A short bibliographic review concerning biomethane production from wastewater sludge.

Biomethane production by anaerobic digestion (AD) of sludge from municipal wastewater treatment is a viable practice to valorise the residues of these plants. However, although the relevant literature is abundant, no comprehensive reviews have been recently published on this topic. Detailed information concerning the factors influencing the AD process and values of biomethane production from the sludge from municipal wastewater treatment plants (MWWTPs) on the global scale may support technicians and researchers in both the planning and the design steps of an AD process. This study proposes a systematic review and a meta-analysis of the factors that noticeably influence biomethane yield deriving from AD of sludge from MWWTP. The reported values were systematically analysed compared to the main factors driving AD, including publication year, geographical area of each study, type of digested sludge, treatment in the water line of the MWWTP, possible sludge pre-treatments, type of digestion process, hydraulic retention time (HRT) and temperature regime of the AD process. A higher biomethane production was registered in North American plants compared to countries in other continents. Older studies published between 2001 and 2005 reported lower mean values compared to the more recent experiments. A gradient of 'primary sludge' > 'mixed sludge' > 'wastewater activated sludge' was found for the mean biomethane yield in relation to the digested sludge type. The mean biomethane yields for different types of sludge on a global scale are 0.425, 0.296 and 0.176 Nm3 kg VS-1 for primary sludge, mixed sludge and waste activated sludge, respectively. Overall, the study demonstrates: (i) the very large variability of biomethane yields from AD of the residues from MWWTPs (mainly due to the different characteristics of sludge) and (ii) the non-significance of some factors (i.e. treatment in the water line, pre-treatments, type of process, HRT and temperature regime) on energy yields from the AD process.

Wastewater treatment in lagoons: A systematic review and a meta-analysis.

This study has carried out a systematic review of 36 scientific papers (reporting 63 case studies) published in the last 15 years about the treatment of industrial, agri-food and municipal wastewater in lagoons. A concentration of studies from a few countries (Italy, Algeria and Iran) and about municipal wastewater (70% of papers) was revealed by the bibliographic analysis. Aeration was supplied in more than 50% of case studies; the storage capacity of lagoons (adopted as a measure of size) was extremely variable (over seven orders of magnitude), while their depth was generally lower than a few metres. The efficiency of lagoon treatments at removing COD was in a wide range (25-98%). Very few studies analysed the energy intensity of treatments in lagoons. The meta-analysis applied to a further selection of 10 papers with 29 case studies revealed significant differences in pH and dissolved oxygen concentration, due to aeration or type of treated wastewater. Treatment efficiency was higher in aerated lagoons compared to non-aerated systems, and did not depend on the type of treated wastewater. Based on the analysis of the reviewed papers, an urgent research need on this topic arises, mainly due to the oldness of most analysed studies. Practical suggestions are given to optimise the depuration performances of lagoons: (i) application of intermittent and night aeration; (ii) reduced air flow rates; (iii) adaptation of microbial biomass to high contents of inhibiting compounds in wastewater; (iv) construction of baffles to keep the planned hydraulic retention time avoiding short-circuit; (v) integration of lagoons with other treatments (e.g., constructed wetlands); (vi) ferti-irrigation of crops with lagoon effluents rather than disposal into water bodies.

Fostering biodiversity research in post-fire biology.

The impacts of wildfire on vegetation and soil erosion have been studied for decades aiming to bring back ecosystems after fire perturbance. However, the influence of fires on above and belowground biodiversity remains far less understood. Biodiversity is critical for supporting ecosystem function, and this data scarcity is hampering managers in adopting effective practices for a proper restoration of burned ecosystems. This limitation could be overcome by future research that should focus post-fire diversity of plants and soil biota, by (i) analysing the environmental factors driving post-fire evolutionary trends; (ii) exploring their interrelations across different spatial and temporal scales; (iii) identifying the variability across fires of different severities and frequency; (iv) ascertaining the post-fire response of individual plant species and soil taxa to fire with or without application of post-fire restoration actions.

Effects of grazing on soil properties in mediterranean forests (Central-Eastern Spain).

Traditional management practices, such as grazing, can have adverse impact on soils. Despite an extensive body of literature exploring the effects of grazing on soil and plants worldwide, there is a notable lack of research on its impacts in Mediterranean forests within the Iberian Peninsula Furthermore, there is a knowledge gap on the enzymatic activities and basal respiration of soil in forest after grazing. To address these gaps, this study aimed to investigate the impact of grazing on various important physicochemical and biological soil properties along with vegetation richness in a Mediterranean forest located in Castilla-La Mancha (Central Eastern Spain). Relative to undisturbed sites, grazing significantly reduced soil water content (-53%) and available water (-59%). However, soil hydraulic conductivity remained unaffected by animal trampling and the soil water repellency observed in ungrazed sites disappeared. Grazed soils experienced a slight increase in pH (+18%). Among the biochemical properties studied, only dehydrogenase showed a significant increase (+100%) while basal respiration exhibited a notable decrease (-24%). Grazing resulted in a reduction of plant species richness (-34%) indicating a loss of biodiversity in grazed areas. The observed significant alterations in key soil and plant properties due to livestock activity suggest that grazing has the potential to modify the overall soil quality of these sites. Certain variables that exhibited noteworthy differences between grazed and ungrazed sites could serve as indicators of grazing impacts in Mediterranean forests. These indicators may be considered proxies for establishing effective land management strategies to mitigate degradation in the Mediterranean forest ecosystem.

Effects of wildfire and post-fire salvage logging on rainsplash erosion in a semi-arid pine forest of Central Eastern Spain.

Rainsplash erosion on forested hillslopes can be increased by both wildfires and post-fire salvage logging, especially under semi-arid Mediterranean conditions. However, few studies have compared rainsplash erosion among forest sites impacted by logging to other forest areas. To fill this gap, this study has evaluated surface runoff and soil erosion in a burnt and logged (manually or mechanically) pine forest of Central-Eastern Spain under simulated rainfall and compared it to unlogged and unburnt plots. Compared to the unburnt plots, surface runoff significantly increased (over 150%) in logged areas, with a peak of 220% on the areas directly subjected to logging machinery. Peak runoff was substantially increased by fire (+130%) and less by logging (+8. Soil loss due to rainsplash erosion was about 235% (manual logging) to 750% (mechanical logging) higher compared to the unburnt plots. Wildfire exerted a much higher soil disturbance compared to salvage logging, with a soil hydrological response that can be up to an order of magnitude higher. The increased runoff and erosion rates in response to wildfire and logging were ascribed to soil compaction, which increased on average 60% on logged plots as well as to the removal of vegetation cover (-80%), whereas soil roughness played a minor role. From these results, we suggest using lightweight machinery in burnt soils, to reduce surface runoff and erosion. The possibility of building contour felled log debris using the burnt wood may also be considered, in order to retain the eroded sediments.

Short-term effects of post-fire soil mulching with wheat straw and wood chips on the enzymatic activities in a Mediterranean pine forest.

Soils of Mediterranean forests can be severely degraded due to wildfire. However, post-fire management techniques, such as soil mulching with vegetal residues, can limit degradation and increase functionality of burned soils. The effects of post-fire mulching on soil functionality have been little studied in Mediterranean forests, and it is still unclear whether the application of straw or wood residues is beneficial. This study explores the changes in important soil chemical and biochemical properties in a pine forest of Central Eastern Spain after a wildfire and post-fire mulching with straw or wood chips. Only basal soil respiration (BSR), dehydrogenase activity (DHA), pH and water field capacity (WFC) significantly changed after the fire and mulching. In contrast, the other enzymatic activities - urease (UA), alkaline phosphatase (Alk-PA) and ß-glucosidase (BGA), - total organic carbon (TOC) and electrical conductivity (EC) were not influenced by these soil disturbances. Time from fire and soil conditions (due to burning and management) were significant variability factors for BSR, pH, BGA, UA, TOC, EC. Mulching increased BSR compared to burned areas, especially in soils with straw (+30 %), thanks to addition of fresh organic residues, quickly incorporated in the soil. Soil pH showed a low variability among the four soil conditions, and TOC was higher in mulched soils (on average + 20 % compared to the burned soils), and this was correlated to the increased BSR. The role of mulching was essential with reference to WFC, as the post-fire management limited its reduction after the fire (on average from -30 % to -20 %). Finally, the Principal Component Analysis coupled to the Analytical Hierarchical Cluster Analysis confirmed the significant influence of the post-fire management on some enzymatic activities, although a sharp discrimination among the four soil conditions was only evident between unburned and burned sites, regardless of the management. Overall, it has been shown that mulching promotes conservation of fragile Mediterranean soils, indicating its effectiveness at preserving soil functionality in areas affected by forest fires.

Limited contribution of post-fire eco-engineering techniques to support post-fire plant diversity.

Eco-engineering techniques are generally effective at reducing soil erosion and restore vegetal cover after wildfire. However, less evidence exists on the effects of the post-fire eco-engineering techniques to restore plant diversity. To fill this knowledge gap, a standardized regional-scale analysis of the influence of post-fire eco-engineering techniques (log erosion barriers, contour felled log debris, mulching, chipping and felling, in some cases with burning) on species richness and diversity is proposed, adopting the Iberian Peninsula as case study. In general, no significant differences in species richness and diversity (Shannon) were found between the forest treated with different post-fire eco-engineering techniques, and the burned and non-treated soils. Only small significant differences were found for some sites treated with log erosion barriers or mulching. The latter technique increased species richness and diversity in some pine species and shrublands. Contour felled log debris with burning slightly increased vegetation diversity, while log erosion barriers, chipping and felling were not successful in supporting plant diversity. This research will help forest managers and agents in Mediterranean forest to decide the best postfire management option for wildfire affected forest, and in the development of more effective post-fire strategies.

Effects of plant species on soil quality in natural and planted areas of a forest park in northern Iran.

Reforestation may help protect the health of endangered forest ecosystems. To implement this action, it is important to evaluate the effects of the planted species on soil quality. Previous studies have demonstrated that soil properties are closely driven by the effects of plant roots and plant remains (quantity and quality) reaching the soil surface. However, little research is available about the effects of plant species on soil quality of reforested sites compared to natural forest ecosystems. This study evaluates the changes in the main soil properties between two 30-40 year-old stand types in forest areas of northern Iran: i) two stands, each one comprising a natural species (Parrotia persica or Pinus taeda); and ii) two stands, each one with planted trees (Quercus castaneifolia or Alnus glutinosa). Compared to reforested sites, the soils with natural trees showed higher root weight density (+43%), pH (+17%), and organic carbon (+64%). These differences led to higher nutrient contents, microbial respiration, aggregate stability, and water retention in soils with natural trees, as confirmed by the correlation analysis. A principal component analysis provided a meaningful combined factor (the first principal component) that showed a clear discrimination in soil quality and fertility among natural and reforested species. The calculation of a soil quality index confirms that planted species may lead to an overall lower quality of soils with planted species compared to natural forest. Since the lower soil quality of planted forests can be also the result of unsuitable management practices, this study suggest that forest operations in reforested areas should be avoided, since this could lead to negative effects on soil quality and contribute to an increase in the risk of soil degradation.

Influence of forest stand age on soil water repellency and hydraulic conductivity in the Mediterranean environment.

The hydrological response of forest soil in the Mediterranean environment is characterised by high runoff and erosion rates, mainly due to low infiltration and high repellency of soils. However, little literature exists about the effects of forest ages on soil water repellency (SWR) and hydraulic conductivity (SHC). This study evaluates these hydrological parameters in five Pinus nigra Arn ssp. Salzmannii stands of different ages in Central-Eastern Spain; one of these stands, unmanaged, was chosen as reference system. SWR (measured in terms of water drop penetration time, WDPT) and SHC as well as the main physico-chemical properties and surface characteristics of soils were surveyed in forty-five plots. Water infiltration was higher in the older stands (including the older and unmanaged forest) and lower (by over 60%) in the more recent pine forests. Four of the studied stands did not show water repellency; only the more recent plantation showed a slight SWR. The differences in SHC among the forest ages were mainly driven by the organic matter (OM) and nutrient contents of the soils as well as by the bulk density and quantity of dead wood. SWR was similar among the plots (despite significantly differences in WDPTs), although having variable OM contents. Considering these differences in soil properties, SHC and SWR were simply predicted for each forest stand using on dbRDA models and the following soil properties: (i) OM and total nitrogen contents of soil (for SHC and SWR); (ii) dead wood and bulk density (for SHC); and (iii) clay content and the percentage of bare soil (for SWR). Overall, this study has showed that, when a new forest stand is planted, decreases in water infiltration, with subsequent increases in runoff generation capacity) of the soils, can be expected. Conversely, no water repellency is likely to affect new pine plantations.

Organic matter removal and ammonia recovery by optimised treatments of swine wastewater.

The organic matter and nitrogen contents of swine wastewater (SW) can be reduced and, at the same time, a fertiliser as ammonium salt can be recovered by wastewater treatments. One of the most promising technique is air stripping (AS). However, the operational parameters (pH, temperature and air flow rate) of AS must be optimised, in order to maximise the ammonia recovery and reduce the requirement of chemicals and energy. In this study 27 batch tests at laboratory scale were carried out on real SW, varying (individually or simultaneously) the pH (not adjusted, 8 and 10), temperature (ambient, 40 and 60 °C) and flow rate (0, 1 and 5 Lair LSW-1 min-1) of AS; the changes in soluble COD (sCOD) and total ammonia nitrogen (TAN) concentrations were evaluated in response to the parameters adjustments. For the tests including AS, the ammonium sulphate recovered was also measured. In general (about 50% of the tests), more than 80% of TAN was removed. Most of these tests were carried out with pH and temperature control and AS at the highest flow rate; the highest efficiency was found for a combination of chemical, thermal and aeration treatments. For a few tests with the same process control, an increase (up to 50%) or a very limited (less than 10%) decrease of sCOD were detected; therefore, these treatments can be adopted prior of anaerobic digestion of SW. A high flow rate, which increases the removal efficiency of both sCOD and TAN, should be adopted, when AS is used as pre-treatment of activated sludge or lagooning plants. Very high amounts (over 80% of the theoretical yield) of ammonium sulphate were recovered by AS at the maximum air flow rate (5 Lair LSW-1 min-1), which would provide a nitrogen fertiliser at a sustainable cost.

Assessment of riparian vegetation characteristics in Mediterranean headwaters regulated by check dams using multivariate statistical techniques.

In mountainous torrents of the Mediterranean environment the riparian vegetation is strongly influenced by the presence of engineering control works, since these structures bring heavy modifications in channel geometry, hydraulic regime and bed sediment size. Previous investigations have shown high linear correlations between physical (section shape, profile slope, specific discharge, surface and subsurface size of the channel bed) and vegetation (development, structure and biodiversity) indicators in headwater channels with check dams of Calabrian (Southern Italy) torrents. Based on these findings, this study applies multivariate statistical techniques (Principal Component Analysis and Partial Least Square Regression) to identify in the same study headwaters new synthetic explanatory variables, representative of the different transects (upstream, downstream or intermediate, compared to the check dam location) and develop predictive models of riparian vegetation characteristics. The Principal Component Analysis has provided a simple parameter (the first Principal Component, explaining about 60% of the total variance), which is able to discriminate the physical and vegetal characteristics of the different transects close to check dams, thus reducing the large number of factors influencing the fluvial processes. Moreover, cover, height and transversal variability of riparian vegetation have a very high influence (loadings over 0.73) on this component, while its biodiversity is correlated to the second Principal Component (loadings over 0.63). The Partial Least Square Regression has shown that it is possible to estimate with fair accuracy (minimum r2 of 0.70) the development, structure as well as transversal variability of the riparian vegetation, starting from the physical features of the channel. These models may be important in the planning steps of new check dams, since their effects on the development and growth of vegetation upstream and downstream can be forecasted before their installation, at least for the quantification of the order of magnitude of the check dam impacts on torrent ecology.

Evaluating the effects of check dams on channel geometry, bed sediment size and riparian vegetation in Mediterranean mountain torrents.

In mountain streams possible negative impacts of check dams on soil, water and riparian vegetation due to check dam installation can be noticed. In spite of the ample literature on the qualitative effects of engineering works on channel hydrology, morphology, sedimentary effects and riparian vegetation characteristics, quantitative evaluations of the changes induced by check dams on headwater characteristics are rare. In order to fill this gap, this study has evaluated the effects of check dams located in headwaters of Calabria (Southern Italy) on hydrological and geomorphological processes and on the response of riparian vegetation to these actions. The analysis has compared physical and vegetation indicators in transects identified around check dams (upstream and downstream) and far from their direct influence (control transects). Check dams were found to influence significantly unit discharge, surface and subsurface sediments (both upstream and downstream), channel shape and transverse distribution of riparian vegetation (upstream) as well as cover and structure of riparian complexes (downstream). The actions of the structures on torrent longitudinal slope and biodiversity of vegetation were less significant. The differences on bed profile slope were significant only between upstream and downstream transects. The results of the Agglomerative Hierarchical Cluster analysis confirmed the substantial similarity between upstream and control transects, thus highlighting that the construction of check dams, needed to mitigate the hydro-geological risks, has not strongly influenced the torrent functioning and ecology before check dam construction. Moreover, simple and quantitative linkages between torrent hydraulics, geomorphology and vegetation characteristics exist in the analysed headwaters; these relationships among physical adjustments of channels and most of the resulting characteristics of the riparian vegetation are specific for the transect locations with respect of check dams. Conversely, the biodiversity of the riparian vegetation basically eludes any quantitative relations with the physical and other vegetal characteristics of the torrent transects.

Simulating the hydrological response of a small tropical forest watershed (Mata Atlantica, Brazil) by the AnnAGNPS model.

Given the intrinsic hydrological cycle made of large input of water vapour and intense precipitation producing large volumes of water and sediment, modelling runoff and water losses in humid tropical watersheds is important for forest and water resources management. For instance, reliable simulations of the water cycle in such environments are a prerequisite for predictions of water quality, soil erosion and the climate change effects on water resources. The distributed parameter, physically based, continuous simulation, daily time step AnnAGNPS model, was implemented in almost completely forested (98% of its area, 0.56 km2) Cunha watershed (Brazil) to assess its capability to simulate hydrological processes under tropical conditions. The simulated surface runoff was compared to 4-year observations with statistical indices on several time scales. The model, running with default CN of forest, showed poor predictions of runoff. After increasing CN from 63 to 72 by calibration, the runoff prediction capability of AnnAGNPS was satisfactory on annual, seasonal and monthly scales, while daily runoff predictions were less accurate. Modelling water losses at event scale showed that the effect of forest vegetation on water retention during a single precipitation was more limited than for longer periods (months, seasons and years), since evapo-transpiration and interception account for small shares (>20%) of total precipitation. This study demonstrated that the AnnAGNPS model has reliable runoff prediction capacity in tropical forest watersheds at the annual and seasonal scales (E > 0.73), whereas daily runoff simulations are less accurate (E = 0.44). The use of this model may prove an important tool for water resource and territory management in tropical rainforests.

Depuration in aerated ponds of citrus processing wastewater with a high concentration of essential oils.

Citrus processing wastewater was treated in aerated pilot plants in order to evaluate the following: (a) energy efficiency under different air flow rates and times; and (b) limits of spontaneous microflora in adapting to essential oils. In comparison to permanent air flow, night aeration for 12 hours determined an increase of up to 12% of the monthly removal rate of chemical oxygen demand (COD) and a consequent reduction by 10% of energy consumptions per unit of COD removed from 0.63 to 0.57 kWh/kg(COD). Lowering night aeration from 14 to 7 1/m3/h reduced by only 10% the removal rate of COD; the energy consumption per unit of COD removed (0.32 kWh/kg(COD)) was consequently reduced by more than 40%. Dissolved oxygen was maintained at very low level, rarely exceeding 0.2 ppm, with no bad smell. The consequent high oxygen deficit of 98-99% of saturation induced high oxygen transfer efficiency. The microbial population was characterized mainly by aerobic bacteria; only 5-8% of bacteria were strictly anaerobic. In the deep tank layer under the air diffuser a small amount of sludge settled (0.03-0.04 kg of dry matter per kg of COD removed), containing only 3% of total organic matter detected at the end of the depuration process. The fact that the concentration of essential oils could be progressively increased up to 1400 ppm without noticeably slowing down the biological processes demonstrated the remarkable microbial adaptation.