In the dairy processing, there are a number of volatile fatty acids released into the vapour space as milk products decompose. Some Sulphide and Ammonia are present but the key odour source is the volatile fatty acids present in the waste streams. There are key areas in the waste treatment process where odour is emitted. Generally, the screening system is an area of turbulence that allows odour to be released.
Holding Tanks incorporate mixing to maintain solids in suspension, there is a constant drain off and filling of the wastewater tanks. This creates enough turbulence to release odour. The filling process in batch systems promotes the emission of odours.
The DAF unit will release significant amount of odour as the air present in the system will allow volatile fatty acids to be stripped away. None of the odorous compounds will be oxidised by the presence of air as the air volume is insufficient.
The aeration process, if adequately arranged will operate in conditions where the average organic strength is very low and will have little residual volatile compounds present to be released. There will be some odour emitted from the aeration process and it needs to be appropriately considered. Only on very high strength waste will the aeration process require covers and extraction.
Sludge collected from the Dissolved Air Flotation (DAF) unit and from the secondary clarifier will generate significant odour when held in the sludge holding tanks. The odour will typically be from Ammonia and Hydrogen Sulphide with some mercaptans and Dimethyl Sulphide from the decomposition protein.
The sludge generated if the system is large are feed into Anaerobic Digestors. This is done generally to generate methane for energy management of the system and reduce the total solids sludge volume. The Methane will be in the form of Biogas (65% Methane, 34% Carbon Dioxide with traces of Ammonia and Hydrogen Sulphide). The biogas can be used either to operate boilers or once cleaned used for Combined Heat and Power systems (CHP). Inline Scrubbers are required to reduce the levels of Hydrogen Sulphide in the Biogas.
Biofilters are the most common approach to deal with the odorous air from dairy waste. The Biofilters are able to adapt to wide spectrum of volatile fatty acids. Where high quality emissions are required, then Activated Carbon is required to polish the treated air from the Biofilter. If the concentrations are high then BioTrickling Filters can be considered.