The production of a large amount of surplus sludge is an important issue facing the activated sludge process. Since 2003, China's surplus sludge production has exceeded 10 million tons (National Bureau of Statistics, etc., 2010); During the large-scale construction during the "Eleventh Five-Year Plan" period, China's sludge output was about 21.88 million tons in 2011. It is estimated that by 2015, China's sludge output will exceed 30 million tons (Fu Tao et al., 2010). Effective treatment and disposal will produce secondary pollution, directly or indirectly threaten environmental safety and public health, and at the same time greatly reduce the environmental benefits of sewage treatment facilities. Surplus sludge reduction technology has attracted widespread attention from researchers at home and abroad, and currently mainly includes Physical methods (such as mechanical action, heat treatment, microwave, ultrasonic, radiation, etc.), chemical methods (such as acid-base treatment, ozone oxidation, Fenton reagent oxidation, supercritical water oxidation, chemical agent uncoupling, etc.) and biological methods (such as biological Predation, biological enzymes, multifunctional microbial preparations, etc.) The reduction effect of single sludge reduction technology is often limited. Ye Fenxia et al. (2004) used 3, 3 ′, 4 ′, 5-tetrachlorosalicylide (TCS) As Uncoupling agent, when the dosage is 0.5 mg ， g-1 (based on VSS), the sludge reduction is about 30%; Zhu Yishu et al. (2008) used ultrasonic wave for aerobic digestion of sludge, so that The sludge TSS reduction is about 40%. The combination of sludge reduction technology and activated sludge treatment system can achieve further reduction or even zero discharge of residual sludge. The main reduction mechanisms are dissolution-hidden growth, uncoupling metabolism , Maintaining metabolism, etc. (Wei et al., 2003).
The sludge ozone reduction technology is to lyse activated sludge by ozone treatment, and then return it to the biological treatment system. Microorganisms in the system use this part of the material to perform hidden growth, thereby achieving the purpose of reducing weight (Chu et al. ,, 2009). Yasui et al. (1996) conducted a 9-month study on the technology in a small urban sewage treatment plant in Japan, and there was no residual sludge discharge during operation. At the same time, researchers focused on ozone treatment of activated sludge. A series of studies have been carried out on the effects of sludge reflux and the biological treatment system after ozone treatment. The study found that after ozone treatment, the sludge concentration decreased and the soluble COD increased (Yang et al., 2011); VSS / The TSS, pH, and bound water content decreased, and the Zeta potential increased (Bougrier et al., 2006). The higher ozone dosage reduced the sludge particle size (Zhang et al., 2009); sludge The biodegradability has been improved (Yeom et al., 2002). The lower ozone dosage has no significant effect on the sludge microorganism species, while the higher ozone dosage results in a gradual reduction of microorganism species and even on sludge. Severe activity Bad (Yan et al., 2009). After the ozone-treated sludge is returned to the biological treatment system, COD and nitrogen still have a high removal rate, but the COD and ammonia nitrogen concentrations in the effluent slightly increase (Kou Qingqing et al., 2012 ), The concentration of nitros nitrogen is kept low, and the concentration of nitrite is reduced (Sun Dedong et al., 2006), and the sludge reflux has no significant effect on the denitrification of biological treatment systems (Dytczak et al., 2006), and ozone Dissolved and difficult-to-sedimentary particulate organic matter generated during the treatment can be used as a carbon source for denitrification (Ahn et al., 2002), and SS remains low (Lee et al., 2005). Studies have shown (Saktaywin et al. , 2005), after the combination of sludge ozone treatment and activated sludge process, the removal effect of phosphorus is reduced. This is because biological phosphorus removal is achieved through the discharge of excess sludge. Sludge reduction and even zero emissions make phosphorus in biological treatment The gradual accumulation in the system causes the phosphorus concentration in the effluent to rise, so the ozone reduction of sludge should be combined with the phosphorus removal process.
Based on this, this study investigated the effects of sludge ozone reduction, process control parameters, and the effect of reflux sludge after ozone treatment on the biological treatment system, and explored the optimal conditions for chemical phosphorus removal of the sludge supernatant after ozone treatment, with a view to Provide technical support for the large-scale application of sludge ozone reduction technology.
2 Materials and methods
2.1 Sludge ozone treatment experiment
The sludge used in the experiment was taken from the return sludge of the secondary sedimentation tank of the A / A / O process of the Qinghe Wastewater Treatment Plant in Beijing. The MLSS and MLVSS were 8603 and 6185 mg ， L-1. The original 1400 mL sludge was taken before each experiment. Aerated for 24 h, and then diluted to 3500 mL with deionized water. The sludge ozone treatment experiment uses a semi-continuous reaction mode, that is, the sludge is sequentially fed into the reactor, and ozone is continuously passed into the reactor. A total of 2 are completed. Batch of ozone treatment experiments. The volume of the sludge samples was 500 mL, the ozone gas flow rate was 0.2 L ， min-1, and sand core aeration was used. In the first batch of experiments, the initial pH was 6.8 and the oxidation time was 10 minutes, the dosage of ozone was 0, 26, 63, 154, 227, 268 mg ， g-1 (based on SS); in the second batch of experiments, the ozone gas concentration was 50 mg ， L-1, oxidation The time is 9 minutes, and the initial pH is adjusted to 3.0, 5.0, 7.0, 9.0, 11.0.
The ozone gas concentration is measured by the iodometric method. After the reaction is completed, the residual ozone in the system is blown out with nitrogen and collected with a 1% KI solution. The actual ozone consumption is calculated by formula (1).
In the formula, MO3, cons is the ozone consumption (mg); CO3, gas, in is the ozone concentration at the reactor inlet (mg ， L-1); QO3, gas, in is the ozone gas flow rate (L ， min-1) , Measured by a gas flow meter; T is the ozone oxidation time of activated sludge (min); MO3, gas, out is the amount of ozone (mg) not participating in the reaction.
The sludge dissolution rate R is used to characterize the degree of lysis, and the calculation formula is as follows:
In the formula, [MLSS] o and [MLSS] t represent MLSS (mg ， L-1) of sludge after ozone treatment and after ozone treatment.
2.2 Effect of sludge reflux on biological treatment system after ozone treatment
This experiment uses two sets of small activated sludge test devices, one set as a control system, running with the traditional activated sludge method; the other set adds a sludge ozone treatment unit as a sludge reduction system. Each set of devices consists of one Aeration tank (5.2 L) and a secondary sink (2.6 L), placed in a temperature-controlled water bath at 20 ℃. The hydraulic retention time (HRT) of the control system and the reduction system was 15 h, and the dissolved oxygen (DO) was 3 ~ 5 mg ， L-1, pH = 5 ~ 7, MLSS is (1690 ＼ 50) mg ， L-1 and (1756 ＼ 90) mg ， L-1, sludge age (SRT) of the control system is 20 d The ozone dosage of the sludge ozone treatment unit is 100 mgg-1.
The reaction device was continuously operated for 60 days and divided into two stages. The first stage lasted 15 days, and both sets of reactors were operated using the traditional activated sludge method; the second stage lasted 45 days, and the sludge ozone treatment unit in the reduction system Started, the control system still operates according to the traditional activated sludge method. In the second stage, the daily sludge volume of the control system is 0.25 L, and the reduction system takes 0.4 L of sludge daily for ozone treatment, and then treats the sludge. Injected into the aeration tank, there is no remaining sludge discharge during the operation of the sludge reduction system. The experiment uses artificial wastewater, consisting of peptone, beef extract, ammonium sulfate, potassium phosphate, sodium acetate, magnesium sulfate, calcium chloride, iron chloride and Formulated with trace elements, the main water quality indicators are: COD 363 mg ， L-1, TN 68.73 mg ， L-1, NH3-N 21.2 mg ， L-1, TP 9.6 mg ， L-1, pH = 6.2. Before the system was officially operated, the inoculated sludge was cultured and domesticated with artificial wastewater for 30 days. The treatment effect and activated oxygen sludge specific oxygen consumption rate (SOUR) of the two systems were monitored.
2.3 Dephosphorization experiment of sludge supernatant after ozone treatment
Ca (OH) 2 was selected as the phosphorus removal agent, and different calcium-phosphorus molar ratios (1.7, 3.3, 6.7, 10.0, 13.3) and the phosphorus removal effect on the sludge supernatant after ozone treatment, as well as SCOD, nitrogen, and pH were investigated, respectively. Impact.
2.4 Analysis method
After the sludge samples are mixed uniformly, the total COD (TCOD) is measured. After filtering through a 0.45 μm filter membrane, the soluble COD (SCOD), total nitrogen (TN), ammonia nitrogen (NH3-N), and nitrate in the sludge liquid phase are measured. Nitrogen (NO-3-N), nitrite nitrogen (NO-2-N), total phosphorus (TP), and active phosphorus (PO3-4-P). The numbers of the Hach water quality analysis methods used for the above indicators are respectively : 8000, 10072, 10031, 8039, 8507, 8114, and 10127. The determination of the specific oxygen consumption rate (SOUR) of activated sludge was performed with reference to the literature procedure (Wang Jianlong et al., 1999).
3 Results and discussion
3.1 Ozone treatment of sludge
During the ozone treatment of sludge, the amount of ozone added and the initial pH of the activated sludge will affect the dissolution rate of the sludge and the dissolution of organic matter, nitrogen, phosphorus and other substances, thus affecting the effect of sludge ozone reduction. Therefore, it is reasonable. The amount of ozone added and the initial pH of the sludge are not only beneficial to the sludge ozone reduction, but also can improve the economics of the process.
3.1.1 Effect of ozone dosage on sludge dissolution rate
First, the effect of different ozone dosage on sludge dissolution rate was investigated. As shown in Figure 1, as the ozone dosage increased, the MLSS and MLVSS of sludge gradually decreased, and the sludge dissolution rate gradually increased. When the dosage of ozone is 0 ~ 154 mg ， g-1, the dissolution rate increases rapidly, from 0 to 26%; when the dosage of ozone is 154 ~ 268 mg ， g-1, the dissolution rate of sludge increases. Slow, increased from 26% to 33%. In order to obtain a better sludge reduction effect while reducing the cost of ozone treatment of sludge as much as possible, a reasonable dosage of ozone is recommended to be about 150 mg ， g-1.
Figure 1.Effect of ozone dosage on sludge dissolution rate
3.1.2 Effect of Ozone Dosage on COD, Nitrogen and Phosphorus Dissolution in Sludge
Figure 2 examines the effect of the amount of ozone added on the dissolution of COD, nitrogen, and phosphorus in the sludge. With the increase of the amount of ozone added, the TCOD of the sludge decreases to a certain extent, and SCOD, TN, TP, and PO3- 4-P increased significantly, NH3-N, NO-3-N increased, and NO-2-N remained within a low concentration range (less than 1.5 mg ， L-1). This is consistent with the law reported by Yang et al. (2011) When the ozone dosage reached 227 mg ， g-1, the TCOD of sludge decreased by 7.2%, SCOD increased by 50.5 times, and TN, NH3-N and NO-3-N increased by 3.7, 12.2, and 1.4, respectively. Times, TP and PO3-4-P increased by 11.1 and 10.4 times respectively. This shows that after ozone treatment, the microbial cells rupture and the intracellular substances are released into the sludge liquid phase, which makes the sludge SCOD, TN, TP significantly. Increase; with the increase of the amount of ozone, some organic matter is mineralized or blown off by ozone, which reduces the TCOD of the sludge, and some organic nitrogen is converted into NH3-N, which is then oxidized to NO-2-N and NO. -3-N, part of the organic phosphorus is converted to PO3-4-P.
In this batch of experiments, SCOD and TN reached the highest values when the ozone dosage was 227 mg ， g-1 (1081 and 104 mg ， L-1), and TP was 268 mg ， g-1 At the highest value (25.7 mg ， L-1), the TN increased in the sludge liquid phase was mainly organic nitrogen (about 68%), and the increased TP was mainly PO3-4-P (about 86%). The activated sludge denitrification system (such as A / A / O process) often has the problem of insufficient carbon source. If the ozone-treated sludge is returned to the biological treatment system, it may allow to improve the denitrification effect and achieve sewage. The dual purpose of zero sludge discharge. However, in the case of sludge reduction or even zero discharge, the return of ozone-treated sludge to the biological treatment system will increase the phosphorus load of the system, resulting in the gradual accumulation of phosphorus in the system, and ultimately the The effluent phosphorus concentration exceeds the standard, so additional phosphorus removal measures need to be considered.
Figure 2 Effect of ozone dosage on COD, N and P dissolution in sludge
3.1.3 Effect of initial pH on sludge dissolution rate
Figure 3 examines the effect of different initial pH on the sludge dissolution rate. As the initial pH increases, the sludge dissolution rate first increases (pH = 3 ~ 9) and then decreases (pH = 9 ~ 11). It may be because alkali can destroy microbial cells, leading to the dissolution of intracellular substances and synergistic effects with ozone, and the alkaline conditions are conducive to the formation of OH with strong oxidizing capacity during the ozone reaction, thereby improving the oxidation efficiency of ozone. Too high initial pH is not conducive to sludge ozone oxidation reaction, because too many hydroxide ions will cause browning reaction of sludge (reaction between amino compounds and carbonyl compounds, the reaction product has a certain degree of oxidation resistance ( Abraham et al., 2007).
Figure 3 Effect of initial pH on sludge dissolution rate
3.1.4 Effect of initial pH on COD, nitrogen and phosphorus dissolution in sludge after ozone treatment
Figure 4 shows the effect of initial pH on the dissolution of COD, nitrogen, and phosphorus in sludge after ozone treatment. Before ozone treatment, the TCOD and SCOD of the sludge were 4245 and 18 mg ， L-1, TN, NH3-N, and NO, respectively. -2-N and NO-3-N are 12.00, 0.25, 0.05, 5.05 mg ， L-1, TP and PO3-4-P are 0.68 and 0.60 mg ， L-1, respectively.
Figure 4 Effect of initial pH on COD, N, P dissolution in sludge after ozone treatment
With the increase of the initial pH, the TCOD of the sludge was basically unchanged after ozone treatment. SCOD and TN increased rapidly (pH = 3 ~ 7), then increased slowly (pH = 7 ~ 9), and finally decreased (pH = 9 ~). 11), NO-3-N increased gradually, NH3-N and NO-2-N accounted for a small proportion in TN (0.4% ~ 3.1% and 0.1% ~ 0.7% respectively), and there was no significant change. Therefore, The alkaline conditions are more conducive to the reduction of ozone in the sludge. This may be due to the higher oxidation efficiency of ozone under the alkaline conditions, the enhanced lysis of sludge, and the dissolution of intracellular substances. It is further oxidized to NO-3-N. But as mentioned earlier, the browning reaction of the sludge due to the excessively high initial pH is not conducive to the ozone treatment of the sludge. With the increase of the initial pH, TP and PO3- 4-P gradually decreased (pH = 3 ~ 9) and then increased or remained stable (pH = 9 ~ 11), indicating that acidic conditions are more conducive to the release of phosphorus-containing substances in sludge cells into the sludge liquid phase. . Therefore, the initial pH neutral or weak alkaline conditions are conducive to the ozone treatment of sludge.
3.1.5 pH change of sludge after ozone treatment
Table 1 shows the pH of the sludge after ozone treatment under different initial pH conditions. When the initial pH is slightly acidic, the pH of the sludge increases slightly; when the initial pH is neutral or alkaline, the pH of the sludge after treatment There is a large reduction. Organic matter will be decomposed into a variety of small molecular organic acids under the action of ozone. The alkaline conditions are favorable for the formation of OH, which enhances the oxidation of ozone, and produces more organic acids, resulting in a pH after the reaction. The reduction is relatively large; under acidic conditions, the organic acid produced by the reaction does not contribute much to pH, and part of the organic carbon escapes as CO2 after mineralization, so the pH does not decrease significantly but increases after the reaction.
Table 1 Changes in pH of sludge after ozone treatment
3.2 Impact of sludge reflux on biological treatment system after ozone treatment
The above experimental results show that after the sludge is treated with ozone, organic matter, nitrogen, phosphorus and other substances will be released into the sludge liquid phase to achieve a certain degree of sludge reduction. If the ozone-treated sludge is returned to the biological treatment system In the use of hidden growth of microorganisms, further reduction of sludge can be achieved, and even the purpose of zero sludge discharge. The following experiments comprehensively evaluate the impact of sludge reflux after ozone treatment on biological treatment systems.
3.2.1 Effects of sludge reflux after ozone treatment on removal of COD, nitrogen, phosphorus and SS in biological treatment systems
Figure 5 shows the removal rates of COD, TN and TP in the sludge reduction system and the control system. In the first stage (first 15 d), the sludge reduction system and the control system use the same operation mode, so the COD of the two systems , TN and TP removal rates are very close. In the second stage (after 45 d), the sludge reduction system started the ozone treatment process, and its COD and TN removal rates remained basically the same as those of the control system, while the TP removal rate decreased significantly. Other scholars The results of this study also prove this (Lee et al., 2005; Kou Qingqing et al., 2012). During the entire operation period, the COD of the effluent of the two systems was between 15 and 35 mg ， L-1, and the removal rate was above 90%. The effluent TN is between 50 ~ 60 mg ， L-1, and the removal rate is about 20%. In the second stage, the NH3-N in the effluent of the sludge reduction system slightly increased, and the NO-3-N was about 30 mg. ， L-1, NO-2-N is less than 2 mg ， L-1.
Figure 5 COD, TN and TP removal rates in the sludge reduction system and the control system
In this study, the traditional activated sludge method was used, and the denitrification process could not be performed in the bioreactor. However, due to the long hydraulic retention time of the secondary sedimentation tank (about 7.5 h), partial denitrification reactions can be performed, so it has a certain denitrification function The concentration of various forms of nitrogen in the effluent remained basically the same between the two operating stages and between the two treatment systems, indicating that the effect of sludge reflux after ozone treatment on the nitrification process was not obvious. The sum of nitrogen concentrations is approximately equal to the TN concentration, indicating that most of the organic nitrogen is converted to inorganic nitrogen after the biological treatment system; NH3-N still maintains a certain concentration level, indicating that the nitrification process in the system is incomplete. Sludge reduction system and control The SS of the effluent of the system is 7.5 and 15 mg ， L-1, which indicates that the sludge reflux after ozone treatment has a certain improvement on the SS of the effluent of the activated sludge system. This is because the sludge returned to the biological system after ozone treatment can It is beneficial to change the sludge particle size distribution balance in the bioreactor (Bohler et al., 2004) and increase the organic load of the microorganisms in the bioreactor (ie, the F / M ratio is increased). Microorganisms produce more extracellular polymers (EPS) (Dytczak et al., 2006), thereby improving sludge sedimentation performance. The reduction of phosphorus removal capacity of the sludge reduction system indicates that the sludge reduction process and phosphorus removal need to be combined The combination of processes can achieve long-term stable operation of the sludge reduction system.
3.2.2 Effect of sludge reflux after ozone treatment on microbial activity in biological treatment system
Figure 6 is the oxygen consumption rate curve of activated sludge. The specific oxygen consumption rate SOUR of activated sludge is obtained based on the oxygen consumption rate and MLSS. The SOUR of the ozone sludge reduction system is about 0.19 mg ， g-1 ， min-1 ( SS), the SOUR of the control system is about 0.23 mg ， g-1 ， min-1, which is not much different, indicating that the sludge reflux after ozone treatment has no significant effect on the biological activity of the biological system. The sludge SVI is very close, about 82 mL ， g-1, indicating that the sedimentation performance of activated sludge in both systems is good.
Fig. 6 Curve of oxygen consumption rate of activated sludge in reduction system and control system
3.3 Preliminary study on phosphorus removal from sludge supernatant after ozone treatment
As mentioned before, when the remaining sludge of the biological treatment system is zero-discharged, the phosphorus concentration in the effluent will gradually increase. Therefore, it is necessary to increase the phosphorus removal process to achieve the TP of the effluent. The effect of the effect. As can be seen from the figure, as the molar ratio of calcium and phosphorus increases, TP and PO3-4-P in the supernatant gradually decrease. When the molar ratio of calcium and phosphorus is 3.3 to 10.0, the phosphorus removal rate increases faster. At 0 ~ 3.3 and 10.0 ~ 13.3, the removal rate increased slowly. When the molar ratio of calcium and phosphorus was above 10.0, the removal rates of TP and PO3-4-P both exceeded 80%.
Fig. 7 Effect of calcium-phosphorus molar ratio on phosphorus removal effect
Figure 8 examines the effect of calcium-phosphorus molar ratio on SCOD, nitrogen, and pH in the sludge supernatant after phosphorus removal. As can be seen from the figure, with the increase of calcium-phosphorus molar ratio, SCOD, TN, and NO-3-N have Reduced, NH3-N and NO-2-N are kept in a low concentration range (less than 1.7 and 0.2 mg ， L-1, respectively). When the molar ratio of calcium and phosphorus is 10.0, SCOD, TN and NO-3-N are reduced respectively 20.2%, 11.2%, and 48.6%. After the phosphorus removal, the sludge supernatant is returned to the biological treatment system to provide the carbon source for the nitrogen removal of microorganisms. With the increase of the calcium-phosphorus molar ratio, the sludge supernatant The pH rises rapidly first, then slowly rises, and then gradually stabilizes between 12-13. This is because when the molar ratio of calcium to phosphorus is large, most of the phosphorus has precipitated, and the supernatant has excess Ca (OH) 2. This alkaline The sludge supernatant may need to be adjusted back to pH before being returned to the biological treatment system. The effect of the pH value of the sludge supernatant on the biological treatment system after phosphorus removal.
Figure 8 Effect of calcium-phosphorus ratio on SCOD, N and pH of sludge supernatant after phosphorus removal
1) After ozone treatment, the reduction effect is more significant. With the increase of ozone dosage, the MLSS and MLVSS of the sludge gradually decrease, the sludge dissolution rate gradually increases, and the organic matter, nitrogen, phosphorus, etc. in the microbial cells Substances are released into the sludge liquid phase. It is recommended that a reasonable dosage of ozone is about 150 mg ， g-1, and the sludge dissolution rate can reach about 26%. The sludge dissolution rate is higher under the condition that the initial pH is slightly alkaline. It is also conducive to the dissolution of organic matter and nitrogen, while partial acidic conditions are conducive to the dissolution of phosphorus. Considering the effect of the initial pH comprehensively, the ozone treatment of sludge should be performed under the initial pH neutral or weakly alkaline conditions.
2) After the ozone-treated sludge is returned to the biological treatment system, it has no significant effect on the microbial activity, COD and nitrogen removal effect; because there is no residual sludge discharged from the system, phosphorus is gradually accumulated in the system, resulting in phosphorus removal effect Decrease, need to increase the phosphorus removal process.
3) Ca (OH) 2 is used as the dephosphorizing agent of the sludge supernatant after ozone treatment. The high calcium-phosphorus molar ratio is beneficial to the dephosphorization of the supernatant. It is recommended to control its value at about 10.0. At this time, TP and PO3- The removal rates of 4-P were all above 80%; as the molar ratio of calcium and phosphorus increased, SCOD and TN in the supernatant decreased, and the pH gradually increased.