Sewage smells naturally and under septic condition generates obnoxious hydrogen sulphide gas characterized by its rotten egg smell.  In Hong Kong, seawater, with high natural concentration of sulphate, is generally used for toilet flushing.  The high sulphate content in sewage is conducive to the emission of hydrogen sulphide.


Measures to control odour in sewage treatment works

Three types of mitigation measures in reducing odour nuisance from the sewage treatment works are commonly implemented.  These include -
  • dosing of chemicals, like calcium nitrate, ferric chloride, sodium chlorite and other deodourising agents, and injection of oxygen into sewage / sludge to control the generation of odour;
  • covering up of channels, chambers and tanks which are likely to emit odour; and
  • installing deodourisation units like activated carbon system, chemical scrubbers and biofilters at appropriate locations to clean up the collected foul gases from odour sources in the plants.
Depending on the type of treatment operation in a sewage treatment works, and the characteristics of its surrounding and the incoming sewage flow, one or a set of combinations of the above three types of measures in the sewage treatment works is adopted with a view to meeting the odour standard.
To ensure that all odour control systems are in proper working condition, their performance is closely monitoring and proper maintained, like timely replacement of odour absorption media such as activated carbon in the deodourisers is ensured.  The provision of odour mitigation measures together with good operational practices have been proven to be very effective in controlling odour nuisance from sewage treatment facilities.
However, sewage treatment works occasionally experience shock odour load arising from the fluctuating composition of incoming sewage.  This may result in short term strong odour emission.  The source of such shock load is very difficult to trace.  If the sources can be identified, appropriate actions to avoid reoccurrence will be taken.

Odour monitoring

Odour perception is subjective and the measurement in terms of odour unit is a complicated process involving testing by a group of selected and trained people.  As hydrogen sulphide gas has a strong correlation with the odour in sewage and can be measured conveniently and accurately by means of analyzers, it is taken by many countries as a substitute indicator.  DSD also adopts hydrogen sulphide measurement for odour monitoring in sewage treatment works, in additional to environmental monitoring & audit measurements required under the Environmental Impact Assessment Ordinance.
Hydrogen sulphide levels is regularly measured in sewage treatment works to monitor the performance of the odour control measures.  Operational adjustment, like adding more chemical or increasing the air changing rate of the deodourisers, would be done in the sewage treatment works to tackle any variations in operating environment.  In case there are or will be major variations which could not be handled by operational adjustments alone, upgrading works will be considered.

Way forward

We put strong emphasis on mitigating odour nuisance in delivering quality sewage treatment services to the community.  The technologies and operational practices adopted are proven and effective to meet the statutory requirement.  The management system also allows us to identify public concern or operational abnormality as far as odour nuisance is concerned, so that prompt operational adjustment or enhancement works could be put in place.
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