In most municipal wastewater treatment plants, there is need for the removal of nitrogen, which usually takes place using the combined nitrification - denitrification process. Vigorous recirculation between the aeration and the anoxic tanks is enforced, to ensure complete denitrification. The scope of the present work was to investigate the possibility for nitrification-denitrification process in once-through systems (i.e.: without recirculation), without the need for the addition of extra carbon source (i.e. using the BOD in wastewater as carbon source), using encapsulated microorganisms. The primary aim was to increase the concentration of nitrifiers in the aerated reactor with parallel operation at hydraulic retention times (HRTs) below the doubling time of heterotrophic microorganisms, thus ensuring high ammonia oxidation rate and minimal reduction of organic carbon. The preserved organic carbon may be then used as carbon source at the downstream anoxic reactor. Coagulated and clarified wastewater from the effluent of the primary clarifier of a municipal wastewater treatment plant was used as feed to the system.

The system comprised of two reactors (with 2 L working volume each) configured in series. The first one (aerated tank) contained encapsulated nitrifiers, while the second one (anoxic tank) contained encapsulated denitrifiers. The system operated at HRTs 8, 4 and 3 h (calculated individually for each reactor). The experiments indicated that at HRT equal to 8 h, almost all N-NH4+ was converted to N-NO3- in the aerated reactor, while the total nitrogen (TN) concentration was below 2 mg L-1 at the exit of the system. At HRT of 4 h a slight decrease in N-NH4+ removal was observed at the exit of the aerated tank (N-NH4+ concentration was measured 3.7 ± 0.1 mg L-1). At HRT equal to 4 h, N-NH4+ concentration did not change significantly during the downstream treatment in the anoxic tank, while N-NO3- concentration at the exit of the system was 1.4 ± 0.1 mg L-1. At the lowest HRT (3 h), N-NH4+ concentration was measured between 10 and 11 mg L-1, both, at the exit of the aeration tank and at the exit of the system; while, N-NO3- was measured 2.6 ± 0.2 mg L-1 at the exit of the system. On the other hand, BOD and TOC removal in the aeration tank decreased with the decrease of the HRT. BOD concentration at the exit of the anoxic tank measured 30.3 ± 2.2 and 19.4 ± 1.7 mg L-1 for HRTs 8 h or 4 h, respectively, while it was measured 51.6 ± 7.6 mg L-1 at HRT 3 h. On the other hand, TOC concentration at the outlet was measured 17.5 ± 1.2 and 13.2 ± 0.6 for HRTs 8 or 4 h, respectively, while it was measured 31.1 ± 5.6 mg L-1 at HRT equal to 3 h. Analysis of variance (ANOVA) showed significant variations of all measured parameters with the applied HRT, apart from N-NO3- concentration at the exit of the aerated tank. The latter was attributed to the complete oxidation oft N-NH4+ in the aerated tank at all HRTs.

Based on the efficiency of the system, the volume of the aeration and denitrification tanks of a wastewater treatment plant using encapsulated microorganisms may be designed 16 times smaller, compared to conventional activated sludge plants, while the need for recirculation between the aerated and anoxic tanks may be completely eliminated.

在大多数市政废水处理厂中，需要去除氮，这通常是通过结合硝化-反硝化过程进行的。强制在曝气池和缺氧池之间进行强力再循环，以确保完全脱氮。当前工作的范围是研究一次通过系统中硝化-反硝化过程的可能性（即：无需再循环），而无需添加额外的碳源（即在废水中使用BOD作为碳源），使用封装的微生物。主要目的是在水力停留时间（HRT）低于异养微生物倍增时间的情况下，通过平行操作提高曝气反应器中硝化剂的浓度，因此，可以确保较高的氨氧化速率，并减少有机碳的减少。然后可以将保存的有机碳用作下游缺氧反应器的碳源。来自市政废水处理厂一级澄清池中的凝结和澄清废水用作系统的进料。

该系统由两个串联配置的反应器（每个反应器的工作容积为2 L）组成。第一个（充气罐）包含封装的硝化器，而第二个（厌氧罐）包含封装的反硝化器。该系统在HRT第8、4和3小时运行（每个反应器分别计算）。实验表明，在HRT等于8 h时，充气反应器中几乎所有N-NH4 +都转化为N-NO3-，而系统出口处的总氮（TN）浓度低于2 mg L-1。在HRT为4公顷时，在充气罐出口处观察到N-NH4 +的去除量略有下降（测得N-NH4 +的浓度为3.7±0.1 mg L-1）。在HRT等于4 h的情况下，在缺氧池的下游处理期间N-NH4 +的浓度没有明显变化，而系统出口处的N-NO3-浓度为1.4±0。1毫克L-1。在最低的HRT（3小时）下，在曝气池出口和系统出口处测得的N-NH4 +浓度在10至11 mg L-1之间。而在系统出口处测得的N-NO3-值为2.6±0.2 mg L-1。另一方面，随着HRT的降低，曝气池中BOD和TOC的去除量也随之减少。在HRT 8 h或4 h时，缺氧槽出口处的BOD浓度分别为30.3±2.2和19.4±1.7 mg L-1，而在HRT 3 h时为51.6±7.6 mg L-1。另一方面，对于HRT 8或4 h，出口处的TOC浓度分别为17.5±1.2和13.2±0.6，而在HRT等于3 h时，其TOC浓度为31.1±5.6 mg L-1。方差分析（ANOVA）显示，所应用的HRT在所有测量参数上均存在显着变化，除了曝气池出口处的N-NO3-浓度外。后者归因于所有HRT充气池中N-NH4 +的完全氧化。

基于系统的效率，使用包封微生物的废水处理厂的曝气池和反硝化池的容量可以设计为比传统的活性污泥厂小16倍，而在曝气池和缺氧池之间可能需要再循环被彻底淘汰。