Sludge Loading Research Articles

Sludge Deposit Mapping for Steam Generators

During plant operation, corrosion products from feedwater, drain and condensate system accumulate on the secondary side of steam generator in form of scaled deposits around the tubes and sludge piles on top of the tubesheet. These deposits increase potential for corrosion, affect fluid flow and reduce heat transfer efficiency of the steam generator. Using data obtained during periodical eddy current examination of steam generator tubing, information on sludge deposit location and thickness is extracted from low frequency absolute channels. Sludge deposits can be detected over the whole length of the inspected tube. Using automatic analysis deposit results are provided within 36 hours after the inspection. Results are imported in Sludge Mapping software and presented in 3D visualization of steam generator. Both PWR and VVER steam generator visualization is supported. Sludge mapping software provides visual information on sludge deposit thickness using color code and by calculation position of sludge indication, software draws the indication on the location within the steam generator where indication was found. Sludge mapping software provides information on heavy sludge loading areas of steam generator and can help with tracking of sludge build-up over time that can be used for optimizing steam generator maintenance. This paper presents INETEC’s Sludge Mapping solution, its functionality and features for visualization of sludge deposit location and thickness within the steam generator.

Primary Sludge from Dairy and Meat Processing Wastewater and Waste from Biomass Enzymatic Hydrolysis as Resources in Anaerobic Digestion and Co-Digestion Supplemented with Biodegradable Surfactants as Process Enhancers

Incorporation of various alternative resources as co-digestion substrates aids to reduce the consumption of agricultural crops for biogas production. However, the efficiency and limitations of these co-substrates is still not fully understood. Use of biomass waste remaining after enzymatic hydrolysis for high value chemical fermentation, meat processing and dairy wastewater primary sludge as co-substrates in an agricultural resource anaerobic digestion plant is tackled within this study. The results showed that anionic surfactants (<200 ppm) can be used to improve fat, oil and grease (FOG) solubility in water and, at the same time, enhance the biomethane potential of FOG-containing sludge by increasing it from 1374.5 to 1765 mLCH4/gVS for meat processing wastewater primary sludge, and from 534 to 740 mLCH4/gVS for dairy wastewater primary sludge, when agricultural digestate is used as a substrate and sludge loading is not more than 10% from the volatile solids loaded. At the same time, only 549.7 mLCH4/gVS was produced as 30-day BMP when 5% biomass hydrolysis waste was used. Biomass hydrolysis waste co-digestion with primary sludge from dairy and meat processing wastewaters has an antigenic effect, and separate substrate anaerobic digestion gave a better results, thus, showing that excessive combination of various waste resources can be inhibitory for biogas production and the appropriate substrate selection and combination is a technical challenge for the biogas industry.

Increasing of Sludge Loading Rate Induces Gel Fouling in Membrane Bioreactors Treating Real Sewage

Increasing of Sludge Loading Rate Induces Gel Fouling in Membrane Bioreactors Treating Real Sewage

Literature Review on Membrane Biofouling Occurring in MBR and Its Related Technologies for Greywater or Wastewater Treatment

This review provides an overview of membrane biofouling of the present MBR and its advanced related technologies to treat greywater and wastewater for solving the global water reuse problem. MBR is a competitive technology for wastewater treatment. HMBR, MBMBR, OMBR have shown a better membrane performance and fouling resistance than MBR. Most of the studies considered in this review were discussing factors causing membrane fouling and the fouling mechanism. In MBR and its related technologies, key microbial species in wastewater biological treatment are different from each other. There are several kinds of bacteria that could adhere to membrane surface to induce severe membrane fouling while treating domestic water such as greywater and blackwater. However, the ones causing membrane fouling are all related to filamentous species, which could cause negative effects on membrane fouling. The biomass growth in HMBR and MBMBR is both suspended and attached, while the biomass in MBR and OMBR are just suspended. Autotrophic-heterotrophic bacteria ratio and anaerobic-aerobic ratio are key parameters for microbial community and contaminants removal. Different microbial species prefer to remove different contaminants. They are affected not only by influent source but also by some environmental parameters. Nowadays, the reason for membrane fouling focuses on extracellular polymeric substances (EPS) and soluble microbial products (SMP), mostly excreting by microorganism in MBR system. EPS/SMP could help bioflocs attachment on the membrane surface in the first stage fouling and cause the accumulation on membrane surfaces and within the pore structure for cake formation and pore blocking respectively, which are the dominant fouling modes. To control EPS, it’s a good way to control factors influencing biomass growth rate and biomass performance, such as sludge loading rate, HRT and organic loading rate, and control filamentous bacteria overgrowth. To control SMP, it can be achieved by adjustment of operation parameters (SRT, HRT, DO concentration, temperature, aeration) and addition of adsorbents or coagulants. Membrane fouling, which still remains a major problem for all membrane bioreactors, still needs further discussion.

DISSEMINATING TEXTILE SLUDGE INDUCED HEAVY METAL CONTAMINATION AND ASSOCIATED HEALTH RISKS IN CONTEXT OF DHAKA, BANGLADESH

This study investigates the present scenario of heavy metal contamination in textile sludge along with their associated health problems, their removal efficiencies, alternative usage, and challenges to handling them in the perspective of Bangladesh. Textile sludge is considered to be the biggest source of hazardous elements and has the potential characteristics to create different diseases in human beings including cancer. Since the sludge is associated with a high load of heavy metals, it poses threat to both environment and human health if they remain untreated in an open environment. The methodology of this study focuses on a systematic review of data related to sludge-induced heavy metal contamination around Dhaka city. Results showed that Cr, Ni, Cu, As, Pb, and Cd were found to be dominant elements in the sludge that may pose serious threats to human health. This study also pointed out some treatment methods to remove heavy metals from textile sludge load, but however, still, there is a long way to implement them on a large scale with reduced economic, health, and environmental costs. This study concludes that further analysis is needed to remove heavy metals and encourages the emerging application of textile sludge for sustainable development in the health economic sector of the country.

Occurrence, seasonal variation, mass loading and fate of pharmaceuticals and personal care products (PPCPs) in sewage treatment plants in cities of upper Ganges bank, India

The data on the environmental concentration of pharmaceuticals and personal care products (PPCPs) is hardly investigated for major cities of the upper Ganges basin, India. In this study, fifteen PPCPs were analyzed in four sewage treatment plants (STPs) of Haridwar and Rishikesh cities located at the bank of River Ganges, India. Eleven PPCPs were detected with an average total concentration of 147.20 ng L−1 in influent, 105.06 ng L−1 in the effluent, and 87.75 ng kg−1 in sludge from STPs. The average concentration (ng L−1) of individual PPCPs was recorded highest for Caffeine (695.86) followed by, ibuprofen (387.53), acetaminophen (152.81), triclosan (98.93), tetracycline (75.39), ketoprofen (69.47), DEET (46.59), salicylic acid (45.02), erythromycin (32.98), ciprofloxacin (20.16) and metoprolol (13.61) at STPs inlets. Results suggested a spatial and seasonal variation in PPCPs loads in influent, effluent and sludge in STPs studied. The overall PPCPs removal efficiencies of STPs were recorded in the ranges of −154–63%. The total mass load of PPCPs studied ranged between 0.02 and 448.64 mg/d/1000 population. This study concluded that STPs are an active source of PPCPs in the River Ganges, which may pose a grave threat to the health of the river ecosystem and human population.

Study on the inhibition of sludge bulking in A2/O coupled system at low temperature by segmented inlet water mode

ABSTRACT This study aimed to propose a segmented influent method to inhibit sludge bulking. The sludge bulking phenomenon was observed in a A2/O coupled system treating municipal wastewater under low temperature (15 ± 0.5)°C. Adopting the segmented inlet water process, the distribution ratio of the inlet flow in the anaerobic zone and the aerobic zone were 2:1 and 1:1, the sludge bulking phenomenon was suppressed. The sludge loading rate (F/M) analysis showed that the F/M of the anaerobic zone with single-point inflow was only 0.15 kg COD·(kg MLSS·d)−1, which was prone to induce sludge bulking. However, the F/M concentration gradient of the system under segmented inlet water conditions was obvious, which could inhibit the sludge bulking caused by low F/M. The effluent removal results showed that the system had high removal rates of COD, NH4 +-N, TN, and TP at a flow distribution ratio of 2:1, with average removal rates of 88.85% ± 2.94%, 91.26% ± 6.68%, 76.60% ± 5.60%, and 96.80% ± 2.17%, respectively. This study confirmed that the segment inlet method inhibited sludge bulking, while the flow distribution ratio of 2:1 also ensured efficient pollutant removal capacity of the system.

SARS-CoV-2 and other pathogens in municipal wastewater, landfill leachate, and solid waste: A review about virus surveillance, infectivity, and inactivation

This review discusses the techniques available for detecting and inactivating of pathogens in municipal wastewater, landfill leachate, and solid waste. In view of the current COVID-19 pandemic, SARS-CoV-2 is being given special attention, with a thorough examination of all possible transmission pathways linked to the selected waste matrices. Despite the lack of works focused on landfill leachate, a systematic review method, based on cluster analysis, allows to analyze the available papers devoted to sewage sludge and wastewater, allowing to focalize the work on technologies able to detect and treat pathogens. In this work, great attention is also devoted to infectivity and transmission mechanisms of SARS-CoV-2. Moreover, the literature analysis shows that sewage sludge and landfill leachate seem to have a remote chance to act as a virus transmission route (pollution-to-human transmission) due to improper collection and treatment of municipal wastewater and solid waste. However due to the incertitude about virus infectivity, these possibilities cannot be excluded and need further investigation. As a conclusion, this paper shows that additional research is required not only on the coronavirus-specific disinfection, but also the regular surveillance or monitoring of viral loads in sewage sludge, wastewater, and landfill leachate. The disinfection strategies need to be optimized in terms of dosage and potential adverse impacts like antimicrobial resistance, among many other factors. Finally, the presence of SARS-CoV-2 and other pathogenic microorganisms in sewage sludge, wastewater, and landfill leachate can hamper the possibility to ensure safe water and public health in economically marginalized countries and hinder the realization of the United Nations' sustainable development goals (SDGs).

Nitrous oxide emission from full-scale municipal aerobic granular sludge

The nitrous oxides emission was measured over 7 months in the full-scale aerobic granular sludge plant in Dinxperlo, the Netherlands. Nitrous oxide concentrations were measured in the bulk liquid and the off-gas of the Nereda® reactor. Combined with the batch wise operation of the reactor, this gave a high information density and a better insight into N2O emission in general. The average emission factor was 0.33% based on the total nitrogen concentration in the influent. The yearly average emission factor was estimated to be between 0.25% and 0.30%. The average emission factor is comparable to continuous activated sludge plants, using flocculent sludge, and it is low compared to other sequencing batch systems. The variability in the emission factor increased when the reactor temperature was below 14 °C, showing higher emission factors during the winter period. A change in the process control in the winter period reduced the variability, reducing the emission factors to a level comparable to the summer period. Different process control might be necessary at high and low temperatures to obtain a consistently low nitrous oxide emission. Rainy weather conditions lowered the emission factor, also in the dry weather flow batches following the rainy weather batches. This was attributed to the first flush from the sewer at the start of rainy weather conditions, resulting in a temporarily increased sludge loading.

Full-scale vacuum degassing of activated sludge – A case study over 2-years of operation

The effect of vacuum degassing of activated sludge on biological wastewater treatment over a total period of 4 years was examined. In half of the period studied, the treatment system was classic, where wastewater, after a biological reactor with nitrification-denitrification and biological dephosphatation, was directed to secondary settlers. In the second period, the treatment system was modified so that the sludge from the biological reactor was directed to the vacuum degasser, followed by the secondary settling tanks (vacuum degassing activated sludge system; VDAS). As a result of using the VDAS, a significant improvement in sludge settling ability was recorded, which allowed the plant operators to increase the amount of biomass in the biological reactor. Differences in the quantity and quality of the wastewater influent were found during both periods. However, the quality of treated wastewater and the efficiencies of nutrient removal and removal rates were usually better in the system with vacuum degassing. An improvement was achieved not only for the elimination of the different forms of nitrogen, which by definition were to be removed as ammonia and gaseous nitrogen, but also for organic compounds expressed as COD and phosphorus. As a result of the lower sludge loading, sludge production expressed in g SS per g COD removed was significantly lower (20 %) with the VDAS.

Effect of Sewage Sludge Compost Usage on Corn and Faba Bean Growth, Carbon and Nitrogen Forms in Plants and Soil

Sewage sludge is an effective fertilizer in many soil types. When applied as an amendment, sludge introduces, in addition to organic matter, plant nutrients into the soil. When applied for cropland as a fertilizer, the mass loading of sewage sludge is customarily determined by inputs of N and/or P required to support optimal plant growth and a successful harvest. This study aims to examine the changes in organic matter contents and nitrogen forms in sludge-amended soils, as well as the growth of corn and faba bean plants. The main results indicated that there were higher responses to the corn and faba bean yields when sludge was added. Levels of organic carbon in soil were higher after maize harvest and decreased significantly after harvesting of beans, and were higher in sludge amended soils than unmodified soils, indicating the residual effect of sludge in soil. NO3−-N concentrations were generally higher in the soil after maize harvest than during the plant growth period, but this trend was not apparent in bean soil. The amounts of NH4+-N were close in the soil during the growth period or after the maize harvest, while they were higher in the soil after the bean harvest than they were during the growth period. Total nitrogen amounts were statistically higher in the soil during the growth period than those collected after the corn harvest, while they were approximately close in the bean soil. The total nitrogen amount in corn and bean leaves increased significantly in plants grown on modified sludge soil. There were no significant differences in the total nitrogen levels of the maize and beans planted on the treated soil.

Adsorption Purification of Phenol-Containing Wastewater from Oil Refineries

In this paper, the possibility of adsorptive purification of industrial wastewater from oil refineries from phenols by industrial wastes is considered. It is proposed to use carbonate sludge for water treatment of natural water as one of the most accessible materials, which is a large-tonnage waste of energy, for sewage treatment. Phenols in wastewater are hazardous toxic substances. In legal order, sanitary and hygienic standards establish the maximum permissible concentration (MPC) of phenol in water, which is 0.001 mg / dm3. Industrial wastewater containing phenols has a number of difficulties faced by technologists when selecting a technological scheme for cleaning these pollutants, or their disposal. The adsorption isotherm, the kinetic curve of phenol adsorption, the output curve of phenol adsorption under dynamic conditions were constructed, the dynamic sorption capacity of the “GrSM-1” sorption material was determined, which prove the effectiveness of using carbonate sludge as an adsorbent. A flowchart for the purification of wastewater from phenols is proposed, the calculation of the adsorption filter with the loading of carbonate sludge in the scheme for the purification of wastewater from phenols is performed.

Explore the sludge stabilization process in sludge drying bed by modeling study from mesocosm experiments

Sludge drying bed (SDB) treatment is a valuable alternative to conventional sludge treatment methods. However, changes in sludge hydrotexture during dewatering present a barrier for direct modeling of the SDB process. This study proposes a modeling strategy to simulate the sludge stabilization process in SDB treatment by separating sludge dewatering and sludge solids stabilization into independent processes. Two cell decay theories widely used by activated sludge models (ASM), death-regeneration concept and endogenous respiration theory, are compared to describe the biokinetic processes of sludge digestion. Both cell decay theories are found to adequately describe effluent total COD, NH4–N, NO3–N, and sludge layer composition, but have pronounced differences in describing effluent COD compositions. Results show that natural aeration does not maintain adequate aerobic/anoxic sludge digestion within the sludge layer to fully nitrify NH4–N released by cell decay. Results also indicate that the kinetics of sludge digestion are adaptable over time, indicating the need to adopt lumped values for biokinetic simulations. While lowered sludge dewatering rates (outflow) can increase biodegradable COD for cell metabolism, increased sludge loading rates (inflow) lead to higher effluent COD and NH4–N concentrations. Contrary to conventional judgement, this study demonstrates the merit of sludge layer formation to reduce leaching loss of biodegradable COD. Overall, the proposed modeling strategy is proven capable of simulating deposited sludge digestion processes in an SDB.

Remediation of Cu(II) and its adsorption mechanism in aqueous system by novel magnetic biochar derived from co-pyrolysis of sewage sludge and biomass

The novel magnetic biochar (MBC), derived from co-pyrolysis of sewage sludge and biomass loading nanosized iron oxide particles, was used as an environmentally friendly adsorbent. The loading of magnetic particles was in favor of increasing the adsorption capacity and separation from aqueous system for biochar (BC). The physical/chemical characteristics of MBC were revealed by elemental analysis, VSM, SEM-EDS, XRD, FTIR, zeta potential, and batch adsorption-desorption experiments. The nanosized γ-Fe2O3 particles grown on the surface of biochar showed ferromagnetic property. For the remediation of Cu(II) contamination, MBC-5 showed remarkable adsorption capacity of 67.68mg/g, and presented a wide pH range of 3.0-6.0. The Langmuir isothermal and pseudo-second-order model could describe adsorption process well. The adsorption mechanism of Cu(II) involved physical adsorption, ion exchange, and electrostatic surface complexation on the surface of MBCs. In the desorption experiments, MBC-5 holds the adsorption efficiency of 81.09% after fifth recycle still, which illustrated a remarkable performance of cyclic utilization by the solid waste of sewage sludge and biomass.

Integrated Community Onsite Sanitation System for Close Loop Faecal Management

The achievement of safe sanitation access is about 7.42 % in 2018, that indicates high risk of faecal contamination from unmanaged onsite sanitation system and has not been fully supported by faecal sludge service. This paper described the study of characteristic of onsite sanitation and faecal sludge for review potential of integrating of sludge handling with improved communal wastewater treatment. Faecal sludge quality were observed from pit latrines and communal septic tank, then analysis method apply cluster analysis and through close loop approach consider treatment and disposal for resource recovery. The pilot model of combined treatment of wastewater and faecal sludge was developed by anaerobic digester with a maximum 5% sludge loading, then further treatment by biofilter system and sludge drying bed. The quality of treated water can meet COD and nitrogen as effluent standard, biosolid reached a stabilization index of 40 - 60%, water content of 50-60% and less Coliform bacteria content. The characteristics of faecal sludge strength to be handled will be influenced bytechnical aspect or environment. The integrated system could be applied to manage wastewater and faecal sludge and its by products, mainly for rural area has not been catered by faecal sludge service.

Assessment of sludge management strategies in wastewater treatment systems using a plant-wide approach

The objective of this paper is to use plant-wide modeling to assess the net impacts of varying sludge management strategies. Special emphasis is placed on effluent quality, operational cost and potential resource recovery (energy, nutrients). The study is particularly focused on a centralized bio-solids beneficiation facility (BBF), which enables larger, more capital intensive sludge management strategies. Potential barriers include the ability to process reject streams from multiple donor plants in the host plant. Cape Flats (CF) wastewater treatment works (WWTW) (Cape Town, South Africa) was used as a relevant test case since it is currently assessing to process sludge cake from three nearby facilities (Athlone, Mitchells Plain and Wildevoelvlei). A plant-wide model based on the Benchmark Simulation Model no 2 (BSM2) extended with phosphorus transformations was adapted to the CF design / operational conditions. Flow diagram and model parameters were adjusted to reproduce the influent, effluent and process characteristics. Historical data between January 2014 and December 2019 was used to compare full-scale measurements and predictions. Next, different process intensification / mitigation technologies were evaluated using multiple criteria. Simulation values for COD, TSS, VSS/TSS ratio, TN, TP, NH4+/NH3, HxPO43-x, NOx alkalinity and pH fall within the interquartile ranges of measured data. The effects of the 2017 severe drought on influent variations and biological phosphorus removal are successfully reproduced for the entire period with dynamic simulations. Indeed, 80% of all dynamically simulated values are included within the plant measurement uncertainty ranges. Sludge management analysis reveals that flow diagrams with thermal hydrolysis pre-treatment (THP) result in a better energy balance in spite of having higher heat demands. The flow diagram with THP is able to i) increase biodegradability/solubility, ii) handle higher sludge loads, iii) change methanogenic microbial population and iv) generate lower solids volumes to dispose by improving sludge dewaterability. The study also reveals the importance of including struvite precipitation and harvesting (SPH) technology, and the effect that pH in the AD and the use of chemicals (NaOH, MgO) may have on phosphorus recovery. Model-based results indicate that the current aerobic volume in the water line (if properly aerated) would be able to handle the returns from the sludge line and the contribution of a granular partial nitritation/Anammox (PN/ANX) reactor on the overall nitrogen removal would be marginal. However autotrophic N denitrification generates a much lower sludge production and therefore increases AD treatment capacity. The study shows for the very first time in Africa how the use of a (calibrated) plant-wide model could assist water utilities to decide between competing plant layouts when upgrading a WWTW.

Performance of sludge degradation, mineralization and electro-energy harvesting in a sludge treatment electro-wetland: Insight into the sludge loading rate

In order to enhance the performance of sludge treatment wetland (STW), microbial electrochemical system (MES) was introduced into a conventional sludge treatment wetland. A sludge treatment electro-wetland (STEW) was specially developed and investigated for its performance in sludge treatment and bioenergy production. The study investigated the performance of the STEW in terms of sludge organic degradability, sludge stabilization and the capability of bioelectricity generation at different sludge loading rates (SLR) of 50, 95, 125 kg TS/year·m2. The quality of the final residual sludge product for agricultural reuse was also characterized. Results revealed that the highest power output (0.790 V of voltage and 0.229 W/m3 of power density) of STEW was achieved with the highest SLR (125 kg TS/year·m2). The best sludge treatment efficiency (7.07 % of dewatering rate, 40.43 % of volatile solid (VS) removal and 80.95 % of total chemical oxygen demand (TCOD) removal) of STEW were achieved with proper SLR of 95 kg TS/year·m2. After resting period, the nutrient concentrations in the residual sludge were quite low (total nitrogen of 2.86 % total solid (TS) and total phosphorous of 0.93 %TS), and heavy metals and faecal bacteria (Salmonella spp. and Escherichia coli) concentrations remained within the current legal limits for land application of the sludge. Experiments showed that the STEW system demonstrated a satisfactory efficiency for sludge dewatering and mineralization in spite of the high sludge loading rate and recycled the bioenergy simultaneously. The results also illustrated that the STEW was a promising technology for simultaneous sludge treatment and energy recovery.

Performance of sludge drying reed beds for the leachate purification: Effects of sludge loading frequencies and plant species

Sludge drying reed beds (SDRBs), as a natural biological technology, have positive effects on surplus sludge treatment. However, few studies focus on the sludge leachate purification in the SDRBs regarding the wetland plant species and sludge loading rates. In this study, four SDRBs planted with two wetland plant species (Phragmites australis, Typha angustifolia) were investigated for leachate purification under six sludge loading volumes and feeding frequencies (9L/3d, 6L/3d, 4L/d, 3L/d, 2.5L/d, and 2L/d). Results showed that the lowest Escherichia coli content of 630 number/mL was determined in the P. australis SDRBs, with 86.2–92.8% lower than those in the T. angustifolia controls. However, similar removal efficiencies of chemical oxygen demand (COD), total nitrogen (TN), ammonium (NH4+), total phosphorus (TP), and heavy metals were obtained in the SDRBs with both plant species. Moreover, the optimum sludge loading volume and feeding frequency of 3L/d was determined in the P. australis system, with pollutants (COD, TN, NH4+, and TP) mass removal efficiencies in the leachate being over 94.9%. In addition, the principal component analysis indicated that water loss and oxidation-reduction potential had positive influences on pollutant removal in the planted SDRBs. Overall, the best leachate removal efficiency was obtained in the P. australis SDRBs under the sludge loading volume and feeding frequency of 3L/d.

Can additional air supply enhance decomposition processes in sludge treatment reed beds?

This work was designed to investigate the influence of artificial aeration on the sludge decomposition process in mesocosm sludge treatment reed beds (STRBs). In addition to the typical STRB design, where ventilation is mainly provided by a drainage pipe, passive aeration via a "chimney" and active aeration via a blower were introduced. During the entire observation period of 1.5 years, O2 concentrations in the upper part of the filter were significantly higher in the artificially aerated beds. To determine decomposition rates, a study with decomposition bags, measurements of CO2 emissions from the STRB and isotopic partitioning of CO2 emissions were performed. The results indicate an accelerated sludge degradation process in both active and passive beds. However, this effect was limited to part of the season and could not be demonstrated by episodic measurements of CO2 efflux. The CO2 efflux showed a seasonal pattern. Average CO2 efflux was below 3.0 μmol m−2 s−1 in the winter months and reached 43 μmol m−2 s−1 in the warmer months. The low sludge load and drought period in summer 2018 resulted in an extremely low CO2 efflux in August 2018. Isotopic analyses revealed changes in decomposition dynamics for certain parts of the season, differences in contributions of sludge and plant derived CO2 to total CO2 emissions from differently aerated beds. Overall, passive aeration proved to be similarly efficient as active aeration and could therefore be considered for application in a full-scale system.

Monitoring the variation of soil quality with sewage sludge application rates in absence of rhizosphere effect

Agricultural soils in semi-arid regions have frequently been degraded due to adverse climatic conditions, organic matter depletion, and poor farming practices. To enhance soil quality, this study examines the reuse of sewage sludge (SS) as an available source of organic matter in a typical Mediterranean sandy-loam soil. Accordingly, we studied the cumulative effect of two annual applications of 40, 80 and 120 tons of sludge per ha on soil quality in absence of vegetation. The dose-dependent improvement of organic matter content was the most significant event that reflected sludge application rates, and consequently influenced other soil properties. Accordingly, soil structural stability increased by 13.3%, 28.8% and 59.4% for treatments SS-40, SS-80 and SS-120 respectively as compared to unamended control. Structural stability improvement was also confirmed by the dose-dependent variation of other edaphic factors including calcium content, the microbial quotient as well as Welt and C:N ratios. These parameters are involved in cementing soil aggregates by cation bridging, the formation of microbial mucilage, and clay-humic complexes. Soil magnetic susceptibility (SMS) was measured in situ as a possible rapid tool to evaluate soil condition. SMS showed significant correlation with sludge dose and stability amelioration testifying to the aggregation role that can play Al2O3 and particularly Fe2O3 minerals added by the hematite-rich sludge. Besides, analytical results and field observations revealed no trends of soil salinization or acidification by excessive sludge amounts. By avoiding the rhizosphere effect, outcomes could reflect the resilience and intrinsic capacity of the soil to cope with excessive sludge loads.