Mechanical properties of dewatered sewage sludge

The mechanical properties of dewatered, anaerobically digested sewage sludge were determined from soil laboratory tests. The sludge material is largely composed of organic clay sized-particles, a sizable fraction of which is in an active state of biological digestion which can continue over many years under field conditions. Moderately digested sludge material was found to have a typical specific gravity of solids value of 1.55, and loss on ignition (LOI) value of 70% dry mass. Strongly digested sludge, produced by digesting the liquid sludge further at 35 degrees C in the laboratory, was found to have a lower LOI value of 55% dry mass, and a higher specific gravity of solids value of about 1.72. The maximum dry density of 0.56 tonne/m3 for the dried sludge material was produced using standard Proctor compaction at roughly 85% moisture content (54% solids content). Air-dried, compacted sludge material was tested in quick-undrained triaxial compression and vane shear. Undrained shear strength-moisture content plots are presented. Shear strength values measured in triaxial compression and vane shear were consistent. The effective angle of shearing resistance (phi') was determined from consolidated-undrained, triaxial compression tests on pasteurized, normally consolidated samples of the sludge material. The mechanical properties of the sludge material changed with the level of sludge digestion. The phi' value increased from 32 degrees for moderately digested sludge, to 37 degrees for strongly digested sludge. The effective cohesion of the sludge material remained zero throughout. The shrinkage, swelling and adhesion properties of the sludge material were also studied. Significant shrinkage occurred as the compacted material dried. The sludge material lost its adhesion below about 95% moisture content (51% solids content). Re-hydration of the dry material caused the bulk volume to double.     

Use of dewatered municipal sludge on Canna growth in pot experiments with a barren clay soil

The present study evaluated the possibility of using the dewatered municipal sludge for non-agricultural purposes. The sludge was amended with soil and was applied at 0, 165, 330, 495 and 660 t/ha to promote the growth of Canna. The results showed that the Canna growth pattern exhibited a pronounced positive growth response in the range of 165–495 t/ha, and the Canna could not survive at an amendment rate of 660 t/ha. The analysis of chlorophyll fluorescence parameters showed that sludge did no harm to Canna, while under the conditions of barren soil alone, the plants were put into nutrients stress conditions. Due to the application of sludge, the concentration of heavy metals (Cu, Zn, Cr, Cd, Pb and Ni) in soil increased. However, by planting of Canna, contents of Cd, Ni and Zn showed trends of decline; Cd and Ni have shown a significant decline in concentration, while Zn had only limit response. As a result, dewatered sludge might be used to amend the barren soil and Canna could be used for phytoremediation of sludge.     

Characteristics of municipal solid waste and sewage sludge co-composting

The purpose of this work is to study the characteristics of the co-composting of municipal solid waste (MSW) and sewage sludge (SS). Four main influencing factors (aeration pattern, proportion of MSW and SS, aeration rate and mature compost (MC) recycling) were systematically investigated through changes of temperature, oxygen consumption rate, organic matters, moisture content, carbon, nitrogen, carbon-to-nitrogen ratio, nitrogen loss, sulphur and hydrogen. We found that a continuous aeration pattern during composting was superior to an intermittent aeration pattern, since the latter delayed the composting process. A 3:1 (v:v) mixture of MSW and SS was most beneficial to composting. It maintained the highest temperature for the longest duration and achieved the fastest organic matter degradation and highest N content in the final composting product. A 0.5L/minkgVS aeration rate best ensured rapid initiation and maintained moderate moisture content for microorganisms. After the mature MC was recycled to the fresh materials as a bulking agent, the structure and moisture of the initial materials were improved. A higher proportion of MC resulted in quicker decrease of the temperature, oxygen consumption rate and moisture. Therefore a 3:1:1 (v:v:v) proportion of MSW: SS: MC is recommended.     

Anaerobic co-digestion of swine and poultry manure with municipal sewage sludge

The anaerobic digestion of municipal sewage sludge (SS) with swine manure (SM) and poultry manure (PM) was undertaken. It was found that a mixture of sewage sludge with a 30% addition of swine manure gave around 400 dm³/kgVS of biogas, whereas the maximal biogas yield from ternary mixture (SS:SM:PM = 70:20:10 by weight) was only 336 dm³/kgVS. An inhibition of methanogenesis by free ammonia was observed in poultry manure experiments. The anaerobic digestion was inefficient in pathogen inactivation as the reduction in the number of E. coli an Enterobacteriaceae was only by one logarithmic unit. A substantial portion of pathogens was also released into the supernatant.     

Simulation of co-incineration of sewage sludge with municipal solid waste in a grate furnace incinerator

Incineration is one of the most important methods in the resource recovery disposal of sewage sludge. The combustion characteristics of sewage sludge and an increasing number of municipal solid waste (MSW) incineration plants provide the possibility of co-incineration of sludge with MSW. Computational fluid dynamics (CFD) analysis was used to verify the feasibility of co-incineration of sludge with MSW, and predict the effect of co-incineration. In this study, wet sludge and semi-dried sludge were separately blended with MSW as mixed fuels, which were at a co-incineration ratios of 5 wt.% (wet basis, the same below), 10 wt.%, 15 wt.%, 20 wt.% and 25 wt.%. The result indicates that co-incineration of 10 wt.% wet sludge with MSW can ensure the furnace temperature, the residence time and other vital items in allowable level, while 20 wt.% of semi-dried sludge can reach the same standards. With lower moisture content and higher low heating value (LHV), semi-dried sludge can be more appropriate in co-incineration with MSW in a grate furnace incinerator.     

Batchwise mesophilic anaerobic co-digestion of secondary sludge from pulp and paper industry and municipal sewage sludge

Residues from forest-industry wastewater-treatment systems are treated as waste at many pulp and paper mills. These organic substances have previously been shown to have potential for production of large quantities of biogas. There is concern, however, that the process would require expensive equipment because of the slow degradation of these substances. Pure non-fibrous sludge from forest industry showed lower specific methane production during mesophilic digestion for 19 days, 53 ± 26 Nml/g of volatile solids as compared to municipal sewage sludge, 84 ± 24 Nml/g of volatile solids. This paper explores the possibility of using anaerobic co-digestion with municipal sewage sludge to enhance the potential of methane production from secondary sludge from a pulp and paper mill. It was seen in a batch anaerobic-digestion operation of 19 days that the specific methane production remained largely the same for municipal sewage sludge when up to 50% of the volatile solids were replaced with forest-industry secondary sludge. It was also shown that the solid residue from anaerobic digestion of the forest-industry sludge should be of suitable quality to use for improving soil quality on lands that are not used for food production.     

Geotechnical properties of municipal solid waste at different phases of biodegradation

This paper presents the results of laboratory investigation conducted to determine the variation of geotechnical properties of synthetic municipal solid waste (MSW) at different phases of degradation. Synthetic MSW samples were prepared based on the composition of MSW generated in the United States and were degraded in bioreactors with leachate recirculation. Degradation of the synthetic MSW was quantified based on the gas composition and organic content, and the samples exhumed from the bioreactor cells at different phases of degradation were tested for the geotechnical properties. Hydraulic conductivity, compressibility and shear strength of initial and degraded synthetic MSW were all determined at constant initial moisture content of 50% on wet weight basis. Hydraulic conductivity of synthetic MSW was reduced by two orders of magnitude due to degradation. Compression ratio was reduced from 0.34 for initial fresh waste to 0.15 for the mostly degraded waste. Direct shear tests showed that the fresh and degraded synthetic MSW exhibited continuous strength gain with increase in horizontal deformation, with the cohesion increased from 1 kPa for fresh MSW to 16–40 kPa for degraded MSW and the friction angle decreased from 35° for fresh MSW to 28° for degraded MSW. During the triaxial tests under CU condition, the total strength parameters, cohesion and friction angle, were found to vary from 21 to 57 kPa and 1° to 9°, respectively, while the effective strength parameters, cohesion and friction angle varied from 18 to 56 kPa and from 1° to 11°, respectively. Similar to direct shear test results, as the waste degrades an increase in cohesion and slight decrease in friction angle was observed. Decreased friction angle and increased cohesion with increased degradation is believed to be due to the highly cohesive nature of the synthetic MSW. Variation of synthetic MSW properties from this study also suggests that significant changes in geotechnical properties of MSW can occur due to enhanced degradation induced by leachate recirculation.     

Effect of mixing ratio on sewage sludge and septage co-digestion

Journal of Material Cycles and Waste Management - Septage, which is made up of liquid and solid materials pumped from septic tanks, can be a potential co-digestion resource for sewage sludge. This...     

Waste paper and clinoptilolite as a bulking material with dewatered anaerobically stabilized primary sewage sludge (DASPSS) for compost production

Environmental problems associated with sewage sludge disposal have prompted strict legislative actions over the past few years. At the same time, the upgrading and expansion of wastewater treatment plants have greatly increased the volume of sludge generated. The major limitation of land application of sewage sludge compost is the potential for high heavy metal content in relation to the metal content of the original sludge. Composting of sewage sludge with natural zeolite (clinoptilolite) can enhance its quality and suitability for agricultural use. However, the dewatered anaerobically stabilized primary sewage sludge (DASPSS) contained a low concentration of humic substances (almost 2%), and the addition of the waste paper was necessary in order to produce a good soil conditioner with high concentrations of humics. The final results showed that the compost produced from DASPSS and 40-50% w/w of waste paper was a good soil fertilizer. Finally, in order to estimate the metal leachability of the final compost product, the generalized acid neutralization Capacity (GANC) procedure was used, and it was found that by increasing the leachate pH, the heavy metal concentration decreased. The application of the sequential chemical extraction indicated that metals were bound to the residual fraction characterized as a stabilize fractions.     

Moisture retention of municipal solid waste mixed with sewage sludge and ash in a semi-arid climate.

Mechanisms involved in moisture storage in refuse are explored using data from four sets of experiments in a semi-arid climate. Two laboratory series of experiments contained municipal solid waste (MSW) amended with sewage sludge, one with higher proportions of ash in the MSW than the other. Outdoor experiments contained waste streams with different proportions of ash. Field cells compared moisture retention of refuse and MSW co-disposed with sewage sludge. Sewage sludge at high loads was found to increase the moisture storage relative to unamended MSW. Belt-pressed sludge retained water as bound water that was released by decay and changing pH. Sun-dried sludge also retained more moisture than MSW alone. In gravimetric terms, ash reduced the storage potential of MSW, in laboratory and outdoor experiments. However, outdoor experiments released less leachate from ash-rich refuse than middle-income waste with no ash fraction.     

Comparative assessment of municipal sewage sludge incineration,gasification and pyrolysis for a sustainable sludge-to-energy management in Greece

For a sustainable municipal sewage sludge management, not only the available technology, but also other parameters, such as policy regulations and socio-economic issues should be taken in account. In this study, the current status of both European and Greek Legislation on waste management, with a special insight in municipal sewage sludge, is presented. A SWOT analysis was further developed for comparison of pyrolysis with incineration and gasification and results are presented. Pyrolysis seems to be the optimal thermochemical treatment option compared to incineration and gasification. Sewage sludge pyrolysis is favorable for energy savings, material recovery and high added materials production, providing a ‘zero waste’ solution. Finally, identification of challenges and barriers for sewage sludge pyrolysis deployment in Greece was investigated.     

Legislative and environmental issues on the use of ash from coal and municipal sewage sludge co-firing as construction material

For the economy of any co-firing process, it is important that the common waste management options of ash remain practical. Ash from bituminous coal combustion is typically handed to the construction industry. This paper describes the current European legislation on use of ash for construction purposes. Also, it presents an experimental study on the suitability of fly ash from combustion of mixtures of bituminous coal and municipal sewage sludge as additive to cement and concrete, and for use in open-air construction works, based on the ash chemical composition and the characteristics of the extract of the ash. Presently, two European standards forbid the use of ash from co-firing as additive to cement or concrete. This study shows that ash derived from coal and sewage sludge co-firing contains generally less unburned carbon, alkali, magnesium oxide, chlorine, and sulfate than coal ash. Only the concentration of free lime in mixed ash is higher than in coal, even though, at least up to 25% of the thermal input, still below the requirements of the standards. This ash also meets the requirements for the use of fly ash in open-air construction works--concentration and mobility of few elements--although this management option is forbidden to ash from co-firing. The leaching of Cd, Cr, Cu, Ni, Pb and Zn was investigated with three leaching tests. The concentration of these metals in the extracts was below the detection limit in most cases. The concentration of Cu and Zn in the extract from fly ash was found to increase with increasing share of sewage sludge in the fuel mixture. However, the concentration of these two metals in the extract is not regulated. This study indicates that excluding a priori the use of ash from co-firing as a suitable additive for construction material could cause an unnecessary burden on the environment, since probably ash would have to be disposed of in landfill. However, allowing this requires the modification of current European standards to include limitations on all elements and compounds, absent in coal but which might be present in other fuels, that are deleterious for the quality of construction materials.     

Influence of temperature on product distribution and biochar properties by municipal sludge pyrolysis

Biochar has the potential to amend degraded soil and improve crop yield. An experiment involving municipal sludge pyrolysis was carried out in a horizontal quartz reactor over the temperature range of 300–700 °C. The aim of this work was to investigate the influence of pyrolysis temperature on product distribution and biochar properties required for agronomic applications. Results of the experiment showed that yield and energy conversion efficiency of biochar decreased as pyrolysis temperatures rose, while bio-oil and syngas increased gradually. Biochar aromaticity barely changed, while polarity gradually decreased and specific surface area increased with a rise in pyrolysis temperature. Nutritive elements showed different enriching characteristics in the sludge pyrolysis process: nitrogen failed to enrich in biochars, whereas both phosphorus and potassium were enriched. Heavy metals showed good stability in the pyrolysis process except chromium; the contents of all metals used in the biochar conformed to Chinese control standards for agronomic application.     

Effects of municipal solid waste compost and sewage sludge on chemical and spectroscopic properties of humic acids from a sandy Haplic Podzol and a clay loam Calcic Vertisol in Portugal

The effects of amendment with municipal solid waste compost (MSWC) and anaerobically digested sewage sludge (SS) on the compositional and structural features of soil humic acids (HAs) were investigated. For this purpose, HAs were isolated from MSWC, SS, and two different Portuguese soils, a sandy Haplic Podzol and a clay loam Calcic Vertisol, which were either unamended or amended with MSWC or SS at a rate of 60tha(-1). The isolated HAs were analyzed for elemental and acidic functional group composition, and by ultraviolet/visible, Fourier transform infrared (FT IR), and fluorescence spectroscopies. The application of MSWC and especially SS to soils determined an increase of C, N, H, and S contents and E(4)/E(6) ratios (i.e., ratios of absorbances at 465 and 665nm), and a decrease of O, COOH, and phenolic OH contents and C/N, C/H, and O/C ratios of soil HAs. The FT IR and fluorescence results showed that the organic amendments, especially SS, caused an increase of the aliphatic character and a decrease of the degrees of aromatic polycondensation, polymerization, and humification of amended soil HAs. Both MSWC and SS affected more markedly the clayey soil HAs than the sandy soil HAs, possibly due to less extended mineralization processes and the protective action of clay minerals on amended soil HAs.     

Mechanical and physical properties of cement blended with sewage sludge ash

The aim of this paper is to evaluate the compatibility of sewage sludge ash (SSA) with various types of commercially available cements (CEM I and CEM II types, cements with several proportions of clinker). The behaviour of mortars fabricated with various percentages (10-30% by weight) of the cement replaced by SSA has been analyzed in terms of workability, mechanical strength, porosity and shrinkage/expansion. SSA exhibits moderate pozzolanic activity; the highest compressive strengths were obtained with 10% of the cement replaced by SSA. The CEM II/B-M (V-LL) 42.5R cement is considered ideal for preparing mortars containing SSA. Shrinkage data demonstrate that sulphates present in SSA are not reactive towards cement.     

Evaluation of an organic soil amendment generated from municipal solid waste seeded with activated sewage sludge

This study investigated the feasibility of using fresh activated sewage sludge as inoculum for the microbial valorization of segregated municipal solid waste and evaluated the quality of organic soil amendment generated. Organic fraction of municipal solid waste, which consisted of vegetative (vegetable, fruit and flower) wastes was seeded with activated sewage sludge and processed by rapid aerobic microbial treatment. Efficacy of microbial valorization process and quality of final product were assessed by physico-chemical analysis. Suitability of final product was assessed with regard to heavy metal content, pesticide residues, microbiological quality and phytotoxicity. Quality of the soil amendment generated was compared with the control product generated with a commercial microbial inoculum. Phytotoxicity experiments indicated the stimulatory effect of sewage sludge seeded soil amendment on plant growth but inhibition was observed in closed growth test due to the evolution of gaseous phytotoxic agents. The study suggests that segregated municipal solid waste can be effectively valorized with activated non-dewatered sewage sludge as inoculum and the quality of soil amendment generated was comparable to compost intended for unrestricted applications.     

Physical and chemical properties study of the activated carbon made from sewage sludge

Preparation of activated carbon from sewage sludge is a promising way to produce a useful adsorbent for pollutants removal as well as to dispose of sewage sludge. The objective of this study was to investigate the physical and chemical properties of the activated carbon made from sewage sludge so as to give a basic understanding of its structure. The activated carbon was prepared by activating anaerobically digested sewage sludge with 5 M ZnCl2 and thereafter pyrolyzing it at 500 degrees C for 2 h under nitrogen atmosphere. The properties investigated in the present study included its surface area and pore size distribution, its elemental composition and ash content, its surface chemistry structure and its surface physical morphology. Furthermore, its adsorption capacities for aqueous phenol and carbontetrachloride were examined. The results indicated that the activated carbon made from sewage sludge had remarkable micropore and mesopore surface areas and notable adsorption capacities for phenol and carbon-tetrachloride. In comparison with commercial activated carbons, it displayed distinctive physical and chemical properties.     

Co-digestion of grease trap sludge and sewage sludge

Redirection of organic waste, from landfilling or incineration, to biological treatment such as anaerobic digestion is of current interest in the Malmö-Copenhagen region. One type of waste that is expected to be suitable for anaerobic digestion is sludge from grease traps. Separate anaerobic digestion of this waste type and co-digestion with sewage sludge were evaluated. The methane potential was measured in batch laboratory tests, and the methane yield was determined in continuous pilot-scale digestion. Co-digestion of sludge from grease traps and sewage sludge was successfully performed both in laboratory batch and continuous pilot-scale digestion tests. The addition of grease trap sludge to sewage sludge digesters was seen to increase the methane yield of 9–27% when 10–30% of sludge from grease traps (on VS-basis) was added. It was also seen that the grease trap sludge increases the methane yield without increasing the sludge production. Single-substrate digestion of grease trap sludge gave high methane potentials in batch tests, but could not reach stable methane production in continuous digestion.     

Effect of compost treatment of sewage sludge on nitrogen behavior in two soils

This work aims to evaluate the effects of compost treatment of digested sewage sludge on nitrogen behavior in two soils, a Spodosol and an Oxisol soil. Digested sewage sludge was composted with sawdust and woodchips, diluting the total nitrogen to one-fourth (dry mass basis) of its original value. Then, sludge and compost were added to the two soils on an equivalent dry weight basis to consider the risk of NO3- -N leaching. Compost treatment of sewage sludge has slowed down the release of mineral-N to half in the Spodosol and to one-third in Oxisol soil. As a result, NO3- -N concentrations in soils incubated with compost were less than half of the amounts found from soils incubated with digested sludge. Estimates were made of the maximum monthly nitrate to leach from the four combinations of soil and sludge treatment. Application of digested sludge, at a higher nitrogen application rate, resulted in a higher nitrate leaching potential than application of the compost product. Soil type also played an important role, with the Oxisol having slightly higher estimated leaching potential than the Spodosol. The higher nitrate release rate in the Oxisol is counterbalanced by its higher field capacity to lessen the expected difference between the two soils.     

Effect of thermochemical pretreatment on sewage sludge and its impact on carboxylic acids production

This paper investigates the potential of converting sewage sludge into a useful product, namely carboxylic acids. To potentially enhance acid yields, the effect of pretreatment using 0.3 g lime/g dry biomass and water at 100 °C for 10–240 min was studied. The pretreated sludges were anaerobically fermented to mixed-acids using a mixed culture of microorganisms; methanogens were suppressed using iodoform. Batch fermentations were performed at 55 °C using ammonium bicarbonate buffer. The first batch experiments compared treated and untreated sludge as the only substrate. The second batch experiments used a mixture of sludge plus lime-treated bagasse (20:80 by weight). Analysis of liquor shows that the pretreatment were effective in solubilizing constituent compounds of sewage sludge. Nitrogen content and carboxylic acids increased with increasing pretreatment time. However, the soluble sugars peaked at 60 min, and then decreased with longer pretreatment time, showing that the solubilised sugars were undergoing intermolecular reactions, such as Maillard reactions. Fermentation experiments were a good indicator of the biodegradability of the pretreated sludges. Results clearly showed that lime-treating sludge, using even the minimum pretreatment time (10 min), negatively impacted acid production. The likely causes of this observation are attributed to the production of recalcitrant complexes and toxic compounds. Batch fermentation of untreated sludge yielded 0.34 g total acids/g VS fed, whereas sludge with 240-min lime pretreatment yielded only 0.20 g total acids/g VS fed. Co-fermentation of untreated sludge with pretreated bagasse gave a yield of 0.23 g total acids/g VS fed.