The sludge was characterized according to ABNT NBR /, following the procedures for obtaining leached extract (ABNT NBR /), to classify . Brazilian Association of Technical Norms, in ABNT NBR Procedure for obtention leaching extract of solid wastes (Rio de Janeiro, ), p. 16 8. Brazilian Association of Technical Regulation. Procedures for obtainment of leaching extract from solid residues. Rio de Janeiro: ABNT (NBR ).
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Water and sewage treatment plants STP and WTP generate as byproduct a significant amount of sludge with environment harmful elements. Sending to landfills or depositing on the ground or rivers are respectively expensive and dangerous alternatives. In this scenario, the use of this waste in paving processes is a promising alternative for disposal thereof. In this study, we focused on characterizing sludge and evaluating its use in paving, which showed satisfactory results for use in base and sub-base floors.
In Brazil, the generation of solid waste reaches significant amounts, since there are few solutions for final disposal. The water and sewage treatment plants are nbe responsible for environmental damages due to the amount and toxicity of the waste produced in sludge form.
Unused solid materials form such residues during wastewater treatment process.
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The sanitation in this country is still precarious. Hence, in the coming years, the amount of sludge produced every day tends to increase due to the implementation of new service stations. There are no accurate data on total domestic production of sludge, but it is estimated that it exceeds three hundred thousand tons per year for each type of sludge.
For a water or sewage treatment system be regarded as effective, it is required that final disposal of sludge is performed properly, since it has environment harmful elements in its composition, such as heavy metals, pathogenic microorganisms and organic pollutants.
In case of improper disposal, these materials may contaminate soil and groundwater, bringing damages to the environment and population. Among the main waste destinations in use nowadays, we may cite storage in large tanks method that requires large areasdisposal into landfills process considered uneconomic and launching into waterways and the ground environmentally incorrect. Because of the large volume of sludge produced, residue destination planning should consist 100005 a program to guide final disposal taking into account technical, environmental and economic points of view.
Thus, alternatives for disposal and application of sludge as agricultural inputs should be investigated to mitigate these issues. The alternatives that have been used worldwide to discard sludge from sewage treatment are burning, landfill, land farming, heavy clay ceramic industry and agricultural recycling. These solutions have as limiting factors groundwater and soil contamination organic fertilizer and land farmingcosts landfill and incineration and nnr of mechanical strength clay pottery and fine aggregate.
In this scenario of great immediacy, new studies on sludge management are being developed; among them, there is its use as a power source, which despite being promising has not demonstrated viability yet. Regarding the waste treatment and disposal, FONSECA jbr out that it is essential to reuse them instead of simply storing it temporarily due to economic, environmental and area availability causes.
The most acceptable technologies are those that provide reincorporation of wastes into nature, with no harmful properties to the local flora and fauna.
In a WTP, the disposal of sludge consists in its application in heavy clay ceramic industry or as backfilling ditches. Even though promising, there are no studies indicating any economic or environmental viability of these uses, since these byproducts have resistance loss compared to those already marketed.
Adding industrial residues to the soil to make them less toxic is called stabilization of granules. It is regarded as a pre-treatment, in which hazardous constituents of wastes are processed and kept in less soluble forms, being confined in capsules, particles or blocks.
According to GONDIMstabilization promotes higher soil resistance and bulk density aside from lower permeability.
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The paving, for its extension and large volumes of mobilized soil, constitutes in an alternative of waste reuse, particularly those with certain potential for stabilization. Industrial waste or byproducts available near the areas where they are produced generally result in cost-effective solutions. Collection was performed in Februaryis collecting around 90 kg sludge that was packed in plastic drums of 30 kg.
This WTP has a nominal capacity of 2, L s -1 and flow produced of 1, L s -1with the following treatment units: In this station, the largest sludge production occurs between May and June, which is a rainy period in Recife’s Metropolitan Area; therefore, surface materials are massively carried to water bodies.
Due to this fact, water turbidity and color changes, resulting in increasing levels of aluminum sulphate and sludge generation.
Sampling occurred in Februarycollecting about 90 kg sludge and packaging in 30 kg plastic barrels. This plant has a nominal capacity of about 80,m 3 sewage per day.
A primary treatment is carried out on domestic sewage to remove settleable solids by decantation and part of suspended organic matter. Subsequently, sludge is treated under anaerobic digesters, followed by dehydration through sun exposure in drying beds for 15 days. Sludge production and composition vary with season and daytime. Rainy seasons tend to produce higher amounts due to water infiltration, as well as STPs near industrial parks, where treated effluent are deposited.
Moreover, earlier in the day abjt night, productions are higher compared to other times. Surface horizon consists of granular soil – sandy bould gravel – with a depth of around two meters. We chose a granular soil A given the nature of the organic residues; thus, avoiding associating certain residue behaviors with clay particles’ ones. Testing of physical and chemical characterization of materials were made and mechanical testing of soil and residue mixtures, to evaluate the potential use of these residues in pavement bases and sub-bases.
Compression tests were performed through tests of California bearing ratio CBR and Resilience Modulus RM to assess potential use of the residue in base and sub-base paving. Soil stabilization was checked by mixing sludge and soil classified as A silty sand by the TRB soil classification Nhr Research Board. Through these curves, we could see that texture of WTP sludge and soil are similar, indicating great physical compatibility when 10050 to each other whether compared to mixtures with STP sludge.
Table 1 shows the results of soil and sludge residue consistency indexes through liquidity limit LLplasticity limit PLhumidity hdensity, void ratio e and porosity n. In compliance with the TRB rating, an A soil type is regarded as good to excellent for paving uses. The aluminum sulphate added over treatment has waterproofed soil completely, reducing plasticity, what could also explain non-plasticity of WTP residues.
By adding water to the residue, evaporation bbr repelling of water particles occur. By analyzing these values, we can infer that WTP residues can be classified as alumina silicate of high silica content and significant amounts of Al 2 100005 3 and Fe 2 O 3with a red loss of Thus, we can assert that the amount of aluminum-sulfate based coagulant added in the water treatment we assessed was higher than in other WTPs that have already surveyed in this country.
The sample showed high silica content and significant amounts of Al 2 O 3 and Fe 2 O 3with red loss of bnr The presence of other compounds can be explained by the presence of impurities because of the residue nature. In Avnt 2 are shown respectively the X-ray diffraction spectra of water and sewage abny residues.
Graphs showed a few well-defined peaks owning to the large amount of organic matter within the residues. The presence of Al and Si comes from using aluminum sulphate coagulants during water treatment. Moreover, the composition presented suspended materials such as sand and clayey material. The other reflections indicating the presence of anorthite [Ca Bbr 2 Si 2 O 8 ] and illite. Tables 3 and 4 display the chemical constituents found in leachates and extract solutions analyzed by atomic absorption spectroscopy.
According to the results, both of them were classified as non-hazardous wastes at “Class II A” non-inert. Hence, they are classified as hazardous by such organizations.
The Figures 3 and 4 show the results of abng thermal analysis and thermogravimetry for the sludge from STP and WTP, respectively. Compaction tests were performed at intermediate compression pressure for soil — WTP sludge mix and under interchanged pressures for soil —STP sludge mix. Figures 5a and 5b shows the results obtained in carrying out the compaction assay, as in Figure 6 are the results of the CBR testing.
This decrease was expected due to the nature of the samples, which nbf organic matter in their composition. It is noteworthy the similarity of the curves, a most likely explanation for this would be that both residues have similar reactions with soil, based on their similar rate of SiO2. As the soil under study is an A type, its behavior is related to particle size and compactness.
We can hypothesize that both residues fill the voids similarly and even with same chemical compositions, the show resembling chemical and mineralogical interactions. It is noted that reference 10005 are part of the service specifications for being used in roads with hbr traffic, abtn. Therefore, to use as pavement base, all values are lower than those recommended for heavy traffic roads are. This behavior ngr that most elevated concentrations of aluminum sulphate can be harmful to stabilization.
The mixture soil and STP residue could jbr used in light traffic road bases. Samples were deformed abnnt that the LVDT’s were not able to perform displacement measurements, interrupting testing continuity. This fact was attributed to the low resistance of mixtures of soil and residue and pure soil, causing specimen collapse before the test begins. In order to select the RM model, classical model applicability was evaluated.
The model must faithfully describe the resilient behavior of the material, assessing its deformation, accurately. Analyzing the parameters of the different models, we found that the correlation coefficients for nvr models according to the confining stress and stress invariant were not representative for soil mixed with WTP sludge, since there is no single behavior pattern.
Regarding the mixture with STP sludge, it was the best described by the model as a function of confining stress. Since the previous models did not represent precisely resilient behavior of the studied samples, we evaluated the use of a model compound to the mixtures, represented by equation 4.
The composite model relates the RM values as a function of confinement and deviation stresses, being represented by three-dimensional graph. The parameters of the compound models referring to the mixtures are shown in Table 6.
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Another way to analyze the compound model is through surface graphics of RM values for a better visual assessment of variations in resilient parameters and mixtures thereof. The surfaces set by equations of the compound model for mixtures of soil with both sludge residues are shown in Figure 7. A written python program with the aid of a matplotlib math library plotted graph.
Figure 7 plots that the RM of both mixtures are susceptible to large influence of confining stress and little deviation one except for lower deviation stress values. The dispersion of the results may have been affected by the granular nature of the soil, which requires further care with samples during the process to avoid damages, mainly with regard to laying of the specimens into the triaxial chamber.
The sludge residue derived from WTPs has a great amount of quartz, kaolinite and little of hematite. These minerals are typical of material that has undergone treatment in WTPs. Chemically, the residue showed larger amounts than those found in the literature of both aluminum and iron oxides, indicating that the use of coagulants exceeded the values of other WTPs, due to impurities present in these waters.
The atomic absorption spectroscopy corroborated results found from chemical analysis since highlighted certain elements such as manganese, iron and aluminum in quantities above those permitted by the NBR No. Even though the leachate analysis has classified the residue as non-hazardous for being non-inert, care is essential with its storage and disposal, to avoid contamination of water resources and even the soil with heavy metals.
The STP sludge residue has high organic matter content, which was confirmed by chemical analysis that found different oxides within the sample. Spectroscopy indicated the presence of heavy metals in amounts greater than the maximum limit set by standards. This result occurs because many industries discharge effluents into public sewer irregularly and unmanageably, increasing the concentration of these elements.
American Association of Highways and Transportation Officials. Rio de Janeiro, Rio de Janeiro, a. Rio de Janeiro, b. Standard methods for the examination of water and wastewater.
American Public Health Association,p.