day 7, with the Mg-modified biochar exhibiting a substantial increase in phosphorus adsorption, but with increased variation. The initial drop in phosphorus adsorption on day 3 for Mg-modified biochar may be attributed to competing ions, clogging the biochar surface, or microbial biofilm formation. The later surge in phosphorus adsorption by day 7 suggests that the biochar underwent physicochemical changes, improving its adsorption capacity. Further data on competing ions and biofilm activity could help confirm the exact mechanism.

A similar trend is seen for the adsorption of nitrogen. Non-modified char started with noticeably higher concentration levels than the modified char. Over time, particularly after day 5, the modified biochar is shown to greatly increase nitrogen concentration (Figure 2).

While pH is relatively constant for the Mg-modified biochar, the unmodified biochar shows a decrease with time. This decrease is likely due to a combination of nutrient adsorption, microbial activity, and chemical changes in the biochar. Initially, biochar’s alkaline functional groups provide some buffering, but as phosphorus and nitrogen species adsorb, proton release lowers the pH. Microbial processes, particularly nitrification, further contribute to acidification by generating additional H⁺. Over time, the gradual leaching of carbon compounds and the depletion of alkaline minerals (e.g., K₂O and CaO) reduce the biochar’s buffering capacity, leading to further pH decline. The Mg-modified biochar could also be slightly more acidic in nature due to the slight hydrolysis of magnesium species and the increase in acidic functional groups formed during the pyrolysis step.

The modified biochar shows a gradual increase in total nitrogen with a p-value of 0.0028, indicating the significance of the rise in nitrogen over time. On the other hand, the unmodified biochar remains relatively flat, showing less nitrogen adsorption. Both the modified and unmodified biochar water treatment systems show a significant increase in phosphorus adsorption as well, with p-values of 0.0059 and 0.0082, respectively. The modified biochar significantly improves nitrogen, nitrate, and phosphorus adsorption over time. The unmodified biochar shows weaker trends and decreases pH. These findings suggest that the modified biochar is a more effective long-term solution for nutrient adsorption.

Magnesium levels stay constant in the unmodified char as there is no significant amount of Mg in the pre-processed biochar. Mg bonded to the modified char is shown to drastically decrease shortly after exposure to the effluent. This could be due to the insufficient bonding of the magnesium to the biomass/biochar during processing. The amount of biochar near the end of experimentation is almost comparable to the unmodified biochar. Further work should be put into optimizing Mg/C binding mechanisms to increase nutrient adsorption capacity.

For both the modified and unmodified biochars, similar correlations are observed in the heatmaps (Figure 3), although the magnitudes differ slightly. The correlation study used for generating the heatmaps was based on Pearson’s correlation coefficient. This is the most common method for measuring linear relationships between numerical variables. A strong positive correlation between nitrate-nitrogen and phosphorus adsorption suggests that these nutrients tend to be retained simultaneously, likely due to the inherent surface properties of the biochar. Similarly, a positive correlation between pH and potassium oxide (K₂O) indicates that higher pH levels enhance potassium retention, possibly through improved cation exchange capacity.

Conversely, a strong negative correlation between nitrate-nitrogen adsorption and total carbon content implies that surface chemical modifications occurred, altering nitrogen adsorption behavior. Exposure to wastewater may have induced oxidation or protonation of functional groups, leading to shifts in surface charge distribution and affecting nitrate binding. Additionally, the adsorption of anions like nitrate could have influenced electrostatic interactions, potentially reducing the availability of active sites for further nitrogen retention. These findings suggest that the observed changes in nitrogen adsorption are more likely due to biochar surface chemistry. Additionally, the data indicate that higher nitrate retention is associated with a decrease in pH, potentially due to the release of H⁺ ions during the adsorption process. Table S1 presents the results of field experiments.

Techno-economic results: In the proposed techno-economic analysis, 29,400 metric tons of pinewood are processed annually to produce 11,760 metric tons of modified biomaterials. Under the base-case scenario—with a processing capacity of 10 metric tons of ground, dry forest residue per day—the total cost, which encompasses both capital and variable expenditures, is estimated at USD 51,709 per year over a 10-year period. This results in a unit cost of USD 436 per metric ton of biomaterials. Detailed breakdowns of the capital and operational costs for each pathway are provided in

Table 4. Capital and operational expenditures of each process.

Process	Capital (USD/yr)	Variable (USD/yr)
Collection (Wood, MgCl2)	4445	4361
Grinding	10,912	11,653
Drying	13,976	17,254
Portable refinery	59,976	4954
Biomaterials storage	6076	2116
Transportation	9076	6914
Fish farm	-	4457
Total	104,461	51,709

, with approximately 67% of the total cost attributed to capital expenses, primarily due to the high initial investment required for the mobile pyrolysis refinery. The use of magnesium chloride for biochar modification had a significant impact on operational costs compared to the previous study.

Environmental impact results: The largest contributing factor to the global warming potential in this process is the upstream production of the magnesium chloride additive (

Table 5. Life cycle impact assessment data using CML baseline.

Impact Category	Per Metric Ton Biochar	Unit
Global Warming Potential (GWP100)	692	kg CO2-eq
Acidification Potential	2.4	kg SO2-eq
Ecotoxicity Potential	0.18	kg 1,4-DCB-eq
Human Toxicity Potential	0.36	kg 1,4-DCB-eq
Photochemical Oxidation Potential	1.44	kg C2H4-eq
Eutrophication	−1124	kg PO4-eq

). Although it represents only about 5% of the feed by weight, its synthesis is highly energy-intensive and generates a disproportionate amount of CO₂-eq emissions (~75%). This chemical production stage drives a significant portion of the overall GWP because the emissions per unit mass of magnesium chloride are much higher than those associated with the biomass itself.

In addition, the energy required to initially heat the pyrolysis reactor—primarily via natural gas combustion—also contributes notably to the process’s carbon footprint. However, when comparing the two, the emissions from the chemical production of the additive remain the dominant source of GWP. This means that efforts to reduce the overall environmental impacts of the process should focus on either improving the efficiency of the additive’s production or reducing the required dosage while maintaining biochar performance.

5. Conclusions

The primary objectives of this study are as follows: (a) improving water quality and pollution reduction in downstream water from aquaculture facilities using raw and magnesium (Mg)-modified biochar water treatment systems from pinewood, (b) estimating water quality using data from laboratory and field experiments, as well as molecular and spectroscopy analysis, and (c) conducting characterization and statistical analysis to identify the primary causes that drive the fish production away from being sustainable. Particularly, this study involves the investigation for developing a sustainable methodology to remove the main eutrophication agents, i.e., phosphorous and nitrogen, from the effluents of aquaculture facilities. The key results of this study include the following: (i) modified biochar removed 56.3% more NO₃-N and 13.3% more P₂O₅ compared to unmodified biochar, making it significantly more effective for nutrient removal from fish farm effluent; (ii) the total global warming potential (GWP) for 1 ton of modified biochar was estimated at 1750 kg CO₂-eq, with 400 kg CO₂-eq sequestration in the soil phase, reducing its overall footprint; (iii) pH and K₂O adsorption (r ≈ 0.92) showed a high positive correlation, suggesting that increased phosphorous and nitrogen retention enhances pH stabilization; and (iv) NO₃-N and P₂O₅ adsorption (r ≈ 0.82) were strongly correlated, indicating that biochar modification enhances the simultaneous retention of these nutrients. This study provided the following outcomes:

• Understanding the complexities of modified biochar production, reaction mechanisms, and multi-functional performance in water treatment.
• Generating valuable data and a base of knowledge to effectively assess water quality in the Thousand Springs and Hagerman areas of Idaho.
• Developing an environmentally friendly and economical method to remove the eutrophic contaminants from the farm effluents to sustain and improve the productivity of fish farming in Idaho and the nation.
• Reducing water pollution generated from aquaculture production facilities to enhance sustainability benefits to the surrounding area and aquaculture industry using engineered biochar water treatment systems.
• Increasing fish production within the EPA permit limits, and consequently stabilizing employment and increasing profitability.
• The collected nutrient-rich biochar after water treatment has the potential as a commercial slow-release fertilizer to improve soil fertility and crop health. The possible supplementary benefits could result in the recycling of excess nutrients from downstream water through repurposing as soil amendments.