4. Odour analysis
The relative intensity of odour emissions from three different types of sewage sludge cake during application to land and after spreading were assessed using standard techniques:
1. Lime treated dewatered cake (LIMED)
2. Anaerobically digested and dewatered sludge cake (AD)
3. Anaerobically digested and dewatered sludge cake following pre-treatment by a thermal
hydrolysis process (THP)
Odour emissions from the LIMED cake were in excess of an order of magnitude higher than from the AD and THP treated cakes. These differences were also reflected in indicative H2S emission rate measurements. The implications are that there are significantly higher risks of adverse odour emissions from land applications of LIMED cake than from applications of conventional AD or THP digested cakes.
The substantially higher odour emission rates from lime treated cake demonstrate that much more rigorous odour mitigation measures must be used than for digested cake (with or without preliminary THP) if land spreading odour impacts are to be controlled or mitigated. Examples of such additional controls could include:
a) Selecting application sites which are remote from residential settlements and housing,
b) Restricting applications to small areas of land at any one time,
c) Not applying limed cake to grassland or other areas which preclude ploughing-in or cultivation other than in very remote locations.
d) Ploughing- in or incorporating more or less immediately after land spreading to minimise the surface areas of material exposed between spreading and incorporation.
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