Aerobic Sewage Sludge Research Articles

PERFORMANCE OF Calendula officinalis L. IN SUBSTRATES DEVELOPED WITH SOLARIZED SEWAGE SLUDGE AND AGRO-INDUSTRIAL WASTES

Efficient products can be developed for the ornamental plant sector using agro-industrial waste. This study evaluated the agronomic performance and ornamental quality of calendula plants (Calendula officinalis) cultivated in substrates formulated with solarized aerobic sewage sludge (SASS), rice husk ash (RHA), and sugarcane bagasse (SB). Five additive rates were used: S2: 33.33% SASS + 33.33% RHA + 33.33% SB; S3: 16.65% SASS + 41.67% RHA + 41.67% SB; S4: 12.48% SASS + 43.76% RHA + 43.76 % SB; S5: 8.32% SASS + 45.84% RHA + 45.84 % SB; S6: 4.16% SASS + 41.07% RHA + 54.76% SB; and S1: commercial substrate (reference). Raw materials and substrate formulations were characterized and agronomic and ornamental aspects were evaluated in calendula plants cropped in all substrates. The lowest agronomic performance was observed in S6 (lowest SASS rate), while the highest occurred in S3 compared to the other treatments. Regarding the ornamental potential, S3 and S4 and the commercial substrate (S1) were classified as excellent. This study highlights the successful use of organic raw materials (SASS and agro-industrial waste) to compose substrates for calendula production in pots, with the recommended proportions ranging from 12.48 to 16.65% of SASS combined with RHA and SB.

Evaluation of Dairy Wastewater Treatment Systems Using Carbon Footprint Analysis

Modernisation of municipal and industrial wastewater treatment plants (WWTPs) should be carried out, taking into account its impact on global warming, e.g., through carbon footprint (CF) analysis. An important industrial sector in Poland is the dairy industry. In dairy WWTPs, the aerobic sewage sludge stabilisation applied thus far is being replaced by the anaerobic process. This change is positive due to the possibility of energy production, but it is unclear how it affects greenhouse gases (GHG) emissions. The aim of the research was to perform CF analysis for two scenarios of dairy WWTP operation. The analysis was based on the real operating data of the current system (current scenario) and project of its modernisation (alternative scenario). The current scenario consists of mechanical and dissolved air flotation (DAF) treatment, biological treatment in sequence batch reactors (SBRs), aerobic sewage sludge stabilisation and its final farmland usage. The alternative scenario assumes replacing aerobic stabilisation with anaerobic stabilisation and a combined heat and power (CHP) system. The CF calculations were based on empirical models, taking into account different emission input parameters, expressed in CO2 equivalents (CO2e). The total CF of the current scenario was 22 kg CO2e PE−1 year−1, while the alternative was 45 kg CO2e PE−1 year−1. The largest share in the current scenario belongs to emissions from WWTPs and energy use, while in the alternative, there is the addition of emissions from biogas use.

Structure and indicators of electric energy consumption in dairy wastewater treatment plant

The aim of the research was to determine the indicators of electricity consumption in every stage of the dairy sewage treatment process in relation to the sewage flow and the load of removed organics (BOD5, COD) and nutrients (TN, TP). The research was conducted in a dairy wastewater treatment plant (WWTP) consisting of mechanical treatment, averaging tank, dissolved air flotation (DAF) and biological treatment with sequence batch reactors (SBRs). Energy consumption was measured with the help of transducers. Indicators of unit electricity consumption were determined on the basis of 95 measurement series of energy consumption, sewage flow and removed load. The mean value of total unit energy consumption relating to the flow for the entire WWTP was 2.29 kWh·m−3, while for biological treatment 1.17 kWh·m−3 and 0.05 kWh·m−3 for DAF. The mean values of indicators relating to removed pollutants load for the entire WWTP were: 1.89 kWh·kgrem BOD5−1, 1.30 kWh·kgrem COD−1, 48.61 kWh·kgrem TN−1 and 160.01 kWh·kgrem TP−1. During biological treatment, energy consumption indicators were on average: 1.65 kWh·kgrem BOD5−1 and 1.19 kWh·kgrem COD−1, 52.90 kWh·kgrem TN−1 and 141.26 kWh·kgrem TP−1, while for DAF: 0.12 kWh·kgrem BOD5−1, 0.09 kWh·kgrem COD -1, 3.85 kWh·kgrem TN−1 and 16.17 kWh·kgrem TP−1. It was found that the biological treatment in SBRs was responsible for 54.1% of the total energy consumption of dairy WWTP. Aerobic sewage sludge treatment accounted for 17.0% of total consumption, mechanical treatment 17.1%, deodorization 2.6%, and other (social, lighting etc.) 6.9%, while DAF only 2.3%. The real-time electricity metering system enabled the optimisation of the electricity consumption in the WWTP, taking into account its consumption in unit processes and the removed pollutants load. The application of this system enabled to make corrections that reduced energy consumption while maintaining the required treatment efficiency.

Organic versus inorganic fertilizers: Response of soil properties and crop yield

<abstract> <p>The decrease in soil productivity and quality caused by the continuous and abusive use of mineral fertilizers makes necessary to adopt more sustainable agricultural soil management strategies that help to maintain soil edaphic fertility. In light of these considerations, we have evaluated the effect of organic vs. inorganic fertilization on soil microbial communities, soil quality, and crop yield in a melon crop. The following treatments were tested: i) aerobic sewage sludge from a conventional wastewater treatment plant (WWTP) using aerobic bacteria (SS); ii) aerobic sewage sludge from a WWTP using a bacteria-microalgae consortium (B); iii) N-P-K mineral fertilizer (M); iv) a treatment in which 50% of the N was contributed by SS and 50% by mineral fertilizer (M + SS); v) a treatment in which 50% of the N was contributed by B and 50% by mineral fertilizer (M + B); and vi) a no-fertilized control soil. Melon yield and fruit quality were determined in addition to several soil physical, chemical, biochemical and microbiological parameters. Organic fertilizers (SS and B) increased the percentage of soil water-stable aggregates (52 and 60% respectively) as well as the content of organic C (18 and 31%), water soluble C (21 and 41%), N (15 and 41%) and available P content (41 and 82%) compared to inorganic fertilization. They also stimulated bacterial and fungal abundance to a greater extent than mineral fertilizers (189 and 242% vs 85%, and 57 and 122% vs 29%, respectively), as well as soil respiration, and dehydrogenase, β-glucosidase, phosphatase, urease, and glycine aminopectidase activities. The analysis of principal components with parameters linked to soil quality clearly showed that organic fertilizers cause a greater improvement in soil characteristics and microbial community than mineral fertilizers. Results demonstrate that organic and combined fertilization could be used as substitutes for nitrogen mineral fertilizers in melon crop, since these treatments led to similar melon production and quality while improving soil characteristics and microbial population size and activity.</p> </abstract>

Insights into pyrolysis characteristics of Brazilian high-ash sewage sludges using thermogravimetric analysis and bench-scale experiments with GC-MS to evaluate their bioenergy potential

This paper is the first comprehensive report about the renewable fuels produced from the pyrolytic conversion of anaerobic and aerobic Brazilian high-ash sewage sludges. The study was divided into three steps: (1) an in-deep evaluation of physicochemical characteristics of both sewage sludges; (2) thermogravimetric analysis under inert conditions for determining bench-scale pyrolysis conditions; (3) characterization of volatile pyrolytic products from fixed bed reactor using GC/MS analysis. The anaerobic and aerobic sewage sludges present substantial high heating values and high contents of ash and sulfur, which are adverse properties for solid fuels. Regarding the pyrolytic behavior, the sewage sludge samples presented a two-step thermal decomposition process. The bio-oil produced from pyrolysis indicates that the high-ash sewage sludges are a source of high value-added chemicals, such as aliphatic and monoaromatic hydrocarbons. In addition, the pyrolysis can promote the production of a hydrogen-rich syngas for thermal boilers or engines use. In this research, it was demonstrated that the pyrolysis process of Brazilian high-ash sewage sludge can be applied for the production of useful biofuels - as bio-oil and bio-syngas - also adding value to this waste, thus contributing to the establishment of renewable source of bioenergy.

Application of SS-VF Bed for the Treatment of High Concentrated Reject Water from Autothermal Termophilic Aerobic Sewage Sludge Digestion

Application of SS-VF Bed for the Treatment of High Concentrated Reject Water from Autothermal Termophilic Aerobic Sewage Sludge Digestion

Renewable Alkenes from the Hydrothermal Treatment of Polyhydroxyalkanoates-Containing Sludge.

Polyhydroxyalkanoates (PHA) are a key constituent of excess sludge produced by Aerobic Sewage Sludge Treatment plants. The accumulation of significant amount of PHA inside aerobic microbial cells occurs when a surplus of an easily degradable carbon source (e.g., volatile fatty acids, VFA) is found in combination with other nutrients limitation. Herein, hydrothermal treatment (HT) of PHA-containing sludge at 300 and 375 °C was demonstrated to be effective in converting most (>70% w/w) of the bacterial PHA stored inside microbial cells into alkene/CO2 gas mixtures. Simultaneously, most of non-PHA biomass was converted into water-soluble compounds (50% carbon yield) that were acidogenic fermented to produce volatile fatty acids, ideal substrate to feed aerobic bacteria and produce more PHA. According to results here presented, HT of excess sludge with moderate (13%) PHA content can produce about 50 kg of alkenes per tonne of suspended solids treated, with a significant reduction of sludge mass (80% reduction of wet sludge volume) and consequent disposal cost.

Evaluation of energy consumption during aerobic sewage sludge treatment in dairy wastewater treatment plant

The subject of the research conducted in an operating dairy wastewater treatment plant (WWTP) was to examine electric energy consumption during sewage sludge treatment. The excess sewage sludge was aerobically stabilized and dewatered with a screw press. Organic matter varied from 48% to 56% in sludge after stabilization and dewatering. It proves that sludge was properly stabilized and it was possible to apply it as a fertilizer. Measurement factors for electric energy consumption for mechanically dewatered sewage sludge were determined, which ranged between 0.94 and 1.5 kWhm−3 with the average value at 1.17 kWhm−3. The shares of devices used for sludge dewatering and aerobic stabilization in the total energy consumption of the plant were also established, which were 3% and 25% respectively. A model of energy consumption during sewage sludge treatment was estimated according to experimental data. Two models were applied: linear regression for dewatering process and segmented linear regression for aerobic stabilization. The segmented linear regression model was also applied to total energy consumption during sewage sludge treatment in the examined dairy WWTP. The research constitutes an introduction for further studies on defining a mathematical model used to optimize electric energy consumption by dairy WWTPs.

Application of subsurface vertical flow constructed wetlands to reject water treatment in dairy wastewater treatment plant

ABSTRACTThe paper presents the effects of applying subsurface vertical flow constructed wetlands (SS VF) for the treatment of reject water generated in the process of aerobic sewage sludge stabilization in the biggest dairy wastewater treatment plant (WWTP) in Poland. Two SS VF beds were built: bed (A) with 0.65 m depth and bed (B) with 1.0 m depth, planted with reeds. Beds were fed with reject water with hydraulic load of 0.1 m d−1 in order to establish the differences in treatment efficiency. During an eight-months research period, a high removal efficiency of predominant pollutants was shown: BOD5 88.1% (A) and 90.5% (B); COD 84.5% (A) and 87.5% (B); TSS 87.6% (A) and 91.9% (B); TKN 82.4% (A) and 76.5% (B); N-NH4+ 89.2% (A) and 85.7% (B); TP 30.2% (A) and 40.6% (B). There were not statistically significant differences in the removal efficiencies between bed (B) with 1.0 m depth and bed (A) with 0.65 m depth. The research indicated that SS VF beds could be successfully applied to reject water treatment in dairy WWTPs. The study proved that the use of SS VF beds in full scale in dairy WWTPs would result in a significant decrease in pollutants’ load in reject water. In the analyzed case, decreasing the load of ammonia nitrogen was of greatest importance, as it constituted 58% of the total load treated in dairy WWTP and posed a hazard to the stability of the treatment process.

Gaseous emissions from sewage sludge combustion in a moving bed combustor

Substantial increase in sewage sludge generation in recent years requires suitable destination for this residue. This study evaluated the gaseous emissions generated during combustion of an aerobic sewage sludge in a pilot scale moving bed reactor. To utilize the heat generated during combustion, the exhaust gas was applied to the raw sludge drying process. The gaseous emissions were analyzed both after the combustion and drying steps. The results of the sewage sludge characterization showed the energy potential of this residue (LHV equal to 14.5MJkg−1, db) and low concentration of metals, polycyclic aromatic hydrocarbons (PAH), polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF). The concentration of CO, NOx, BTEX (benzene, toluene, ethylbenzene and xylenes) emitted from the sludge combustion process were lower than the legal limits. The overall sludge combustion and drying process showed low emissions of PCDD/PCDF (0.42ng I-TEQNm−3). BTEX and PAH emissions were not detected. Even with the high nitrogen concentration in the raw feed (5.88% db), the sludge combustion process presented NOx emissions below the legal limit, which results from the combination of appropriate feed rate (A/F ratio), excess air, and mainly the low temperature kept inside the combustion chamber. It was found that the level of CO emissions from the overall sludge process depends on the dryer operating conditions, such as the oxygen content and the drying temperature, which have to be controlled throughout the process in order to achieve low CO levels. The aerobic sewage sludge combustion process generated high SO2 concentration due to the high sulfur content (0.67wt%, db) and low calcium concentration (22.99gkg−1) found in the sludge. The high concentration of SO2 in the flue gas (4776.77mgNm−3) is the main factor inhibiting PCDD/PCDF formation. Further changes are needed in the pilot plant scheme to reduce SO2 and particulate matter emissions, such as the installation of exhaust gas-cleaning systems. According to previous studies, the efficient operation of such cleaning systems is also effective for metals emission control, which makes the combustion of sewage sludge a feasible treatment method from both energetic and environmental perspectives.

The Chemophytostabilisation Process of Heavy Metal Polluted Soil.

Industrial areas are characterised by soil degradation processes that are related primarily to the deposition of heavy metals. Areas contaminated with metals are a serious source of risk due to secondary pollutant emissions and metal leaching and migration in the soil profile and into the groundwater. Consequently, the optimal solution for these areas is to apply methods of remediation that create conditions for the restoration of plant cover and ensure the protection of groundwater against pollution. Remediation activities that are applied to large-scale areas contaminated with heavy metals should mainly focus on decreasing the degree of metal mobility in the soil profile and metal bioavailability to levels that are not phytotoxic. Chemophytostabilisation is a process in which soil amendments and plants are used to immobilise metals. The main objective of this research was to investigate the effects of different doses of organic amendments (after aerobic sewage sludge digestion in the food industry) and inorganic amendments (lime, superphosphate, and potassium phosphate) on changes in the metals fractions in soils contaminated with Cd, Pb and Zn during phytostabilisation. In this study, the contaminated soil was amended with sewage sludge and inorganic amendments and seeded with grass (tall fescue) to increase the degree of immobilisation of the studied metals. The contaminated soil was collected from the area surrounding a zinc smelter in the Silesia region of Poland (pH 5.5, Cd 12 mg kg-1, Pb 1100 mg kg-1, Zn 700 mg kg-1). A plant growth experiment was conducted in a growth chamber for 5 months. Before and after plant growth, soil subsamples were subjected to chemical and physical analyses. To determine the fractions of the elements, a sequential extraction method was used according to Zeien and Brümmer. Research confirmed that the most important impacts on the Zn, Cd and Pb fractions included the combined application of sewage sludge from the food industry and the addition of lime and potassium phosphate. Certain doses of inorganic additives decreased the easily exchangeable fraction from 50% to 1%. The addition of sewage sludge caused a decrease in fraction I for Cd and Pb. In combination with the use of inorganic additives, a mobile fraction was not detected and an easily mobilisable fraction was reduced by half. For certain combinations of metals, the concentrations were detected up to a few percent. The application of sewage sludge resulted in a slight decrease in a mobile (water soluble and easily exchangeable metals) fraction of Zn, but when inorganic additives were applied, this fraction was not detected. The highest degree of immobilisation of the tested heavy metals relative to the control was achieved when using both sewage sludge and inorganic additives at an experimentally determined dose. The sequential extraction results confirmed this result. In addition, the results proved that the use of the phytostabilisation process on contaminated soils should be supported.

Dissipation, metabolism and sorption of pesticides used in fruit-packaging plants: Towards an optimized depuration of their pesticide-contaminated agro-industrial effluents

Wastewaters from the fruit-packaging industry constitute a serious point source contamination with pesticides. In the absence of effective depuration methods, they are discharged in municipal wastewater treatment plants or spread to land. Modified biobeds could be an applicable solution for their treatment. We studied the dissipation of thiabendazole (TBZ), imazalil (IMZ), ortho-phenylphenol (OPP), diphenylamine (DPA) and ethoxyquin (EQ), used by the fruit-packaging industry, in anaerobically digested sewage sludge, liquid aerobic sewage sludge and in various organic substrates (biobeds packing materials) composed of soil, straw and spend mushroom substrate (SMS) in various volumetric ratios. Pesticide sorption was also determined. TBZ and IMZ showed higher persistence especially in the anaerobically digested sewage sludge (DT50=32.3–257.6d), in contrast to OPP and DPA which were rapidly dissipated especially in liquid aerobic sewage sludge (DT50=1.3–9.3d). EQ was rapidly oxidized mainly to quinone imine (QI) which did not persist and dimethyl ethoxyquinoline (EQNL, minor metabolite) which persisted for longer. Sterilization of liquid aerobic sewage sludge inhibited pesticide decay verifying the microbial nature of pesticide dissipation. Organic substrates rich in SMS showed the highest dissipation capacity with TBZ and IMZ DT50s of ca. 28d compared to DT50s of >50d in the other substrates. TBZ and IMZ showed the highest sorption affinity, whereas OPP and DPA were weakly sorbed. Our findings suggest that current disposal practices could not guarantee an efficient depuration of effluents from the fruit-packaging industry, whereas SMS-rich biobed organic substrates show efficient depuration of effluents from the fruit-packaging industry via accelerated dissipation even of recalcitrant fungicides.

Effect of pre-heating on microwave enhanced advanced oxidation process

Aerobic sewage sludge and dairy manure were pre-treated by conventional heating (CH) (40, 50 and 60°C) followed by microwave enhanced advanced oxidation process (MW/H2O2-AOP) at 100 and 120°C for solids disintegration and nutrient release. Increasing CH and MW temperatures resulted in an increase in soluble chemical oxygen demand concentration and volatile fatty acids in the treated solution. The maximum ratio of soluble to total chemical oxygen demand obtained for sewage sludge and dairy manure was 60% and 22%, respectively. The synergistic effect of microwave irradiation and hydrogen peroxide (H2O2) was more pronounced for sewage sludge at high CH and MW temperatures. Temperature and treatment time played a significant role in releasing orthophosphates. A maximum orthophosphate-to-total phosphate ratio of 55% and 85% was obtained for sewage sludge and dairy manure, respectively. A simple energy balance reveals that the CH step proves to be advantageous during continuous real-time operation of MW/H2O2-AOP technology.

Exoelectrogenic bacterium phylogenetically related to Citrobacter freundii, isolated from anodic biofilm of a microbial fuel cell.

An electrogenic bacterium, named Citrobacter freundii Z7, was isolated from the anodic biofilm of microbial fuel cell (MFC) inoculated with aerobic sewage sludge. Cyclic voltammetry (CV) analysis exhibited that the strain Z7 had relatively high electrochemical activity. When the strain Z7 was inoculated into MFC, the maximum power density can reach 204.5 mW/m(2) using citrate as electron donor. Series of substrates including glucose, glycerol, lactose, sucrose, and rhammose could be utilized to generate power. CV tests and the addition of anode solution as well as AQDS experiments indicated that the strain Z7 might transfer electrons indirectly via secreted mediators.

Crescimento de mudas de <i>Prunus brasiliensis</i> (Cham. & Schltdl.) D. Dietr. em substratos à base de lodo de esgoto compostado e fertilizante mineral

http://dx.doi.org/10.5902/198050987555A crescente quantidade de resíduos sólidos produzidos pelo homem gera a necessidade de seu aproveitamento. Uma oportunidade é o uso de lodo de esgoto para atendimento da demanda por insumos alternativos no meio agrícola e florestal. Dessa maneira, o objetivo do presente trabalho foi avaliar o crescimento de Prunus brasiliensis (Cham. & Schltdl.) D. Dietr. (pessegueiro-bravo) em substratos de lodo de esgoto aeróbio compostado com restos de podas de árvores trituradas, com diferentes níveis de fertilizante, e comparar o crescimento destas mudas com as produzidas em substrato comercial, normalmente utilizado em viveiros florestais. O experimento foi conduzido em casa de vegetação entre julho e outubro de 2008 e em área de rustificação entre novembro de 2008 e fevereiro de 2009, pertencentes à Companhia de Saneamento do Paraná - SANEPAR, localizado no município de Araucária, PR. Foram testados três níveis (0; 2,7 e 4 g dm-3) de fertilizante granulado de liberação lenta (N; P2O5; K2O - 15-9-12) em três tipos de substratos: substrato comercial de casca de pinus (Pinus sp.) compostada e vermiculita e, substratos de resíduos de podas de árvores trituradas compostadas com lodo de esgoto aeróbio nas proporções 2:1 e 3:1 (v:v). Aos sete meses de idade das mudas, foram mensuradas as seguintes variáveis: altura, diâmetro de colo e biomassa seca aérea (folhas e ramos). Os compostos à base de lodo de esgoto indicam crescimento para Prunus brasiliensis maiores aos obtidos pelos tratamentos utilizando substrato comercial à base de casca de Pinus compostada e vermiculita. Para ambos os compostos à base de lodo de esgoto, o nível intermediário de fertilizante (2,7 g dm-3) apresenta resultados similares ao nível superior (4 g dm-3) para a maioria das variáveis testadas.

Land application of aerobic sewage sludge does not impair methane oxidation rates of soils

The aim of this study was to measure and compare methane oxidation rates of arable and grassland soils that received 7.5tha−1y−1 of noncontaminated aerobically treated sewage sludge for ten years. Arable soils showed generally lower methane oxidation rates (from 6 to 15∗10−3h−1) than grassland soils (from 26 to 33∗103h−1). Oxidation rate constants (k) of soils amended with sewage sludge were remarkably close to their respective untreated controls, but a soil, that had received a tenfold sewage sludge application (i.e. 75tha−1y−1), showed a statistically significantly higher k-value. Laboratory addition of up to 1000mgPbg−1 soil to this soil did not cause any significant change in methane oxidation, but caused a decrease from 13.9 to 10.9×10−3h−1 in the control soil. Addition of Zn was much more toxic than Pb, with a significant decrease at 300μgg−1 soil rate and an almost complete inhibition at 1500μgg−1 soil rate. Higher resistance was evident of sewage sludge treated soil in comparison to control soil, for both biomass C and CH4 oxidation activity.

Evaluation of the suitability of using large amounts of urban wastes for degraded arid soil restoration and C fixation

The addition of organic wastes is being used increasingly as a suitable method for the recovery of degraded semiarid soils. This, in turn, can contribute to the disposal of considerable amounts of these wastes. The aim of this work was to study how the application rate and maturity of the added organic waste influence the dynamics of the organic carbon (Corg) and its fixation and partitioning in different soil particle‐size fractions. For this, a semiarid degraded soil (S) was amended, at rates of 1.1 (d1) and 3.3 (d2) g Corg 100 g soil−1, with a mixture of organic domestic waste and aerobic sewage sludge at two different maturities (fresh, FR; composted, MC) and incubated for 9 months under controlled conditions. The association of Corg with different soil particle‐size fractions was determined at the start and end of the incubation period. The organic C mineralization rates were greatest in soils amended at the larger rate. Even so, after 9 months of incubation, the soils amended at the greater rate had significantly (P < 0.05) larger values of Corg than the soils amended at the smaller rate. The greater rate of addition of composted material resulted in the greatest C fixation, although the efficiency of fixation was greatest with the smaller rate; composted material was better than fresh material and the C was associated increasingly with the finer fractions as incubation progressed.

Influence of the acid buffering capacity of different types of Technosols on the chemistry of their leachates

The preparation of tailor-made Technosols from wastes may be a novel and prospective option for the re-use of wastes and restoration of degraded areas. A two-month study with pilot columns was conducted to evaluate the influence of the acid buffering capacity of different Technosols on the chemistry of their leachates. The Technosols were made from mixtures of organic and inorganic wastes at a ratio of 56:44 (w/w). The organic components used were an anaerobic (AN) and an aerobic (AE) sewage sludge. The inorganic wastes used – referred to as “conditioners” – were Linz-Donowitz slag (LD) and foundry sand (FS). A mixture of the two conditioners at a ratio of 50:50 (w/w) was made to provide a third type of conditioner (LD + FS). Controls consisted of columns filled with organic waste only (either AN or AE sludges). Changes in pH, electrical conductivity, concentrations of major ions and dissolved organic carbon in the leachates were evaluated periodically. The main processes determining the pH of the systems were nitrification and leaching, but organic matter decomposition and carbonation may also have had an influence. Nitrification was strongly retarded in the AN sludge (attributed to the probable absence of nitrifiers in this waste after the AN wastewater treatment) and was impeded in those mixtures in which LD was used as a component (due to the liming effect). Final pH values ranged from 5.0 and 5.4 (in AE and AE + FS, respectively) to 11.1 (in AN + LD). The pH of the other mixtures finally ranged between 7 and 8. In formulating mixtures of wastes, their acid buffering capacity should be taken into account in addition to the nutrient contents and the limits of contaminants established by local regulations.

Monitoring of di-(2-ethylhexyl)phthalate, nonylphenol, nonylphenol ethoxylates, and polychlorinated biphenyls in anaerobic and aerobic sewage sludge by gas chromatography–mass spectrometry

The occurrence of di-(2-ethyhexyl)phthalate (DEHP), nonylphenol and nonyphenol mono- and diethoxylates (NPEs) and seven polychlorinated biphenyl (PCB) congeners in different types of sludge samples is reported. The analysis of these compounds was carried out by sonication-assisted extraction and analytical determination by gas chromatography coupled with a mass spectrometry detector, following a previously described method. The applicability of the method was tested by monitoring the organic pollutants in primary, secondary, mixed, and digested-dehydrated sludge samples from two wastewater treatment plants (WWTPs) based on aerobic and on anaerobic biological stabilization. The occurrence of these compounds in sewage sludge and the influence of sludge stabilization process on the further farmland application of the sludge were evaluated. DEHP and NPEs were detected in all analysed sludge samples from both WWTPs at concentration levels in the range of 22.3–601 mg kg−1 and 136–2357 mg kg−1 dm (dry matter), respectively. PCBs were detected in all types of sludge analysed from the anaerobic WWTP but was not detected in any sludge sample from the aerobic WWTP. Concentration levels of the sum of the seven PCBs congeners were up to 1.5 mg kg−1 dm. The concentration of DEHP, sum of NPEs, and sum of the seven PCB congeners were higher than the limits fixed in the third draft of the future Sludge Directive for land application of sludge in the 67%, 100%, and 11% of samples from the anaerobic WWTP and in the 83%, 92%, and 0% of samples from the aerobic WWTP, respectively.

Composting anaerobic and aerobic sewage sludges using two proportions of sawdust

Sawdust has been proven to be a good bulking agent for sludge composting; however, studies on the most suitable ratio of sludge:sawdust for sludge composting and on the influence of the sludge nature (aerobic or anaerobic) on the composting reaction rate are scarce. In this study two different sewage sludges (aerobic, AS, and anaerobic, ANS) were composted with wood sawdust (WS) as bulking agent at two different ratios (1:1 and 1:3 sludge:sawdust, v:v). Aerobic sludge piles showed significantly higher microbial activity than those of anaerobic sludge, organic matter mineralization rates being higher in the AS mixtures. The lowest thermophilic temperatures during composting were registered when the anaerobic sludge was mixed with sawdust at 1:1 ratio, suggesting the presence of substances toxic to microorganisms. This mixture also showed the lowest decreases of ammonium during composting. All this matched with the inhibitory effect on the germination of Lepidium sativum seeds of this mixture at the first stages of composting, and with its low values of microbial basal respiration. However, the ANS + WS 1:3 compost developed in a suitable way; the higher proportion of bulking agent in this mixture appeared to have a diluting effect on these toxic compounds. Both the proportions assayed allowed composting to develop adequately in the case of the aerobic sludge mixture, yielding suitable composts for agricultural use. However, the ratio 1:1 seems more suitable because it is more economical than the 1:3 ratio and has a lower dilution effect on the nutritional components of the composts. In the case of the anaerobic sludge with its high electrical conductivity and ammonium content, and likely presence of other toxic and phytotoxic substances, the 1:3 ratio is to be recommended because of the dilution effect.