This study discusses the effect of application of High Rate Algal Pond (HRAP) as a pre-treatment unit on nitrogen transformation and removal in Sub-surface flow Gravel bed Constructed Wetland (SGCW). Experiments were carried out at the University of Dar es Salaam main campus wastewater treatment facility in two different settings. In Phase I, effluent from primary facultative pond was fed directly to unplanted gravel bed SGCW, with a discharge of 0.69 m³/d. During Phase II, the HRAP with 0.6 m depth, received effluent from primary facultative pond at a flow rate of 1.2 m³/d and discharged it to unplanted gravel bed SGCW with 0.8 m deep, filled to 10 cm above water level with 20 mm diameter aggregates, at a flow rate of 0.69 m³/d. Both HRAP and SGCW were operated at a retention time of 5 days. The results showed that improved nitrogen removal occurred with inclusion of HRAP unit. In Phase I, an average nitrogen removal of 41.3% was achieved, while in Phase II total nitrogen reduction in SGCW improved to 57.3%. These results show the significance of HRAP for better performance of nitrogen removal in gravel bed SGCW. It was further observed that the removal efficiency of nitrate-nitrogen was superior in SGCW phase II (68.2%) compared to SGCW phase I (20.7%). Hence this research proved that provided the influent is fully nitrified in the pre-treatment system such as HRAP, denitrification becomes the major contributor to the nitrogen removal process in SGCW.

本研究探讨了将高速率藻类池（HRAP）用作预处理单元对地下流砾石床人工湿地（SGCW）中氮转化和去除的影响。在达累斯萨拉姆大学的主要校园废水处理设施的两个不同环境中进行了实验。在第一阶段中，来自初级兼性池的废水直接喂入未种植的砾石床SGCW，排放量为0.69 m ³ / d。在第二阶段中，深度为0.6 m的HRAP以1.2 m ³ / d的流量从初级兼性池中流出，然后将其排入深度为0.8 m的未种植砾石床SGCW，填充至水位以上10 cm的20 mm直径为0.69 m ^3的骨料/ d。HRAP和SGCW的保留时间均为5天。结果表明，加入HRAP单元可以改善除氮效果。在第一阶段，平均脱氮量达到41.3％，而在第二阶段，SGCW中的总氮减少量提高到57.3％。这些结果表明，HRAP对于改善砾石床SGCW中的脱氮性能具有重要意义。进一步观察到，相比于SGCW I期（20.7％），SGCW II期（68.2％）的硝酸盐氮去除效率更高。因此，这项研究证明，只要进水在HRAP等预处理系统中被完全硝化，反硝化就成为SGCW中脱氮过程的主要推动力。