Ultrasonic Phosphor Recovery from Sewage Sludge
- The worldwide demand for phosphor is increasing, whilst the supply of natural phosphorus resources are getting scarce.
- Sewage sludge and sewage sludge ash are rich in phosphorus and can be therefore used as source to reclaim phosphorus.
- The ultrasonic wet-chemical processing and precipitation improves the recovery of phosphate from sewage sludge as well as from the ash of incinerated sludge and makes the recovery significantly more economical.
Phosphorus
Phosphorus (phosphor, P) is a non-renewable resource, which is heavily used in agriculture as fertilizer as well as in many industries, where phosphorus is a valuable additive (e.g., paints, laundry detergents, flame retardants, animal feed). Sewage sludge, incinerated sewage sludge ash (ISSA), manure and dairy effluents are rich in phosphorus, making them a source for phosphorus recovery in regards of the finite resource of phosphorus as well as of environmental concerns.
Phosphorus recovery rates from the liquid waste water streams can reach 40 to 50%, while recovery rates from sewage sludge and sewage sludge ash can reach up to 90%. Phosphorus can be precipitated in many forms, one of them being struvite (valued as a high-quality, slow-release fertilizer). In order to make the reclamation of phosphorus economical, the recovery process must be improved. Ultrasonication is a process intensifying method that accelerates the process and increases the yield of recovered minerals.
Ultrasonic Phosphorus Recovery
Under sonication, valuable materials such as struvite (magnesium ammonium phosphate (MAP)), calcium phosphate, hydroxyapatite (HAP) / calcium hydroxyapatite, octacalcium phosphate, tricalcium phosphate, and dicalcium phosphate dihydrate can be recovered from waste streams. The ultrasonic treatment improves the wet-chemical extraction as well as precipitation and crystallisation (sono-crystallization) of valuable materials from sewage sludge and from the ash of incinerated sludge.
Whilst the content of phosphorus (8-10%), iron (10-15%), and aluminium (5-10%) in ash of mono-incinerated sewage sludge is quite high, it also contains toxic heavy metals such as lead, cadmium, copper, and zinc.
Phopshorus Recovery – A Two-Step Process
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- Acid Extraction
The first step of phosphor recovery is the extraction or leaching of phosphorus from sewage sludge or incinerated sewage sludge ash (ISSA) using an acid such as sulfuric acid or hydrochloric acid. Ultrasonic mixing promotes the wet-chemical leaching by increasing the mass transfer between the acid and the ISSA so that a complete leaching of phosphorus is achieved rapidly. A pre-treatment step using ethylenediaminetetraacetatic acid (EDTA) may be used to improve the extraction procedure.
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- Precipitation of Phosphorus
Ultrasonic crystallisation enhances the precipitation of phosphates significantly by increasing the seeding points and accelerating the adsorption and aggregation of molecules in order to form a crystal. Ultrasonic precipitation of phosphorus from sewage sluge and ISSA can be achieved e.g., by using magnesium hydroxide and ammonium hydroxide. The resulting precipitate is struvite, a compound made up of magnesium, ammonium, phosphorus and oxygen.
Sonocrystallization of Struvite
Ultrasonic dispersing promotes the mass transfer between phases and initiates the nucleation and crystal growth for phosphates (e.g., struvite / MAP).
Ultrasonic inline precipitation and crystallisation of struvite allows for the treatment of large volume strams on industrial scale. The issue of processing a large sewage sludge stream can be resolved by a continuous ultrasonic process, which accelerates the crystallization of struvite and improves the crystal size producing smaller, more uniform phosphate particles. The size distribution of precipitated particles is determined nucleation rate and the subsequent crystal growth rate. Accelerated nucleation and inhibited growth are the key factors for the precipitation of cristalline phosphate particles, i.e. struvite, in an aqueous solution. Ultrasonication is a process intensifying method that improves the blending in order to obtain a homogenous distribution of reactive ions.
Ultrasonic precipitation is known to give narrower particle size distribution, smaller crystal size, controllable morphology and as well as fast nucleation rate.
Struvite crystals precipitated from swine effluent (source: Kim et al. 2017)
Good precipitation results can be achieved for example with PO3-4 : NH+4 : Mg2+ at a ratio of 1 : 3 : 4. The pH range of 8 to 10 leads to maximum phosphate P release
Ultrasonication is a highly efficient process intensifying technique to promote the precipitation of valuable materials such as calcium phosphate, magnesium ammonium phosphate (MAP) and hydroxyapatite (HAP), calcium hydroxyapatite, octacalcium phosphate, tricalcium phosphate, and dicalcium phosphate dihydrate from waste water. Sewage sludge, manure and dairy effluents are known as nutrient-rich waste water, which is suitable for the production of valuable materials via ultrasonically assisted precipitation.
Struvite crystal formation:
Mg2+ + NH+4 + HPO2-4 + H2O –> MgNH4PO4 ∙ 6H2O + H+
High-power ultrasonic processors from lab to pilot and industrial scale.
Industrial Ultrasonic Equipment for Leaching and Precipitation
High-performance ultrasonic systems and reactors are required to treat incinerated sewage sludge ash (ISSA) and sewage sludge on industrial scale. Hielscher Ultrasonics is specialized in the design and manufacturing of high-power ultrasonic equipment – from lab and bench-top to fully industrial units. Hielscher ultrasonicators are robust and built for the 24/7 operation under full load in demanding environments. Accessories such as flow cell reactors with various geometries, sonotrodes (ultrasonic probes) and booster horns allow for the optimal adaption of the ultrasonic system to the process requirements. In order to process large volume streams, Hielscher offers 4kW, 10kW and 16kW ultrasonic units, which can be easily combined in parallel to ultrasonic clusters.
Hielscher’s sophisticated ultrasonicators feature a digital touch display for easy operation and precise control of the process parameters.
User-friendliness and an easy, safe operation are key features of Hielscher ultrasonicators. The remote browser control allows the operation and control of the ultrasonic system via PC, smart phone or tablet.
The table below gives you an indication of the approximate processing capacity of our ultrasonicators:
| Batch Volume | Flow Rate | Recommended Devices |
|---|---|---|
| 10 to 2000mL | 20 to 400mL/min | UP200Ht, UP400St |
| 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 |
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Literature/References
- Dodds, John A.; Espitalier, Fabienne; Louisnard, Olivier; Grossier, Romain; David, Rene; Hassoun, Myriam; Baillon, Fabien; Gatumel, Cendrine; Lyczko, Nathalie (2007): The effect of ultrasound on crystallisation-precipitation processes: Some examples and a new segregation model. Particle and Particle Systems Characterization, Wiley-VCH Verlag, 2007, 24 (1), pp.18-28
- Kharbanda, A.; Prasanna, K. (2016): Extraction of Nutrients from Dairy Wastewater in the Form of MAP (Magnesium Ammonium Phosphate) and HAP (Hydroxyapatite). Rasayan Journal of Chemistry Vol. 9, No. 2; 2016. 215-221.
- Kim, D.; Jin Min, K.; Lee, K.; Yu, M.S:; Park, K.Y. (2017): Effects of pH, molar ratios and pre-treatment on phosphorus recovery through struvite crystallization from effluent of anaerobically digested swine wastewater. Environmental Engineering Research 22(1), 2017. 12-18.
- Rahman, M., Salleh, M., Ahsan, A., Hossain, M., Ra, C. (2014): Production of slow release crystal fertilizer from wastewaters through struvite crystallization. Arab. J. Chem. 7, 139–155.
Facts Worth Knowing
How Does Ultrasonic Precipitation Work?
Ultrasonication impacts nucleation and crystal growth, a process known as sonocrystallization.
Firstly, the application of ultrasound allows to influence the nucleation rate, where there solid crystals form from a liquid solution. High-power ultrasond creates cavitation, which is the growth and implosion of vacuum bubbles in a liquid medium. The implosion of the vacuum bubbles introduces energy into the system and reduces the critical excess free energy. Thereby, seeding points and nucleation are initiated at a high rate and at the earliest time. At the interface between cavitation bubble and the solution, half of a solute molecule is solvated by the solvent, while the other half of the molecule surface is covered by the cavitation bubble, so that the solvation rate is decreased. The re-dissolution of the solute molecule is prevented, while the coagulation of molecules in the solution is increased.
Secondly, sonication promotes the crystal growth. Ultrasonic mixing promotes the growth of crystals by incresing the mass transfer and aggregation of molecules.
The results achieved by sonication can be controlled by the sonication mode:
Continuous sonication:
Continuous ultrasonic treatment of the solution produces many nucleation sites, so that a large number of small crystals are created
Pulsed sonication:
The application of pulsed / cycled sonication allows for the precise control over crystal size
Sonication to initiate nucleation:
When ultrasound is applied only during the beginning of the crystallization process, a finite number of nuclei are formed, which then are grown to a larger size.
Using ultrasonication during crystallization, the growth rate, size, and shape of the crystal structures can be influenced and controlled. The various options of sonication make sono-crystallization processes precisely controllable and repeatable.
Ultrasonic Cavitation
When high-intensity ultrasound cross a liquid medium, high-pressure (compression) and low-pressure (rarefaction) waves are alternating through the liquid. When the negative pressure caused for an ultrasonic wave crossing a liquid is large enough, the distance between the molecules of the liquid exceeds the minimum molecular distance required to hold the liquid intact, and then the liquid breaks down so that vacuum bubbles or voids are created. Those vacuum bubbles are also known as cavitation bubbles.
Cavitation bubbles used for power ultrasonic applications such as mixing, dispersing, milling, extraction etc. occur under ultrasound intensities higher than 10 Wcm2. The cavitation bubbles grow over several acoustic low-pressure / high-pressure cycles until they reach a dimension where they cannot absorb more energy. When a cavitation bubble has reached its maximum size, it implodes violently during a compression cycle. The violent collaps of an transient cavitation bubble creates extreme conditions such as very high temperatures and pressures, very high pressure and temperature differentials and liquid jets. Those forces are the source for chemical and mechanical effects used in ultrasonic applications. Each collapsing bubble can be considered as a microreactor in which temperatures of several thousands degrees and pressures higher than one thousand atmospheres are created instantaneously [Suslick et al 1986].
Ultrasonic extraction is based on acoustic cavitation and its hydrodynamic shear forces
Phosphorus
Phosphorus is an essential, non-regenerable resource and experts already predict that the world will hit “phosphor peak”, i.e. the time from which supply can no longer meet the increased demand, in approx. 20 years. The European Commission has already classified phosphorus as a critical raw material.
Sewage sludge is often used as fertilizer spread on the fields. However, since sewage sludge not only contains valuable phosphate but also harmful heavy metals and organic pollutants, many countries such as Germany, restrict by legislation how much sewage sludge can be used as fertilizer. Many countries such as Germany have stringent fertilizer regulations, which limit the contamination with heavy metals strictly. Since phosphorus is a finite resource, the German Sewage Sludge Regulation from 2017 requires sewage plant operators to recycle phosphates.
Phosphorus can be recovered from wastewater, sewage sludge, as well as from the ash of incinerated sewage sludge.
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. Inorganic phosphates are mined to obtain phosphorus for use in agriculture and industry. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid.
Don’t confuse the name phosphorous with the element phosphorus (chemical symbol P). They are two different things. A multivalent nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks.
Organic phosphates are important in biochemistry and biogeochemistry.
Phosphate is the name of the ion PO43-. Phosphorous acid, on the other hand, is the name of the triprotic acid H3PO3. This is a combination of 3 H+ ions and one phosphite (PO33-) ion.
Phosphorus is the chemical element that has the symbol P and atomic number 15. Phosphorus compounds are also widely used in explosives, nerve agents, friction matches, fireworks, pesticides, toothpaste and detergents.
Struvite
Struvite, also referred to as magnesium ammonium phosphate (MAP), is a phosphate mineral with the chemical formula NH4MgPO4·6H2O. Struvite crystallizes in the orthorhombic system as white to yellowish or brownish-white pyramidal crystals or in platlet-like forms. Being a soft mineral, struvite has a Mohs hardness of 1.5 to 2 and a low specific gravity of 1.7. Under neutral and alkaline conditions struvite is hardly soluble, but can be easily dissolved in acid. Struvite crystals form when there is a mole to mole to mole ratio (1:1:1) of magnesium, ammonia and phosphate in wastewater. All three elements – magnesium, ammonia and phosphate – are normally present in waste water: magnesium coming mainly from the soil, seawater and drinking water, ammonia is broken down from the urea in wastewater, and phosphate coming from food, soaps and detergents into the wastewater. With these three elements present, struvite is more likely to form at higher pH values, higher conductivity, lower temperatures, and higher concentrations of magnesium, ammonia and phosphate. Recovery of phosphorus from waste waterstreams as struvite and recycling those nutrients as fertilizer for agriculture is promising.
Struvite is a valuable slow-release mineral fertilizer used in agriculture, which has the advantages of being granular, easy-to-use, and odour-free.