Pollutants transformation in saline sewage sewer-experimental investigation, process study and model development

In Hong Kong, seawater is used in most of the flush toilets. This results in the production of high concentrations of hydrogen sulfide in the saline sewage sewers which leads to serious corrosions of the sewer pipes. To tackle this problem, nitrate dosing is a method practiced in the Tung Chung area currently. Since chemical dosing involves large amounts of human and financial resources, a cost-effective dosing approach should always be sought and the optimal dosage of chemicals required should be determined. However, with the constraints of site investigations and the complexities of sewer pollutant transformation processes, it would be difficult to obtain the optimal dosing amount of nitrate for the best control of hydrogen sulfide emission. Therefore an effective sewer process model would be useful for the prediction of sulfate and nitrate pollutant transformations in the saline sewage sewers. Furthermore, this model should also be able to evaluate the organic removal capacity of a gravity sewer as an interim solution to water pollution control in rural areas where no sewage treatment facilities are available. Although a number of sewer process models were proposed in the last 15 years, most of them overlooked the role of sewer biofilm and none of them explored the salinity effect and the sulfate reducing reaction. With these in mind, we developed a new and comprehensive sewer process model to fully describe various pollutants transformations in a saline sewage sewer. In order to achieve this goal, we conducted a series of studies over the last seven years, including: 1) the investigation of sewage quality transformation in a full-scale saline sewage sewer, 2) the study of sewer biofilm population, structure and bio-kinetics, 3) the development of mathematical models for the predictions of sulfate, nitrate, and organic matter transformations in both sewage and biofilm phases, as well as in the entire sewer system, 4) the verification of the proposed mathematical biofilm model using spatial profiles of various pollutant concentrations within the sewer biofilm, measured by relevant microelectrodes, 5) the verification of the proposed sewer process model with full-scale sewer measurements, and 6) the application of the proposed sewer process model to the real saline sewage sewer in the Tung Chung area for the control of hydrogen sulfide production. The main conclusions of this study can be drawn as follows: (1) A dynamic one-dimensional model for the sewage phase of the pollutants transformation was proposed, with consideration of sewer hydraulics, pollutants transport, dispersion, re-aeration and microbial transformations. This model accurately simulated the sewage quality transformation in the sewage phase of a saline sewage sewer. (2) The microbial transformation bio-kinetics in the sewer were developed from both theoretical and experimental studies. A new method was developed for simultaneous determination of various parameters in the bio-kinetics. These parameter values were based on limited batch experimental data. (3) A dynamic sewer biofilm process model was proposed. It included biofilm attachment and detachment, substrate diffusion and microbial transformation in the biofilm of a saline sewage sewer. This new model not only simulated sulfate reduction, sulfide oxidation, denitrification, and organic degradation, but also dealt with the variations of biofilm thickness and density in a real sewer. (4) Various verification approaches for the proposed sewer biofilm model were attempted, including comparisons between model predictions and measurements of biofilm thickness and density; comparisons between model predictions and spatial profiles of H2S, NH4-N, O2, NOx-N within the biofilm using relevant microelectrodes; and comparisons between model prediction and spatial profile of sulfate reducing bacteria (SRB) within the biofilm using specific gene probe. All these comparisons verified the proposed sewer biofilm model successfully. (5) A saline sewage sewer quality transformation process model was developed by integrating the sewage phase model and the biofilm model. This new sewer process model was verified by the measurements of the variation of dissolved oxygen concentration in a full-scale saline sewage gravity sewer. (6) The sewer process model was applied to a real sewer in Tung Chung. The simulation showed that the optimal dosage of nitrate for the control of hydrogen sulfide production was 84 g-N m-3 under a continuous dosing condition. Key Words: Saline sewage; sewer biofilm; sewage quality transformation; model development and verification; hydrogen sulfide control