Various Solutions for Phosphorus Recovery from Municipal Sewage Sludge

Phosphorus is a critical resource mineral, which natural supply is diminishing rapidly. Consequently, the German government enacted by decree that from 2029 phosphorus must be largely recovered from sewage sludge. The implementation of power ultrasound opens various options to intensify the recovery of phosphorus from municipal sewage sludge.

Sewage Sludge Treatment Plants and Phosphorus Recycling

Sewage sludge is rich in phosphorus. Various ultrasonic treatment options are available to intensify the recovery of phosphorus from sewage sludge.Phosphorus is a critical element, widely used as fertilizer and raw material in fine chemical and pharmaceutical industry. Due to its rapidly shrinking available resources, the German government has implemented a law by which all municipal waste water treatment plant must establish measurement to recover phosphorus from the municipal sewage sludge. The implementation of high-performance ultrasound offers various treatment options, which increase the efficiency of phosphorus recovery significantly.

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High-power ultrasound system for the inline treatment of waste water and sewage sludge. Sonication promotes biomass disintegration for better anaerobic sludge digestion and nutrient recovery (e.g., phosphorus, nitrogen, magnesium, potassium, calcium etc.)

Sewage sludge treatment benefits heavily from the application of ultrasound: Sonication improves disintegration and nutrient recovery (e.g., phosphorus)

Waste water and sewage sludge treatment plants use ultrasound for improved sludge digestion and nutrient recovery (e.g., phosphorus)

Waste water plants use ultrasound for improved sludge digestion and recovery of nutrients such as phosphorus.

Ultrasonic Sewage Sludge Disintegration for Improved Phorsphorus Recovery

Irradiating sewage sludge with powerful ultrasound waves has been clearly demonstrated to improve the disintegration of the biomass in sewage sludge. Numerous research studies have shown the benefits of ultrasonic waste activated sludge treatment and various industrial ultrasound systems are running in municipal waste water treatment plants in Germany.
Ultrasonic pretreatment of substrates in sewage sludge has several beneficial effects for anaerobic digestion processes. These benefits include reducing the particle size, increasing the rate of hydrolysis, and shortening hydraulic retention time.
Cavitational shear forces generated by high-intensity / low-frequency ultrasound waves cause the breakup of sludge floc morphology and the microbial structure of sludge.
The research team of Nguyen demonstrated in their studies “that the application of ultrasound is very effective in reducing the particle size of biomass, achieving a reduction to an average particle size of >78.78% proportional to the length of time and intensity of ultrasonic irradiation exposure. This indicated that the sludge particles disintegrated and sludge particle size decreased, based on an inverse relationship between the sonication time and floc particle size. The application was highly effective, despite the fact that sludge floc observations before treatment revealed that the sludge flocs were dense and highly compact, composed of many sub-compartments with compact cores, cell clusters, bacterial colonies, protozoa, and filamentous bacteria, among other factors. Analysis of the effluent shows that the ultrasonic process significantly disintegrated the structural integrity of sludge flocs of all sizes. Floc pieces were reduced to as little as ≤6.5 μm under optimal treatment conditions, and were dissolved in the sludge slurry after 5–10 min of ultrasonic treatment with a low ultrasound frequency of 20 kHz.” (Nguyen et al., 2015)
An efficiently disintegrated sewage sludge, i.e. sonicated sludge, shows significantly improved separation properties allowing to remove phosphorus-rich fractions from biomass and gel-like fractions as well as water. Ultrasonic cavitation disrupts cell structures of the biomass in sewage sludge and facilitates the subsequent fractionation into three fractions of (i) cellulose-rich fibres, (ii) a nutrient-rich gel, and (iii) a liquid that is easily fermentable. These three fractions of the sludge can be further processed, e.g. phosphorus recovery, heavy metal removal etc.

Advantages of Ultrasonic Sewage Sludge Treatment

  • Improved anaerobic digestion
  • Smaller particle size of floc pieces
  • Improved recovery of phosphorus, minerals and (heavy) metals
  • Chemical-free alternative to conventional adsorption

Ultrasonically Intensified Wet-Chemical Precipitation of Phosphorusn

Ultrasonication is a well known technique used since long times in chemistry for the siccessful and efficacious precipitation of minerals, particles and crystals. For the phosphoris recovery from sewage sludge, sonication is applied in order to promote and accelerate the precipitation of struvite. Struvite (magnesium ammonium phosphate) is a phosphate mineral with formula NH4MgPO4·6H2 O, which offers a simple solution to remove phosphorus in bound form from waste sludge.
During the struvite crystallization /precipitation process, the ions PO43-, NH4+, and Mg2+ are removed from the liquid phase due to struvite precipitation, until the reaction reaches the equilibrium.

Struvite crystals can be precipitated from sewage sludge. Sonication improves the recovery process. Sonication promotes the precipitation process and makes recovery of phosphorus bound in struvite significantly more efficient.

Struvite crystals precipitated from swine effluents.
(picture and study: ©Kim et al. 2017)

When ultrasonication is applied as sludge pre-treatment prior to struvite precipitation process, the ultrasonically induced cavitation provides a thourough mixing on molecular level providing active surface area for crystal growth. An increase in the solubilization rate of extracellular and intracellular substances, further benefits the struvite precipitation by increasing the bioavailability of NH4+ and PO43- ions. The sonication treatment results in a facilitated mass transfer through the slurry, which is caused by ultrasonic cavitation.
Sonication can be also applied to hydrothermal carbonization liquor, where phosphorus is recycled as struvite from the hydrothermal carbonization liquor through ultrasonically assisted extraction and precipitation.
Read more about large-scale struvite precipitation for phosphorus recovery from sewage sludge!

Ultrasonic flow cell with 4x 4kW power-ultrasound for sewage sludge treatment such as improved disintegration and phosphorus recovery.

Industrial ultrasonic system for large scale sewage sludge treatment in continuous flow mode.

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Sono-Fenton for Phosphorus Release from Sewage Sludge

The study of Gong et al. (2015) demonstrated the efficiency of a combined ultrasound–Fenton (also known as sono-Fenton) pre-treatment for sewage sludge disintegration. Applying the ultrasound–Fenton treatment significantly increased carbon, nitrogen and phosphorus release. The sono-Fenton treatment increased the total nitrogen (N) and phosphorus (P) by 1.7- and 2.2-fold, respectively, when compared to the Fenton treatment alone. After the sono-Fenton treatment, sludge showed a considerably finer particle size and looser microstructure based on scanning electron microscopy.Using electron spin resonance, the highest OH• signal intensity increased from 568.7 by the Fenton treatment to 1106.3 after the sono-Fenton treatment. This demonstrated that the sono-Fenton treatment induces disintegration of sludge and improves the release of organic carbon, nitrogen and phosphorus significantly.

Ultrasonically Intensified Microbial Fuel Cells

Ultrasonic pretreatment of sewage sludge promotes the solubilization of various organic compounds from the sludge matrix, which subsequently accelerates the microbial electrohydrogenesis.
More and Ghangrekar (2010) contribute the beneficial effects of sonication pre-treatment on microbial fuel cells to enhanced enzymatic activities and the availability of extracellular proteins, polysaccharides and enzymes, which are released from inner layers of sludge flocs to outer layers by ultrasonic sonoporation and cell disruption, resulting in improved substrate utilization efficiency. For improving the electricity harvesting from the microbial fuel cells, improvement in the electron transfer capability of the bacteria to an extracellular solid substance is necessary. This extracellular electron transfer can occur either in direct contact between the cell surface and the solid surface or indirectly via the so-called exogenous and endogenous mediators. For direct electron transfer between bacteria and electrode surface, the electron should reach the outer membrane of the cell. This phenomenon might have occurred due to ultrasonication pre-treatment given to the inoculum when the specific supplied energy was adequate; hence, favouring higher Coulombic efficiency.
An ultrasonically intensified microbial electrolysis process could be combined with subsequent fermentation processes for sludge treatment.

Sono-Electrochemical Sewage Sludge Treatment

Sonoelectrochemical equipment for large scale phosphorus recovery from sewage sludgeElectrocoagulation is a simple process, which can be easily applied to waste water and allows the treatment of large volumes efficiently and cost-effective. Ultrasonication helps to overcome the main drawback of electrocoagulation processes by preventing the formation of passivating layers from the electrodes. Passive films forming on the electrode surface over time, drastically reduce the efficiency of electrocoagulation systems and ultrasound is a facile method to remove these passivating layers continuously during operation, whilst at the same time sonication increases the turnover of electrocoagulation systems. Ultrasonic waves breaking down sediments formed at the electrode surface and generate high amounts of radical species to remove pollutants by creating high-pressure points inside the solution during the cavitation phenomenon. By combining electrocoagulation with ultrasonic, due to the creation of new surfaces on the electrode by cavitation or micro-streaming, the decrease in the thickness of the distribution layer is exacerbated by increasing mass transfer rates. Sono-electrocoagulation increases the amount of coagulant formation compared to electrocoagulation, and the flocculation is enhanced by extreme mixing and oxidation by free radical formation; therefore reach the desired efficiency is possible in the shortest time.
(Moradi et al., 2021)

The Advantages of Sonoelectrochemistry and Sonoelectrocoagulation

“Sonoelectrochemistry is the combination of ultrasonic energy in an electrochemical system that offers several advantages, including gas bubble removal at the electrode surface, solution degassing, disruption of the Nernst diffusion layer, enhancement of the mass transport of electroactive species through the double layer, and the activation and cleaning of the electrode surface. These benefits in electrochemistry lead to improved process efficiencies (electrode and current efficiencies), increased electrochemical rates and yields, decreased cell voltages and electrode overpotentials, improved electrodeposited materials in terms of hardness, quality, porosity and thickness, and the suppression of electrode fouling and degassing at the electrode surface.” (Foroughi et al., 2021)
Read more about sono-electrochemical equipment and its applications!

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Industrial Ultrasonic Systems for Sewage Sludge Treatment

Hielscher Ultrasonics high-performance systems are a reliable work-horse in wastewater and sewage sludge treatment plants. Hielscher ultrasonic systems deliver high-power ultrasound at 20kHz, which creates intense acoustic cavitation. Effects of our high-power ultrasonic treatment include cell disruption and disintegration, high mass transfer, disinfection, decomposition of polymers, release of enzymes and homogenization of the sludge. High-intensity sonication improves treatment efficiency through the generation of oxidative radicals and by the increase in nucleation sites, ensuring better coagulation and flocculation. The reliable and continuous generation of these intense ultrasonic cavitation forces and its effects allow to integrate our powerful ultrasonic processors for various applications in order to improve the energy- and nutrient-harvest from sewage sludge.

  • Ultrasonic disintegration of sewage sludge
  • Ultrasonically intensified anaerobic digestion
  • Recovery of valuable nutrients (phosphorus, nitrogen, magnesium, calcium, potassium, etc.)
  • Removal of contaminants such as heavy metals

Offering ultrasonic processors with flow-through reactors at various sizes, Hielscher Ultrasonics has the ideal ultrasonic waste waster and sludge treatment systems for your sewage sludge facility.
The modular design allows for the simple instalment of several ultrasonic units in parallel, which makes a reliable and efficient treatment of any volume possible.
Hielscher high-power / high-throughput ultrasonic systems boost the efficiency and yield in the treatment of biomass in sewage sludge.
High-intensity ultrasonic probe Cascatrode™️ in reactor - optimized for superior sonication effectsAll our ultrasonicators feature cleaning-in-place (CIP).
Suitable for demanding treatments: Sewage sludge often contains fibrous materials with high cellulose contents, which are hard to disintegrate. Hielscher Ultrasonics’ industrial ultrasonic processors can deliver very high amplitudes. Amplitudes of up to 200µm can be easily continuously run in 24/7 operation. For even higher amplitudes, customized ultrasonic sonotrodes are available. Such high amplitudes are an efficacious way to disrupt the cellular structure of celluloses, lignin and robust cell walls of other materials. The optimized design of our flow cell reactors ensures ideal flow patterns and the uniform, efficient ultrasonic treatment of the fed sludge.

We Work with System Integrators and Engineers of Sewage Sludge Plants

As manufacturer and system builder of high-performance ultrasonic modular units, Hielscher Ultrasonics works together with system integrators. Most operators of waste water and sewage sludge treatment plants work with system integrators, which are well-experienced in the design and automation of such purification plants. Our team of design engineers and technical engineers recommend the optimal ultrasonic configuration for the targeted sludge volume, provide detailed system informations, CAD drawings as well as installation service and operational training. This enables a constructive and goal-oriented implementation of the integration of high-performance ultrasound in sewage treatment plants. Please contact us if you are interested in integrating our ultrasonic processors for improved sludge treatment!

Advantages of Hielscher Ultrasonics Sewage Sludge Treatment Systems

  • High efficiency due to intense ultrasound cavitation
  • Uniform sonication due to superior flow cell design
  • Linear scale-up to any volume / flow rate
  • High energy-efficiency / low energy cost
  • High operation safety
  • Fulfilling common industrial standards
  • 7/24 operation under full load
  • No requirement for harsh chemicals
  • Low maintenance / no disruption to work
  • Fast RoI
  • Simple integration and retro-fitting
  • Environmental-friendly
Quality, Reliability & Safety

  • Superior quality: Designed and built in Germany
  • Technical service, training and support from our well-experienced staff
  • User-friendly and Safe Operation
  • 24/7 operation
  • Outstanding robustness
  • Pre-set operation options
  • Easily programmable settings
  • Browser remote control
  • Automatic data protocolling
  • Low maintenance / low downtime for maintenance
  • CIP (cleaning-in-place)

The table below gives you an indication of the approximate processing capacity of our ultrasonicators:

Batch Volume Flow Rate Recommended Devices
0.1 to 20L 0.2 to 4L/min UIP2000hdT
10 to 100L 2 to 10L/min UIP4000hdT
n.a. 10 to 100L/min UIP16000
n.a. larger cluster of UIP16000

Contact us now to get more details about Hielscher ultrasonic sewage sludge treatment systems, installation options and technical information!

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Ultrasonic high-shear homogenizers are used in lab, bench-top, pilot and industrial processing.

Hielscher Ultrasonics manufactures high-performance ultrasonic homogenizers for mixing applications, dispersion, emulsification and extraction on lab, pilot and industrial scale.



Literature / References

Facts Worth Knowing

Additional Benefits of Ultrasonic Sewage Sludge Treatment

Besides the above presented advantages of ultrasonic treatment of municipal sewage sludge, the application of high-power ultrasound waves shows additional positive effects such methanogenesis.
Ultrasonication has been proved to be promising in terms of methanogenic inhibition as it provides a consistent result. Ultrasonication treatment might have enhanced enzymatic activities of the exoelectrogens, and the permeability and selectivity of cell membrane which accelerated proteins, polysaccharides and enzymes transport from inner layers of sludge flocs to outer layers resulting in improved substrate utilization and electricity generation. (cf. Raychaudhuri and Behera, 2020)

Coulombic Efficiency

The Coulomb efficiency or Coulombic efficiency is usually used to describe the released battery capacity. It refers to the ratio of the discharge capacity after the full charge and the charging capacity of the same cycle. It is usually a fraction of less than 1.
Coulombic Efficiency (CE %) is the ratio of discharge capacity (mAh/g) to charge capacity (mAh/g) multiplied by 100.


High performance ultrasonics! Hielscher's product range covers the full spectrum from the compact lab ultrasonicator over bench-top units to full-industrial ultrasonic systems.

Hielscher Ultrasonics manufactures high-performance ultrasonic homogenizers from lab to industrial size.