STP/ETP Odour Control

Odour Control in STP

Most of the new sewage treatment plants (STPs) are built in Building Basement area. The whole facility will be enclosed in a single building. The STP system is a combination of:

• Pre-treatment,
• primary treatment,
• secondary (biological) treatment and
• tertiary treatment.

Sewage Treatment Plants produce Odourous compounds during the treatment process. These compounds are generated by biological processes at different steps within in the plant. Typical areas with odour emissions are the inlet pump station with bar screen chamber, oil & grease trap, biological process, sludge handling, sludge drainage.

There is a wide spectrum of possible inorganic and organic molecules, which can create unpleasant Odours. The most common are hydrogen suphide, ammonia, sulphur compounds, mercaptans, amines, aldehydes, ketones etc. Especially for treatment plants located in or nearby residence/commercial buildings these emissions lead to problems with residents. Therefore regulations are in place to reduce the emissions and set limits for selected parameters/compounds.

Hydrogen Sulphide and Ammonia are two main parameters restricted by regulations.

The concentration of H2S found in the exhaust air of Sewage Treatment Plant is in the range of 5 – 15 ppm. Threshold limit of H2S = 0.002 – 0.15 ppm

The emission level for ammonia is typical in the range of 50 to 150 ppm. Threshold Limit for  NH3 is = 5 ppm

Due to existing limits it is typically required to gather the polluted air via exhaust equipment and to install a treatment system. Normally Reduction of Odourous gas requires the dilution with outside air. For this large capacity of fresh air supply duct is required which will dilute the polluted air. Generally 5 – 7 fresh air changes are required for STP Exhaust.

The integration of Air Ozonation System and injection of Ozone into Air Handling System is the latest and most popular technology to reduce H2S and NH3 from enclosed STP Exhaust.

Role of Ozone:

Ozone is a powerful oxidant which rapidly oxidizes Odourous gases such as Hydrogen sulphide and ammonia.

Ozone Reactivity with Hydrogen Sulphide and Ammonia:
H2S + O3 → SO2 + H2O
2 NH3 + 4 O3 → NH4NO3 + 4 O2 + H2O.

Advantages:

• Ozone oxidizes Odourous gases instantly.
• Ozone can kill bacteria, viruses, fungi etc. in air.
• Ozone is a very powerful oxidizing agent,
• Ozone generators (Ozonators) are very easy to install, compact, and can run 24×7 effciently
• Ozone gas can be generated from ambient air oxygen on site itself.
• Easy to control ozone concentration by sensors.
• Adjustable ozone sensor (0.02 – 0.1 ppm) (Optional)
• Saving Intake of Fresh Air from outside for dilution is greatly reduced (By about 80%). Due to Ozone saving of energy which in cooling outside Air and Dramatic saving in cost.

Salient Features:

• Ozone generator system is suitable for achieving reduction in volatile organic compounds, organic Odours in indoor environment.
• Ozonator does not produce any hazardous by‐products.
• Feed gas to ozone generator can be medical grade dry pure oxygen having 80- 85% purity or can be treated fresh air with low humidity, due point and oil free.
• MOC of Ozone Cell is Stainless/Aluminum with high temperature dissipation design.
• Dielectric Tubes is fabricated of ceramic to provide high efficiency dielectric.
• Ozone Electrode (Cell) are water cooled or air cooled.
• The Ozone Generator is suitable for operation on 230V‐ 50Hz Single phase input power.

Deliverables of Ozonation System:

• Ozone generator is placed in STP Plant room & connected to Exhaust duct line through ozone injector.  Ozone will be injected at entry point of exhaust air duct.
• Ozone will be dosed between 1.0 – 2.5 ppm (depending on site conditions).
• The ozone generator system is designed for continuous operation with optional auto ON/OFF mode

 Important Sources of Odour Pollution and Odourous Compounds Emitted

Digester,
Black Liquor Storage Tank,
Evaporator,
Recovery Boiler,
Smelt Dissolving Tank,
Lime kiln

CH3 SH,
CH3SH,
CH3’2S,
H2S,
CH3’2S2,
CH3’2SCH3SH,
CH3’2SH2S,
CH3SH,
CH3SH,
SO2

Raw hides and skins storage / waste fleshing,
Beam house operation,
Finishing operation,
ETP: Collection Tanks,
ETP: Primary Treatment Units,
ETP: Sludge Dewatering System,
ETP: Anaerobic Lagoons.