In this study, a modified horizontal settling tank with rotating biological discs was developed to treat slightly polluted surface water, and its performance on the simultaneous removal of turbidity, organics, and ${\mathrm{NH}}_4^{\mathrm{+}}$-N was investigated on a lab scale. Results show that the effluent quality of the modified settling tank is stable in more than two months of continuous operation. At a hydraulic retention time (HRT) of 2.0 h, 73.65 ± 5.15% turbidity, 53.98 ± 5.17% TOC, and 77.01 ± 10.02%, ${\mathrm{NH}}_4^{\mathrm{+}}$-N could be removed by the modified settling tank with an average of 1.96 NTU turbidity, 1.98 mg/L TOC, and 0.46 mg/L ${\mathrm{NH}}_4^{\mathrm{+}}$-N residue in the effluent. Due to the improvement in DO supply, higher removal efficiencies of both organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N were achieved with increased disc rotating speed (r < 4 r/min). Further study showed that the genus Hyphomicrobium dominant on the posterior discs and the genus Nitrospira dominant on the anterior discs mainly contributed to the enhanced bio-oxidation of organics and ${\mathrm{NH}}_4^{\mathrm{+}}$-N, respectively.