Biomass conversion strategies and wastewater reuse: a deep focus …

Biomass conversion strategies and wastewater reuse: a deep focus …

A wide variety of eco-friendly and zero-waste techniques have been developed for biomass conversion and valorization of its residues and by-products, including the water fraction and organic residues. The wastewater reuse is one of the best strategies for water security, sustainability, and resilience. Today, the innovative technologies for biomass conversion and energy production allow the recovery of wastewater with better and safer features than traditional municipal effluents.

Depending on the moisture content of the starting feedstock, the hydrothermal liquefaction (HTL) process can generate up to 95% of wastewater (HTL–WW) that is generally rich in nitrogen, phosphorus, sulfate, micronutrients, and minerals. While HTL–WW is currently recycled through biological systems like microalgae cultivation and anaerobic digestion, the possibility of using it as irrigation water for agriculture represents a promising approach to reduce the increasing pressure on freshwater resources.

Biomass Conversion Strategies

Different approaches are commonly used for biomass conversion, including direct combustion, biochemical processes (e.g., anaerobic digestion, hydrolysis, and fermentation), and thermochemical processes (e.g., pyrolysis, gasification, and liquefaction).

Direct combustion involves burning biomass in the presence of oxygen at high temperatures (800–1650 °C) to produce heat, electric power, and fuel gas. However, this method is not commonly used due to its high costs and low-quality products.

Biochemical processes like anaerobic digestion generate a biogas mixture primarily composed of methane and carbon dioxide, as well as a nutrient-rich sludge that can be used as a fertilizer or substrate for microalgae cultivation. Hydrolysis and fermentation break down complex polysaccharides into sugars, which are then transformed into ethanol and other chemicals by microorganisms.

Thermochemical processes include gasification, which transforms biomass into syngas (e.g., carbon monoxide, hydrogen, and methane), and pyrolysis, which rapidly heats biomass in the absence of oxygen to produce bio-oil, char, and pyrolytic liquid. Another thermochemical technology is hydrothermal liquefaction (HTL), which is considered the most cost-effective and eco-friendly for converting organic biomass.

Wastewater Reuse

The wastewater generated during HTL, known as HTL–WW, is not only a potential waste stream but also a valuable by-product due to its unique chemical composition, which is free of pathogens and harmful substances. The characteristics of HTL–WW depend on the feedstock used, but it is generally rich in organic and inorganic compounds, including nitrogen, phosphorus, and sulfur, as well as micronutrients and minerals.

Agricultural Irrigation

One of the promising avenues for HTL–WW valorization is its use as irrigation water in agriculture. Compared to traditional municipal wastewater, HTL–WW has the advantage of being free of human pathogens due to the high temperatures and pressures involved in the HTL process, which can also degrade hazardous compounds and emerging contaminants.

The chemical composition of HTL–WW, with its high concentrations of organic matter, nutrients, and salts, makes it a potential source of irrigation water and crop fertilizers, helping to reduce the use of synthetic fertilizers and the associated environmental impacts. However, to comply with the regulatory guidelines for wastewater reuse in agriculture, reliable dilution or filtration strategies may be required to address parameters such as chemical oxygen demand (COD) and electrical conductivity.

Industrial Applications

In addition to agricultural irrigation, HTL–WW can also be considered for other industrial applications, such as aquifer recharge and water treatment processes. The recovery of valuable compounds from HTL–WW, such as organic acids, alcohols, and phenols, can further contribute to the circular economy approach.

Sustainable Waste Management

The valorization of HTL–WW is a crucial step in the development and widespread adoption of this eco-friendly technology, as it increases its overall sustainability. This is particularly important in the context of the European Green Deal, which aims to foster the transition towards a climate-neutral economy by reducing greenhouse gas emissions and promoting the use of renewable energy sources.

Circular Economy

The concept of “circular bioeconomy” combines the principles of the circular economy (reusing, repairing, and recycling) with the bioeconomy, which utilizes renewable biological resources to produce energy and other valuable products. In this context, the efficient valorization of HTL–WW as a source of water, nutrients, and other compounds contributes to the circular economy approach by reducing waste and creating new revenue streams.

Integrated Biorefineries

The integration of HTL technology with other biomass conversion processes, such as anaerobic digestion and microalgae cultivation, can further enhance the overall efficiency and sustainability of the system. By recycling the HTL–WW through these biological systems, the organic content can be converted into renewable energy (e.g., biogas), while the nutrient-rich streams can be used to cultivate microalgae, which can then be fed back into the HTL process.

Renewable Energy Production

The HTL process is designed to produce high-value bio-oil, which can be used as a renewable transportation fuel or as a feedstock for the production of bio-based chemicals. By integrating the HTL technology with efficient wastewater management and valorization strategies, the overall energy efficiency and greenhouse gas reduction potential of the system can be significantly improved.

Environmental Impacts

The adoption of HTL and the reuse of HTL–WW in agriculture and other industrial applications can have a positive impact on the environment, contributing to water conservation, soil remediation, and the reduction of greenhouse gas emissions.

Water Conservation

The use of HTL–WW for agricultural irrigation can help to reduce the pressure on freshwater resources, especially in regions facing water scarcity, such as the Mediterranean basin. This approach can also provide economic benefits by avoiding the costs associated with groundwater extraction and traditional wastewater treatment.

Soil Remediation

The organic matter and nutrients present in HTL–WW can be beneficial for soil health and fertility, potentially reducing the need for synthetic fertilizers and improving the sustainability of agricultural practices.

Greenhouse Gas Emissions

The integration of HTL technology with efficient wastewater management and renewable energy production can contribute to the overall reduction of greenhouse gas emissions, supporting the European Union’s goal of achieving carbon neutrality by 2050.

In conclusion, the valorization of HTL–WW through its use in agricultural irrigation, industrial applications, and integrated biorefinery systems represents a promising approach to enhancing the sustainability and circular economy potential of biomass conversion technologies. By leveraging the unique chemical properties of HTL–WW, stakeholders in the European energy and agricultural sectors can work towards a more sustainable and resilient future.

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