There are two general approaches to sludge treatment inside a boiler: coagulation and dispersion.
When the total quantity of sludge is high, as a result of high feed water hardness, it is preferable to coagulate the sludge to form foculeous particles.

These can be removed by draining. Coagulation can be achieved by precise dosing of the alkaline products, phosphates and organic compounds used for treatment.

When the quantity of sludge is not too high (low feed water hardness), it is preferable to use a product with a high phosphate content.
Phosphates form sludge particles. An organic dispersant is then used to keep the particles dispersed in the boiler water.

Scaling in boilers is caused by impurities having precipitated out of the water directly onto the heat transfer surfaces, or by suspended matter settling on the metal and becoming hard and sticky. Evaporation in boilers increases the concentration of impurities. These interfere with heat transfer and can cause hot spots leading to local overheating.
Common feedwater contaminants that can form deposits in boilers include calcium, magnesium, iron, aluminum and silica. Scaling is formed by salts with limited solubility. These salts circulate in water in soluble or precipitated form.

Scaling is mainly due to the presence of calcium and magnesium salts (carbonates and sulfates), which are less soluble hot than cold, or due to the presence of high silica concentrations in relation to the alkalinity of the boiler water.
If left untreated, scaling leads to a gradual reduction in boiler efficiency, acting as an insulator. Eventually, the composition of the scale will cause overheating and breakage.

Deposits in the boiler can also lead to clogging or partial obstruction due to corrosive attack beneath the deposits. In general, deposits can affect operating efficiency, producing damage, unscheduled boiler downtime, and increased cleaning costs.

The carbonate deposit is generally granular and sometimes porous. It is dense and uniform. The use of an acid solution can easily identify a carbonate deposit generating carbon dioxide bubbles by reaction.

The sulfate deposit is harder and denser than a carbonate deposit because the crystals are smaller and more united. It does not pulverize easily, nor does it effervesce in the presence of acid.

Silica can vaporize in steam at relatively low pressures. Its solubility in steam increases proportionally with temperature. Thus, silica becomes more soluble if the steam is superheated.

Depending on boiler conditions, silica is distributed in boiler water and steam in a defined ratio. This ratio depends on two factors: boiler pressure and water pH. The value of this index increases almost logarithmically with pressure and decreases inversely in proportion to pH.
If silica is present in the boiler water, the usual corrective action is to increase boiler blowdown to an acceptable level, thus correcting the condition that is causing silica contamination.

A strong silica deposit is very hard, resembling porcelain. The silica crystal is extremely small, forming a very dense, impermeable scale. This scale is extremely fragile and very difficult to pulverize. It is not soluble in hydrochloric acid, and is usually highly colored.