How to ensure the influent COD concentration of sewage treatment plants?
The "Implementation Plan for Deepening the Battle against Urban Black and Malodorous Water Bodies" has clearly stated that by 2025, the "urban domestic sewage centralized collection rate should strive to reach over 70%" and the "proportion of urban domestic sewage treatment plants with influent BOD concentration higher than 100mg/L should reach over 90%." However, during grassroots inspections, the author found that there is still considerable pressure to achieve these goals, and the phenomenon of "clean water in, clean water out" occurs to varying degrees in many places. Many centralized domestic sewage treatment plants have influent COD concentrations that are too low, often less than 100mg/L, far below the design requirements of 200mg/L to 400mg/L, resulting in wasted investment.
To ensure the influent COD concentration of sewage treatment plants, it is necessary to understand the reasons for the low influent COD concentration. The influent COD concentration is affected by various factors such as residents' water use habits, lack of separation between rainwater and sewage, incorrect or leaking pipeline connections, and groundwater intrusion. Among them, for most areas, rainfall is a short-term occasional event, and the continuous flow of groundwater into damaged pipes is unlikely. Therefore, the author believes that the main reasons for the long-term low influent COD concentration of sewage treatment plants are the "non-pollution" of sewage source components and the poor sealing of sewage pipelines.
As for domestic sewage, the sewage generated from daily washing, bathing, and cleaning does not add much COD. In addition, many residential areas are equipped with septic tanks, and toilet water has been purified to a certain extent, reducing the COD flowing into the sewage pipeline. It is normal for the COD of domestic sewage to be low. Theoretically, as long as these domestic sewage is sealed and transported in pipelines to sewage treatment plants, the influent COD concentration will not be high. However, in reality, due to imperfect pipelines or phenomena such as incorrect connections and leaks, sewage often leaks midway. This part of sewage is a problem of "not eating enough" for treatment plants. Once it is directly discharged into surface water, soil, or groundwater, it will bring a series of secondary environmental pollution problems such as odor and blackening of water bodies.
The author believes that to ensure the influent COD concentration of sewage treatment plants, it is necessary to improve the overall integrity and sealing of pipelines. As for rainwater and sewage separation, priority should be given to laying sewage pipelines to transport harmful sewage for treatment.
As for industrial wastewater, during the environmental impact assessment of construction projects, each enterprise is required to achieve standard emissions, which greatly reduces the industrial COD and BOD from the source. So how to ensure the influent COD concentration of sewage treatment plants? Many places have tried, for example, wastewater from breweries and solid-state fermentation in the Baijiu manufacturing industry does not contain heavy metals and toxic and harmful organic substances. It is a high-quality carbon source. Co-treatment with sewage can help improve the operating efficiency of sewage treatment plants.
The author believes that for sewage pipelines with good sealing, if the source COD is low and there are industrial wastewater sources in the area that do not contain heavy metals and toxic and harmful organic substances, the feasibility of substituting their carbon sources can be demonstrated, reducing the COD emission standards of these enterprises. This not only reduces the burden of decentralized pollution control for enterprises but also improves the efficiency of centralized sewage treatment, which is an effective carrier for coordinated carbon reduction. However, this approach should be cautiously and orderly implemented. First, it should only apply to industrial enterprises with absolutely safe water quality. Second, attention should be paid to the fairness of policies. Third, measures should be taken to prevent moving to the other extreme, resulting in high influent COD concentrations and overloaded operation of sewage treatment plants.
Source: China Environment News
