The raw water from natural resources is high in impurities, turbidity, dissolved gases, and mineral content. Sometimes algae, mud and oil could be present and silica is particularly a problem in water from lakes, and this scenario complicates treatment further. Lack or inadequate water treatment is dangerous as it may cause scaling and corrosion which affect the reliability, efficiency, and safe operation of the boiler.

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In general, boiler water treatment consists of external and internal water treatment. External water treatment refers to conditioning boiler feedwater by removing impurities, hardness, oil, oxygen, dissolved and suspended solids, outside the boiler and usually accomplished by mechanical means such as continuous clarifier, pressure sand filter, water softener, and deaerator. Because it is not possible to obtain a perfect boiler feed water by external water treatment, an internal water treatment, which mainly by chemical means, shall be employed. All boiler engineers must be aware that the external treatment of boiler feedwater must be emphasized since more chemical addition in the boiler is undesirable.
In industrial boilers, the removal of turbidity and suspended matters is already done by the municipal water treatment system. This is however a different story in palm oil mills. The external treatment for boiler makeup (feedwater) consists of pH adjustment, coagulation, flocculation, sedimentation, filtration, water softening, and finally, deaeration. Soda ash is added if the pH of the water is low. Soda ash acts as pH adjustment because coagulant functions at pH ranges over 5.5 to 8.0. The coagulation chemicals are dosed into the inlet pipe leading to the bottom of a continuous water clarifier. Coagulation and flocculation are the basic steps in boiler water treatment to reduce turbidity, organic substances, and color of raw water. Coagulation neutralizing the negative charges on colloid surfaces, allowing the particles to agglomerate to form floc, which is slow settling. The most common coagulants are aluminum sulfate (alum), sodium aluminate, and polyaluminum chloride (PAC). Synthetic polymers called polyelectrolites have been developed for coagulation process. Flocculation is further agglomeration of slowly-settling coagulated particles into large rapidly-settling floc with the addition flocculant such as organic polymer (starch) or synthetic polymer such as polyacrylamide (PAM) to attach and bridge between particles to form larger agglomerates.