LODELLING THE DISPERSION OF HYDROGEN SULPHIDE AT A WASTEWATER TREATMENT PLANT

Abstract

For a small, urbanised city such as Singapore, where land is a scarce commodity, much effort and money have been directed towards the control of odorous emissions at sewage treatment plants so as to minimise the expensive buffer zones typically set aside for such facilities. Existing process units at a number of municipal wastewater treatment plants in Singapore are currently being upgraded into covered units and odorous air within each covered unit is collected and treated at odour treatment scrubbers. An objective of this study is to illustrate the impact of the cover and treatment strategy at one of the wastewater treatment plants - Kranji Wastewater Treatment Plant (KWWTP). Modelling investigation of the dispersion characteristics of hydrogen sulphide (H2S), a representative odorous air pollutant emitted from municipal wastewater treatment in Singapore is carried out for the conditions before and after the various process units are covered. Appropriate impact assessment can then be made from such modelling. KWWTP is located in the North of Singapore and treats an average daily sewage flow of about 76 ML/day. It is designed to operate based on the conventional activated sludge process. The source emission rates from various uncovered process units comprising point, area, and volume sources are determined from a field inventory study and these rates are used as input to the United States Environmental Protection Agency (USEPA) dispersion model - Industrial Source Complex Short Term (ISCST3) dispersion model.

The hourly maximum H2S concentration levels in the vicinity of the plant under five years meteorological conditions are modelled and compared against the situation after covering of the process units. Modelled results indicated that the area impacted by the plume is significantly reduced after the covering of the process units. Efforts have been devoted towards establishing a novel relationship between the area, A, impacted by the odorous H2S plume emitted from the plant with the magnitude of the ground level concentration, C, obtained in the vicinity of the plant. It has been found that the novel equation of the form A=q Cn, where q and n are empirical coefficients, best described this relationship. Comparison study between Singapore and USA meteorological data shows that the "n" values are statistically similar to each other. However, the "q" values are statistically different from each other. Thus it is logically deduced that if wastewater treatment plants of the same capacity are located in Singapore and USA respectively, their plume will influence different area. The main impacting factors on the novel equation including meteorological conditions and source configuration have also been investigated in this study. It is found that the "n" value is sensitive to the stability category while the "q" value is sensitive to the wind speed and the source strength.