Effects of biogeochemical interactions between cementitious materials and sewage on the durability of wastewater treatment plant facilities

Wastewater treatment plants (WWTPs) are critical environmental solutions for sanitation management in many cities and municipalities. The construction of these facilities uses cementitious materials (e.g., concretes) due to their low cost, high strength and excellent watertightness properties. However, the long-term performance of these materials in WWTPs is affected by deterioration influenced by the formation of new (secondary) cement minerals. These secondary minerals are formed as a result of biogeochemical interactions between cementitious materials and wastewater microbial communities. The literature shows a lack of consensus on the mechanisms involved in the biogeochemical mechanism of sewage and cementitious materials in WWTP facilities. As a result, many civil and water engineers are unaware of its adverse effects on the sustainability of WWTP facilities, and as a consequence, the operation of many WWTP facilities costs billions of dollars in repair and maintenance due to concrete failure. This study studies the possible processes of biogeochemical interactions between sewage and cementitious materials in WWTPs and their subsequent mineral alteration and formation.

An in-situ experiment exposed 48 cement specimens of ordinary Portland cement and calcium sulfoaluminate cement to the sewage pumping station and sand-trap structures. The research involves: (1) geochemical analysis (SEM and XRD) to study the change of cement materials, (2) engineering analysis to study their mechanical change and (3) microbiological investigations to explore the microbial communities involved in the biogeochemical interaction.

The preliminary results of the study: (a) change of color from light grey to a mixture of yellow and brown for cement pastes exposed in the sewage pumping station, whereas the samples from the sand-trap maintained their original grey color. (b) the appearance of secondary minerals such as gypsum (CaSO₄.2H₂O), ettringite (Ca₆Al₂(OH)₁₂(SO₄)₃·26H₂O), and thaumasite (Ca₃Si(OH)₆ (CO₃) (SO₄)₁₂.H₂O) which are characterized as expansive process causing several cracks in the concrete structures. (c) the main mechanism for the formation of these sulfur-related minerals (i.e., gypsum, ettringite, and thaumasite) involves sulfide adsorption and its subsequent oxidation to form biogenic H₂SO₄ which eventually attack the cement alkaline mineral phases such as portlandite (Ca(OH)₂ and calcium silicate hydrate (C-S-H). Another biogeochemical mechanism for sewage-cement interaction observed in this work was the carbonation process, which resulted in the formation of calcite mineral in hydrated cement.