Assessment of biodegradability of cellulose and poly(butylene succinate)-based bioplastics under mesophilic and thermophilic anaerobic digestion with a view towards biorecycling.

Despite the increasing interest in bioplastics, there are still contradictory results on their actual biodegradability, which cause difficulties in choosing and developing appropriate sustainable treatment methods. Two biofoils (based on poly(butylene succinate) (PBS37) and cellulose (Cel37)) were anaerobically degraded during 100-day mesophilic (37 °C) and thermophilic (55 °C) tests (PBS55, Cel55). To overcome low degradation rates in mesophilic conditions, alkaline pre-treatment was also used (Pre-PBS37, Pre-Cel37). For comprehensive understanding of biodegradability, not only methane production (MP), but also the structure (topography, microscopic analysis), tensile properties, and FTIR spectra of the materials undergoing anaerobic degradation (AD) analysed. PBS37 and Pre-PBS37 were visible in 100-day degradation, and the cumulative MP reached 25.5 and 29.3 L/kg VS, respectively (4.3-4.9% of theoretical MP (TMP)). The biofoils started to show damage, losing their mechanical properties over 35 days. In contrast, PBS55 was visible for 14 days (cracks and fissures appeared), cumulative MP was 180.2 L/kg VS (30.2% of the TMP). Pieces of Cel were visible only during 2 days of degradation, and the MP was 311.4-315.0 L/kg VS (77.3-78.2% of the TMP) at 37 °C and 319.5 L/kg VS (79.3% of the TMP) at 55 °C. The FTIR spectra of Cel and PBS did not show shifts and formation of peaks. These findings showed differences in terms of the actual biodegradability of the bioplastics and provided a deeper understanding of their behaviour in AD, thus indicating limitations of AD as the final treatment of some materials, and also may support the establishment of guidelines for bioplastic management.

Multi-faceted analysis of thermophilic anaerobic biodegradation of poly(lactic acid)-based material.

Currently, the production of bio-based polymeric materials, of which poly(lactic acid) (PLA) is the most popular, has been increasing, causing the growth of PLA waste in municipal waste. Thus, it is necessary to develop sustainable methods for treating it. Methane production, resulting from anaerobic digestion (AD), is a potential end-of-life scenario for PLA waste that needs to be investigated. To obtain high efficiency of AD, thermophilic fermentation was applied, and to overcome low rates of biodegradation, hydrothermal (HT) and alkaline (A) pretreatments were used. For a deep insight into the process, differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and microscopic and microbial analyses (based on 16S rDNA) were applied. For both untreated (PLA) and pretreated (PLAHT, PLAA) samples a high maximal methane production (MP) of 453 L/kg volatile solids (VS) was obtained, almost 100 % of the theoretical methane yield from PLA. The use of pretreatment allowed shortening of the time for obtaining maximal MP, especially the hydrothermal pretreatment, which shortened the overall time of MP 1.3-fold, and methane was produced at an almost 10 % higher rate (8.35 vs 7.79 L/(kg VS·d)). However, DSC and microscopic analyses revealed that, in all cases, methane was intensively produced i) after the reduction of the molecular mass of the PLA material and ii) also when PLA pieces were not visible. This should be considered when designing the operational time for the AD process. Parallel to the gradual biodegradation of PLA, the abundances of Firmicutes, Thermotogae, and Euryarcheota increased. With PLAHT, Syntrophobacteraceae, Thermoanaerobacteraceae, and methanogens were identified as potential key thermophilic PLA biodegraders.

Alginate-like polymers from full-scale aerobic granular sludge: content, recovery, characterization, and application for cadmium adsorption.

Aerobic granular sludge (AGS) is a proven resource for the recovery of biopolymers like alginate-like polymers (ALP). This is the first report on the dynamics of ALP produced by AGS (ALP-AGS) in a full-scale wastewater treatment plant (WWTP), optimization of ALP recovery from AGS, and adsorption of cadmium (Cd2+) by ALP. Recovery of ALP was highest when using 120 mL of 0.2 M Na2CO3 at 70 °C for 45 min. Seasonal (1.5 years, over 3100 cycles) and intra-cycle changes in ALP-AGS in the WWTP were monitored. The ALP content in AGS increased in the transition period between winter and spring, reaching over 150 mg/g MLSS. In the batch reactor cycle, the ALP-AGS level peaked 2 h after the start of aeration (mean peak level: 120 mg/g MLSS), then decreased about two-fold by the end of the cycle. The ALP-AGS had a small surface area and a lamellar structure with crystalline outgrowths. The optimal conditions of Cd2+ adsorption with ALP were a dosage of 7.9 g d.m./L, a pH of 4-8, and an equilibrium time of 60 min. Carboxyl and hydroxyl groups were the key functional groups involved in Cd2+ adsorption. According to the Sips model, the maximum Cd2+ adsorption capacity of ALP-AGS was 29.5 mg/g d.m., which is similar to that of commercial alginate. AGS is a richer source of ALP than activated sludge, which ensures the cost-effectiveness of ALP recovery and increases the sustainability of wastewater treatment. Information on the chemical properties and yields of ALP from full-scale WWTPs is important for downstream applications with the recovered ALP.

Valorization of full-scale waste aerobic granular sludge for biogas production and the characteristics of the digestate.

Despite the dynamic development of aerobic granular sludge (AGS) technology in wastewater treatment, there is limited data on how the different properties of AGS and activated sludge (AS) translate into differences in waste sludge management. Waste sludge generated in both AGS and AS technology is the biggest waste stream generated in wastewater treatment plants (WWTPs). This study aimed to assess biogas production from waste AGS from a full-scale system. Additionally, the properties of the digestate were investigated in terms of its management in line with the assumptions of a circular economy. Both aspects are important because the characteristics of AGS differ from those of AS. Its dense, extracellular-polymer-rich granule structure makes the susceptibility of AGS to anaerobic stabilization lower than that of AS. Given the advantages of AGS for sustainable wastewater treatment and its increasing popularity, waste AGS management will pose a serious challenge for WWTP operators. Therefore, AGS from a full-scale municipal WWTP was valorized for biogas production by increasing the accessibility of the organics in the sludge by homogenization or ultrasound pretreatment. Ultrasound pretreatment released about an order of magnitude more organics from the biomass than homogenization and significantly improved the production of methane-rich biogas (455 L/kg VS, about 66% of CH4). The digestion time of pretreated AGS was reduced by 25% in comparison with that of untreated AGS making anaerobic digestion of AGS a feasible solution for sludge management. The AGS digestate was rich in Ca (77.0 g/kg TS), Mg (10.9 g/kg TS), N (35.1 g/kg TS) and P (32.4 g/kg TS), whereas its heavy metal levels and biochemical methane potential were low. AGS digestate is not only environmentally safe, but it can serve as a rich source of organics and elements essential for soil fertility and stability.

Can the biological stage of a mechanical-biological treatment plant that is designed for mixed municipal solid waste be successfully utilized for effective composting of selectively collected biowaste?

Although the requirements for overall recycling rates can only be met when organic recycling is not overlooked, information is scarce regarding adaption to biowaste composting of existing mechanical-biological treatment (MBT) plants originally designed for stabilization of organic municipal solid waste (OFMSW). Thus, this study aimed to assess the suitability of the operational conditions in the biological part of a full-scale MBT plant now used for stabilization of OFMSW (working line: closed-module-covered-pile-open-pile) with a view to producing compost from biowaste. Temperatures above 75 °C were maintained in the closed module and reached again in the covered pile, indicating that intensive organic-matter mineralization occurred in both stages. In the covered pile, the temperature sharply decreased, indicating depletion of easily biodegradable organic matter. An aerobic 4-day respiration test (AT4) value below 10 mg O2/g dry matter, the cut-off for assessing compost stability, was obtained after 8 weeks. However, a high content of humic substances (HS), reflecting compost maturity, was obtained only after 120 days. The increase in HS content proceeded in two phases. In the first phase (45-84 day), the rate constant and the rate of HS formation were lower than in the second phase (84-120 day) (0.072 vs. 0.087 day-1, 1.97 vs. 3.06 mg C/(g organic matter·d)). All the above-mentioned indicators and the nutrient content (N, P, K, Mg, Ca) in the compost indicates that the biological stage of an MBT plant can successfully treat biowaste. This is in accordance with a circular economy and will contribute to increasing recycling rates.

Quality of heavy metal-contaminated soil before and after column flushing with washing agents derived from municipal sewage sludge.

Removal of heavy metals (HMs) from soil is a priority in soil washing/soil flushing. However, for further management of remediated soil, it should be characterized in detail. This study presents, for the first time, an evaluation of soil quality after column flushing with new-generation washing agents (WAs) recovered from municipal sewage sludge (dissolved organic matter, DOM; soluble humic-like substances, HLS; soluble humic substances, SHS) and Na2EDTA as a standard benchmark. Sandy loam soil was spiked with industrial levels of Cu, Pb and Zn, then flushed in a column reactor at two WA flow rates (0.5 and 1.0 ml/min). Soil quality was assessed by determining both physico-chemical (pH, total HMs and their mobility, soil organic matter, OM, humic substances, HS and their fractions, macroelements) and biological indicators (dehydrogenase activity, DHA; germination rate, GR; and inhibition factors for roots and shoots of Triticum aestivum). Total residual HMs contents and HMs contents in the mobile fraction were significantly lower in soil flushed at 1.0 ml/min than in soil flushed at 0.5 ml/min. With all WAs, the decrease in Cu content was larger than that of the other HMs, however this HM most effectively was removed with DOM. In contrast, Pb most effectively was removed by HLS and Na2EDTA, and DOM should not be used to remediate Pb-contaminated soil, due to its very low effectiveness. Flow rate did not appear to affect the fertilizing properties of the soil, DHA activity or soil toxicity indicators. Soil flushing with all SS_WAs increased OM, HS, and exchangeable P, K and Na content in remediated soils, but decreased exchangeable Ca content, and in most cases, exchangeable Mg content. Soil flushing substantially improved DHA activity and GR, but only slightly improved the shoot and root inhibition factors.

Kinetics of Cu, Pb and Zn removal during soil flushing with washing agents derived from sewage sludge.

This paper presents the first tests of Cu (7875 mg/kg), Pb (1414 mg/kg) and Zn (566 mg/kg) removal from contaminated soil with sewage-sludge-derived washing agents (SS_WAs) (dissolved organic matter, DOM; soluble humic-like substances, HLS; soluble humic substances, SHS) and Na2EDTA (as a standard benchmark) in column experiments. Flow rates of 0.5 ml/min and 1 ml/min were used. Using a 1. order kinetic model, the kinetic constant (k), the maximum concentrations of each metal removed (Cmax), and the initial rates of metal removal (r) were established. At both flow rates, stable flow velocity was maintained for approximately eight pore volumes, for flushing times of 8 h (1.0 ml/min) and 16 h (0.5 ml/min). Although the flow rate did not influence k, it influenced Cmax: at 1 ml/min, Cmax values were higher than at 0.5 ml/min. For Cu and Zn, but not Pb, k was about twofold higher with Na2EDTA than with SS_WAs. Although Na2EDTA gave the highest kCu, Cmax,Cu was highest with DOM (Na2EDTA, 66%; DOM 73%). For Pb removal, HLS was the most effective SS_WA (77%; Na2EDTA was 80% effective). kZn was about twofold higher with Na2EDTA than with SS_WAs. Cmax,Zn was highest with HLS. The quick mobilization of Cu, Pb and Zn with most of the WAs corresponded to efficient metal removal from the exchangeable (F1) fraction.

Thermophilic and mesophilic biogas production from PLA-based materials: Possibilities and limitations.

Recently, the use of bio-based products, including biodegradable poly(lactic acid) (PLA), has increased, causing their rapid growth in municipal waste streams. The presence of PLA in biowaste may increase biogas production (BP). However, the rate of PLA biodegradation, which affects the time frame of anaerobic digestion, is a key parameter for an efficient process. In this study, detailed kinetics of BP from PLA were determined at 58 °C and 37 °C. At both temperatures, lag phases were observed: 40 days at 37 °C, and 10 days at 58 °C. After the lag phase BP proceeded in two phases, differed in process rate. At 58 °C, during the 1st phase (up to day 30), the rate of BP (rB1,58) equaled about 35 L/(kg OM·d). At the end of this phase, the amount of biogas was 710 L/kg OM, which constituted 84% of the maximal BP (831-849 L/kg OM). In the 2nd phase (10 days), only 13% of maximal BP was produced (rB2,58 of 16.1 L/(kg OM·d)). At 37 °C, maximal BP (obtained after 280 days) was 1.5-fold lower (558-570 L/kg OM) than at 58 °C. In the 1st phase (100 days), rB1,37 was 1.4 L/(kg OM·d); at the end of this phase, BP constituted merely 14% of the maximal BP. A majority of biogas was produced in the 2nd phase (the next 180 days), and rB2,37 doubled to 2.6 L/(kg OM·d)). At 58 °C, intensive biogas production took place when PLA pieces were still visible. At 37 °C, in contrast, biogas was mainly produced when the PLA pieces had been disintegrated. Although PLA anaerobically biodegrades and produces a high yield of biogas, the time frame of PLA digestion is much longer than that of biowaste and, in thermophilic conditions requires separate digesters. In mesophilic conditions, however, is unacceptable at technical scale.

Suitability of environmental indices in assessment of soil remediation with conventional and next generation washing agents.

Remediation of soils contaminated with metal must ensure high efficiency of metals removal, reduce bioavailability of residual metals and decrease ecological risk. Thus, for comprehensive environmental soil quality assessment, different indices must be used. In this study, suitability of 8 indices was used for soil highly contaminated with Cu (7874.5 mg kg-1), moderately with Pb (1414.3 mg kg-1) and low with Zn (566.1 mg kg-1), washed in batch and dynamic conditions with both conventional and next-generation washing agents. The following indices were used: modified contamination factor (mCf), modified contamination factor degree (mCdeg), mobility factor (MF), reduced partition index (IR), potential ecological risk factor (Er,Z), modified potential ecological risk factor (Er,m), potential ecological risk index (RIZ) and modified ecological risk index (RIm). For mCf, mCdeg and IR own classification scale was proposed. It was proven that most useful indices for assessment of soil pollution with metals were mCf and mCdeg. The mCf together with the IR allow to simultaneous assessment of soil pollution and stability for individual metals. These indices were appropriate for soil contaminated with different concentrations of metals, washed under both hydrodynamic conditions using various washing agents and different effectiveness of metals removal. Thus, they may be considered as most useful for evaluation of remediation method, feasibility of washing agent and assessing soil quality after washing.

New-Generation Washing Agents in Remediation of Metal-Polluted Soils and Methods for Washing Effluent Treatment: A Review.

Soil quality is seriously reduced due to chemical pollution, including heavy metal (HM) pollution. To meet quality standards, polluted soils must be remediated. Soil washing/soil flushing offers efficient removal of heavy metals and decreases environmental risk in polluted areas. These goals can be obtained by using proper washing agents to remove HMs from soil. These washing agents should not pose unacceptable threats to humans and ecosystems, including soil composition. Currently, it is desirable to use more environmentally and economically attractive washing agents instead of synthetic, environmentally problematic chemicals (e.g., ethylenediaminetetraacetic acid (EDTA)). The usefulness of novel washing agents for treatment of heavy metal-contaminated soils is being intensively developed, in terms of the efficiency of HM removal and properties of washed soils. Despite the unquestionable effectiveness of soil washing/flushing, it should be remembered that both methods generate secondary fluid waste (spent washing solution), and the final stage of the process should be treatment of the contaminated spent washing solution. This paper reviews information on soil contamination with heavy metals. This review examines the principles and status of soil washing and soil flushing. The novel contribution of this review is a presentation of the sources and characteristics of novel washing agents and chemical substitutes for EDTA, with their potential for heavy metal removal. Methods for treating spent washing solution are discussed separately.

Adsorption - Membrane process for treatment of stabilized municipal landfill leachate.

The aim of this study was to investigate the efficiency of removal of difficult-to-biodegrade organic compounds from real stabilized landfill leachate with a membrane process alone and in combination with powdered-activated-carbon (PAC) adsorption. For filtration, ceramic membranes were used. The characteristics of the raw leachate were 788 mg COD/L and color of 0.4458 cm-1. With all combinations of PAC-adsorption and a membrane process (MF, UF, fine-UF) and with fine-UF alone, leachate treatment was highly efficient. For each membrane, treatment was more efficient when the membrane process was combined with PAC addition. This means that adsorption (PAC dose 3 g/L, adsorption time 30 min) made the largest contribution to leachate treatment (COD and color removal efficiency of 73.1% and 94.4%, respectively). In all cases, organic particles bigger than 100 kDa were removed most efficiently, whereas particles smaller than 3 kDa were removed least efficiently. The efficiency of leachate treatment with PAC + MF, PAC + UF and PAC + fine-UF did not differ significantly (>87% COD and > 97% color). With regard to membrane flux, however, these combinations can be ranked in the following order: PAC + MF > PAC + UF > PAC + fine-UF. Therefore, PAC + MF (0.3 MPa) was selected as the most effective solution (COD and color removal efficiencies of 87.8% and 97.2%, respectively; permeate flux of 167.6 L/(m2∙h)), as it combined efficient pollutant removal with low membrane pressure.

Stabilisation of municipal solid waste after autoclaving in a passively aerated bioreactor.

Autoclaving of unsorted municipal solid waste is one of the solutions in waste management that maximises the amount of waste for recycling. After autoclaving, however, a large part of the waste is composed of unstabilised biodegradable fractions (organic remaining fraction, ORF), which may comprise up to 30% of autoclaved waste and cannot be landfilled without further stabilisation. Thus, the aim of this study was to investigate the effectiveness of aerobic stabilisation in a passively aerated reactor of organic remaining fraction after full-scale autoclaving of unsorted municipal solid waste. The organic remaining fraction had a volatile solids content of ca. 70%, a 4-day respiration activity test (AT4) of ca. 26 g O2 kg-1 total solids and a 21-day gas formation test (GP21) of ca. 235 dm3 kg-1 total solids. Stabilisation was conducted in a 550 L reactor with passive aeration (Stage I) and a periodically turned windrow (Stage II). The feedstocks consisted entirely of organic remaining fraction, or of organic remaining fraction with 10% inoculum (ORF + I). Inoculum constituted product of stabilisation of organic remaining fraction. During stabilisation of organic remaining fraction and ORF + I, thermophilic conditions were achieved, and the decreases of volatile solids, AT4 and GP21 could be described by 1 order kinetic models. The rate constants of volatile solids removal (kVS) were 0.033 and 0.068 d-1 for organic remaining fraction and ORF + I, respectively, and the thermophilic phase was shorter with ORF + I (25 days vs. 45 days). The decrease in GP21 corresponded to volatile solids decrease, but AT4 decreased sharply during the first 10 days of waste stabilisation in the reactor, indicating that the content of highly biodegradable organic matter decreased during this time.

Tannic acid for remediation of historically arsenic-contaminated soils.

Soil washing effectively and permanently decreases soil pollution. Thus, it can be considered for the removal of the most toxic elements, for example arsenic (As). In this study, historically As-contaminated soils (2041-4294 mg/kg) were remediated with tannic acid (TA) as the washing agent. The scope of this study included optimization of the operational conditions of As removal, determination of As distribution in soil before and after double soil washing, and measurement of TA loss during washing. The optimum conditions for As removal were 4% TA, pH 4 and 24 h washing time. The average As removal after single and double washings was 38% and 63%, respectively. TA decreased As content in amorphous and poorly crystalline oxides by >90%. Although TA increased the amount of As in the easily mobilizable As fraction, the stability of As in washed soils increased, with reduced partition indexes of 0.52-0.66 after washing. The maximum capacity of the soils to adsorb TA (qmax) was 50.2-70.4 g C/kg. TA sorption was higher at alkaline than at acidic conditions. Only TA removes As from soils effectively if the proportion of As in amorphous and poorly crystalline oxides is high. Thus, it can be considered for remediation of historically contaminated soils.

Effect of barley straw and coniferous bark on humification process during sewage sludge composting.

This study investigated how different amendments (barley straw, coniferous bark) influence organic matter (OM) removal kinetics and humification during sewage sludge composting. With bark, high temperatures, intensive OM degradation and humification were achieved later than with straw. The rate of OM degradation was lower with bark than with straw (15.18 g/kg OM·d vs 24.07 g/kg OM·d) and the time needed for intensive HS formation was longer with bark (140 days vs 60 days). The kinetic constants for humic substances (HS) and humic acids (HA) formation were lower with bark than with straw (kHS 0.025 d-1 vs 0.047 d-1, kHA 0.022 d-1 vs 0.044 d-1). With bark, however, the increase in HS concentration during composting (Cmax,HS) was higher (178 mg C/g OM vs 84 mg C/g OM), and the fulvic fraction predominated in HS (80%), whereas with straw, humic acids predominated (82% of HS).

Ecological risk assessment of sewage sludge from municipal wastewater treatment plants: a case study.

The aim of this study was to characterize sewage sludge from mechanical-biological wastewater treatment plants (WWTPs) in terms of total concentrations of heavy metals (HMs) and their distribution. Moreover, HM mobility, stability and ecological risk were assessed by calculating the mobility factor (MF), the reduced partition index (Ir) and the ecological risk index (ERI). Hierarchical cluster analysis (CA) was used to group the HMs in the sludges on the basis of their MF, Ir and ERI. The concentration of HMs decreased in the following order: Zn > Cu > Ni > Pb > Cd. HM mobility, as indicated by the MF values, was as follows: 18.7%-30.6% (Zn), 8.1%-36.4% (Ni), 6.8%-11.1% (Cu), 3.2%-32.2% (Pb) and 0% (Cd). Based on the average Ir values, the stability of the metals in the sludge increased in this order: Zn (0.23)=Pb (0.23)

Suitability of humic substances recovered from sewage sludge to remedy soils from a former As mining area - a novel approach.

Batch washing experiments were performed to evaluate the feasibility of using a solution of humic substances (HS) extracted from municipal sewage sludge as a washing agent to remove As from soils at a former As mining area. Soils (S1, S2, S3) differed in organic matter content, pH and As concentration. At pH 4 and a HS concentration of 4000mgTOCL-1, As removal efficiency ranged from 18% (S2) to 27% (S3). In all cases, As removal proceeded according to pseudo-second-order kinetics (equilibrium As concentrations ranged from 625mgkg-1 (S3) to 1250mgkg-1 (S3); rate constants, from 1.02×10-5kgmg-1min-1 (S1) to 2.05×10-5kgmg-1min-1 (S3). The time needed to reach equilibrium was 12h. With double washing, the efficiency of As removal was 1.5-times higher (on average) than with single washing. Double washing increased As stability, as indicated by the reduced partition index, especially in soils S1 and S3. Moreover, HS effectively decreased the content of the most toxic As(III) (by 95-97%).

Microfauna community during pulp and paper wastewater treatment in a UNOX system.

This study characterized the microfauna community during treatment of pulp and paper wastewater in a UNOX system aerated with pure oxygen, and with a high organic loading rate (0.4±0.06kg BOD/kg MLSSday), low sludge retention time (3.73±0.33 day), and high oxygen concentration (≤20mg O2/L) in comparison to municipal treatment systems. In the aeration tank, temperatures were high, averaging 35.7°C (March-May), then 38.9°C (June-August). Effluent quality was acceptable: 180±22mg COD/L, 7.2±2.1mg BOD5/L, and 33±5mg TSS/L. At 35.7°C, 5 taxa were identified in the activated sludge: small flagellates (flagellates <20µm), attached ciliates (Vorticella infusionum, Vorticella octava), crawling ciliates (Chilodonella uncinata) and free-swimming ciliates (Sathrophilus muscorum). During this period, the SBI (Sludge Biotic Index) was 6-7, corresponding to Quality Class II. At 38.9°C, two taxa co-dominated (Vorticella infusionum, Sathrophilus muscorum). When Vorticella infusionum dominated, the SBI was 5 or 7 (Quality Class III/II); when Sathrophilus muscorum dominated, the SBI was 0 (Quality Class IV). Slight changes in the abundance of two opposing Madoni keygroups impeded proper classification of activated sludge. However, effluent quality remained the same, showing that these indicators of activated sludge quality do not always reflect effluent quality.

Behaviors of heavy metals (Cd, Cu, Ni, Pb and Zn) in soil amended with composts.

This study investigated how amendment with sewage sludge compost of different maturation times (3, 6, 12 months) affected metal (Cd, Cu, Ni, Pb, Zn) bioavailability, fractionation and redistribution in highly contaminated sandy clay soil. Metal transformations during long-term soil stabilization (35 months) were determined. In the contaminated soil, Cd, Ni and Zn were predominately in the exchangeable and reducible fractions, Pb in the reducible fraction and Cu in the reducible, exchangeable and oxidizable fractions. All composts decreased the bioavailability of Cd, Ni and Zn for up to 24 months, which indicates that cyclic amendment with compost is necessary. The bioavailability of Pb and Cu was not affected by compost amendment. Based on the reduced partition index (IR), metal stability in amended soil after 35 months of stabilization was in the following order: Cu > Ni = Pb > Zn > Cd. All composts were more effective in decreasing Cd, Ni and Zn bioavailability than in redistributing the metals, and increasing Cu redistribution more than that of Pb. Thus, sewage sludge compost of as little as 3 months maturation can be used for cyclic amendment of multi-metal-contaminated soil.

Kinetics of organic matter removal and humification progress during sewage sludge composting.

This study investigated the kinetics of organic matter (OM) removal and humification during composting of sewage sludge and lignocellulosic waste (wood chips, wheat straw, leaves) in an aerated bioreactor. Both OM degradation and humification (humic substances, HS, and humic acids, HA formation) proceeded according to 1. order kinetics. The rate constant of OM degradation was 0.196 d(-1), and the rate of OM degradation was 39.4 mg/g OM d. The kinetic constants of HS and HA formation were 0.044 d(-1) and 0.045 d(-1), whereas the rates of HS and HA formation were 3.46 mg C/g OM d and 3.24 mg C/g OM d, respectively. The concentration profiles of HS and HA indicated that humification occurred most intensively during the first 3 months of composting. The high content of HS (182 mg C/g OM) in the final product indicated that the compost could be used in soil remediation as a source of HS for treating soils highly contaminated with heavy metals.

Feasibility of using humic substances from compost to remove heavy metals (Cd, Cu, Ni, Pb, Zn) from contaminated soil aged for different periods of time.

There is a need for inexpensive, readily-available and environmentally-friendly soil washing agents to remediate polluted soils. Thus, batch washing experiments were performed to evaluate the feasibility of using a solution of humic substances (HS) extracted from compost as a washing agent for simultaneous removal of Cu, Cd, Zn, Pb and Ni from artificially contaminated soils aged for 1 month, 12 months and 24 months. The efficiency of metal removal in single and multiple washings and kinetic constants (equilibrium metal concentration qe and rate constant k from the second-order kinetic equation) were determined. On average, triple washing removed twice as much metal as that removed with a single washing. At pH 7 and a HS concentration of 2.2 g C L(-1), metal removal from all soils decreased in this order: Cd (79.1-82.6%) > Cu (51.5-71.8%) > Pb (44.8-47.6%) > Ni (35.4-46.1%) > Zn (27.9-35.8%). However, based on qe (mg kg(-1)), metal removal was in this order: Pb > Zn ≈ Cu > Ni > Cd. This difference was due to different concentrations of metals, which is typical for multi-metal contaminated soils. Regardless of washing mode, removal of Cd and Pb was not affected by soil age, whereas removal of Cu, Ni and Zn was higher in soils that had been aged for a shorter time. These results indicate that HS are suitable for remediating soil contaminated with multiple heavy metals in extremely high concentrations.