Strategies for phosphorus recovery from wastewater by struvite crystallization

Resumo

Phosphorus is an essential element to sustain life, and is especially important in increasing agricultural productivity and guaranteeing food security to the world¿s growing population. Currently, modern agriculture is becoming increasingly dependent on the availability and use of chemical phosphorus fertilizers, but, these phosphorus fertilizers are produced using a non-renewable source, phosphate rocks. The lifetime of its phosphorus reserves has been predicted by several studies. Most of these predictions stated that the phosphorus reserves may be depleted over the next several decades. Most of the global phosphorus mined is used in the production of food, and about 90% of this phosphorus mined that enters in the food system is lost during the process of food production: in fertilizer industry, agricultural application and food processing. Besides, almost 100% of phosphorus consumed in food by humans is lost excreted in urine and faeces, and it enters into the wastewater treatment system. So, most of the phosphorus consumed by humans usually ends up in water bodies via wastewater treatment systems. When high concentrations of phosphorus and nitrogen are discharged to water bodies, these nutrients can stimulate or enhance the development and proliferation of algae and other water plants, resulting in eutrophication of aqueous environment. The current use of phosphorus is very inefficient at numerous stages of the phosphorus cycle, provoking environmental problems such as water pollution and a predictable depletion of phosphorus reserves. In this regard, phosphorus scarcity and eutrophication could be solved simultaneously with technologies for phosphorus recovery from solid wastes and wastewater. The traditional path to recover phosphorus from wastewater has been the direct application of sewage sludge from WWTPs to agricultural lands. However, the direct application of sewage sludge is being increasingly questioned due to increasing concerns about the presence of heavy metals and organic contaminants in sewage sludge. In this way, when phosphorus recycled by the use of sewage sludge to agricultural lands is restricted, the incineration of sludge can be applied. If sewage sludge is incinerated, the resulting ash contains the highest available phosphorus content within the wastewater stream, however, also contains heavy metals and other inorganic contaminants. In order to obtain a suitable phosphorus product from sewage sludge ashes it requires treatment, such as chemical extraction processes, to remove these contaminants. Moreover, the conventional technologies for the removal of phosphorus from wastewater are chemical phosphate precipitation by adding metal salts or biological phosphorus removal processes. However, in these technologies, the phosphorus is not recovered as a sustainable product, but rather as a waste sludge that needs to be treated or disposed in landfills. In light of above, an important requisite for phosphorus recovery technologies is generating a final product with high quality, high purity and free from pollutants. Thus, the aim of phosphorus recovery technologies must be produce a phosphorus product that has a potential to be used as fertilizer or as raw material for phosphorus industry. Furthermore, the technologies of phosphorus recovery should not only be technically feasible but they should also be cost-effective. The economic viability is therefore crucial to decide on the implementation of a phosphorus recovery technology. On the basis of all the aforementioned, in the present thesis the recovery of phosphorus from wastewater by struvite crystallization was studied. The crystallization of struvite (magnesium ammonium phosphate, MgNH4PO4¿6H20) could be a sustainable alternative to recover the phosphorus from wastewater, especially at high phosphate concentrations. Struvite is a white crystalline substance with a distinctive orthorhombic crystal structure, which crystallizes as a result of a reversible reaction. In contrast to traditional systems for phosphorus removal from wastewater, such as chemical phosphate precipitation by adding aluminium or iron salts, struvite crystallization is not only used for removing phosphate from wastewater but also generates a product with commercial value as a fertilizer. However, the economic viability of struvite crystallization is highly influenced by the availability of magnesium in the raw wastewater, the cost of alkali used or the cost magnesium source used. This is because wastewater usually contain low amount of magnesium compared with phosphate and nitrogen and it is necessary to supply a magnesium source to promote the crystallization of struvite. With the aim of evaluating the use of low cost magnesium sources to promote phosphorus recovery by struvite crystallization from wastewater, studies for recovering phosphorus as struvite in different reactor configurations, at laboratory scale and later at full-scale, have been developed under this Thesis. This information is essential in order to perform a cost-effective struvite crystallization process. Taking the existent knowledge as starting point (Chapter 1), in the present Thesis the recovery of phosphorus from wastewater by struvite crystallization has been studied. Special attention was paid to the magnesium and alkali sources used to promote phosphorus recovery as struvite. The use of seawater as a cheap magnesium source for struvite crystallization was proposed in Chapter 3. A chemical equilibrium model (Visual Minteq) was used in order to obtain information about the conditions in which struvite crystallization can be promoted instead of precipitates of amorphous magnesium and calcium phosphates that also are observed in saline industrial wastewater. Then, the process of struvite crystallization from a thermodynamic and kinetic point of view was studied in order to obtain a struvite crystallization model (Chapter 4). Later, the effectiveness of two low cost industrial grade magnesium products, MgO and Mg(OH)2, as magnesium and alkali sources to recover the phosphorus by struvite crystallization was investigated (Chapter 5). Successively in Chapter 5, industrial Mg(OH)2 was used in two different struvite crystallization systems, two-phase fluidized bed reactor and continuous stirred tank reactor, in order to compare both reactor configurations. Finally, a full-scale struvite crystallization plant was designed and operated in a municipal WWTP for the treatment of the centrate obtained from the sludge anaerobic digester (Chapter 6). Additionally, the feasibility of the industrial grade Mg(OH)2 as a cheap magnesium and alkai sources for promoting struvite crystallization at full scale was also investigated in Chapter 6. The main contents of each one of the chapters of the present thesis and the achieved objectives are detailed in the following sections: In Chapter 1, the main motivations that drive to the need of phosphorus recovery from wastewater are presented. Subsequently, a brief description of the technologies commonly used and potential streams for phosphorus removal/recovery from wastewater is provided, to give an overview on the state of the art of phosphorus removal and recovery technologies in WWTPs. Later, the process of crystallization of struvite is described, including aspects as struvite characteristics, the chemistry and mechanisms of the formation of struvite and Key operational factors which influence the struvite crystallization, such as pH, temperature, supersaturation, presence of seed crystals, as well as the use of magnesium sources. In Chapter 2, the material and methods used during in the different experiments performed along the following chapters are described. Either conventional parameters used for the characterization of the wastewater liquid fraction or those used for the characterization of the solid phase (precipitates) in terms of chemical composition and appearance. Furthermore, the protocol for the preparation of the industrial magnesium hydroxide used to crystallize struvite as well as the methods to characterize this magnesium product are also described. Finally, the methodology applied to use a chemical equilibrium software (Visual Minteq) is described. This chemical equilibrium software was used to simulate different conditions under which the phosphorus can be precipitated from wastewater in order to predict the equilibrium concentrations of the different ions in the effluent as well as the mineral species that can be precipitated during struvite crystallization. In Chapter 3, the performance of a struvite crystallization system treating the wastewater from a frozen fish processing industry was studied. These industries, mainly situated in coastal zones, can produce wastewater that contains high salinity levels and also high amounts of nutrients due to use seawater as process water. In this regard, the use of seawater could be act as magnesium source during struvite crystallization. In this Chapter, a three-phase fluidized bed reactor (FBR) was operated at laboratory scale (2.4 L) to recover phosphorus from a saline wastewater. The industrial wastewater used was taken from a frozen fish processing factory located in the Ria of Vigo (Galicia, NW Spain). This saline wastewater presented a high concentration of magnesium and also high amounts of calcium which could interfere with the struvite crystallization by forming different calcium and/or magnesium phosphates. Therefore, a chemical equilibrium software, Visual Minteq, was used during this study in order to verify whether thermodynamics or kinetics rule the formation of the different phosphates that were observed in a precipitation reactor. Also, Visual Minteq was used to determine the conditions in which struvite crystallization can be promoted. This study determined that the formation of these precipitates was ruled by the thermodynamics of the process and not by kinetics. The nature of phosphate precipitates obtained during phosphorus recovery depends on wastewater characteristics and the precipitation operating conditions. In addition, the composition of the solid phase and effluent predicted were in accordance with the precipitates detected and the composition effluent during the reactor operation, respectively, regardless of the saline wastewater treated. Visual Minteq may be used to optimize the seawater dose in struvite crystallization plants. Thus, it was demonstrates that phosphorus precipitation may be predicted, making it possible to obtain information about the operational conditions in which the crystallization of struvite can be promoted. Finally, this study revealed that the concentration of ammonium has an important role in struvite crystallization. Therefore, a high molar ratio of ammonium to phosphate is highly beneficial to obtain pure struvite crystals instead of calcium and/or magnesium phosphates precipitates. The objective of Chapter 4 was to study the crystallization of struvite from a thermodynamic and kinetic point of view in order to obtain a struvite crystallization model. This study was carried out in a continuous stirred batch reactor (1 L). During this study, different sets of experiments were developed in which struvite was either dissolved (undersaturated) or precipitated (oversaturated). Experiments performed at different temperatures (25, 30 and 35 ºC) and pH values (8.2, 8.5 and 8.8) were developed to determine the kinetics of struvite precipitation and dissolution. Struvite precipitation was modelled as a reversible reaction. Results showed that an increase in the reaction temperature causes an increase in the rates of struvite precipitation and dissolution. Additionally, it was observed that struvite precipitation and dissolution are relatively fast reactions. In fact, the struvite equilibrium was achieved within a few minutes in the undersaturated and supersaturated aqueous solutions. Moreover, experiments performed at different temperatures revealed a clear relation between the temperature and struvite solubility product. Between 25 and 35 ºC, the solubility product of struvite increases with temperature. Thus, the thermodynamics and kinetics of the struvite crystallization process are influenced by temperature. Finally, struvite crystallization can be represented by a reversible kinetic model, obtaining a good fit between the experimentally measured phosphate concentration and those predicted by the kinetic model. In Chapter 5, the use of two industrial grade magnesium products, MgO and Mg(OH)2, as magnesium and alkali sources to recover the phosphorus by struvite crystallization was proposed. Both magnesium products were used and compared to determine which is the best source of magnesium and alkali to use for struvite crystallization. MgO was a powdered product provided by one of the two Spanish producers (Magnesitas de Rubián, Galicia, NW Spain). This industrial magnesium product contained 80.5% of MgO. While the Mg(OH)2 was prepared by hydrating the aforementioned industrial MgO in water. This study was carried out in a two-phase fluidized bed reactor with a volume of 1.8 L treating a synthetic medium simulating the centrate of anaerobic sludge dewatering stage in a municipal WWTP. The composition of influent was similar during the operation of reactor. The phosphate concentration in the influent was 1.7 mmol PO43-·L-1, and the molar ratio of ammonium to phosphate was 25. This influent also contained calcium and bicarbonate ions in order to evaluate the effect of ions composition on the efficiency of crystallization reactor. With the information obtained from the present work it is inferred that the industrial Mg(OH)2 was more effective than MgO for recovering the phosphorus by struvite crystallization. Additionally, experimental results revealed that ammonium and bicarbonate ions have affected the requirements of the alkali source added to recover the phosphorus as struvite. In the same way, the purity of the precipitates was affected by the presence of calcium and bicarbonate ions. Subsequently in this chapter, a comparative study of two different struvite crystallization systems at laboratory scale, two-phase fluidized bed reactor (FBR, 1.8 L) and continuous stirred tank reactor (CSTR, 2.15 L), has been developed. In this study, the previously used industrial Mg(OH)2 was added to promote struvite crystallization. Stable operation of both crystallization systems was maintained, the efficiency of phosphorus recovery was above 70%. Phosphorus was entirely recovered as struvite. Experimental results indicated that CSTR was the most cost effective for recovering phosphorus as struvite using the industrial Mg(OH)2. Therefore, in comparison to other magnesium sources used, the use of industrial Mg(OH)2 could be an economical alternative to recover the phosphorus as struvite. However, if the aim of struvite crystallization process is to recover the phosphorus into granular product, like pellets, FBR can be a good alternative. In Chapter 6, a full-scale struvite crystallization plant was designed and operated for the treatment of the centrate obtained from the sludge anaerobic digester in a municipal WWTP. The plant of struvite crystallization was located in the municipal WWTP of Guillarei (Galicia, NW Spain). The crystallization plant was composed by a two-phase fluidized bed reactor connected in series to a settler. The fluidized bed reactor promoted the growth of larger crystals; whilst the settler enhanced the growth and recovery of fine crystals. Centrate obtained after centrifuging the effluent from anaerobic sludge digester was fed to the plant. The concentration of phosphorus in the centrate was between 1.1 and 2.2 mmol PO43-¿L-1 during the operation of struvite crystallization system. Also, this centrate fed had a high ammonium concentration (70.2 mmol NH4+¿L-1) and also a high level of alkalinity (65.3 mmol HCO3-¿L-1). During the operation of crystallization plant, operating pH was controlled adding the industrial grade Mg(OH)2 slurry studied in Chapter 5. Stable operation of crystallization system was maintained, the efficiency of phosphorus recovery was around 77% of the phosphorus present in the influent. This study determined that the phosphorus concentration in the influent was an important role in the efficiency of struvite crystallization system, when the concentration of phosphorus in the influent was increased, the efficiencies of phosphorus recovery and the performance of Mg(OH)2 slurry were increased. Experimental results showed a certain relation between phosphorus concentration in the effluent and operating pH was observed. Effluent phosphate concentration diminished by increasing the operating pH. Regarding to the solid phase, visual observations determined that most of the precipitates were pellets. The size range of these pellets was from 0.5 to 5.0 mm, and at least 60% of pellets had a size larger than 0.5 mm. Struvite was the most abundant mineral present in these pellets. This produced struvite was suitable to be used as an agricultural fertilizer. Thus, the aforementioned results underlined that it is possible to recover phosphorus by struvite crystallization using an industrial grade Mg(OH)2 at full-scale in a municipal WWTP. Finally, this struvite crystallization system was patented (ES 2455740 B2) as a result of the work developed in this chapter.