Coagulation and sedimentation of particulate pollutants in marine waters after wastewater outfall discharge

Outfall discharge to seawater is generally used as a practical means of wastewater disposal in many coastal cities. However, deposition of solid pollutants from the wastewater on the seabed would affect the benthic communities. In the present study, both laboratory experiments and numerical simulations were carried out on the role of particle coagulation in the transport and sedimentation of sewage solid materials in marine waters after outfall discharge. Actual sewage samples were collected from two treatment plants in Hong Kong for the experimental study. A flocculator-imaging system was developed for characterization of the particle size distribution (PSD) dynamics during flocculation and dilution of sewage particles in seawater. The system consisted of an external flow-through cell, a microscopic CCD video recorder and an image analyzer. The laboratory results demonstrated that coagulation played an important role in regulating the transport of solid pollutants in marine waters. Flocculation of 30–45 min could shift the PSD considerably, transforming small particles to larger, fast-settling particle aggregates. With a growth in particle size by flocculation, the average settling velocity of the sewage particulates increased by a factor of 2 or more. In addition to the laboratory study, a mathematical model was developed to simulate the transport dynamic of wastewater particles in seawater after outfall discharge. The results of numerical simulations compared well with the experimental observations. Both laboratory and simulation results suggest the important role of coagulation in the transport and sedimentation dynamics of sewage particles in marine waters, especially in the early phase of outfall discharge. As a result, suspended solids from the wastewater discharge would deposit more closely in the vicinity of the outfall sites.