**6.5 Biorefinery feedstock: bioenergy and biofertilizer production**

Sustainable energy sources and organic fertilizers are essential to meet human energy consumption and agricultural needs, as hydrocarbon sources are depleting at an alarming rate [24]. WH can be explored for the production of bioenergy and bio-fertilizer due to its high hemicellulose and cellulose content, rapid growth on the water surface, accessibility to various tropical and sub-tropical areas, and quick proliferation after being gathered. WH offers many benefits as it can flourish on the water without competing for crops and vegetables [28]. Some techniques for valorizing WH biomass as bioenergy and organic fertilizer resource feedstock incorporate fuel briquette by biomass densification through mechanical conversion, biogas through anaerobic digestion; biohydrogen and bioethanol through hydrolysis and fermentation; syn-gas, biochar, and bio-oil through thermochemical conversion via pyrolysis, gasification, and hydrothermal liquefaction (HTL); and organic fertilizer creation by composting [14]. However, fresh WH biomass may contain up to 95% water, which may complicate harvesting and processing [6]. The biofuel yields from thermal processes such as combustion, gasification, and pyrolysis generally suffer from wet biomass resources, which necessitates pretreatment and dewatering [16]. As such, the scope of this chapter is limited to the biochemical and mechanical conversion of WH biomass via fuel briquette, bioethanol, and biogas production. This is based on the intention of this chapter to focus on economical benefits for rural communities, which require that approaches are less technologically complex yet economically feasible, as well as being based on local socio-economic capacity and the existing energy situation in Ethiopia.

### *6.5.1 Fuel briquette production*

Briquetting is the densification of biomass to enhance the properties with more added values by increasing the energy density of different biomass residues [31]. The possibility of WH transformation to the briquettes has been evaluated for two decades, which was reviewed recently [73]. In terms of combustion characteristics, WH briquette showed a higher calorific value in comparison to the mangrove and firewood [79]. Carnaje [23] produced and tested the highest calorific value (16.6 MJ/ kg) with 30:70 charcoal/molasses ratio WH briquette. Charcoal briquetting from WH weed is practiced by Kenyan researchers based on Lake Victoria WH biomass source and they proposed that development as a suitable technology for the briquetting of charcoal dust from the pyrolysis of WH [24]. It is suggested that a small-scale WH charcoal briquetting industry could have several beneficial aspects for the lakeside communities; for instance, providing an alternative income, source of biomass, improvement of the lake shore environment through the removal of WH, improved access to the lake and less risk to maritime transport, and reduced health risk associated with the presence of WH. It also serves as a potential alternative to firewood and charcoal, alleviation of pressure on other biomass fuel sources, such as wood, thereby reducing deforestation and associated soil erosion.

#### *6.5.2 Ethanol production*

Bioethanol is made from the fermentation of biomass and is a promising alcoholic biofuel existing in the market today because of its clean combustion. WH, because of its low lignin content is an alluring source of biomass, as cellulose and hemicellulose

### *Invasive Water Hyacinth Challenges, Opportunities, Mitigation, and Policy Implications… DOI: http://dx.doi.org/10.5772/intechopen.106779*

are all the more effectively changed over to fermentable sugar accordingly bringing about a lot of utilizable biomass for the biofuel business [28]. Production of bioethanol from WH demonstrates that in parallel with the physical control approach of gathering and landfill, it is financially viable to deliver bioethanol from the gathered biomass [28]. Nevertheless, some studies have reported high bioethanol yield in the absence of cellulase [24, 26]. The study conducted by Manivannan et al. [80] revealed that bioconversion of WH to bioethanol using two sequential steps of acid hydrolysis followed by fermentation with Candida intermedia NRRL Y-981 produced a maximum bioethanol yield of 0.21 g/g with a productivity of 0.01 g/l/h. The yield can be improved by integrating low-cost pretreatments followed by fermentation with improved bioethanol-producing microorganisms and will play a critical role in making the process economically viable [16, 24].

Several investigations proposed that pretreatment of WH biomass for ethanol generation is important and requires a moderately high temperature and strong acid/ alkali pretreatments given that WH has low sugar and high lignocellulose substance; thus, energy cost is generally high, making it hard to accomplish a positive energy balance [6, 24, 26, 28]. Notwithstanding its commitment to energy enhancement, the creation of bioethanol utilizing WH as a feedstock can not just control the fast invasion of WH yet can likewise add to carbon discharge reduction and water quality improvement. While the production cost of bioethanol is high, ecological qualities assume a significant job in the financial support of the generation. The coupled utilization of WH as a phytoremediation plant and bioethanol feedstock is a potential reaction to green advancement techniques [28].

#### *6.5.3 Biogas production*

Many scientists have suggested that the valorization of invasive WH plants for biogas production plays a significant role in controlling weed infestation [24]. One of the options is anaerobic (AD) digestion, which takes place in a reactor or digester in the absence of oxygen, and the produced biogas can be used as a heat source for cooking, lighting, or heat energy to provide shaft power for generating electricity [6]. In addition to biogas, digestate from the AD process is utilized as a biofertilizer for soil conditioning and mushroom cultivation media [6, 81, 82]. WH biomass contains much water, rich in crude protein, and rich in nitrogen, cellulose, hemicellulose, and other natural substances with the C:N ratio (10-30:1) [4, 6]. Due to the reliance of the AD process on the activities of the microbial consortium, the maximum yield of biogas production depends on several parameters [30, 61]. Pretreatment [6], optimization of process parameters, suitable digester design, stimulation of microbial populations, and co-digestion with other organic wastes have all been used to increase the biogas generation yield of WH biomass [29, 83].

Furthermore, WH's energy potential is significant and encouraging. According to Castro and Agblevor, 2020 [84], one tonne of fresh biomass can produce 846.5 MJ of energy, with just 6.8 percent of the total energy required for mechanical harvesting. As a result, while power is necessary to collect the WH biomass from its infested water body, the total energy provided by the biomass is more than enough to keep operations running. Annually, 50 kg/m<sup>2</sup> ash-free WH biomass can be produced, with daily biomass productivity of 0.04–0.08 kg/m<sup>2</sup> [85]. The annual energy potential of one tonne of collected biomass is comparable to 13.3 m<sup>3</sup> biogas or 18.35–18.75 kWh electricity. Up to 75%, higher methane levels can be obtained, increasing electricity production by 25 MJ/kg [86]. That is an incredible opportunity for communities that have been severely affected by the WH invasion and are economically dependent on agriculture to manage weeds, generate income, and profit from self-sustaining energy resources.

#### *6.5.4 Biofertilizer*

The agricultural sector is in crisis because of the lack of cheap and accessible sources of organic nutrients to sustain the growing demand for food caused by overpopulation, especially in developing countries [6]. Biofertilizers are organic material of natural origin that provides one or more nutrients essential to plants for their growth. One of the most available strategies for soil fertility remediation is the valorization of WH weeds. The presence of phosphorus (P), potassium (K), and nitrogen in the WH biomass, and the C/N ratio make it a suitable substrate for composting as a biofertilizer [86, 87].

Composting is a high-temperature aerobic microbial disintegration process that is one of the most widely used ways of producing organic fertilizer from WH biomass [47]. WH Compost is an excellent natural supplement for sandy soils due to its hygroscopic nature and high moisture retention properties [64, 86]. A study by Vedya and Girish [17] shows that the WH plant can be used as a biofertilizer when mixed into the soil, boosting the performance of wheat plants. The study included control experiments that did not employ WH compost; physical and chemical properties were studied. Physical parameters such as root length, percentage germination, shoot length, and biomass content; shoot ratios were studied. The study also assessed chemical characteristics such as chlorophyll, reducing sugar, and protein content. According to the findings, both physical and chemical metrics exhibited greater values when compared to the control.

Therefore, composting WH biomass is a potentially feasible solution for WH biomass valorization on a large scale or in a commercial setting [88]. It is also a step toward overcoming the increasing food demand caused by overpopulation, particularly in developing countries, by utilizing the cost-free sustainable biofertilizer derived from WH biomass, increasing socio-economic benefits to rural communities while also assisting in the control of the WH invasion [86, 87].
