The decentralized onsite wastewater system is comprised of a series of elements that each serve a purpose in the treatment process. These elements differ based on region, wastewater strength and desired effluent quality standards, but the large majority of systems all start with one essential structure: the septic tank. The design of this tank is key to the effectiveness of the entire system. It not only provides an environment for physical separation through floating and settling, but should also serve as the catalyst for initial biological treatment as well. During this course, we will examine how tank designs have evolved and what are the essential key aspects for an effective septic tank. We will discuss the importance of watertightness and how to maintain it. We will identify factors that can maximize retention time to lower demand on downstream processes. We will go beyond conventional tanks and discuss newer technologies that can be incorporated into the septic tank to increase its effectiveness and expand its capacities. We will review national and international standards that apply to septic tanks.