Phosphorus Migration and Transformation in Activated Sludge by Ultrasonic Treatment

Abstract

Ultrasonic treatment is an effective method to disintegrate sludge and extract organic matter and nutrients, including nitrogen and phosphorus, from the sludge. This study investigated the transformation of phosphorus species during the ultrasonic treatment of sludge, to reveal the mechanism of phosphorus migration in the activated sludge structure. The experimental results indicated that power density and ultrasonic time were critical parameters affecting the energy input for sludge integration. The optimal phosphorus release performance was achieved at 2.5 W/mL 10 min. The release of phosphorus showed as a layer-by-layer pattern from the inner sludge core to the outer sphere of the multiple-layer structure of the sludge. The complex sludge structure played an important role in buffering the ultrasonication process and transfer of phosphorus. Mg-P, Ca-P, and organic phosphorus are the main phosphorus species that can be extracted from the sludge core into the supernatant. The three-stage process of phosphorus migration through the sludge layers, including dissolution, reprecipitation, and equilibrium, has been revealed.

1. Introduction

Phosphorus is an important element for human beings due to its irreplaceable role in global agriculture, especially in the food supply and security [1]. However, phosphorus is a non-renewable resource, of which nearly 90% is obtained from phosphate rock. With the rapid growth of the world’s population, a shortage of phosphorus resources has been predicted to occur within the next 100 years [2]. Currently, many countries cannot meet their needs of mineral phosphorus for national economic development. Thus, it is a global challenge to achieve the sustainable development of phosphorus resources.

Waste activated sludge (WAS) from wastewater treatment plants is rich in phosphorus and may cause eutrophication when disposed of improperly, while proper use can make it a valuable resource. It is reported that a large amount of phosphorous is present in sewage sludge, which can satisfy 12–15% of the total phosphorus demand, after recovery [3,4]. In China, the sludge recovery rate is only 25%, which means that up to 17.32 g/kg-TS of phosphorus in municipal sludge is wasted [5]. Thus, WAS could be an effective supplement for phosphate rock, if we can only achieve a significant extraction of phosphorus from the solid phase. To achieve this purpose, a variety of physical, chemical, and biological methods have been developed for sludge resource recovery, such as thermal treatment, acid treatment, ultrasonic treatment, and anaerobic digestion [6].

WAS can be structurally divided into three parts [7,8]: (1) the sludge core, which is a solid phase including microbial cells and inorganic components such as sand and chemical precipitation; (2) extracellular polymeric substances (EPS), which include tightly bound EPS (TB-EPS, located close to the sludge core) and loosely bound EPS (LB-EPS, located in the outer layer of EPS). EPS have complex components, rich in organic matter (e.g., polysaccharides and proteins), in which divalent metal ions (e.g., Ca²⁺ and Mg²⁺) act as ionic bridges; and (3) soluble microbial products (SMP), which are the spatial distribution of the phosphorus in sludge that is closely related to this structure. Due to the presence of chemical phosphorus removal, the coagulant transfers the dissolved phosphorus into the solid phase [4]. While EPS wrap around microorganisms and provide a buffer area for the storage of metabolites for phosphorus absorption and the release of phosphorus-accumulating organisms (PAOs), in addition to organic phosphorus (OP), the metal ions of EPS also adsorb inorganic phosphorus (IP) [9]. Most of the phosphorus in sludge supernatant exists in the form of dissolved inorganic phosphorus. Obviously, phosphorus can only be effectively utilized if it is transferred from the inside to the outside along with the disintegration of the sludge; thus, it is important to choose an appropriate sludge treatment method.

Ultrasonic treatment has been proven to be an effective means of sludge treatment, which has the advantages of a lack of pollution, short action time, etc. Le et al. [10] have summarized 31 full-scale applications of ultrasound for sludge treatment worldwide, most of which aim to promote anaerobic digestion for gas production. When ultrasound propagates in the medium, it generates cavitation, causing a local high-temperature and high-pressure environment. The ability of ultrasound to promote the release of intracellular substances can be attributed to four aspects [11,12]: (1) sludge disintegration due to water-mechanical shear; (2) cell lysis due to thermal effects; (3) oxidation of free radicals; (4) and a stirring effect of the sound flow. In ultrasound treatment, power density and ultrasonic time are considered as critical parameters. When the threshold of the power density is exceeded, the release of organic matter, nitrogen, and phosphorus from the sludge is accelerated, and the thermal effects at a high power density promote the conversion of phosphorus forms [13]. When the power density exceeds 0.24 W/mL, phosphorus would start to be released [14]. The release of total phosphorus continues over time, but reprecipitation may also occur. In addition, high MLSS will hinder the formation of cavitation bubbles and weaken the ultrasonic effect.

In fact, studies on ultrasound have focused on the release of organic matter and the effect on subsequent gas production and dewatering, with little focus on phosphorus migration and transformation. Chu et al. [15] have divided the release of organic matter into three processes; Xiao et al. [16] have demonstrated a decrease in the polysaccharide/protein ratio during ultrasound. However, Liu et al. [17] have found that the release of phosphorus showed different patterns from organic matter and nitrogen. It is still worthwhile to investigate the mechanism of phosphorus migration in sludge during ultrasonic treatment. Moreover, it deserves attention that the role of extracellular polymeric substances (EPSs) in phosphorus release has been neglected, and the metal–phosphorus interrelationship is not clear.

In this study, we investigated the transformation of phosphorus species, metal ions, and organic matters in different layers of sludge structure during ultrasonic treatment. This study aimed to bridge the existing research gaps in the field of the ultrasonic-enhanced phosphorus-release mechanism in WAS. The relationship of ultrasonic energy input and sludge disintegration was analyzed, and the role of EPS and metal ions during phosphorus release was revealed. This work not only clarified the mechanism of sludge ultrasonication but also provided recommendations for subsequent resource recovery from WAS.

2. Materials and Methods

2.1. Experimental Set-Up

The WAS was collected from the secondary sedimentation tank of anaerobic-anoxic–aerobic (A²/O) system in Liede Sewage Plant in Guangzhou, China. The main characteristics of raw sludge are shown in

Table 1. Characteristics of the raw sludge.

Characteristics	SS	VSS	TCOD	TP	TN	pH	Al	Fe	Mg	Ca
Raw sludge	35.4 g/L	20.6 g/L	11,857 mg/L	834 mg/L	964 mg/L	6.45	53.05 mg/g	5.02 mg/g	3.38 mg/g	11.04 mg/g

.

Batch experiment was conducted in 500 mL beakers, which were filled with 250 mL sludge. The ultrasound experiments were conducted by Ningbo Xinzhi Ultrasonic Crusher (SCIENTZ-IIN). During ultrasonic treatment of sludge, a probe was placed in the center of the beaker, which should be 1.0~2.0 cm below the sludge level. The effects of power density and ultrasonic time during ultrasound treatment were investigated through orthogonal experiments, with total of 25 experimental groups and 1 blank group (parameter settings: power density 0, 0.5, 1.0, 1.5, 2.0, and 2.5 W/mL; ultrasonic time 0, 10, 20, 30, 45, and 60 min). The transformation of phosphorus species in WAS was investigated in four sections distributed in the sludge structure, including SMP, LB-EPS, TB-EPS, and sludge core. All the experiments were conducted in triplicate.

2.2. Analytical Methods

Sludge samples collected after ultrasonic treatment were centrifuged at 8000 rpm for 15 min. Then supernatant samples were filtered by 0.45 μm cellulose nitrate membrane filters. Phosphorus (total phosphorus and soluble phosphorus), and ammonia (NH₄⁺-N) were determined spectrophotometrically (Hach, DR 2800, Ames, IA, USA), based on ammonium molybdate and salicylic acid methods, respectively. Total nitrogen (TN) was determined spectrophotometrically (Persee, T9, Beijing, China), after alkaline potassium per sulfate oxidation. Chemical Oxygen Demand (COD) was determined after digestion (Hach, DRB200, USA) followed by dichromate titration. Suspended solid (SS) and volatile suspended solid (VSS) were determined according to weight method. The metal ions (Mg²⁺, Ca²⁺, Fe³⁺, Al³⁺) were detected by ICP-OES after being extracted, passed through 0.45 μm filter membrane, and diluted with 2% HNO₃.

EPS was extracted by heating method [18], which reduced the interference caused by the introduction of other substances. The steps were as follows. Firstly, 40 mL of sludge was centrifuged at 4000× g at 4 °C for 15 min, and then it was filtered by 0.45 μm cellulose nitrate membrane filters to separate SMP out. Secondly, take the filtered sludge residue, restore the sludge to the original volume with 0.05% NaCl at 70 °C, shake it evenly, and then centrifuge and filter the sludge under the same conditions to separate LB-EPS out. Finally, repeat the second step to obtain TB-EPS, but before centrifugation, 60 °C water bath heating for 30 min. After EPS extraction, the extracted sludge was dried at 35 °C and stored in desiccators for subsequent testing. In addition to the conventional indicators, polysaccharides and proteins were detected in the extracted EPS. Polysaccharides were determined spectrophotometrically based on anthrone-H₂SO₄ method, and proteins were determined spectrophotometrically based on Bradford method.

The phosphorus species in solid phase were analyzed according to the Standards in Measurements and Testing (SMT) Programme extraction protocol [19,20,21]. Based on the SMT extraction method, P in WAS can be divided into five fractions: (1) total solid P (TSP), extracted by 3.5 M HCl; (2) inorganic P (IP), extracted by 1 M HCl; (3) OP, the residual after IP extraction was treated at 450 °C, then extracted by 1 M HCl; (4) non-apatite inorganic P (NAIP, the P associated with oxides and hydroxides of Al, Fe, Mg, and Mn); and (5) apatite P (AP, the P specie associated with Ca). Therefore, TSP = IP + OP, and IP = NAIP + AP.

The degree of sludge disintegration was assessed by determining the COD in sludge supernatant. A reference (100%) was defined as the aqueous phase COD obtained by chemical sludge disintegration in 0.5 mol/L NaOH for 22 h at 20 °C. The degree of disintegration (DD) is calculated as the ratio of COD increase by sonication to the COD increase by chemical disintegration [22]:

$$\mathrm{DD}=\frac{{\mathrm{COD}}_{\mathrm{ultrasound}}-{\mathrm{COD}}_0}{{\mathrm{COD}}_{\mathrm{NaOH}}-{\mathrm{COD}}_0}\times 100\%$$

(1)

The total energy input can be express as specific energy (SE). SE is a function of ultrasonic time (T), power density (P), and initial total suspended solid (SS), according to the following equation [23,24]:

$$\mathrm{SE}=\frac{\mathrm{P}\times \mathrm{T}}{\mathrm{SS}}$$

(2)

3. Results and Discussion

3.1. Sludge Disintegration via Ultrasonic Treatment

The effects of ultrasound on the sludge disintegration are shown in Figure 1. During ultrasonic treatment, the disintegration of sludge floc and the cells lysis accounted for the release of organic matter, which could be reflected in the increasing DD [17]. In all cases, the increase in power density could promote sludge disintegration. It was observed that for experiment groups with a reaction time higher than 30 min, the DD rose rapidly at the beginning and became stable after the threshold of power density, while for a reaction time less than 30 min, there was not a significant power density threshold. The threshold of power density at an ultrasonic time of 30 min was 2.0 W/mL, at 45 min it was 1.5 W/mL, and at 60 min it was 1.0 W/mL, which showed a rising trend. The threshold was influenced by the total energy input to the system. The total energy input can be express as SE, which is a function of ultrasonic time, power density, and solid mass. The increased energy input promoted cavitation, enhanced water-mechanical shear, and rapidly destroyed sludge flocs. For SE under 150,000 kJ/kg-SS, sludge disintegration increased strongly. For supplied energy above 150,000 kJ/kg-SS, the DD remained at around 60%, suggesting the maximumly efficient SE. If the SE was above this value, a further increase in the power density contributed negligibly to sludge disintegration. Thus, as the ultrasonic time increased, the power density threshold arrived earlier. The highest DD at 2.5 W/mL for 60 min was up to 60.89%, which was very close to 1.5 W/mL for 60 min and 2.0 W/mL for 60 min. Another view was that for sludge with high SS (up to 35.37 g/L in this research), a large amount of energy was wasted in the transfer between solid phases, leading to poor efficiency of acoustic energy utilization, especially at a high energy input [25]. From the above results, power density and ultrasonic time are two important factors of the total energy input to the system, which greatly affected the sludge disintegration.

During sludge disintegration, polysaccharides and proteins were released from the sludge core to the outer sphere, eventually leading to a large increase in SMP and LB-EPS (Figure 2). The raw sludge supernatant contained small amounts of polysaccharides and proteins, which were mainly present in the sludge core. After ultrasonic treatment, these polysaccharides and proteins went through two layers (LB-EPS and TB-EPS) to enter the SMP. With the increase in power density, the release of polysaccharides and proteins showed an upward trend.

As shown in Figure 3, the effect of ultrasonic time on EPSs was different with the power density. Increasing the ultrasonic time could only promote the dissociation and transformation of polysaccharides into SMP and LB-EPS and had little effect on TB-EPS. The fraction of proteins were greatly elevated at all parts but declined after 30 min of ultrasonic treatment, which might be caused by the high temperature denaturation of protein [15].

In general, the increase in SE elevated the concentration of polysaccharides and proteins in SMP, which was mainly caused by sludge disintegration. The spatial structure of sludge played an important role in this process, as the outer structure trapped the polysaccharides and proteins released from the inner structure [26]. This trapping phenomenon was due to the rheological properties of the sludge itself, where the viscosity of the layers slows down the outward transmission of substances.

3.2. Phosphorus Release during Ultrasonic Treatment

The effects of ultrasound on the phosphorus release are shown in Figure 4. For ultrasonic time, the concentration of total phosphorus in the supernatant rose rapidly during the first 20 min of the treatment, followed by a small decrease, and then it became stable at the later stage, which might be caused by the dissolution and reprecipitation of the phosphorus that occurred in the process [4]. With the disintegration of the floc, the metal ions in the EPS were released, and the soluble phosphate might be reprecipitated into the solid phase as metal–phosphorus sediments.

For short-time ultrasonic treatment (10 min), the increase in power density from 0 to 2.5 W/mL strongly enhanced the release of phosphorus, which mainly existed as orthophosphate. It can be speculated that loosely bounded phosphorus, such as adsorbed phosphate, in the sludge floc could be released easily under the action of mechanical shear at the beginning of the ultrasonic treatment [12]. However, when ultrasonic time lasted more than 20 min, there was no significant increase in TP concentration by the further enhancement of power density. Although the TP concentration becomes relatively stable, the fraction of orthophosphate decreased with increasing power density. This evidence suggested that intracellular organic phosphorus was extracted via cell disruption at a high energy input; meanwhile, inorganic orthophosphate re-precipitated, which might be induced by the dissolved metal ions from disintegrated EPS [27].

The maximum TP concentration in the supernatant achieved around 100 mg/L, under the conditions of 10 min of 2.5 W/mL, and a similar result could be achieved by increasing the ultrasonic time at low density (60 min of 0.5 W/mL). Similar to sludge disintegration, phosphorus release from WAS during ultrasonic treatment was correlated with specific energy, which was influenced by both ultrasonic time and power density. The maximum TP concentration was reached with the input specific energy between 40,000 to 50,000 kJ/kg-SS (Figure 5), which is much lower than the optimal specific energy required for sludge disintegration (150,000 kJ/kg-SS). Therefore, the optimal phosphorus release performance was reached at 2.5 W/mL 10 min. To further reveal the mechanism of phosphorus migration in WAS, the transformation of phosphorus species was investigated in the following section.

3.3. Transformation of Phosphorus Species in Sludge Structure

In the extracellular sludge, the phosphorus fraction is closely related to the sludge structure. Phosphorus in SMP, LB-EPS, and TB-EPS spatially migrated and transformed with the disintegration of EPS. The release of phosphorus was actually affected by the EPS structure, which provided a buffering effect for the release of the inner layer materials [26]. With the ultrasonic treatment, the distribution of TP in the sludge structure changed greatly (Figure 6a). The amount of total extracellular phosphorus increased from 38.09 mg/L to 151.46 mg/L with the increasing power density, mainly contributed by TP in LB-EPS and SMP.

After extraction of the extracellular contents (SMP, LB-EPS, and TB-EPS), the residual phosphorus species in the sludge core were analyzed under different power densities (Figure 6b). The changes of TP in the extracellular fraction were mainly contributed by the sludge core, since the sludge core contained the vast majority of phosphorus in the sludge, which was present in different forms, including NAIP, AP, and OP. According to the speciation analysis of sludge core, the amounts of IP (NAIP and AP) and OP were decreased. The reprecipitation of NAIP was observed with a power density higher than 1.5 W/mL, due to the large release of metal ions. Meanwhile, ultrasound raised the sludge temperature, created free radicals to convert OP into IP, and increased the percentage of IP, which were beneficial for phosphorus recovery [28].

With the increasing ultrasonic time, phosphorus species in each layer of sludge varied (Figure 7a). The TP concentration in SMP climbed up in the first 30 min, then decreased, which was accompanied by the reprecipitation of metal ions and phosphorus. The trend of TP in LB-EPS was similar to that in SMP: as the ultrasonic time increased, the inner-layer TP release process was adsorbed and trapped by LB-EPS, causing a small increase in TP concentration, and a decreasing trend was also observed after 30 min. However, the TP in TB-EPS rose gradually during the 60 min treatment. This evidence suggested that a long ultrasonic time will lead extracellular phosphorus to reprecipitation and then to re-enter the inner structure [29]. Therefore, to maximize phosphorus release, ultrasonic time should be limited and optimized.

Metal ions played an important role in the migration and transformation of phosphorus during ultrasonic treatment. Mg and Ca are important components in the EPS bridging of negatively charged biopolymers. The disintegration of sludge flocs led to the release of Ca and Mg. A significant increase in Ca and Mg in the LB-EPS layer was observed in the 5th min of treatment (Figure 7). Then the concentrations dropped back. This evidence indicated that the mechanical shear force of ultrasound extracted Ca and Mg layer by layer. Eventually, the increasing concentration of metal ions and orthophosphate lead to reprecipitation, as metal–P compounds at the latter stage of treatment.

Although the Ca concentration in SMP increased strongly (Figure 7c), the fraction of AP (phosphorus bonding to Ca) in the sludge core was not significantly reduced (Figure 7b). This trait suggested that the extracted Ca was not from Ca-P but was from other Ca species. In this study, Al and Fe were not detected in the SMP. Thus, it is difficult to extract Al-P and Fe-P by ultrasound, due to the strong bonds between Al/Fe and phosphorus [30]. Therefore, the reduction fraction of NAIP (phosphorus bonding with Fe, Al, or Mg) in the sludge core was mainly contributed by the release of Mg. The maximum Mg concentration in TB-EPS, LB-EPS, and SMP was achieved at 0.5, 1.0, and 1.5 W/mL, respectively (Figure 6d). There was a hysteresis effect on Mg release from the inner to the outer layer of the sludge structure. The LB-EPS and TB-EPS in the intermediate layer have buffering and adsorption effects in the mass transfer process, which make the migration and transformation of metal ions and phosphorus have a hysteresis effect.

3.4. Mechanism for P Migration in Activated Sludge by Ultrasonic Treatment

According to the study of the migration and transformation process of phosphorus species via ultrasonic treatment, the mechanism of phosphorus release from sludge has been summarized in Figure 8. The complex structure of raw sludge can be divided into the sludge core, TB-EPS, LB-EPS, and SMP from the inner core to the outer sphere. When external energy, such as ultrasonic energy, is input into the sludge system, homeostasis is disrupted, and substances (phosphorus and organic matter) would be released from the sludge core into the supernatant layer by layer. During the process of sludge disintegration, the outer layer structure acted as a buffering system, where the release of the inner layer material is slowed down by adsorption and retention due to the viscosity and absorbability of EPS. Greater energy input is required to destroy the structure of EPS, to further release inner-layer material. The multiple-layer structure of sludge is an important factor that limits the release of phosphorus. Therefore, breaking this structure requires high ultrasonic energy input, which is closely related to power density and ultrasonic time.

Under the influence of spatial structure, phosphorus in sludge had its unique release pattern. The phosphorus-release process over time can be divided into the following stages.

3.4.1. Dissolution Stage

The rapid disintegration of sludge accompanies the dissolution of organics and metal ions. At this stage, the Mg-P and loosely bonded phosphorus can be released greatly from the sludge core into the supernatant through EPS layers, while the Al-P and Fe-P were hardly dissolved. The migration of phosphorus was affected by the adsorption and retention of polysaccharides and proteins in EPS. The multiple-layer structure of EPS led to a hysteresis effect upon phosphorus release.

3.4.2. Reprecipitation Stage

Metal ions as well as orthophosphate can be continuously extracted from the sludge core. Along with ultrasonic treatment, the change of temperature and pH affects the solubility of metal ions. The increasing concentrations trigger the reprecipitation reaction and form metal–P sediments into the inner layer. The migration of phosphorus became a two-way transfer, and dissolution and reprecipitation co-existed in this stage.

3.4.3. Equilibrium Stage

Organic phosphorus will be slowly hydrolyzed into a soluble phase during lipolysis and cell disintegration. The reactions of the phosphorus release and metal–P reprecipitation achieve equilibrium. The TP concentration in the supernatant becomes stable.

4. Conclusions

Ultrasonic treatment is an effective method to achieve sludge disintegration and can also be applied to facilitate phosphorus release. Power density and ultrasonic time are two critical factors determining the ultrasound energy input for material release. The multiple-layer structure of sludge led to a layer-by-layer release pattern of phosphorus and triggered a hysteresis effect. The process of phosphorus migration through sludge layers was divided into three stages: dissolution, reprecipitation, and equilibrium. Therefore, controlling the phosphorus migration process in the dissolution stage by optimizing the ultrasonic energy input could improve phosphorus release and recovery performance.