An anaerobic lagoon wastewater treatment system has a flow-splitter tank for accurately dividing a flow of raw, untreated wastewater. The system is designed as an earthen basin or as a tank and includes at least one partitioning element for providing two connected wastewater treatment zones. The first wastewater treatment zone in the direction of wastewater flow has a larger volume and is more efficient in the removal of biological oxygen demand (BOD) than the second wastewater treatment zone. The flow-splitter tank divides the incoming wastewater flow into two streams, which are conducted to the first and second wastewater treatment zones, respectively. A predetermined ratio of carbon to nitrogen is maintained within an effluent pumped from the second wastewater treatment zone by adjusting the relative amount of raw wastewater added directly to the second wastewater treatment zone.