[N2O Emission from the Processes of Wastewater Treatment: Mechanisms and Control Strategies].

As a direct carbon emission source, the amount of nitrous oxide (N2, which is actually caused by AOB denitrification. To control the N2O emission during biological N-removal, complete HND and NO2- accumulation for AOB denitrification should be avoided to a large extent. For this purpose, DO in aerobic tanks should be controlled at a normal level (approximately 2 mg·L-1), and solid retention time (SRT) should be extended, up to 20 d, which would avoid accumulating N2O for AOB denitrification. Additionally, external carbon should be supplemented in time to promote HDN approaching the end, N2. This review summarizes the mechanisms of all the mentioned N2O emission pathways and discusses the control strategies of N2O emission according to the associated mechanisms.

Plants boost pyrrhotite-driven nitrogen removal in constructed wetlands.

Pyrrhotite is a promising electron donor for autotrophic denitrification. Using pyrrhotite as the substrate in constructed wetlands (CWs) can enhance the nitrogen removal performance in carbon-limited wastewater treatment. However, the role of plants in pyrrhotite-integrated CW is under debate as the oxygen released from plant roots may destroy the anoxic condition for autotrophic denitrification. This study compared pyrrhotite-integrated CWs with and without plants and identified the effects of plants' presence in nitrogen removal, pyrrhotite oxidized dissolution, and microbial community. The results show that plants enhanced the TN removal significantly (from 41.6 ± 3.9 % to 97.1 ± 2.6 %). Plants can accelerate the PAD in CW through the strengthening of pyrrhotite dissolution. Enriched functional (Thiobacillus and Acidiferrobacter) and a more complex bacterial co-occurrence network has been found in CW with plants. This study identified the role of plants in PAD acceleration, providing an in-depth understanding of pyrrhotite in CW systems.

Making Waves: A sea change in treating wastewater - Why thermodynamics supports resource recovery and recycling.

Entropy is a concept defined by the second law of thermodynamics. Applying this concept to the world we live in, entropy production must be minimized and negentropy (negative entropy production) should be accelerated, in order to produce a healthy and stable ecological system. The present wastewater treatment, however, contributes to entropy production. This means that conventional wastewater treatment, without recovery of resource and energy, will gradually but inevitably contribute to a deteriorating ecological balance. When the self-cleaning ability of the natural ecological system is limited, the need to develop sustainable wastewater treatment in order to delay entropy production and accelerate negentropy becomes urgent. Resource and energy recovery from wastewater should be the first priority, as they can contribute significantly towards minimizing entropy production and accelerating negentropy. Sustainable wastewater treatment must focus on recovering recyclable high value-added organic chemicals from wastewater and/or excess sludge to minimize entropy production caused by methane (CH4, once combusted, is converted into CO2 - an even higher substance in entropy) via anaerobic digestion. Instead of CH4, thermal energy present in the effluent can be utilized for heating/cooling buildings and also for drying excess sludge towards incineration to recover more energy. Overall, this can lead to a carbon-neutral operation and even creating a "carbon sink" could be possible for wastewater treatment.

Recovery of value-added products by mining microalgae.

Microalgae blooms are always blamed for the interruption of the aquatic environment and pose a risk to the source of drinking water. Meanwhile, microalgae as primary producers are a kind of resource pool and could benefit the environment and contribute to building a circular economy. The lipid and polyhydroxybutyrate (PHB) in the cells of microalgae could be alternatives to fossil fuels and plastics, respectively, which are the culprits of global warming and plastic pollution. Besides, some microalgae are rich in nutrients, such as proteins and astaxanthin, which make themselves suitable for feed additives. As wastewater is rich in nutrients necessary for microalgae, thus, value-added product recovery via microalgae could be an approach to valorizing wastewater. However, a one-size-fits-all approach deploying various wastewater for the above products cannot be summarized. On the contrary, specific technical protocols should be tailored regarding each product in microalgae biomass with various wastewater. Thus, this review is to summarize the research effort by far on wastewater-cultivated microalgae for value-added products. Wastewater type, regulation methods, and targeted product yields are compiled and discussed and are expected to guide future extrapolation into a commercial scale.

Treating carbon-limited wastewater by DWTR and woodchip augmented floating constructed wetlands.

Floating constructed wetlands (FCWs) have attained tremendous popularity for water purification purposes. However, FCW functions establishment in nutrients removal from carbon-limited wastewater, especially in cold weather, is still a challenge. Here, two drinking water treatment residual (DWTR) based biocarriers (B-I: DWTR cakes, B-II: DWTR cakes combined with woodchips) have been augmented into FCW to enhance the nutrients (N and P) removal performance. Compared to the traditional FCW, the intensified FCWs simultaneously achieved higher N and P removal efficiencies, with average pollutants removal of 52.16 ± 11.51% for TN and 92.72 ± 1.61% for TP in FCW-I and 57.65 ± 9.43% for TN and 92.17 ± 2.55% for TP in FCW-II, respectively, while their removal in FCW-III of 27.74 ± 7.11% for TN and 17.91 ± 9.27% for TP. B-II performed best in overcoming the negative influence of low temperature in nutrients removal. Mass balance budget indicated that most P was enriched in DWTR based biocarriers. Thus it is feasible to recycle and recover P from the surface water. Furthermore, P in the sediment can be changed from active P to stable P, mitigating the internal P release risk. This study can help to expand the understanding of the intensified FCWs and promote the practical application of FCWs.

Capabilities and mechanisms of microalgae on removing micropollutants from wastewater: A review.

Micropollutants in wastewater are a set of compounds receiving a growing concern to the environment and human health. As a green and low-cost process, microalgae-based systems (MBSs) have already been demonstrated the ability of micropollutant removal. In the present review, 114 micropollutants and 16 microalgae species in total are summarized and analyzed to present an overview capability of the MBSs. The analysis shows that MBSs can eradicate most of the included micropollutants with 94 compounds (82% of total) being removed by ≥ 50%. Regarding the reliability of removal efficiency, those from hormone active substances, macrolides, and cephalosporins are consistently removed at a high level (≥80%). Herein, biodegradation is the predominant removal pathway for most micropollutants, particularly, bearing electron-donating groups. Besides, the large family of microalgae species and unique phototrophic ability enables broad ecological niches and extra abilities over activated sludge systems to remove some recalcitrant micropollutants, e.g. pesticides. In the future study, optimization on the reactor configuration and operation parameters is expected to improve the stability of MBSs before extrapolating to full-scale deployment.

Dual role of macrophytes in constructed wetland-microbial fuel cells using pyrrhotite as cathode material: A comparative assessment.

In the recent years many studies have shown that wetland plants play beneficial roles in bioelectricity enhancement in constructed wetland-microbial fuel cell (CW-MFC) because of the exudation of root oxygen and root exudates. In this study, the long-term roles of plants on the bioelectricity generation and contaminant removal were investigated in multi-anode (Anode1 and Anode2) and single cathode CW-MFCs. The electrode distances were 20 cm between Anode1-cathode and 10 cm between Anode2-cathode, respectively. Additionally, the employment of natural conductive pyrrhotite mineral as cathode material was firstly investigated in CW-MFC system. A cathode potential of -98 ± 52 mV to -175 ± 60 mV was achieved in the unplanted (CW-MFC 1), and planted CW-MFCs with Iris pseudacorus (CW-MFC 2), Lythrum salicaria (CW-MFC 3), and Phragmites australis (CW-MFC 4). The maximum power densities of Anode1-cathode and Anode2-cathode were 8.23 and 15.29 mW/m2 in CW-MFC 1, 8.51 and 1.67 mW/m2 in CW-MFC 2, 5.67 and 3.15 mW/m2 in CW-MFC 3, and 7.59 and 14.71 mW/m2 in CW-MFC 4, respectively. Interestingly, smaller power density was observed at Anode2-cathode, which has shorter electrode distance than Anode1-cathode in both CW-MFC 2 and CW-MFC 3, which indicates the negative role of oxygen released from the flourished plant roots at Anode2 micro-environment in power production. Therefore, recovering power from commercial CW-MFCs with flourished plants will be a challenge. The contradiction between keeping short electrode distance and avoiding the interference from plant roots to maintain anaerobic anode may be solved by the proposed modular CW-MFCs.

Relieving the inhibition of humic acid on anaerobic digestion of excess sludge by metal ions.

Anaerobic digestion (AD) is an effective approach to recovering chemical (organic) energy from excess sludge, but the conversion efficiency for energy is usually not very high. One of the obstacles comes from the severe inhibition of humic acid (HA) on both hydrolytic and methanogenic process on the AD. Therefore, it is necessary to ascertain some effective approaches to relieving the inhibition of HA for obtaining a high methane (CH4) yield. With the "clean" sludge (cultured by synthetic wastewater) containing almost no HA and metal ions, the inhibition of HA on the AD process was designed by dosing HA at 15% VSS, and relieving the inhibition by metal ions was also designed by dosing the different amounts of Ca2+ and Al3+. Based on the batch AD experiments, solo Ca2+=100 mg /L or Al3+=70 mg/L added realized the highest relieved efficiency of 65%, respectively. Interestingly, dual metal ions added at the low concentrations (Ca2+=50 mg/L and Al3+=10 mg/L) could reach up to 80 % of the relieved efficiency, which was attributed to the synergistic effect of 1+1>2. The mechanisms behind the phenomena could be that metal ions might interact with HA via electrostatic force, cation exchange and sweep flocculation. Thus, some key hydrolytic and methanogenic enzymes could indirectly be reactivated and degradation of organic substances could be enhanced in the AD process. In wastewater treatment plants, metal ions contained in excess sludge would "inherently" relieve the inhibition of HA to an extent, which depends on the effective and/or optimal concentration of metal ions at a free (unabsorbed and/or unwrapped) state.

Where do we stand to oversee the coronaviruses in aqueous and aerosol environment? Characteristics of transmission and possible curb strategies.

By 17 October 2020, the severe acute respiratory syndrome coronavirus (SARS-CoV-2) has caused confirmed infection of more than 39,000,000 people in 217 countries and territories globally and still continues to grow. As environmental professionals, understanding how SARS-CoV-2 can be transmitted via water and air environment is a concern. We have to be ready for focusing our attention to the prompt diagnosis and potential infection control procedures of the virus in integrated water and air system. This paper reviews the state-of-the-art information from available sources of published papers, newsletters and large number of scientific websites aimed to provide a comprehensive profile on the transmission characteristics of the coronaviruses in water, sludge, and air environment, especially the water and wastewater treatment systems. The review also focused on proposing the possible curb strategies to monitor and eventually cut off the coronaviruses under the authors' knowledge and understanding.

Research advances of Tetrasphaera in enhanced biological phosphorus removal: A review.

The processes of enhanced biological phosphorus removal (EBPR) have been widely applied in wastewater treatment plants (WWTPs). However, meeting the increasingly stringent effluent discharge standards requires a more stable EBPR performance. Under the circumstances, the identification of genus Tetrasphaera as potential phosphate accumulating organisms (PAOs) has aroused much research interests on them. In practice, a large biovolume of genus Tetrasphaera has been reliably observed in a number (up to 80) of WWTPs around the world. Tetrasphaera show a phenotype of aerobic polyphosphate (poly-P) accumulation at the condition of assimilating glucose and/or amino acids anaerobically in advance. Moreover, Tetrasphaera also present versatile physiologies, of which some show no net orthophosphate removal. While there are certainly some contradictory results and gaps in our knowledge concerning Tetrasphaera, this review summarizes the discovery, abundance in WWTPs, functions on EBPR, and biochemistry of the genus Tetrasphaera in the existing literature. It is expected to present the state-of-art progress about the genus Tetrasphaera, and to guide future R & D work.

Energy recovery from wastewater: Heat over organics.

In dealing with wastewater, chemical energy has traditionally been perceived as the only source of recoverable energy in moving towards the carbon-neutral operation of wastewater treatment plants. Based on an estimation of practically recoverable energy embedded in municipal wastewater, however, the potential for thermal energy (90% recovery from wastewater) is much higher than for chemical energy (COD, 10% recovery). The carrier of chemical energy (COD) has a high exergy value which should, from a sustainability point of view, be utilized to the greatest extent possible. Rather than being converted into methane (and subsequently into carbon dioxide), carbon (COD) contained in wastewater should be converted into highly valuable organic products. Thermal energy could be utilized for district heating/cooling, agricultural greenhouses, and even for drying dewatered sludge. In this way, thermal energy can indirectly offset the energy demand for wastewater treatment. The limitations in utilizing thermal energy are not generally based on technical difficulties; in fact, they can be mainly attributed to supply distances and governmental policies. It would, therefore, be greatly beneficial if municipal authorities would work together to jointly plan utilization of this thermal energy.

Adaptation of semi-continuous anaerobic sludge digestion to humic acids.

Anaerobic digestion (AD) is a technology for recovering chemical energy as methane from excess sludge/waste. Unfortunately, humic acids (HA) contained in excess sludge can have the effects of inhibiting the efficiency of energy conversion. Based on a batch experiment, the impact of HA on a semi-continuous AD process was sequentially investigated, with the impact on the associated enzymes and microorganisms being measured. The results of this semi-continuous experiment indicate that the inhibition of the microbial community increased with an increased HA:VSS ratio. Long-term cultivation did not result in the adaption of methane production to the presence of HA. Moreover, at HA:VSS = 20%, the strongest inhibition (74.3%) on energy conversion efficiency was observed in the semi-continuous experiment, which was two-fold higher than that recorded in the batch experiment. This is attributed to serious and irreversible inhibition of both acidogenic and methanogenic microorganisms, as well as the physical-chemical reactions between HA and the associated enzymes which, it was concluded, were the dominant mechanisms of inhibition in the batch experiment.

Environmental impacts of resource recovery from wastewater treatment plants.

Conventional wastewater treatment plants (WWTPs) clean wastewater and minimize water pollution; but, while doing so, they also contribute to air pollution and need energy/material input with associated emissions. However, energy recovery (e.g. anaerobic digestion) and resource recovery (e.g. water reuse) allow us to offset the adverse environmental impacts of wastewater treatment. Life cycle assessments (LCA) have been used more and more to evaluate the environmental impacts of WWTPs and to suggest improvement options. There is a need to search for resource recovery applications that genuinely realize a net-zero impact on the total environment of WWTPs. In this work, a scheme with highly efficient energy and resource recovery (especially for thermal energy) is proposed and evaluated. The environmental impact of a conventional WWTP in comparison with the scheme proposed here, with energy/resource recovery included, was calculated, and discussed with reference to LCA methodology. In the process of using LCA, it was necessary to choose a regional situation to focus on. In this case, a Chinese situation was focused as a reference, but the qualitative information gained is of worldwide relevance. The results clearly revealed that conventional WWTP does not benefit the total environment as a whole while the new scheme benefited the total environment via resource/energy recovery-based processes. Among others, thermal energy recovery played a significant role towards a net-zero LCA analysis (contributing around 40%) which suggests that more attention and research should be focused on it.

Adsorption of phosphorus with calcium alginate beads containing drinking water treatment residual.

Aluminum-based drinking water treatment residuals (DWTR) were encapsulated by alginate to develop a pelletized media (DWTR-CA beads) for phosphorus (P) adsorption. The beads were successfully manufactured to uniform size and shape requirements. The effects of DWTR powder concentration and particle size, and bead mean size on P adsorption, were investigated. The DWTR was found to be an important component in the beads for P adsorption, while the calcium alginate shell contributed little for P adsorption. The maximum P adsorption capacity of the DWTR-CA bead was 19.42 mg P/g wet beads, corresponding to a bead diameter of 3.1 ± 0.2 mm and DWTR concentration of 2% (1% weight/volume (W/V)), mg/mL). The adsorption data fit well with the intra-particle diffusion model and the pseudo-second-order kinetic model, while both the Langmuir and Freundlich adsorption isotherms described the adsorption process well. Furthermore, the study on the effect of pH on P adsorption showed that acidic conditions resulted in a better P adsorption and the DWTR-CA beads have the function of pH neutralization. The findings of this study show that the DWTR-CA beads are a promising adsorbent/substrate for P removal.

Enhanced Nutrients Removal Using Reeds Straw as Carbon Source in a Laboratory Scale Constructed Wetland.

The low carbon/nitrogen (C/N) ratio and high nitrate content characteristics of agricultural runoff restricted the nitrogen removal in constructed wetlands (CWs). To resolve such problems, the economically- and easily-obtained Phragmites Australis (reeds) litters were applied and packed in the surface layer of a surface flow CW as external carbon sources. The results demonstrated that the introduction of the reeds straw increased the C concentration as a result of their decomposition during the CW operation, which will help the denitrification in the ensuing operation of an entire 148 days. The total nitrogen (TN) and Chemical Oxygen Demand (COD) () in the effluent reached the peak level of 63.2 mg/L and 83 mg/L at the fourth and the second day, respectively. Subsequently, the pollutants in the CW that were filled with straw decreased rapidly and achieved a stable removal after 13 days of operation. Moreover, the present study showed that the N removal efficiency increased with the increase of the hydraulic retention time (HRT). Under the HRT of four days, the CW presented 74.1 ± 6%, 87.4 ± 6% and 56.0 ± 6% removal for TN, NO3⁻, and TP, respectively.

Global development of various emerged substrates utilized in constructed wetlands.

Substrate selection is one of the key technical issues for constructed wetlands (CWs), which works for wastewater treatment based mainly on the biofilm principle. In recent years, many alternative substrates have been studied and applied in CWs, and a review is conducive to providing updated information on CW R&D. Based on the intensive research work especially over the last 10 years on the development of emerged substrates (except for the three conventional substrates of soil, sand, and gravel) in CWs, this review was made. The substrates are categorized depending on their main roles in pollutant removal as ion-exchange substrates, P-sorption substrates, and electron donor substrates. Among these, reuse of various waste products as substrates was suggested due to their competitive pollutant removal efficiency and minimized waste disposal. Regarding substrate development, future research on avoiding substrate clogging to extend their lifetime in CWs is needed.

Long-term operation with an insight into a newly established green bio-sorption reactor: Can it achieve "1 + 1 > 2"?

An eco-friendly system of green bio-sorption reactor (GBR), constructed by embedding alum sludge-based constructed wetland (AlCW) into a conventional activated sludge process to achieve "1 + 1 > 2", was evaluated under a long-term operation basis. Insight into the pollutants removal, particularly the role of the AlCW in the GBR, was explored and discussed. The results showed that the GBR could achieve 90% and 95% removal for TN and TP (Stage 4), respectively, under the hydraulic and nitrogen loading rate of 2.07 m3/(m3·d) and 166.2 gN/(m3·d), respectively. Intriguingly, despite the P adsorption, the AlCW enlarged the size of the activated sludge flocs which benefited the simultaneous nitrification and denitrification. Subversively, the embedding AlCW brings about dual-intensification in both capacity and efficiency. In addition, the GBR as an ecological engineering system can be employed closely to residential area in line with its green and pleasing appearance.

Energy capture and nutrients removal enhancement through a stacked constructed wetland incorporated with microbial fuel cell.

To improve the sustainability of constructed wetlands (CWs), a novel tiered wetland system integrated with a microbial fuel cell (MFC) was developed in this study. Compared to the single stage CW, chemical oxygen demand (COD) removal efficiency was improved from 83.2% to 88.7%. More significantly, this tiered system significantly enhanced total nitrogen removal efficiency (an increase from 53.1% to 75.4%). In terms of MFC integration, a gradually decreased performance in electricity production was observed during its 3 months of operation (the voltage dropped from nearly 600 mV to less than 300 mV), which resulted in a reduction of power density from around 2 W/m3 to less than 0.5 W/m3. The deterioration in performance of the air-cathode is the main reason behind this, since the electrode potential of the cathode under open circuit reduced from 348.5 mV to 49.5 mV while the anode potential kept constant at around -400 mV. However, in spite of its electrical performance reduction, it was proved that MFC integration enhanced COD removal and the nitrification process. Further work is needed to improve the stability and feasibility of this new system.

A fancy eco-compatible wastewater treatment system: Green Bio-sorption Reactor.

A novel concept was proposed and preliminarily investigated by embedding alum sludge-based constructed wetland into conventional activated sludge system in terms of Green Bio-sorption Reactor (GBR). This novel GBR inherited the aesthetic value of constructed wetland and owned the robust phosphorus (P) adsorption along with the benefit of carriers' addition (dewatered alum sludge). The preliminary demonstration was conducted in a lab-scale sequencing batch reactor (SBR) system without biological phosphorus removal process. The novel process achieved averagely 96%, 99% and 90% for BOD, TP and TN removal with piggery wastewater as influent, demonstrating for the first time of its promising performance. Moreover, the coexistence of biofilm and suspended sludge also achieved 55-88% simultaneous nitrification and denitrification efficiency, higher than biofilm only. Overall, alum sludge-based GBR could achieve reliable pollutants removal and provides a novel and sustainable pathway to upgrade conventional activated sludge system.

Embedding constructed wetland in sequencing batch reactor for enhancing nutrients removal: A comparative evaluation.

In the present study, a novel green bio-sorption reactor (GBR) was firstly proposed and preliminarily investigated by embedding constructed wetland (CW) into the aeration tank of the conventional activated sludge (CAS). This integrated novel system owns the striking features of adding carriers of wetland substrate (i.e. the dewatered alum sludge in this case) in CAS for robust phosphorus adsorption and enriching the biomass. Meanwhile, the "green" feature of this GBR imparted aesthetic value of CW to the CAS system. The preliminary 3-month trial of GBR based on a sequencing batch reactor (GB-SBR) with diluted piggery wastewater demonstrated an average removal of 96%, 99% and 90% for BOD, TP and TN, respectively. The comparison with moving bed biofilm reactor (MBBR) and integrated fixed-film activated sludge (IFAS) reflected the advantages of GBR over purification performance, aesthetic value and potential carbon sink. Moreover, the carriers used in the GBR are dewatered alum sludge which is in line with the policy of "recycle, reuse and reduce". Overall, this GBR undoubtedly offered a more sustainable and economical solution for retrofitting the aging CAS.