Experimental Study of Increase of Biogas Production from Lagoon Station's Sludge by Alkaline Pretreatment

Abstract

The present research focuses on the experimental study of the effect of alkaline pretreatment with NaOH on biogas production. Different concentrations of NaOH, i.e. 1, 2.5 and 5% (w/w), were tested on the lagoon station's sludge (dry basis) at room temperature for 24 h. The results obtained after 60 days of digestion, through the cumulative volume of biogas recorded, clearly indicate a positive effect of the chemical alkaline pretreatment on the anaerobic digestion since the amount of biogas produced increased by 42.6% when the NaOH concentration was close to 2.5%. This concentration is considered optimal under the chosen conditions. Indeed, inhibition of the methanogenic activity and a blockage of the digestion process are observed beyond this concentration. These results suggest that the alkaline pretreatment can improve the energy efficiency of the obtained biogas (CH₄ content) and reduce the residence time.

Keywords

anaerobic digestion lagoon station's sludge biogas enhance alkaline pretreatment NaOH

1. Introduction

In recent decades, population growth, industrial development and rapid urbanization worldwide have been accompanied by an increase in waste discharge and a significant demand for conventional energy. In addition, the growth of human activities in cities generates solid waste and liquid effluents of various kinds.

Anaerobic digestion (AD), also known as bio-methanation, is considered one of the most efficient and advanced techniques for treating various organic wastes such as municipal solid waste, sewage sludge, agricultural residues, animal manure and food waste through the action of various types of anaerobic microorganisms.¹

This process makes it possible to stabilize these wastes, reduce their volumes, and finally produce useful biogas with high energy value. This renewable energy source can be used in various domestic and industrial applications. In addition, the AD process can produce solid and liquid fertilizers.² It is worth noting that the latter niche is all the more important since Saharan agricultural soils (such as the study area) are skeletal, salty and very poor in organic matter.³ In addition, the addition of digestate to agricultural soils allows for substantial water savings during crop irrigation. This is very important in arid areas, which is the interest of our studies.

The methane yield and biogas potential produced during the AD process can be limited by several factors, namely pH, temperature, substrate composition, dilution rate, and microorganisms.⁴

To obtain the best biogas yield, different categories of pretreatment methods, such as physical (mechanical, thermal, irradiation, ultrasound, microwaves, etc.), chemical (ozonation, acids, basics, oxidative, ionic liquids, inorganic salts, etc.), biological (enzymatic, fungal) or combined, have been employed to improve the biodegradation rate of lignocellulosic biomass and biogas production from the anaerobic digestion process.^5–7

In addition, chemical pretreatments are generally the most widely used because they are simpler, faster, more effective and more economical than other pretreatment methods such as physical and biological methods.⁸ Chemical pretreatment is defined as the destruction of organic compounds before their incorporation into the digester by strong oxidants, acids, or alkalis to facilitate their biological degradation and improve biogas production.⁹ The effect of chemical pretreatments on the AD process depends on the substrates’ characteristics and the applied method's type and conditions.

In most cases, the AD process requires pH adjustment by increasing alkalinity. Therefore, alkaline pretreatment has been reported and proven to be the most effective chemical pretreatment for its many advantages.¹⁰ The main advantages of alkaline pretreatment are the high efficiency of hemicellulose and lignin removal, accelerated solubilization of hemicelluloses and proteins, and reduced cellulose crystallinity during the AD process.⁴

In addition, alkaline pretreatment induces saponification and solvation reactions that result in increased surface area, increasing substrate availability for enzymatic degradation by microbes.¹¹ Furthermore, alkaline pretreatment is the preferred and widely used chemical method for lignocellulosic substrates and wastewater sludge.¹² What is more, the alkaline solution can be recovered by liquid-solid separation and then reused to adjust pH, which cannot be achieved with acids.

Alkaline pretreatment is conducted at a low temperature and under pressure, using various strong bases. Its treatment time is of the order of a few hours to a few days. Sodium hydroxide,¹³ hydrogen peroxide,¹⁴ ammonium hydroxide,¹⁵ calcium hydroxide,¹⁶ magnesium hydroxide,¹⁷ potassium hydroxide¹⁸ and calcium peroxide¹⁹ are commonly used. Among these, several studies have shown that sodium hydroxide is the most widely used alkali in alkaline pretreatment and the most effective for lignin removal and biogas production, of which the ranking order of these alkalis according to several authors is as follows: NaOH > KOH > Mg(OH)₂> Ca(OH)₂.²⁰ This reagent induces the saponification of the ester bonds to present between lignin and carbohydrates. This induces the following series of solubilization: hemicelluloses > lignin >> cellulose. According to fructose > glucose > mannose, the reactivity of sugars towards the action of soda for their solubilization is: fructose > glucose > mannose.

However, no previous studies are available detailing the effect of these pretreatment methods on the potential for biogas production from lagoon station's sludge, which is the novelty of this study. Furthermore, a previous study showed the feasibility of converting lagoon station's sludge of the City of Adrar into biogas.³ Thus, the main objective of the current research is to optimize biogas production from this substrate.

The operational factors that influence alkaline pretreatment are temperature, time and alkali concentration.⁴ Therefore, it is important to optimize the concentration of alkali pretreatment to reduce alkali consumption, improve the decay effect, and increase the efficiency of the AD process.²¹ In this context, the objective of the present study is to evaluate the effect of alkaline pretreatment with NaOH on the biogas production potential of AD of lagoon station's sludge and find the optimal concentration of NaOH for this type of substrate.

2. Materials and methods

2.1 Collection and preparation of substrates

The substrate used in this study to feed the digesters is sludge (dry basis) collected from the lagoon station of the city of Adrar. The physico-chemical characteristics of the sludge used are presented in Table 1.

Table 1.
Characteristics of the substrates used in the BMP tests.

Parameter Value

pH 6.45

Volatile solids (%) 53.33

Rate of organic carbon (%) 31.00

Total solids (%) 98.37

Total nitrogen (‰) 1.11

C/N 27.92

CaCO₃ (%) 5.40

Parameter	Value
pH	6.45
Volatile solids (%)	53.33
Rate of organic carbon (%)	31.00
Total solids (%)	98.37
Total nitrogen (‰)	1.11
C/N	27.92
CaCO₃ (%)	5.40

The first step after collection, often necessary, is separating the organic part of the substrate to be digested from the mineral or inert part.²²

After collection, the sludge was dried by sunlight under room temperature and stored. To ensure a homogeneous particle size in all digesters, to reduce the substrate volume in the digester, to optimize the biogas production, kinetics and to reduce the residence time, the substrate was subjected to a mechanical pretreatment consisting of a simple grinding to a size of about 1 mm.²³

2.2 NaOH pretreatment method

An alkaline pretreatment method was applied to our substrate using sodium hydroxide. The pretreatment conditions chosen in this research were based on previous studies on alkaline pretreatment with NaOH on different substrates.^11,24

A 1 g sample of the prepared biomass was mixed uniformly with 1 mL of 0.25, 0.625, and 1.25 M-NaOH solution. The corresponding concentrations of NaOH on the substrate solids were 1, 2.5 and 5% (w/w), respectively. Then, the pretreated substrates were covered with plastic films and stored at room temperature, for 24 h.

2.3 Experimental setup for anaerobic digestion

The biomethane potential (BMP) tests were performed in batch digesters. They are equipped with two ports, one for the intake of liquid samples, and the other for exhausting the produced biogas whose volume is supposed to be measured. The useful volume of the reactors is 2.5 L.

The digesters used were fed with diluted sludge, to obtain a dry matter concentration of about 30 g/L. The digesters were maintained in a thermostatically controlled heated water bath under mesophilic conditions (37 °C). The reactors were manually agitated two to three times per day.²⁵ The experiment was performed for a retention time of 60 days.

3. Methods of analysis

The stability of the process was evaluated by the most commonly used parameters that can inform us about the operation of the digesters, such as pH, total alkalinity (TA), volatile fatty acids (VFAs), VFA/TA ratio, volume and quality of the biogas produced, were monitored during this study. The pH values were measured using a HANNA pH meter (HI 3220). Total alkalinity was estimated by the method described by C. Berthe (2006).²⁶ VFAs content was determined by the method described by A. Sow (1990).²⁷ The volume of biogas produced was measured daily using a hydraulic (liquid displacement) system.³ The biogas produced was analyzed using the PerkinElmer Clarus 500 CPG device. The flammability test of the obtained biogas was performed according to the method described by Mokobia et al. (2012).²⁸

The two most common purification parameters, which help to find pollutants in wastewater, were also monitored; this procedure allowed to estimate the purification rate of the substrate. These two parameters are the initial and final chemical oxygen demand (COD), measured according to the standard method AFNOR T90-101, and the initial and final biochemical oxygen demand (BOD₅) determined with the help of the photometer OxiTop® WTW. In addition, the effect of pretreatment on solids removal was also performed by determining volatile solids before and after AD.

3. Results and discussions

The total solid (TS) and volatile solid (vs.) values were, respectively, 98.37 and 53.33% for the lagoon station's sludge. The pH, C/N ratio, TS, VS, CaCO₃ and total nitrogen of raw materials were outlined in Table 1.

In light of these results, it is clear that these sludges are complex mixtures consisting mainly of organic matter (more than 50%), making them more suitable for methanization.

3.1 Changes in pH

pH is one of the most important stability parameters that shows the status of the anaerobic digestion process.³ This parameter is considered a critical factor for the decomposition of the anaerobic digester as it can cause disruptions to the microbiome dynamics and subsequent metabolomic pathways.²⁹

Fermentative microorganisms can operate in a wider pH range between 4.0 and 8.5.³⁰ It is highly dependent on VFAs and buffering capacity.²⁴ Variations in the pH of the medium within the digesters are shown in Figure 1.

Figure 1.

Changes in pH values during AD process.

As seen, initial pH values ranged from 6.72 to 10.46. A decrease in pH values was observed between 1 and 14 days of digestion, as the pH value varied from 6.72 on day 1 to a minimum value of 6.51 on day 14^th for the untreated digester, from 7.38 to 6.49 for the digester treated with 1% NaOH and from 8.76 to 6.81 for the digester treated with 2.5% NaOH. This decrease continued until day 35^th for the digester treated with 5% NaOH, from 10.46 to 7.60. The decrease in pH values may be partly due to the accumulation of volatile fatty acids in the medium and the low buffering capacity of these digesters early in the process.³¹

Considering these values, we can observe the difference between the different digesters in terms of pH decrease. Indeed, decreases of 0.21, 0.89, 1.95 and 2.86 were recorded for the untreated digesters and those treated with 1, 2.5 and 5% NaOH, respectively. These differences are attributed to differences in the amount of VFAs released into the medium.³⁰

In addition, after this part, an obvious increase in the pH value was observed from the 14^th day until the end of the digestion, from 6.77 to 7.2 for the untreated digester, from 6.63 to 7.24 for the one treated with 1% NaOH and from 6.99 to 7.33 for the digester treated with 2.5% NaOH. For the digester treated with 5% NaOH, this part was observed from the 42^nd day, from 6.63 to 7.24. Several researchers have reported similar results.³²

3.2. Evolution of volatile fatty acids

First, it is important to know that VFAs concentration has been identified as a valuable indicator of the gap and must be maintained below the inhibition level for efficient operation.³³ VFAs represent the readily biodegradable organic matter available during the anaerobic process.³⁴ The VFAs concentration is a characteristic parameter of the early stages of biogas, and monitoring their evolution allows estimating the degradation status of the substrate. The variation of their concentrations in the environment directly affects the alkalinity and pH of the medium.³³ Figure 2 illustrates the results obtained for the variation of VFAs concentrations in the different digesters.

Figure 2.

Variation of VFAs concentrations in the different digesters.

As can be clearly seen, there was no significant accumulation of VFAs in the culture medium. A similar trend in VFAs variation was observed for all digesters.

At the beginning of the process, a rapid increase in the VFAs concentration in the medium resulted from rapid hydrolysis and acidogenesis, without observing any change in the stability of the processing system, presumably due to the good and sufficient buffer system of these digesters.

It should be noted that although the mass of the sample added to each digester is the same, the degradation state of the sample is not the same in all digesters, resulting in the production of varying amounts of volatile fatty acids.

In the present study, higher VFAs productions were observed compared to the reference experiment with the untreated digester. Indeed, the maximum VFAs recorded was 0.73, 0.75, 1.43 and 2.1 g/L for the untreated digester and those treated with 1, 2.5 and 5% NaOH, respectively. This is most likely due to the alkalizing power of the NaOH solution and its ability to facilitate the biodegradability of the complex organic matter contained in the substrate. It is generally accepted and proven that alkaline pretreatment is very effective in modifying and altering the structure of lignin, in the solubilization of hemicelluloses and proteins, and consequently in facilitating the accessibility of carbonaceous substances such as cellulose through its partial decrystallization.³⁵ In addition, higher concentrations of NaOH during pretreatments are practically much more effective in breaking down complex organic matter and transforming chemical components.³⁶

These same observations have already been made in several studies conducted on the effect of alkaline pretreatment on anaerobic digestion.³⁶

3.3. Variation of the total alkalinity (TA) of the medium

Alkalinity is defined as the ability of an aqueous solution to neutralize acids.³⁷ This parameter is very important as it assesses the stability of anaerobic digesters.²⁹ The variation in alkalinity of digesters is presented in Figure 3.

Figure 3.

Variation in alkalinity during AD process in the different digesters.

In this experiment, low alkalinity was detected in all reactors at the beginning of the process. The TA values started to increase during the experiment until the end of the process for all digesters. Most of these values were in the optimal digestion range, i.e. between 1 and 3 g of CaCO₃/L. The increase in alkalinity was normally due to the activity of methanogenic bacteria, which could produce alkalinity in the form of carbon dioxide, ammonia and bicarbonate.³⁸ These values were sufficient and resilient to pH changes caused by VFAs production in the media concentrations to counteract and prevent pH drops.³⁹

The differences in values recorded at the beginning of digestion are due to the difference in hydroxide ion concentrations, responsible for the increase in TA.²⁴

3.4. Variation of the VFA/TA ratio

The stability of the anaerobic process can be assessed by the VFA/TA ratio.⁴⁰ This ratio has been identified as one of the potential auxiliary indicators to diagnose imbalances in the AD system.⁴¹ A ratio between 0.3–0.5 is generally considered acceptable for the anaerobic digestion.³⁶ Above this value, it is an indicator of instability. The ratios from this study for the different digesters are presented in Figure 4.

Figure 4.

Variation in VFA / TA ratio for different concentrations.

As shown in Figure 4, the VFA/TA ratio values for all digesters at the initial stage of anaerobic digestion fluctuated between 0.28 and 0.38. After that, our results show that the VFA/TA ratios increased with time with maximum values of 0.682, 0.625, 0.84, and 0.88 for the untreated digester and for those treated with 1, 2.5, and 5% NaOH, which is considerably above the optimal upper limit, especially for the two digesters treated with 2.5 and 5% NaOH, may be due to the high concentration of VFAs produced by these digesters.³ Despite this, the pH did not decrease during this period. This can be attributed to the buffer system described above.

At the end of digestion, this ratio decreases in all digesters, due to the decrease of VFAs concentrations in the medium, on the one hand, and the increase of the alkalinity of the medium on the other hand. This confirms the observations already made by Song et al.³⁶ in their work on optimizing alkaline pretreatment of rice straw.

3.5 Biogas production

Biogas production is the most reliable criterion for a good operation of anaerobic digestion. The daily and cumulative biogas production are shown in Figures 5 and 6, respectively.

Figure 5.

Daily biogas production for different concentrations.

Figure 6.

Cumulative biogas production for untreated and pretreated digesters.

Thus, it is clear that the biogas production of the pretreated digesters was higher than that of the untreated digester. The final cumulative biogas results show that the maximum amount (2799 mL) was achieved by the digester treated with 2.5% NaOH, 42% more than the untreated (1962.5 mL). The same was achieved by the digester pretreated with 1% (2163 mL), which was 10.2% higher than the untreated, as shown in Figure 6. In addition, Figure 5 shows that the same pretreated digester had the highest daily biogas production.

The increase in cumulative biogas volume is due to the effect of alkaline pretreatment, which allows for the solubilization of organic matter and its utilization by anaerobic microorganisms and increases the surface area available for enzymatic activity.³⁰ The medium alkalinization resulted in a higher amount of VFA, directly influencing the volume of biogas produced.

The results obtained are consistent with studies conducted by other researchers under similar conditions. Ray et al.⁴² reported that an increase in biogas production of 29–112% during anaerobic digestion was observed after pretreatment of waste activated sludge with 20 meq NaOH/L for 24 h. Comparatively, the highest methane production of 35% was obtained by Monlau et al.⁴³ after pretreatment at 55 °C with 4% NaOH for 24 h was found by their work of the effect of alkaline pretreatment on anaerobic digestion of sunflower stalks. In a similar study by Zhu et al.,²⁴ pretreatment of corn stover using 5% NaOH (w/w) for 24 h was sufficient to increase biogas production by 37%, compared to the untreated substrate after 40 days of digestion. In the same lineage, the highest methane yield was obtained at 35 °C, 2% NaOH for 24 h by Kang et al.⁴⁴ in their work of improving methane production from anaerobic digestion of Pennisetum Hybrid by alkaline pretreatment. Recently, an increase in cumulative biogas of 34.8% (407.09 mL/gVS at pH = 10), compared to 301.9 mL/gVS of the untreated, was reported by Dasgupta and Chandel¹¹ in their work on the effect of alkaline pretreatment using the organic fraction of municipal solid waste.

However, a process inhibition was noticed in the digester treated with 5% NaOH. The biogas yield decreased by 3.1% compared to the untreated case. This phenomenon can be attributed to several reasons:

- Rapid hydrolysis and consequently to early acidogenesis, which caused the methanogenesis process to be blocked²⁴

- Extreme pH conditions or high concentration of Na, a widely reported inhibitor of anaerobic digestion⁴⁵

- This could also be due to the hindrance of bacterial growth by inhibiting microbial growth.⁴⁶

Therefore, it can be concluded that the optimal NaOH concentration that allows good digestion of the studied sludge and suitable biogas production is 2.5% NaOH.

3.6 Biogas quality

Biogas production was monitored and quantified during digestion, and its flammability was determined. Tables 2 and 3 below show the composition of the biogas obtained on the 40^th day of digestion; they also present the monitoring of the flammability of the biogas obtained in each digester.

Table 2.
The composition of the biogas obtained on the 40^th day in each digester.

Compound Concentration %

Untreated 1.0% NaOH 2.5% NaOH 5.0% NaOH

Hydrogen (H₂) 3.18 1.66 0.84 2.07

Carbon dioxide (CO₂) 35.51 31.06 27.54 34.78

Hydrogen sulfide (H₂S) 1.08 1.12 1.22 1.09

Oxygen (O₂) 0.08 0.03 0.05 0.06

Nitrogen (N₂) 5.83 6.92 5.58 6.61

Methane (CH₄) 54.32 59.21 64.77 55.39

Compound	Concentration %
Hydrogen (H₂)	3.18	1.66	0.84	2.07
Carbon dioxide (CO₂)	35.51	31.06	27.54	34.78
Hydrogen sulfide (H₂S)	1.08	1.12	1.22	1.09
Oxygen (O₂)	0.08	0.03	0.05	0.06
Nitrogen (N₂)	5.83	6.92	5.58	6.61
Methane (CH₄)	54.32	59.21	64.77	55.39

% volumetric percentage.

Table 3.

The monitoring of the flammability of the biogas obtained in each digester.

Digester	Flammability status
Untreated	Flammable from the 15^th day
1% NaOH	Flammable from the 12^th day
2.5% NaOH	Flammable from the 10^th day
5% NaOH	Flammable from the 21^th day

Moreover, alkaline pretreatment is also beneficial for the quality of the obtained biogas. The results that alkaline pretreatment with NaOH can improve the energetic quality of the produced biogas. In other words, it can improve the CH₄ content of the biogas.

The highest methane contents were obtained in the pretreated cases. In the present case, the methane content was equal to 59.21 and 64.77% for the digesters treated with 1 and 2.5% NaOH, respectively, while it was 54.32% for the untreated digester.

For the digester treated with 5% NaOH, a CH₄ decrease was noted (55.39% only). This may be due to the toxicity of high sodium concentration to the methanogenic bacteria. Several researchers have reported similar results Penaud et al.⁴⁷ and Zhu et al.²⁴

3.7 Volatile solids removal rate

The purpose of determining volatile solids removal was to examine the efficiency of degradation and the correspondence with the biogas produced. Figure 7 below shows the volatile solids content before and after digestion.

Figure 7.

Volatile solids content before and after AD process.

As shown in Figure 7, the results obtained also indicate that the reduction of volatile solids increases when the NaOH loading increases from 1.0% to 2.5%, implying a high transformation of sludge to biogas. The greatest volatile solids reduction (17.4%) was achieved with 2.5% NaOH pretreatment, compared to 12.4 and 13.7% for the untreated and 1% NaOH pretreated digesters, respectively, during the 60-day experiment.

This percentage correlates with the effect of alkaline pretreatment and the high biodegradability of the organic matter and demonstrates the high metabolic activity and biodegradability of the substrate used.^10,40 The lowest removal was obtained in the digester treated with 5% NaOH with a rate of 11.8%.

These results are consistent with what was previously reported: the highest rate of volatile solids reduction corresponds to the highest volume of biogas produced. Similar results have been reported by Dai et al.⁴⁸ and Khalid et al.¹⁰

3.8 CODs and BOD₅removal efficiency

CODs and BOD₅ removal is a parameter that represents the degree of hydrolysis and solubilization of volatile solids achieved by the acidogenic bacteria during pretreatment. Figure 8 and Figure 9 show the CODs and BOD₅ values obtained for each digester before and after digestion.

Figure 8.

CODs values obtained for each digester before and after AD process.

Figure 9.

BOD₅ values obtained for each digester before and after AD process.

The performance of the NaOH pretreatment can be evaluated based on the efficiency of COD and BOD₅ removal. As shown in Figure 8 and Figure 9, compared to the four bioreactors, the concentration of COD and BOD₅ increased in the bioreactor with the increase in sodium hydroxide dosage during the AD start-up period. During the hydrolysis step, complex compounds such as carbohydrates, lipids, and proteins break down into monomorphic sugars and other simpler compounds, resulting in increased COD and BOD₅ values. ¹⁸

The efficient biodegradation in the digesters was highlighted in terms of CODs removal efficiency, which reached the range of 30–41% in the digesters pretreated with NaOH compared to the untreated 25% removal digesters. The same results were obtained for BOD₅ removal efficiency with a range of 25–30% compared to the untreated 25% removal. This result was consistent with the versus removal efficiency.³ Therefore, NaOH pretreatment offers the advantage of achieving greater volumetric removal of organics. The result was similar to other researchers.¹¹

It will be more interesting and beneficial for the follow-up of this type of pre-treatment study to evaluate the solubilisation rate, i.e., to calculate the fraction of the total COD that has been converted to soluble COD after the alkaline pre-treatment.⁴⁹

3.9 Limitations and future perspectives

At the end of this study, it is important to point out that the application of this type of pretreatment with these conditions only remains insufficient to optimize the biogas production and its methane content in particular. However, it would be very interesting to deepen this research to determine the optimal case for other parameters such as time, temperature and solid-liquid ratio.

However, the use of certain alkaline agents, such as potassium, may be of interest for agronomic tests to assess effects of digestate application to the soil. In addition to increasing the accessibility of the material, the enrichment/addition of the value of the digestate from the digestion of this biomass should be considered.

Future work in the field could include digestate characterization, economic analysis, life cycle assessment and comparaison with other pretreatment technologies such as thermal treatment, combined alkali-thermal treatment, icing-thawing, ultrasonication and biochar addition to increase biogas yield from lagoon station's sludge.⁴⁹

4. Conclusions

Different concentrations of NaOH were tested on the mesophilic AD of lagoon station's sludge. Based on the results obtained after 60 days of digestion, it was found that chemical pretreatment with NaOH for 24 h was very beneficial to the anaerobic digestion. Indeed, compared to the control, an increase of 42.2% of the cumulative biogas production was observed when the NaOH concentration was close to 2.5% (w/w). Under the conditions considered, this concentration was optimal compared to the case of the untreated digester. Above this concentration, inhibition of methanogenesis and blockage of the digestion process is observed. Furthermore, it is important to note that the alkaline NaOH pretreatment improved the energy yield of the biogas, i.e. it increased its CH₄ content. Improved removal efficiencies accompanied this improvement in the energy and volume yields of the biogas for volatile solids content, COD and BOD₅.

Footnotes

Acknowledgments

The authors would like to thank the Directorate General for Scientific Research and Technological Development (DGRSDT) for their support. Also, they would like to express their appreciation for the valuable time that anonymous reviewers have dedicated to the review process.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Mohamed El Amine Dahou

Mohamed El Amine DAHOU holds a doctoral degree in Process Engineering. He is a Lecturer at Department of Hydrocarbons and Renewable Energies, Ahmed Draia University, Adrar-Algeria. He conducts research on bio-energy which includes biogas digesters and biomass gasification.

Mohammed HADJKOUIDER is a Lecturer in the Ahmed Draia university-Adrar-Algeria since 2014. He received the Magister in earth sciences (remote sensing and surface geochemistry) from Kasdi Merbah University-Ouargla-Algeria (UKMO) in 2012 and He received the PhD in earth sciences (Hydrogeochemistry) from Kasdi Merbah University-Ouargla-Algeria (UKMO) in 2019. Since 2013, speaker at international conferences in Algeria, Tunisia, and Turkey, before 2014 he has been an engineer at Sonatrach Algerian Oil and Gaz.

Siham DEHMANI is a Lecturer at the Department of Hydrocarbon and Renewable Energies, Ahmed Draia University, Adrar-Algeria from February 2020 until today. She is a researcher who is interested in Sustainable Renewable Energies, Wastewater Treatments, and Microalgae Biomass Production.

Abdelmadjid HABCHI holds a doctoral degree in Process Engineering and currently Lecturer in the Faculty of Science and Technology; Department of matter science at the University of Ahmed Draia, Adrar, Algeria

Said SLIMANI holds a doctoral degree in hydraulics engineering. He does research on Environmental Engineering, Civil Engineering and Irrigation and Water Management. His current project is 'Assessment of aquifer vulnerability region of Adrar.

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Compound	Concentration %
Compound	Untreated	1.0% NaOH	2.5% NaOH	5.0% NaOH
Hydrogen (H₂)	3.18	1.66	0.84	2.07
Carbon dioxide (CO₂)	35.51	31.06	27.54	34.78
Hydrogen sulfide (H₂S)	1.08	1.12	1.22	1.09
Oxygen (O₂)	0.08	0.03	0.05	0.06
Nitrogen (N₂)	5.83	6.92	5.58	6.61
Methane (CH₄)	54.32	59.21	64.77	55.39