Abstract
Abstract Bioenergy remains the largest branch of renewable energy, and microalgae are a promising object of research among other types of biomasses whose scale for energy purposes is increasing. On the other hand, the growth of global energy production and urbanization, which results in high rates of municipal waste and wastewater generation, requires the development of integrated technologies that allow waste to be disposed of as fully as possible. Sustainable investments in the production of energy by various technologies are one of the methods to solve this complex problem. In this chapter, we study the methods of microalgae utilization of nutrients from wastewater and by-product liquid waste of sustainable investments from microalgae by hydrothermal liquefaction (HTL) technology. Wastewater has a complex composition, and the treatment of nitrogen and phosphorus and other biogenic elements, as well as heavy metals, using biological objects is optimal and cost-effective. Also the water phase after HTL is a by-product that has limited energy value. Biofuel investments have higher growth rates and at the same time do not compete with the investments in fossil fuels. Biofuel investments' cost of seaweed fuel can be reduced through high-value-added related products, such as food and feed additives, and pharmaceutical and cosmetic products.