Abstract
M.Dip.Tech.(Environmental Health)
At the onset of this project the aim was to utilise fly ash, supplemented with lime, to stabilise
sewage sludge, in so doing create an artificial soil. This would be an environmentally friendly
method of disposing of both the fly ash 'and the sewage sludge which are both considered
wastes. The sewage sludge was pasteurised by the temperature produced by the exothermic
reaction between the moisture ofthe sewage sludge and the high pH ofthe fly ash and the lime.
Initially very high concentrations of lime were utilised to achieve the pasteurisation of the
sewage sludge. These levels proved to be adequate to completely sterilise the sludge, rather
than pasteurise it, as temperatures in excess of 1000e were obtained and maintained for a
prolonged period. The product, SLASH, should ideally maintain some of the non-pathogenic
microorganisms to facilitate natural degradation processes in the soil. .It was thus decided to
minimise the lime addition and maximise the ash usage without losing the necessary soil-like
texture of the SLASH product and still insuring pasteurisation. Various 10kg tests were
conducted on differing ratios of sewage sludge, ash and lime. The mixtures were contained in
insulated 220 litre drums in order to ensure a minimum of interference from environmental
factors. The tests were conducted with the drums sealed and unsealed, as well as with coarse
and finely milled lime-stone. These laboratory scale tests led to the establishment of a
preferred 6:3:1 ratio ofsewage sludge to ash to lime.
After an international visit to a plant manufacturing a similar product, it was suggested that the
ratio be amended to 6.3:3.4:0.6 of sewage sludge to ash to lime. Tests showed that although
the temperatures obtained when using this ratio were insufficient for pasteurisation,
pasteurisation did occur. This was attributed to the maintenance of the pH of the product
above 12 for more than 24 hours. This severely stressed the microorgansims and they died.
No pathogenic bacteria were detected in the analyses from these tests, but the total aerobic
bacterial count indicated the presence of some microorganisms which would become part of the
soils natural flora.