Spatial and temporal variations of dissolved oxygen in Cha-Am municipality wastewater treatment ponds using GIS Kriging interpolation

Abstract

This study investigated the spatial and temporal variations in dissolved oxygen (DO) in the Cha-Am wastewater treatment ponds to assess treatment dynamics and to identify possible areas where the treatment train could be improved. Cha-Am is a small resort town with extensive beaches, located on the west coast of the Gulf of Thailand. The wastewater treatment system for Cha-Am consists of four ponds in sequence: aeration pond, sedimentation pond, extended aeration pond, and evaporation pond. Two YSI 6920 datasondes were installed near the inlet of the aeration pond and in the sedimentation pond, to measure dissolved oxygen (DO), pH, conductivity, temperature, and turbidity at 30 min time intervals over a 3 month period. DO averaged 3.09 mg/L and 3.33 mg/L, respectively in the aeration pond and in the sedimentation pond. DO generally varied over a diel cycle with higher values observed in midafternoon and lower values observed after midnight. DO often increased after a rainfall event. Ordinary Kriging (OK) interpolation in ArcGIS10.1 was used to map the spatial distribution of DO at different depths based on YSI spot measurements. OK indicated the highest DO concentrations were near the surface (0.5 m to 1.0 m); averaging 18.53 mg/L, 20.5 mg/L, 17.31 mg/L and 9.7 mg/L in the four ponds, but sometimes the concentrations were <2 mg/L near the bottom of the ponds. Two of the ponds are used as a wild catch fishery and low DO seems to negatively impact the fish. The spatial trend of DO shows that normally DO is lower at the inlet of the aeration pond than at its outlet even though mechanical aerators are operated through part of the day. Improved aeration and sunlight penetration through enhanced particle settling may be of benefit.