Sewage sludge: odour emissions assessment

1. Summary

Scope

One of the main utilisation routes for sewage sludge cake (or biosolids) is recycling to agricultural land as an organic fertiliser and a variety of regulations and codes of practice are in place to ensure it is undertaken safely. However, land applications can be problematic in terms of public perception if sludge recycled to agriculture generates 'excessive' odour emissions on farmland and particularly during and after land spreading.

This appraisal and report was commissioned by The James Hutton Institute to help assess the relative intensity of odour emissions which might be generated from three different types of sewage sludge cake during application to land and after spreading. Area specific odour emission rates expressed as European odour units per second per square metre of cake surface (ouE/s/m2) were measured from disturbed field- stored heaps of sludge cakes produced using the following three treatment systems:

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)

Findings

Effects of Sludge Cake Type - After taking account of potential measurement uncertainties and errors, 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 the 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.

Effects of Sludge Cake Age - The differences in odour emission rates between different sludge cake ages were not large in relation to the measurement uncertainties, although there was a trend for higher emissions from the most recently processed limed and conventional AD cake. Hydrogen sulphide emission rates for these two cakes were also higher from the most recently processed cake. This effect might be expected if there is a degree of additional biological or chemical stabilisation in cake which has been stockpiled for longer periods. On the other hand, NH3 emissions were slightly higher from the older limed and AD cakes than the more recently treated materials.

Implications for the biosolids industry

This work concerns a small number of different sludge cakes from a limited number of different sewage treatment works, but on the evidence of the measured emissions rates the following conclusions can be drawn:

1. There are much higher risks of adverse odour effects from the land applications of limed cakes than there are from the application of anaerobically digested cakes, either with or without preliminary thermal hydrolysis.

2. 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.