Use of sewage/fecal sludge grit as a partial replacement in production of concrete pavers

Abstract

This research project aimed at examining the feasibility of using grit as a partial replacement for sand in the manufacture of concrete pavers. Grit is a byproduct of the initial pretreatment process of faecal sludge and water-water i.e., screening. The essence of the grit removal process is to reduce accumulation of the fines downstream that could potentially cause blockage and eventual breakdown of equipment. In this study, the grit used was sampled from the different sources at Lubigi WWTP that is; grit from waste water transported through municipal sewers, grit from fecal sludge deposited by vacuum trucks and grit from manual fecal sludge collection systems. The chemical and physical properties of the grit in its raw form were investigated which included the quantity of grit loads deposited at the plant, its pH, moisture content, E-coli concentration, Particle size distribution, Particle density and organic content. From the results of the above investigation, grit from each of the sources had high organic content and fecal coliform contamination hence it required treatment and washing. Thermal treatment was applied on the raw grit causing a 90% reduction in E-coli contamination and upon washing and sieving, 80% reduction in organic content was realized. After the treatment and washing procedure, the chemical and physical properties of the treated grit were compared to river sand that indicated similar range of results for particle size distribution and particle density (1.9 g/cm3 – 2.4 g/cm3 ). A design mix of M40 concrete was used to prepare the samples of concrete pavers with the control having no grit added to it. Experimental mixes were then prepared with 10%, 20%, 30%, 40% and 50% replacement of river sand with grit. The mechanical and hydrologic properties of the pavers were tested and the results showed reduction in both compressive strength and split tensile strength of the pavers with increase of grit in the mix. An optimum percentage replacement of 30% of river sand with grit in the mix was recommended since it provided compressive strength of 15MPa and 0.8MPa split tensile strength that are sufficient for paving units used for light traffic and compound design. It was further observed that the water absorption of the concrete pavers increased with the percentage replacement of sand with grit in the mix designs.