Before sampling, it is necessary to understand the production and treatment process flow, discharge patterns and treatment measures related to the discharged wastewater, so as to formulate a sampling plan, determine the existing interfering factors and take necessary pretreatment measures.
When sampling wastewater, special attention should be paid to the representativeness of the samples. For Class I pollutants in industrial wastewater, samples should be taken at the discharge outlet of the workshop or the workshop treatment facility. For Class II pollutants and other organic items, samples should be taken at the total discharge outlet of the factory. Domestic sewage is sampled before it enters the sewage network, while sampling points are set up at the discharge outlets of hospital sewage. Sampling points - once confirmed, shall not be changed at will.
The actual sampling position should be at the center of the sampling section. When the water depth is greater than 1 meter, sampling should be conducted at a depth of 1/4 below the surface layer. When the water depth is less than or equal to 1 meter, samples should be taken at half of the water depth.
When sampling, it is necessary to pay attention to removing debris, garbage and other floating objects on the water surface.
When collecting wastewater samples, it is recommended to measure the flow rate simultaneously as the basis for determining the composition ratio of the mixed sample and calculating the discharge volume.
The wastewater samples collected in 06 are mainly instantaneous samples and proportionally mixed samples. If the production processes of some pollutant-discharging units are continuous and stable, and the instantaneous samples have good representativeness, then the instantaneous sampling method can be adopted. For pollution sources with sewage treatment facilities that are operating normally or with regulating tanks, if the wastewater is discharged stably, instantaneous wastewater samples can also be collected during monitoring. For wastewater discharged unstably, samples should be taken in time units to form mixed samples for analysis. When the variation in wastewater flow is less than 20% and the pollutant concentration changes relatively little over time, water samples of equal volume are collected at equal time intervals and mixed.
07 Automatic sampling is carried out using an automatic sampler, which includes time proportional sampling and flow proportional sampling. When the sewage discharge volume is relatively stable, time proportional sampling can be adopted; otherwise, flow proportional sampling must be used.
For monitoring items that are significantly affected by suspended solids, automatic sampling should be conducted 5cm below the water surface of the sewage channel (channel, ditch), at half of the distance from the edge of the channel (channel, ditch) and the center point of the waterway. Manual sampling is the same as oil sampling. Uniform water samples containing suspended solids should be collected.
The monitoring items for sewage have different requirements according to the industry type. When collecting samples in different time units, samples for determining pH value, chemical oxygen demand, biochemical oxygen demand, dissolved oxygen, sulfides, oil, organic matter, residual chlorine, fecal coliform, suspended solids, radioactivity and other items cannot be mixed and can only be sampled separately.
The container materials to be selected for different monitoring items, the preservatives added and their dosages and storage periods, the volume of water samples to be collected, and the washing methods of the containers.
The composition of wastewater samples is often quite complex, and their stability is usually worse than that of surface water. Efforts should be made to determine them as soon as possible.
When sampling directly with the sample container, the sample must be rinsed three times with water before sampling, except for special items. However, when there is floating oil on the water surface, the oil extraction containers cannot be flushed.
Water samples used for the determination of suspended solids, five-day biochemical oxygen demand, sulfides, oils and residual chlorine must be sampled separately with fixed volume and all used for determination.
When choosing a special dedicated sampler (such as an oil sampler), sampling should be carried out in accordance with the usage method of that sampler.
When sampling, the "Sampling Record Form" should be filled out carefully. The form should include the following information: the name of the pollution source, the purpose of monitoring, the monitoring item, the sampling point, the sampling time, the sample number, the nature of the sewage, the sewage flow rate, the name of the sampler, and other relevant matters.
The collection requirements for organic matter, bacteriology and other samples, as well as other precautions, can be referred to surface water.