On the potential of water hyacinth as a biomass briquette for heating applications

The spread of the fast-growing water hyacinth weed in Harare’s Lake Chivero and other open water sources is threatening the capital city’s main water supply. Environmentalists blame the spread of the weed on the discharge of effluent, particularly raw sewage, into the Lake and its tributaries. The weed has spread, to many other water bodies throughout Zimbabwe. The plant grows rapidly and produces enormous amounts of biomass. Due to its flat top leaf structure, it covers extensive areas of open waterways.

The water weed causes ecological and economic problems by impeding navigation, fishing and recreational activities. The weed’s rapid growth rate creates chronic shortage of dissolved oxygen required by the water’s fauna and the flora [1, 2]. Perna and Burrows [3] noted that the water hyacinth cover on water bodies reduces the gaseous exchanges that take place at the air–water interface and reduce the photosynthetic activity of submerged plants by hindering the penetration of the sun’s rays. Figure 1 shows the clogged Manyame river, one of the tributaries of Lake Chivero.

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In slow-moving water bodies, water hyacinth mats physically slow the flow of water, causing suspended particles to be precipitated, leading to siltation. The reduced water flow can also cause flooding and adversely affect irrigation schemes by clogging the canals and piping systems. Displacement of water by water hyacinth can mean that the effective capacity of water reservoirs is reduced by up to 400 m³ of water per hectare, causing water levels in small reservoirs to fall more rapidly in dry periods [4].

Methods for controlling the water hyacinth weed include manual and mechanical removal as well as chemical and biological interventions. Chemical use is the fastest but it has gross negative impact on the environment and water quality. The chemicals will also have to be removed to make the water safe for human consumption. Although it may be used in conjunction with other control methods, biological control is an essential option in any management plan to control the hyacinth water weed. Biological control is relatively cheap and safe for the environment giving long-term sustainable control with minimal maintenance once the weevil is established. The biological method is ideal for large areas, environmentally sensitive areas, sites where access is not possible for spraying or harvesting, sites susceptible to re-infestation and sites where water is being used for stock watering. The weevils will also disperse to other areas, including inaccessible or difficult terrain. With biological control, there is no danger of off-target damage to, or removal of other desirable plants which can occur with herbicides or manual harvesting.

However, biological control should not be used where eradication is to be attempted unless it is to thin out a large area prior to eradication. Eradication is rarely possible with biological control, and carries the danger that the insect population will die out, and the weed will then return. The ideal situation is to achieve a dynamic equilibrium where both the plant and the insect survive at low levels without causing economic damage [5]. Large-scale mechanical processes usually require machines powered by electricity or liquid fuels. This tends to increase the process costs and damage the environment. The manual process using hand-held tools is slow considering the rate at which the water weed multiplies. Nevertheless, for the purposes of productive plant utilisation, it is the best way to harvest the weed.

It has been reported that water hyacinth weed cannot be used as stock feed because of low protein content. It cannot also be used to make paper because of its short stalk length. However, it can be utilised to remove heavy metals from sewage and as an energy source in the form of briquettes. Substantial research work has been directed towards searching for various ways of realising value in the weed.

The weed is not only affecting water supply to cities and towns in Zimbabwe, but is also negatively affecting the livelihoods of people living close to the infested water bodies. These communities usually consist of the poor and vulnerable who have limited access to basic energy services. In most cases, they use wood and paraffin leading to deforestation, indoor pollution, and increased procurement costs. It is also worth noting that urban dwellers in Zimbabwe have also resorted to the traditional energy sources such as wood due to frequent electricity outages. The country’s sole utility, the Zimbabwe Electricity Supply Authority (ZESA) generates about 60 % of the Country’s peak demand and imports power from South Africa, Mozambique and Democratic Republic of Congo in an attempt to address the shortfall [6]. Urban dwellers are also increasingly using Liquefied Petroleum Gas (LPG) for their heating needs. Imported fuels are, however, beyond the reach of many. The rural people have virtually deforested their environs in search of energy sources. The briquetting of the biomass, especially the foliage which is normally left to waste or burnt in open fires, has the potential to fill the energy gap and ensure energy sustenance in rural communities located close to the infested water bodies.

The abundance of the rapidly growing hyacinth water weed presents both environmental challenges and opportunities. It is envisaged that water hyacinth is a potential biomass material for the production of briquettes because of its high growth yield and availability in large amounts throughout the year. The present research investigates the potential use of water hyacinth from nearby watercourses as biomass material for making briquettes. The calorific value of the water weed briquettes will be compared to other briquette samples made from other leaf species.

The research motivation is how best an environmental challenge can be utilised to alleviate the energy crisis experienced by communities living in and around hyacinth water weed-infested water courses. The results of the study are expected to influence environmental and energy policy decisions on hyacinth water weed harvesting and utilisation.

In this project, fresh water hyacinth plants and other leaf samples were manually collected and briquetted in the laboratory. A molasses binder was used to improve the strength and compactness of the briquettes. A bomb calorimeter was used to measure the heat of combustion of the briquette samples. The GCV of the different briquette samples varied from 14.51 to 20.57 MJ/kg. The water hyacinth briquette had a GCV of 14.55 MJ/kg. This compares favourably to the GCV of wood which varies from 14.40 to 17.40 MJ/kg on dry mass basis. The GCV of briquetted water hyacinth samples was about 70 % of the GCV of the lowest coal grade used for heating applications in agriculture and industry in Zimbabwe.

The water hyacinth briquette samples had average water content of 7.8 %, ash content of 12.4 %, volatile matter 65.7 % and fixed carbon of 21.9 %. The fixed carbon content of about 20 % is in the range of other woody and herbaceous plants. This exercise has shown that water hyacinth biomass can be processed into convenient solid fuel pellets using briquetting technology. The environmentally unfriendly biomass material found in various water ways can be harvested and serve as an alternative energy source.

The production of biomass briquettes from water hyacinth is recommended. This could help alleviate the challenges posed by this water weed. Successful commercialisation of the briquettes can lead to increased incomes, job creation and improved environmental management. The use of briquettes may help reduce the prevalence of the water weed in aquatic bodies and also help reduce the intensive use of wood fuel which usually leads to deforestation.