The large yellow tankers were seen by the waste treatment plant and a manhole near Harbour Park in Windmill Road, sparking questions as to why they were there.
In response, a Southern spokesman said: “Residents may have noticed one of our tankers near Littlehampton seafront. We are currently exploring a number of investment schemes to enhance the sewage system in the area.
“While investigative and planning work is underway we are managing wastewater through a combination of measures, including regular tankers to remove any excess waste. We do appreciate this may be inconvenient and we are grateful for our customers support and understanding.”
The spokesman added the tanks and planned work were not connected to the flooding in May, which overwhelmed the sewer system and led to dozens of homes being flooded.