Submerged vegetation removal promotes shift of dominant phytoplankton functional groups in a eutrophic lake

Historical data indicate that the dominance of submerged plants in Dianchi Lake in the 1960 s was characterized by low algal density with dominance of non-toxic group J（Scenedesmus,Pediastrum,etc.）. The removal of submerged plants,which began in the 1970 s,resulted in the expansion of bloom-forming Microcystis（group M）. Laboratory experiments suggested that Microcystis aeruginosa was inclined to grow and develop at elevated temperatures. The growth of Scenedesmus obliquus was slower than that of co-cultivated M. aeruginosa in the absence of Ceratophyllum demersum,especially at higher temperatures. The existence of submerged plant C. demersum could inhibit the growth of the harmful algae M. aeruginosa and this inhibitory effect by C. demersum was enhanced with an increase in temperature. Instead,with C. demersum,the growth of S. obliquus was not inhibited,but the co-cultivated M. aeruginosa was eliminated in a short time. Combined with the historical data and laboratory experiments,it was indicated that the submerged plants might play important roles in the dominance of the non-toxic group J in the historical succession. Consequently,the introduction of the submerged plant such as C. demersum might alter the dominant phytoplankton functional groups from M to J and benefit the restoration of the eutrophic lake.     

Contaminant removal from low-concentration polluted river water by the bio-rack wetlands

The bio-rack is a new approach for treating low-concentration polluted river water in wetland systems. A comparative study of the efficiency of contaminant removal between four plant species in bio-rack wetlands and between a bio-rack system and control system was conducted on a small-scale (500 mm length × 400 mm width × 400 mm height) to evaluate the decontamination effects of four different wetland plants. There was generally a significant difference in the removal of total nitrogen (TN), ammonia nitrogen (NH₃-N) and total phosphorus (TP), but no significant difference in the removal of permanganate index (COD_Mn) between the bio-rack wetland and control system. Bio-rack wetland planted with Thalia dealbata had higher nutrient removal rates than wetlands planted with other species. Plant fine-root (root diameter ≤ 3 mm) biomass rather than total plant biomass was related to nutrient removal efficiency. The study suggested that the nutrient removal rates are influenced by plant species, and high fine-root biomass is an important factor in selecting highly effective wetland plants for a bio-rack system. According to the mass balance, the TN and TP removal were in the range of 61.03--73.27 g/m² and 4.14--5.20 g/m² in four bio-rack wetlands during the whole operational period. The N and P removal by plant uptake constituted 34.9%--43.81% of the mass N removal and 62.05%--74.81% of the mass P removal. The study showed that the nitrification/denitrification process and plant uptake process are major removal pathways for TN, while plant uptake is an effective removal pathway for TP.     

Effect of dissolved oxygen on nitrate removal using polycaprolactone as an organic carbon source and biofilm carrier in fixed-film denitrifying reactors

Nitrate-nitrogen(NO_3~--N) always accumulates in commercial recirculating aquaculture systems(RASs) with aerobic nitrification units. The ability to reduce NO_3~--N consistently and confidently could help RASs to become more sustainable. The rich dissolved oxygen(DO)content and sensitive organisms stocked in RASs increase the difficulty of denitrifying technology. A denitrifying process using biologically degradable polymers as an organic carbon source and biofilm carrier was proposed because of its space-efficient nature and strong ability to remove NO_3~--N from RASs. The effect of dissolved oxygen(DO) levels on heterotrophic denitrification in fixed-film reactors filled with polycaprolactone(PCL) was explored in the current experiment. DO conditions in the influent of the denitrifying reactors were set up as follows: the anoxic treatment group(Group A, average DO concentration of 0.28 ± 0.05 mg/L), the low-oxygen treatment DO group(Group B, average DO concentration of 2.50 ± 0.24 mg/L) and the aerated treatment group(Group C, average DO concentration of 5.63 ± 0.57 mg/L). Feeding with 200 mg/L of NO_3~--N, the NO_3~--N removal rates were 1.53, 1.60 and 1.42 kg/m3PCL/day in Groups A, B and C, respectively. No significant difference in NO_3~--N removal rates was observed among the three treatments. It was concluded that the inhibitory effects of DO concentrations lower than 6 mg/L on heterotrophic denitrification in the fixed-film reactors filled with PCL can be mitigated.     

The plant-enhanced bio-cathode: Root exudates and microbial community for nitrogen removal

A plant bio-electrochemical system(PBES) was constructed for organic pollutant removal and power generation. The bio-cathode, composed of granular activated carbon(GAC), stainless wire mesh and a plant species(Triticum aestivum L.), was able to catalyze cathodic reactions without any requirement for aeration or power input. During the 60-day-long operation, an average voltage of 516 m V(1000 Ω) and maximum power density(Pmax) of 0.83 W/m~3 were obtained in the PBES. The total nitrogen removal and total organic carbon removal in the PBES were 85% and 97%, respectively. Microbial community analyses indicated that bacteria associated with power generation and organic removal were the predominant species in the bio-cathode, and plant-growth-promoting rhizobacteria were also found in the PBES. The results suggested that the coupling of plants with the GAC cathode may enhance the organicmatter degradation and energy generation from wastewater and therefore provide a new method for bio-cathode design and promote energy efficiency.     

An efficient way to enhance the total nitrogen removal efficiency of the Anammox process by S0-based short-cut autotrophic denitrification

In order to reduce the amount of NO_3~-–N generated by the Anammox process, and alleviate the competition between denitrification and Anammox for NO_2~-–N in a single reactor, the preference of S~0 for reacting with coexisting NO_2~-–N and NO_3~-–N in the sulfur autotrophic denitrifying(SADN) process and the coupling effect of short-cut SADN and the Anammox process were studied. The results showed that S~0 preferentially reacted with NO_3~-to produce NO_2~-–N, and then reacted with NO_2~-–N when NO_3~-–N was insufficient, which could effectively alleviate the competition between SADN bacteria(SADNB) and Anammox bacteria(An AOB) for NO_2~-–N. After 170 days of operation, coupling between short-cut S~0-SADN and the Anammox process was first successfully achieved. SADNB converted the NO_3~-–N generated by the Anammox process into NO_2~-–N, which was once again available to An AOB. The total nitrogen removal efficiency eventually stabilized at over 95%, and the effluent NO_3~-–N was controlled within 10 mg/L, when high NH_4~+–N wastewater was treated by the Anammox process. Microbial community analysis further showed that Candidatus Brocadia and Thiobacillus were the functional microorganisms for An AOB and SADNB.     

Persistence and growth of faecal culturable bacterial indicators in water column and sediments of Vidy Bay, Lake Geneva, Switzerland

The aims of this study was to investigate the persistence and the growth of culturable bacterial indicators (CBI) including total coliforms (TC) and faecal coliforms represented by Escherichia coli, enterococcus (ENT), and aerobic mesophilic bacteria (AMB) in the surface sediments and the water column of Vidy Bay (Lake Geneva, City of Lausanne, Switzerland). The study was carried out for 60 d using microcosms containing Sewage Treatment Plant (STP) e uent and nonsterile water without CBI, as well as contaminated and non-contaminated sediments. The e ects of water temperature and of organic matter associated with sediments on the survival of CBI in the sediments and the water column were observed. The number of CBI colonies in the contaminated sediments of Vidy Bay and in the STP e uent was almost identical in the order of 105–107, 104–106, 103–105, and 104–107 CFU/100 g sediment or /100 mL water for TC, E. coli, ENT, and AMB respectively. A degradation of CBI was observed in the sediments where organic mater content was low and in the water column at a temperature of 10°C after 5 d of experimentation. In addition, a growth of CBI was observed in the sediment which is rich in organic matter at 20°C. The results of this study indicate: (1) the higher concentrations of the CBI observed in di erent points in the water column of Vidy Bay may not be explained only by the recent contribution of the three potential sources of the Bay contamination including STP and the Chamberonne and Flon Rivers, but also by the persistence, removal from sediment and multiplication of CBI in the sediment and water column; (2) the sediment of Vidy Bay constitute a reservoir of CBI and can even support their growth; and (3) the CBI not only survive in sediments, but also can be remobilized and increased in the water column, therefore, it become a permanent microbiological pollution in Vidy Bay.     

Evaluation of a cost e ective technique for treating aquaculture water discharge using Lolium perenne Lam as a biofilter

Wastewater stabilization ponds generate low cost by-products that are useful for agriculture. The utilization of these by-products for soil amendment and as a source of nutrients for plants requires a high level of sanitation and stabilization of the organic matter, to maintain acceptable levels of soil, water and air quality. In this study, two aquaculture wastewater treatment systems; recirculating system and a floating plant bed system were designed to improve the quality of irrigation water in local communities with low income. In both systems the grass species Lolium perenne Lam was used as a plant biofilter while vegetable specie Amaranthus viridis was used to evaluate the performance of the system and the suitability of the phyto-treated water for irrigation. It was found that the harmful material removal rate for recirculating system was 88.9% for TAN （total ammonia nitrogen）, 90% for NO2--N, 64.8% for NO3--N while for floating plant bed system 82.7% for TAN, 82% for NO2--N and 60.5% for NO3--N. Comparative analysis of the efficiency of waste element removal between the two systems revealed that both systems performed well, however, plant growth was not robust for floating plant bed system while recirculating system is energy consuming. Although both systems did not attain sufficient levels of TN （total nitrogen） and TP （total phosphorus） load reduction, the treatment with L. perenne remarkably improved the irrigation water quality. A. viridis plants irrigated with the phyto-treated discharge water had lesser concentrations of heavy metals in their tissues compared to those irrigated with untreated discharge. The control plants irrigated with untreated discharge were also found to be highly lignified with few stems and small leaves.     

Accelerated biodegradation of nitrophenols in the rhizosphere of Spirodela polyrrhiza

We investigated the biodegradation of 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), and 2,4-dinitrophenol (2,4-DNP) in the rhizosphere of Spirodela polyrrhiza plants by conducting degradation experiments with three river water samples supplemented with each nitrophenol (NP). We then isolated NP-degrading bacteria both from the S. polyrrhiza roots and from the river water. In the river water samples, removal of the three NP was accelerated in the presence of S. polyrrhiza plants. The three NPs persisted in an autoclaved solution with sterile plants suggests that NP removal was accelerated largely by bacterial NP biodegradation rather than by adsorption and uptake by the plants. We isolated 8 strains of NP-degrading bacteria: 6 strains from the S. polyrrhiza roots and 2 strains from river water without the plants. The 2-NP- and 2,4-DNP-degrading bacteria were isolated only from the S. polyrrhiza roots. The 4-NPdegrading bacteria different from those isolated from the river water samples were also found on S. polyrrhiza roots. The 2-NP- and 4-NP-degrading strains isolated from the roots utilized the corresponding NP (0.5 mmol/L) as the sole carbon and energy source. The 2,4-DNP-degrading strains isolated from the roots showed substantial 2,4-DNP-degrading activity, but the presence of other carbon and energy sources was required for their growth. The isolated NP-degrading bacteria from the roots must have contributed to the accelerated degradation of the three NPs in the rhizosphere of S. polyrrhiza. Our results suggested that rhizoremediation with S. polyrrhiza may be effective for NP-contaminated surface water.     

Algal uptake of dissolved organic nitrogen in wastewater treatment plants

The algal uptake of dissolved organic nitrogen(DON) in the anaerobic–anoxic–oxic(A_2O)process was investigated in this study. Anaerobic, aerobic and effluent DON samples from two wastewater treatment plants(WWTPs) were separated into hydrophilic and hydrophobic fractions using a DAX-8 resin coupled with an anion exchange resin and a nanofiltration(NF)pretreatment. Hydrophilic DON accounted for 66.66%–88.74% of the entire DON for the two plants evaluated. After a 15-day incubation, 16.95%–91.75% DON was bioavailable for algal growth, and untreated samples exhibited higher DON bioavailability, with 52.83% DON average uptake rates, compared with the hydrophilic and hydrophobic fractions(45.53% and 44.40%, respectively) because the pretreatment caused the inorganic salt to be resistant to algae. Anaerobic untreated samples, hydrophilic fractions and hydrophobic fractions showed higher DON reduction rates and higher biomass accumulation compared with the other DON fractions due to the decomposition of resistant organics by anaerobic and anoxic bacteria.DON in aerobic and effluent samples of plant A was more bioavailable than that of plant B with usages of 27.49%–55.26% DON. DON bioavailability in the anaerobic–anoxic–oxic process decreased in the following order: anaerobic effluent aerobic. The DON contents were reduced after anaerobic treatment in the two plants. The EEM-PARAFAC model identified three DON components, including two humic acid-like substances and one protein-like substance in plant A and two protein-like substances and one humic acid-like substance in plant B.     

Interference of iron as a coagulant on MIB removal by powdered activated carbon adsorption for low turbidity waters

Powered activated carbon (PAC) is widely used in water treatment plants to minimize odors in drinking water. This study investigated the removal of 2-methylisoborneol (MIB) by PAC adsorption, combined with coagulation using iron as a coagulant. The adsorption and coagulation process were studied through different case scenarios of jar tests. The analysis evaluated the effect of PAC dosing in the liquid phase immediately before or after the coagulant addition. Ferric sulphate was used as the coagulant with dosages from 10 to 30 mg/L, and PAC dosages varied from 10 to 40 mg/L. The highest MIB removal efficiency (about 70%) was achieved without the coagulant addition and with the highest PAC dosage (40 mg/L). Lower MIB removal efficiencies were observed in the presence of coagulant, showing a clear interference of the iron precipitate or coagulant in the adsorption process. The degree of interference of the coagulation process in the MIB removal was proportional to the ratio of ferric hydroxide mass to the PAC mass. For both cases of PAC dosing, upstream and downstream of the coagulant injection point, the MIB removal efficiency was similar. However, MIB removal efficiency was 15% lower when compared with experiments without the coagulant application. This interference in the MIB adsorption occurs potentially because the coagulant coats the surface of the carbon and interferes with the MIB coming in contact with the carbon’s surface and pores. This constraint requires an increase of the PAC dosage to provide the same efficiency observed without coagulation.