Investigations of operating conditions to improve the UV transmittance in chemically enhanced primary treatment

Chemically enhanced primary treatment (CEPT) has been favored in treatment of wastewater in recent years for its much higher efficiencies in removing particles and other contaminants comparing to the conventional primary treatment and lower costs than the secondary treatment. Siu Ho Wan Sewage Treatment Work (SHWSTW) employed the CEPT process in 2005 and was upgraded with an UV disinfection unit since 2007. However, the CEPT process using ferric chloride at SHWSTW produces effluents with UV transmittance (UVT) constantly low as 10% to 15%, which is quite unfavorable for UV disinfection. The current remedial reaction of increasing the resident time of the disinfection facilities is highly energy-consuming and it’s doubtful that this action would be successful at the plant’s full capacity. This study proposed methods to enhance UVT of the CEPT effluent by identifying the relative importance of various UVT-influencing parameters under different operating conditions. Jar tests were utilized to simulate the CEPT process under various operating conditions and data were used to construct a mathematical model describing the relationship between the effluent UVT and those UVT-influencing parameters, including TSS with different particles size ranges, DOC and dissolved iron. Through sensitivity analysis, the predominance of these parameters on UVT was determined. Together with the evaluations on the operating parameters, strategies to enhance effluent UVT were obtained and verified through jar tests. In general, TSS played the most important role in influencing effluent UVT. Increasing the flocculation time and speed at the plant’s current poor rapid mixing conditions could improve the effluent UVT to some degree. However these two parameters affected the effluent UVT much less when a near-ideal rapid mixing was provided. On the other hand, increases in rapid mixing speed and coagulant dose exerted great positive effects on the effluent UVT. In addition, the developed model provided a convenient tool for predicting the effluent UVT under different operation conditions