Causes and countermeasures of low influent concentration in urban sewage treatment plants

The influent quality is an important basis for determining the process flow of sewage treatment plant, and is the main parameter for accounting the actual emission reduction, which directly affects the construction cost and operation cost. till/extremely2014A sewage plant was built in the urban area of a city in, with a designed treatment capacity ofeightten thousandm3,And equipped withfiveA lifting pumping station;The main purpose of the sewage treatment plant is to play an important role in reducing the total emission of major pollutants in the region. However, with the official operation of the sewage treatment plant, a series of problems are gradually exposed.,One of the most important problems is water inflow.CODThe concentration is generally low. In view of this situation,The author makes an investigation and analysis.,The following results are obtained.

oneMethods and contents of investigation

①Collect information.Contact relevant units to collect relevant information and understand the relevant situation.,Including the design and completion drawings of drainage pipe network in the service scope of sewage plant, urban population, meteorological climate and other information.;Drainage system adopted by the city, groundwater situation and pipe network maintenance, etc. Analyze the pipe network diagram in detail,Determine the sampling point in combination with field investigation.

②Carry out major pollutants on each drainage unit, including domestic sewage raw water, sewage interception head, branch pipes at all levels, sewage pumping station, etc.CODSampling monitoring of concentration.Select representative residential areas to investigate the treatment efficiency of septic tanks and the situation of residents’ own water.

③Investigate and analyze the regional drainage pipe network, including sewage pumping station and sewage interception engineering pipe network.According to the population and water consumption units in the service range of the pipe network, the theoretical displacement is estimated and compared with the actual displacement in the measured range to determine the pollution acceptance rate of the drainage pipe network.

④Find out the specific location where the external water source enters the pipe network, and record its flow andCODConcentration, and continuous monitoring for many days.

⑤According to the experimental results and collected data, the causes of low influent concentration in sewage plants are analyzed and summarized, and targeted suggestions and rectification measures are put forward.

2Present situation of actual inlet water quality of water supply and drainage and sewage plant in a city

According to the survey, the average water consumption of urban residents in this city is8.91ten thousandm3,The peak period of water use can reach near9.7ten thousandm3. Its drainage system is mainly intercepting combined system. The sewage treatment plant reflects the basic situation of the water quality of the sewage treatment plant on that day with the mixed samples of the early, middle and late inflow. Through the mixed inlet water of the sewage treatment plant in the morning, the middle and the eveningCODMonitoring the concentration, and obtaining the change diagram of the inlet water quality, as shown in the figure.l.

pictureoneVariation diagram of influent concentration of sewage treatment plant

Youtuoneknowable,The influent concentration of sewage treatment plant is mainly concentrated in80~100mg/L,In a long-term low state, far from reaching the design value.300mg/L.

threeInvestigation on background value of municipal domestic sewage

The residential areas with concentrated residents, administrative center areas, representative commercial dining areas, hotel-intensive areas and hospital sewage were selected as the investigation objects of urban domestic sewage background value. The monitoring results are shown in the table.one.

tableoneMonitoring results of background value of municipal domestic sewage

YoubiaooneAccording to the monitoring results in.,The concentration of sewage in residential areas and commercial dining areas is relatively high, while the concentration of sewage in hospitals is low, and the concentration of sewage in municipal central areas is similar to that in hotel-intensive areas. Calculate the domestic sewage according to the proportion of various domestic sewage.CODThe weighted average concentration is189.34mg/L.

fourcause analysis

4.1Problems of drainage system

The new urban area of this city adopts the complete rain and sewage diversion system.,However, the main sewage pipe has not yet been built.,Sewage can only be combined with rainwater pipes and discharged into the nearest river channel.,Polluted water environment;The drainage system in this city is mainly the combined system of rain and sewage. At present, the old city is still using the way of collecting sewage by culvert instead of pipeline. In addition, the city has a subtropical monsoon humid climate, with frequent rain and heavy rainfall in the rainy season. In recent years, the average annual rainfall in this city is about1800mm,This greatly increases the dilution effect of rainwater on sewage concentration. It can be seen that the rainwater and sewage confluence system has a great influence on the influent concentration of sewage plants.

4.2Exogenous lowCODInfluence of concentrated water source entering sewage pipeline

During the investigation, it was found that there were many foreign water sources entering the sewage pipe network in the whole sewage pipe network collection system. These foreign countries are lowCODConcentrated water sources include river water, swimming pool water, construction water and groundwater. The survey results show that the average amount of water flowing from the inner river into the pumping station reaches2.
2ten thousandm3,About the total sewage volume1/4,thatCODThe average concentration is60mg/L; There are small and medium-sized swimming pools in many communities in the city, and the water replaced by them is basically discharged into the urban sewage pipeline, just like the water used for construction.,These all feed the sewage plant.CODThe concentration has a certain impact.;Located in the south bank of the middle reaches of the Yangtze River, the city is rich in water resources and has a high groundwater level.,However, most drainage pipes are made of concrete pipes, even rigid joints in the old city. Therefore, groundwater and surface water can easily penetrate or flow into the drainage pipe network.,The concentration of sewage is dilute.

4.3Problems of septic tank setting

Septic tank is a simple decentralized facility for primary sewage treatment, which can play a certain role in sewage treatment. Therefore, when there is no sewage treatment plant in the urban area, septic tanks are built in many residential areas to treat domestic sewage.,To some extent, it can reduce the pollution of water. But after the sewage treatment plant was established,,The pipe network collected the sewage pretreated by septic tanks.,It also directly reduces the amount of sewage entering the sewage treatment plant.CODConcentration. The survey results show that the septic tank treatment efficiency of the whole residential area in this city is as follows35%about,Finally, the supernatant of septic tank is discharged into the sewage pipe network.

4.4Low pollution acceptance rate

Since the establishment of sewage treatment plant in this city,The laying of urban drainage pipe network is relatively backward.,Domestic sewage in many areas can’t be effectively collected to the main sewage pipe and discharged to the sewage treatment plant. In the urban area, some main pipes are still idle and not connected to the main pipe, and sewage collection pipes are not laid in many areas, thus reducing the effective collection area of the drainage pipe network and the low pollution acceptance rate. In some areas of the city, the main sewage is directly discharged into the nearby water body, so the amount of sewage to be treated cannot be reached.,The phenomenon of pumping river water to pretend to be treated, which not only leads to the low pollution acceptance rate of sewage plants and the low influent concentration, but also causes the pollution of environmental water bodies, which does not reflect the value of building sewage plants at all.

4.5Residents’ water use habits

With the continuous improvement of the city’s water supply system and the continuous improvement of residents’ living standards, the city’s water price is low and local residents’ awareness of saving water is not strong.,As a result, the per capita water consumption has increased year by year. According to the survey, the increase of residents’ water consumption is mainly reflected in the increase of water consumption for bathing and washing.,The organic matter concentration of these domestic sewage is low, which will inevitably lead to the low influent concentration after entering the sewage plant.

fivecounter-measure

5.1Pay attention to the preliminary investigation and planning and design of urban drainage system construction

The basic data of urban drainage system planning and design is the decisive factor to evaluate the planning and design quality and social and environmental impact. The main basic data include the overall urban planning, existing urban drainage systems and facilities, local sewage irrigation, and the status quo of sewage reuse. If the sewage plant does not adjust the sewage quality in the collection area, the current sewage pipe network in the collection area, the population and water consumption in the sewage collection area, etc.

Check and design, which will lead to the uncertainty of the quantity and quality of water entering the sewage plant after its completion.

5.2Improve the drainage system

According to the national drainage code, "different drainage systems are allowed in the same town", the intercepting combined drainage system can be used for transition in the near future, and the separated drainage system can be gradually transformed with the urban construction in the long term, and the original rainwater and sewage combined pipe can be changed into a single rainwater pipe.,No longer connected to the intercepting main pipe, at the same time, the intercepting main pipe is used as the sewage main pipe and its overflow port is closed, and the sewage branch pipe is paved again to connect to the sewage main pipe, so that in the long run,

The rain and sewage are completely separated. The building density in the old city of this city is high.,The laying of pipe network is also complicated, and it is difficult to transform. Therefore, areas that are easy to transform, such as areas with low building density and low pipe network density, can be transformed into a complete rain and sewage separation system first, and then combined with the planning department.,For the old city which is difficult to transform, the drainage system is gradually changed to the rain and sewage complete separation system.

5.3Reduce external water sources entering the pipe network

During the investigation, it is found that the external water sources entering the pipe network in this city mainly include rainwater, river water (including a large number of internal rivers entering the collection pool of pumping stations) and groundwater. Therefore, measures should be taken as far as possible to reduce the entry of foreign water sources into the sewage pipe network, and even to prevent river water from entering the pipe network. In this regard, the following measures are proposed:①The main measure to prevent rainwater from entering the sewage pipe network is to gradually change the drainage system into the rain and sewage complete separation system.; ②For several drainage outlets connecting the sewage pipe network and rivers in the urban area, as well as culverts serving as sewage main pipes to collect domestic sewage, under the condition of not affecting flood discharge,,It can be blocked to prevent rainwater and other low-concentration external water sources from entering the pipe network.;For the overflow port where river water is easy to flow backward, a suitable flap can be installed to prevent it from flowing backward. (3) do a good job of leakage prevention and impermeability. When designing urban drainage network, we should investigate the groundwater and surface water conditions in various areas of the city.,In order to do a better job of leakage prevention and impermeability, so as to achieve the purpose that the influent concentration of sewage plant is least affected by the infiltration of groundwater and surface water.

5.4Gradually transform septic tanks

If there is no septic tank, feces and other debris will directly enter the sewage pipeline, which will easily cause pipeline blockage, so it is not recommended to completely dismantle the septic tank.,Considering the influence of septic tank on the influent concentration of sewage plant, it can be reformed. Most communities or buildings in this city are equipped with three-stage septic tanks, which have many stages and long sewage retention time.,To reduce the ability of septic tank to treat organic matter, the number of stages can be reduced, and the retention time of sewage and anaerobic digestion time can be shortened.,A simple precipitation effect can be achieved. Corresponding measures can be taken to reduce the number of septic tanks and reduce the volume of septic tanks.

5.5Improve the construction of sewage pipe network and improve the pollution acceptance rate

The problem that often exists in urban sewage plants is that although the sewage plants are completed and put into operation as planned, only part of the sewage main pipes and branch pipes are built in the sewage pipe network.,Not all the sewage in the planned area is collected, resulting in insufficient actual treatment capacity of the sewage plant and the phenomenon that the uncollected sewage is directly discharged into the river to pollute the environment. The sewage treatment plant in this city also has similar problems, so it is necessary to re-plan the drainage pipe network to fully connect with the old drainage system.,Try to ensure that all sewage in the planned area will eventually be discharged into the sewage plant.

5.6Strengthen residents’ awareness of water saving,Improve water use efficiency

The city is rich in water resources, the water price is not high, the residents’ water consumption is increasing year by year, and the habit of wasting water is also formed, so it is particularly important to strengthen the residents’ awareness of water saving. It is suggested to start from the following two aspects.:①Do a good job in publicity, encourage residents to save water, and encourage residents to use more water, such as collecting water for washing vegetables and bathing as toilet water. (2) adjust the water fee or the planned water consumption is the same as the fixed water supply, and control the water consumption of residents, so that residents can gradually form the habit of saving water.