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

Sewage sludge ash (SSA) contains the critical raw material phosphorous. International research shows that SSA may be utilized as a secondary resource in concrete, bricks or lightweight aggregates, but the high phosphorous content calls on recovery before such use. In the present investigation, phosphorous was recovered from SSA by electrodialytic separation (EDS), and the treated ash (SSA-EDS) was tested as partial clay substitute in brick manufacturing.

The experimental work was made with fired brick discs of 2 cm in diameter. For comparison, similar tests were conducted with SSA-EDS and untreated SSA. During EDS, 84–91% phosphorous was recovered. The pH of the SSA was reduced from 10.2 to 3.7 during EDS, while the content of iron oxides increased.

The brickwork clay contained about 17% carbonates, a major cause for firing mass loss. The reference brick discs were yellow. As the carbonate content decreased with increasing clay substitution (0–50% per weight), the firing mass loss decreased accordingly. The SSA was reddish due to a high iron oxide content, and the colour of the brick discs turned increasingly reddish with an increasing percentage of SSA and especially SSA-EDS.

Thereby it may act as a colouring agent, to obtain desired red bricks, for which natural clays are becoming scarce. When fired at 1000 °C, all brick discs had apparent densities and open porosities within the accepted 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 formation of the glassy phase resulted in a decrease in open porosity with increasing substitution with SSA-EDS for the discs fired at 1050 °C, which was the reversed order compared to firing at 1000 °C.