2.1 Introduction
Various compounds containing the element nitrogen are becoming increasingly important inwastewater management programs because of the many effects that nitrogenous materials inwastewater effluent can have on the environment. Nitrogen, in its various forms, can depletedissolved oxygen levels in receiving waters, stimulate aquatic growth, exhibit toxicitytoward aquatic life, affect chlorine disinfection efficiency, present a public health hazard.and affect the suitability of wastewater for reuse. Biological and chemical processes whichoccur in wastewater treatment plants and in the natural environment can change thechemical form in which nitrogen exists Such change may eliminate one deleterjous effect ofnitrogen while producing, or leaving unchanged, another effect. For example, by convertingammonia in raw wastewater to nitrate, the oxygen-depleting and toxic effects of ammmoniaare eliminated, but the biostimulatory effects may not be changed signiicantly.
lt is important, therefore, prior to the detailed discussions of nitrogen removal processeswhich form the principal content of this manual, to review the chemistry of nitrogen andthe effects that the various compounds can have. Several speciic aspects are discussed inthis chapter. First, the nitrogen cycle for both surface water and soil/groundwatercnvironments is described, with emphasis on the important compounds and reactionsassociated with cach. Second, sources of nitrogen, both natural and man-caused, arediscussed. Important elements of the latter category include domestic and industrialwastewater, urban and suburban runoff, surface and subsurface agricultural drainage, andemissions to the atmosphere which may eventually enter the aquatic environment througlprecipitation or dustfall Then, the effects of nitrogen discharge to Surface watergroundwater, and land are summarized. And finally, introductory to the following chapters.a brief discussion is presented on the relationship between the various nitrogen compoundsand process removal efficicncy.
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2.1 引言
含氮元素的各種化合物在廢水管理計(jì)劃中變得越來(lái)越重要，因?yàn)閺U水中氮素材料對(duì)環(huán)境可能產(chǎn)生的影響很多。氮在其各種形式下會(huì)降低接收水體的溶解氧水平、促進(jìn)水生生物生長(zhǎng)、對(duì)水生生物具有毒性、影響氯消毒效率、構(gòu)成公共衛(wèi)生危害，并影響廢水再利用的適用性。在廢水處理廠和自然環(huán)境中發(fā)生的生物和化學(xué)過(guò)程可以改變氮的存在形式。這種改變可能消除一種有害氮素的影響，同時(shí)產(chǎn)生或保持另一種影響不變。例如，通過(guò)將原始廢水中的氨轉(zhuǎn)化為硝酸鹽，可以消除氨的耗氧和毒性作用，但生物刺激作用可能不會(huì)顯著改變。
因此，在詳細(xì)討論氮去除過(guò)程之前，本手冊(cè)的主要內(nèi)容之前，回顧氮的化學(xué)和其可能的影響至關(guān)重要。本章節(jié)討論了幾個(gè)具體方面。首先，描述了表面水體和土壤/地下水環(huán)境的氮循環(huán)，重點(diǎn)關(guān)注與每種循環(huán)相關(guān)的重要化合物和反應(yīng)。其次，討論了氮的來(lái)源，包括自然和人為因素。后者的重要元素包括家庭和工業(yè)廢水、城市和郊區(qū)徑流、表面和亞表面農(nóng)業(yè)排水以及通過(guò)降水或灰塵沉降最終進(jìn)入水生環(huán)境的大氣排放。然后，總結(jié)了氮排放對(duì)表面水、地下水和土壤的影響。最后，在下一章之前，簡(jiǎn)要討論了各種氮化合物與處理效率之間的關(guān)系。

2.2 氮循環(huán)

22. The Nitrogcn Cycle
Nitrogen exists in many compounds becatse of the high number of oxidation states it canassume. In ammonia or organic compotnds, the form most closely associated with plantsand animals, its oxidation state is minus 3. At the other extreme its oxidation state is plus 5when in the nitrate form. In the environment, changes from one oxidation state to anothercan be brought about biologically by living organisms The rclationship between the variouscompounds and the transformations which can occur are often presented schematically in adiagram known as the nitrogen cycle. Figure 2-1 shows a common manner of presentation.The atmosphere serves as a reservoir of N2 gas from which nitrogen is removed naturally by electrical discharge and nitrogen-fixing organisns and artificially by chemical manufacturing. Nitrogen gas is returned to the atmosphere by the action o denitrifying organisms. Inthe fixed state, nitrogen can undergo the various reactions shown. A general desciption olthe nitrogen cycle is presenfed here, and aspects of particular importance to surface waterand soil/groundwater environmtents are discussed in the following sections.
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氮循環(huán)

氮的氧化狀態(tài)可以假定很高，因此它存在于許多化合物中。在植物和動(dòng)物所密切關(guān)聯(lián)的氨或有機(jī)化合物中，它的氧化狀態(tài)為負(fù)3；而在硝酸鹽形式中，它的氧化狀態(tài)則是正5。在環(huán)境中，生物可以通過(guò)生物途徑將一種氧化態(tài)轉(zhuǎn)換成另一種；其中化合物間的關(guān)系及其轉(zhuǎn)化通常以氮循環(huán)的方式圖示呈現(xiàn)。圖2-1展示了常見(jiàn)的圖例呈現(xiàn)方式。大氣作為氮?dú)獾膬?chǔ)量，自然地通過(guò)電放電和固氮微生物以及人工地通過(guò)化學(xué)制造來(lái)去除氮。通過(guò)反硝化微生物作用，氮?dú)夥祷卮髿狻Ｔ诠潭顟B(tài)中，氮可以經(jīng)歷各種反應(yīng)。本文介紹了氮循環(huán)的一般描述，并探討了對(duì)地表水和土壤/地下水環(huán)境至關(guān)重要的方面。

Transformation reactions? of importance include fixation, ammonification, assimilationnitrification and denitrification.2 These reactions can be carried out by particularmicroorganisms with either a net gain or loss of energy; energy considerations often play animportant role in determining the reaction which occurs. The principal compounds olconcern in the nitrogen cycle are nitrogen gas, ammonium, organic nitrogen, and nitrate.These compounds and their oxidation states are shown below:
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重要的轉(zhuǎn)化反應(yīng)包括固定、氨化、同化、硝化和反硝化。2 這些反應(yīng)可以由特定的微生物進(jìn)行，其能量?jī)粼鲆婊騼魮p失；能量考慮經(jīng)常在確定發(fā)生的反應(yīng)時(shí)發(fā)揮重要作用。氮循環(huán)中的主要化合物是氮?dú)?，銨，有機(jī)氮和硝酸鹽。下面顯示了這些化合物及其氧化態(tài)：

It is important to note that at neutral pH values there is very little molecular ammonia(NH3) in wastewater as most is in the form of the ammonium ion (NH4). The distributionof ammonia and ammonjum as a function of pH is discussed in Section 6.1.1.
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需要注意的是，在中性pH值下，廢水中分子氨（NH3）含量非常少，而大部分都是以銨離子（NH4）的形式存在。關(guān)于氨和銨在pH值變化下的分布情況詳見(jiàn)第6.1.1節(jié)。

Fixation of nitrogen from N2 gas to organic nitrogen is accomplished biologically byspecialized microorganisms. This reaction requires an investment of energy. Biologicalfixation accounts for most of the natural transformation of nitrogen to compounds whichcan be used by plant and animal life, Lightning fixation has been estimated to account forapproximately 15 percent of the total which occurs naturally.3 Industrial fixation wasinitially developed in the early 20th Century for manufacture of both fertilizer andexplosives. Presently, nitrogen fixed by industry is about half the amount that is removedfrom the atmosphere by natural means.
Ammonification is the change from organic nitrogen to the ammonjum (NHa/NH) formThis occurs to dead animal and plant tissue and to animal fecal matter.
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生物學(xué)上由特定微生物通過(guò)將N2氣體轉(zhuǎn)化為有機(jī)氮實(shí)現(xiàn)氮固定。這種反應(yīng)需要投入能量。生物學(xué)固定過(guò)程占了氮轉(zhuǎn)化為可以供植物和動(dòng)物利用的化合物的自然過(guò)程的大部分。閃電固定被估計(jì)占了自然過(guò)程總量的約15％。20世紀(jì)初，工業(yè)固定最初被開(kāi)發(fā)用于生產(chǎn)肥料和炸藥。目前，工業(yè)固定的氮量約為自然方式從大氣中移除的氮量的一半。腐爛是有機(jī)氮轉(zhuǎn)化為銨（NH4 / NH）形式的過(guò)程，這發(fā)生在死亡的動(dòng)物和植物組織以及動(dòng)物糞便中。

Nitrogen in urine exists principally as urea. Urea is hydrolyzed by the enzyme urease toammonium carbonate.
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尿液中的氮主要存在于尿素中，尿素會(huì)被酶酰胺酶水解生成碳酸銨。

Assimilation is the use of ammonium or nitrate compounds to form plant protein and othernitrogen-containing compounds.
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同化是利用銨或硝酸鹽化合物形成植物蛋白質(zhì)和其他含氮化合物的過(guò)程。

Animals require protein from plants or from other animals. With certain specific exceptions.
they are incapable of converting inorganic nitrogen forms into organic forms.
The term “nitrification” is applied to the biological oxidation of ammonium, first to thenitrite, then to the nitrate, form. The bacteria responsible for these reactions are termedchemoautotrophic because they use inorganic chemicals as their source of energy. Generally.the Nitrosomonas genera are involved in conversion of ammonium to nitrite under aerobicconditions as follows:
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動(dòng)物需要從植物或其他動(dòng)物中獲取蛋白質(zhì)，除一些特殊例外外，它們無(wú)法將無(wú)機(jī)氮形式轉(zhuǎn)化為有機(jī)形式。術(shù)語(yǔ)“硝化”適用于氨的生物氧化，首先形成亞硝酸鹽，然后形成硝酸鹽。負(fù)責(zé)這些反應(yīng)的細(xì)菌被稱(chēng)為化能自養(yǎng)細(xì)菌，因?yàn)樗鼈兪褂脽o(wú)機(jī)化學(xué)物質(zhì)作為能源。通常，在有氧條件下，氨轉(zhuǎn)化為亞硝酸鹽的過(guò)程中，Nitrosomonas屬參與其中，具體過(guò)程如下：

硝酸鹽則通常由亞硝酸鹽經(jīng)由Nitrobacter菌株氧化而來(lái)，其反應(yīng)式為：

整個(gè)硝化反應(yīng)式如下：

To oxidize 1 mg/l of ammonia-nitrogen requires about 4.6 mg/l of oxygen when synthesis ofnitrifiers is neglected. The nitrate thus formed may be used in assimilation as describedabove to promote plant growth, or it may be used in denitrification, wherein throughbiological reduction, first nitrite and then nitrogen gas are formed, A fairly broad range ofbacteria can accomplish denitrification,including Psuedomonas, Micrococcus， Achromobacter. and Bacillus In simplified form, the reaction steps are as follows:
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氨氮1毫克每升的氧化需要約4.6毫克每升的氧氣，假設(shè)不考慮硝化細(xì)菌的合成。因此產(chǎn)生的硝酸鹽可用于同化，如上所述，以促進(jìn)植物生長(zhǎng)，也可用于反硝化作用，其中通過(guò)生物還原作用，首先形成亞硝酸鹽，然后形成氮?dú)?。相?dāng)廣泛的細(xì)菌可完成反硝化作用，包括銅綠假單胞菌、微球菌、黃色桿菌和芽孢桿菌。簡(jiǎn)化形式的反應(yīng)步驟如下：

Here methanol is used as the example organic carbon source, although many natural andsynthetic organic compounds can serve as the carbon source for denitrification.
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在這里，甲醇被作為有機(jī)碳源的示例，雖然許多天然和合成的有機(jī)化合物都可以作為反硝化的碳源。

Oxidation of organic atter to carbon dioxide and water furnishes energy for bacteria.Either oxygen or nitrate may be used for the oxidation, but the use of oxygen results in therelease of more energy. When both oxygen and nitrate are present, bacteria preferentiallyuse oxygen. Therefore, use of nitrate for denitrification can only occur under anoxicconditions, an important consideration when attempting to remove nitrate from wastewater.
Nitrite, since it is an intermediate in the nitrification and denitrification processes, can linkthe nitrification and denitrification steps directly without passing through nitrate. First.nitrite is formed from oxidation of ammonium by Nitrosomonas, then nitrite can bedenitrified to nitrogen gas. By this route less oxygen is required for nitrification and lessorganic matter (energy) is required for denitrification. This is a special case, however, andnot broadly applicable to municipal wastewater treatment.
In discussing the nitrogen cycle, it is useful to differentiate between the surface water andsediment environment and the soil/groundwater environment. This aids in understanding theroles that nitrogenous compounds play in each and the problems which can be encountered
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有機(jī)物質(zhì)氧化成二氧化碳和水為細(xì)菌提供能量。氧氣或硝酸鹽都可以用于氧化，但使用氧氣會(huì)釋放更多的能量。當(dāng)同時(shí)存在氧氣和硝酸鹽時(shí)，細(xì)菌優(yōu)先使用氧氣。因此，在嘗試將硝酸鹽從廢水中去除時(shí)，硝化只能在缺氧條件下進(jìn)行，這是一個(gè)重要的考慮因素。

硝酸鹽中間產(chǎn)物亞硝酸鹽可以直接將硝化和反硝化步驟聯(lián)系起來(lái)而不經(jīng)過(guò)硝酸鹽。首先，氨經(jīng)Nitrosomonas氧化生成亞硝酸鹽，然后亞硝酸鹽可以被反硝化成氮?dú)?。通過(guò)這種方式，硝化所需的氧氣較少，反硝化所需的有機(jī)物（能量）也較少。但這是一個(gè)特殊情況，不適用于城市廢水處理。

在討論氮循環(huán)時(shí)，區(qū)分水表和沉積物環(huán)境與土壤/地下水環(huán)境是有用的。這有助于理解氮化合物在各種情況下所扮演的角色以及可能遇到的問(wèn)題。