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

31.What are suspended solids?
Suspended solids SS are also called non-filterable substances. The measurement method is to filter the water sample with a 0.45μm filter membrane and then evaporate and dry the filtered residue at 103oC ~ 105oC. Volatile suspended solids VSS refers to the mass of suspended solids that volatilizes after burning at a high temperature of 600oC, which can roughly represent the content of organic matter in suspended solids. The remaining material after burning is non-volatile suspended solids, which can roughly represent the content of inorganic matter in the suspended solids.
In wastewater or polluted water bodies, the content and properties of insoluble suspended solids vary with the nature of the pollutants and the degree of pollution. Suspended solids and volatile suspended solids are important indicators for wastewater treatment design and operation management.
32. Why are suspended solids and volatile suspended solids important parameters in wastewater treatment design and operation management?
Suspended solids and volatile suspended solids in wastewater are important parameters in wastewater treatment design and operation management.
Regarding the suspended matter content of the secondary sedimentation tank effluent, the national first-level sewage discharge standard stipulates that it shall not exceed 70 mg/L (urban secondary sewage treatment plants shall not exceed 20 mg/L), which is one of the most important Water Quality control indicators. At the same time, suspended solids are an indicator of whether the conventional sewage treatment system is operating normally. Abnormal changes in the amount of suspended solids in the water from the secondary sedimentation tank or exceeding the standard indicate that there is a problem with the sewage treatment system, and relevant measures must be taken to restore it to normal.
The suspended solids (MLSS) and volatile suspended solids content (MLVSS) in activated sludge in the biological treatment device must be within a certain quantity range, and for sewage biological treatment systems with relatively stable Water Quality, there is a certain proportional relationship between the two. If MLSS or MLVSS exceeds a specific range or the ratio between the two changes significantly, efforts must be made to return it to normal. Otherwise, the quality of the effluent from the biological treatment system will inevitably change, and even various emission indicators, including suspended solids, will exceed standards. In addition, by measuring MLSS, the sludge volume index of the aeration tank mixture can also be monitored to understand the settling characteristics and activity of activated sludge and other biological suspensions.
33. What are the methods for measuring suspended solids?
GB11901-1989 specifies the method for the gravimetric determination of suspended solids in water. When measuring suspended solids SS, a certain volume of wastewater or mixed liquid is generally collected, filtered with a 0.45 μm filter membrane to intercept the suspended solids, and the filter membrane is used to intercept the suspended solids before and after. The mass difference is the amount of suspended solids. The common unit of SS for general wastewater and secondary sedimentation tank effluent is mg/L, while the common unit for SS for aeration tank mixed liquid and return sludge is g/L.
When measuring water samples with large SS values such as aeration mixed liquor and return sludge in wastewater treatment plants, and when the accuracy of the measurement results is low, quantitative filter paper can be used instead of the 0.45 μm filter membrane. This can not only reflect the actual situation to guide the operation adjustment of actual production, but also save testing costs. However, when measuring SS in secondary sedimentation tank effluent or deep treatment effluent, a 0.45 μm filter membrane must be used for measurement, otherwise the error in the measurement results will be too large.
In the wastewater treatment process, suspended solids concentration is one of the process parameters that needs to be frequently detected, such as inlet suspended solids concentration, mixed liquid sludge concentration in aeration, return sludge concentration, remaining sludge concentration, etc. In order to quickly determine the SS value, sludge concentration meters are often used in wastewater treatment plants, including optical type and ultrasonic type. The basic principle of the optical sludge concentration meter is to use the light beam to be scattered when it encounters suspended particles when passing through the water, and the intensity is weakened. The scattering of light is in a certain proportion to the number and size of the suspended particles encountered. The scattered light is detected by a photosensitive cell. and the degree of light attenuation, the sludge concentration in the water can be inferred. The principle of the ultrasonic sludge concentration meter is that when ultrasonic waves pass through wastewater, the attenuation of the ultrasonic intensity is proportional to the concentration of suspended particles in the water. By detecting the attenuation of the ultrasonic waves with a special sensor, the sludge concentration in the water can be inferred.
34. What are the precautions for the determination of suspended solids?
When measuring and sampling, the effluent water sample of the secondary sedimentation tank or the activated sludge sample in the biological treatment device must be representative, and large particles of floating matter or heterogeneous clot materials immersed in it should be removed. In order to prevent excessive residue on the filter disc from entraining water and prolonging the drying time, the sampling volume is preferably to produce 2.5 to 200 mg of suspended solids. If there is no other basis, the sample volume for suspended solids determination can be set as 100ml, and it must be thoroughly mixed.
When measuring activated sludge samples, due to the large suspended solids content, the amount of suspended solids in the sample often exceeds 200 mg. In this case, the drying time must be appropriately extended, and then moved to a dryer to cool to the equilibrium temperature before weighing. Repeated drying and drying until constant weight or weighing loss is less than 4% of the previous weighing. In order to avoid multiple drying, drying, and weighing operations, each operation step and time must be strictly controlled and completed independently by a laboratory technician to ensure consistent techniques.
The collected water samples should be analyzed and measured as soon as possible. If they need to be stored, they can be stored in a 4oC refrigerator, but the storage time of the water samples should not exceed 7 days. In order to make the measurement results as accurate as possible, when measuring water samples with high SS values such as aeration mixed liquid, the volume of the water sample can be appropriately reduced; while when measuring water samples with low SS values such as secondary sedimentation tank effluent, the test water volume can be appropriately increased. Such volume.
When measuring the concentration of sludge with high SS value such as return sludge, in order to prevent filter media such as filter membrane or filter paper from intercepting too much suspended solids and entraining too much water, the drying time must be extended. When weighing at constant weight, it is necessary to Pay attention to how much the weight changes. If the change is too large, it often means that the SS on the filter membrane is dry on the outside and wet on the inside, and the drying time needs to be extended.