Using Coagulants Poly Aluminum Chloride and Aluminum Sulfate to Optimize Phosphorus and Turbidity Removal from Secondary Effluent

Transcription

1 Using Coagulants Poly Aluminum Chloride and Aluminum Sulfate to College of Environmental and Resource, Southwest University of China, Chongqing, , China Abstract There are many factors affecting the turbidity and phosphorus removal performance, historical study had focus more on developing efficient chemical coagulants, optimize exist coagulant system and application of coagulation-flocculation method. Despite the drastic variations in such kind of field, researches of the usage of coagulation-flocculation remains neglected. Purpose of this paper is based on the deeply lab experiment and try to give sum detail discussions relate to secondary wastewater treatment. It also discussed some topics which other researches never done or few discussed before, for example, the optimum ratio of reagents, and the difference between different levels of TP. The experiment shows that the proposed method is effective. Keywords: Chemical Coagulants, Poly Aluminum Chloride, Aluminum Sulfate 1. Introduction In the status qua, as the world spreading phenomenon of eutrophication of water bodies deteriorate so lead to a series of related environmental and ecological problems [1]. This phenomenon of 'premature ageing' of aquatic systems can result in the undesirable presence of algal blooms. Such kind of aberrant growth of algae adversely impairs downstream water treatment (WWTP) processes and restricts recreational activities in the vicinity. Meanwhile, the main cause for eutrophication in water bodies is an accumulation of nutrients such as phosphorus and nitrogen [2][3]. Historically, phosphorus removal from wastewater is a complex process due to differences in its forms and limited chemical as well as physical condition during the secondary treatment. In consequence, although secondary treatment may remove more than 85.0% of the BOD5 and suspended solids, it does not remove significant amounts of nitrogen, phosphorus, or heavy metals. So it is urging to discover new methods to promote phosphorus removal [4][5]. According to the research on existing municipal wastewater treatment plants, phosphorus removal was under 40.0% at cost for influent with a average phosphorus concentration of 2.7 mg/l which is much lower than anticipate [6][7]. The alternative approach to removal phosphorus fall into three main categories: physical, chemical, and biological. Physical methods have proved to be either too expensive, as in the case of reverse osmosis, or inefficient, removing only 10.0% of the TP [8][9]. Enhanced biological treatment can remove up to 97.0% of the TP,, however this process can be highly variable due to operational difficulties. Chemical removal techniques, using metal salt, are reliable and wellestablished processes. Further world wide land scale researches include 37 Full-scale phosphorus removal plants indicate that more than 97.0% plants use metal salt treatment and among those approximately 85.0% of treatment plants directly chose alum as the chemical coagulant For instance USA secondary treatment, majority of Canada, Germany, Norway and Sweden[10]. Those three dominant chemicals available for phosphorus removal are calcium, aluminum and iron. Based on the experiment, the chemical reagents selected were: aluminum sulfate and poly aluminum chloride (PAC). For [Al 2 (SO 4 ) 3 18H 2 O], historical research found that average 87.5% of turbidity removal can be done and maximum removal efficiency observed was 97.4% for ortho-phosphorus and 95.6% for TP counterpart. Under optimum condition, best removal proportion for PAC could reach to 96.3% and 93.9% for ortho-phosphorus and TP respectively. Further experiments showed the cooperation of two coagulants occurred the ideal turbidity and phosphorus removal efficiency, usually the dosage composed of inorganic coagulant alum or ferrous and polymer coagulants (either organic or International Journal of Digital Content Technology and its Applications(JDCTA) Volume6,Number17,September 2012 doi: /jdcta.vol6.issue

2 inorganic ). Such kind of mixture dosage renders theoretically accordance, immediately after the alum or ferrous into wastewater, it charge processes of charge neutralization between two particles nearby, followed with the dosage of high polymer like PAC which relate to further reaction of absorption and binding action among those micro-flocs. With those two processes compensate each other so that mitigate some inappropriate physical or chemical obstacles in other words, ph, mixture intensity etc. Experiment had done for that which saw a maximum phosphorus removal efficiency of 92.1% for secondary municipal effluent with aluminum chloride and PAM under ph=7.0. The problem is there are many factors affecting the turbidity and phosphorus removal performance, the common parameters include ph, alkalinity, speed of flask mixing and other interfering substances. Although some of these factors have already been understand thoroughly in several historical works. Ebeling indicated that flocculation and mixing speed played only minor role in both phosphorus and solid removal processes, so there are other factors vary drastically with the practical condition include ph, coagulant dose. Thereby, experiments relate to those factors are necessity as well as it should design to optimize such conditions. Historical study had focus more on developing efficient chemical coagulants, optimize exist coagulant system and application of coagulation-flocculation method. Despite the drastic variations in such kind of field, researches of the usage of coagulation-flocculation remains neglected. Purpose of this study was based on the deeply lab experiment and try to give sum detail discussions relate to secondary wastewater treatment. It also discussed some topics which other researches never done or few referred before, for example, the optimum ratio of reagents, and the difference between different levels of TP. The rest of the paper is organized as follows. Section 2 is the description of materials and methods. Section 3 focuses on experiment results and discussion. Section 4 focuses on conclusions. 2. Materials and methods 2.1 Raw water origin All chemicals used in this study were analytical grade reagents. The raw wastewater used in this experiment was origin from Ilsan wastewater treatment plant, Goyang-si, Gyeonggi-do, South Korea. Turbidity, TP and ph were 3.0 NTU, 0.73 mg/l, 6.6, respectively. By contrast, the artificial wastewater acted as the comparison group with higher TP. The reagents were potassium di-hydrogen phosphate (KH 2 PO 4 ), which had been dried in oven under 120 for 2 hours. In accordance with the phosphorus concentration of 5.0 mg/l, add mg to ml distill water and diluted to 5.0 L. ph=5.5 was adjusted use chemical buffer: sulfate acid (H 2 SO 4 ) ph=4.0 and sodium hydroxide (NaOH) ph=10.0. The basic chemical characteristics are turbidity equal to 0, TP equal to 5.0 mg/l and ph adjusted to 5.5, by counterpart. The chemical coagulants of PAC and aluminum sulfate [Al 2 (SO 4 ) 3 18H 2 O] were prepared. 2.2 Predetermination of ph The works handled on the principles of chemical coagulation-flocculation. Several comparisons had been done within the effluent from natural wastewater treatment plant (WWTP) and artificial high phosphorus concentration wastewater, aluminum sulfate [Al 2 (SO 4 ) 3 18H 2 O], aluminum chloride (PAC) and the mixture compound were the basic chemical reagents. Finally optimum conditions of those coagulants had been determined and illustrated in graphs. Turbidity removal and phosphorus removal efficiency were two indicators been tested. Turbidity was showed as unit NTU and tested use turbidimeter while phosphorus was analyzed in accordance to the standard methods. Determine the optimum ph for removal the turbidity and TP in secondary effluent, it is obviously that ph control coagulation is one of the most important factor governing phosphorus removal, as phosphorus always combine with sludge particles, here simply test removal rate of turbidity and suppose the removal of phosphorus was inconsistent with it. Experiments undertook by drop-wise 0.05 ml PAC and 3.0 ml aluminum sulfate and mixture compound each into ml wastewater, high speed mixture of 250 r/min for 0.5 min and medium speed of 200 r/min for 1.0min, low speed mixture of 30 r/min for 10.0 min finally sediment for 30 min, sample water were directly taken from the supernatant water. 430

3 Turbidity was tested. 2.3 Coagulation-flocculation procedures In each of the tests, optimum condition was met and idea ph for each groups reached, ml sample water took into consideration and coagulant was added under stirring. For each group of experiment (secondary effluent and artificial wastewater), high velocity mix of 250 r/min took place for 30 sec, followed by medium mix velocity of 200 r/min for 1min at the same time drop-wise the coagulant, then slow mix of 30 r/min for 10 min, finally the settling period lasted for 30 min. In accordance with test remains phosphorus, before test, supernatant of secondary effluent need chemical digestion in autoclave for 30 min with aid of sulfuric acid and potassium persulfate (GB/T ). Coagulants amount in each single experiment Table 1: Table 1. Coagulants amount in each single experiment UNIT VOLUME PAC(ml) PH 4.5 Alumina(ml) PH 5.5 PAC/alumina(1:1) 0.05:3 0.1:6 0.15:9 0.2: :15 PH 5.0 (ml:ml) PAC/alumina(2:3) 0.03:3 0.06:6 0.1:9 0.15: :15 PH 5.0 (ml:ml) PAC/alumina(3:2) 0.08:3 0.15:6 0.25:9 0.3:12 0.5:15 PH 5.0 (ml:ml) PAC/alumina(3:1) (ml:ml) 0.15:3 0.3:6 0.45:9 0.6: :15 PH 5.0 Secondary effluent turbidity: NTU, TP: mg/l Artificial wastewater turbidity: 0 NTU, TP: 5.0 mg/l Lab experiment temperature: Experiment Results and Discussion Under the optimum condition, two crucial parameters turbidity and TP were examined based on two different water samples. The central points were the comparison of three different dosages in each experiment and evaluation outcome performance. So, several experiments were carried out until yielding a maximum removal percent. 3.1 Result for predetermination of ph Determine the optimum ph condition for each group of experiment had been carried out as the first step because ph is among the most important parameters and turbidity removal efficiency could be used to reflect the circumstances within phosphorus, result showed in figure 1 below: 431

4 3 Turbidity (NTU) PAC aluminium sulfate mixture PAC/Al ph(25 celsus) Figure 1. Turbidity removal efficiency according to ph (PAC, aluminum sulfate, mixture PAC/Al) Table 2. Data of remain turbidity under different ph ph PAC NTU NTU NTU NTU NTU Aluminum sulfate NTU NTU NTU NTU NTU Mixture PAC/Al NTU NTU NTU NTU NTU 3.2 Turbidity removal According to the experiment schedule, turbidity removal efficiency was one of the two important wastewater parameters, the sludge remained in the effluent over regulate standard, the sludge settled in the downstream will intense high pressure to treatment plant. The inner principles for turbidity removal are colloidal potential neutralization or absorption: sludge particles attached several times to the same aluminum ion, and also can attach to other aluminum ions. In this way, such molecules are linked together through aluminum bridges to form large, often insoluble complexes. It is manifest that the turbidity removal efficiency were much higher using chemical coagulants, results showed in Figure 2: Turbidity removal % UNIT VOLUME PAC aluminium sulfate PAC/Al(1:1) PAC/Al(2:3) PAC/Al(3:2) PAC/Al(3:1) Figure 2. Turbidity removal efficiency for each different coagulant (secondary effluent) Summarize from those figures, obviously when compare the PAC dosage and alumina dosage, generally alumina saw a higher removal efficiency of average 85.0%. However, it is yield that mix dosage of PAC/Al enjoyed average removal percent of 87.5%, which the highest percent even could 432

5 reach 90.5%. Retrospect to historical study, low absorption rate occurred by simple add dosage of PAC as the turbidity in raw water was not high enough to leaves the sludge particles unsaturated to form perfect bridge. Besides, the removal rate would be further undermined by the over amount of PAC particle (PAC>0.15 ml), the removal efficiency reduced from 87.3% to 81.6%. By contrast, when use alumina, the alumina molecule ions neutralized electrical potential with sludge particle and generated micro-flocs facilitate the sedimentation so that the efficiency for alumina was higher than PAC. Therefore, PAC and alumina could compensate each other when used as amalgamation to palliate the side effects, for the simple reason of the combination of two basic principles. As high proportion of PAC would deteriorate the flocculation and repel the nearby micro-flocs with the phenomena of 'colloidal protection', the data for group of PAC/Al (3:1) was the lowest. In consequence, in this experiment, the optimum removal efficiency was approximately 90.5% reached by the chemical aid between PAC/Al 2:3~3:2 contain 1:1. Taking into grant practical influences, generally PAC/Al (0.15 ml: 12 ml) was the ideal dosage ratio in this study. Further study may revise this data more accurately. 3.3 Phosphorus removal As illustrated in Figure 3, both those groups remain TP were less than 0.5 mg/l except the group PAC/Al (3:1), so under the lab condition, removal of phosphorus use chemical coagulants were extremely ambient, the maximum removal efficiency observed on the experimental domain was reached to 82.1% with the dose of aluminum sulfate 15 ml and PAC/Al (0.18 ml: 15 ml). The average removal efficiency reaches nethermost to 65.0% with the origin TP mg/l More specifically, dosage of aluminum sulfate was more efficiency than PAC at the same alumina concentration. It is manifest that mix dosage obtained optimum removal efficiency compare to simple aids except group PAC/Al (3:1) which summarized the average data was about 70.0%. Both three groups of PAC/Al 1:1, 2:3, 3:2 showed tendency of gradually ascend. Among the scale of experiment, the optimum dosage of chemical removal of phosphorus should be settled as 2:3~3:2 under ph 5.0. Although the removal efficiency in this experiment were less than historical study which obtained average efficiency of 89.3%, considering the reality low phosphorus concentration of secondary effluent, this is acceptable. During this experiment, as the samples were effluent from WWTP, TP were composed of orthophosphate and insoluble organic phosphorus which exist accompany with sludge particles. The phosphorus remain in effluent would first reacts with the orthophosphate. Based on this principle, the removal of orthophosphate should be more efficiency than insoluble phosphorus simply because the insoluble part must be removed together with organic sludge by a complicated combination of interaction and adsorption to form 'bridge'. In the present study of the Pearson correlation between Phosphorus removal and turbidity removal, a correlation of showed in the secondary effluent which addressed the previous concern. Such kind of chemical structure is not as stable as the precipitation of AlPO 4 which is insoluble in aquatic. But when formed such kind of colloidal, it will act reciprocal on AlPO 4 and facilitate the precipitation of it. That is one of the reason for the over dosage of PAC would deteriorate the final removal efficiency as mentioned before. When use coagulationflocculation to removal effluent from waste treatment plant, not the more aids the better. All in all, take into grant the economic and removal of turbidity simultaneously, the optimum removal dosage is PAC/Al (0.15 ml : 12 ml) in the experiment concentration for ml sample. 433

6 TP removal percent % UNIT VOLUME TP removal by PAC TP removal by alluminium sulfate TP removal by PAC/Al(1:1) TP removal by PAC/Al(2:3) TP removal by PAC/Al(3:2) TP removal by PAC/Al(3:1) Figure 3. TP removal efficiency from secondary effluent According to the artificial wastewater, the experiment data showed in Table 3. For the sample water high in phosphorus concentration, the average removal efficiency observed to be more than 75.0%, and the highest removal efficiency reached about 88.9% with the dosage of aluminum sulfate for amount of 9.0 ml. But the optimum average performed dosage among those groups was the simple aid of PAC, which enjoyed average removal percent of 83.3%. The graph line of PAC/Al (1:1) and PAC/Al (2:3) were almost parallel and overlap and after a slight increase, two lines maintained a turn point and decrease to about 60.0%, much lower than average removal efficiency among all the groups. The least efficiency of phosphorus remove was happened in the group of PAC/Al (3:2), with the average data was 65.0%. Overall, both PAC and aluminum sulfate are well performed on treating high phosphorus wastewater which meant that alumina is a good coagulant for WWTP to handle secondary effluent phosphorus. On the other hand, with optimum ph 5.5, in this experiment the inconsistent dosage for removal of phosphorus decided in this experiment is PAC: Phosphorus (1.62:1) with the optimum efficiency reached to 88.9%. When applied to industrial use, nonetheless, the best choose is not the optimum one while take consider the economic. As PAC is much more expensive than aluminum sulfate, the optimum dosage should be PAC/Al 2:3 (0.06 ml: 6.0 ml). TP removal percent % UNIT VOLUME TP removal by PAC/Al(1:1) TP removal by PAC/Al(2:3) TP removal by PAC/Al(3:2) TP removal by PAC TP removal by aluminium sulfate Figure 4. Phosphorus removal efficiency for artificial wastewater Table 3. Data of phosphorus removal efficiency for artificial wastewater PAC(ml) origin TP(mg/L) ph remain TP(mg/L) removal percentage %

7 Al(ml) PAC/Al(1:1) 0.05: : : : : PAC/Al(2:3) 0.03: : : : : PAC/Al(3:2) 0.08: : : : : Compared the phosphorus removal efficiency between secondary effluent and artificial wastewater, apparently it is better perform in the high phosphorus wastewater with average removal efficiency reached to more than 75.0%. The amount of coagulant for artificial water to optimize the process was lower than secondary effluent which meant that orthophosphate was easier to treat than TP. Nonetheless, average remain phosphorus concentration in treated secondary effluent was mg/l (municipal wastewater treatment plant effluent standard TP<0.5 mg/l). For artificial wastewater of high phosphorus concentration, the data was mg/l counterpart superior the effluent water standard, so water reclaimed and advanced wastewater treatment method need be applied. 4. Conclusions Based on the results of the experiments in this study, the following conclusions can be drawn: Aluminum salts acting as coagulants in WWTP secondary effluent are kinds of highly effective and chemical coagulation-flocculation methods in treating turbidity and phosphorus is generally acceptable. The maximum turbidity and phosphorus removal efficiency under the optimum dosage condition from secondary effluent was 90.5% and 78.8%, which inconsistent with the residual turbidity NTU, phosphorus residual concentration mg/l. The optimum dosage of removal of turbidity and phosphorus for the secondary effluent under ph 5.0 generated from this experiment was PAC/Al ( ml: 12.0 ml), for alumina concentration mg/l and mg/l respectively. According to the artificial wastewater used in this experiment as a contrast group, optimum dosage been determined was PAC/Al (0.06 ml: 6.0 ml). Under this condition, the optimum phosphorus removal efficiency reached highly to 82.3%, higher than efficiency in secondary effluent. 435

8 Overall, the mix coagulant of PAC and aluminum sulfate were more effective than the simple dosage of either one, PAC had a wide adaptability in ph and other kinds of physical situation than aluminum sulfate, the optimum dosage ratio of PAC and aluminum sulfate in treat turbidity and phosphorus in low phosphorus water and high phosphorus water was both around 1:1.5, (mol: mol) for aluminum concentration ratio took grant of economic influence in the experiment scale. 5. References [1] A.I. OMOIKE, G.W. VANLOON, "Removal of phosphorus and organic matter removal by alum during wastewater treatment", Water Research, Elsevier, Vol.33, No. 17, pp. 3617~3627, [2] T. STEPHENSON, "Phosphorus removal by chemical precipitation in a biological aerated filter", Water Research, Elsevier, Vol. 31, No. 10, pp. 2557~2563, [3] J. E. GREGOR, "Optimizing natural organic matter removal from low turbidity waters by controlled PH adjustment of aluminum coagulation", Water Research, Elsevier, Vol. 31, No. 12, pp. 2949~2958, [4] Kai Yang, "Municipal wastewater phosphorus removal by coagulation", Environmental technology, Taylor & Francis, Vol. 31, No. 6, pp. 601~609, [5] T. Clark, Stephenson, "Development of a jar testing protocol for chemical phosphorus removal in activated sludge using statistical experiment design", Water Research, Elsevier, Vol. 33, No. 7, pp ~ 1734, [6] Xiaoxia Niu, "Preparation and coagulation efficiency of poly aluminum ferric silicate chloride composite coagulant from wastewater of high-purity graphite production", Journal of environmental sciences, Elsevier, Vol. 23, No. 7, pp ~ 1128, [7] Yong-Wei Jin, Ya-Mei Dong, Zhen-Jia Zhang, "The Simultaneous Removal of DOC and Ammonia Nitrogen from Surface Water by Hybrid Process", In Proceeding of 2009 International Conference on Environmental Science and Information Application Technology, Vol. 2, pp.36~39, [8] Xufeng Mi, "Study of Performance and Mechanism of Sandwich Protection Devices for Bridges", International Journal of Advancements in Computing Technology, Advanced Institute of Convergence Information Technology, Vol. 4, No. 5, pp. 280 ~ 286, [9] Wang Nianqin, Chen Liang, Cai Qianqian, Zhang Wansong, Liu Zhuannian, "Study on the Performance of a Novel Inorganic Polymeric Flocculant", In Proceeding of 2011 International Conference on Computer Distributed Control and Intelligent Environmental Monitoring, pp.1827~1829, [10] Yung-Hao Kao, Chi-Chung Lee, "Use of Artificial Immune Algorithm with Memory and Suppressor Cells to Optimize the Placement of RFID Portal Reader Antennas", International Journal of Digital Content Technology and its Applications, Advanced Institute of Convergence Information Technology, Vol. 6, No. 6, pp. 312 ~ 321,