How to monitor the water quality of tailwater from sewage treatment plants?
Basic physical and chemical indicators
pH value: pH value is an important indicator for measuring the acidity and alkalinity of water. It is generally measured by a pH meter, inserting a pH electrode into the tailwater sample and reading the value directly. The normal tailwater pH value should usually be between 6.5 and 8.5. If the pH value exceeds this range, it may have an adverse effect on the ecological environment of the receiving water body, such as excessive acidity will corrode pipes, and excessive alkalinity may cause precipitation of certain metal ions in the water.
Temperature: Temperature affects the rate of chemical reactions and biological activity in water. Use a thermometer to measure, and generally place the thermometer probe in the tailwater body for a period of time before reading the data. Changes in temperature may affect the metabolic activity of microorganisms in the tailwater, and also affect the solubility of substances such as dissolved oxygen in the water.
Dissolved oxygen (DO): Dissolved oxygen refers to oxygen molecules dissolved in water. It can be measured by a dissolved oxygen meter. Sufficient dissolved oxygen is essential for the survival of aquatic organisms in receiving water bodies. Generally speaking, the dissolved oxygen content of good water should be above 5mg/L. Too low dissolved oxygen may cause aquatic organisms to suffocate and die, and may also be a signal that the water body is polluted by organic matter.
Conductivity: Conductivity is an indicator of ion concentration in water. It is measured with a conductivity meter, which can reflect the amount of electrolytes such as salts dissolved in water. Higher conductivity may mean that the water contains more inorganic salts, such as sodium chloride, sodium sulfate, etc.
Nutrient indicators
Ammonia nitrogen (NH₃ – N): Ammonia nitrogen is nitrogen in the form of free ammonia (NH₃) and ammonium ions (NH₄⁺) in water. It can be determined by Nessler’s reagent spectrophotometry or salicylic acid-hypochlorite spectrophotometry. Ammonia nitrogen is an important indicator for measuring eutrophication of water bodies. High concentrations of ammonia nitrogen can lead to excessive reproduction of aquatic plants such as algae. In tail water, the ammonia nitrogen content should generally be controlled at a low level. For example, in the tail water discharge standard of urban sewage treatment plants, the ammonia nitrogen content is generally required to be below 5mg/L (first-level A standard).
Total phosphorus (TP): Total phosphorus is the sum of various forms of phosphorus in water, including orthophosphate, condensed phosphate and organic phosphate. It is usually determined by ammonium molybdate spectrophotometry. Phosphorus is an important nutrient element for plant growth. Excessive total phosphorus content in tail water can also cause eutrophication of water bodies. The total phosphorus discharge standard of tail water of general sewage treatment plants may be around 0.5mg/L (first-level A standard).
Chemical oxygen demand (COD): Chemical oxygen demand refers to the amount of oxidant consumed when a water sample is treated with a strong oxidant under certain conditions, expressed in mg/L of oxygen. It is usually determined by the potassium dichromate method. It reflects the degree of contamination of water by reducing substances, which are mainly organic matter. The lower the COD content of tail water, the less organic matter there is in the water. The Class A standard for tail water from general urban sewage treatment plants requires COD≤50mg/L.
Biochemical oxygen demand (BOD): BOD refers to the amount of dissolved oxygen required for the biochemical process of microbial decomposition of organic matter in water under aerobic conditions. It is generally determined by the five-day culture method (BOD₅), that is, the water sample is cultured at 20℃ for 5 days, and the difference in dissolved oxygen before and after the culture is determined. BOD is an indicator of the content of organic matter that can be decomposed by microorganisms in the water body, and the lower the value, the better. Usually the BOD₅ of tail water is required to be below 10mg/L.
Pollutant indicators
Heavy metals: Common heavy metals that need to be monitored include mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr), etc. Atomic absorption spectroscopy or inductively coupled plasma mass spectrometry (ICP-MS) can be used for detection. Even if the content of these heavy metals in water is very low, they may be enriched in aquatic organisms and cause harm to human health through the food chain. Generally, the content of heavy metals in tail water is required to be very low, such as mercury content is generally required to be less than 0.001mg/L.
Suspended solids (SS): Suspended solids refer to solid matter suspended in water, including inorganic matter, organic matter, mud, clay, microorganisms, etc. that are insoluble in water. The weight method is used for determination, that is, the water sample is filtered through filter paper, the filter paper and filter residue are dried and weighed, and the content of suspended solids is calculated. Suspended solids will affect the transparency and appearance of the water body, and may also carry pollutants. Generally, the content of suspended solids in tail water should be controlled below 10mg/L.
Petroleum and animal and plant oils: Detected by infrared spectrophotometry. These substances may come from grease in industrial wastewater or domestic sewage. They will form an oil film on the water surface, affecting the reoxygenation capacity of the water body and the breathing of aquatic organisms. Petroleum and animal and plant oils in tail water are generally required to be controlled at a low level, such as below 1mg/L.
Microbial indicators
Total bacteria count: measured by plate counting method, that is, the tail water sample is diluted and inoculated on nutrient agar medium, and the number of colonies formed is counted after incubation at a suitable temperature for a certain period of time. The total bacteria count can reflect the degree of microbial contamination of the water body. Generally speaking, the total bacteria count in the tail water should be controlled within a certain range. For example, the tail water discharged near the drinking water source has more stringent requirements for the total bacteria count.
Fecal coliform group count: measured by multi-tube fermentation method or membrane filter method. Fecal coliform group mainly comes from human and animal feces and is an important indicator of fecal contamination of water bodies. If the number of fecal coliform group in the tail water is too high, it may cause harm to human health and aquatic life. The general sewage treatment plant tail water discharge standard requires that the number of fecal coliform group does not exceed 1000/L (level A standard).
Select monitoring method
On-site rapid detection: For some indicators that need to understand the water quality in a timely manner, such as pH value, dissolved oxygen, temperature, etc., portable water quality detection instruments can be used for on-site rapid detection. These instruments are easy to operate and can get results in a short time, which is convenient for staff to grasp the basic situation of tailwater in time. For example, at the outlet of the sewage treatment plant, a portable dissolved oxygen meter can be used to detect the dissolved oxygen content every once in a while.
Laboratory analysis and testing: For more complex indicators such as nutrient indicators, heavy metals, and microbial indicators, it is necessary to collect tailwater samples and bring them back to the laboratory for analysis and testing. Laboratory testing methods are highly accurate and sensitive, but require a certain amount of time and professional equipment and technicians. For example, when using an atomic absorption spectrometer to detect heavy metal content, the sample needs to be pretreated, including digestion, filtration and other steps, and then analyzed on the instrument. This method can accurately determine the type and content of trace heavy metals in the tailwater.
Determine the monitoring frequency
The monitoring frequency depends on many factors, such as the treatment scale of the sewage treatment plant, the stability of the treatment process, and the environmental sensitivity of the receiving water body. For large sewage treatment plants, some basic indicators (such as pH, dissolved oxygen, and suspended solids) can be monitored 1-2 times a day when the treatment process is complex and stable. For nutrient indicators (such as ammonia nitrogen, total phosphorus) and pollutant indicators (such as heavy metals), they can be monitored once a week or every two weeks. For microbial indicators, due to the relatively complex detection process, they can be monitored once a month or quarterly.
If the treatment process of the sewage treatment plant is abnormal, such as equipment failure, sudden changes in the influent water quality, etc., it is necessary to increase the monitoring frequency, timely understand the changes in the tail water quality, and take corresponding measures to adjust. For example, when the heavy metal content of the influent suddenly increases, it is necessary to increase the frequency of tail water heavy metal monitoring to ensure that the tail water meets the discharge standards.
Sampling method and location
Sampling location: Generally, sampling is carried out at the final drainage outlet of the sewage treatment plant. The water sample at this location can represent the final discharged tail water quality. At the same time, if the sewage treatment plant has multiple drainage outlets, sampling is required at each drainage outlet. In addition, in order to better understand the changes in the water quality of tail water in the drainage pipe, sampling can also be carried out at different locations of the drainage pipe (such as near the outlet of the treatment facility, near the drainage outlet, etc.).
Sampling method: For uniform tail water, a simple random sampling method can be used, using a water sampler to collect a certain volume of water samples at a specified depth and location. If the tail water is stratified at the outlet (such as stratification caused by different temperatures and densities), a stratified sampling method is required to collect water samples at different depths and mix them as analysis samples. During the sampling process, care should be taken to avoid contamination of water samples, use sterilized water sampling tools, and ensure that the water samples are stored in a suitable container in a timely manner after sampling, and are stored according to the requirements of the test indicators (for example, some water samples require the addition of preservatives, and some require low-temperature storage, etc.).