Low Total Nitrogen Research Articles

Using sulfide as nitrite oxidizing bacteria inhibitor for the successful coupling of partial nitrification-anammox and sulfur autotrophic denitrification in one reactor

The partial nitrification-anammox (PNA) process, as a new “green” biological nitrogen removal technology, is completely autotrophic and has great economic and environmental benefits. However, the applied bottleneck of this process is the inability to inhibit nitrite oxidizing bacteria (NOB) activity and the low total nitrogen removal efficiency (TNRE) due to nitrate accumulation. In this study, the addition of sulfide was utilized to selectively screen NOB while promoting the sulfur autotrophic denitrification (SAD) process in one sequencing batch biofilter granular reactor. The results showed that when the influent concentration of NH4+-N and S2− were 184.6 ± 4.4 mg/L and 153.9 ± 3.1 mg/L, the average TNRE reached 89.6 ± 3.6 %. At this stage, aerobic ammonia oxidizing bacteria (AOB) and anaerobic ammonium oxidation bacteria (AnAOB) showed good activities (specific oxygen uptake rate of AOB and specific anammox activity could reach to 0.09 ± 0.00 mg O2/g SS/h and 0.47 ± 0.06 mg TN/g SS/h, respectively), while NOB activity was significantly suppressed by sulfide, with specific oxygen uptake rate of 0.003 ± 0.00 mg O2/g SS/h, which further demonstrating the feasibility and long-term application of this sustainable and efficient process. Anammox was the predominant nitrogen removal pathway contributing approximately 89.0 % of nitrogen removal. Typical AOB (Nitrosomonas), AnAOB (Candidatus Kuenenia), and SAD bacteria (Thiobacillus, Pseudoxanthomonas, and Limobacter) coexisted in the PNA-SAD system. The syntrophic interactions among sulfur-oxidizing, sulfate-reducing, and anammox bacterial populations of this system were revealed by PICRUST2 and network analysis. Sulfides were most likely oxidized to biologically produced elemental sulfur first and then converted to sulfate via the Sox system. Our study provided a new reaction scheme and theoretical reference for efficient nitrogen and sulfur removal in a single reactor.

Start‐up and operation of anammox biofilter in treating low‐strength ammonia wastewater prepared by tail water from a municipal wastewater treatment plant

AbstractBACKGROUNDEconomic technology of advanced nitrogen removal of municipal wastewater is urgently needed. In this study, anammox biofilter (A‐BF) as a carbon‐ and energy‐saving advanced nitrogen removal process was tested under the conditions of sewage secondary effluent.RESULTSThe results showed that A‐BF could be successfully started up under total nitrogen (TN) of both 25 and 75 mg L−1 within 30 days at 25 °C. It achieved a nitrogen removal rate of 0.62 ± 0.08 kg‐N m−3 d−1 with nitrogen loading rate of 1.35 kg‐N m−3 d−1 at TN of 17 mg L−1. The ceramsite bed was conducive to the formation of biofilm and enrichment of functional microorganisms. The volatile suspended solid reached 1.39–7.33 g L−1, in which the abundance of the typical anammox bacteria Candidatus_Brocadia and Candidatus_Kuenenia occupied 0.96–1.35% and 0.29–0.92%, respectively.CONCLUSIONSOverall, under low TN concentration and high nitrogen load, A‐BF could be a candidate technique for advanced nitrogen removal of municipal sewage. © 2023 Society of Chemical Industry.

Impact of fertilization and grazing on soil N and enzyme activities in a karst pasture ecosystem

The biggest obstacle to economic development in karst areas comes from the ecological crisis, so healthy soil and adequate food are important. Soil nitrogen (N) is an important factor in maintaining soil quality. Understanding the changes in soil N fractions and enzyme activities under different management practices is important to maintain sustainable pastureland in karst areas. In this study, we investigated how grazing and fertilization affect different soil nitrogen fractions and enzyme activities of karst pastureland. The four treatments were unused land (UL, undisturbed utilization), abandoned-grazing pastureland (APL, grazing for three years followed by abandonment for six years), grazing pastureland (GPL, controlled grazing without fertilizer) and fertilized-grazing pastureland (FGPL, controlled grazing + fertilizer), with the aim of determining the management practices that is most beneficial to soil N in the area. Our results indicated that FGPL is the most beneficial in enhancing soil N fractions. This is reflected in greater total nitrogen (TN), alkaline driven nitrogen (NOH), inorganic nitrogen (TON), anaerobic mineralized nitrogen (The average increase in each soil layer over UL was 1.43 times; TMN), microbial biomass nitrogen (The average increase in each soil layer over UL was 3.51 times; MBN), amino acid nitrogen (AAN), amino sugar nitrogen (ASN), acidolyzed unknown nitrogen (TUN) and unacidified nitrogen (AIN) in FGPL soils. Likewise, greater enzyme activities were also found in nitrogen conversion-related enzymes (soil urease (S-UE), soil N-acetyl-β-D-aminoglucosidase (S-NAG), nitrate reductase (S-NR), and soil acid protease (S-ACPT)). In addition, the combination of grazing and balanced nutrient inputs slowed soil acidification and maintained high levels of total phosphorus (TP) and total potassium (TK). In contrast, the soil N fractions responded significantly to APL (low TN and plant effective nitrogen), while insufficient conversion capacity (enzyme activity, MBN) was the main reason for the compromised soil N availability at GPL. Government policy makers and livestock producers should encourage controlled grazing with proper fertilizer supplementation, and avoid prolonged abandonment of grazing lands.

Sludge granulation in PN/A enhances nitrogen removal from mainstream anaerobically pretreated wastewater

Treating anaerobically pretreated wastewater using partial nitritation/anammox (PN/A) process faces severe challenges because of the complex syntrophic and competitive relationship among various bacteria. Results of this study suggested a continuous low dissolved oxygen (DO) concentration failed to sustain NH4+ removal (<80 %), whereas moderate DO concentrations with high aerobic periods suppressed anammox reaction. Through implementing a moderate DO concentration with low aerobic periods (MDO-LA), NH4+ and total nitrogen removal efficiency reached 91.5 ± 5.5 % and 71.3 ± 2.8 % respectively. The specific activities of ammonium-oxidizing bacteria (AOB) and anaerobic ammonium-oxidizing bacteria (AnAOB) reached 0.942 ± 0.030 and 0.277 ± 0.010 g nitrogen per gram mixed liquor volatile suspended solids, respectively, mainly because MDO-LA favored Thiothrix (filamentous bacteria) wash-out and promoted Nitrosomonas growth. Moreover, sludge granules covered by a thin exterior rim with abundant AOB were formed, favoring Ca. Brocadia growth (5.4 % to 13.2 %) and mass transfer between AOB and AnAOB, which consequently increased the expression of genes coding hydroxylamine oxidase and hydrazine synthase. Overall, achievements in this study provide a promising operating strategy for PN/A treating anaerobically pretreated wastewater.

Nitrogen and phosphorus losses in Nordic and Baltic agricultural monitoring catchments – Spatial and temporal variations in relation to natural conditions and mitigation programmes

Nitrogen (N) and phosphorus (P) losses via agricultural drainage water have negative impacts on receiving water bodies and large-scale programmes to reduce nutrient losses have been established in the Nordic and Baltic countries, together with agricultural catchment monitoring programmes. This study evaluated time series (9–40 years) of data from 34 selected Nordic-Baltic catchments for spatial and temporal variations in area-specific water discharge (mm) and in concentrations and transport of total nitrogen (TN) and total phosphorus (TP).Water discharge from the catchments varied from 125 mm (Denmark) to > 1000 mm (Norway). Catchments with low TN concentrations (≤3 mg L-1) were dominated by clay or grass leys or were undrained with reduction of nitrate (NO3) in shallow groundwater. Catchments with high TN concentrations (≥10 mg L-1) had loams and cereal crops. TP concentrations were highest (≥0.45 mg L-1) in catchments with erosive soils, relatively high water discharge and cereal crops, and lowest (≤0.07 mg L-1) in catchments with permeable soils.Generalised additive mixed model (GAMM) analysis of time series of transport and flow-weighted concentrations of TN and TP for temporal patterns revealed decreases in TN concentrations in seven catchments and increases in eight, while four had periods with opposing trends. TN concentrations decreased in Denmark and Sweden in 1990–2010, following introduction of mitigation programmes. TP concentrations decreased in eight catchments and increased in six, while one showed opposing trends. Decreases in TP coincided with improved P balance in catchments with sand and loam. To further reduce N and P losses, a tailored set of mitigation measures is needed for each combination of soil, climate, geohydrology and agricultural production. Intensive monitoring of small catchments can reveal how N and P losses relate to natural conditions and to changes in agricultural production.

Quality assessment and evaluation of irrigation water and soil used for maize (Zea mays L.) production in Boloso Sore district, southern Ethiopia

Poor quality of irrigation water and soil are among the major factors determining maize productivity in Ethiopia. This study assessed and evaluated the quality of irrigation water and soil under maize production in Soke and Woybo irrigation schemes in Boloso Sore district, Ethiopia. Four water samples per site per season were collected from the first point of the irrigation schemes and farm gate for dry and rainy seasons in 2019/2020. Soil samples of 108 were collected from 36 points, from which 18 composited samples were taken for laboratory analysis. Results show that irrigation water of the two schemes is non-saline (electrical conductivity <0.2 dS m−1) and in the normal pH range (6.5–7.5). Maximum concentration of cations in irrigation water was in the order of sodium (22.3 mg l−1) > potassium (7.3 mg l−1) > calcium (6.2 mg l−1) > magnesium (3.1 mg l−1). Moderate to severe sodicity (sodium adsorption ratio of 10.9) was also recorded. Sulfate, nitrate, and phosphate contents in water were trace, and increased during rainy seasons in downstream. Textural classes of soils are clay loam to clay, and less compact to restrict root penetration (bulk density ≤1.4 g cm−3), have slow infiltration rate (≤0.13 cm h−1), and medium level of total available water (≤178 mm m−1). Soils are strongly acidic to neutral (pH: 5–6.5), salt-free, and have low soil organic carbon (≤2.1%), low total nitrogen (≤0.1%), low available phosphorus and sulfur, and low Ca2+: Mg2+ ratio. It can be concluded that the irrigation water in the study area has cation imbalance (poor quality) which affects soil quality and maize productivity. Likewise, soils of the study area have poor quality. Lime application, efficient fertilizer use, and organic matter applications can be suggested. Further study on optimizing fertilizer rates and irrigation levels has to be conducted to improve maize productivity.

Performance and mechanism study of g-C3N4/rGO heterojunction enhanced NO3− reduction by nZVI under visible light irradiation

Zero valent iron (ZVI) is a low-cost, earth-abundant reactive metal, and it has a strong reducing ability with a standard redox potential of − 0.44 V. Therefore, it is an effective and widely used reducing agent for pollutant degradation. Since nanoscale zero valent iron (nZVI) shows a larger specific surface area which leads to the faster reaction rate and higher conversion efficiency, it has been widely used in NO3− nitrogen reduction reaction, however, it shows disadvantages of poor chemical stability, narrow applicable pH, and low total nitrogen (TN) removal efficiency. To solve these problems, in this paper, the nZVI nanoparticles were deposited on the g-C3N4/rGO composite, and its NO3− removal efficiency under visible light irradiation was investigated. The effects of nZVI loading, initial pH of solution, catalyst dosage and initial NO3− concentration on NO3− reduction performance were studied. The results showed that the g-C3N4/rGO/nZVI material achieved 100% of NO3− reduction efficiency in 60 min. And the TN removal efficiency of g-C3N4/rGO/nZVI was 82.8%, superior to nZVI and g-C3N4/nZVI (13.3% and 38.2%, respectively). Finally, the denitrification mechanism of g-C3N4/rGO/nZVI was proposed as follows: Under light irradiation, the photogenerated electrons on g-C3N4 transferred via rGO to the nZVI surface, thus improving the reactivity and stability of nZVI. The photogenerated electrons and active electrons on the surface of nZVI reduced NO3− to N2, NH4+, and other products, while the photogenerated holes further oxidized part of NH4+ to N2 and other gaseous products. During this process, the rGO material could inhibit the oxidation of nZVI by the photogenerated holes from g-C3N4.

Total nitrogen and pH are key variables for functional structure of zooplanktonic communities in an agropastoral landscape

Changes in land use are one of the main impacts on aquatic ecosystems. The conversion of natural areas into agropastoral, such as pasture or monocultures, can affect the limnological characteristics of the water that in turn influence the structure of aquatic communities. But the impact is still unclear, particularly on zooplankton communities. The objective of this study was to evaluate the effect of water parameters from eight reservoirs inserted in an agropastoral landscape on the functional structure of zooplankton. The functional characterization of the zooplankton community was based on four traits: body size, feeding type, habitat type, and trophic group. Functional diversity indices (FRic FEve and FDiv) were estimated and modeled water parameters, using generalized additive mixed models (GAAM). The water parameters investigated were total nitrogen (TN) and total phosphorus (TP), dissolved oxygen (OD), temperature and pH. Moreover, we used RDA to analyze the influence of these environmental variables on the sharing of traits among the sample sites. FRic was high in the reservoirs with low TN concentration and low pH values. FEve was also high in low pH and high TP concentration. FDiv was high with unsharp increases of pH and high concentrations of TN and DO. Our analyses showed pH as a key variable for functional diversity, since it was related to the variation in all indices. Data also pointed out changes in diversity functional with small pH changes. The big and medium size, raptorial-cop and filtration-clad functional traits were positively associated with high concentrations of TN and alkaline pH. The small size and filtration-rot were negatively associated with high concentrations of TN and alkaline pH. Density of filtration-rot was lower in pasture landscapes. In conclusion, our study indicated that pH and TN are key variables for the functional structure of zooplanktonic communities in an agropastoral landscape.

Integrated Hydroponics-Microbial Electrochemical Technology (iHydroMET) is promising for Olericulture along with domestic wastewater management

Integrated Hydroponics-Microbial Electrochemical Technology (iHydroMET) has emerged as a promising decentralized option for efficient domestic wastewater management. Considering its intended application at households and the aim of offering useful plant cultivation options to the end users, this study focused on testing the Olericulture feasibility in sewage-treating iHydroMET design in wastewater-saturated (SW) and unsaturated (UW) reactor conditions. The systems operated outdoors in drip-hydroponics batch recirculation mode achieved efficient COD (87.4 ± 5 % and 82.1 ± 4.4 %) and turbidity (98.8 ± 0.5 % and 95.8 ± 0.9 %) but low total nitrogen (14.9 ± 3 % and 36.7 ± 6.7 %) removal in UW and SW conditions, respectively. Spinach and Chilli yield in UW and SW conditions were 21.1 ± 5.9 and 4.4 ± 1.2 g/plant and 32.7 ± 3.1 and 12.8 ± 2.4 g/plant, respectively. Chilli quality was compromised marginally in wastewater; however, Spinach yield was similar to that obtained in control with nutrient-rich Hoagland solution. These findings validate iHydroMET's benefits for growing leafy vegetables besides wastewater management at households.

Impacts of adding FeSO4 and biochar on nitrogen loss, bacterial community and related functional genes during cattle manure composting

This study investigated the impacts of adding FeSO4 and biochar to cattle manure and rice straw composts on functional genes controlling nitrogen loss, bacterial community, nitrification, and denitrification. Four treatments were established, including a control group (CP), and CP mixtures that included 4% biochar (TG1), 4% FeSO4 (TG2), or 2% FeSO4 and 2% biochar (TG3). Compared to CP, TG1–3 had a lower total nitrogen loss rate, and TG3 resulted in reduced NH3 (52.4%) and N2O (35.6%) emissions to mitigate nitrogen loss. The abundance of amoA and narG gene in TG3 was higher than in the other groups, and TG3 was beneficial to the growth of Proteobacteria and Actinobacteria. According to redundancy and Pearson analysis, TG3 had a positive effect on the nitrification process by increasing the abundance of amoA and narG. Thus, biochar and FeSO4 addition mitigate nitrogen loss by regulating the nitrification processes.

Can the Odontotermes horni (Wasmann) subterranean termite significantly alter the macronutrient status of various host plants?

The study aimed to analyze the impact of O.horni foraging on different types of organic litters such as maize straw, finger millet straw, cattle dung, farm waste, grass, silver oak leaves, eucalyptus leaves, and bark. The results showed that the earthen sheetings of O.horni had significantly lower total nitrogen content compared to the various organic litters. Moreover, there were variations in the organic carbon, organic matter, total nitrogen, and available N, P2O5, K, CEC, and C: N ratio content among the earthen sheetings of different organic litters. Interestingly, the earthen sheetings had slightly higher nutrient content than the surrounding subsoils.

Achieving high-level nutrient removal and wastewater treatment capacity through iron-mediated microbial process and sludge granulation

This study aims to develop and demonstrate a novel integration of anammox with zero-valent iron (Fe0) for highly efficient wastewater treatment. The process enables not only enhanced phosphorus removal but also favored sludge granulation to reduce footprint, and more importantly, leads to a combination of microbial iron (Fe)-based denitrification for high-level nitrogen removal. In an integrated laboratory system, superior performances of nitrogen and phosphorus removal and treatment capacity were achieved over one year. With low-strength mainstream wastewater (NH4+-N = 58.2 ± 0.6 mg/L) as influent, total nitrogen (TN) removal efficiency stabilized at 97.4 ± 1.6 %, with effluent TN concentration of <2.0 mg N/L. Phosphorus removal was also high above 90 %, attributed to the formation of chemical precipitates in sludge granulation with a mean volume diameter of ∼1 mm. The formation of granular sludge was due to that the Fe0 served as carriers to support microbial attachment and growth, and stimulated the secretion of extracellular polymeric substances to accelerate microbial aggregation. A 16S rRNA gene sequencing analysis, together with a series of batch activity tests revealed that the added Fe0 enriched nitrate removal pathway and improved the activity of denitrifiers, thereby successfully polishing effluent with a very low TN level. Overall, this proposed integrated process is proved to be a promising technology of simultaneously achieving high-level nitrogen, phosphorus removal and capacity for next-generation wastewater treatment.

A multi-agent AI reinforcement-based digital multi-solution for optimal operation of a full-scale wastewater treatment plant under various influent conditions

Optimal operating systems have been widely employed to improve the economic and environmental performance of wastewater treatment plants (WWTPs) for efficiently treating discharged water pollutants. However, the multi-optimization approach applied to WWTPs has been inadequate, resulting in a low optimization performance owing to varying influent conditions and inherent complex interactions between manipulated variables. Therefore, this study developed a digital multi-solution for optimal operation of a WWTP based on multi-agent reinforcement learning. Dynamic influent conditions with low, normal, and high influent chemical oxygen demands and total nitrogen composition ratios were generated using a k-means clustering algorithm to accurately reflect operating conditions. A game abstraction method based on a two-stage attention network (G2ANet) algorithm was employed to simultaneously search for three operational setpoints: dissolved oxygen, external sludge recycling, and external carbon dose. The adaptability of the G2ANet-based digital multi-solution for determining setpoints was evaluated by employing newly measured one-year influent data. The results demonstrated the ability of an intelligent G2ANet-based digital multi-solution in identifying optimal setpoints to improve the performance of WWTPs and outperform manual operating systems under varying influent conditions. It reduced aeration energy by 25 % and improved effluent quality by 7 %, while maintaining a similar pumping energy. The G2ANet-based digital multi-solution can innovatively contribute to the smart operation of WWTP operation systems owing to its environmental stability and adaptability.

Soil carbon and nitrogen sequestration and associated influencing factors in a sustainable cultivation system of fruit trees intercropped with cover crops

We comprehensively quantified the influence of cover crops on soil carbon and nitrogen storage in Chinese orchards by a meta-analysis based on the literature published during 1990 to 2020. The factors influencing soil carbon and nitrogen storage were also analyzed. The results showed that compared with clean tillage, soil carbon and nitrogen storage under the cultivation of cover crops considerably increased by 31.1% and 22.8%, respectively. Intercropping legumes enhanced soil organic carbon storage by 4.0% and total nitrogen storage by 3.0% compared with non-legumes. The effect of mulching duration was most prominent at 5-10 years, with 58.5% and 32.8% increases in soil carbon and nitrogen storage, respectively. The largest increase in soil carbon and nitrogen storage (32.3% and 34.1%, respectively) occurred in areas with low initial organic carbon (<10 g·kg-1) and total nitrogen (<1.0 g·kg-1). In addition, suitable mean annual temperature (10-13 ℃) and precipitation (400-800 mm) markedly contributed to soil carbon and nitrogen storage in the middle and lower reaches of the Yellow River. The findings indicated that multiple factors influence the synergistic changes in soil carbon and nitrogen storage in orchards, while intercropping with cover crops is an effective strategy for enhancing soil carbon and nitrogen sequestration.

Performance of pilot-scale membrane aerated biofilm reactors integrated with anoxic nano-biotechnological reactor for domestic wastewater treatment

Membrane-aerated biofilm reactors (MABRs) have aroused increasing attention due to their excellent performance in treating wastewater, where the membranes behave as bio-carriers for microorganisms and bubbleless air diffusers. The MABR technology has not been fully commercialized due to reactor design and low total nitrogen (TN) removal efficiency at short hydraulic retention times (HRT). In this study, a hybrid system of MABR 1 integrated with an anoxic nano-biotechnological reactor filled with Granulated Nanoscale Oxyhydroxides of Fe (GNOF) media was evaluated to assess the improvement in nitrogen removal performance at 12, 10, and 4 h of HRTs. At the same time, another MABR (MABR 2) was operated individually at 12, 10, 8, 6, 4, and 2 h of HRTs to assess the influence of HRT on nitrogen removal performance. An enhancement in removal performance was reported in the hybrid MABR-GNOF, achieving the highest removal efficiencies of 74.3 ± 3.1% for ammonium nitrogen (NH4+-N), 69.8 ± 2.1% for total nitrogen (TN), and 90.9 ± 1.7% for chemical oxygen demand (COD), at 12 h HRT. The hybrid MABR-GNOF system attained 18% higher nitrogen removal than the MABR-only system at 12 h of HRT. A simultaneous anoxic nitrification-denitrification and COD oxidation might be developed for the removal of COD, NH4+-N, and TN from domestic wastewater by using GNOF as an electron acceptor in the hybrid MABR-GNOF unit. The findings in this study confirmed the possibility of integration of GNOF and MABR on a pilot scale and are promising for the application of this hybrid system on a full scale.

Biochar as a water protection tool in peatland forestry – nutrient recovery from runoff water

Water quality is significantly affected by the forest logging activities that are carried out in drained peatlands, which causes a notable enhancement in sediment loading and nutrient export to water bodies. Furthermore, the seasonal fluctuations in nutrient concentrations in the runoff further underscore the need for efficient water protection tools in peatland forestry. To address these issues, biochar-based adsorption methods can potentially offer an effective alternative for water protection in peatland forestry. The current thesis investigates the potential for adsorption-based nutrient recovery from clear-cut peatland runoff water using Norway spruce and Silver birch biochars. In particular, the aim is to i) study the adsorption characteristics in relation to biochar properties and nutrient compounds in runoff waters in a small-scale laboratory experiment (Paper I); ii) investigate the adsorption characteristics of nitrogen (N) compounds from runoff water in a meso-scale laboratory experiment that utilises biochar reactors (Paper II); and iii) investigate the dynamics of biochar adsorption and desorption under fluctuating total nitrogen (TN) concentrations in runoff water (Paper III). Biochar made from birch and spruce were shown to efficiently adsorb N compounds from the water. Across all experiments, the TN content declined significantly, irrespective of the scale, or whether the runoff volume ranged from litres (I and III) to hundreds of litres (II). Furthermore, the TN content at the beginning of the experiment declined at the fastest rate. The results indicate that cases of relatively low adsorption capacity were attributed to the low initial TN concentrations in the water. In addition, TN adsorption occurs above a threshold concentration in natural runoff water. The spruce did not adsorb TN when the concentrations in the runoff water fell below 0.4 mg L-1. In this thesis, biochar emerges as a compelling water protection solution, particularly in regions where clear-cut peatlands release substantial quantities of nutrients.

Biochar as a water protection tool in peatland forestry – nutrient recovery from runoff water

Water quality is significantly affected by the forest logging activities that are carried out in drained peatlands, which causes a notable enhancement in sediment loading and nutrient export to water bodies. Furthermore, the seasonal fluctuations in nutrient concentrations in the runoff further underscore the need for efficient water protection tools in peatland forestry. To address these issues, biochar-based adsorption methods can potentially offer an effective alternative for water protection in peatland forestry. The current thesis investigates the potential for adsorption-based nutrient recovery from clear-cut peatland runoff water using Norway spruce and Silver birch biochars. In particular, the aim is to i) study the adsorption characteristics in relation to biochar properties and nutrient compounds in runoff waters in a small-scale laboratory experiment (Paper I); ii) investigate the adsorption characteristics of nitrogen (N) compounds from runoff water in a meso-scale laboratory experiment that utilises biochar reactors (Paper II); and iii) investigate the dynamics of biochar adsorption and desorption under fluctuating total nitrogen (TN) concentrations in runoff water (Paper III). Biochar made from birch and spruce were shown to efficiently adsorb N compounds from the water. Across all experiments, the TN content declined significantly, irrespective of the scale, or whether the runoff volume ranged from litres (I and III) to hundreds of litres (II). Furthermore, the TN content at the beginning of the experiment declined at the fastest rate. The results indicate that cases of relatively low adsorption capacity were attributed to the low initial TN concentrations in the water. In addition, TN adsorption occurs above a threshold concentration in natural runoff water. The spruce did not adsorb TN when the concentrations in the runoff water fell below 0.4 mg L-1. In this thesis, biochar emerges as a compelling water protection solution, particularly in regions where clear-cut peatlands release substantial quantities of nutrients.

Micro(nano)plastic size and concentration co-differentiate the treatment performance and toxicity mechanism in aerobic granular sludge systems

Microplastics (MPs) and nanoplastics (NPs) released into wastewater may pose threats to the biological treatment process. Aerobic granular sludge (AGS), a prospective biotechnology for wastewater treatment, can effectively intercept MPs and NPs, but the effect of these emerging contaminants on the AGS system lacks investigation. This study showed that low concentrations of MPs and NPs (1 mg/L) posed negligible inhibitory effects on the characteristics and performance of the AGS system. Yet, increasing MPs concentrations to higher levels (20 and 100 mg/L) impaired the granular integrity and diminished the total nitrogen (TN) removal efficiency by 3.8 %–17.8 %, especially for the denitrification process. Compared to MPs, prolonged exposure to NPs at the same levels showed greater adverse effects on the AGS, as evidenced by overgrowth of filamentous bacteria and lower TN removal efficiency (decreased by 11.4 %–22.8 %). This was possibly due to biofilm formation on the surface of MPs and excessive secretion of extracellular polymeric substances served as barriers to mitigate the toxic effects of MPs. In contrast, microorganisms were directly exposed to the toxicity of NPs, resulting in greater oxidative stress, impaired cytomembrane structure, deteriorated cell viability, and decreased proteins production. Metagenomic analysis further revealed that the populations of key functional bacteria (e.g. denitrifiers Acidovorax and Flavobacterium) decreased after long–term exposure to MPs and NPs. The key enzymes and genes (e.g. nirK, napAB, nirS, norBC and nosZ) related to nitrogen transformation processes, electron donor and energy production (e.g. mdh) were declined accordingly, unravelling the inherent mechanism for the decreased nitrogen removal. This study indicated that NPs have more detrimental impacts on AGS system, which needs to be taken into account when treating plastic containing wastewaters.

Spatiotemporal Changes in Water Quality Parameters and the Eutrophication in Lake Erhai of Southwest China

To understand the lake status and reasons of eutrophication at Lake Erhai in recent years, water quality, including water temperature (T), pH, dissolved oxygen (DO), total nitrogen (TN), total phosphorus (TP) and chlorophyll-a (Chl-a) from 2016 to 2020 was monitored and analyzed. The results showed no obvious thermocline in the vertical direction at Lake Erhai, while Chl-a demonstrated obvious spatiotemporal distribution characteristics in Lake Erhai. Chl-a concentrations increased to a maximum in summer in August with the low TN:TP value, leading to algal blooms, most notably in the southern lakes. Low pH and DO appeared due to the thermocline of Erhai Lake (August 2016). A large area of algae distribution due to the increase of total phosphorus appeared in the northern lake area of Lake Erhai in December 2016, with a tendency of mesotrophic to light eutrophic in summer by the nutritional evaluation of Lake Erhai, especially in the central lake area and the northern lake area. Pearson’s correlation coefficient and principal component analysis showed a significant positive correlation between Chl-a and T (r = 0.34, p ≤ 0.01) and TP (r = 0.31 p ≤ 0.01) in the mesotrophic Lake Erhai, indicating that TP content was one of the triggering factors for the algal blooming. Based on the spatiotemporal changes in water quality parameters and their relationship with eutrophication, scientific agencies should implement management strategies to protect Lake Erhai, supplemental to the costly engineering measurements.

Effect of Bio-Electrochemical Treatment of Hydroponic Effluent on the Nutrient Content

This study examined the effect of bio-electrochemical treatment processes on nitrogen and phosphorus removal, but it also evaluated the impact of the treatment processes on the concentration of other nutrients present in hydroponic effluent. A bio-electrochemical reactor (BER) was used in the experiment to treat effluent from the hydroponic cultivation of tomatoes. It was stated that the nitrogen removal efficiency decreased with increasing current density. The study showed that an electric current density of 0.63 A/m2 ensured the lowest effluent nitrate concentration. The nitrogen removal efficiency ranged from 41.6%-R1 (density of 0.63 A/m2) to 8.9%-R4 (density of 5 A/m2). Electric current density higher than 1.25 A/m2 resulted in lower total nitrogen removal efficiency. The total phosphorus removal efficiency increased with increasing electric current density. The phosphorus removal efficiency was the lowest—95.1%—in the R1 reactor, whereas it was the highest in R4—99.1%. The concentration of the other elements in the effluent was determined. The content of molybdenum, boron, sulphates, and potassium did not meet the acceptable norms for discharging hydroponic effluent into the environment. The study showed that bio-electrochemical processes taking place in BER caused secondary contamination of hydroponic wastewater with molybdenum ions.