This　study　focuses　on　the　effectiveness　of　calcium　(Ca2+)　improving　ferric　(hydro)oxides　precipitation　and　its　subsequent　effects　on　arsenic　co-precipitation　with　ferric　(hydro)oxides.　The　effects　of　Ca2+　on　surface　charge　characteristics　and　precipitating　behavior,　which　are　respectively　represented　as　zeta　(zeta)　potential　and　R-PDA,　are　investigated　for　ferric　(hydro)oxides　precipitates.　The　presence　of　Ca2+　increases　the　potential　of　ferric　(hydro)oxides,　and　a　more　significant　effect　is　observed　at　higher　pH　conditions.　Calcium　apparently　facilitates　ferric　(hydro)oxide　floc　aggregation,　increasing　the　maximum　R-PDA　from　3.19　to　5.27%　(in　the　presence　of　3.5　mg/L　silicate　as　silicon).　These　positive　effects　contribute　to　reduce　the　adverse　effects　resulting　from　the　presence　of　silicate　and　enhance　arsenic　removal　via　ferric　(hydro)oxides　co-precipitation　under　different　conditions.　Furthermore,　the　effect　of　Ca2+　facilitating　ferric　precipitation　(and　therefore　providing　more　precipitated　solids　for　arsenic)　is　predominant　in　favoring　arsenic　removal　compared　with　that　of　increasing　surface　charge.　Calcium　plays　an　important　role　in　arsenic　co-precipitation　with　ferric　(hydro)oxides　in　the　presence　of　silicate.