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

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.

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.

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 PO₄^3-, NH₄⁺, and Mg²⁺ are removed from the liquid phase due to struvite precipitation, until the reaction reaches the equilibrium.

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 PO₄^3- 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!

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

Electrocoagulation 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!

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.
All 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!

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

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