Biogas Generation from Co-Digestion Waste Systems: The Role of Water Hyacinth

Abstract

Using biomass as a renewable energy source has earned tremendous interest from researchers in recent decades, especially because the technology is environmentally benign. This article reviews the recent methods for generating biogas from water hyacinth (WH, Eichornia crassipes), arguably the world’s most evasive aquatic macrophyte. Therefore, various economic, environmentally benign, and renewable procedures that enhance biogas production from WH biomass are reviewed. WH has been co-digested with numerous waste types, including poultry droppings, municipal wastes, animal tissue wastes, pig wastes, cow dungs, etc., recording varying success degrees. Other studies focused on optimizing the operation parameters, such as mixing ratio, contact time, pH, temperature, organic loading rate, etc. We observed that most attempts to generate biogas from WH alone were not promising. However, when co-digested with other biomasses or wastes, WH either increases the process rate or improves the methane yield content. Also, the potential of WH as a phytoremdiator-cum-biogas source was investigated. This chapter provides mathematical models, scale-up installation models, and specific experimental results from various studies to guide future study plans toward optimizing CH4 generation from WH co-digestion.

1. Introduction

Biogas, an energy source comprising CH₄, CO₂, and traces of some gaseous impurities, is generated via biomethanation, i.e., anaerobic digestion of substrates. Irrespective of the substrate, typical biogas is composed of 50–80% CH₄, 20–50% CO₂, 5–10% of H₂, 1–2% of N₂, ≈0.3% water vapor, and traces of H₂S and H₂O_(g) [1, 2]. Regardless of their proportions, CO₂ and H₂S are the major impurities in biogas. Therefore, post-production cleanup processes are required to remove them for optimum performance of the final product. Usually, CO₂ is absorbed into hydroxides of Ca, K, or Ba (Eq. (1)), while CuSO₄ removes H₂S, FeSO₄, Pb(NO₃)₂, or FeCl₃ (Eq. (2)). For CO₂ removal, NaOH is an efficient absorbent, although KOH is 27% more effective, using only 125 kWh/Tor CO₂ energy [3]. Otherwise, to minimize the cost and avoid additional waste generation, the pristine gas stream could be bubbled through water to remove both gases, albeit with less efficiency [4].

$$\mathrm{Ca}{\left(\mathrm{OH}\right)}_{2\left(\mathrm{aq}\right)}+{\mathrm{CO}}_{2\left(\mathrm{g}\right)}\to {\text{CaCO}}_3+{\mathrm{H}}_2\mathrm{O}$$E1

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2{\mathrm{Pb}}_{\left(\mathrm{aq}\right)}+{\mathrm{H}}_2{\mathrm{S}}_{\left(\mathrm{g}\right)}\to {\text{2CH}}_3{\text{COOH}}_{\left(\mathrm{aq}\right)}+{\mathrm{PbS}}_{\left(\mathrm{s}\right)}$$E2

2. Biodigestion process

A typical biodigester is made of concrete, metal, or other material that permits anaerobic biomass fermentation [5]. For optimum performance, the operational and ambient conditions must be diligently considered. Several factors that affect biogas production efficiencies include pH, temperature, type and quality of the substrate, mixing speed and consistency organic loading, formation of highly volatile fatty acids, and inadequate alkalinity [6]. The retention (turn-over) time is the period required for organic materials to be decomposed entirely toward achieving maximum biogas yield. Fertilizers and mineralized water are the usual valuable by-products of this process [5].

Research into biogas technology in Africa gained momentum in the last decade. For instance, in Nigeria, biogas production from Bambara nut chaff [6], agricultural waste [7], and abattoir waste [8], and the performance evaluation of a biogas stove for cooking [9] have been reported. Furthermore, biogas generation from co-digested substrates, such as spent grains and rice husk [10], banana and plantain peels [11], pig waste and cassava peels [12], sewage and brewery sludge [13], have also been experimented. In most cases, co-digestion enhances methane yield by ≈60%. Similar studies were carried out in other African countries such as Uganda [14], South Africa [15], Sudan [16], etc.

Generally, plant-based biofuels are environmentally clean energy, with a high potential of lowering fossil fuel consumption to the barest minimum in the near future [17]. Over the past decade, several studies have focused on producing biomethane using lignocellulosic residues of high abundance and low cost [18, 19]. According to Bekkering et al. [20] and Holm-Nielsen et al. [21], biogas can be used as fuel and fuel cells to generate heat, steam, electricity, produce chemicals, upgrade natural gas grids via injection, etc. Elsewhere, Jantsch and Mattiasson [22] discussed how anaerobic digestion could treat wastewater and organic wastes, yielding biogas as a valuable by-product. The four major sources of biogas production are livestock waste, landfill gas (LFG), activated sludge from wastewater treatment plants, and IIC (industrial, institutional, and commercial waste) [22, 23, 24].

Biomethanation occurs in four main steps (Figure 1) viz. hydrolysis [23] , acidogenesis [24], acetogenesis [26], and methanogenesis [27]. Methane is the main component of biogas (50–70%). Other components include CO₂ (30–40%) and traces of H₂S and H₂O_(g) [28]. The respective equations for the four steps are provided as Eqs. (3)–(6):

Figure 1.

Process flow of the degradation of organic material through anaerobic digestion [24, 25].

$$\text{Hydrolysis}:\left({\mathrm{CsH}}_{10}{\mathrm{O}}_5\right)+{\mathrm{nH}}_2\mathrm{O}\to \mathrm{n}\left({\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\right)$$E3

$$\text{Acidogenesis}:\mathrm{n}\left({\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\right)\to {\text{3nCH}}_3\text{COOH}$$E4

$$\text{Acetogenesis}:{\mathrm{CH}}_3\text{COOH}\to {\mathrm{CH}}_4+{\mathrm{CO}}_2$$E5

$$\text{Methanogenesis}:{\mathrm{CO}}_2+{\text{4H}}_2\to {\mathrm{CH}}_4+{\text{2H}}_2\mathrm{O}$$E6

Four major microbial groups are respectively involved: the hydrolytic-fermentative bacteria (hydrolyze complex organic compounds into simple ones), fermentative bacteria (convert the simple organic compounds into volatile fatty acids, yielding H₂ and CO₂), acetogenic bacteria (convert the fatty acids into acetic acid), and methanogenic archaea (produce CH₄ either from acetate or from H₂ and CO₂) [24, 25].

3. Factors affecting biogas yield and production

The quality of manure influences the methanogenic diversity in a reactor, and the overall conversion efficiency of manure to CH₄ is influenced by the retention time [29], pH, oxygen level, NH₃-N, and volatile fatty acids (VFA) contents, and temperature [30, 31]. Biogas can be produced under psychrophilic (10–30^oC), mesophilic (20–50°C), and thermophilic (50–75°C) conditions [2]. Mesophilic and thermophilic conditions present different reactor designs, operational advantages, and drawbacks. Most anaerobic digesters are designed to operate at mesophilic (40^oC) or thermophilic (55^oC) temperature to maximize biogas yield [32], whereas, between 40 and 50°C, methanogens are inhibited. Elsewhere, anaerobic digestion temperature was optimized at 25–38°C (mesophilic conditions), with temperatures near 38°C showing more excellent (≥95%) digestion stability. Likewise, a mesophilic treatment at 38°C reportedly destroys 99.9% of pathogens [33].

Similarly, the C/N [34, 35], slurry concentration, mixing rate, and bacteria type (starter) are other crucial parameters that influence biogas quality and yield [36]. Typically, the organic loading rate (OLR) ranges between 0.5 and 3 kg VS (volatile solids) per m³ per day [29]. Table 1 lists the average C/N of various substrates used for biogas production [70, 71, 72]. Typical C/N values/ranges for biogas production are as follows: liquid cattle manure (6–20), chicken manure (3–10), liquid swine manure (5), straw (50–150), grass (12–26), potatoes (35–60), sugar beet/beet foliage (35–46), cereals (16–40), fruits and vegetables (7–35), mixed food waste (15–32), slaughterhouse waste—soft tissue (4), slaughterhouse waste—guts (22–37), food waste (3–17), distillery waste (8), etc. An increasing C/N (10–30) increases the formation of fatty acids in the process [34, 35]. If the fatty acid concentrations are not sufficiently high, methanogenesis could result.

S/N	Starting material	CH4 yield	Substrate mixing ratio/concentration	Biogas yield	Observation	Experimental condition	Ref.
WH only
1	WH only	72.53%	5–30 g/L		Shoots only generated 6.86% extra methane	39°C substrate; concentration 25 g/L	[37]
2	WH only		C/N 35	202 L/kg TS	TS not a significant factor	TS 1.59% ; 60 days	[38]
3	WH only	50–65%	C/N 16		No link between C/N ratio and production	51 days	[39]
4	WH only		1:4 (WH: Wate); C/N: 15	245 L/kg VS		60 days; 30–37°C	[40]
5	WH only	65%		380 L/kg VS	Pretreatment with NaOH yielded most methane composition (71%)	35°C; size 2 cm; 60 days	[41]
6	WH only		C/N: 25.9	75 L/kg TS		pH: 6.65; TS: 8%; size = 2 cm	[42]
7	WH only			360.1 L/kg TS		40 days; Size: 1 cm; 45°C	[43]
8	WH only			221 L/kg TS	Increase of 75.61%	Size = 1 cm; pretreated at 60°C for 24 h	[44]
Co-digested WH
9	WH + cow dung		C/N : 32.0	108 L/kg TS	Microbial consortium; 7.26%	60 days; size = 15 cm; TS 5–10%	[45]
10	WH + MW1			16 L/kg		36-37°C ; pH 6.0–7.4	[46]
11	WH + MW	60.5%	4:1 (WH: waste)	230 L/kg VS		TS = 4%; 15 days	[47]
12	WH + other biomass	68.67%		237.4 L; CH4/kg VS		F:M = 1:1; 60 days	[48]
13	WH + other biomass		Duckweed: WH = 7:3; C/N = 16.4	20.55 L/kg VS		8% TS; size < 6 mm	[49]
14	WH + other biomass		Salivinia: WH = 0.5:1	406 L/kgVS	VFA affected WH in 3:1		[50]
15	WH + other animal wastes	52.8%	1:2 (WH: Buffalo dung)	2.86 L/day	Pretreatment increased biogas production by 102%; methane by 51%	Size: 6 mm	[51]
16	WH + poultry droppings		2:8 9; (WH: poultry manure)	34.65 L/kg		40 days	[52]
17	WH + cow dung	49–53% upgraded to 73%	3:1 (WH: cow dung)	3.2 L/kg		22.8–36.6°C	[53]
18	WH + cow dung	63.7%			Optimal OLR un-pretreated		[54]
19	WH + MW		F/M = 10.01 : 0.03	152 L/kg; TS (daily)	TS = 6.76%		[55]
20	WH + cow dung	65%		270 L/m^3	22%	Size = 5 cm; 10 days	[56]
21	WH + cow dung	56.4%	2:1 (WH: cow dung) C/N 10:1	3050 L/day		40 days; size = 2–5 cm; 28–36.7°C; pH = 6.5–7.8	[57]
22	WH + pig waste	88.3%	1:3 (WH: Pig waste)			27–34°C	[58]
23	WH + pig waste	1.4 kg : 1L (piggery waste:WH) C/N = 30:1				TS = 14.02%; pH = 6.0–7.2; 12 days	[59]
24	WH + MAW2	64.9%	3:8:9 (pig dung: WH: Poultry droppings)	307 L/kg		pH = 6.5; TS = 9.09%; 52 days	[60]
25	WH + MAW	62.14%	2:2:1 (WH: cow dung: poultry dropping)			0.02 m	[61]
26	WH + MAW			0.255 kg/m^3	OLR of 1.5 kg/m^3 yielded most biogas	36–37°C; TS = 9.98%; 60 days; pH = 5.0–7.4	[62]
27	WH + animal waste	68%	3:7 (WH: animal waste)	14.09 L/kg	Increasing temperature from 24 to 32 increased production by 186%	24°C	[63]
28	WH + animal waste + others		4:4:2 (waste WH: cow manure)	60 ppm CH4; 10,744 ppm		21 days; size = 2 cm	[64]
29	WH + animal waste		C/N = 20/1; 5:3:2 (Prosopis juiflora pods: Duckweed: WH	96.6 L/kg			[65]
30	WH + animal wastes + others			1 L/day		35°C; size = 3–5 cm; TS = 5.6%; OLR3 = 50 g/L	[66]
31	WH + animal wastes + others			273.3 L/kg		40 days	[67]
32	Phytoreme diation		C/N 26.9	5195 m^3			[68]
33	Phytoreme diation			23,650 cc/kg dry weight	Growing in 20% effluent increased production	35°C; 21 days	[69]

Table 1.

Recent studies on generating biogas (CH₄) from water hyacinth (WH) or water hyacinth-based (co-) digestion.

¹

MW = municipal waste.

²

MAW = multiple animal wastes.

³

OLR = organic load ratio.

Methanogens are sensitive to rapid temperature change, while thermophilic methanogens are more temperature-sensitive counterparts. Therefore, temperature should be kept exactly at ±2°C [2].

Some researchers have investigated the optimal pH for microbial performance during anaerobic digestion. According to Yadvika et al. [73], the pH within the digester should be kept within 6.8–8.0, whereas Thy et al. [74] concluded that 6–8 pH range is the optimal pH. At the onset of the acid-forming stage of the digestion, the pH may be <6.0. However, it could be >7.0 during methane formation and maintained because it is sensitive to acidity. In a properly operating anaerobic digester, a pH of 6.8–7.2 converts volatile acids to CH₄ and CO₂ [75]. The pH is the most suitable indicator for plausible digester instability after gas production [29]. Initially, the pH would decrease as the organic matter undergoes acetogenesis. However, as the methanogens rapidly consume the acids, the pH rises, stabilizing the digester performance. Fermentative bacteria require a pH > 5.0 to become enzymatic, while methanogenic activity takes place at a pH range of 6.2–8.0, optimized at ±7.1 [29, 76]. In addition, other phenomena, such as the dissociation of important compounds (ammonia, sulfide, organic acids, etc.), are directly affected by pH [32]. Methanogenic bacteria are generally susceptible to pH and do not thrive at pH < 6.0 [77].

Homogeneous mixing within the digester improves the contact between the microorganisms substrate, improving the bacterial ability to obtain required nutrients. Also, by homogenization, scum formation and temperature increase within the digester are minimized. However, excessive mixing can disrupt the microorganisms; therefore, slow mixing is preferred [78]. According to Kossman et al. [79], with other parameters fixed, a well-agitated substrate can increase biogas production by 50%.

4. Various biomasses for biogas production

The anaerobic fermentation of manure for biogas production does not compromise the quality of the fertilizer supplement because the nitrogen and other substances remain in the treated sludge [80]. In the absence of appropriate disposal methods, animal dungs can cause various environmental and health problems, such as pathogenic contamination, odor pollution, and greenhouse gas emission [81]. Rain may flush these wastes into neighboring water bodies or percolate underground, springs, and wells used for sanitation and domestic purposes [82]. Poultry and livestock wastes often contain high concentrations of human pathogens, spilled feed, bedding materials, fur, wastewater, feed residues, feces, and urine. Therefore, the waste should be effectively managed to minimize environmental and public health risks. Such practices might result in acute gastrointestinal upset (e.g., nausea, diarrhea, and vomiting). Also, contact with affected surface waters during recreational activities can cause skin, ear, or eye infections.

4.1 Recent advances with WH only

Some researchers have investigated biogas generation from WH, either solely or co-digested with other waster materials (Table 1). Being tagged as a waterway menace, WH has been identified as a substrate for economically feasible biogas production [83, 84].

The effect of substrate concentration, particle size, and incubation period of dry WH shoots (WHS) and whole WH (WWH) plants on biogas production has been reported [37]. The CH₄ yield increased with substrate concentration till the sixth day (25 g/L) before declining. WHS consistently had a higher CH₄ yield than WWH, especially for every particle size. Recently, Syafrudin et al. [38] evaluated the optimization of biogas production using liquid anaerobic digestion. They used central composite and complete factorial design to determine the optimal values of enzyme concentration, C/N ratio, and total solid that generates the highest biogas volume. The optimum conditions for the C/N ratio were within 30–40 and 6% of the enzyme, with no significant effect of total solids. In another research, Patil et al. [40] chopped and ground WH to a fine paste and mixed with water in five ratios to evaluate the optimal level of dilution to produce the highest volume of methane. They reported that the slurry with WH 1:4 water had the highest volume of gas.

The effect of microwave pretreatment of fresh and dried WH on biogas production was also studied [42]. It was observed that the optimum condition is 560 W and 9 min contact time on fresh WH. However, in these conditions, CH₄ production is inhibited. In addition, Rozy et al. [43] studied how various parameters affected biogas production rate and volume. Their maximum biogas yield was achieved at 45°C and a pH of 7, with 1 cm particle size and 40% inoculum concentration and 0.2 mM of MnCl₂. Later, a study on the effect of organic (citric) acid pretreatment of WH on biogas production was carried out [44]. Later, response surface methodology was used to optimize the pretreatment parameters. About 76% increase in biogas yield was achieved. For optimum utilization of WH biomass, Hudakorn et al. [48] evaluated the production of biogas and biomass pellets from WH. They stated that, although biogas production commenced from the first day, flammable biogas didn’t start to yield till the 10th day.

4.2 Recent advances with co-digested WH

4.2.1 WH + municipal wastes

Nugraha et al. [45] studied the effect of food to microbial (F/M) ratio on biogas yield from WH. They stated that the optimum F/M ratio and TS level were 10.0 and 6.76%, respectively. They concluded that biogas production reduces inversely with the F/M ratio. The same year, Ukwuaba [46] evaluated the performance of biogas yield from co-digesting kitchen wastes and WH. Temperature was identified as the optimal parameter. The highest and lowest gas pressure was observed at the 25th and 37th days, respectively. Previously, Hernandez-shez et al. [47] had investigated the potential of generating biogas from co-digesting WH with fruit and vegetable waste. They optimized the biogas production in terms of total solids concentration. Co-digestion increased the biogas produced. A total solid concentration of 80:20 (WH/food waste), corresponding to a C/N ratio of 20, was the optimal condition to avoid pH correction. However, for a continuous co-digestion, they recommended an organic loading rate of 2 kg VS m⁻³ d⁻¹ and 15 days retention time.

4.2.2 WH + poultry droppings

Some co-digestion of WH with poultry droppings has been carried out. Ojo et al. [52] studied the best mix of WH with poultry manure (PM) that produces maximum biogas. The authors calculated for the optimum biogas production rate, a factor of the data collected using the following equation:

$${{\mathrm{G}}^1}_{\max }=-\mathit{abc}{\left[1-c\right]}^{\mathrm{c}-1}$$E7

where G¹_max = biogas production rate, a = ultimate biogas production, b = pseuso-biogas production velocity (rate constant), c = shape factor.

They observed that mixing WH and PM at 2:8 produced the highest volume of biogas. Furthermore, the volume of biogas produced increases slightly with temperature. Also, the highest biogas yield was observed on the 18th day. It was concluded that 2 WH: 8 PM is the best-aided WH digestion mix in daily biogas production, a cumulative volume of biogas produced, and a maximum biogas production rate. Elsewhere, Patil et al. [41] studied the effect of different pretreatment on biogas yield from WH. Notably, alkali treatment had no significant effect on the biogas produced from WH blended with poultry waste.

4.2.3 WH + cow dungs

Cow dung is a popular co-stock material for WH biodigestion is a popular one. Nugraha et al. [55] used response surface methodology (RSM) to study the optimization of biogas production by solid-state anaerobic digestion to discover the optimum total solids (TS), C/N, and microbial consortium (MC) for biogas production from a mixture of WH and cow dung. They then discovered that TS and MC had the most and least effect on biogas yield, respectively. The maximum biogas yield was obtained at TS concentration range 5–10%, C/N of 32.09, and MC of 6%. Somewhere else, Adegunloye et al. [58] evaluated the optimal ratio of variation of WH to pig dung to generate the maximum methane amount.

The ambient temperature affected the temperature in the digester as the temperature in the digester was higher than the ambient temperature by 1–3°C. The authors observed that 1:3 of WH to pig dung produced the highest amount of CH₄. In another report, Jayaweera et al. [39] evaluated the biogas production from WH grown under different nitrogen concentrations. The author carried out this study for four months at mesophilic temperatures using batch-fed anaerobic reactors. WH was grown in various folds of total nitrogen then co-digested with CD. They mentioned that WH roots contain high fiber and lignin content, thus making them unsuitable as a substrate. They recommended a retention time of 27–30 days for optimum results.

A process by which volatile fatty acids (VFAs) were extracted from WH and the VFAs laden slurry was developed [56]. The extracts were then used as a feed supplement to the conventional biogas digesters. The authors discussed that WH contains 60 g/kg of TS, requiring a large digester for significant biogas production. The VFAs were extracted by charging acid-phase reactors with a mixture of WH and cow dung slurry. The reactors were aerated and the pH kept within 5.5–7.0 to enhance acidogenic bacteria growth but that of methanogenic bacteria. Figure 2 is a pictorial explanation of the experiment. For the same TS input, the VFA supplemented the feed, yielding 22% higher biogas amount. However, no significant changes happened to methane yield.

Figure 2.

Schematic representation of the process developed by the authors [56].

Eltawil et al. [85] studied the effect of stirring, dry oxidation, and water scrubbing processes on biogas quality from different substrates. Using five digesters equipped with handle stirrers (Figure 3), the gas produced from the digester was flushed through scrubbers to reduce H₂S and CO₂ concentrations of the biogas. They observed that stirring increased the biogas production rate by 45% for WH and cow dung mixtures but did not significantly impact the CH₄ volume of the biogas. We gathered that water scrubbing and dry oxidation removed 95% CO₂ and 97% H₂S. Therefore, this technology is recommended for developing countries where low-cost technology is needed.

Figure 3.

Schematic diagram of the digester with stirring blade [85].

Similarly, Akinnuli et al. [59] studied the performance of pig dung and WH for biogas production. The output gas was passed through KOH and anhydrous CaCl₂ to remove CO₂ and moisture, respectively. The authors stated that mixing pig dung and water hyacinth in the ratio 1.4:1 was optimum for biogas production. They recommended the digestion be carried out during the summer because low temperatures lead to low biogas generation. Elsewhere, a fixed dome digester was designed for biogas production using cow dung and WH (Figure 4) [57]. The digester is a semi-batch reactor composed of a fermentation chamber, feed and digestate pipes and, a fixed dome on top for biogas storage. This configuration was recommended as a cheaper alternative for natural gas production.

Figure 4.

Schematic diagram of fixed dome digester [57].

4.2.4 WH + multiple animal wastes

Akindele et al. [60] reportedly co-digested pig dung, poultry droppings, and WH anaerobically. Lignocellulosic materials and animal manure co-digestion enhanced digestibility, biogas production, and equipment utilization. The respective mixing ratio of 3:9:8 was optimum for methane yield. Also, no gas production in the first four days as the enzymes adapt to a new environment. CH₄ production lapsed from 8th to 16th day. The biogas production increased with fermentation until the 40th day, with the highest biogas production observed on the 52th day.

Earlier, Fadairo et al. [61] co-digested WH with cow dung and poultry droppings. Their optimum mixing ratios were 2:2:1 and 1:1:0. The lower the WH dosage, the lower the biogas generated. However, the substrate containing WH alone produced the least biogas. Cow dung influenced biogas production than poultry droppings, attributed to the ammonium ions in the latter.

The effect of Organic Loading Rate (OLR) on biogas production systems has also been researched [62]. The rate of adding feedstock required alteration for optimal growth of methanogens, which directly influence biogas produced. The authors noted that direct charging above 1.5 kg/m³ inhibits the growth of the methanogens. Recently, the co-digestion of (WH) biomass with ruminal slaughterhouse waste (RSW) was evaluated [63]. The highest and lowest biogas yields were with the substrate of solely slaughterhouse waste and WH, respectively. Also, the co-digestion of the waste with WH (5–50%) significantly reduced the retention time by 26 days, whereas if the proportion is >50%, no further impact on retention time will occur. The study recommended co-digestion of 30% waste and 70% WH at 32°C.

In some cases, WH and animal wastes are dosed with other waste materials. Sa’adiah et al. [64] evaluated biogas production from co-digesting Tofu waste, WH, and cow manure. They observed that adding more WH inhibited the production of biogas. They then recommended that mixing WH, tofu, and CD at 2:4:2 for optimal biogas production. Also, Prabhu et al. [65] investigated the anaerobic co-digestion of Prosopis juliflora pods with WH, dry leaves, and cow manure, modeled the biogas production kinetics using a modified Gompertz equation to examine the cumulative methane production (Eq. (8)).

$$Y=M\ast \mathit{exp}\left\{-\mathit{exp}\left[\frac{\mathit{Rm}\ast e}{M}\left(\lambda -1\right)+1\right]\right\}$$E8

where Y = cumulative methane production (L at time t), M = maximum methane production potential (L-CH₄), Rm = maximum methane production rate (L-CH₄/d), λ = Lag phase time (day), E = constant (2.71).

The authors noticed that methane composition was higher in biogas yielded by WH-rich mixtures than other mixtures. E.g, WH + dry (2:3) achieved the maximum methane yield of 80%. The coefficient of determination (R²) between the experimental data and the model ranged as 0.991–0.999.

Moreover, Shah et al. [66] explored the potential of three plants (WH, giant reed, and maize) and poultry waste for biogas generation, using WH with 13% hemicellulose and poultry waste as inoculum. WH had the highest volatile solids, soluble solids and, a better C/N ratio. Thus, it was a relatively superior biogas substrate. The highest biogas yield occurred on the 11th day. From the four substrates, WH contributed the highest to biogas production. Likewise, Otaraku et al. [86] modeled the cumulative biogas produced from sawdust, cow dung, and WH. They concluded that the polynomial model best fitted the cumulative biogas production at any given day, with R² > 0.9. Similarly, the potential of biogas production from mixtures of WH, cassava peels, and cow dung using standard microbial techniques has been reported [67]. The highest total biogas yield from co-digesting the three substrates was noted. It was concluded that the prescribed treatment combinations could be facilitated with or without starter culture.

4.2.5 WH + other biomasses

To co-digest WH with other biomasses, Ogunwande et al. [49] constructed biodegradation and maximum biogas yield models based on first-order kinetics to describe and predict maximum biogas yields from the co-digestion of duckweed (DW) and WH. They made three assumptions: there was a correlation between the volatile solid and degradation of biogas yield at any time; a certain quantity of volatile solids in the substrates was recalcitrant to degradation within the retention time allowed; there was no lag time before the beginning of volatile solids degradation. They noted that biogas production started within the first day of digestion. The following biodegradation model was provided:

$${\mathrm{C}}_{\mathrm{t}}=\left({\mathrm{C}}_0–{\mathrm{C}}_{\mathrm{e}}\right){\mathrm{e}}^{–\mathrm{kt}}+{\mathrm{C}}_{\mathrm{e}}\mathrm{at}0\le \mathrm{t}$$E9

where C₀ is VS concentration in the substrates at the beginning of the experiment (%, db), C_t is the VS concentration in the substrates at any moment (%, db), t is the time, k is the VS degradation rate constant based on the quantity of VS in the substrate (D⁻¹), C_e is the remnant VS concentration after retention time (%, db). Also, the researchers provided a biogas yield model equation as follows:

$${\mathrm{Y}}_{\mathrm{t}}={\mathrm{Y}}_{\mathrm{m}}\left(1-{\mathrm{e}}^{-\mathrm{kt}}\right)$$E10

where Y_t is the biogas yield at time t and Y_m is the maximum biogas yield.

Elsewhere, Bhui et al. [50] explored the role of volatile fatty acids (VFAs) in WH and Salvinia plant digestion. The biogas production from both plants was highest at an inoculum to substrate ratio of 3:1. It was concluded that acetonic, propionic, and butyric acid were the common VFAs found in the plants that played a major role in biogas production. In the same year, the effects of hydrothermal pretreatment on biogas yield were investigated [51]. A dramatic surge in biochemical oxygen demand (BOD) occurred after the first 30 min of the pretreatment. The increasing BOD revealed that the microbes have larger access to cellulose, a substrate for biogas production. The biogas yield rate started to increase at 30 min of pretreatment, peaking 60 min. Longer hydrothermal pretreatment could reduce the methane yield. Also, the WH:buffalo dung had no significant effects on biogas yield without hydrothermal pretreatment. They recommended a 1:2 WH and cow dung mixing ratio for optimum biogas yield.

4.3 Phytoremediation with WH

Phytoremediation of polluted waster using WH has been researched widely [87]. However, some researchers have delved into adopting post-phytoremediation WH biomass for biogas generation. Singhal et al. [69] co-digested WH and channel grass used to phytoremediate paper mill and distillery factory effluents for biogas production. The plants grown in the effluents were chopped, sun-dried, and oven-dried at 60°C, before pulverizing to fine particles. It was then mixed with cow dung slurry and digested. The digester feed used for phytoremediation produced more biogas than pristine ones. Likewise, the effect of temperature and feedstock size on biogas production of WH used for phytoremediation was reported [88]. It was observed that improved biodegradation of organic matter occurred at high temperatures. Therefore, the digestion of WH should be done at thermophilic conditions with smaller particle size. Similarly, Kumar et al. [68] assessed the biogas production potential of WH that was initially used for phytoremediation of sugar mill effluent. They concluded that the biomass had high potential for biogas production than virgin counterparts.

5. Conclusions and recommendations

The invasive presence of water hyacinth (WH) on our waterways often hinders numerous socioeconomic, agricultural, and ecological processes, tagging the macrophyte an environmental nuisance. However, we have identified that it has some inherent benefits when exploited appropriately. WH is potential biomass for biogas production, and this fact has been adequately studied.

Biogas generation from WH is temperature-dependent, taking place between 25 and 50°C, provided the temperature is kept steady at ±2°C because methanogens are sensitive to abrupt temperature change. Also, pH can be used as a performance indicator due to acetogenesis bacteria’s pH requirement of >5.0 pH, while methanogens require pH in the range of 6.2–8.0 to form CH₄. Overall, WH aids some other biomass biogas generation potential while it retards some others. By co-digesting Salivinia grass and WH at 1:0.5, the highest volume of biogas (406 L/kg VS) generated was reported, whereas co-digestion with pig dung generated the highest CH₄ content (88.3%).

Moreover, co-digesting WH with cow dung (most popularly researched) produced more biogas than with poultry droppings and buffalo dung. Although homogenization aids biogas yield by 50%, it showed no significant effect on CH₄ content. Also, in most cases, increasing C/N and F/M ratios inhibits CH₄ formation. Optimally performing wet scrubbers and dry oxidation for cleanup could remove up to 95% CO₂ and 97% H₂S from the raw biogas generated. It was also identified that digesting WH shoots without the roots could up biogas yield significantly.

Finally, because hydrothermal pretreatment before digestion has recently been identified to enhance biogas generation, we recommend that the effects of various pretreatments for methane generation from WH be further researched.