Biological Treatment and Clarification System combines the biological treatment system and clarification into a compact design utilizing common wall construction. The biological treatment is located at the periphery in a ring-shaped area while the clarification stage is in the central region. The treatment system is fully expandable for complete biological nutrient removal. The system can also be designed to incorporate aerobic sludge holding, flow equalization, post aeration and chlorine contact chamber. The treatment system will be complete with mixers for the anaerobic and anoxic zones (if applicable), fine or coarse bubble aeration for the biological portion, coarse bubble aeration for the aerobic holding tank, piping, valves, pumps and blowers. The system will incorporate a clarifier with spiral blades for rapid solids removal. The controls will provide a single point for operation of the equipment and incorporate dissolved oxygen probes and TSS probes for automating operation of the system. The clarifier includes scum removal and scum removal pumps. The digester is designed for a fill and decant cycle. A decant pump is included to return the supernatant back to the plant for treatment. The Biological Treatment and Clarification System can be installed in concrete tanks, or field-erected steel tanks.