1. Start and run
The anaerobic reactor used the digested sludge of a municipal wastewater treatment plant in Tianjin as a seed sludge. The inoculation began on June 10, 2002. Continuous inflow of water began on June 20, gradually increasing the reactor load according to the general UASB starting law.] . Because the production of paper has not yet reached the designed output. Therefore, the operating load of the circulating water treatment is lower than the design load and fluctuates between 3.5-7.0 kg COD/m3.d.
Determination of COD concentration, volatile fatty acids (VFA), suspended solids (SS), pH, and temperature of circulating water (influent from the biological treatment system), anaerobic and aerobic effluent, and measurement of different sites from time to time during operation The water BOD, alkalinity, hardness, and sludge concentration in the reactor record the amount of water entering the treatment system, the amount of clean water used in the production, the amount of electricity used, and the amount of paper produced.
From the beginning of anaerobic operation, papermaking workshops gradually increased the amount of treated water to reduce the amount of fresh water. Because the workshop uses an independent cost accounting, the worker's enthusiasm for water conservation is high. Figure 2 shows that since the anaerobic treatment system was operating, the plant's clean water usage has been reduced to below 2.5m3/t paper (all process water except boiler water), and no wastewater has been discharged. The workshop improved the operation of the paper machine, which enabled the speed of the rotary paper machine to reach 130m/min, which was much higher than the design speed of more than 70 meters per minute of the equipment. However, the production did not have an important impact, indicating that the water quality can meet the production needs. Although for the waste paperless pulp and paper mill without deinking, the national wastewater discharge standard (GB3544-2001) allows the discharge of 150m3/t paper, and the water consumption of many factories is still quite high. With drainage management, after proper use of large amounts of recycled water, the wastewater discharge per ton of paper has been gradually reduced to about 7 cubic meters; after the new anaerobic and aerobic treatment system was put into operation for 3 months, it finally achieved its expectation. The zero discharge target of waste water, and the amount of water required per ton of paper is only 2.4t, while the clean water consumption of similar zero-emission factories in Europe is below 4m3/t paper.
2. Removal of organic pollutants
Table 2 shows the average daily removal of organics (indicated as COD) from the anaerobic-aerobic biological treatment system after continuous influent water.
In the paper production scale does not reach the predetermined design level, one of the two reactors is mainly operated (the other only enters a small amount of water to maintain the operation), and the load of the normal operation of the reactor is only 3.5-7 kg/m3.d. between. It can be seen from Table 2 that after one month of continuous inflow of anaerobic reactor, the amount of COD removed per day has reached 1000kg per day. After the anaerobic reactor is started up, the anaerobic treatment system basically reaches a stable state in October. The average amount of COD removed in a month was 2536 kg/d in October (October), and the statistics in November and December were 2,967 kg/d and 2,639 kg/d, respectively. Based on the monthly paper production, circulating water produced per ton of paper removed 26.89 kg of COD during the anaerobic phase.
Since most of the biodegradable organics have been removed during the anaerobic phase, the removal rate of anaerobic effluent in the subsequent aerobic treatment is greatly reduced, and the COD removal rate in the aerobic SBR reactor is only in the same period. About 24%, COD removal amounted to 430kg per day, and 4.56kg per ton of paper. Total anaerobic and aerobic COD removal was 31.45kg/t paper. The anaerobic part removed COD accounted for 85.5% of the total, and aerobic only accounted for 14.5%. Since the anaerobic system removes so much COD, the amount of aeration required for aerobic treatment is greatly reduced, so circulating water can be processed at a very low cost.
Further information can be obtained from the BOD removal in Table 3. It can be seen that the system has a high removal rate of BOD, with an average of 96.5 %, of which only anaerobic fraction is removed by 92.3%; after anaerobic treatment The effluent BOD is very small, and the aerobic system removes about 50% of the residual BOD, but for the total BOD removal, anaerobic accounts for 96% of it, and the aerobic part only accounts for 4%. The high removal rate of BOD is still stable after all recycled water is reused.
The above results show that the biological treatment system can effectively remove the organic matter in the circulating water. Most of these organic substances are removed in the anaerobic system.
In addition, from the perspective of the whole plant, when the easily biodegradable organic matter is removed, the biorefractory organic matter in the completely closed circulating water system will accumulate (for example, lignin and its derivatives), and the inert components in the circulating water will be much larger than The proportion of closed water used before, so after the start of the start, the anaerobic treatment COD removal rate will gradually decline from a higher level, and then stabilized at a level, this removal rate is necessarily less than the wastewater treatment system for the purpose of meeting the discharge standards. Since the amount of organic matter removed by the organics introduced into the system during the production process and the processing system must be balanced, the size of the removal rate also depends on the amount of water entering the treatment system. The greater the amount of water processed, the smaller the removal rate.
3. Control of organic matter concentration in circulating water
Different from the standard discharge of general industrial water treatment, the goal of water treatment in a zero-emission factory is to maintain the water quality suitable for the production water under the condition that the water is completely recycled. According to foreign literature, in the production of corrugated paper and zero-emissions plants, the experience of Germany’s Z黮pich, who first adopted this type of process, can meet the water quality requirements of corrugated paper production with circulating water maintained at 8000 mg COD/l. [1,6]. However, after Z€黮pich in Germany, the Belgian VPK Oudegem mill, which started operation at the end of 1999, controlled the circulating water concentration below 5000mgCOD/l. [7]
In this example, the reuse ratio of recycled water increases until it is completely reused. However, due to the effective treatment, the COD concentration of circulating water reaches a balanced range, and it is stable and does not continue to increase. As can be seen from Fig. 3, due to the accumulation of inert substances, the effluent COD concentrations of anaerobic and aerobic treatments have increased at the initial stage, but have not risen to a certain extent. At the same time, we saw that the concentration of circulating water dropped from 7000 mg/l and was finally well controlled between 4000-5000 mg COD/l.
During this period, the factory's three cylinder paper machines operated at a speed of 130m/min (designed at a speed of only 70m/min and more) without serious web dewatering problems and other anionic trash and secondary stickies. The problem. Although Figure 3 also shows that the residual COD in the effluent is still above 1000mg/l, these CODs are basically inert components, and most of them come from lignin components, which have not had much impact on the papermaking process.
In the management, the factory adopts the independent accounting of the workshop, and the water consumption is allocated to the cost. The local water resource fee per square meter is 1.8 yuan and the electricity fee is 0.18 yuan, which is 1.98 yuan/m3 in total. Therefore, each workshop actively reduces the amount of clean water, but due to the difficulty of water scheduling, the water may be reused in the initial stage, sometimes the processing system does not have enough water, and the added water capacity of nearly 2000m3 in the treatment system requires supplementary water. The average water consumption per ton of paper can only be gradually reduced. In October, a larger reservoir was built and the water scheduling problem was further solved. After November, the water consumption was reduced to less than 2.2-2.3m3/t paper. There was no wastewater discharge at all, and the added clean water included all process water. And the clean water in the workshop is mainly used to supplement the water evaporated during the papermaking process and the moisture discharged by the sludge and slag discharge, as well as the amount of supplementary water in the newly added pool.
4. Removal of low molecular organic acids
Organic acids are important components of COD and BOD in waste water. In the treatment of papermaking recycled water, the low molecular weight part of organic acids, mainly volatile organic acids such as acetic acid, propionic acid, and butyric acid (VFA), becomes a foul paper. The main source of taste is the quality problems that are difficult to overcome with the product, and it also deteriorates the operating environment.
Cellulose, hemicellulose, and added excipients such as starch and carboxymethyl cellulose in papermaking raw materials can all serve as a source of low molecular weight organic acids. After a closed cycle, microbial activity increases the number of low molecular weight organic acids. The speed of formation, according to the literature, if there is no biological treatment system, the concentration of waste paper production corrugated paper VFA can be accumulated to 10000-13000mg/l, accounting for about 40% of COD [6].
The removal rate of VFA in this system is very high (Table 4). After a period of operation, the concentration of VFA in the circulating water before treatment can be maintained at 23-35 mmol/l, and anaerobic treatment can remove 90% of the volatilization. Acid, after aerobic treatment, the volatile acid in the water is almost completely removed. Therefore, the papermaking process has no odor-induced environmental problems and product quality problems.
5. Biological softening of circulating water
One of the other technical advantages of anaerobic is its demineralization of calcium. The anaerobic treatment produces a large amount of CO2, and the CO2 in the water is in an oversaturated state, so that Ca2O3 is precipitated with Ca+2 ions in the water, and the hardness of the water is decreased, thereby solving the problem of accumulation of Ca+2 ions in the circulating water. The CaCO3 precipitation actually takes place in an aerobic treatment after anaerobic, and the rise of pH caused by the aeration action causes the CaCO3 precipitate to be generated. This process is similar to the sedimentation of lake bottom carbonate caused by bio-softening in natural lakes. This effect effectively suppresses the increase in water hardness, which is controlled within a certain range. Habets et al. [7] suggested that the carbonate concentration of recycled water should be less than 500mg/l.
As shown in Figure 4, in this project example, after the anaerobic biological system was operated to a certain stage, the water's calcium carbonate concentration was reduced to less than 500 mg/l. The average CaCO3 removal rate of incoming and outgoing water is about 25%, effectively controlling the water hardness accumulation.
6. Effect of UASB on aerobic treatment
Before the anaerobic system was operated, the SBR reactor was forced to extend the operating cycle due to severe sludge expansion and influent concentration, and the influent water was diluted with fresh water.
After the anaerobic system was operated, the sludge volume ratio of SBR had been reduced to 15% in less than one month, and SBR operation has never experienced sludge expansion problems since then. After the SBR stops aerating, the water clarification speed is very fast. This is consistent with the widely accepted theory that anoxic or anaerobic processes prior to aerobic treatment can inhibit sludge expansion during subsequent aerobic processes. The speed of sedimentation of aerobic effluent was significantly accelerated due to the fact that calcium carbonate in the sludge increased the sedimentation performance of the sludge.
Since the sludge expansion problem has been solved, the operating cycle of SBR is reduced from 16 hours to 8 hours, and the aeration time is reduced to 4 hours. The daily treatment water volume of SBR is rapidly increased to 600-700 m3/d. Although not all anaerobic water has been treated, it has been several times larger than the original treatment volume. Although the SBR feed water concentration and the aerobic COD removal rate were lower than before the anaerobic operation, the total removal of COD by SBR was higher than before.
Fourth, the recycling water treatment costs and economic benefits
1. Cost of circulating water treatment
The cost of circulating water treatment based on the actual cost of the plant operation is shown in Table 4.
The cost of this recycling water treatment is quite low. The main reasons include the following:
* Most of the organics are removed in the anaerobic section, and the anaerobic treatment does not require aeration, so the recycled water treatment per ton of paper saves 15-20 kWh compared to the aerobic treatment.
* Drug costs are greatly reduced, and most of the nutrients in the closed loop remain in the system. Therefore, in practice, only a small amount of pharmaceuticals should be added based on the actual concentration of water, such as nitrogen and phosphorus. In addition, anaerobic treatment consumes only one-third of aerobic nutrients per unit of COD even if nutrients in the closed cycle are not considered.
* The amount of sludge produced by anaerobic is very small, theoretically equivalent to only 1/6-1/10 of the same organic matter removed [5]. In actual operation, sludge was discharged only once in November from the start of operation to the end of the year, and the amount discharged was 5 tons (dry matter). The organics removed by aerobic treatment accounted for only a small part (only 14.5% of the total COD removal as described above), so the cost of sludge disposal was much lower.
2. Other economic benefits
* Saving water, Tianjin water resource fee is 1.8 yuan/m3, and the reduction in water consumption produces obvious economic benefits.
* Energy production Biogas production is 13 m3/t paper, equivalent to 270 MJ per ton of recycled water.
* The increase of fiber and raw material recovery rate. According to Tianjin's experience, the consumption of raw materials is greatly reduced compared with the biological treatment without circulating water (including aerobic and anaerobic components). Fiber and raw material recovery accounted for 6.9% of the output. The raw material consumption of paper is reduced to less than 115 tons.
Source: Journal Author: He Yanling, Huang Fu Hao,