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

35.What is water turbidity?

Water turbidity is an indicator of the light transmittance of water samples. It is due to the small inorganic and organic matter and other suspended matter such as sediment, clay, microorganisms and other suspended matter in the water that cause the light passing through the water sample to be scattered or absorbed. Caused by direct penetration, the degree of obstruction to the transmission of a specific light source when each liter of distilled water contains 1 mg SiO2 (or diatomaceous earth) is generally regarded as a turbidity standard, called the Jackson degree, expressed in JTU.

The Turbidity Meter is made based on the principle that suspended impurities in water have a scattering effect on light. The turbidity measured is the scattering turbidity unit, expressed in NTU. The turbidity of water is not only related to the content of particulate matter present in the water, but also closely related to the size, shape, and properties of these particles.

High turbidity of water not only increases the dosage of disinfectant, but also affects the disinfection effect. The reduction of turbidity often means the reduction of harmful substances, bacteria and viruses in the water. When the turbidity of water reaches 10 degrees, people can tell that the water is turbid.

36.What are the methods for measuring turbidity?

The turbidity measurement methods specified in the national standard GB13200-1991 include spectrophotometry and visual colorimetry. The unit of the results of these two methods is JTU. In addition, there is an instrumental method for measuring water turbidity using the scattering effect of light. The unit of the result measured by the Turbidity Meter is NTU. The spectrophotometric method is suitable for the detection of drinking water, natural water and high turbidity water, with a minimum detection limit of 3 degrees; the visual colorimetry method is suitable for the detection of low turbidity water such as drinking water and source water, with a minimum detection limit of 1 Spend. When testing turbidity in the secondary sedimentation tank effluent or advanced treatment effluent in the laboratory, both the first two detection methods can be used; when testing turbidity on the effluent of the sewage treatment plant and the pipelines of the advanced treatment system, it is often necessary to install online Turbidimeter.

The basic principle of the online turbidity meter is the same as that of the optical sludge concentration meter. The difference between the two is that the SS concentration measured by the sludge concentration meter is high, so it uses the principle of light absorption, while the SS measured by the turbidity meter is lower. Therefore, by using the principle of light scattering and measuring the scattering component of the light passing through the measured water, the turbidity of the water can be inferred.

Turbidity is the result of the interaction between light and solid particles in the water. The size of turbidity is related to factors such as the size and shape of impurity particles in the water and the resulting refractive index of light. Therefore, when the content of suspended solids in the water is high, generally Its turbidity is also higher, but there is no direct correlation between the two. Sometimes the suspended solids content is the same, but due to different properties of the suspended solids, the measured turbidity values are very different. Therefore, if the water contains a lot of suspended impurities, the method of measuring SS should be used to accurately reflect the degree of water pollution or the specific amount of impurities.

All glassware in contact with water samples must be cleaned with hydrochloric acid or surfactant. Water samples for turbidity measurement must be free of debris and easily sedimentable particles, and must be collected in stoppered glass bottles and measured as soon as possible after sampling. Under special circumstances, it can be stored in a dark place at 4°C for a short period of time, up to 24 hours, and it needs to be shaken vigorously and returned to room temperature before measurement.

37.What is the color of water?

The chromaticity of water is an index specified when measuring the color of water. The chromaticity referred to in Water Quality analysis usually refers to the true color of water, that is, it only refers to the color produced by dissolved substances in the water sample. Therefore, before measurement, the water sample needs to be clarified, centrifuged, or filtered with a 0.45 μm filter membrane to remove SS, but filter paper cannot be used because the filter paper can absorb part of the color of the water.

The result measured on the original sample without filtration or centrifugation is the apparent color of the water, that is, the color produced by a combination of dissolved and insoluble suspended matter. Generally, the apparent color of water cannot be measured and quantified using the platinum-cobalt colorimetric method that measures the true color. Characteristics such as depth, hue, and transparency are usually described in words, and then measured using the dilution factor method. The results measured using the platinum-cobalt colorimetric method are often not comparable to the colorimetric values measured using the dilution multiple method.

38.What are the methods for measuring color?

There are two methods for measuring colorimetry: platinum-cobalt colorimetry and dilution multiple method (GB11903-1989). The two methods should be used independently, and the measured results are generally not comparable. The platinum-cobalt colorimetric method is suitable for clean water, lightly polluted water and slightly yellow water, as well as relatively clean surface water, groundwater, drinking water and reclaimed water, and reused water after advanced sewage treatment. Industrial wastewater and seriously polluted surface water generally use the dilution multiple method to determine their color.

The platinum-cobalt colorimetric method takes the color of 1 mg of Pt (IV) and 2 mg of cobalt (II) chloride hexahydrate in 1 L of water as one color standard unit, generally called 1 degree. The preparation method of 1 standard colorimetric unit is to add 0.491mgK2PtCl6 and 2.00mgCoCl2?6H2O to 1L of water, also known as platinum and cobalt standard. Doubling the platinum and cobalt standard agent can obtain multiple standard colorimetric units. Since potassium chlorocobaltate is expensive, K2Cr2O7 and CoSO4?7H2O are generally used to prepare a substitute colorimetric standard solution in a certain proportion and operating steps. When measuring color, compare the water sample to be measured with a series of standard solutions of different colors to obtain the color of the water sample.

The dilution factor method is to dilute the water sample with optically pure water until it is nearly colorless and then move it into a colorimetric tube. The color depth is compared with that of optically pure water of the same liquid column height on a white background. If any difference is found, dilute it again until Until the color cannot be detected, the dilution factor of the water sample at this time is the value expressing the color intensity of the water, and the unit is times.