This book offers a clear, authoritative, and practice-oriented guide to modern trickling filter technology as a best practice, positioning it as an ecological, economical, and resilient solution for municipal and industrial wastewater treatment. Bridging more than a century of innovation, the book demonstrates why trickling filters remain a cornerstone of sustainable biological treatment systems worldwide. Through rigorous analysis and applications, the book explains the innovation in fundamental principles of biofilm systems, highlighting their efficiency, robustness, and role in achieving high-performance biological purification under diverse climatic and operational conditions. It critically examines classical and modern design philosophies, operational strategies, and process optimization techniques for modern-generation trickling filter systems. The book provides in-depth coverage of integrated treatment approaches, combining trickling filter with activated sludge configurations applied worldwide, as well as novel hybrid systems that incorporate adsorption processes and bioaugmentation. Special attention is given to the removal of emerging contaminants and micropollutants, supported by pilot-scale studies and operational performance data Industrial applications are addressed through detailed discussions of biotower systems for food and beverage wastewater, while decentralized, resource-efficient solutions are explored through combinations of trickling filter anaerobic digestion. The book further evaluates design parameters, operational costs, performance indicators, and future development pathways, linking trickling filter technology to circular economy concepts, water reuse strategies, and sustainable development goals, with relevance to the MENA region. By reviewing theoretical models and current research trends, the book connects engineering practice with public health protection and sustainable water governance. It serves as an essential reference for academics, practicing engineers, decision-makers, regulators, and postgraduate students, and is equally valuable for professionals engaged in environmental engineering and water management worldwide.