A comparison of purification efficiencies of various constructed ecosystems (aquatic,semi-aquatic and terrestrial) receiving urban wastewaters

Tests were carried out under controlled conditions in the Experimental Plant of Viville (Arlon, Belgium) to enhance the purification of urban wastewater by natural means. The results demonstrate the need to structure treatment systems in a series of different artificial ecosystems (or a Hierarchical Mosaic of Artificial Ecosystems — MHEA in French). The first two levels we used were made up of an unplanted aquatic ecosystem (stabilization pond) followed by a semi-aquatic ecosystem planted withTypha latifolia L. in which the water flows over the substrate. At a flow rate of 4 m²/PE (1 PE=150 1/day of typical urban wastewaters in Belgian rural zones), this first stage substantially reduces suspended solids (SS), COD and BOD₅, a significant amount of tot-N and tot-P, and reduces pathogens by 100-fold. Further, the system is easy to manage (sludge is eliminated in the first stage and biomass is collected in the second stage) and the treatment system does not clog up.Nevertheless, real and sustainable environmental protection demands even higher performance rates, and these first two stages, both in terms of design and dimension, can only be considered as a satisfactory part of a MHEA system. Artificial aquatic, semi-aquatic, and terrestrial ecosystems were systematically compared at the third and fourth stage of the system to increase the overall removal efficiency.The most complete and efficient system in our tests (i.e., the one that provides the most successful primary (SS), secondary (COD and BOD₅) and tertiary (N and P) treatment and the best pathogens removal rates) was made up of 3 sequential series of ecosystems: an aquatic ecosystem whose flow went into a plantedTypha latifolia system (surface water flow), that flowed into a terrestrial ecosystem planted withAlnus glutinosa (L.) Gaertn (vertical subsurface water flow). A total surface area (stages 1–4) of 8 m²/PE ensured a high performance level whose outflow conformed to the strictest European norms.     

A sequencing batch reactor system for high-level biological nitrogen and phosphorus removal from abattoir wastewater

A sequencing batch reactor (SBR) system is demonstrated to biologically remove nitrogen, phosphorus and chemical oxygen demand (COD) to very low levels from abattoir wastewater. Each 6 h cycle contained three anoxic/anaerobic and aerobic sub-cycles with wastewater fed at the beginning of each anoxic/anaerobic period. The step-feed strategy was applied to avoid high-level build-up of nitrate or nitrite during nitrification, and therefore to facilitate the creation of anaerobic conditions required for biological phosphorus removal. A high degree removal of total phosphorus (>98%), total nitrogen (>97%) and total COD (>95%) was consistently and reliably achieved after a 3-month start-up period. The concentrations of total phosphate and inorganic nitrogen in the effluent were consistently lower than 0.2 mg P l⁻¹ and 8 mg N l⁻¹, respectively. Fluorescence in situ hybridization revealed that the sludge was enriched in Accumulibacter spp. (20–40%), a known polyphosphate accumulating organism, whereas the known glycogen accumulating organisms were almost absent. The SBR received two streams of abattoir wastewater, namely the effluent from a full-scale anaerobic pond (75%) and the effluent from a lab-scale high-rate pre-fermentor (25%), both receiving raw abattoir wastewater as feed. The pond effluent contained approximately 250 mg N l⁻¹ total nitrogen and 40 mg P l⁻¹ of total phosphorus, but relatively low levels of soluble COD (around 500 mg l⁻¹). The high-rate lab-scale pre-fermentor, operated at 37°C and with a sludge retention time of 1 day, proved to be a cheap and effective method for providing supplementary volatile fatty acids allowing for high-degree of biological nutrient removal from abattoir wastewater.     

Anaerobic treatment of baker's yeast effluent using a hybrid digester with polyurethane as support material

Summary A high-strength baker's yeast effluent was anaerobically treated using a hybrid digester under mesophilic conditions. The digester was subjected to a substrate COD concentration of 21 767 mg/I at three different HRTs. At HRTs of 3.0, 2.0 and 1.0 d, the digester reduced the substrate COD by 76, 61 and 33%, respectively. Although the best COD removal was obtained at an OLR of 7.30 kg COD/m³.d, the highest COD removal rate (6.51 kg COD/M³-d) was found at 10.65 kg COD/m³.d at an HRT of 2.0 d. The low methane yield and VFA accumulation found in the digester effluent, indicated inhibition on methanogenic level and this was considered to be the rate-limiting step during the anaerobic treatment process. The overall efficiency of the digester indicated that this digester design and support medium was suitable for the treatment of a high-strength, sulfate-rich baker's yeast effluent.     

Optimization for growth of Rhodocyclus gelatinosus in seafood processing effluents

Tuna condensate was a better substrate than shrimp-blanching water or effluent from a frozen-seafood plant for growing Rhodocyclus gelatinosus under anaerobic conditions in the light. One strain out of four examined, R7, gave the highest biomass (4.0 g/l), cell yield (0.32 g cell/g COD), and COD removal (78%) in 1:10 (v/v) diluted tuna condensate. Shrimp-blanching water added to the tuna condensate further increased growth rate, biomass and COD removal. Optimal growth was at pH 7.0 and 3000 Lux light intensity. Acetate, pyruvate, glucose, glutamate, propionate or malate added to the tuna condensate did not increase cell yield, carotenoid or bacteriochlorophyll content or biomass protein. A maximum cell mass of 5.6 g/l (containing 50% protein) and 86% COD removal were obtained after 5 days' incubation under optimal conditions.     

Autotrophic denitrification and inhibitory effect caused by the injection of spent sulfidic caustic in a modified Ludzack-Ettinger process

As spent sulfidic caustic (SSC) from petroleum plants contains a high concentration of alkalinity and sulfur compounds, SSC can be applied in sewage treatment system as an electron donor for autotrophic denitrification. In our previous study, the reuse of SSC in the biological nitrogen process was successful, and some neutralization may be required for stable treatment performance. In this study, the pH of SSC was neutralized to 12.0 from 13.3, and the modified Ludzack-Ettinger process was conducted for 90 days with the municipal wastewater. Some toxic effects of SSC on microorganisms were tested via a specific oxygen uptake rate (SOUR) assay. According to the SOUR assay, as compared with no SSC injection condition, SOUR was reduced by approximately 5.4% when 4 mL SSC/L was injected and the effective concentration of a toxicant causing 50% inhibition of the microorganism’s activity (EC₅₀) was 22.6 mL/L. During the days of operation, the COD removal and nitrification efficiency were over 53.0 and 98.2%, respectively. The TN removal efficiency was 56.6% and the nitrogen removal rate (NRR) was 0.15 kg/m³·d when the hydraulic retention time (HRT) in the anoxic tank was 3 h. The ratio of nitrifying bacteria was unaffected by the HRT, and Nitrobacter spp. and Nitrospira genus existed at similar ratios. The ratio of T. denitrificans increased after the injection of SSC and was approximately 6.5%.     

A case of unusually high oxygen demand in a eutrophic lake

An unusually high hypolimnetic water column BOD (WCBOD), roughly 40 times higher than the sediment oxygen demand (SOD), was observed in a small eutrophic lake and an adjoining lagoon. The mean 5-day WCBOD during thermal stratification in the lake was 29 and 49 g/m² at 10 and 20 °C, respectively, while in the lagoon it was even higher (47 and 87 g/m² at 10 and 20 °C, respectively). The soluble fraction comprised about two-thirds of the WCBOD. WCBOD in the lake was much less during the unstratified period (5-day = 5 g/m²). The SOD rates at two depths in both the lake (0.31 and 0.2 g/m²-d) and lagoon (0.41 and 0.28 g/m²-d) were not unusually high. The ultimate whole BOD (UWCBOD + USOD) was approximately 96 g/m² in the lake and 136 g/m² in the lagoon and UWCBOD formed over 90% of the ultimate whole BOD in both water bodies. A possible cause for these abnormally high WCBODs, in addition to the normal autochthonous production, is an allochthonous source from loosely aggregated and flocculant mats of the bog moss, Sphagnum, which surrounds the lake-lagoon system. Storm water per se was clearly insignificant, but would have contributed indirectly through nutrients for autochthonous production. Such high short-term BOD rates may greatly over-estimate the demand to be satisfied by continuous aeration.     

Effect of pH on phase separated anaerobic digestion

A pilot scale experiment was performed for a year to develop a two-phase anaerobic process for piggery wastewater treatment (COD: 6,000 mg/L, BOD: 4,000 mg/L, SS: 500 gm/L, pH 8.4, alkalinity 6,000 mg/L). The acidogenic reactor had a total volume of 3 m³, and the methanogenic reactor, an, anaerobic up-flow sludge filter, combining a filter and a sludge bed, was also of total volume 3 m³ (1.5 m³ of upper packing material). Temperatures of the acidogenic and methanogenic reactors kept at 20°C and 35°C., respectively. When the pH of the acidogenic reactor was controlled at 6.0–7.0 with HCl, the COD removal efficiency increased from 50 to 80% over a period of six months, and as a result, the COD of the final effluent fell in the range of 1,000–1,500 mg/L. BOD removal efficiency over the same period was above 90%, and 300 to 400 mg/L was maintained in the final effluent. The average SS in the final effluent was 270 mg/L. The methane production was 0.32 m³ CH₄/kg COD_removed and methane content of the methanogenic reactor was high value at 80–90%., When the pH of the acidogenic reactor was not controlled over the final two months, the pH reached 8.2 and acid conversion decreased compared with that of pH controlled, while COD removal was similar to the pH controlled operation. Without pH control, the methane content in the gas from methanogenic reactor improved to 90%, compared to 80% with pH control.     

Constructed wetlands for the treatment of landfill leachates: The Slovenian experience

Constructed wetlands are widely recognized as an economical, efficient and environmentally acceptable means to treat many different types of wastewater. Six systems have been constructed in Slovenia for the treatment of landfill leachates. This paper describes the early stages of two treatment systems operating from 1990 to 1993 that were used to treat leachates from municipal landfills. System S1 consisted of a sedimentation lagoon and a 600 m² reed bed. System S2 was designed with a sedimentation lagoon and two reed beds (total=450 m²). The subsurface flow in both was horizontal and fluctuated widely in S2, but was a constant 0.2 1 s^–1 in S1. Peat, soil, sand and gravel were in system S2, while gravel with a hydraulic conductivity of 5×10^–4 m s^–1, was in S1.The unanticipated fluctuations of hydraulic and organic loadings influenced their performance. The efficiency in reduction of organic matter, N, P, metals and fecal coliforms varied through the year. In 3 years of operation, the average removal efficiencies for COD, BOD₅ and TSS were 38%, 61%, and 81%, respectively, in S1, and 53%, 45%, and 47%, respectively, in 1.5 years of operation for S2. The reduction of ammonium did not reach the effluent standard of 10 mg l^–1 for either S1 or S2. As, Zn, Pb, and Cu accumulated in roots, and Cu, Fe, As, Ni and Pb accumulated in rhizomes after one year of operation in S2.     

Nutrient removal from polluted stream water by artificial aquatic food web system

For the removal of nutrients from eutrophic stream water polluted by non-point sources, an artificial aquatic food web (AAFW) system comprising processes of phytoplankton growth and Daphnia magna grazing was developed. The AAFW system was a continuous-flow system constructed with one storage basin of 3 m³ capacity, one phytoplankton tank of 3 m³ capacity, and one zooplankton growth chamber of 1.5 m³ capacity. The system was optimized by setting hydraulic retention time of phytoplankton tank as 3 days and D. magna density as 740–1000 individual l⁻¹. When the system was operated on eutrophic stream water that was delivering 471 g of total nitrogen (TN) and 29 g of total phosphorus (TP) loadings for 45 days, 250 g (53%) of TN and 16 g (54%) of TP were removed from the water during its passage through the phytoplankton tank. In addition, 64 g (14%) of TN and 4 g (13%) of TP were removed from the water by harvesting zooplankton biomass in the zooplankton growth chamber, resulting in significant overall removal rates of TN (69%), nitrate (78%), TP (73%), and dissolved inorganic phosphorus (94%). While the removal efficiency of the AAFW system is comparable to those of other ecotechnologies such as constructed wetlands, its operation is less limited by the availability of space or seasonal shift of temperature. Therefore, it was concluded that AAFW system is a highly efficient, flexible system for reducing nutrient levels in tributary streams and hence nutrient loading to large aquatic systems receiving the stream water. Handling editor: J. Padisak     

Enhancing digestion efficiency of POME in anaerobic sequencing batch reactor with ozonation pretreatment and cycle time reduction

Ozonation pretreatment was applied to palm oil mill effluent (POME) prior to anaerobic digestion using the anaerobic sequencing batch reactor (ASBR). Ozonation increased BOD/COD by 37.9% with a COD loss of only 3.3%. At organic loads of 6.48-12.96 kg COD/m³/d, feeding with non-ozonated POME caused a system failure. The ozonated POME gave significantly higher TCOD removal at loadings 6.52 and 9.04 kg COD/m³/d but failed to sustain the operation at loading 11.67 kg COD/m³/d. Effects of cycle time (CT) and hydraulic retention time (HRT) were determined using quadratic regression model. The generated response surface and contour plot showed that at this high load conditions (6.52-11.67 kg COD/m³/d), longer HRT and shorter CT gave the ASBR higher organic removal efficiency and methane yield. The model was able to satisfactorily describe the relationship of these two key operating parameters.     

Design of acidogenic reactors for the anaerobic treatment of the organic fraction of solid food waste

Volatile Fatty Acids (VFA) production by anaerobic fermentation of organic solid wastes was studied at laboratory scale. The influence of initial substrate concentration was evaluated on VFA production. Completely mixed reactors (0.9?l) were used at mesophilic temperature (35?°C). Food wastes had 43.8% Total Solids content. Three dilutions of substrate (1/25, 1/10 and 1/5) corresponding to 1.75%, 4.38% and 8.76% of Total Solids and five values of Organic Loading Rates: 2, 5, 10, 12.5 and 25?kg COD/m³?d were studied. It was found that substrate 1/10 led to 14?g VFA/l at a loading rate of 12.5?kg COD/m³?d and an hydraulic retention time of 3.7 d. The main VFA produced were especially acetate and butyrate. Substrate diluted 1/5 led to 26.1?g VFA/l at a loading of 5?kg COD/m³?d and an hydraulic retention time of 15.1 d, but biomass production was not optimal. In a second study, a cascade of three reactors was used. An effluent with 42?g VFA/l was obtained at steady-state conditions at a loading of 12.5?kg of COD/m³?d and an hydraulic retention time of 12.5?d. The distribution of VFA was the following: 36% of propionate, 34% of acetate and 22.5% of butyrate.     

Dissolved oxygen and nutrient budgets in a phytotreatment pond colonised by <Emphasis Type="Italic">Ulva</Emphasis> spp.

Net daily budgets of dissolved oxygen (O₂), dissolved inorganic carbon (DIC), dissolved inorganic nitrogen (DIN = NH₄⁺+NO₂⁻+NO₃⁻) and soluble reactive phosphorus (SRP) were determined in a pond colonised by Ulva spp. This pond received wastewater from a land-based fish farm and was used as a phytotreatment plant. Three consecutive 24-h cycles of measurements were performed with 8–14 samplings per day. Water samples were collected at the inlet and outlet of the pond and budgets were estimated from differences between inlet and outlet loadings. The first cycle was started when Ulva biomass was 8 kg m⁻², as wet weight. The second cycle was performed after the harvest of ~20% of the macroalgal biomass and the third after the harvest of another ~20% of the remaining biomass. Ulva removal was very fast (<1 h) and samplings for cycles 2 and 3 were started two hours after harvesting, so that the whole experiment lasted ~80 h. When Ulva biomass was at its maximum, the aquatic system was heterotrophic with an O₂ demand of 519 mol d⁻¹ and a net regeneration of DIC (2686 mol d⁻¹), NH₄⁺ (49 mol d⁻¹) and SRP (2.5 mol d⁻¹). The DIC to O₂ ratio was an indicator of persistent anaerobic metabolism. Following the first harvest intervention, this system displayed a prompt response and shifted toward a lower O₂ demand (from −519 to −13 mol d⁻¹), with a lesser regeneration degree of NH₄⁺ (11.4 mol d⁻¹) and DIC (1066 mol d⁻¹). After the second Ulva removal the net budget of SRP became negative (−1.0 mol d⁻¹). By integrating these results over the three days cycle we estimated that in order to operate an efficient nutrient control and maintain macroalgal mats in a healthy status the optimal Ulva biomass should be well below ~4 kg m⁻² as wet weight. Above this threshold, self-limitation would render most of the algal mat unable to exploit light and nutrients. An efficient removal of nitrogen and phosphorus could be attained through the management of macroalgal biomass only with an optimisation of recipient surface to nutrient loading ratio.     

Cultivation and carrageenan yield and quality ofKappaphycus alvarezii in the waters of Vietnam

The carrageenan-producing red algaKappaphycus alvarezii (Doty) Doty was brought to Vietnam from Japan in 1993. Branch fragments of this species were cultivated in a pond, lagoon, inlet and offshore in Vietnam for the first time. The best daily growth rate (DGR) of plants grown in the lagoon area attained 9–11 % day^–1 in May to June (cold season). The water temperature and salinity in this area ranged from 27.2–32.4 °C and 31.4–33.7 °C, respectively. DGR of plants grown in the inlet ranged from 7 to 9% day^–1 in June. Grazing by fish has been observed to occur in this area. The DGR of plants grown in the pond ranged from 5–6% in January–July, but decreased to less than 4% day^–1 in August (hot season). K. alvarezii in Vietnam showed a carrageenan yield of 18.8–24.6% and gel strength of 1566–1712 g cm^–2. These values are similar ones obtained fromK. alvarezii cultivated in the Philippines and Indonesia.     

Effect of nutrient loading and retention time on performance of high rate algal ponds

Small pilot ponds in a glasshouse at the Scottish Agricultural College (Auchincruive) were used to investigate the effects of changing C:N:P loading rate and retention time on pond performance as measured by nutrient removal and dry matter biomass. One experiment investigated ponds operated at two C:N:P ratios: low (9:7:1) and high (104:10:1) and two retention times (4 and 7 days θ. Increasing retention time from 4 to 7 days increased the concentration of total (dry matter) and algal (chlorophyll a) biomass and the degree of nitrification. It also increased removal of phosphorus, but had no effect on nitrogen or COD removal. Cyanobacteria predominated in ponds operated at both 4 and 7 days, and the density of cyanobacteria increased with increased retention time. Nitrogen removal was independent of C:N:P ratio; indeed the lower C:N:P ratio favoured increased nitrification. A high C:N:P ratio increased phosphorus and COD removal and increased the concentration of algal biomass (chlorophyll a), but had little effect on total biomass (dry matter). A second experiment varied COD loading rate (600, 350 and 100 kg COD ha-¹ d-¹) while maintaining a constant retention time (either 5 or 7 days θ). Species composition was independent of retention time. The longer retention time increased both total and algal biomass concentration and also percentage of nitrogen removed. Nitrification was independent of retention time. Increasing loading rate increased dry matter production and resulted in a predominance of cyanobacteria over Chlorophyceae. Increased loading rate was related to increase in nitrogen removal, however more complete nitrification occurred at low COD loading rates. Phosphorus removal in the pond with 5-day (θ) remained constant independent of loading rate, but in the pond with 7-day θ phosphorus removal increased with increased COD loading. COD removal was independent of both retention time and loading rate. This revised version was published online in August 2006 with corrections to the Cover Date.     

Annual productivity of Spirulina (Arthrospira) and nutrient removal in a pig wastewater recycling process under tropical conditions

An evaluation was made of the annual productivity of Spirulina (Arthrospira) and its ability to remove nutrients in outdoor raceways treating anaerobic effluents from pig wastewater under tropical conditions. The study was based at a pilot plant at La Mancha beach, State of Veracruz, Mexico. Batch or semi-continuous cultures were established at different seasons during four consecutive years. The protein content of the harvested biomass and the N and P removal from the ponds were also evaluated. Anaerobic effluents from digested pig waste were added in a proportion of 2% (v/v) to untreated sea-water diluted 1:4 with fresh water supplemented with 2 g L^–1 sodium bicarbonate, at days 0, 3 and 5. A straight filament strain of Spirulina adapted to grow in this complex medium was utilized. A pH value 9.5 ± 0.2 was maintained. The productivity of batch cultures during summer 1998 was significantly more with a pond depth of 0.10 m than with a depth 0.065 m. The average productivity of semi-continuous cultures during summer 1999 was 14.4 g m^–2 d^–1 with a pond depth of 0.15 m and 15.1 g m^–2 d^–1 with a depth of 0.20 m. The average annual productivity for semi-continuous cultures operating with depths of 0.10 m for winter and 0.15 and 0.25 m for the rest of the year, was 11.8 g m^–2 d^–1. This is the highest value reported for a Spirulina cultivation system utilising sea-water. The average protein content of the semi-continuous cultures was 48.9% ash-free dry weight. NH₄-N removal was in the range 84–96% and P removal in the range of 72–87%, depending on the depth of the culture and the season.     

Larval abundance of Chironomus circumdatus in relation to biotic and abiotic factors

Larval abundance of Chironomus circumdatus in sewage canal and pond systems was studied during 1988–1990. Monthly changes in the morphometric features of the pond revealed that both total and littoral areas progressively decreased from 1063 and 107 m² in December 1988 to 151 and 43 m² in May '89; the decrease during the year 1989–1990 was from 1116 and 92 m² in October to 109 and 31 m² in May. A significant negative correlation (r= – 0.52) was obtained for the relation between littoral area and larval density in the pond. Larval density and biomass depended generally on the nature of the substrate and quantity of organic matter. Larval density of Ch. circumdatus was positively correlated with O₂, bacterial count and organic matter content, but negatively correlated with CO₂ level. Daily removal of organic matter by the larvae ranged from 20 to 31 % of the available organic matter in the sewage canal and from 3 to 11 % in the pond.     

Production ecology of phyto- and zooplankton in a eutrophic pond dominated byChaoborus flavicans (Diptera: Chaobolidae)

Primary production of phytoplankton and secondary production of a daphnid and a chaoborid were studied in a small eutrophic pond. The gross primary production of phytoplankton was 290 gC m⁻² per 9 months during April–December. Regression analysis showed that the gross primary production was related to the incident solar radiation and the chlorophylla concentration and not to either total phosphorus or total inorganic nitrogen concentration. The mean chlorophylla concentration (14.2 mg m⁻³), however, was about half the expected value upon phosphorus loading of this pond. The mean zooplankton biomass was 1.60 g dry weight m⁻², of whichDaphnia rosea and cyclopoid copepods amounted to 0.69 g dry weight m⁻² and 0.61 g dry weight m⁻², respectively. The production ofD. rosea was high during May–July and October and the level for the whole 9 months was 22.6 g dry weight m⁻².Chaoborus flavicans produced 10 complete and one incomplete cohorts per year. Two consecutive cohorts overlapped during the growing season. The maximum density, the mean biomass, and the production were 19,100 m⁻², 0.81 g dry weight m⁻², and 11.7 g dry weight m⁻²yr⁻¹, respectively. As no fish was present in this pond, the emerging biomass amounted to 69% of larval production. The production ofC. flavicans larvae was high in comparison with zooplankton production during August–September, when the larvae possibly fed not only on zooplankton but also algae.     

Nutrient removal from sewage by an artificial food web system composed of phytoplankton and Daphnia magna

In order to remove nutrients from sewage, ecotechnology with an artificial food web composed of phytoplankton and Daphnia magna was used. To optimise performance of the system, phytoplankton growth, zooplankton growth, and a continuous-flow system were used. For phytoplankton growth, stirring was 6.7 times faster than the settling in growth rate of Scenedesmus. Zooplankton growth was not influenced by phytoplankton succession, and the specific production coefficient of D. magna was 110.4 mg Daphnia dry weight (DW) per mg chlorophyll a (Chl a). Results indicated that removal of nutrients was better in a long hydraulic residence time (HRT) system than in a system with short HRT. The optimum retention time was found to be 3 days for the phytoplankton chamber and 1.5 days for the subsequent D. magna chamber, respectively, with total retention time of the combined chambers being kept at 4.5 days. When a pilot plant was operated under these conditions, the removal rates of total nitrogen (TN) and total phosphorus (TP) were 68 and 56%, respectively. In the material budget of TN, 32% of inputs passed on to effluent, 39% to sludge, 27% to air and 2% to harvested Daphnia. For TP, 44% of inputs passed on to effluent, 51% to sludge and 4% to Daphnia.     

Influence of wastewater composition on nutrient removal behaviors in the new anaerobic–anoxic/nitrifying/induced crystallization process

In this study, the new anaerobic–anoxic/nitrifying/induced crystallization (A₂N–IC) system was compared with anaerobic-anoxic/nitrifying (A₂N) process to investigate nutrient removal performance under different influent COD and ammonia concentrations. Ammonia and COD removal rates were very stable in both processes, which were maintained at 84.9% and 86.6% when the influent ammonia varied from 30 mg L⁻¹ to 45 mg L⁻¹ and COD ranged from 250 mg L⁻¹ to 300 mg L⁻¹. The effluent phosphorus always maintained below 0.2 mg L⁻¹ in A₂N–IC, whereas in A₂N the effluent phosphorus concentration was 0.4–1.7 mg L⁻¹, demonstrating that A₂N–IC is suitable to apply in a broader influent COD and ammonia concentration range. Under higher influent COD (300 mg L⁻¹) or lower ammonia conditions (30 mg L⁻¹), the main function of chemical induced crystallization was to coordinate better nutrient ratio for anoxic phosphorus uptake, whereas under high phosphorus concentration, it was to reduce phosphorus loading for biological system. Under the similar influent wastewater compositions, phosphorus release amounts were always lower in A₂N–IC. To clarify the decrease procedure of phosphorus release in the A₂N–IC, the equilibrium between chemical phosphorus removal and biological phosphorus removal in A₂N–IC was analyzed by mass balance equations. During the long-term experiment, some undesirable phenomena were observed: the declining nitrification in post-aerobic tank and calcium phosphorus precipitation in the anaerobic tank. The reasons were analyzed; furthermore, the corresponding improvements were proposed. Nitrification effect could be enhanced in the post-aerobic tank, therefore ammonia removal rate could be increased; and biologically induced phosphorus precipitation could be inhibited by controlling pH at the anaerobic stage, so the phosphorus release and recovery could be improved.     

Prevalence and transmission of antimicrobial resistance among <Emphasis Type="Italic">Aeromonas</Emphasis> populations from a duckweed aquaculture based hospital sewage water recycling system in Bangladesh

In order to investigate the influence of a duckweed aquaculture based hospital sewage water recycling plant on the prevalence and dissemination of antibiotic resistance, we made use of an existing collection of 1,315 Aeromonas isolates that were previously typed by the biochemical fingerprinting PhP-AE system. In these treatment plant, hospital raw sewage water is first collected in a settlement pond (referred to as sewage water in this study) and is then transferred to a lagoon, where the duckweed (Lemnaceae) is grown (referred to as lagoon). The duckweed is harvested and used as feed for the fish in a separate pond (referred to as fish pond). From this collection, representatives of 288 PhP types were subjected to antibiotic susceptibility testing for eight antimicrobials by broth microdilution method. The overall resistance rates among Aeromonas isolates from the treatment plant were highest for ampicillin (87%) and erythromycin (79%) followed by cephalothin (58%), nalidixic acid (52%), streptomycin (51%), tetracycline (31%), chloramphenicol (13%) and gentamicin (8%). A significantly lower prevalence of antibiotic resistance was found in Aeromonas from environmental control water, patient stool samples, duckweed and fish compared to sewage water isolates. The prevalence of resistance in the sewage water was not significantly reduced compared to the lagoon water and fish pond. Throughout the treatment system, the frequencies of resistant strains were found to diminish during the sewage water purification process, i.e. in the lagoon where sewage water is used to grow the duckweed. However, the frequency of resistant strains again increased in the fish pond where sewage grown duckweed is used for aquaculture. Among the selected isolates, two multiresistant clonal groups of Aeromonas caviae HG4 were identified that exhibited indistinguishable PhP and amplified fragment length polymorphism fingerprints and shared a common plasmid of approximately 5 kb. Representatives of both groups were recovered from almost every part of the sewage treatment plant but not in the control ponds nor in human samples, which suggests that specific multiresistant Aeromonas clones are able to persist and spread throughout the entire purification process.