Demissie Dawana Kechea,
Zemed Menberu Fetanub,
Wudinesh Zawuga Babisob and
Akiber Chufo Wachemo*b
aArba Minch University Water Resource Research Center, P. O. Box 21, Arba Minch, Ethiopia
bFaculty of Water Supply and Environmental Engineering, Arba Minch Water Technology Institute (AWTI), Arba Minch University, P. O. Box 21, Arba Minch, Ethiopia. E-mail: akiberchufo@yahoo.com; Tel: +251911813633
First published on 17th March 2022
Water hyacinth (Eichhornia crassipes) is characterized as an aquatic plant that can grow very rapidly and freely float over water, and covers the top surface of water. It prevents the penetration of sunlight and reduces nutrients and oxygen from water bodies, and can adversely affect the aquatic ecosystem; however, it can be used in the biogas production as the sole feedstock for anaerobic digestion due to high contents of carbohydrates and biodegradable matter. Thus, this study was aimed to remove water hyacinth from Lake Abaya as a renewable energy resource for biogas production through anaerobic digestion. A lab-scale anaerobic batch reactor was applied to assess its biogas and methane generation potential. The results showed that the total solids of 91.9% at dry basis and volatile solids of 84.82% of TS. The biogas and methane production potential of 474.92 mL g−1 VS and 213.92 mL g−1 VS was recorded during 45 days HRT at 37 ± 1 °C. The daily methane production discloses accelerated increment starting from day 8 up to the peak point on day 12 (2170 mL d−1). The biochemical conversion of substances in water hyacinth to biogas was recorded as follows: total volatile solids (96.1%), cellulose (92.5%), hemicellulose (88.2%), and chemical oxygen demand (99.3%). Based on results, E. crassipes can be the sole feedstock for the anaerobic reactor to produce biomethane, while the effluents can be applied as organic fertilizers.
Now a day's serious socio-economic and environmental problems are the encroachment of invasive plants, such as water hyacinths, in lakes, ponds, and rivers. Water hyacinth (WH) is an aquatic plant that can grow very rapidly and free-floating over the water’ surface. This is due to the leaves and stem containing air-filled sacs, which help them to stay afloat on water. The invasive potential of this plant has created massive environmental problems in many areas. It has shown an extremely high growth rate and the coverage of the whole body of water by forming a thick compact carpet. This thick carpet prevents the penetration of sunlight underwater, and thus the oxygen level in water decreases4 and numerous problems, including damage of aquatic plants and animals, irrigation problems preventing water transportation, and a breeding place for mosquitoes leading to rise in the mosquito population.5
Ethiopia is one of the countries facing the existence of WH in its water bodies.6 The invasion of water hyacinth on the water bodies in Ethiopia was noticed in 2012 on Lake Tana. Since then, the plant has shown tremendous expansion on Lake Tana. Due to its quick expansion, it is becoming a serious challenge for society to get rid of it, which is now not only in Lake Tana, but also in Lake Abaya. Ethiopia is endowed with huge resources of water and wetland ecosystems such as rivers, lakes, swamps, floodplains, and man-made reservoirs.7 These bodies of water are in danger of being infected by WH due to its rapid propagation if the condition remains unchanged. Attempts to control the weed have caused high costs and labor requirements, leading to nothing but temporary removal of the water hyacinths.8 Therefore, this leads to the search for another alternative method of water hyacinth management, which can be easily implemented locally.
WH is rich in essential nutrients, nitrogen, and has a high content of fermentable matter and hence the locally grown WH is a potential biomass for biogas production. Apart from bio-gas, the digestate obtained after AD is rich in macronutrients needed for the healthy growth of plants; hence, it is a useful soil conditioner and manure with no detrimental effects on the environment.9 Thus, the application of WH as a source of renewable energy to substitute fossil fuels will provide effective management of this invasive plant on water bodies, and in the meantime, the effluent after AD could be used as a suitable organic fertilizer for plant nourishment. Therefore, this study aims to determine the biomethane potential and substance conversion during anaerobic digestion while analyzing the gradual changes in system stability from the water hyacinth collected from Lake Abaya, Ethiopia.
Parameters | Water hyacinth | Inoculum |
---|---|---|
Total solids of dry basis (%) | 91.9 ± 0.12 | 3.0 ± 0.07 |
Total volatile solids (%) | 84.82 ± 13.04 | 56 ± 0.21 |
Ash contents (%) | 18.49 ± 0.07 | 0.72 ± 0.04 |
Total nitrogen contents (%) | 1.82 ± 0.32 | 1.96 ± 1.19 |
Total organic carbon (TOC)% | 37.73 ± 2.00 | — |
COD (mg L−1) | 11![]() |
182 ± 14.14 |
pH | _____ | 7.6 |
Ammonia nitrogen (NH3–N), mg L−1 | 767.2 | 641.2 |
Preliminary tests were conducted before doing the main research to know about the optimum pretreatment chemical dose and optimum pretreatment time intervals that would be used. Four preliminary tests were conducted using urea pretreatment applied for hydrolysis of long chained lignocellulosic compounds in water hyacinth. Organic loading of 65 g L−1 water hyacinth was mixed using distilled water in the ratio of 1:
10 (w/w) to make a slurry according to the pervious study of ref. 11. Then, the urea solution with a different concentration was added to the slurry to make an optimum concentration for pretreatment ahead of the AD process. This was done by measuring the drop of pH until its changes became approximately constant. This hydrolysis experiment was done using 2%, 3%, 4%, and 6% urea solution based on the dry matter content of water hyacinth. Daily changes in pH were measured to increase the accuracy of preliminary alkaline hydrolysis experimental results. In this study, urea was used as a pretreatment reagent because it has an additional benefit in improving the C/N ratio in AD reactors, increases digestate quality to use as fertilizer, and is less corrosive to the equipment.12
Based on the preliminary results of the experiment, the optimum pretreatment time and optimum dose of urea suitable to hydrolyze lignin bond was found to be five days and 4% urea. Accordingly, the required amount of urea was dissolved in distilled water prior to the actual experiment. The solution was then added into different 1.5 L anaerobic digesters and thoroughly mixed with water hyacinth in the prepared digesters. These digesters were then sealed and kept in the laboratory at ambient temperature for five days.
The system was checked for airtightness before starting the experiment in order to prevent the leakage of gas into and out of the reactor system, and it was fed and withdrawn once in the entire reaction period.
Fig. 2 shows the experimental set-up used to measure the volume of biochemical methane potential in the anaerobic reactor through adsorption of CO2 in alkaline solution; in the reactor, a 1 L graduated measuring cylinder was inverted in the container, and in the intermediate container filled with 0.1 M Ca(OH)2, which traps CO2 to produce CaCO3 precipitate and allow CH4 to pass through the glass bottle having a volume of 1.5 L. Ca(OH)2 was used to trap CO2 because of its economic feasibility and less corrosive nature for the testing environment.
Moisture content in the sample was determined gravimetrically by hot air oven-drying at 80 °C, according to ref. 17.
![]() | (1) |
Analysis of lignocellulosic components was conducted using the direct method adapted from ref. 18. Two g of sample was taken and boiled with ethanol (4 times) for 15 min, after that thoroughly washed with distilled water, and retained in the oven for dry weight at 40 °C overnight, then allocated into two parts in which one part was considered as fraction “A”. The other part of the residue was treated with 24% potassium hydroxide for 4 h at 25 °C, washed thoroughly with distilled water, dried at 80 °C overnight, and the dry weight was taken as fraction “B”. 2 g of samples was taken again and treated with 72% H2SO4 for 3 h to hydrolyze the cellulose and refluxed by 5% H2SO4 for 2 h. Distilled water was used to remove H2SO4 by washing it completely, dried at 80 °C in the oven overnight, and the dry weight was taken as fraction “C”.
Cellulose = B − C | (2) |
Hemicellulose = A − B | (3) |
Lignin = C itself | (4) |
The determination of organic carbon (OC) in the sample was performed using Walkley & Black chromic acid wet oxidation method ref. 19. Total organic carbon content in the sample was determined using eqn (5) as shown below:
![]() | (5) |
Total nitrogen content was determined by digestion with strong sulfuric acid in the presence of a catalyst within Kjeldahl digestion setup, and ammonium nitrogen was distilled. Ammonium nitrogen was measured using acid titration from the distillate.20
The content of crude protein was determined using the modified micro-Kjeldahl method using the formula below ref. 21.
Crude protein content (%) = micro-Kjeldahl nitrogen content (%) × 6.25 (based on the assumptions that nitrogen contains 16% of protein) | (6) |
The method consists of allowing the biogas produced to pass through a 0.1 M Ca(OH)2 solution, which was filled in a separate container in order to capture and convert the CO2 fraction present in the biogas into CaCO3, and then, the measuring cylinder was submerged in a container having the same liquid, inside of the measuring cylinder was attached a plastic tube, which was connected with a separate reactor at the other end through which volumetrically the fraction of methane produced bubbles in the liquid and filled the cylinder, replacing the liquid, and the gas volume can then be read as shown in Fig. 2. Moreover, the cylinder was supported by an iron stand. The amount of liquid replaced corresponds to the volume of methane produced.23 Furthermore, phenolphthalein solution was added to the barrier solution. Phenolphthalein imparts pink color in alkaline solutions, and a change in color indicates whether Ca(OH)2 is available to absorb CO2 or not.22
Once the cumulative biogas and methane production were determined, then the composition of biogas was calculated stoichiometrically as CH4 and CO2 in liters and converted to normalized volume using standard temperature and pressure (STP).24 The CH4 yield was calculated to measure the volume of methane produced relative to the amount of volatile solids added (VS) based on the formula given in (eqn (5)).
![]() | (7) |
Mineralization of lignocellulose takes place in an anaerobic digestion process under complex biochemical conditions. In this work, evaluation of the daily performance of the anaerobic reactor showed that there were irregularities in process routines, which caused fluctuating biogas production trends. Anaerobic digestion of urea pretreated water hyacinth at mesophilic conditions (37 ± 1 °C) was performed to assess continuous gas production trends and related featuring objects during the process. Fig. 3 shows daily biogas production, and Fig. 4 shows cumulative biogas yield of water hyacinth.
![]() | ||
Fig. 3 The average daily volume of biogas and methane gas generated from day 1 to day 45 at STP. The experiment was conducted at 37 °C. |
The daily biogas production fluctuates during the entire digestion period, and the system achieves biogas production in between two major peak points. The daily biogas production showed rapid increment on the first day to the 5th day and then reached a minor peak point on the 7th day. The change continued, and it decreased from the 8th day to the 11th day before it reached a major peak point on the 12th day, and then it was gradually decreased up to a stable point. More specifically, the first minor peak biogas production (1800 ± 28.28 mL d−1) was recorded on the 7th day of the anaerobic digestion period. However, the reactor performance directly came down after 4 days to the biogas production volume of 500 ± 70.71 mL d−1 on day 11. The good thing was the process recovered quickly from the shocking decrease in biogas production and raised the production up to the major peak point (2170 ± 28.28 mL d−1) of daily biogas production on day 12. The daily methane yield followed the same style in the parallel way like daily biogas production except in the first six days, where the methane production was very slow compared to the rest of the digestion periods. Carbon dioxide is the 2nd abundant gas constituent in biogas produced from AD of lignocellulose.27 In the present study, the composition of CO2 dominated in the initial stage of digestion periods to reach the highest value on day five (1180 mL d−1), but after day 6, the CO2 concentration gradually decreased in biogas and became relatively constant after the 35th day of digestion time.
The cumulative biogas production within the digestion period is shown in Fig. 4. Thus, the biogas production was continued up to 45 days. The total volume of methane gas production within 45 days' digestion period was 12000 mL at STP. Whereas the corrected cumulative biogas yield of the experiment was 474.92 (mL g−1 VS−1), while the cumulative methane potential yield of the experiment was 213.92 (mL g−1 VS−1). This result was comparable with a similar study reported by ref. 28, and it was done in the Silpakorn University of Thailand where biogas and methane yields (458.44 L kg−1 of VS and 237.37 L kg−1 VS, respectively) and the components of CH4 and CO2 were 63.2% and 36.7%, respectively, from water hyacinth. The percentage composition of this study is comparable with a similar study reported by ref. 29 with the composition of CH4 and CO2 (68.89% and 29.08%, respectively).
pH is one of the significant parameters used to evaluate the performance of anaerobic microorganisms during the process of bio-methane production. There are different kinds of microbial groups taking parts in the AD process, which have varied optimum pH values for their optimal-growth rates. For example, a pH range of 5.0 to 6.0 is suitable for acidogens, while pH from 6.5 to 8.5 is more convenient for the methanogens group.30 In most cases, pH value generally drops after a few days of microbial incubation. The pH of WH was in the acidic range, whereas inoculum was in the neutral range, so the pH of the substrate was adjusted to the optimum range before AD with 1 N Ca(OH)2 solution.
In this study, pH of the water hyacinth at the initial stage was 5.91, so the initial pH was adjusted before AD by 1 N Ca(OH)2 solution. There was a partial change in the pH value during the digestion period, at the initial AD period, which was in the range of 7.0–7.2. To investigate the system stability, nine additional reactors were arranged in the same condition to check the system sensitivity every five days, discarding one reactor and measuring the sensitive parameters throughout the digestion period.
In the beginning, a slight decrement in pH was shown but the system sustained its stability for methane conversion, and the pH at the end of AD was 7.9. This might be the influence of inoculum and urea pretreatment that played significant roles in sustaining the pH-buffering of the reactor without the addition of alkali to the reactor.
The alkalinity of the anaerobic reactor can be defined as the tendency of the intermediate solutions to maintain stable buffer capacity caused by existing organic acids.31 To achieve the constancy and neutralize VFAs, the total alkalinity of the system must be higher than 2000 mg L−1,32 unless it could affect the action of microbial groups during methane production. In this study, the alkalinity of the system at the beginning was 2200 mg L−1, and it was in the range, but it fluctuated during the digestion period, as shown in Fig. 5.
![]() | ||
Fig. 5 System stability and sensitivity indicating parameters measured every five days during anaerobic digestion periods. |
The optimum content of the ammonium ion (as well the ammonia) inside the reactor has a great role in the stability of the AD process. The optimal ammonia concentration increases the stability process by ensuring adequate buffer capacity of the methanogenic medium.33 Due to the use of urea as a pretreatment chemical, it buffers pH drops during digestion periods. In this study, ammonia concentration before AD was 762.5 mg L−1, and it fluctuated during the digestion period and after AD was 785.8 mg L−1, as shown in Fig. 5.
The total solid (TS) and volatile solid (VS) at dry basis of WH and inoculum were 91.9% and 84.82%; and 3.0% and 56%, respectively. Furthermore, the detailed properties of the starting materials are shown in Tables 1 and 2.
Parameter | Water hyacinth | Literature results | Reported by |
---|---|---|---|
Moisture content (%) | 89.31 ± 0.12 | 90.00 ± 5% | 41 |
Dry matter (DM) (%) | 8.81 ± 0.14 | 8.56 | 26 |
Total solids of dry basis (%) | 91.90 ± 0.12 | ||
Total volatile solids (%) as percentage of TS | 84.82 ± 13.04 | 89.13% | 37 |
Ash content (%) as percentage of TS | 18.49 ± 0.07 | 20.2 | 46 |
pH | 5.91 ± 1.00 | 5.80 | 41 |
Cellulose (%) | 25.39 ± 0.73 | 24 | 47 |
Hemicellulose (%) | 26.89 ± 2.51 | 30 | 47 |
Lignin (%) | 19.05 ± 2.64 | 16 | 47 |
Total organic carbon (%) | 37.73 ± 2.00 | 38.4 | 47 |
C/N | 20.73 ± 1.20 | 25 | 48 |
Crude protein contents (%) | 11.38 ± 0.70 | 11.8% | 49 |
Food to microbe ratio, TVS (g kg−1)/inoculum (TVS g kg−1) | 1.51 ± 0.12 | 1.5 | 50 |
Sodium (%) | 0.78 ± 0.01 | 0.58 ± 0.40 | 18 |
Potassium (%) | 2.60 ± 0.02 | 2.78 | 47 |
Calcium (mg L−1) | 32.10 ± 2.20 | — | — |
Magnesium (mg L−1) | 48.10 ± 4.21 | — | — |
Chloride (mg L−1) | 728.74 ± 5.89 | — | — |
Total alkalinity as CaCO3 (mg L−1) | 2200.00 ± 56.50 | — | — |
Phosphate (PO43−) (%) | 12.00 ± 0.02 | 15.07 | 46 |
Nitrate-nitrogen (NO3−–N) (%) | 0.37 ± 0.01 | — | — |
COD (mg L−1) | 11![]() |
15![]() |
29 |
The initial physicochemical analysis of the inoculum and substrate are important factors to be analyzed as it influences the overall anaerobic system stability and biogas production.
The high contents of volatile solids and COD of water hyacinth indicate that water hyacinth is favorable for the AD process to biogas production.
Carbon/nitrogen (C/N) is an essential parameter that determines biogas production. The C/N ratio should be in the range of 15 to 30 for biogas production operating at full potential. The carbon/nitrogen ratio of WH in this study was 20.73, and it was found to be in the optimal range for bio-methane production as recommended by ref. 34. This shows that WH collected from Lake Abaya can serve as a sole feedstock for bio-methane production without co-digestion or mixing it with other substrates.
The results showed that the total solids and total volatile solids of the WH were 91.9 ± 0.12 and 84.82 ± 13.04%, respectively. The volatile solid content was higher than the biodegradable range of 75–80% stated by ref. 35. Volatile matter (VM) is the most flammable biomass component.36 This indicates that large content of WH was biodegradable during the AD process, and hence it can serve as an important substrate for bio-methane production. The content of total solids and VS in WH is comparable with a similar study reported by ref. 37, which is 7% and 89.13%, respectively. The contents of total solids and VS of rice straw were 93.90% and 84.31%, respectively, as reported by ref. 38. The ash content of this biomass is the second largest content with the value of 18.49 ± 0.07% in this study. The ash content is influenced by the inorganic elements, which are part of biomass.39 The result obtained in this study was comparable with a similar study reported by ref. 40, which is 20.1%.
Physically, water hyacinth is rich in terms of moisture content. In this study, the moisture content was 89.31 ± 0.12% of its total solids, and it is comparable with a similar study reported by ref. 41, which is 90 ± 5%.
The components of lignocellulosic compounds in WH (lignin, hemicellulose, and cellulose) were measured based on the direct method. From the result, the hemicellulose content is 26.89 ± 2.51%, and hemicellulose is associated with cellulose and contributes to the structural component of the biomass, and cellulose content from the result is 25.39 ± 0.73%. Cellulose is the main structural component in the cell wall, which shows that WH has a good potentiality for biogas production, and it is comparable with a similar study reported by ref. 25, in which the hemicellulose content is 30%, and cellulose content is 24%.
Parameter | After AD | % removal efficiency |
---|---|---|
Total solids (%) | 4.30 ± 0.06 | 95.3 |
Total volatile solids (%) | 3.30 ± 0.11 | 96.1 |
Cellulose (%) | 1.91 ± 0.09 | 92.5 |
Hemicellulose (%) | 3.17 ± 0.66 | 88.2 |
Lignin (%) | 16.59 ± 2.11 | 12.9 |
Crude protein contents (%) | 0.16 ± 0.02 | 98.6 |
COD (mg L−1) | 82.00 ± 1.22 | 99.3 |
Phosphate, PO43− (%) | 5.00 ± 2.02 | 58.3 |
Nitrate-nitrogen, NO3−–N (%) | 0.23 ± 0.01 | 37.8 |
Calcium (mg L−1) | 16.03 ± 3.11 | 50.1 |
Magnesium (mg L−1) | 1.64 ± 0.24 | 96.6 |
Potassium (%) | 2.10 ± 0.2 | 19.2 |
Chloride (mg L−1) | 339.40 ± 44.00 | 53.4 |
Total alkalinity as CaCO3 (mg L−1) | 420.00 ± 11.02 | 80.9 |
Ammonia nitrogen (NH3–N) (mg L−1) | 641.20 ± 12.03 | 16.4 |
pH | 7.89 ± 0.01 | — |
Lignocellulose components | Before pretreatment | After pretreatment |
---|---|---|
Cellulose | 25.39 ± 0.73 | 23.41 ± 0.42 |
Hemicellulose | 26.89 ± 2.51 | 22.67 ± 0.91 |
Lignin | 19.05 ± 2.64 | 14.24 ± 1.1 |
The removal efficiency of total volatile solids and COD is an essential parameter to determine the stability and efficiency of the AD system. Normally after hydrolysis, cellulose, and hemicellulose of the lignocellulosic substrate are converted into lower molecular mass organic compounds or COD (usually into simple soluble sugars). The higher the removal, the higher the stability of the system.41 In this study, total volatile solid and COD removal efficiencies were 96.1% and 99.3%, respectively. It is indicated that the higher removal rate of COD/TVS had higher methane production.30
The main components of carbonaceous compounds in water hyacinth are cellulose, hemicellulose, and lignin. The effectiveness of the pretreatment applied and AD performance can be evaluated based on the rate of lignocellulose degradation.42 But the hydrolysis and cellulose crystallinity was basically hindered by an unmanageable lignin network. The differences in lignocellulose concentration before and after AD are given in Table 3. The total composition of lignocellulose in water hyacinth was 71.33%. Out of this value, hemicellulose, cellulose, and lignin accounted for 26.89%, 25.39%, and 19.05%, respectively. However, the bio-methane production would be more affected by the accessibility and degradability of hemicellulose, cellulose, and the extent of lignin inter cross-linkage. In the present study, the conversion rate of hemicellulose, cellulose, and lignin were 88.2%, 92.5%, and 12.9%, respectively. The substance conversion efficiency of AD of WH was calculated as per the equation below:
![]() | (8) |
All major plant nutrients such as nitrogen, phosphorous, and potassium were retained in the reactor during the AD process. The previous study conducted by ref. 45 has shown that the digestate of WH can be taken as a bio-fertilizer when incorporated into the soil, which increases the performance of plant growth. With high nutrient content, the digestate from AD was considered to be an effective alternative fertilizer and environmentally friendly relating to chemical fertilizer.29
Three different groups were designed, and these were replicated ten times, giving a total of thirty experimental groups. Replication was done to increase the accuracy of the experiment.43 The tomato crop was planted using digestate as organic fertilizer, inorganic fertilizer (DAP), and control. The control treatment was without both bio-fertilizer and DAP application. The performance of water hyacinth as a bio-fertilizer was compared to the chemical fertilizer (DAP). The indicators of physical appearance performance were studied like plant height, the yield of fruit at maturation, and weight of fruits per plant (kg).
The height of the plant increased consistently from week one to week twelve for all the treatments, as shown in Fig. 6. Even though the control had shown the highest increment in height within four weeks, but the stem was very thin and had a probability of breaking compared with both treatments, having an average height of 80 cm. After week four, the treatment (using bio-fertilizer) had shown an increment in growth stage up to 124 cm, and it proved to be more effective with high strength, branched, and high yield than using chemical fertilizer. Application of bio-fertilizer statistically led to improvement of the plant height and strength.
![]() | ||
Fig. 6 Comparison of the growth performance among bio-fertilizer, DAP, and control on tomato height. |
The harvesting stage of the tomato plant started in the 10th week. The average yield of the plant treated by bio-fertilizer per plant was 0.4 kg, whereas the yield of the plant treated with DAP and control started in week eleven. In week eleven, the yields treated with bio-fertilizer, DAP, and neutral were 0.8, 0.5, and 0.2 kg, respectively, per plant and also in week twelve, the yields treated with bio-fertilizer, DAP, and neutral were 0.7, 0.4, and 0.2, respectively, as shown in Fig. 7.
In general, the result related to physical appearance (Fig. 8) and the product yield showed that the application of digestate from water hyacinth significantly influenced the plant height and production yield among the treatment performance of tomato plants in all the parameters evaluated. In general, the performance of the crops using digestate was very good.
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Fig. 8 Experimental pots with tomato plants after ten weeks: (A) bio-fertilizer and (B) inorganic fertilizer. |
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