With the continuous improvement of science, technology and quality of life in China, the discharge of sewage is increasing gradually, so it is necessary to effectively solve the problems of water pollution and water shortage. In this case, the technology of reclaimed water development and reuse has developed rapidly, and has been widely used in the United States, Japan, India, Britain and other countries (especially Japan), which is of great significance to realize the sustainable utilization of water resources. In domestic universities, Tsinghua University adopts membrane bioreactor integrated process to treat bath water, and all the reclaimed water is used to flush the toilets in students' dormitories. The net benefit of reclaimed water reuse project reached 1, 304,438+0,000 yuan. MBR technology was adopted in the reclaimed water reuse project of Youshi University in China, with a direct economic benefit of 525,000 yuan [1].
It is understood that there are about 0.2m3 college students in China at present, and the reclaimed water is 4.6 million cubic meters/day [1]. These domestic sewage are discharged into the urban sewage pipe network and treated by the urban sewage treatment plant. However, campus greening and flushing toilets in student apartments consume a lot of tap water, resulting in waste of energy and resources, insufficient number of water-saving campuses, and uneven management level and water-saving benefits [2]. Taking Zhengzhou University as an example, the water quality characteristics of domestic sewage in university dormitories were studied, and appropriate processes were selected for treatment and reuse according to the water quality characteristics. In this study, the new process of "grid-primary sedimentation tank -A/O tank-biological contact oxidation tank-secondary sedimentation tank-surface flow constructed wetland" was selected to treat some campus dormitory domestic sewage, which reached the standards of urban miscellaneous water use and landscape reuse, not only reducing the sewage discharge to the drainage system, saving the operating cost of urban drainage facilities and the sewage treatment cost paid by the school, but also effectively alleviating the shortage of campus water supply [3].
1 quality analysis and process selection of domestic sewage in colleges and universities
1. 1 Quality Analysis of Domestic Sewage in Colleges and Universities
According to the field investigation, there are about 40,000 students in Zheng Da New District, each of whom can produce about 70L domestic sewage every day and about 2800m3 domestic sewage every day. Student accommodation is divided into four parks, namely Liuyuan, Lotus Garden, Chrysanthemum Garden and Songyuan Garden. There are about 14000 students in Liuyuan, and some floors in Liuyuan are equipped with sewage reuse devices, which can be reused as toilet flushing water after simple treatment, so it is not considered for the time being. There are about 26,000 people in the other three parks, so * * can generate about 1.800 m3 of domestic sewage every day. If we have a normal holiday in February, July and August, about 500,000 m3 of domestic sewage will be generated every year. At the same time, Humei Lake in the new campus of Zhengzhou University is an artificial lake in the campus, with large area and large water demand. If the domestic sewage of campus dormitory can be reused in this artificial lake, it can not only realize the effective reuse of sewage, but also reduce the cost of reusing Hu Mei Lake in schools.
1. 1. 1 water quality monitoring indicators and methods (table 1)
1. 1.2 sewage quality characteristics
The characteristics of water consumption in colleges and universities are that students' water consumption is greatly influenced by season and temperature, and the coefficient of change of water consumption in colleges and universities is large and has regularity [4]. The quality of domestic sewage in colleges and universities is relatively stable and the pollution is low. After long-term monitoring of the domestic sewage quality of some dormitories in the new campus of Zhengzhou University, the water quality is shown in Table 2:
The domestic sewage of college students' dormitory does not contain kitchen drainage, but only bathroom and bathroom drainage, which belongs to high-quality miscellaneous drainage and can be completely treated and reused by colleges and universities themselves.
Process selection of 1.2
According to the principle of process selection: ① advanced technology and stable treatment effect; ② Low investment and operation cost; ③ Simple management and reliable operation. Determine the process of domestic sewage treatment and reuse in university dormitories in this study as shown in figure 1:
1) Primary sedimentation tank: Primary sedimentation tank can remove settleable and floating objects in wastewater. After preliminary sedimentation, the wastewater can remove about 50% of settleable, grease and floating objects and 20% of BOD. According to the calculation of removing BOD5 or solid matter per unit mass, the primary sedimentation tank is the most economical purification step.
For domestic sewage and industrial sewage with high suspended solids, the primary sedimentation tank should be used for pretreatment (Figure 1).
2)A/O tank: A/O process connects the first anoxic stage and the second aerobic stage in series, in which the DO in the A stage is not more than 0.2mg/L and the DO in the O stage is 2 ~ 4 mg/L. In the anaerobic stage, anaerobic bacteria hydrolyze suspended pollutants such as starch, fiber, carbohydrate and soluble organic matter in sewage into organic acids, so that macromolecular organic matter is decomposed into small molecular organic matter and insoluble organic matter is converted into soluble organic matter. These products after anoxic hydrolysis enter an aerobic tank for aerobic treatment, which can improve the biodegradability and oxygen efficiency of sewage. In the anoxic stage, heterotrophic bacteria ammoniate pollutants such as protein and fat (N in organic chain or amino in amino acid) and release ammonia (NH3 and NH4+). Under the condition of sufficient oxygen supply, the nitrification of autotrophic bacteria oxidizes NH3-N(NH4+) into NO3-, and returns to the A pool through reflux control. Under anoxic conditions, denitrification of heterotrophic bacteria reduces NO3- to molecular nitrogen (.
3) Biological contact oxidation tank: filler is set in the aeration tank as the carrier of biofilm. After being oxygenated, the wastewater to be treated flows through the filler at a certain flow rate and contacts with the biofilm, and the biofilm interacts with the suspended activated sludge to purify the wastewater.
4) Secondary sedimentation tank: The secondary sedimentation tank is an important part of the activated sludge system, and its main function is to separate sludge, clarify, concentrate and reflux mixed liquor. Its working effect directly affects the effluent quality and return sludge concentration of activated sludge system.
2 experimental equipment and content
2. 1 experimental device
In this experiment, the process flow shown in Figure 1 is adopted, and the pilot plant is shown in Figure 2. The main components are: primary sedimentation tank, A/O tank, biological contact oxidation tank and secondary sedimentation tank, and the water treatment capacity is 30-40L/h. ..
1)A/O: It consists of two parts, with the ratio of 1: 3, namely, anoxia before and aerobic after. Including tank, filler, agitator, aeration device, etc. The anoxic tank has an inner diameter of 800mm and a height of 900mm, and the aerobic tank has an inner diameter of 1200mm and a height of 1500mm.
2) Biological contact oxidation tank: The structure includes tank body, filler, water distribution device and aeration device. The pool type is a cuboid; The tank size is 460mm long, 400mm wide, 8mm thick, with total height 1400mm and superelevation of 50 mm. 3) Primary sedimentation tank: the tank type is cylindrical; The size of the tank is 340mm in outer diameter, 8mm in wall thickness, 540mm in total height and 50mm in superelevation.
4) Secondary sedimentation tank: the tank type is cylindrical; The size of the tank is 340mm in outer diameter, 8mm in wall thickness, 600mm in total height and 80mm in superelevation.
2.2 Determination of process parameters
In this paper, the domestic sewage in the dormitory of the new campus of Zhengzhou University is taken as the research object, and its specific water quality indicators are COD concentration 100 mg/L ~ 394 mg/L, ammonia nitrogen concentration 10 mg/L ~ 40 mg/L, total phosphorus concentration 2 mg/L ~ 4 mg/L, and pH = 7 ~ 9. Taking the removal efficiency of COD, ammonia nitrogen and TP as the main index.
The selected process is used to treat domestic sewage in colleges and universities. The main factors affecting this process are pH, DO, HRT, SRT, reflux ratio, anoxic-aerobic reaction time and so on. According to literature review, it is determined that MLSS is 3000 ~ 3500 mg/L, dissolved oxygen in aeration tank is 2.0 ~ 3.5 mg/L, sludge reflux ratio is 75%, hydraulic retention time is 12h[5], anoxic-aerobic HRT is 6h and 12h, and sludge reflux ratio and nitrification liquid reflux ratio are 65433 respectively. In biological contact oxidation, the optimal gas-water ratio is 16: 1, and the optimal hydraulic load is 5.0m3/(m3? Chapter VI.
3 Analysis of experimental results
Bacteria are cultured by inoculating sludge from sewage treatment plant, and COD, NH3-N and TP are removed by running pilot plant, as shown in Figure 3 ~ Figure 5:
The COD removal effect of the reactor is shown in Figure 3. The influent COD fluctuated greatly, ranging from 109. 1 ~ 328.5mg/L, with an average of 214.1mg/L. The effluent COD of the system was relatively stable, ranging from13.6 ~ 29.5 mg/L. As can be seen from the figure, the removal rate of COD is relatively stable, ranging from 74.0% to 94.5%, with an average removal rate of 85.9%, indicating that the reactor has a good removal effect on COD. The suspended sludge floc in the reactor contains a large number of bacterial micelles with compact structure, which has strong biosorption and oxidation ability for organic matter and greatly promotes the removal of COD. A large number of filamentous bacteria grow in the sludge floc on the surface of suspended filler, which is beneficial to the adsorption of bacterial micelles, which not only improves the sludge settling performance, but also effectively promotes the oxidative decomposition of organic matter.
The removal effect of NH3-N by the reactor is shown in Figure 4. The concentration of ammonia nitrogen in dormitory sewage is low, and the influent ammonia nitrogen is in the range of 18.40 ~ 35.20 mg/L, with an average of 28.02 mg/L; The effluent ammonia nitrogen is in the range of 5.94 ~ 9.39 mg/L, with an average value of 7.95mg/L, which meets the urban miscellaneous water standard. As can be seen from the figure, the removal rate of ammonia nitrogen is relatively stable, ranging from 62.05% to 76.64%, and the average removal rate is 7 1. 1 1%, indicating that the removal effect of ammonia nitrogen by the system is general. It is considered that the biofilm formation time is too short and the biofilm formation is insufficient, which leads to the fact that although the filler provides good attachment conditions for the growth of nitrifying bacteria, the biomass per unit volume in the reactor is not sufficient and the nitrification capacity is not too high.
The removal effect of total phosphorus by the reactor is shown in Figure 5. The influent TP concentration is 2. 12 ~ 3.60 mg/L, and the average influent TP concentration is 2.85 mg/L; The TP concentration in the effluent is 0. 16 ~ 0.48 mg/L, and the average TP concentration in the effluent is 0.3 1mg/L, which meets the urban miscellaneous water standard. The removal rate of TP is 85.33% ~ 9 1.20%, and the average removal rate is 89.28%, which shows that this process has a good removal effect on TP. It is considered that the filler in the anoxic tank prevents the surface air from entering the anoxic tank, reduces the oxygen mass transfer efficiency, causes the anaerobic microenvironment in the anoxic section, and forms a micro-anaerobic/anoxic/aerobic system. Phosphorus-accumulating bacteria release phosphorus in anaerobic environment, absorb phosphorus through O-stage aerobic, and then discharge it with shed biofilm and suspended sludge, thus achieving phosphorus removal effect. At the same time, the system regularly discharges sludge through the bottom hopper, and a large amount of phosphorus-containing sludge is discharged with the sludge accumulated at the bottom.
4 Conclusion and prospect
4. 1 conclusion
(1) By analyzing the characteristics of domestic sewage in university dormitories, it is determined that the treatment process is "grid-primary sedimentation tank -A/O tank-biological contact oxidation tank-secondary sedimentation tank-surface flow constructed wetland".
(2) According to the actual situation and process design, a pilot plant of "grid-primary sedimentation tank -A/O tank-biological contact oxidation tank-secondary sedimentation tank" was designed. When the MLSS is 3000-3500mg/L and the dissolved oxygen in the aeration tank is 2.0-3.5mg/L, the sludge reflux ratio is 75% and the hydraulic retention time is 65438+. In biological contact oxidation, the optimal gas-water ratio is 16: 1, and the optimal hydraulic load is 5.0m3/(m3? D) As operating parameters, the results show that the removal rates of COD, NH3-N and TP are 93.77% ~ 94.69%, 62.05% ~ 76.64% and 85.33% ~ 93.82% respectively. The effluent COD is 4.98 ~ 7.83 mg/L, and NH3-N is 5.94 ~ 5.95 mg/L..
(3) Class C water quality standards for landscape and entertainment water stipulate that COD≤30mg/L, NH3-N≤0.5mg/L and TP≤0.05mg/L, and urban miscellaneous water quality standards stipulate that COD≤50mg/L and NH3-N ≤10 mg/L. Because the effluent index of NH3-N exceeds the "Landscape"
4.2 Prospects
(1) Because the removal rate of ammonia nitrogen is too low, the expected goal of reuse for landscape water quality labeling has not been achieved. The analysis reason should be that the operating parameters of the pilot plant in this experiment are the best operating parameters obtained by consulting the literature, and the best operating parameters suitable for this technological process were not found during the experiment, which led to the failure to reach the best state during the operation; It is also possible that the biofilm formed by biological contact oxidation pool is not perfect, so more attention should be paid in future research.
(2) Because the pilot plant is not equipped with constructed wetland, the effluent quality does not meet the standard of landscape water reuse. However, in the practical engineering application, in the follow-up study, the artificial lake can be transformed, and a large number of wetland plants such as reeds, water lilies and cattails can be planted to build a surface-flow artificial wetland, which can make full use of school resources and improve water quality, so as to reduce the groundwater recharge of the artificial lake and give people a pleasant landscape value.
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