Feasibility study of biogas upgrading coupled with nutrient removal from anaerobic effluents using microalgae-based processes

The present research was conducted to simultaneously optimize biogas upgrading and carbon and nutrient removal from centrates in a 180-L high-rate algal pond interconnected to an external CO₂ absorption unit. Different biogas and centrate supply strategies were assessed to increase biomass lipid content. Results showed 99 % CO₂ removal efficiencies from simulated biogas at liquid recirculation rates in the absorption column of 9.9 m³ m^?2 h^?1, concomitant with nitrogen and phosphorus removal efficiencies of 100 and 82 %, respectively, using a 1:70 diluted centrate at a hydraulic retention time of 7 days. The lipid content of the harvested algal–bacterial biomass remained low (2.9–11.2 %) regardless of the operational conditions, with no particular trend over time. The good settling characteristics of the algal–bacterial flocs resulted in harvesting efficiencies over 95 %, which represents a cost-effective alternative for algal biomass reutilization compared to conventional physical–chemical techniques. Finally, high microalgae biodiversity was found regardless of the operational conditions.     

Wastewater treatment and biofuel production through attached culture of <Emphasis Type="Italic">Chlorella vulgaris</Emphasis> in a porous substratum biofilm reactor

The feasibility of attached culture Chlorella vulgaris in a porous substratum biofilm reactor (PSBR) for simultaneous wastewater treatment and biofuel production was investigated. The characteristics, including algal biofilm growth, lipid yield, nutrient removal, and energy efficiency of the outdoor cultures, were investigated under the influence of both inoculum densities and the percent submerged area. A maximum biofilm productivity of 57.87 g m^?2 d^?1 with 81.9 % adhesion was achieved under optimal conditions (inoculum density of 18 g m^?2 and the percent submerged area of 5.7 %). The lipid content and lipid yield were 38.56 % and 27.25 g m^?2 d^?1, respectively. Meanwhile, the algae removed 99.95 % ammonia, 96.05 % total nitrogen (TN), and 99.83 % total phosphorus (TP). Further, the energy life cycle for the PSBR was analyzed. The biomass productivity per unit irradiance was up to 4.6 g MJ^?1 (photosynthetic efficiency of 10.65 %). The PSBR was considered to be economically feasible due to the net energy ratio of 1.3 (>1).     

Microalgae-based agro-industrial wastewater treatment: a preliminary screening of biodegradability

The potential of algal–bacterial symbiosis for the removal of carbon, nitrogen and phosphorus from five agro-industrial wastewaters was investigated in enclosed batch biodegradation tests using a mixed microalgae consortium and activated sludge as model microorganisms. The target wastewaters were obtained from potato processing (PW), fish processing (FW), animal feed production (MW), coffee manufacturing (CW) and yeast production (YW). The initial C/N/P ratio of the agro-industrial wastewater was correlated with its biodegradability. Thus, the highest removals of total organic carbon (TOC) and nitrogen were recorded in two fold diluted FW (64?±?2 % and 85?±?1 %, respectively), while the maximum P-PO₄ ^3? removal achieved was 89?±?1 % in undiluted PW. The biodegradable TOC was in most cases the limiting component in the treatment of the wastewaters evaluated. This study confirmed the potential of coupling carbon and nutrient recovery from agro-industrial effluents with the production of a valuable algal–bacterial biomass, despite their poor biodegradability.     

Algal turf scrubber (ATS) floways on the Great Wicomico River,Chesapeake Bay: productivity,algal community structure,substrate and chemistry1

Two Algal Turf Scrubber (ATS) units were deployed on the Great Wicomico River (GWR) for 22 months to examine the role of substrate in increasing algal productivity and nutrient removal. The yearly mean productivity of flat ATS screens was 15.4 g · m^?2 · d^?1. This was elevated to 39.6 g · m^?2 · d^?1 with a three‐dimensional (3‐D) screen, and to 47.7 g · m^?2 · d^?1 by avoiding high summer harvest temperatures. These methods enhanced nutrient removal (N, P) in algal biomass by 3.5 times. Eighty‐six algal taxa (Ochrophyta [diatoms], Chlorophyta [green algae], and Cyan‐obacteria [blue–green algae]) self‐seeded from the GWR and demonstrated yearly cycling. Silica (SiO₂) content of the algal biomass ranged from 30% to 50% of total biomass; phosphorus, nitrogen, and carbon content of the total algal biomass ranged from 0.15% to 0.21%, 2.13% to 2.89%, and 20.0% to 25.7%, respectively. Carbohydrate content (at 10%–25% of AFDM) was dominated by glucose. Lipids (fatty acid methyl ester; FAMEs) ranged widely from 0.5% to 9% AFDM, with Omega‐3 fatty acids a consistent component. Mathematical modeling of algal produ‐ctivity as a function of temperature, light, and substrate showed a proportionality of 4:3:3, resp‐ectively. Under landscape ATS operation, substrate manipulation provides a considerable opportunity to increase ATS productivity, water quality amelioration, and biomass coproduction for fertilizers, fermentation energy, and omega‐3 products. Based on the 3‐D prod‐uctivity and algal chemical composition demonstrated, ATS systems used for nonpoint source water treat‐ment can produce ethanol (butanol) at 5.8× per unit area of corn, and biodiesel at 12.0× per unit area of soy beans (agricultural production US).     

Effects of different influent C/N ratios on the performance of various earthworm eco-filter systems: nutrient removal and greenhouse gas emission

In this study, we sought to identify influent carbon-to-nitrogen (C/N) ratios that yield relatively high nutrient removal efficiency with relatively low greenhouse gas (GHG) emissions. The earthworm eco-filter (EE) system, which is composed of earthworms and plants (EP group), was found to be optimal for maximizing nutrient removal while reducing GHG emissions. In this EE system, the optimal influent C/N ratio for nutrient removal and GHG emission under C2N treatment conditions. Nutrient removal efficiency under this condition was 85.19 ± 6.40 % chemical oxygen demand, 71.99 ± 11.28 % total nitrogen, and 77.91 ± 8.51 % total phosphorus, while the CO₂ emission rate was 678.89 ± 201.87 mg m^?2 h^?1. Moreover, the highest nutrient removal and GHG emission rates were both achieved in late summer (August). Thus, carbon variation, season, system variation, as well as synergistic interaction between system variations and seasons, significantly affect nutrient removal efficiencies and GHG emissions.     

Impact of nitrite on aerobic phosphorus uptake by poly-phosphate accumulating organisms in enhanced biological phosphorus removal sludges

Impact of nitrite on aerobic phosphorus (P) uptake of poly-phosphate accumulating organisms (PAOs) in three different enhanced biological phosphorus removal (EBPR) systems was investigated, i.e., the enriched PAOs culture fed with synthetic wastewater, the two lab-scale sequencing batch reactors (SBRs) treating domestic wastewater for nutrient removal through nitrite-pathway nitritation and nitrate-pathway nitrification, respectively. Fluorescence in situ hybridization results showed that PAOs in the three sludges accounted for 72, 7.6 and 6.5 % of bacteria, respectively. In the enriched PAOs culture, at free nitrous acid (FNA) concentration of 0.47 × 10^?3 mg HNO₂-N/L, aerobic P-uptake and oxidation of intercellular poly-β-hydroxyalkanoates were both inhibited. Denitrifying phosphorus removal under the aerobic conditions was observed, indicating the existence of PAOs using nitrite as electron acceptor in this culture. When the FNA concentration reached 2.25 × 10^?3 mg HNO₂-N/L, denitrifying phosphorus removal was also inhibited. And the inhibition ceased once nitrite was exhausted. Corresponding to both SBRs treating domestic wastewater with nitritation and nitrification pathway, nitrite inhibition on aerobic P-uptake by PAOs did not occur even though FNA concentration reached 3 × 10^?3 and 2.13 × 10^?3 mg HNO₂-N/L, respectively. Therefore, PAOs taken from different EBPR activated sludges had different tolerance to nitrite.     

Growth and nutrient removal properties of a freshwater microalga Scenedesmus sp. LX1 under different kinds of nitrogen sources

Microalgae have received much attention for the inorganic nutrient removal in tertiary treatment of domestic wastewater. Effect of different kinds of nitrogen sources on the growth and nitrogen/phosphorus removal properties of a newly isolated freshwater microalga, Scenedesmus sp. LX1, from a low-nutrient environment condition was studied and reported in this paper. The order of specific growth rate of the microalga with different nitrogen sources was NH₄-N > urea-N > NO₃-N. With nitrate or urea as nitrogen source, the microalga could grow well and remove both nitrogen and phosphorus efficiently (90% nitrogen and nearly 100% phosphorus were removed). However, with ammonium as the nitrogen source, the maximum algal density was relatively low, and the nitrogen and phosphorus removal efficiencies were as low as 31.1% and 76.4%, respectively. This was caused by the inhibitory effect of algal culture's acid pH due to H⁺ releasing from NH₄⁺ during algal cultivation process.     

Effects of stationary phase elongation and initial nitrogen and phosphorus concentrations on the growth and lipid-producing potential of Chlorella sp. HQ

Higher lipid production and nutrient removal rates are the pursuing goals for synchronous biodiesel production and wastewater treatment technology. An oleaginous alga Chlorella sp. HQ was tested in five different synthetic water, and it was found to achieve the maximum biomass (0.27 g L^?1) and lipid yield (41.3 mg L^?1) in the synthetic secondary effluent. Next, the effects of the stationary phase elongation and initial nitrogen (N) and phosphorus (P) concentrations were investigated. The results show that the algal characteristics were affected apparently under different N concentrations but not P, which were verified by Logistic and Monod models. At the early stationary phase, the algal biomass, lipid and triacylglycerols (TAGs) yields, and P removal efficiency increased and reached up to 0.19 g L^?1, 46.7 mg L^?1, 14.3 mg L^?1, and 94.3 %, respectively, but N removal efficiency decreased from 86.2 to 26.8 % under different N concentrations. And the largest TAGs yield was only 6.4 mg L^?1 and N removal efficiency was above 71.1 % under different P concentrations. At the late stationary phase, the maximal biomass, lipid and TAGs yields, and P removal efficiencies primarily increased as the initial N and P concentrations increase and climbed up to 0.49 g L^?1, 99.2 mg L^?1, 54.0 mg L^?1, and 100.0 %, respectively. It is concluded that stationary phase elongation is of great importance and the optimal initial N/P ratio should be controlled between 8/1 and 20/1 to serve Chlorella sp. HQ for better biodiesel production and secondary effluent purification.     

NUTRIENT SEQUESTRATION,BIOMASS PRODUCTION BY MICROALGAE AND PHYTOREMEDIATION OF SEWAGE WATER

The present work was aimed at analysing the role of inoculated microalgae in nutrient dynamics, bioremediation and biomass production of sewage water. Preliminary microscopic analyses of sewage water revealed the presence of different algal groups, with predominance of Cyanophyta. Among the inoculated strains, Calothrix showed highest dry cell weight (916.67 mg L^?1), chlorophyll and carotenoid content in tap water + sewage water (1:1) treatment. Significant removal of NO₃-N ranging from 57–78% and PO₄-P (44–91%) was recorded in microalgae inoculated tap water + sewage water. The total dissolved solids and electrical conductivity of tap water + sewage water after incubation with Calothrix sp. decreased by 28.5 and 28.0%, accompanied by an increase in dissolved oxygen from 4.4 to 6.4 mg L^?1 on the 20th day. Our investigation revealed the robustness of Calothrix sp. in sequestering nutrients (N and P), improving water quality and proliferating in sewage water.     

Evaluation of microalgal consortia for treatment of primary treated sewage effluent and biomass production

The present investigation was aimed towards analyzing the potential of consortia of native filamentous microalgal strains (MC2), native unicellular microalgal strains (MC3), and selected microalgae from germplasm (MC1) in terms of nutrient removal, water quality improvement, and biomass production using primary treated sewage water. Highest NO₃-N (90 %) and PO₄-P (97.8 %) removal was obtained with MC2-inoculated sewage water. Highest decrease in total dissolved solids to 806 from 1,120 mg L^?1 and highest increase in dissolved oxygen of 9.0 from 0.4 mg L^?1 were obtained using MC2-inoculated sewage water on the sixth day. The biomass production was also highest in MC2 (1.07 g L^?1) followed by MC1 and MC3 (0.90 and 0.94 g L^?1, respectively) on the sixth day. The consortium of filamentous strains from native environment not only proved promising in nutrient removal efficiency but also led to enhanced biomass. The present study highlighted the utility of such a consortium for sewage wastewater treatment and the promise of sewage water as a growth medium for biomass production.     

Seasonal variation in light utilisation,biomass production and nutrient removal by wastewater microalgae in a full-scale high-rate algal pond

There has been renewed interest in the combined use of high-rate algal ponds (HRAP) for wastewater treatment and biofuel production. Successful wastewater treatment requires year-round efficient nutrient removal while high microalgal biomass yields are required to make biofuel production cost-effective. This paper investigates the year-round performance of microalgae in a 5-ha demonstration HRAP system treating primary settled wastewater in Christchurch, New Zealand. Microalgal performance was measured in terms of biomass production, nutrient removal efficiency, light absorption and photosynthetic potential on seasonal timescales. Retention time-corrected microalgal biomass (chlorophyll a) varied seasonally, being lowest in autumn and winter (287 and 364 mg m^?3day^?1, respectively) and highest in summer (703 mg m^?3day^?1), while the conversion efficiency of light to biomass was greatest in winter (0.39 mg Chl- a per μmol) and lowest in early summer (0.08 mg Chl- a per μmol). The percentage of ammonium (NH₄–N) removed was highest in spring (79 %) and summer (77 %) and lowest in autumn (47 %) and winter (53 %), while the efficiency of NH₄–N removal per unit biomass was highest in autumn and summer and lowest in winter and spring. Chlorophyll-specific light absorption per unit biomass decreased as total chlorophyll increased, partially due to the package effect, particularly in summer. The proportional increase in the maximum electron transport rate from winter to summer was significantly lower than the proportional increase in the mean light intensity of the water column. We concluded that microalgal growth and nutrient assimilation was constrained in spring and summer and carbon limitation may be the likely cause.     

Optimisation of biomass and lipid production by adjusting the interspecific competition mode of Dunaliella salina and Nannochloropsis gaditana in mixed culture

The high cost of algal cultivation has been a barrier associated with the commercialisation of algal biodiesel. Therefore, this study aimed to enhance lipid production by optimising the nutrient supply to benefit the coexistence of Dunaliella salina and Nannochloropsis gaditana. The effects on biomass and lipid production of using different proportions of D. salina and N. gaditana, urea and NaHCO₃ were optimised by response surface method with a 17-run Box–Behnken design. The optimal conditions for the algal growth are 58 % of D. salina in the mixture at OD₆₈₀, 150 μL day^?1 urea (0.0044 g day^?1) and no addition of NaHCO₃. The biomass concentration and lipid production reached 1.00 and 0.383 g L^?1, respectively, which are exceeded by the amount before optimisation, indicating the efficiency of the model obtained by response surface method.     

Novel MBR_based main stream biological nutrient removal process: high performance and microbial community

For municipal wastewater treatment, main stream biological nutrient removal (BNR) process is becoming more and more important. This lab-scale study, novel MBR_based BNR processes (named A₂N-MBR and A₂NO-MBR) were built. Comparison of the COD removal, results obtained demonstrated that COD removal efficiencies were almost the same in three processes, with effluent concentration all bellowed 30 mg L^?1. However, the two-sludge systems (A₂N-MBR and A₂NO-MBR) had an obvious advantage over the A²/O for denitrification and phosphorus removal, with the average TP removal rates of 91.20, 98.05% and TN removal rates of 73.00, 79.49%, respectively, higher than that of 86.45 and 61.60% in A²/O process. Illumina Miseq sequencing revealed that Candidatus_Accumulibacter, which is capable of using nitrate as an electron acceptor for phosphorus and nitrogen removal simultaneously, was the dominant phylum in both A₂N-MBR and A₂NO-MBR process, accounting for 28.74 and 23.98%, respectively. Distinguishingly, major organism groups related to nitrogen and phosphorus removal in A²/O system were Anaerolineaceae_uncultured, Saprospiraceae_uncultured and Thauera, with proportions of 11.31, 8.56 and 5.00%, respectively. Hence, the diversity of dominant PAOs group was likely responsible for the difference in nitrogen and phosphorus removal in the three processes.     

Nutrient limitation of epilithic and epixylic biofilms in ten North American streams

1. Nutrient diffusing substrata were used to determine the effect of added inorganic nitrogen (N) and phosphorus (P) on the development of epilithic and epixylic biofilms in 10 North American streams. Four treatments of diffusing substrata were used: Control (agar only), N addition (0.5 m NaNO₃), P addition (0.5 m KH₂PO₄), and N + P combined (0.5 m NaNO₃ + 0.5 m KH₂PO₄). Agar surfaces were covered with glass fibre filters (for epilithon) or discs of untreated white oak wood veneer (for epixylon). 2. We found that if algae showed significant response to nutrient addition, N limitation (either N alone or N with P) was the most frequent response both on GF/F filters and on wood. Despite the low dissolved nutrient concentrations in our study streams, more than a third of the streams did not show any response to N or P addition. In fact, P was never the sole limiting nutrient for algal biofilms in this study. 3. Nutrient addition influenced algal colonisation of inorganic versus organic substrata in different ways. The presence of other biofilm constituents (e.g. fungi or bacteria) may influence whether algal biomass on wood increased in response to nutrient addition. Algae on organic and inorganic substrata responded similarly to nutrient addition in only one stream. 4. Fungal biomass on wood was nutrient limited in six of 10 study streams. N limitation of fungal biomass (with or without secondary P limitation) was most frequent, but P limitation did occur in two streams. 5. Our results show that biomass responses to nutrient addition by the heterotrophic and autotrophic components of the epixylic biofilm were different, though both experienced the same stream nutrient conditions. For algae and fungi growing on wood, limiting nutrients were rarely similar. Only three of nine streams showed the same biomass response to nutrient addition, including two that showed no significant change in biomass despite added nutrients.     

A two-stage system coupling hydrolytic acidification with algal microcosms for treatment of wastewater from the manufacture of acrylonitrile butadiene styrene (ABS) resin

Objectives

To demonstrate the effectiveness of a novel two-stage system coupling hydrolytic acidification with algal microcosms for the treatment of acrylonitrile butadiene styrene (ABS) resin-manufacturing wastewater.

Results

After hydrolytic acidification, the BOD₅/COD ratio increased from 0.22 to 0.56, showing improved biodegradability of the wastewater. Coupled with hydrolytic acidification, the algal microcosms showed excellent capability of in-depth removal of COD, NH₃–N and phosphorus with removal rates 83, 100, and 89%, respectively, and aromatic pollutants, including benzene, were almost completely removed. The biomass concentration of Chlorella sp. increased from 5 × 10⁶ to 2.1 × 10⁷ cells/ml after wastewater treatment.

Conclusions

This two-stage coupling system achieved deep cleaning of the benzene-containing petrochemical wastewater while producing greater algae biomass resources at low cost.

     

Response of performance and bacterial community to oligotrophic stress in biofilm systems for raw water pretreatment

Understanding the dynamics of performance and bacterial community of biofilm under oligotrophic stress is necessary for the process optimization and risk management in biofilm systems for raw water pretreatment. In this study, biofilm obtained from a pilot-scale biofilm reactor was inoculated into a pilot-scale experimental tank for the treatment of oligotrophic raw water. Results showed that the removal of NH₄ ⁺–N was impaired in biofilm systems when influent NH₄ ⁺–N was less than 0.35 mg L^?1 or NH₄ ⁺–N loading rate of less than 7.51 mg L^?1 day^?1. The dominant bacteria detected in biofilm of different carrier were obvious distinct from phylum to genus level under oligotrophic stress. The dominant bacteria in elastic stereo media carrier changed from Proteobacteria (51.1%) to Firmicutes (32.7%), while Proteobacteria was always dominant in suspended ball carrier after long-term operation under oligotrophic conditions. Oligotrophic stress largely decreased the functional bacteria for the removal of nitrogen and organics including many genera in Proteobacteria and Nitrospirae, but increased several genera with spore forming organisms or potential bacterial pathogens in ESM carrier mainly including Bacillus, Mycobacterium, Pseudomonas, etc.     

Biofilm producing indigenous bacteria isolated from municipal sludge and their nutrient removal ability in moving bed biofilm reactor from the wastewater

In the present study, improved moving bed biofilm reactor (MBBR) was applied to enhance the nutrient removal ability of the municipal wastewater. A total of 18 indigenous bacterial isolates were screened from the sewage sludge sample and nitrate reductase, nitrite reductase and hydroxylamine oxidase was analyzed. The strains Pseudomonas aeruginosa NU1 and Acinetobacter calcoaceticus K12 produced 0.87 ± 0.05 U/mg and 0.52 ± 0.12 U/mg hydroxylamine oxidase, 1.023 ± 0.062 U/mg and 1.29 ± 0.07 U/mg nitrite reductase, and 0.789 ± 0.031 U/mg and 1.07 ± 0.13 U/mg nitrate reductase. Nitrogen and phosphate removal improved by the addition of nutrient sources and achieved > 80% removal rate. pH and temperature of the medium also affected nutrient removal and improved removal was achieved at optimum level (p < 0.05). MBBR was designed with R1 (aerobic), R2 and R3 (anoxic) reactors. MBBR reactors removed acceptable level phosphorus removal properties up to 7.2 ± 3.8%, 42.4 ± 4.6%, and 84.2 ± 13.1% in the R1, R2, R3 and R4 reactors, respectively. Denitrification rate showed linear relationship at increasing concentrations nitrogen content in the reactor and denitrification rate was 1.43 g NO₂-N /m²/day at 1.5 g NO₂-N /m²/day. Dehydrogenase activity was assayed in all reactors and maximum amount was detected in the aerobic biofilm reactor. Based on the present findings, MBBRs and the selected bacterial strains are useful for the degradation domestic wastewater with minimum working area.     

Comparison of the Bacterial Communities in Anaerobic, Anoxic, and Oxic Chambers of a Pilot A2O Process Using Pyrosequencing Analysis

A₂O process is a sequential wastewater treatment process that uses anaerobic, anoxic, and oxic chambers for nitrogen and phosphorus removal. In this study, the bacterial communities among these chambers were compared, and the diversity of the bacteria involved in nitrogen and phosphorus removal was surveyed. A pilot-scale A₂O process (50 m³ day^?1) was operated for more than 6 months, and bacterial 16S rRNA gene diversity was analyzed using pyrosequencing. A total of 7,447 bacterial sequence reads were obtained from anaerobic (1,546), anoxic (2,158), and oxic (3,743) chambers. Even though there were differences in the atmospheric condition and functionality, no prominent differences could be found in the bacterial community of the three chambers of the pilot A₂O process. All sequence reads, which were taxonomically analyzed using the Eztaxon-e database, were assigned into 638 approved or tentative genera. Among them, about 72.2 % of the taxa were contained in the phyla Proteobacteria and Bacteroidetes. Phosphate-accumulating bacteria, Candidatus Accumulibacter phosphatis, and two other Accumulibacter were found to constitute 3.1 % of the identified genera. Ammonia-oxidizing bacteria, Nitrosomonas oligotropha, and four other phylotypes in the same family, Nitrosomonadaceae, constituted 0.2 and 0.9 %, respectively. Nitrite-oxidizing bacteria, Nitrospira defluvii, and other three phylotypes in the same family, Nitrospiraceae, constituted 2.5 and 0.1 %, respectively. In addition, Dokdonella and a phylotype of the phylum Chloroflexi, function in nitrogen and/or phosphate removal of which have not been reported in the A₂O process, constituted the first and third composition among genera at 4.3 and 3.8 %, respectively.     

Nitrate removal in two relict oxbow urban wetlands: a 15N mass-balance approach

A ¹⁵N-tracer method was used to quantify nitrogen (N) removal processes in two relict oxbow wetlands located adjacent to the Minebank Run restored stream reach in Baltimore County (Maryland, USA) during summer 2009 and early spring 2010. A mass-balance approach was used to directly determine the flow of ¹⁵NO₃ ^? to plants, algae, and sediments, with unaccounted for ¹⁵N assumed to be denitrified. During the summer, plant and algal uptake accounted for 42%, of the added ¹⁵NO₃ ^? in oxbow 1, less than 1% remained in the water column and 57% was unaccounted for. In oxbow 2 during the summer, plant and algal uptake accounted for 63% of the added ¹⁵NO₃ ^?, with <1% remaining in the water column and 38% unaccounted for. During the early spring, plant and algal uptake were much lower in both oxbows, ranging from 0.05 to 13.3% of the ¹⁵N added, with 97 and 87% was unaccounted for in oxbow 1 and 2, respectively. The amount of unaccounted for ¹⁵N was equivalent to estimated areal denitrification rates of 12 and 6 mg N m^?2 d^?1 in the summer and 78 and 15 mg N m^?2 d^?1 in the spring, in oxbow 1 and oxbow 2, respectively. However, the uncertainty of these estimates is high as it was difficult to detect accumulation of ¹⁵N in the sediments which could have accounted for a very large percentage of the added ¹⁵N. Our results suggest that the two relict oxbow wetlands are sinks for NO₃ ^? during both summer and spring but that the pathways of removal vary with plants and algae playing a major role in summer but not in spring.     

Hectare-scale demonstration of high rate algal ponds for enhanced wastewater treatment and biofuel production

High rate algal ponds (HRAPs) are shallow, paddlewheel-mixed open raceway ponds that are an efficient and cost-effective upgrade for the conventional wastewater treatment ponds used by communities and farms the world over. HRAPs provide improved natural disinfection and nutrient removal and can be further enhanced by carbon dioxide (CO₂) addition to promote algal growth which is often carbon limited. This paper discusses the construction and operation of a 5-ha demonstration HRAP system treating primary settled wastewater at the Christchurch wastewater treatment plant, New Zealand. The system consisted of four 1.25-ha HRAPs that were constructed from an existing conventional pond. Algae were harvested from the HRAP effluent in specially designed settlers, which concentrated the algal/bacterial biomass to 1–2% organic solids for conversion to bio-crude oil following dewatering. Performance data from the first 15?months of HRAP operation (without CO₂ addition) are presented. The four demonstration HRAPs had reasonable replication of both treatment performance and algal/bacterial productivity with similar annual average wastewater treatment efficiency (~50% removal of BOD₅, ~87% removal of fBOD₅, ~65% removal of ammoniacal-N, ~19% removal of dissolved reactive phosphorus and ~2 log removal of Escherichia coli), algal species composition and algal/bacterial biomass production (~8?g?m^?2?day ?1 volatile suspended solids). These results were in good agreement with the results for pilot-scale HRAP without CO₂ addition in New Zealand. This study provides further indication of the potential for energy efficient and effective wastewater treatment using HRAP, while biofuel conversion of the harvested algal bacterial biomass could provide a valuable niche distributed energy source for local communities.