This paper presents the results of a pilot scale investigation of coagulation using aluminium based coagulants (polyaluminium chloride sulphate (PACL1) and polyaluminium chloride (PACL2)), combined with pre-ozonation and pH adjustment, on natural organic matter (NOM) removal from groundwater. Tests were conducted at a semi-industrial pilot plant, using groundwater with a high content of NOM (5.1 ± 0.8 mg C/L dissolved organic carbon (DOC); 0.211 ± 0.007 cm−1 UV absorbance at 254 nm (UV254)) and trihalomethane formation potential (THMFP (273 ± 73 μg/L)). It was determined that both coagulants showed similar efficacy in reducing DOC (∼50%), UV254 (∼60%) and THMFP (∼65%). Application of pre-ozonation enhanced total coagulation efficiency (up to 61% and 80% for DOC and UV254, respectively), probably due to its microflocculation effect. THMFP in the treated water fluctuated depending on ozone dose, coagulant type and pH. The pre-treatment resulted in NOM oxidation to lower molecular weight hydrophilic organic substances. PACL1 coagulation did not efficiently remove these new THM precursors, whereas PACL2 was able to reduce THMFP to 77 μg/L at a low ozone dose of 0.4 mg O3/mg DOC, suggesting differences in the coagulation mechanism for the two coagulants. This difference confirms the necessity of optimizing the combined treatment depending upon the choice of coagulant applied.