Geotechnics of municipal sludges and residues for landfilling

Abstract

This paper presents a comprehensive review of reported field and laboratory investigations and theoretical work concerning the geotechnical behaviour/properties of water treatment residue (WTR), biosolids and sewage sludge materials, which are by-products of municipal water and wastewater treatment processes. After describing the backgrounds to the generation of these materials and various disposal options (with a focus on landfilling), including associated geotechnical issues, their physical, geotechnical index, compaction, undrained strength and effective-stress strength properties are described, including the effects of oxidation, biodegradation and viscous gellike pore fluid for biosolids/sewage sludge material. Characteristic compressibility, consolidation and permeability behaviour/properties are also presented for these materials and linked with microstructure and the chemicals added during the treatment processes. The paper then focuses on various geotechnical issues pertinent to landfill (monofill) disposal of these materials, including undrained strength requirements, design strength, on-site and laboratory strength measurements and water content-strength correlations. Since such correlations are material specific, with the geotechnical properties of biosolids, sewage sludge and WTR materials varying between water/wastewater treatment plants, they cannot be applied more widely with confidence. Alternative approaches to predicting undrained strength are described, including a power-law strength relationship with water content, which can be applied more generally. , 2008a). The type and amount of chemicals added depends on the nature of the source water, but primarily on its turbidity and hardness. The coagulants cause the colloidal particles present to aggregate into flocs (with the polyelectrolyte also acting as a binding agent to increase their inherent shear strength) that are more readily separated out by settling processes, removing not only the impurities but also the chemical additives. NotationThe residue so formed is categorised as aluminium sulfate (alum) or iron WTR material depending on the coagulant salt, with the former being the most widely used coagulant in the water treatment industry. Lime-softening sludge, composed of typically 80-95 wt% calcite (Raghu and Hsieh, 1986), is the result of softening hard water before its distribution as drinking water.Biosolids and sewage sludge materials are highly organic residue by-products of municipal waste water treatment, with the organic fraction originating principally from human faeces (primary sludge) and bacterial biomass (secondary sludge) and the inorganic fraction derived from materials such as soil, sediment and inorganic residuals (Haynes et al., 2009). According to the US Environmental Protection Agency (EPA) (2015), sewage sludge refers to the solids separated out during the treatment processes, whereas biosolids refers to treated sewage sludge material that meets regulatory pollutant-and pathogen-control requirements for land applica...

Measurement, interpretation and recommended use of laboratory strength properties of fibrous peat

“…In other words, a tenfold decrease in strain rate produced a 28% decrease in the mobilised s u TC value. High f 0 TC and strain rate dependency values (m = 0·20-0·39 for N = 57-70%, where N is the ignition loss) have also been deduced from CU TC testing of municipal sludge and residue materials (O'Kelly, 2016(O'Kelly, , 2017) -for example, the alum-water-treatment residue material investigated had f 0 CIUC ¼ 39°.…”

Use of fall cones to determine Atterberg limits: a review

“…Vardanega & Haigh (2014b) demonstrated from analysis of a large database of British standard (30°-80 g) fall-cone test results that, for any given soil, acceptable linear correlations could be drawn between both the logarithm of undrained strength and liquidity index and the logarithm of undrained strength and the logarithmic liquidity index. While the ratios of strengths at the PLs and LLs varied between soils, defining any two (or more) points on these linear relationships would give good predictions of undrained strengths at intermediate water contents (O'Kelly, 2013(O'Kelly, , 2016b). The measurement of PL 100 together with the LL FC would achieve this.…”

“…μ = 0·1, where μ is the rate dependence parameter). It is, however, not uncommon for the rate of strength increase to range between 5 and 15% (Ladd & Foott, 1974), with values of up to 30% measured for soils with high organic content (O'Kelly, 2014(O'Kelly, , 2016b. Hence, for soil having a greater rate dependence of strength, the average undrained strength value mobilised over the course of the cone penetration into the soil test specimen would be lower than that deduced from analysis of the fall-cone data using equation (1), and vice versa.…”

Geotechnical laboratory testing and data interpretation for biosolids and sewage sludge

“…Their organic fractions principally originate from human faeces (primary sludge) and bacterial biomass (secondary sludge), with their inorganic fractions derived from materials such as soil, sediment and inorganic residuals (O'Kelly, 2016a). Sewage sludge refers to the slurry material separated out during the primary and secondary treatment processes.…”

“…The dewatered and (or) solar-dried materials are disposed of by various means, including: their beneficial uses as agricultural fertilisers, in forestry, for composting and soil redevelopment and in mine reclamation; their indefinite storage in single-purpose disposal pits or lagoons; landfilling at dedicated deposition areas or co-disposal with municipal solid waste at sanitary landfills or dedicated monofills; and incineration (O'Kelly, 2016a). In limited circumstances, biosolids and sewage sludge amended with additives such as lime, fly ash, cement or slag can be used as construction materials -for example as landfill cover or as structural fill material in road embankments , with special protection required in the event that there is potential leaching (flow) to adjoining water bodies.…”