Tài liệu Study on phosphorus recovery from digested piggery wastewater by struvite precipitation

HANOI UNIVERSITY OF SCIENCE TECHNISCHE UNIVERSITÄT DRESDEN Master Thesis STUDY ON PHOSPHORUS RECOVERY FROM DIGESTED PIGGERY WASTEWATER BY STRUVITE PRECIPITATION Vu Phuong Thuy Supervisor: Professor. Dr. Cao The Ha Hanoi, November 2011 ACKNOWLEDGEMENTS I would like to thank Professor Doctor Cao The Ha, my supervisor, and greatly appreciate his supervision, advice, and guidance from the early stage of this research as well as giving me experiences throughout the work. During the time I was doing the research, he also gave the great opportunity, the best support and conditions in many ways for me to go to National Institute for Environment Studies, Japan to participate in the training course “Sustainable Landfill Management”. Many thanks to all the staffs in the Center for Environmental Technology and Sustainable Development and the Faculty of Chemistry of Hanoi University of Science for helping me with analytical techniques. I would like to thank to Dr. Kazuyoshi Suzuki, Dr. Anton Perera, Prof. Phil Westerman for your helps with answering my questions. Finally, I would like to thank to my family, my friends who are always supportive and encourage me during difficult times, especially my mom who helped me to take care of the experiment that was performed in my house. ABSTRACT This research studied on struvite precipitation as a method to remove and recovery phosphorus in the real digested piggery wastewater to form a product as fertilizer. There are many factors that affect to the precipitation of struvite such as pH, ion molar ratios, aeration, mixing energy, temperature… In this study, series of jar-test experiments were conducted to investigate the influence of pH, Mg:P molar ratio, Ca ions and mixing speed to the phosphorus removal efficiency and the crystallization of struvite. Experiments with lab-scale reactor in batch and continuous mode were also conducted to investigate the kinetic of phosphorus removal, the controlled struvite crystallization and the quality of the generated product. The results showed that, a) At pH at 9.25, the phosphorus removal efficiency was the highest. b) Increasing Mg:P molar ratio (from 1 to 2.5) and stirring speed (from 50 rpm to 100 rpm) led to enhanced phosphorus removal efficiency. c) Ca ions play important role in both phosphorus removal efficiency and crystallization of struvite. A presence of Ca ions at Mg:Ca of 4:1 to 2:1 will increase the phosphorus removal efficiency and affect the morphology of struvite crystals compared to Mg:Ca of 1:1, 2:1 or 1:0. d) The rate constant of phosphorus removal in the pilot-scale experiment with digested piggery wastewater when Mg ions added estimated as 0.050 min-1 or 3.00 hr-1. e) It is possible to attach and grow struvite crystals on stainless steel mesh in order to recover it as fertilizer. KEYWORDS Struvite, stirring speed, crystallization. digested piggery wastewater, phosphorus removal, TABLE OF CONTENTS Page List of figures...................................................................................................................3 List of tables.....................................................................................................................4 INTRODUCTION..........................................................................................................5 1 REVIEW OF LITERATURE ...............................................................................7 1.1 Introduction to phosphorus and its applications..................................................7 1.2 Impacts of excessive phosphorus in water streams..............................................8 1.3 Recovery of phosphorus.......................................................................................8 1.4 Phosphorus removal and recovery techniques.....................................................9 1.5 Struvite precipitation..........................................................................................13 1.5.1 Background.................................................................................................13 1.5.2 Struvite chemistry in wastewater................................................................14 2 GOAL AND OBJECTIVES.................................................................................17 2.1 Goal....................................................................................................................17 2.2 Objectives...........................................................................................................17 3 MATERIALS AND METHODS..........................................................................19 3.1 Wastewater collection and analysis...................................................................19 3.2 Jar-test experiment procedure............................................................................20 3.2.1 Impact of pH...............................................................................................20 3.1.2 Impact of magnesium addition....................................................................20 3.1.3 Impact of Calcium ions...............................................................................21 1 3.2.4 Impact of stirring speed..............................................................................22 3.3 Bench-scale experiments....................................................................................22 3.3.1 Reactor design and batch mode experiment...............................................22 3.3.2 Accumulation device design and continuous mode experiment.................25 3.4 Analytical and assessment methods...................................................................28 3.4.1 Analytical techniques..................................................................................28 3.4.2 Kinetic model design for phosphorus removal...........................................28 4 RESULTS AND DISCUSSIONS..........................................................................30 4.1 Characteristics of the digested piggery wastewater...........................................30 4.2 Jar-test experiments - Impact of factors.............................................................30 4.2.1 pH............................................................................................................... 30 4.2.2 Magnesium addition....................................................................................32 4.2.3 Calcium ions…...........................................................................................34 4.2.4 Stirring speed..............................................................................................41 4.3 Bench-scale experiments....................................................................................43 4.3.1 Batch mode experiment..............................................................................43 4.3.2 Continuous mode experiment.....................................................................46 5 CONCLUSIONS AND RECOMMENDATIONS..............................................53 5.1 Conclusions…................................................................................................... 53 5.2 Recommendations............................................................................................. 54 REFERENCES ............................................................................................................55 2 LIST OF FIGURES Figure 3-1 Schematic diagram of batch mode experiment......................................................23 3-2 Photo of batch mode experiment...........................................................................24 3-3 Schematic diagram of continuous mode experiment.............................................26 4-1 Effect of pH to %P-Removal.................................................................................32 4-2 Effect of magnesium addition to %P-removal.......................................................33 4-3 Effect of Mg:Ca ratio to %P-removal....................................................................36 4-4 (a), (b), (c) Photos of struvite crystals by light microscope..................................39 4-4 (d), (e), (f). Photos of struvite crystals by light microscope..................................40 4-5 Residual P-PO4 concentration at different stirring speed versus time...................42 4-6 Residual P-PO4 concentration and pH value versus time......................................44 4-7 Linear of kinetic model of phosphate removal......................................................45 4-8 (a), (b), (c) Photos of struvite crystal growth on stainless steel mesh....................48 4-9 Photos of struvite crystals on inner and outer mesh..............................................49 4-10 (a), (b) Photos of struvite crystals taken out of the mesh.......................................50 4-11 X-RAY diffraction of recovered struvite crystals..................................................52 3 LIST OF TABLES Table 1-1 Summary of phosphorus removal technologies, Morse et al. (1998) ...................10 1-2 Summary of phosphorus recovery technologies, Morse et al. (1998)...................11 4-1 Characteristic of the digested piggery wastewater (19/07/2011)...........................30 4-2 Effect of pH to %P-Removal.................................................................................31 4-3 Effect of magnesium addition to %P-removal.......................................................33 4-4 Solid species examined and selected as primary precipitation..............................34 4-5 Effect of Mg:Ca ratio to %P-removal....................................................................35 4-6 Residual P-PO4 concentration at different stirring speed versus time...................42 4-7 Residual P-PO4 concentration and pH value versus time......................................43 4-8 Characteristic of wastewater before and after the experiment...............................45 4 INTRODUCTION Livestock industry is a very important sector of Vietnam which has the strong growth especially in pork production in recent years. However, this can lead to many serious environmental issues as well. According to Department of Livestock Husbandry, Ministry of Agriculture and Rural Development, until October 2009, the number of pigs in over Vietnam was 27.6 mil and predicted to be 36.9 mil by 2015. In 2006, there were 18,000 castle farms in Vietnam in which 6,000 were piggery farms. Each year, 20-30 mil m3 liquid waste is discharged from pig farms and 80% of that is discharged directly into the environment without any pretreatment and caused many pollutions. Livestock wastewater contains high content of nutrients. In Vietnam, the most common treatment method for livestock wastewater is biogas tank and lagoon. Biogas tanks can treat COD and lagoons can treat N, P but with long retention time. The wastewater quality after treatment by these methods is not good enough for discharging into the environment yet. Especially, there are lack of management of phosphorus and ammonium which are elements that can cause pollutions of surface water bodies by extra eutrophication. Besides, phosphorus is an irreplaceable element essential for every living thing on earth which global reserve is going to run out in 60-100 years. Removal of phosphorous by precipitation of struvite has been studied and used somewhere in Europe and Asia for both animal manures and municipal wastewater. Struvite is a mineral formed from three specific components: magnesium, ammonium and phosphate. Struvite (magnesium ammonium phosphate) precipitates as a compact crystal. Therefore, a small amount of easily settleable solids is generated in a struvite precipitation process. Unlike other chemical phosphorus precipitation methods (e.g. using aluminum or ion salt etc.) where only phosphorus alone is removed, in struvite 5 precipitation both phosphorus and ammonium are removed, this this important in the case of piggery wastewater containing large amount of both phosphorus and ammonium. In addition, struvite is easily transported and can be used as a slow-release fertilizer. (NYSERDA, 2006) This report presents the study on struvite precipitation from digested piggery wastewater. The study includes discussion of the relevant background information and a literature review of struvite (Section 1), Goals and objectives of the study (Section 2); Material and methods with series of jar-test experiments and lab-scale reactor experiments (Section 3); Results and discussions (Section 4); and Conclusions and recommendation (Section 5). 6 1 REVIEW OF LITERATURE 1.1 Introduction to phosphorus and its applications Together with Nitrogen and Potassium, Phosphorus is an important element for living organisms. The vast majority of phosphorus sources are consumed as fertilizers in agriculture in over the world. Phosphorus is necessary for the promotion of plant growth and hence, improves crop yield, seed formation and quality of fruit. To balance the nutrients in soils, phosphorus can be added periodically under the form of inorganic fertilizers or manures. Phosphorus is also one of the vital elements needed for livestock as well. It is consumed through the diet of animals. Phosphorus has the function in preventing health problems, improve bone strength and muscle production in animal bodies. Besides, phosphorus is also used in other applications such as detergent production, food processing, chemical production,… However, phosphorus can be lost from the source and discharged into the environment by different ways naturally or due to the lack of proper management. For example, it can be lost from soils by some ways such as crop uptake and removal, runoff and erosion, or leaching. Or it can also be lost from animal feeding source through the waste streams because not 100% of intake phosphorus will remain in animal bodies. According to ICM, 2000 (http://www.ipm.iastate.edu/ipm/icm/2000/8-7- 2000/pbasics.html), 14% of phosphorus in corn and 31% of phosphorus in soybean can be digested by swine, hence, a large percentage of intake phosphorus is excreted into 7 waste streams. Due to the mismanagement of this mineral element, there are many issues related to environmental problems, especially water environment. 1.2 Impacts of excessive phosphorus in water bodies Phosphorus is not toxic, but it can affect the biological activity in clean water bodies. Once phosphorus level in surface water exceeds the critical value for aquatic plant growth, it can cause eutrophication to happen and degrade the water quality. Advanced eutrophication can also lower dissolved oxygen and increasing the BOD (biological oxygen demand) and hence, reduce the aquatic wildlife populations and species diversity in the water body. The USEPA (United States Environmental Protection Agency) recommends that total phosphorus should not exceed 0.1 mg/L in streams and that should not exceed 0.05 mg/L in streams where they enter a lake or reservoir. 1.3 Recovery of phosphorus Phosphorus in phosphate rocks is an irreplaceable resource. In order to meet the increasing demand of fertilizers for agriculture and other purposes, phosphate resources are mined with an increasing rate. In order to contribute to the security of phosphate resource on earth as well as to prevent the potential environmental issues, phosphate should be removed and recovered from waste streams before discharged into the surface water. This includes extracting phosphates from the sources in the forms that can be used in industry as a renewable resource or as fertilizers in agriculture. 8 Numerous of studies and applications of phosphorus recovery have been conducted with effort of recovery this resource from waste streams of which precipitation is the most common concerned method. Phosphorus can be extracted from the waste streams (i.e., sewage treatment plant, livestock waste…) in the forms of precipitated compounds such as calcium phosphate, magnesium phosphate, magnesium ammonium phosphate or struvite. Recovery of phosphorus in waste streams will help to minimize environmental damages, contribute to balance phosphate rock resource on earth and offer economic returns of the recovered products. Phosphorus removal and recovery has been the concerned topic all over the world for several decades. Many methods and technologies have been researched and introduced until now, however its applications in reality are still limited. In the world, the industrial use of recovered phosphorus from waste streams has been successfully implied in several plants in developed countries (Japan, Netherland). Phosphorus removal and recovery techniques still continue to be an attractive subject all over the world for national and international authorities, scientific institutions, industry and other interested parties… In the next part of this report, an overview of existing techniques on phosphorus removal and recovery will be described in details. 1.4 Phosphorus removal and recovery techniques According to Morse et al. 1998, a wide range of technologies for phosphorus removal and recovery were developed including chemical precipitation, biological phosphorus removal, crystallization, novel chemical precipitation approaches and a number of wastewater and sludge-based methods. 9 Morse et al, 1998 reported treatment technologies for phosphorus in wastewater streams into two main groups which are removal technologies and recovery technologies, as shown in summary in Table 1-1 and Table 1-2 below. Table 1-1. Summary of phosphorus removal technologies, Morse et al. (1998) 10 Table 1-2. Summary of phosphorus recovery technologies, Morse et al. (1998) Chemical precipitation, biological phosphorus removal, crystallization, advanced chemical precipitation, and ion exchange technologies are the most common for phosphorus removal and recovery from wastewater. Chemical methods are flexible and can be applied in any stage of wastewater treatment process because it can extract phosphorus from wastewater and sludge in form of metal salts (Morse et al., 1998). Meanwhile in biological methods, phosphorus can be taken up from wastewater by activated sludge and without any chemical addition (Morse et at., 1998). Crystallization for phosphorus removal produces a marketable end-product in the form of calcium phosphate, and the crystallization occurs by adding either caustic soda or milk of lime (Morse et al., 1998). Advanced chemical precipitation is referred to as 11 HYPRO and occurs by the crystallization of P, organic matter and hydrolysis providing carbon and energy in an available form (Morse et al., 1998). Morse et al. (1998) report the ion exchange technology produces struvite. During the ion-exchange precipitation process, P and NH3 ions produce struvite when removed from the wastewater (Morse et al., 1998). Other less common technologies for phosphorus removal include magnetic technology, adsorbents, and tertiary filtration technologies. For magnetic technology, calcium phosphate is precipitated in the form of magnetite by the use of lime and separated using a magnetic field (Morse et al., 1998). Adsorbents also have the ability to remove P from wastewater without additional sludge being produced (Morse et al., 1998). Tertiary filtration for phosphorus removal is incidental leaving the recovered sludge unsuitable for recycling (Morse et al., 1998). Recovery of phosphorus means to precipitate or crystallize it from wastewater in some form of products that can be used for other purposes. Calcium phosphate and struvite are most common forms of recovered phosphorus from wastewater. Compared to calcium phosphate and other recovered phosphorus, struvite has many benefits. One of the main advantages of struvite is that it can be utilized as slow released fertilizer in agriculture. Numerous of researches and applications of this method have been done all over the world, especially in some developed countries. However, in order to introduce this technology into real application, it requires the study on many aspects such as process kinetic and control, reactor design, economic… The next part of the thesis will introduce and discuss in more details on this method. 12 1.5 Struvite precipitation 1.5.1 Background Struvite or magnesium ammonium phosphate hexahydrate (MAP) MgNH4PO4.6H2O was found in 1939 because of the deposition in pipes in a wastewater treatment plant and it became well known since then. In wastewater treatment plant, it is mostly found in some special places such as areas of high turbulence (fouling pumps, aerators, screens,…) (Ohlinger et al., 1998). When it deposits, it can block the pipes that makes an increasing cost for pumping, maintenance or replacement of equipments. Therefore, it is necessary to remove phosphorus in wastewater to prevent such problems in treatment plants... Much of the literature and concern with struvite have been in how to avoid struvite formation in wastewater treatment plants. As the concern developed further in nutrient management (nitrogen and phosphorus) wastewater, researches and practical application in controlled struvite formation has increased over the world, especially in some developed countries. Holland, Australia, Japan are among the first countries that have concerned struvite formation from waste streams, and hence, numerous of researches and applications have been done on this field. In Holland and Japan, there are proprietary struvite recovery from domestic wastewater and industrial wastewater. Also in Holland, there is non-proprietary struvite recovery process in treating veal manure at a full-scale. (NYSERDA, 2006) Struvite recovered from real wastewater has the advantages as following: (1) application of struvite to the plants in ratios beneficial to plant growth and it is a product of commercial value as it may be used as a fertilizer and soil conditioner (Bishop, 2006); (2) Reducing the phosphorus and nitrogen load of side stream and sludge liquors recirculated to the head of wastewater treatment works (Etter, et al., 2010), (Quintana, et al., 2005); (3) Struvite precipitation as the pretreatment was 13 proven to be satisfactory in enhancing biological performance in activated sludge system in aspects of the removals of organic matter, nitrogen and phosphorus, in addition, the proposed process was found to be advantageous in treating swine wastewater (Hong-Duck, et al., 2010); (4) The recovery of phosphorus as struvite was reported to reduce sludge volumes under specific conditions by up to 49% when compared to chemical phosphorus removal processes (Quintana, et al., 2005); (5) Struvite precipitation processes reduced the heavy metal content (As, Cr, Ni, Fe, Zn, Hg, Zn, Cr and Ni), the high reduction efficiencies of these heavy metals indicated their precipitation together with struvite (Uysal, et al., 2010). 1.5.2 Struvite chemistry in wastewater Composition of struvite includes nitrogen (N), phosphorus (P) and magnesium (Mg). Struvite usually precipitates as a stable white orthorhombic crystals in a (1:1:1) molar ratio (El Diwani, et al., 2006): Mg 2  NH 4  PO43  6H 2O  MgNH 4 PO4 .6H 2O Formation of struvite can occur when certain conditions are met. At elevated pH condition and when the concentrations of magnesium, ammonium, phosphate ions exceed the solubility product for struvite, Ksp, struvite precipitation can occur. Ksp=[Mg2+][NH4+][PO43-] pKsp=13.26 (Ohlinger, 1998) There have been many studies focus on kinetics of struvite formation in order to determine the factors affect to the process including pH, molar ratios, stirring speed, temperature, impurities, induction time, etc while other researches focused on different controlled process such as aeration, chemical addition, seeding, or reactor design in order to make best conditions for struvite precipitation and recovery. 14 There are many different ions in wastewater which can influence to the kinetic of struvite formation. Phosphate in wastewater can be in the forms of PO43-, HPO42-, H2PO4-, or MgHPO4-, ammonium can be in forms of NH3, NH4+, magnesium can be MgOH+, Mg2+, etc. Besides, there are Ca2+ and CO32-, HCO3-… as well. If pH in wastewater changes, it can influence to the concentration of the mentioned ions. Previous studies showed that optimal pH for struvite formation is around 9 or 9.5. Burn et al. (2003) studied the influence of Mg:P molar ratio to the process and found that with Mg:P of 1.6:1, phosphate removal efficiency is 91% at pH 9, meanwhile Beal et al. (1999) showed the rate of 88% at Mg:P of 2:1. The struvite crystallization process also has been investigated. Researches focused on factors such as ions in solution, molar ratio, suspended solids, reactor design,...which affect to the nucleation and growth of crystals. Impact of Ca ions or molar ratio Mg:Ca on struvite crystallization has been investigated and reported in several reports. It has been shown that the presence of Ca ions in solution has a significant impact on struvite crystallisation in terms of size, shape, and purity of the product recovered (Kristell et al., 2004). Several different types of struvite precipitation reactors have been studied and designed for removing and recovering phosphorus in wastewater including sophisticated ones to produce high quality struvite (in Holland and Japan) and simple reactors for industrial wastewater and animal waste industry (NYSERDA, 2006). In application for treatment of livestock waste, there are several types of reactors of which most common are fluidized-bed reactor, air agitated column, stirred reactor. The most known works that have been reported are Battistoni (1998), Ueno and Fujii (2001), Münch et al. (2001), Kumashiro et al.(2001), Piekema and Giesen (2001), Mitani et al. (2001), Ohlinger et al. (1999), Suzuki et al. (2005). 15 Fluidized bed can use a seed material that allows the struvite to form as a pellet within the reactor and to be removed periodically. Magnesium salt is added just upstream of the reactor or directly into the reactor. The influent flow can be introduced into either the top or bottom of the reactor. (NYSERDA, 2006) Mixing is provided by sparging air into the base of the reactor or using the influent flow to fluidize the bed. The heavier, larger pellets move to the bottom of the reactor where they are removed periodically. The operating conditions of the fluidized bed can be set to remove crystals of a uniform size. Pellets removed from the reactor freely drain to a low moisture content. (NYSERDA, 2006) The simpler systems do not use a seed material and result in much variability in the precipitate. The magnesium salt is typically added at the beginning of a rectangular reactor with a rapid mixing section (using either a nozzle or air) followed by a quiescent section to allow the material to settle out in the downstream end of the tank. Thus the same tank is used as a reactor and as a rectangular clarifier. Crystals and precipitations are then taken out from the bottom of reactors. (NYSERDA, 2006) Another significant research on reactor design is from Suzuki et al (2005). The reactor was designed with dual function crystallization through aeration, and separation of formed struvite by accumulation on a stainless steel mesh which was thought to be simple and easy to construct and handle reactor. easy to construct and handle. The recovered struvite needed only air drying, but no dehydration or composting before use since it was approximately 95% pure even without washing and was ready for immediate application to farmland. (Suzuki et al., 2005). 16 2 GOAL AND OBJECTIVES 2.1 Goal The overall goal of this study is to evaluate the phosphate removal efficiency by struvite precipitation and to evaluate the recovered struvite from digested piggery wastewater. 2.2 Objectives To achieve this goal, specific objectives were developed as following: 1 Analyses of physical and chemical characteristics of the digested piggery wastewater (i.e. the wastewater after biogas tanks). The analyses include pH, COD, TSS, TP, P-PO4, N-NH3, Ca2+, Mg2+, K+, Alkality. 2 Evaluate the effects of factors to the phosphate removal efficiency. A series of jar test scale experiments were performed for analyzing the effect of four factors: pH; Mg:P ratio; Calcium ions and stirring speed to phosphorus removal efficiency and struvite crystallization. 3 Investigate the kinetics of phosphate removal by struvite precipitation in digested piggery wastewater. A 6 liters column reactor was designed for phosphate removing and struvite precipitation with aeration and precipitation settling. Batch mode experiment with 5 liters of wastewater was performed to examine the process. 4 Examine the accumulation of struvite crystals on stainless steel mesh for separation of recovered struvite. A continuous mode experiment with a device (which was made of stainless steel and put inside the reactor) was performed to test the accumulation and the quality of recovered struvite. 17