Doses Of Sludge Research Articles

Stabilize Kent Çamuru ve Jips Uygulanmış Tuzlu Sodyumlu Topraklarda Arıtılmış Atıksuyun Islanma-Kuruma Döngülerinin Hidrolik İletkenliğe Etkisi

This study was carried out to determine changes in hydraulic conductivity of a saline sodic soil subjected to different wetting-drying cycles and treated wastewater after stabilized sludge sewage and gypsum application. In a factorial experimental design, the study was conducted with three replication by using 3 treatment sewage sludge doses (50, 100 and 150 t ha-1), 3 wetting-drying cycles (0, 7 and 14 days) and 2 different water types (fresh water and treated wastewater). The hydraulic conductivity values of the saturated saline-sodic soils were measured at 2, 12 and 24 hour intervals by using a constant level ICW laboratory permemeter. The increase in the applied treatment sludge dose significantly affected the hydraulic conductivity value of the soils and significantly depending on the measurement range. It was determined that irrigation waters have different properties caused a significant change in the hydraulic conductivity value of the soils, with measurements made at intervals of 2 and 24 hours. Depending on the measurement interval, it was determined that the increase in sewage sludge doses caused significant effect and very important on the soil hydraulic conductivity. The study results indicated that the application of gypsum and stabilized waste sludge to the soil cause an increase in hydraulic conductivity values. The study results indicated that the application of gypsum and sewage sludge to the soil cause an increase in hydraulic conductivity values of saline sodic soils. The study results also showed that treated wastewater containing low amount of suspended solids can be used safely for irrigation on the land have saline-sodic soils. The study result also indicated that by applying solid and liquid wastes obtained from treatment units to the saline-sodic soils can be significant contribution in terms of waste management and environmental protection.

Turbidity removal from synthetic turbid water using coagulant recovered from water treatment sludge: A potential method to recycle and conserve aluminium

The water treatment sludge (WTS), a byproduct of coagulation-flocculation-sedimentation process in drinking water treatment contains significant concentration of aluminium (Al). The WTS is mostly disposed on land or in sewers without treatment. This practice not only poses a serious threat to environment but also results in permanent loss of metals like Al. In order to prevent the adverse effects, studies are being conducted to find alternative use of WTS, and one of the focus areas is to recover coagulant (a solution containing Al) from the WTS by leaching with high strength acid and reuse it as coagulant for treating industrial wastewater. However, the use of the recovered coagulant (RC) for treatment of drinking water is not yet reported. The main concern being the presence of other metals in the RC along with Al. The RC if added to remove turbidity from drinking water may result in elevated concentration of metals in treated water which is objectionable.In order to address this issue, the present study proposes a method to recover coagulant from WTS, and reuse it safely as a coagulant in drinking water treatment without hampering the treated water quality. Specifically, the study used low strength nitric acid to leach Al from WTS. The leaching studies were performed using a Box-Behnken Design (BBD) with three factors: acid strength (M), reaction time and sludge dose. Each factor was set at three levels: acid strength (0.025 M, 0.1625 M and 0.3 M), reaction time (30 min, 75 min and 120 min) and sludge dose (10 g/l, 12.5 g/l and 15 g/l). As Al is the main element responsible for coagulation, leaching efficiency of Al (AlLE) was fixed as the response variable. Performance of the RC in removing turbidity was evaluated on synthetic turbid water and was compared with turbidity removal obtained with 1% (w/v) solution of commercial alum.The results indicate that a maximum concentration of 432 mg/l of Al could be achieved in the RC with optimum factor settings (obtained from statistical analysis) of 0.25 M acid strength, 11.17 g/l sludge dose and 120 min reaction time. The addition of 1 ml/l of the RC in synthetic turbid water resulted in 74% removal of turbidity (with effluent turbidity concentration < 20 NTU) which was similar to removal efficiency obtained with addition of 0.6 ml/l of 1% (w/v) commercial alum solution.The environmental significance of the study is that, it shows that the RC has potential to be used as a coagulant in drinking water treatment. This will not only prevent the permanent loss of Al due to disposing the WTS in landfills but also minimize the requirement of fresh Al salts for coagulation thus resulting in conservation of a natural resource.

Evaluating the Efficiency of Microwaved Sludge in the Removal of 2, 4-Dinitrophenol from Aqueous Solutions: Equilibrium and Kinetics Studies

Introduction: Nitrophenol compounds are toxic compounds found in industrial wastewaters. 2,4-dinitrophenol is the most dangerous compound among phenolic compounds. The aim of this study was to evaluate the removal of 2,4-DNP from wastewater by microwaved dried sludge adsorbent.&#x0D; Materials and Methods: The results of 2,4-DNP removal were discontinuously obtained by the high performance liquid chromatography (HPLC) at a wavelength of 360 nm with various effective factors, such as contact time, pH, initial concentration of 2,4-DNP, and microwaved sludge dose. Finally, the results were analyzed using the kinetics and isotherm models. The equilibrium time was obtained 120 min. The maximum removal rate was obtained at pH 7.&#x0D; Results: The findings indicated that the removal efficiency increased by increasing the adsorbent dose and decreasing the 2,4-DNP concentration. It was revealed that the removal of 2,4-DNP by microwaved sludge was 86%. The correlation coefficient value of linear and non-linear regression showed that kinetic studies follow the pseudo-second order model and isotherm studies follow the Freundlich isotherm model. The adsorption method relied entirely on pH and affected the adsorbent area attributes, ionization rate, and Delete percentage. When the pH was high, there was competition for the adsorption sites between hydroxide ions (OH) and 2,4-DNP molecules. At first, the adsorption process was high speed and gradually reached a stable level, because after a while, the adsorption sites become saturated.&#x0D; Conclusion: As the absorbent dose increases, the efficiency of the adsorption process increases, because larger amounts of adsorbent cause higher adsorption places.

Climate Change and Silvopasture: The Potential of the Tree and Weather to Modify Soil Carbon Balance

Silvopastoral systems play an important role in climate change mitigation, considering their effect on soil carbon sequestration. In silvopastoral systems, sewage sludge can be used as fertiliser, which is promoted by the Circular Economy Package of the European Commission. This study evaluates the soil chemical properties (pH, carbon), tree growth (top height, canopy cover), and their interactions from 1998 to 2012 in a Pinus radiata D. Don silvopastoral system in northwest Spain. Nine fertilisation treatments were applied: three doses of sewage sludge (160, 320, and 480 kg total N ha−1) or no fertilisation, all with or without liming, and mineral fertiliser with no liming. Soil pH decreased over time due to cations extraction by trees and pine needles deposited in the understory. Tree growth increased light interception, decreasing soil carbon incorporation. The interannual variation of carbon also depended on weather conditions. Initially, fertilisation increased soil pH and carbon, but without compensating cations extraction over time. Therefore, it is advisable to apply amendments in the middle years of the plantation. Tree management is also needed to decrease competitiveness and enhance carbon incorporation. Moreover, control plots should be linked to the next CAP 2023–2027 eco-schemes accounting for soil carbon levels.

Pengaruh Dosis Sludge dan Pupuk MKP Terhadap Pertumbuhan dan Hasil Tanaman Lobak Pada Tanah Gambut

Effect of Sludge and MKP Fertilizer Doses on Radish Plant Growth and Yield on Peatsoils. The development of the horseraean plant in peat land is faced with the number of obstacles in low, low ph, inlet, high c-organics with varying degrees of maturity and low - base base soils that are causing the growth and development of plants to be stunted, That would make a sludge delivery out of MKP fertilizer. The study is aimed at seeing the effect that doses of sludge given and MKP fertilizers have on the growth and growth of turnip plants in the peat soil. The research was conducted on KEP’S Agroland at Jl. Sui Raya Dalam, Pontianak City from March 12th to April 31th 2021. The study uses field experimental methods with comprehensive design prosecutions consisting of 2 treatment factors. The first factor is the sludge with the levels of treatment and the second factor is MKP fertilizer. Each factor is repeated 3 times, each denteronomy consisting of 4 plant samples, making the total number of plants 108. The first factor is the dose of sludge and the second is the dose of MKP fertilizer which has three levels of treatment each. Every treatment is repeated three times. A sludge dose consists of 0, 200, 320 g/polybag (equivalent to 0, 25, 40 tons /ha), while the MKP dosage consists of 0, 2, 4 g/polybag (equivalent to 0, 150, 250 kg/ha). The observed variabel is that of the number of leaves, the fresh weight of the plant, the fresh weight of the bulbous, the length of the tuber diameter and the plant’s dry weight. The research suggests and that there is no interaction between her and MKP for all variables. The treatment of sludge and MKP involving a dose of sludge 25 tons/hectares is equivalent to 200 g/polybag and MKP 150 kg/hectares is equivalent to 2 g/polybag is an efficient gift.

Symbiotic treatment of ammonia-nitrogen wastewater by algae and activated sludge: effects of algae and sludge inoculation rates

ABSTRACT A symbiotic microalgal-bacterial system may be an optional technology for wastewater treatment. It was composed of microalgae and activated sludge and established in the SBR to explore the effect of different dosing ratios of algae and sludge on the removal of nitrogen and phosphorus from simulated wastewater containing ammonium. It can be seen from the result that varied algae–sludge dosing ratios had a higher removal effect on COD removal, but the difference was not significant. The algal-bacterial symbiosis system had a 100% removal rate for ammonium removal on the 8th day. Relatively speaking, the removal of nutrients and related mechanisms vary with environmental conditions (inoculation rate). In general, when the additive ratio was 5:1 (algae: AS), the removal rate of TN and TP was the highest, reaching 53.85% and 85.13% in the shortest time (14 days), among them, the removal rate of ammonium and COD was 100%, and the reduction rates of Nitrite nitrogen and Nitrate nitrogen were 362.99% and 73.42%, respectively. In addition, 16S rDNA gene analysis results demonstrated that the microbial community in the reactor with algal sludge inoculation ratio of 5:1 had differences in three stages of the initial reaction, the middle reaction and the end of the reaction. Comamonadaceae, Flavobacterium, Paenarthrobacter, Mesorhizobium, Nitrobacter were enriched during the reaction operation.

Influence of chromium and sodium on development, physiology, and anatomy of Conilon coffee seedlings.

Some components found in the composition of the tannery sludge are nutrients for the plants; it can be considered an alternative source of fertilization as they have favorable agronomic characteristics. However, it is reported in some studies that the presence of chromium and sodium in this residue causes physiological and anatomical disturbances that inhibit the development of the plants. The objective of this study was to evaluate the influence of chromium and sodium on the physiology, anatomy, and development of Conilon coffee seedlings grown on substrates produced with tannery sludge and equivalent doses of chromium and sodium. The experiment was carried out in nursery using randomized block design, containing 5 treatments and 7 repetitions. The treatments consisted of the application of a 40% tannery sludge dose and equivalent doses of chromium and sodium mixed with a conventional substrate. Notably, the presence of sodium in the substrate caused greater damage to the plants, negatively influencing the physiology, anatomy, and, consequently, development of the plants, while the presence of chromium suggests that it does not influence much the evaluated characteristics. The treatment with tannery sludge, on the other hand, despite containing the same chromium and sodium contents, revealed a more pronounced negative influence on the physiology, anatomy, and development patterns of the seedlings. This shows that sodium and chromium alone are not the only factors responsible for the lowest growth indicators studied.

Enhanced Cr(VI) reduction in biocathode microbial electrolysis cell using Fenton-derived ferric sludge

Hexavalent chromium (Cr(VI)) is one of the major concerns for water environment and human health due to its high toxicicity, while ferric sludge produced from Fenton processes is also a tough nut to crack. In this study, the synergetic impact of ferric sludge derived from the Fenton process on the bioreduction of Cr(VI) in biocathode microbial electrolysis cell was investigated for the first time. As a result, Cr(VI) reduction efficiency at biocathode increased by 1.1–2.6 times with 50 mg/L ferric sludge under different operation conditions. Besides, the Cr(VI) reduction enhancement decreased with the increase of pH and initial Cr(VI) concentration or increased with the increase of ferric sludge dosage. Correspondingly, relatively higher power density (1.027 W/m3 with 100 mg/L ferric sludge while 0.827 W/m3 for control) and lower activation energy and resistance were also observed. Besides, the presence of ferric sludge increased biomass protein (1.7 times higher with 100 mg/L ferric sludge) and cytochrome c (1.4 times higher with 100 mg/L ferric sludge). The evolution of microbial community structure for a higher abundance of Cr(VI) and Fe(III)-reducing microorganisms were exhibited, implying the enhancement of Cr(VI) reduction was due to the formation of Fe(II) from the reduction of ferric sludge. These findings provide insights and theoretical support for developing a viable biotechnology platform to realize waste treatment using waste.

ПРАКТИЧЕСКОЕ ПРИМЕНЕНИЕ ОСАДКА СТОЧНЫХ БЫТОВЫХ ВОД НА ПРИМЕРЕ ВОЗДЕЛЫВАНИЯ САФЛОРА КРАСИЛЬНОГО

Purpose: to study the main indicators of sewage sludge in the town Volzhsky and the possibility of its practical application on farmlands. Materials and methods. The object of research is sewage sludge from waste treatment plants of the town Volzhsky, Volgograd region. The chemical composition of sewage sludge was studied under laboratory conditions. Field experiments were laid in accordance with generally accepted methods. Results and discussions. Experimental studies have determined that high concentration of nitrogen (3.3 %), phosphorus (4.27 %), potassium (0.31 %) was noted in the sewage sludge, the organic matter content was 32.0 %. The water extract analysis data indicate a high concentration of calcium cations (33.25 mmol/100 g of soil) and magnesium (14.25 mmol/100 g of soil), as well as sulfate anions (5.80 mmol/100 g of soil). Heavy metals are present in the sludge, but their amount is much lower than the maximum permissible concentration. When testing sewage sludge in the field, it was found that, a decrease in soil bulk density to 1.10 t/m cub. is achieved in combination with chisel tillage. (dose of sewage sludge is 5 t/ha) and up to 1.07 t/m cub. (dose of sewage sludge is 10 t/ha). The data analysis on the safflower yield shows that the processed sewage sludge application increases the yield of safflower seeds in all years of research. The variant of combined action of sludge at a dose of 10 t/ha and chisel tillage turned out to be preferable, which made it possible to increase the safflower yield to 1.51 t/ha on average over the years of research. Conclusions: under the arid conditions of Volgograd region, it is advisable to use deep chisel tillage in combination with sewage sludge at a dose of 10 t/ha in order to increase the productivity of safflower and reduce the bulk density.

Heavy metals in the "soil-plant" system when using sewage sludge

This article presents the results of studies of the content of chemical elements in the "soil-plant" system when various doses of sewage sludge are introduced into the soil. The degree of soil contamination with heavy metals was assessed, chemical concentration coefficients were calculated, the soils were classified by permissible pollution, with the possibility of use for any agricultural crops, and theoretical calculations of the maximum possible one-time dose of sewage sludge to the soil according to the limiting indicator of the concentration of heavy metals were given. The concentration of heavy metals in the carrot and table beet biomass has been studied. It was found that the use of sewage sludge as fertilizer affected the content of dry matter, ash and chemical elements in carrot, the dynamics of migration of heavy metals into plant biomass correlates with an increase in their concentration in the soil, and the accumulation of chemical elements in the photosynthetic part is more intense.

Risk Assessment of Contaminants in Sewage Sludge Applied on Norwegian Soils

Risk Assessment of Contaminants in Sewage Sludge Applied on Norwegian Soils

Analysis of migration of pathogenic drug-resistant bacteria to soils and groundwater after fertilization with sewage sludge.

The paper discusses the analysis of the effect of using sewage sludge for fertilization on the level of soil and groundwater contamination with drug-resistant bacteria. Other sanitary contaminants in these environments were also analysed. Composted sewage sludge was introduced into the sandy soil over a period of 6 months. The examinations were conducted under conditions of a lysimetric experiment with the possibility of collecting soil leachates (in natural conditions). The following doses of sewage sludge were used: 0, 10, 20, 30 and 40 t/ha calculated per experimental object containing 10 kg of sandy soil. The research were carried out within the time frame of one year. Dactylis glomerata grass was grown on the fertilized soils. In soils and leachates from soils (which may have polluted groundwater) collected from fertilized experimental objects, the sanitary condition and quantity of drug-resistant bacteria (mainly from the families Enterobacteriaceae and Enterococcus) were analysed one year after fertilization. Their drug resistance to selected antibiotics was also analysed based on current recommendations. The study showed that fertilization with sewage sludge (even after stabilization and hygienization) results in contamination of soil and infiltrating waters with many species of drug-resistant pathogenic bacteria. The lowest level of contamination of soil and water environment was found after the application of sewage sludge at a dose of 10 t/ha. The isolated drug-resistant strains of intestinal bacteria were less sensitive to older generations of antibiotics including cefazolin, ampicillin, and co-amoxiclav.

Effect of metal content from sewage sludge on the growth of Orthosiphon stamineus

Municipal sewage sludge can be used as fertilizer as it contains a lot of nutrients. By focusing on copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) contents in municipal sewage sludge from primary oxidation ponds and in plant tissue sections, this study attempts to explain the relationship between plant yield and metal concentrations. Raw sludge samples were collected, air-dried, and ground to powder form at the beginning of fieldwork. The metal concentrations of sewage sludge were copper 6.9 mg/kg; iron 330.2 mg/kg manganese 6.7 mg/kg and zinc 9.1 mg/kg. Orthosiphon stamineus (cat’s whiskers) was selected and the plant received a different quantity of sludge application ranging from 1 g to 4 g per plant weekly for six consecutive weeks while observations were done for eleven weeks. Physical parameters such as shrub width and height were monitored to determine plant growth. Through this study, the determined optimum sewage sludge dosage for cats’ whiskers was between 2 to 3 g per plant.

Precipitation of (Mg/Fe-CTAB) - Layered double hydroxide nanoparticles onto sewage sludge for producing novel sorbent to remove Congo red and methylene blue dyes from aqueous environment

Preparation of new sorbent from precipitation of nano-sized (Mg/Fe-CTAB)- layered double hydroxide (LDH) on the surfaces of sewage sludge byproduct to remove the anionic and cationic dyes was the focal point of this work. The presence of nanoparticles and enlarged of interlayers by CTAB intercalation have increased the sludge surface area from 5.34 to 10.32 m2/g. The CTAB mass 0.03 g/50 mL, sludge dosage 1 g/50 mL and (Mg/Fe) molar ratio 2 were the best preparation conditions required to obtain effective sorbent with efficiencies exceeded 93% for MB and CR dyes. These efficiencies were obtained under operational conditions for batch study of 0.5 g coated sludge per 50 mL colored dye solution, initial pH 3 (for CR) and 12 (for MB), and time 3 h for 10 mg/L dyes at 200 rpm. Models of Langmuir and pseudo second-order have a high capability in the representation of sorption records with maximum capacities of adsorption 163.6 and 132.6 mg/g for CR and MB dye, respectively. The X-ray diffraction analysis proved that the calcite occurred mainly at 2θ = 29.8° while quartz corresponded to the 21, 26.6, 36.4, 36.9, 50.1, 60.01 and 68.4°. Characterization tests showed that nano-sized particles of magnesium/iron were precipitated on the sludge due to the formation of hydrotalcite-like compounds with an increase in the percentages of Mg and Fe from 0.87 and 1.36 to 4.25 and 3.03%, respectively. The results showed that the electrostatic attraction, intra-particle diffusion and hydrogen bonding were predominant mechanisms for removal of CR and MB onto coated sludge.

Antioxidant enzyme responses and metabolite functioning of Pisum sativum L. to sewage sludge in arid and semi-arid environments.

The productivity of plants is a direct variant of the countless biotic and abiotic stresses to which a plant is exposed in an environment. This study aimed to investigate the capabilities of leguminous plant garden pea (Pisum sativum L.) to resist water deficit conditions in arid and semi-arid areas when applied with varied doses of sludge for growth response. The effect of sludge doses was evaluated on crop yield, antioxidant enzymes, viz., ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), superoxide dismutase (SOD), and glutathione reductase (GR), and metabolites (ascorbic acid, glutathione, and total protein content). The effective sludge concentrations with respect to seed weight and crop yield were found to be in the following trend: D2 (6.25%)>D3 (12.5%)>D1 (2.5%)>D0 (control) under organic amendment (OA). Conversely, a high dose of the sludge reduced the seed weight and total crop yield. The sludge doses D2 under arid and semi-arid conditions along with organic amendments (OA) significantly enhance the antioxidant enzyme activity, whereas sludge dose D3 with OA ominously regulates the activity of these enzymes. Besides, seeds depicted a considerable increase in ascorbic acid, glutathione, and total protein content in arid and semi-arid conditions upon the application of sludge with OA. Sewage sludge as a source of nutrients indirectly enhances crop yield, antioxidant enzymes, and antioxidant metabolites. Thus, it improves the defense mechanism, reduces abnormal protein glycation, and depletes the susceptibility of protein to proteolysis.

Use of sewage sludge on Eucalyptus saligna initial growth

This study aimed to evaluate the effect of different doses of sewage sludge on Eucalyptus saligna initial growth. E. saligna seedlings were cultivated for 120 days in pots containing soil (control), soil + 30 Mg ha-1 sewage sludge, soil + 60 Mg ha-1 sewage sludge and soil + 90 Mg ha-1 sewage sludge. The chemical analyses of soils and macro- and micronutrient contents in leaves and biometric, biomass and physiological variables were evaluated. The treatments with sewage sludge were statistically superior to control treatment for all variables. Treatments of 30 and 60 Mg ha-1 were superior to that of 90 Mg ha-1 for plant growth. We recommend the application of 30 Mg ha-1 sewage sludge for the initial growth of Eucalyptus saligna seedlings. Moreover, the use of sewage sludge as substrate conditioner and fertilizer for Eucalyptus saligna seedling cultivation is an adequate and sustainable way to manage this urban waste.

The use of sludge as a micronutrient for the improvement of biogas production from seaweed: the integration of two sources of environmental concern to bring new opportunities

AbstractLarge quantities of seaweed in marine environments and coastal areas can cause serious hygienic and environmental problems. Anaerobic digestion (AD) could provide a solution and could also be useful for the production of bioenergy and fertilizer. However, the AD of algae biomass has some limitations and further work is required on the process. To increase the efficiency of the process, batches of 350 mL feedstock containing seaweed biomass (Sargassum sp.), inoculum, and different dosages of sludge from drinking water treatment (DWTS) as a micronutrient source to improve biogas production were digested in a 500 mL glass reactor and under mesophilic conditions, leading to significantly enhanced methane production. The highest methane yield (199 NmL g−1 VS) was observed when 6 mg L−1 DWTS was added, which showed a 30% improvement compared with the control digester and accounted for a 249.4 kWh increase in net energy per ton. The biodegradability index also increased by 10% compared with the control after the addition of DWTS. © 2021 Society of Chemical Industry and John Wiley &amp; Sons, Ltd

Effect of selected bacteria from biogas sludge on the growth and nutrition of upland rice

This study evaluated the influence of selected superior bacterial isolates (SBI), biogas sludge, and their interactions on growth and nutrient uptake of upland rice grown in Ultisols. We used a randomized block design with two factors and seven replicates from October 2020 to April 2021. The first factor used selected SBI (B0 = untreated, B1 = nitrogen-fixing bacteria isolate (N3), B2 = phosphate solubilizing bacteria isolate (P7), B3 = isolate combination (N3+P7)). The second factor was the dosage of biogas sludge (S0 = untreated, S1 = 157.5; S2 = 315; S3 = 630 ml/polybag). The parameters were determined by ANOVA and followed by Duncan’s multiple range test at Р&lt;0.05. The results showed that the isolate P7 significantly increased the N uptake by 20.77% and crop growth rate (CGR) of upland rice 2.81 times. Biogas sludge doses from 315 to 630 ml/polybag significantly increased plant height, uptake of N and P, total fresh and dry weight, and CGR of upland rice. The interaction between N3 and biogas sludge dosage of 630 ml/polybag significantly increased the CGR of upland rice. The application of isolates N3 and P7 and their combination with biogas sludge of 630 ml/polybag has the potential to increase the CGR of upland rice in acidic soils.

Centralized iron-dosing into returned sludge brings multifaceted benefits to wastewater management

Iron salts (i.e. FeCl3) are the most used chemicals in the urban wastewater system. Iron is commonly dosed into sewage or the mainstream system, which provides multiple benefits such as enhanced phosphorus removal and improved sludge settleability/dewaterability. This study reported and demonstrated a new approach that dosed FeCl3 into returned sludge in order to bring two more benefits to wastewater management: short-cut nitrogen removal via the nitrite pathway and less biomass production. This approach is achieved based on our findings that with similar amount of FeCl3, centralized iron dosing into a sidestream sludge unit generated iron concentration two orders of magnitude higher than the common mainstream dosing (e.g. 10–40 mg Fe/L-wastewater), leading to sludge acidification (pH = 2.1) with Fe (III) hydrolysis. Together with accumulated nitrite in the supernatant of the sludge, ppm-level of free nitrous acid was generated and thus enabled sludge disintegration, cell lysis, and selective elimination of nitrite-oxidizing bacteria (NOB). Long-term effects on nitrifying bacteria and overall reactor performance were evaluated using two laboratory reactor experiments for over one year. The experimental reactor showed stable nitrite accumulation with an average NO2−/(NO2− + NO3−) ratio above 80% and ∼30% observed biomass yield reduction compared to those in control reactors. In addition, the centralized sludge dosing strategy still provided benefits such as improved settleability and dewaterability of sludge and enhanced phosphorus removal.

Variation of soil properties with sampling depth in two different light-textured soils after repeated applications of urban sewage sludge

Semi-arid agricultural soils have increasingly been subjected to urban sewage sludge (USS) applications due to accelerated soil depletion and shortages in manure supply. Research studies addressing USS reuse have mostly been conducted in cropping systems and focused on changes in topsoil properties of a given texture. Therefore, sludge-soil interactions could be largely influenced by the presence of plants, soil particle composition and depth. In this field study, two agricultural soils (sandy, S and sandy loam, SL) received simultaneously four annual USS applications of 40, 80, and 120 t ha−1 year−1 in absence of vegetation. Outcomes showed the increase of carbon and macronutrients in both soils proportionally to USS dose especially in the topsoil profile (0–20 cm). Subsoil (20–40 cm) was similarly influenced by sludge rates, showing comparable variations of fertility parameters though at significant lower levels. The depth-dependent improvement of soil fertility in both layers enhanced the microbiological properties accordingly, with significant variations in soil SL characterized by a higher clay content than soil S. Besides, positive correlations between increases in sludge dose, salinity, trace metals, and enzyme activities in both soils indicate that excessive sludge doses did not cause soil degradation or biotoxic effects under the described experimental conditions. In particular and despite high geoaccumulation indices of Ni in both soils and profiles, the global concentrations of Cu, Ni, Pb, and Zn were still below threshold levels for contaminated soils. In addition, the maintenance of pH values within neutral range and the increase of organic matter content with respect to control would have further reduced metal availability in amended soils. Therefore, we could closely investigate the effects of texture and depth on the intrinsic resilience of each soil to cope with repetitive USS applications.