RockAway®: Deposits don’t stand a chance

The formation of mineral deposits (salt crusts and/or calcium carbonate scales) has long been a major problem in many water-carrying systems. That was simply unacceptable for the developers at P.C.S. They put on their thinking cap and came up with the RockAway© product group - chemical substances developed for our customers with the aim to prevent the formation of deposits. The effectiveness of the crystallisation inhibitors is not in doubt; the products are in demand all over the world. Experts and anyone else interested in learning more about them will find further information below.

The problem

Mineral deposits in water-carrying systems substantially interfere with processes. If conditions are favourable, crystallisation may occur when sludge is dewatered. Deposits form that cause problems in the dewatering aggregates and the downstream pipes and conduits that carry the sludge water.

Crystallisation occurs in particular if centrifuges are used to dewater digested sludge. Crystallisation usually occurs after the sludge water has left the centrifuge. The process is exacerbated by sludge particle deposits that have not precipitated.

That leads to operational disruptions all the way to plant shut-downs, which on top requires additional cleaning and maintenance work.

Our solution

Every application case is different, and it is usually necessary to look at the process as a whole to be able to determine the cause of the problem. In a first step, we map the process. After that, we analyse the water and the deposit. Based on the result, we determine the composition of the product exactly as needed to solve your problem. This is how we proceed:

  • We analyse the sludge water samples you provided to determine the cause; the analysis is free-of-charge
  • We determine the composition and the precise quantity of the product to meet your specific needs
  • We recommend the RockAway® product that is right for you

Your benefits

  • Considerable maintenance, repair and replacement savings
  • Significantly longer lifespan of your water-carrying systems

Target industries

  • Biogas plants
  • Municipal sewage treatment plants
  • Paper industry

Our references

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Understanding RockAway®

Scaling – causes, formation, appearances

Analyses under the microscope of the deposits formed in digested sludge dewatering almost always show the typical layered structure, similar to the one you can see in the centre photo below. These inhomogeneous deposits may cause considerable problems in centrifuges, pumps, pipes and conduits. Frequently, pipes and conduits are blocked almost completely, which in turn creates a centrate backlog all the way back to the centrifuge.

Ions / temperature / pH value

The majority of substances is water-soluble to a limited extent, with solubility levels ranging from infinitely soluble to very poorly soluble. Salts, the main components of water, are ionic compounds consisting of cations and anions. In infinitely soluble salts, the cations and anions are separated or split (dissociated) and present in water as solvated ions, i.e. surrounded by a hydrate layer.

In poorly soluble salts, however, the cations and anions are not separated (undissociated); they form a stable chemical compound (ionic compound). The solubility product can be used to describe the tendency to form poorly soluble salts, whereby the solubility product is defined as the product of the dissolved cation and anion concentrations of a dissociated salt in a saturated solution (above a bottom deposit or solid).

If the solubility product is exceeded, the cations and anions combine to form a poorly soluble compound and precipitate from the solution. Whether the solubility product is exceeded largely depends on the following factors:

  • Type and concentration of ions
  • Temperature
  • pH value

The factors influencing deposit formation are closely interlinked. As such, an increase in temperature causes the water to evaporate and the ion concentration to rise in the water-carrying system, which in turn forces the precipitation of poorly soluble salts.

Type and concentration of ions

If suitable cations (e.g. Mg2+, Ca2+, NH4+) and anions (CO32- , PO43-) are present in an oversaturated solution, poorly soluble salts, e.g. calcium carbonate (calcite) or magnesium ammonium phosphate (MAP), form spontaneously.

Temperature and pH value

Precipitation is temporarily suppressed as limiting values such as, e.g., pH, temperature and ion concentration, are undershot. Thus the ions are held in solution in unstable buffer systems.

Buffer systems

The pH value is of critical importance for precipitation and crystallisation processes. The ions mentioned above require a pH of over 7 - an alkaline medium - to generate precipitation and crystallisation products. In many processes, in particular bio-chemical reactions (fermentation, digestion), a buffer system helps to keep the pH constant within a typical range.

Like salts, gases are water-soluble and dissociate into ions. Of particular significance are oxygen (O2), carbon dioxide (CO2) and ammonia (NH3) because they are polar and easily dissolve in water. In the aqueous system NH3 converts into ammonium ion (NH4+), whereas for CO2 the equilibrium between bicarbonate (HCO3-) and carbonate (CO32-) ions is maintained.

This mixture converts into the natural buffer system, ammonium bicarbonate (NH4HCO3). To a limited extent, this buffer can counterbalance changes in the concentration levels of acids and bases and keep the pH constant.

CO2 is special in that in aqueous systems a range of different reactions creates a lime-carbonic acid equilibrium.

All reactions listed above are equilibrium reactions, i.e. they can take place in both directions. The complicated interaction between gaseous and dissolved carbon dioxide (carbonic acid), the H3O+ concentration (pH value) and the bicarbonate concentration cause the carbonate concentration to increase so that the solubility product of calcium carbonate is exceeded.

If changes are drastic, for instance when dissolved gases are desorbed (stripping effect) by strong turbulences, the buffer may be destroyed, which causes the pH in the aqueous system to fluctuate considerably. A pH leap from the acidic into the alkaline range in particular is disastrous because the pH increase strongly encourages precipitation and crystallisation. The transition line between dissolved metal salts and crystallised metal salts is slim and depends on a very sensitive equilibrium system, which is easily disturbed even if the changes are minimal.

For instance, a minimal rise in the pH value causes the precipitation limit level to shift to low ion concentrations. It is possible to determine critical concentration ranges for every type of deposit.

Scaling – cause analysis

Every application case is different, and it is usually necessary to look at the process as a whole to be able to determine the cause of the problem. In a first step, the process is mapped. After that the water and the deposit are analysed.

Scaling – typical deposits

The deposits vary in colour, structure and hardness. The parts forming the structure (ions) either come in crystalline or amorphous arrangement. Symmetrically arranged molecules such as those found in calcite or magnesium ammonium phosphate (MAP) form hard deposits.

Iron hydroxides or brownstone / manganese dioxides convert into an amorphous arrangement of molecules which usually form soft deposits. Often, ageing causes the deposits to change from an amorphous to a crystalline arrangement. The photo on the left shows the crystal structures that are described below:

  • Calcium sulphate (contaminated with iron salts)
  • Calcium carbonate (with hydrated iron oxide)
  • Magnesium ammonium phosphate (MAP, Struvite)

Formation of hydroxylapatite: Ca5((PO4)3(OH))

This salt is almost insoluble and precipitates from a pH of about 8. In particular it forms if the Ca-ion concentration is > 80 mg/l and the PO4-ion concentration is > 60 mg/l. This salt spontaneously precipitates in the centrate discharge area of the centrifuge.

Formation of calcium carbonate: CaCO3
This salt precipitates at a pH of 7.5. As its solubility is better than that of hydroxylapatite, it tends to form especially strong crusts.

Formation of magnesium ammonium phosphate (MAP): MgNH4PO4 * 6H2O

If phosphate concentrations are > 140 mg/l, MAP can already precipitate at a pH of 7.0. In this case, all plant equipment parts will show crystalline crusts of this salt. If the PO4 concentration is between 90 and 140 mg/l, MAP crystallisation occurs only at a pH of 7.5. If the concentration is below 90 mg/l of PO4, a pH of 8.0 is required for the MAP to crystallise. From pH 8.0 onwards, the formation of hydroxylapatite competes with MAP crystallisation and the probability that MAP will form becomes significantly smaller. MAP precipitates later than the calcium salts, i.e. deposits form in the pipes and conduits rather than in the centrifuge area. MAP often already occurs in the pipes and conduits leading from the digester to the centrifuge.

Formation of iron oxide crusts

If the chemical phosphate removal process releases an excess of iron ions, Fe2+ ions may oxidise to Fe3+ ions in the centrifuge discharge area and in the downstream pipes and conduits, which then precipitate due to hydrolytic activity and form brown hydrated iron oxide crusts.

Formation of vivianite; Fe3(PO4)2 x 8 H2O

Under certain, greatly reducing conditions, trivalent iron phosphate that has formed during the phosphate removal converts into vivianite. Such conditions are frequently encountered in heat exchangers, for instance when digested sludge is mixed with raw sludge. Vivianite forms in particular if, apart from chemically precipitated phosphates, a high proportion of biologically bound phosphate is present. Vivianite (blue iron ore) is a bivalent iron phosphate, the chemical formula of which is Fe3 (PO4)2 x 8 H2O. In the beginning, this compound is colourless or white. If it is exposed to air, iron oxidation will instantly colour it blue.

RockAway® – the solution to the problem

Prevention of scaling

You can solve the problem chemically by using crystallisation inhibitors. The newly developed RockAway® product group comprises modified phosphonates and polycarbonate acids that effectively prevent scaling. Bivalent metal ions in particular, such as calcium, magnesium and iron, have a high affinity to them, which affects the formation of poorly soluble metal salts such that they either do not precipitate at all or precipitate in a way that prevents them from forming deposits.

RockAway® has the following special properties. It:

  • excellently inhibits magnesium, calcium and iron ions;
  • influences the crystals’ shape and size;
  • generates micro-crystalline dispersions; and
  • requires low dosing (in the sub-stochiometric range)

Dosing of RockAway®:

For instance, if you base on a calcium concentration of 100 mg/l, a temperature of 35°C and a pH of 8.0, adding a mere 10ppm of the RockAway® product PX 60 N completely prevents crystallisation. In average sludge water, at temperatures of up to 35°C and pH values of up to 8.5, calcium ions can be fully kept in solution by adding about 30ppm of the product.

The following values are indicative values; they apply to average municipal sludge.

Different substances contained in the sludge and different pre-treatment procedures, however, may considerably influence the effectiveness of RockAway® products.

It is generally recommendable to look at the process as a whole, including where the sludge water originates from.

If you send us a sludge water sample (about 1l) and samples of the deposit, we will perform a laboratory analysis to determine the crystallisation potential and advise you what you can do.

This test is free of charge for you. You will just have to pay for the postal fees to send us the sample.

RockAway® the mechanism behind it

Crystallisation only occurs in oversaturated solutions consisting of cations (e.g. NH4+, Ca2+, Mg2+) and anions (e.g. OH-, CO32-, PO43-). Oversaturated solutions are thermodynamically not stable. They try to eliminate the relevant solution component (water) and transform into a stable low-energy state - the solid phase. During this transition (crystallisation) from a liquid state to a solid aggregate state, two different processes take place: nucleation and nucleate growth.

During nucleation, tiny nuclei spontaneously form on tiny particles or surfaces. They constantly disintegrate and form anew. Such nuclei have a so-called "under-critical" size. When these nuclei have reached an "over-critical" size - and only then - they are stable and grow into bigger crystallites, which marks the onset of nucleate growth. The many tiny nuclei (crystallites, micro-crystals) bond into huge crystal structures which eventually convert into hard deposits and crusts.

RockAway® products consist of water-soluble polymer and ionic constituents that have high adsorption abilities on the surface of the nuclei and crystals where they accumulate.

The adsorption causes the composition of the surface to change and prevents, disrupts or delays further growth. On the one hand, adsorption drastically reduces the rate of nucleation and on the other hand, it greatly impedes the growth and agglomeration of crystals.

Thus it is possible to stabilise supersaturated, aqueous solutions and simultaneously prevent turbidity or precipitations.

RockAway® – the products

Download the RockAway® product range as PDF here. Please contact us, we will be happy to advise you!