Characterization of dairy industry sludge for sustainable eco-friendly construction concrete

Dairy wastewater treatment sludge is commonly disposed of despite its potential for material recovery in construction applications. This study characterized dairy wastewater treatment plant (WWTP) sludge from East Java, Indonesia, to evaluate its preliminary suitability as a supplementary raw material for concrete bricks. X-ray fluorescence (XRF), CHNSO elemental analysis, and Atomic Absorption Spectroscopy (AAS) were used to determine oxide composition, organic elemental characteristics, and selected heavy metal concentrations. XRF analysis showed that the sludge was dominated by Fe₂O₃, Al₂O₃, and SiO₂, with a combined content of 73.9%, indicating chemical relevance for cement based systems. However, this oxide composition was interpreted only as an initial screening indicator and not as direct evidence of pozzolanic or cementitious performance. CHNSO analysis revealed a considerable organic fraction, particularly carbon at 48.72 wt%, confirming that the sludge cannot be treated as a purely mineral additive. Supporting CHNSO results after CaO addition (10–20%) showed decreasing carbon and nitrogen signals, suggesting that alkaline calcium bearing conditions may reduce the effective influence of the organic fraction. Therefore, pretreatment through controlled thermal treatment or chemical/alkaline conditioning should be considered before sludge incorporation. Based on the present compositional findings, future mix design studies should begin with low replacement levels, for example up to approximately 10 wt% sludge, as a conservative initial range rather than a confirmed optimum. AAS results showed that Pb, Cd, and Zn were below selected regulatory reference limits, whereas total Cr requires further evaluation because it cannot be directly interpreted as Cr⁶⁺. Overall, dairy WWTP sludge shows potential for further investigation as a supplementary material for non structural concrete bricks, provided that mix optimization, mechanical performance, chromium speciation, TCLP based leaching, and product level environmental safety assessments are conducted.