Effects of sewage sludge application on unfertile tropical soils evaluated by multiple approaches: A field experiment in a commercial Eucalyptus plantation

Terrestrial bioassays for assessing the biochemical and toxicological impact of biosolids application derived from wastewater treatment plants

Wastewater treatment plants (WWTP) are facilities where municipal wastewater undergoes treatment so that its organic load and its pathogenic potential are minimized. Sewage sludge is a by-product of this process and when properly treated is preferentially called "biosolids". These treatments may include some or most of the following: thickening, dewatering, drying, digestion, composting, liming. Nowadays it is almost impossible to landfill biosolids, which however can well be used as crop fertilizers. Continuous or superfluous biosolids fertilization may negatively affect non-target organisms such as soil macro-organisms or even plants. These effects can be depicted through bioassays on terrestrial animals and plants. It has been shown that earthworms have been affected to various degrees on the following endpoints: pollutants' bioaccumulation, viability, reproduction, avoidance behavior, burrowing behavior. Collembola have been affected on viability, reproduction, avoidance behavior. Other terrestrial organisms such as nematodes and diplopods have also shown adverse health effects. Phytotoxicity have been caused by some biosolids regimes as measured through the following endpoints: seed germination, root length, shoot length, shoot biomass, root biomass, chlorophyll content, antioxidant enzyme activity. Very limited statistical correlations between pollutant concentrations and toxicity endpoints have been established such as between juvenile mortality (earthworms) and As or Ba concentration in the biosolids, between juvenile mortality (collembola) and Cd or S concentration in the biosolids, or between phytotoxicity and some extractable metals in leachates or aquatic extracts from the biosolids; more correlations between physicochemical characteristics and toxicity endpoints have been found such as between phytotoxicity and ammonium N in biosolids or their liquid extracts, or between phytotoxicity and salinity. An inverse correlation between earthworm/collembola mortality and stable organic matter has also been found. Basing the appropriateness of biosolids only on chemical analyses for pollutants is not cost-effective. To enable risk characterization and subsequent risk mitigation it is important to apply a battery of bioassays on soil macro-organisms and on plants, utilizing a combination of endpoints and established protocols. Through combined analytical quantification and toxicity testing, safe use of biosolids in agriculture can be achieved.

Effects of sewage sludge application methods on the transport of heavy metals with runoff and their mechanisms

Woodland utilization is a promising disposal method for sewage sludge (SS). However, the potential risk of heavy metals (HMs) transport with runoff must be considered. Among the various factors influencing HMs loss, SS application methods (Holing application, HA; Broadcasting and mixing application, BM; Broadcasting application, BA) are likely to cause significant effects by altering soil erosion and soil aggregates. This study aimed to determine how SS application methods affect HMs loss, soil aggregates erosion, and how they are related. Accordingly, the losses of HMs in surface runoff, interflow, and sediment were quantified during six simulated rainfalls. The results demonstrated that all methods reduced surface runoff, but BA was the most effective. Additionally, BA significantly reduced the total sediment yield and the total proportion of the <0.05 mm fraction aggregates. Moreover, BA had the smallest cumulative losses of Pb and Cd through surface runoff and Cu, Pb, and Cd through sediment. Sediment was the most important pathway for HMs loss, through which over 76.56 % of HMs were lost. In BA, the <0.05 mm fraction aggregates had the lowest HMs load, whereas in other treatments had the highest (54.33 %-80.33 %). The potential ecological risk coefficient of Cd was beyond "moderate" in all the pathways of BM and "high" in the interflow of each SS treatment. Nonetheless, when the multi-elements were evaluated collectively, the potential ecological risk index for each SS treatment was categorized as "low". Overall, BA not only reduced soil erosion but also posed no risk of HMs pollution. It should be noted that the loss of Cd in the interflow had a great impact, while the <0.05 mm fraction aggregates played a significant role in the HMs load. Thus, the current study not only provides an effective approach for the environmentally safe disposal of SS but also proposes a scientific method for the application of SS in woodlands.

Sewage sludge application stimulated soil N2O emissions with a low heavy metal pollution risk in Eucalyptus plantations

Sewage sludge (SS) has been extensively used as an alternative fertilizer in forest plantations, which are beneficial in supplying timbers and mitigating climate change. However, whether the extra nitrogen (N) applied by SS would enhance the soil nitrous oxide (N₂O) emission, an important greenhouse gas, in forest plantations have not been well understood. The objective of this study is to evaluate the ecological effects of SS application on soils, by investigating the soil N₂O emission and the toxicity of the potentially toxic elements (PTEs) in soil. A field fertilization experiment was conducted in Eucalyptus plantations with four fertilization rates (0 kg m^-2, 1.5 kg m^-2, 3.0 kg m^-2, and 4.5 kg m^-2). The soil N₂O emissions were monitored at a soil depth of 0-10 cm using static chamber method, soil chemical properties, and PTEs were determined at soil depths of 0-10 cm, 10-20 cm, and 20-40 cm. The average soil N₂O emission rate was 8.1 μg N₂O-N h^-1 m^-2 in plots without SS application (control). The application of SS significantly increased the soil N₂O emissions by 7-10 times as to control. The increased N₂O emissions were positively related to the soil total phosphorus and nitrogen and negatively correlated with copper and zinc, which increased with the SS application. However, the potential ecological risk index (E_i) and the comprehensive potential ecological risk index (RI) of PTEs were lower than 40 and 150 respectively, which indicating a low toxicity of PTEs to soil health. After seven months of SS application, the priming effects of SS on soil N₂O emissions gradually diminished. These findings suggest that the application of SS may increase N₂O emissions at the initial stages of application (<7 months) and may have a low PTEs pollution risk, even at a high SS addition rate (4.5 kg m^-2).

Environmentally realistic concentrations of eprinomectin induce phytotoxic and genotoxic effects in Allium cepa

Eprinomectin, a veterinary drug within the family of avermectins, is widely used in the agricultural sector to combat a variety of parasites, mainly nematodes. However, only 10% of the drug is metabolized in the organism, so large quantities of the drug are released into the environment through urine and/or feces. Soil is the first and main environmental compartment to be contaminated by it, and nontargeted organisms can be affected. Thus, the present study aims to evaluate the phytotoxicity (through the evaluation of germination, root development, and germination speed) and genotoxicity (through an assessment of the induction of micronuclei and chromosomal aberrations) of eprinomectin. For the analyses, Allium cepa seeds were germinated in soil contaminated with a range of concentrations of eprinomectin: from 0.5 to 62.5 μg/g for the genotoxicity test and from 0.5 to 128.0 μg/g for the phytotoxicity test. The results showed that seed germination was not affected, but root development was affected at concentrations of 0.5 μg/g, 1.0 μg/g, 4.0 μg/g, 8.0 μg/g, 64.0 μg/g, and 128.0 μg/g, and germination speed was significantly changed at concentrations of 1.0 μg/g, 4.0 μg/g, 16.0 μg/g, 32.0 μg/g, and 64.0 μg/g. Significant differences in the mitotic index and genotoxicity index were observed only at concentrations of 2.5 μg/g and 12.5 μg/g, respectively. Only the 0.5 μg/g concentration did not show significant induction of micronuclei in the meristematic cells, but the damage observed at other concentrations did not persist in F1 cells. According to the results, eprinomectin is both phytotoxic and genotoxic, so the release of eprinomectin into the environment should be minimized.

Effects of Sewage Sludge Application on Plant Growth and Soil Characteristics at a Pinus sylvestris var. mongolica Plantation in Horqin Sandy Land

The application of domestic sewage sludge (SS) may affect plant growth and soil quality through altering nutrient availability. However, the effect of SS application on the plant–soil system in sandy soils is poorly understood. In this study, we established SS application treatment plots (SL, 25 t ha−1) and control treatment plots without sewage sludge application (CK, 0 t ha−1). SS was applied to the soil surface of a Mongolian pine (Pinus sylvestris var. mongolica) plantation in Horqin Sandy Land, Inner Mongolia, China, to assess its potential effects on plants and soil. We analyzed tree growth performances (tree height, basal diameter, and diameter at breast height), understory traits (species diversity, coverage, and aboveground biomass), soil physical and chemical parameters (nutrient content, dissolved organic carbon, soil water content, bulk density, pH), and proxies of ecosystem services (soil organic carbon and total nitrogen stocks). The results showed that SS addition not only significantly increased soil nutrient contents, but also markedly enhanced aboveground productivity and plant coverage. Specifically, SS addition decreased soil bulk density and increased concentrations of soil organic carbon, total nitrogen, and total phosphorus and mineral nitrogen, and it also increased soil carbon and nitrogen stocks. Furthermore, the addition of SS significantly increased soil dissolved organic carbon contents and enhanced the fluorescence intensities of dissolved organic carbon components (humic acid-like and UV fulvic acid-like) in the topsoil (0–5 cm). This study provides evidence that SS is an acceptable, and possibly preferred organic fertilizer for improving the soil quality and tree–grass growth of Mongolian pine plantations.

Toxicity of the sawdust used for phosphorus recovery in a eutrophic reservoir: experiments with Lactuca sativa and Allium cepa

Eutrophication is one of the environmental problems arising from the increase of essential nutrient concentrations, mainly phosphorus and nitrogen. In contrast to excess phosphorus, the depletion of phosphate rock deposits used for the production of fertilizers compromises the food supply. Therefore, the development of technologies that propose the recovery of the phosphorus contained in eutrophic environments for its later use for agricultural fertilization purposes is very important to ensure global food security. This work aimed to evaluate the toxic potential of the sawdust (biosorbent previously used for phosphorus adsorption) in order to enable its application in agriculture. For this, toxicity experiments with Lactuca sativa (lettuce) and Allium cepa (onion) seeds were performed. The phytotoxic potential was assessed by means of the seed germination index and physiological parameters such as radicle and hypocotyl growth. Cytotoxicity, genotoxicity, and mutagenicity tests were also performed on onion seeds. From statistical tests, it was possible to affirm that the sawdust did not promote inhibition of seed germination and radicle and hypocotyl growth. No genotoxicity, cytotoxicity and, mutagenicity were observed, which allowed to state that the sawdust is not toxic to the onion species, which reinforces the possibility of application of the biosorbent for soil fertilization purposes. Therefore, the use of sawdust for phosphorus biosorption with the subsequent agricultural application is promising and quite important from a global food security point of view.

Chemical and ecotoxicological assessment of sludge-based biosolids used for corn field fertilization

Sludge-based biosolids can be used for crop fertilization and for soil enrichment with organic matter. The prerequisites for their application are laid out in 86/278/EEC which sets limits for metals of toxicological concern in the biosolid and in the recipient soil. In this context, three kinds of sludge-based biosolids from a municipal wastewater treatment plant were submitted to the leaching tests EN 12457-2 and ΝΕΝ 7341. The leachates were examined for their physicochemical, chemical, and ecotoxicological characteristics four times within a year. Even in the more aggressive metal leaching method (ΝΕΝ 7341), results were much lower than the limits set in Council Directive 86/278/EEC. Correlations were noted between numerous parameters and principal component analysis grouped them in 3 components explaining 76% of total variation. These leaching tests were also performed on soil-biosolid samples from another investigative research in which two of the three kinds of biosolids had been incorporated in corn fields at doses 0, 20, and 40 tn/ha. EN 12457-2 leaching tests indicated that Ni and Cd leaching was affected by biosolid incorporation in soil, in relation to the type of the receiving soil (clay or sand). The leachates from the soil-biosolid mixtures were practically not toxic for the organisms tested (D. magna, V. fischeri, higher plants). In summary the biosolids were considered of low environmental risk when utilized in the field. A detailed risk assessment encompassing both chemical and ecotoxicological analysis is necessary for integrated evaluation of biosolids.

Composted Sewage Sludge Enhances Soybean Production and Agronomic Performance in Naturally Infertile Soils (Cerrado Region, Brazil)

Naturally infertile soils require large amounts of mineral fertilizers to obtain the desired crop yield. In the Cerrado region of Brazil, there is a need to investigate the potential of organic fertilizers to sustainably increase crop productivity and food security. A field study was conducted over two experimental seasons to evaluate the agronomic effectiveness of composted sewage sludge (CSS) as a fertilizer for soybean cultivation in infertile tropical soils. A 4 × 2 + 2 factorial randomized complete block design was applied with the following treatments: (i) CSS: 5.0, 7.5, 10.0, and 12.5 Mg ha−1 on a wet basis applied according to two different methods: whole area (WA) or between rows (BR); (ii) comparison with two alternative treatments: a control with no CSS and mineral fertilizer application, and an area treated with conventional fertilizers only. All the treatments were compared in terms of micronutrient concentrations in surface soil and plant leaves, plant development, crop productivity, and yield. Bi- (ANOVA, correlation matrix, and polynomial regression analysis) and multivariate (PCA, principal factor analysis) statistics were applied to determine statistical differences and relationships/observed variability among the treatments. Results showed that at higher CSS-WA rates: (i) soil and leaf micronutrient concentrations increased; (ii) there was an increase in soybean yield by 12 and 20%, respectively, as compared to control and conventional fertilization; (iii) soybean yield was 67% higher than the mean soybean yields for Brazil. Research outcomes confirm the benefits of CSS application on infertile agricultural soils in the Cerrado region, representing a strong alternative source of micronutrients in the CSS with respect to conventional fertilizers.

Evaluation of Effects of Municipal Sludge Leachates on Water Quality

Biosolids made from municipal sludge are an attractive solution instead of chemical fertilization. Nevertheless, their effects on the ecosystem should always be considered. In the present study, anaerobically digested sludge was subjected to two leaching methods (EN 12457-2 and NEN 7341) and the main physicochemical parameters were measured in the leachates. The aquatic organisms Daphnia magna and Vibrio fischeri were exposed to the leachates in order to test for adverse effects. Mixtures of biosolid/solid, simulating the high dose of 80 tn/ha, were also created, and the same parameters were measured for EN 12457-2 leachates. The results show a strong seasonal variation for the results for the municipal sludge, even though the sludge did not originate from a touristic area. The biosolid/solid mixtures did not produce toxic responses to the organism tested. Nevertheless, the parameters nitrites and nitrates in the leachates were increased in relation to control and they continued to increase even at Day 40 post-application. This increase was soil-type-dependent. The biosolids in question could be used for field fertilization, however measures should be taken against underground water nitrate pollution.

Assessment of phytoremediation potencial of Allium cepa L. in raw sewage treatment

Whereas the effects of the substances found in domestic sewage on live organisms is important to evaluate the use of plants to remove pollutants from the environment. The objective of this study was to assess the phytoremediation activity of Allium cepa L. (onion) roots exposed to raw sewage, as well as the acute toxic activity of this effluent for the bioindicators A. cepa, through a cytotoxicity test, and Artemia salina L., through a mortality/immobility test. The physicochemical assessments of the sewage were conducted in two scenarios: immediately after collection and after being in contact with onion roots (phytoremediation) for 24 hours. The physicochemical data indicate there was a reduction in nitrogen and phosphorus levels and in biochemical oxygen demand in sewage treated. The results from the cytotoxicity test with A. cepa indicated a reduction in the mitotic cell divisions of the onions treated with the raw sewage. The mortality/immobility test with A. salina indicated that the concentrations with 50 and 100% of raw sewage induced the mortality of the nauplii. Thus, the data suggest new studies that seek greater efficiency, efficacy and viability of onion phytoremediation.

Ionic speciation and risks associated with agricultural use of industrial biosolid applied in Inceptisol

The evaluation of the chemical leaching potential from soils amended with biosolid is of extreme importance for environmental safety of agricultural use of these residues. The objective of this study was to evaluate the polluting potential and possible risks associated with the agricultural use of biosolids generated by the polyethylene terephthalate (PET) fiber and resin industry through ionic speciation and analysis of the activity of chemical species present in the leached solution from Inceptisol treated with rates 0, 6, 12, 18, 24, 48, 96, and 144 Mg ha^-1 on dry basis. The experiment was conducted in a lysimeter and the treatments with three replications were distributed at random. Chemical leaching was made by application of CaC1₂ 0.01 mol L^-1 solutions in a volume fourfold higher than the water retention capacity of the soil, divided into five leaching events: 210, 245, 280, 315, and 350 days of incubation. Chemical species concentrations in collected leachates were used for ionic speciation by geochemical software Visual MINTEQA2 version 4.0. Impact factor of chemical species was calculated as the ratio between maximum concentration in the leach solution in the treated soil and control. Dissolved organic carbon had strong influence on Pb⁺² and Cu⁺² leaching, but these elements in free or complexed forms presented low activities in solution. Leaching of NO₃^-, Zn⁺², and Na⁺ represents the main environmental risk of agricultural use of this residue. However, these risks can be minimized if technical criteria and critical limits for the agronomic use of biosolids were observed.