4 可持续生物除磷脱氮推荐工艺
4.1 推荐工艺
　　以BCFS工艺为代表的反硝化除磷，及CANON工艺为代表的厌氧氨氧化作为可持续除磷脱氮关键技术的蓝本，荷兰-中国大学间合作研究提出了一针对城市污水处理的可持续除磷脱氮推荐工艺，如图7所示。这个推荐工艺突出COD甲烷化、磷酸盐回收以及处理水回用等与可持续性密切相关的内容。

图7　可持续城市污水处理除磷脱氮推荐工艺
　　为了有效转换污水中过剩COD为甲烷，早年德国人开发的AB法[42]中A段被推荐用于浓缩COD。A段采用很短的污泥龄(8-25 h)；以这种方式，细菌快速繁殖，约70％～80％的悬浮状和溶解状的进水COD能被合成为细菌细胞。在A段后生物污泥被沉淀分离，然后被送往污泥消化池进行消化、转化。与来自二沉池的生物污泥相比较，由A段产生的污泥有着较好的可消化性，最终会导致较低的消化剩余污泥(熟污泥)。A段浓缩COD的同时，也必然将相当数量的氮、磷合成于细菌细胞之中。
　　经A段处理之后水／物流被分成两股：①泥水分离后的上清液；②通往消化池的污泥。上清液中所剩COD被刚好用来脱除剩余的氮和磷。显然，对这股水／物流采用BCFS工艺脱除氮、磷能最小化COD的消耗量；从BCFS工艺中产生的高磷含量污泥也被送往消化池消化。消化后产生的消化液一般氮、磷浓度很高，且水温较高(应至少为30℃)。一些以回收磷酸盐为目的的现场试验表明[43--45]，污泥消化液是污水处理场中最理想的磷源回收之处。因此，污泥消化液首先被引入一沉淀单元内，通过投加镁化合物(如氯化镁等)形成磷酸铵镁化合物(MAP，即鸟粪石)而分离出磷。鸟粪石能被用作植物生长所需之磷源。磷被回收后的消化液采用CANON工艺脱除浓度仍然很高的氨氮最为合适，因为消化液几乎不含COD，加之较高水温对获得较完全的厌氧氨氧化率十分有利。
　　虽然经CANON工艺处理后的出水氨氮浓度一般并不能直接达到排放标准，但此股水流流量很小，同来自于BCFS工艺的出水(大流量、低浓度)混合后能被稀释到排放标准以下。考虑回用时，COD，N，P已分别达标(排放标准)的出水只需经简单的后处理便可实现回用于非饮用目的。
4.2 效率分析
　　为了定量说明图7所示推荐工艺所能完成的处理效率，此处以荷兰一正在建设中的城市污水处理厂所接收的水质、水量为例进行处理效率分析。该处理厂所接收的平均污水量为8500 m³／d；平均水质如下：COD=625 mg／l, TKN=60 mg／L，TP=9.5 mg／L。
　　根据经验数据，约有40％的进水总氮在A段中能被合成到细菌细胞中。如果以离心机来分离消化液，高达1200mg／L的氨氮会出现在消化液中[46]。假设来自A段普通生物污泥含氮、磷量分别为8％和2％(来自于BCFS工艺生物污泥磷含量可高达12％)；进入消化池的污泥COD有一半可转化为甲烷(这些假设数据均为实际经验值)。理论上，这意味着高达300mg／L的磷可能存在于消化液中。可见，从消化液中回收磷酸盐是十分必要的。
　　以上述经验数据及15℃水温为依据，可借助于数学模拟以及各处理单元可达到的实际处理效果建立有关COD，N，P的物料平衡，计算结果详见图8。按40％进水氨氮在A段内被合成到细菌体内计算，则相应有74％的进水COD(其中约1／3被氧化供合成能量之用)和60％的进水总磷在A段内被转化。剩余26％的COD被引入BCFS工艺，以去除剩余60％的氮(至出水到8 mg／L)和37％的磷(至出水到0.5 mg／L)。BCFS工艺以20 d污泥龄运行，产生的剩余污泥量很少(仅为进水总COD的6.6％)。

图8　推荐工艺物料平衡
　　因约50％污泥形式的COD在消化阶段被转化为甲烷，所以，相应会有一半的氮、磷由于细胞的分解而释放到环境中(消化液)；另一半COD，N，P则以熟污泥形式存在。如果使用氯化镁作为沉淀剂，消化液中97％的磷能以鸟粪石形式沉淀L441，这相当于有46.2％的进水磷可被回收。
　　CANON工艺被作为最后一个处理单元，处理磷回收后氨氮含量仍然很高的消化液。对CANON工艺的数学模拟表明，在30℃，如果工艺参数控制得当，可以达到90％的总氮去除率[40]。尽管CANONS.艺并不能将消化液中的氨氮去除殆尽，但此股水流流量较小，同来自于BCFS工艺很大出水流量混合后并不会使总的出水浓度升值太高。在此例中，出水总的COD，N，P的浓度分别为30 mg几(非生物降解性)、9mg／L和0.6mg／L。
4.3 可持续性
　　上述举例分析中，COD直接氧化(分解)而生成C02的数量被减低到了最小程度。A段中26％的COD的分解是为另外48％的COD合成提供能量，是生物污泥形成(COD浓缩)所必不可少的条件；A段后剩余26％COD刚好被用于反硝化除磷，而非简单分解。因为仅60％的进水总氮负荷需经反硝化除磷脱除，所以，脱氮所必需的碳源(COD)也相应节省。这样，被节省或过剩的COD(共48％)便能被用于甲烷化。再者，无论是COD氧化、反硝化除磷、还是亚硝化都能节省很多耗氧量，因此，推荐工艺的可持续性显而易见。推荐工艺与传统工艺计算所得重要处理效率数据见表2。

技术参数	可持续工艺	传统工艺
O2消耗量 kg 02／kg N	2.6	4.65
或kg O2/kg COD	0.29	0.6
脱氮COD消耗量 kg COD／kg N	1.4	4-5
甲烷生产量 kg CH4／kg COD	0.28	0
剩余污泥量 kg SS／kg COD	0.29	0.4
磷酸盐回收量 kg P／kg P	0.49	0
C02释放量减少 ％	18	0
处理水回用后处理难易程度	简单	复杂

　　注：表中数据表示去除单位质量的污染物，所消耗的或产生的物质的量，如0.48 kg P／kg P表示去除1 kg P可回收磷酸盐(以P计)0.48 kg。
　　与传统工艺相比较，推荐工艺在表2所列的各个方面均具可观的可持续性。
4.4 工艺评估
　　推荐工艺中虽涉及多个处理单元，但象AB法、BCFS工艺、污泥消化、磷酸盐回收等多已用于工程实际，并取得了良好的运行效果。在建筑结构方面，新颖的BCFS工艺同心圆平面设计成为真正意义上的"UNITANK"构造。尽管CANON工艺仍在研发之中，但试验、现场观察以及对其所做的数学模拟无疑会加速它在工程中的应用。因此，推荐工艺并不存在任何技术方面的难题。
　　多个处理单元被合并于推荐工艺中无疑会导致基础建设(工程建设与材料等)费用增高。然而，推荐工艺之可持续性的贡献必将降低运行管理费用(主要是电力消耗和污泥处置)。此外，磷酸盐回收与处理水回用还能带来潜在的经济价值，对缓解磷与水资源危机有现实意义；显著减少的C02释放量对控制温室效应也有着不可低估的积极作用。
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