Spatial and temporal variation of dissolved oxygen in Cha-Am Municipality wastewater treatment ponds using GIS kriging interpolation

This study investigated the spatial and temporal variation of dissolved oxygen (DO) in the Cha-Am wastewater treat-ment ponds to assess treatment dynamics and identify pos-sible areas where the treatment train could be improved. Cha-Am is a small resort town with extensive beach area located on the west coast of the Gulf of Thailand. The wastewater treatment system for Cha-Am consists of four ponds in sequence; i) aeration pond ; ii) sedimentation pond ; iii) extended aeration pond ; and iv) evaporation pond. Two YSI 6920 datasondes were installed near the inlet of aeration pond and the outlet of sedimentation pond and measured DO, pH, conductivity, temperature, and tur-bidity at 30 minute time intervals over a 3 month period. DO at the aeration pond and the sedimentation pond aver-aged 3.09 mg/L and 3.33 mg/L, respectively. DO generally varied over a diel cycle with higher values occurring in midafternoon and lows occurring after midnight. DO also often increased after a rainfall event. Ordinary Kriging (OK) interpolation in arcGIS10 was used to map the spatial distribution of DO at different depths based on YSI spot measurement. The kriging indicated the highest DO con-centrations were near the surface (0.5 – 1.0 m), averaging 18.09, 20.84, 17.22 and 9.31 mg/L in the four ponds, but sometimes the concentrations were below 2 mg/L near the bottom of the ponds. A couple 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 the aeration pond is lower than the outlet even though mechan-ical aerators are operated through part of the day. Improved aeration and sunlight penetration through enhanced parti-cle settling may be of benefit.