Sewage sludge ash as resource for phosphorous and material for clay brick manufacturing

Sewage sludge ash (SSA) contains the critical raw material phosphorous. Internationally, research is carried out on utilization of SSA as secondary resource in concrete, bricks or lightweight aggregates; however, the high phosphorous content calls on recovery prior to such use. In the present investigation, phosphorous was recovered from SSA by electrodialytic separation (EDS) and the treated ash (SSA-EDS) was tested as clay substitute in brick manufacturing. For comparison, similar tests were conducted with SSA. The SSA had a reddish colour from a high iron oxide content. The SSA was milled to d50 = 37 μm prior to use as clay substitute (0-50%). During EDS, 85-91% phosphorous was recovered, the alkaline pH of the SSA (10.2) was reduced to acidic (3.7), and the content of iron oxides increased. The reference brick material was yellow. The colour turned increasingly reddish with increasing percentage of SSA and especially SSA-EDS, and the reddish colour is seen as an advance. The clay contained about 17% carbonates, a major cause for firing mass shrinkage. As the carbonate content decreased with increasing clay substitution, the shrinkage mass did decrease accordingly. When fired at 1000 °C, all brick materials had apparent densities and open porosities within the range of bricks used in the building industry. SEM micrographs showed that mixes with SSA-EDS fired at 1050 °C contained a glassy phase with isolated spherical pores, which may be related to the high hematite content of SSA-EDS. The open porosity decreased with increasing substitution with SSA-EDS when fired at 1050 °C, the reversed order compared to firing at 1000 °C.