Key points for water quality testing operations in sewage treatment plants part eleven

56.What are the methods for measuring petroleum?

Petroleum is a complex mixture composed of alkanes, cycloalkanes, aromatic hydrocarbons, unsaturated hydrocarbons and small amounts of sulfur and nitrogen oxides. In the Water Quality standards, petroleum is specified as a toxicological indicator and human sensory indicator to protect aquatic life, because petroleum substances have a great impact on aquatic life. When the content of petroleum in water is between 0.01 and 0.1mg/L, it will interfere with the feeding and reproduction of aquatic organisms. Therefore, my country's fishery Water Quality standards must not exceed 0.05 mg/L, agricultural irrigation water standards must not exceed 5.0 mg/L, and the secondary comprehensive sewage discharge standards must not exceed 10 mg/L. Generally, the petroleum content of sewage entering the aeration tank cannot exceed 50mg/L.

Due to the complex composition and widely varying properties of petroleum, coupled with limitations in analytical methods, it is difficult to establish a unified standard applicable to various components. When the oil content in water is >10 mg/L, the gravimetric method can be used for determination. The disadvantage is that the operation is complicated and the light oil is easily lost when petroleum ether is evaporated and dried. When the oil content in water is 0.05~10 mg/L, non-dispersive infrared photometry, infrared spectrophotometry and ultraviolet spectrophotometry can be used for measurement. Non-dispersive infrared photometry and infrared photometry are the national standards for petroleum testing. (GB/T16488-1996). UV spectrophotometry is mainly used to analyze odorous and toxic aromatic hydrocarbons. It refers to substances that can be extracted by petroleum ether and have absorption characteristics at specific wavelengths. It does not include all petroleum types.

57. What are the precautions for petroleum measurement?

The extraction agent used by dispersive infrared photometry and infrared photometry is carbon tetrachloride or trichlorotrifluoroethane, and the extraction agent used by gravimetric method and ultraviolet spectrophotometry is petroleum ether. These extraction agents are toxic and must be handled with caution and in a fume hood.

The standard oil should be petroleum ether or carbon tetrachloride extract from the sewage to be monitored. Sometimes other recognized standard oil products can also be used, or n-hexadecane, isooctane and benzene can be used according to the ratio of 65:25:10. Formulated by volume ratio. The petroleum ether used for extracting standard oil, drawing standard oil curves and measuring wastewater samples should be from the same batch number, otherwise systematic errors will occur due to different blank values.

Separate sampling is required when measuring oil. Generally, a wide-mouth glass bottle is used for the sampling bottle. Plastic bottles must not be used, and the water sample cannot fill the sampling bottle, and there should be a gap on it. If the water sample cannot be analyzed on the same day, hydrochloric acid or sulfuric acid can be added to make the pH value <2 to inhibit the growth of microorganisms, and stored in a 4oC refrigerator. The piston on the separatory funnel cannot be coated with oily grease such as Vaseline.

58. What are the water quality indicators for common heavy metals and inorganic non-metal toxic and harmful substances?

Common heavy metals and inorganic non-metal toxic and harmful substances in water mainly include mercury, cadmium, chromium, lead and sulfide, cyanide, fluoride, arsenic, selenium, etc. These water quality indicators are toxic to ensure human health or protect aquatic life. physical indicators. The National Comprehensive Wastewater Discharge Standard (GB 8978-1996) has strict regulations on wastewater discharge indicators containing these substances.

For sewage treatment plants whose incoming water contains these substances, the content of these toxic and harmful substances in the incoming water and the effluent of the secondary sedimentation tank must be carefully tested to ensure that discharge standards are met. Once it is discovered that the incoming water or effluent exceeds the standard, measures should be taken immediately to ensure that the effluent reaches the standard as soon as possible by strengthening pretreatment and adjusting sewage treatment operating parameters. In conventional secondary sewage treatment, sulfide and cyanide are the two most common water quality indicators of inorganic non-metallic toxic and harmful substances.

59.How many forms of sulfide are there in water?

The main forms of sulfur that exist in water are sulfates, sulfides and organic sulfides. Among them, sulfide has three forms: H2S, HS- and S2-. The amount of each form is related to the pH value of the water. Under acidic conditions When the pH value is higher than 8, it mainly exists in the form of H2S. When the pH value is greater than 8, it mainly exists in the form of HS- and S2-. The detection of sulfide in water often indicates that it has been contaminated. Wastewater discharged from some industries, especially petroleum refining, often contains a certain amount of sulfide. Under the action of anaerobic bacteria, sulfate in the water can also be reduced to sulfide.

The sulfide content of sewage from relevant parts of the sewage treatment system must be carefully analyzed to prevent hydrogen sulfide poisoning. Especially for the inlet and outlet water of the stripping desulfurization unit, the sulfide content directly reflects the effect of the stripping unit and is a control indicator. In order to prevent excessive sulfide in natural water bodies, the national comprehensive wastewater discharge standard stipulates that the sulfide content shall not exceed 1.0mg/L. When using aerobic secondary biological treatment of sewage, if the sulfide concentration in the incoming water is below 20mg/L, the active If the sludge performance is good and the remaining sludge is discharged in time, the sulfide content in the secondary sedimentation tank water can reach the standard. The sulfide content of the effluent from the secondary sedimentation tank must be monitored regularly to observe whether the effluent meets the standards and determine how to adjust operating parameters.

60. How many methods are commonly used to detect sulfide content in water?

Commonly used methods to detect sulfide content in water include methylene blue spectrophotometry, p-amino N, N dimethylaniline spectrophotometry, iodometric method, ion electrode method, etc. Among them, the national standard sulfide determination method is methylene blue spectrophotometry. Photometry (GB/T16489-1996) and direct color spectrophotometry (GB/T17133-1997). The detection limits of these two methods are 0.005mg/L and 0.004mg/l respectively. When the water sample is not diluted, In this case, the highest detection concentrations are 0.7mg/L and 25mg/L respectively. The sulfide concentration range measured by p-amino N,N dimethylaniline spectrophotometry (CJ/T60--1999) is 0.05~0.8mg/L. Therefore, the above spectrophotometry method is only suitable for detecting low sulfide content. Watery. When the concentration of sulfide in wastewater is high, iodometric method (HJ/T60-2000 and CJ/T60--1999) can be used. The detection concentration range of iodometric method is 1~200mg/L.

When the water sample is turbid, colored, or contains reducing substances such as SO32-, S2O32-, mercaptans, and thioethers, it will seriously interfere with the measurement and requires pre-separation to eliminate interference. The commonly used pre-separation method is acidification-stripping-absorption. Law. The principle is that after the water sample is acidified, the sulfide exists in the H2S molecular state in the acidic solution, and is blown out with gas, then absorbed by the absorption liquid, and then measured.

The specific method is to first add EDTA to the water sample to complex and stabilize most metal ions (such as Cu2+, Hg2+, Ag+, Fe3+) to avoid interference caused by the reaction between these metal ions and sulfide ions; also add an appropriate amount of hydroxylamine hydrochloride, which can Effectively prevent oxidation-reduction reactions between oxidizing substances and sulfides in water samples. When blowing H2S from water, the recovery rate is significantly higher with stirring than without stirring. The recovery rate of sulfide can reach 100% under stirring for 15 minutes. When the stripping time under stirring exceeds 20 minutes, the recovery rate decreases slightly. Therefore, the stripping is usually carried out under stirring and the stripping time is 20 minutes. When the water bath temperature is 35-55oC, the sulfide recovery rate can reach 100%. When the water bath temperature is above 65oC, the sulfide recovery rate decreases slightly. Therefore, the optimal water bath temperature is generally selected to be 35 to 55oC.

61. What are other precautions for sulfide determination?

⑴ Due to the instability of sulfide in water, when collecting water samples, the sampling point cannot be aerated or violently stirred. After collection, zinc acetate solution must be added in time to make it a zinc sulfide suspension. When the water sample is acidic, alkaline solution should be added to prevent the release of hydrogen sulfide. When the water sample is full, the bottle should be corked and sent to the laboratory for analysis as soon as possible.

⑵ No matter which method is used for analysis, water samples must be pretreated to eliminate interference and improve detection levels. The presence of colorants, suspended solids, SO32-, S2O32-, mercaptans, thioethers and other reducing substances will affect the analysis results. Methods to eliminate the interference of these substances can use precipitation separation, air blowing separation, ion exchange, etc.

⑶ The water used for dilution and preparation of reagent solutions cannot contain heavy metal ions such as Cu2+ and Hg2+, otherwise the analysis results will be lower due to the generation of acid-insoluble sulfides. Therefore, do not use distilled water obtained from metal distillers. It is best to use deionized water. Or distilled water from an all-glass still.

⑷Similarly, trace amounts of heavy metals contained in the zinc acetate absorption solution will also affect the measurement results. You can add 1mL of newly prepared 0.05mol/L sodium sulfide solution dropwise to 1L of zinc acetate absorption solution under sufficient shaking, and let it sit overnight. , then rotate and shake, then filter with fine-textured quantitative filter paper, and discard the filtrate. This can eliminate the interference of trace heavy metals in the absorption solution.

⑸Sodium sulfide standard solution is extremely unstable. The lower the concentration, the easier it is to change. It must be prepared and calibrated immediately before use. The surface of the sodium sulfide crystal used to prepare the standard solution often contains sulfite, which causes errors. It is best to use large particle crystals and quickly rinse them with water to remove the sulfite before weighing.