化学需氧量COD以及生物需氧量BOD是水环境监测中最重要的指标,主要表征水中还原性污染物质的量.由于有机物的种类很多，欲测出其中各自的含量是办不到的，故常用BOD,COD这个综合指标来表示。
水中有机物的分解是分两个阶段进行的，第一阶段为碳氧化阶段，第二阶段为硝化阶段，碳氧化阶段所消耗的氧化量成为碳化生化需氧量(CBOD).
加以悬浮或溶解状态存在于生活污水和制糖、食品、造纸、纤维等工业废水中的碳氢化合物、蛋白质、油脂、木质素等均为有机污染物，可经好气菌的生物化学作用而分解，由于在分解过程中消耗氧气，故亦称需氧污染物质。若这类污染物质排入水体过多，将造成水中溶解氧缺乏，同时，有机物又通过水中厌氧菌的分解引起腐败现象，产生甲烷、硫化氢、硫醇和氨等恶臭气体，使水体变质发臭。
有机物基本上属于还原性物质，能被化学氧化剂氧化。有机物愈多，消耗的氧化剂量也愈多，因此可以用消耗的氧化剂量（换算成O2的mg/L）来间接反映有机物的含量。但有机物不是全部能被氧化的，如以醋酸为主的低级脂肪酸就几乎不能被氧化。此外，被氧化的污染物质还包括还原性的无机物——Fe2＋、NO2－等。
废水中的有机物主要包括40-60%的单排之，25-50%的碳水化合物以及8-12%的油脂。尿液也是污水中主要的有机物组成部分。因为尿液分解相对较快，因此在除fresh（新鲜?)废水外很难在别的类型的废水中发现urea。 Along with the proteins, carbohydrates, fats and oils, and urea, derived from food and human wastes, wastewater typically contains small quantities of a very large number of different synthetic organic molecules, with structures ranging from simple to extremely complex. Sources of synthetic organic molecules include unused medicine, personal care products, and household cleaning and maintenance products.
什么是COD
COD(化学需氧量)：是在一定的条件下，采用一定的强氧化剂处理水样时，所消耗的氧化剂量。它反映了水中受物质污染的程度，化学需氧量越大，说明水中受有机物的污染越严重。
COD以mg/L表示，通过水质监测仪器检测出的COD数值，水质可分为五大类，其中一类和二类COD≤15mg/L，基本上能达到饮用水标准，数值大于二类的水不能作为饮用水的，其中三类COD≤20mg/L、四类COD≤30mg/L、五类COD≤40mg/L属于污染水质，COD数值越高，污染就越严重。
COD（Chemical Oxygen Demand）——化学需氧量
在一定条件下，水中能被强氧化剂氧化的所有污染物质（包括有机物和无机物）的量，以氧的mg/L表示，叫化学需氧量。
COD怎么测
COD的测定方法分铬法（以重铬酸钾做氧化剂）和锰法（以高锰酸钾做氧化剂）两种，分别记为CODCr和CODMn。高锰酸钾法测定的结果受操作条件影响较大，且高锰酸钾溶液不稳定，对氧化程度也有影响，因而测定结果不能代表水中污染物质的确切含量。而重铬酸钾法则克服了上述缺陷，它具有更强的氧化能力，能将污水中绝大部分有机物和还原性无机物氧化。其溶液非常稳定。该法已被广泛采用。其与猛法之间的比值一般为：CODCr：CODMn=3：2.
COD Test Procedures
Prior to completing the COD test, a series of known standards are prepared using KHP (potassium hydrogen phthalate). Most wastewater samples will fall in the high range, so standards of 100, 250, 500 and 1000 mg/L are typically prepared. COD standards can also be purchased.
A COD reactor/heating (150°C) block and a colorimeter are turned on so that both instruments are allowed to stabilize.
Pre-prepared low-range (3-50 ppm) or high-range (20-1500 ppm) vials are selected for the COD test based on expected results. Both ranges can be used if the expected results are unknown.
One vial is marked as a “blank,” and three or four vials are marked with known standard levels. Two vials are then marked for the wastewater sample to make a duplicate run. Note: If multiple wastewater samples are being run, at least 10% of samples are duplicated.
2 mL of liquid are added to each vial. In the case of the “blank,” 2 mL of DI water are added. 2 mL of each standard are added to the corresponding vials. If the wastewater sample is tested at full strength, then 2 mL is added to the corresponding vial. If dilution is required, then serial dilutions are performed and 2 mL of the diluted sample are added to the corresponding vial.
Each vial is mixed well and placed into the reactor block for two hours. After two hours, the vials are removed from the block to a cooling rack for about 15 minutes.
The colourimeter is set and calibrated per the specific instructions for that unit (i.e., proper wavelength, blank and standards) and each vial is placed in the unit and the COD concentration read.
If the sample was diluted, the corresponding multiplication is made.
什么是BOD
生化需氧量的概念是:在一定条件下,微生物分解存在于水中的某些可被氧化物质,特别是有机物进行的生物化学过程中消耗溶解氧的量.
生化需氧量是指在一定条件下，微生物分解存在于水中的可生化降解有机物所进行的生物化学过程中消耗的溶解氧的数量。以毫克/升或百分率，ppm表示。它是反映水中有机污染物含量的一个综合指标。如果进行生物氧化时间为五天就称为五日生化需氧量，相应的还有BOD10， BOD20.
BOD怎么测
测定水中BOD的微生物传感器是由氧电极和微生物菌膜构成，其原理是当含有饱和溶解氧的样品进入流通池中与微生物传感器接触，样品中溶解性可生化降解的有机物受到微生物菌膜菌种中的作用，而消耗一定量的氧，使扩散到氧电极表面上氧的质量减少。当样品中可生化降解的有机物向菌膜扩散速度（质量）达到恒定时，此时扩散到氧电极表面上氧的质量也达到恒定，因此产生一个恒定电流。由于恒定电流的差值与氧的减少量存在定量关系，据此可换算出样品中生化需氧量。
soluble versus particulate, organics, settleable and floatable solids, oxidation of reduced iron and sulfur compounds, or lack of mixing may affect the accuracy and precision of BOD measurements. Presently, there is no way to include adjustments or corrections to account for the effect of these factors.
水质 生化需氧量(BOD)的测定 微生物传感器快速测定法.中华人民共和国环境保护行业标准 HJ/T 86-2002
水质 五日生化需氧量的测定 稀释于接种法。中华人民共和国国家标准。GB7488-87
上述标准的适用范围是2mg/L 到 6000 mg/L的水样。BOD大于6000 mg/L的水样可以通过稀释进行监测，但稀释会带来误差。标准中明确指出实验得到的结果是生物化学和化学作用共同产生的结果，他们不像单一的，有明确定义的化学过程那样具有严格和明确的特性，但是它能提供用于评价各种水样质量的指标。这个实验结果可能会被水中存在的某些物质所干扰，那些对微生物有毒的物质，如杀菌剂，有毒金属，游离氯都会对其产生抑制作用。水中的藻类或硝化微生物也可能造成虚假的偏高结果。
这个监测的原理描述如下：
将水样注满培养瓶，塞好后应不透气，将瓶置于恒温条件下培养5天，培养前后分别测定溶解氧浓度，由两者的差值可算出每升水消耗掉氧的质量，即BOD5的值。由于多数水样中含有较多的需氧物质，其需氧量往往超过水中可利用的溶解氧，因此再培养前需对水样进行稀释，使培养后剩余的溶解氧符合此规定。一般水质检验所测BOD5之包含碳物质的耗氧量和无极还原性物质的耗氧量。有时需要分别测定含碳物质耗氧量和硝化作用的耗氧量，常用的区别含碳和氮的硝化耗氧的方法使向培养瓶中投加硝化抑制剂，加入适量硝化抑制剂后，所测出的耗氧量即为含碳物质的耗氧量。再5天培养时间内，消化作用的耗氧量取决于是否存在足够数量的能进行此种氧化作用的微生物，原污水或初级处理的出水中这种微生物的数量不足，不能氧化显著量的还原性氮，而许多二级生化处理的出水和受污染较久的水体中，往往含有大量的硝化微生物，因此测定这种水样时应抑制其硝化反映。再测定BOD5的同时。利用葡萄糖和谷氨酸标准溶液完成验证实验。
如果水样中没有足够的合适性的微生物，就需要从别处获得微生物，当待分析水样为含难降解物质的工业废水时，取自待分析水排放口下游约3-8km的水或含微生物适宜于待分析水并经实验室培养过的水。
BOD Test Procedures
organics, settleable and floatable solids, oxidation of reduced
To ensure proper biological activity during the BOD test, a wastewater sample:
Must be free of chlorine. If chlorine is present in the sample, a dechlorination chemical (e.g, sodium sulfite) must be added prior to testing.
Needs to be in the pH range of 6.5-7.5 S.U. If the sample is outside this range, then acid or base must be added as needed.
Needs to have an existing adequate microbiological population. If the microbial population is inadequate or unknown, a "seed" solution of bacteria is added along with an essential nutrient buffer solution that ensures bacteria population vitality.
Specialized 300 mL BOD bottles designed to allow full filling with no air space and provide an airtight seal are used. The bottles are filled with the sample to be tested or dilution (distilled or deionized) water and various amounts of the wastewater sample are added to reflect different dilutions. At least one bottle is filled only with dilution water as a control or "blank."
A DO meter is used to measure the initial dissolved oxygen concentration (mg/L) in each bottle, which should be a least 8.0 mg/L. Each bottle in then placed into a dark incubator at 20°C for five days.
After five days (± 3 hours) the DO meter is used again to measure a final dissolved oxygen concentration (mg/L), which ideally will be a reduction of at least 4.0 mg/L.
The final DO reading is then subtracted from the initial DO reading and the result is the BOD concentration (mg/L). If the wastewater sample required dilution, the BOD concentration reading is multiplied by the dilution factor.