The research goal was to investigate the hydraulic conductivity of compacted lime-softening sludge as a material to be applied to landfill liners. In doing so, the effect of compaction and moulding moisture content on the sludge hydraulic conductivity was assessed. An approximate polynomial k_10meanat hydraulic gradients ≥30 for degree of compaction (0.95–1.05) and moulding moisture content (28%–36%) was determined. The results of short-term tap water permeation tests revealed that all hydraulic conductivity values were less than 2.5•10^–8 m/s. A lowest hydraulic conductivity of 6.5•10^–9 m/s, as well as a corresponding moisture content of 31% were then established. The long-term hydraulic conductivity was measured with tap water, distilled water, NaOH and HCl solutions and municipal waste leachate. The factors of permeating liquids and permeation time significantly affected the initial hydraulic conductivity. The long-term hydraulic conductivity increased for NaOH and HCl solutions and decreased for tap and distilled water. A significant reduction of hydraulic conductivity was observed for leachate permeation. The investigated material met the requirements for the liner systems of inert landfill sites regardless of pH and the limit value for hazardous and non-hazardous waste landfills.

The current practice of managing washed mineral waste from grit chambers under national legislation focuses primarily on its disposal, generating high costs for wastewater treatment plants. Other ways are being sought to enable its use, especially as a by-product in the construction industry. This paper presents the results of laboratory tests of the geotechnical, physical and mechanical parameters of washed mineral waste from grit chambers. Research samples were taken from the largest, in terms of maximum daily capacity, wastewater treatment plant “Czajka” in Poland. The washed mineral waste was characterized by organic matter content (0.36% by Tyurin’s method or 1.04% by the loss on ignition method), fraction content (sand fraction was at least 90%; it corresponds in grain size to uniform-grained medium sand), specific density of solids (2.55 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$), dry density, void ratio and porosity corresponding to the state of the loosest and densest possible composition of soil grains and particles (1.54 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$, 0.656, 0.396 and 1.87 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$, 0.364, 0.267, respectively), sand equivalent (93), passive capillarity (0.20 m), maximum dry density (1.78 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$), optimal moisture content (11.23%), degree of saturation after compaction (0.66) and permeability coefficient ($6.22·{10}^{-3}\mathrm{c}\mathrm{m}/\mathrm{s}$). The mechanical parameters determined included internal friction angle (35.5°) and apparent cohesion (14.27 kPa). The possibility of using washed mineral waste as soil for the backfill of installation trenches, abutments and retaining structures, as well as road embankment material, was evaluated considering current standards and legislation. It was found that the values of the determined parameters of washed mineral waste coincide with the values of the geotechnical parameters of sand, and there is a possibility of using this waste as a material in the indicated applications after fulfilling the appropriate conditions.

Przegląd możliwości wykorzystania odpadów piaszczystych odzyskiwanych w oczyszczalniach ścieków jako surowca alternatywnego w budownictwie Sand is a globally mined

The objective of this study was to investigate the swelling potential of compacted lime-softening sludge for application in landfill liners. The study involved the assessment of the effect of compaction and moulding moisture content (30–40%), corresponding to the Proctor standard compaction test. One-dimensional oedometer swell tests were conducted using distilled water, tap water, and municipal landfill leachate, resulting in the determination of the expansion indices. Moreover, changes in the moisture content and dry density during the swelling process were investigated. The expansion index was significantly influenced by the initial moisture content and liquid chemistry. Subsequently, these factors also affected the sludge dry density decrease, and its moisture content increase, whereas the impact of the initial dry density on expansion was of low importance. An increase in the sludge moulding moisture content, limited swelling in all liquids used. The highest expansion, dry density, and moisture content changes due to swelling were identified for leachate at w < w_opt. It should be underlined that the effect of liquid on the swelling potential faded away along with a further increase in the moisture content w > w_opt. The novelty of the work lies in identifying a significant plunge of the expansion index at w ≈ w_opt for the leachate swelling test. The lime-softening sludge non-swelling moisture content was defined as w_non ≈ (w_opt + 4.0%) − (w_opt + 4.5%). For practical engineering implications, the moisture content between (w_opt + 2.0%) and (w_opt + 4.0%) was provided for the most suitable sludge application in landfill liners.

Recycling washed mineral waste, generated as a byproduct of the mechanical wastewater treatment process, can be a beneficial alternative to widely used natural sand in construction. Studies on material from the Warsaw agglomeration, available in quantities sufficient for construction applications, demonstrated its high usability in specific hydrotechnical applications. Key laboratory tests for material characterization included physical, permeability, mechanical, and chemical property analyses. The tested waste corresponds to uniformly graded medium sands (uniformity coefficient: 2.20) and weakly calcareous (calcium carbonate content: 2.25–3.29%) mineral soils with organic content ranging from 0.24% to 1.49%. The minimum heavy metal immobilization level reached 91.45%. At maximum dry density of the soil skeleton (1.78/1.79 g/cm³) and optimal moisture content (11.34/11.95%), the hydraulic conductivity reached 4.38/7.71 m/d. The mechanical parameters of washed mineral waste included internal friction angle (34.4/37.8°) and apparent cohesion (9.37/14.98 kPa). The values of the determined parameters are comparable to those of natural sands used as construction aggregates. As a result, washed mineral waste has a high potential for use as an alternative material to natural sand in the analyzed hydrotechnical applications, particularly for flood embankment construction, by applicable technical standards and construction guidelines.