Nitrification by microalgal-bacterial consortia for ammonium removal in flat panel sequencing batch photo-bioreactors.

Ammonium removal from artificial wastewater by microalgal-bacterial consortia in a flat-panel reactor (FPR1) was compared with a microalgae-only flat-panel reactor (FPR2). The microalgal-bacterial consortia removed ammonium at higher rates (100±18mgNH4+-NL-1d-1) than the microalgae (44±16mgNH4+-NL-1d-1), after the system achieved a stable performance at a 2days hydraulic retention time. Nitrifiers present in the microalgae-bacteria consortia increased the ammonium removal: the ammonium removal rate by nitrifiers and by algae in FPR1 was, respectively, 50(±18) and 49(±22)mgNH4+-NL-1d-1. Apparently ammonium removal by algae was not significantly different between FPR1 and FPR2. The activity of the nitrifiers did not negatively affect the nitrogen uptake by algae, but improved the total ammonium removal rate of FPR1.

Photo-oxygenation to support nitrification in an algal-bacterial consortium treating artificial wastewater.

Oxygenation by photosynthesis in a mixed culture of algae and nitrifiers was investigated for its potential to support nitrification. An open photo-bioreactor (1L fermentor; 30°C) was inoculated with an enriched culture of nitrifiers obtained from activated sludge and a pure culture of Scenedesmus sp. The reactor was illuminated (60 µmol/m2 s) and operated as a sequencing batch reactor with 50% discharge per cycle and sludge retention time of 15 or 30 days. Full nitrification of synthetic wastewater (50 mgNH4+-N/L) without mechanical aeration was achieved after less than 14 cycles (18.5 h React per 24h cycle) and the biomass steadily increased until a concentration of 1.9 g VSS/L with 29 mg chl-a/L. The maximum observed nitrification rate was 7.7 mgNH4+-N/Lh. A mass balance showed that ammonium removal was primarily by nitrification (81-85%) rather than by ammonium uptake by algae.

Application of Quantitative Microbial Risk Assessment to analyze the public health risk from poor drinking water quality in a low income area in Accra, Ghana.

In Accra, Ghana, a majority of inhabitants lives in over-crowded areas with limited access to piped water supply, which is often also intermittent. This study assessed in a densely populated area the risk from microbial contamination of various sources of drinking water, by conducting a Quantitative Microbiological Risk Assessment (QMRA) to estimate the risk to human health from microorganism exposure and dose-response relationships. Furthermore the cost-effectiveness in reducing the disease burden through targeted interventions was evaluated. Five risk pathways for drinking water were identified through a survey (110 families), namely household storage, private yard taps, communal taps, communal wells and water sachets. Samples from each source were analyzed for Escherichia coli and Ascaris contamination. Published ratios between E. coli and other pathogens were used for the QMRA and disease burden calculations. The major part of the burden of disease originated from E. coli O157:H7 (78%) and the least important contributor was Cryptosporidium (0.01%). Other pathogens contributed 16% (Campylobacter), 5% (Rotavirus) and 0.3% (Ascaris). The sum of the disease burden of these pathogens was 0.5 DALYs per person per year, which is much higher than the WHO reference level. The major contamination pathway was found to be household storage. Disinfection of water at household level was the most cost-effective intervention (<5 USD/DALY-averted) together with hygiene education. Water supply network improvements were significantly less cost-effective.

Nitrogen mass balances for pilot-scale biofilm stabilization ponds under tropical conditions.

Nitrogen removal in biofilm waste stabilization ponds were modeled using nitrogen mass balance equations. Four pilot-scale biofilm maturation ponds were constructed in Uganda. Pond 1 was control; the others had 15 baffles in each of them. Two loading conditions were investigated (period 1, 18.2g and period 2, 26.8 g NH(4)-Nd(-1)). Total nitrogen and TKN mass balances were made. Bulk water and biofilm nitrification rates were determined and used in the TKN mass balance. Results for total nitrogen mass balance showed that for both periods, denitrification was the major removal mechanism. Nitrogen uptake by algae was more important during period 1 than in period 2. The TKN mass balance predicted well effluent TKN for period 2 than period 1. This could be due to fluctuations in algae density and ammonia uptake during period 1, no conclusions on reliability of mass balance model in period 1 was made.

Prevalence of asthma and bronchial hyperreactivity in Danish schoolchildren: no change over 10 years.

AIM: To describe the point prevalence of current physician-diagnosed asthma and bronchial hyperreactivity (BHR) in 2001 among unselected Danish schoolchildren aged 6-17 years, compared with the prevalence from a similar study from 1990 to 1991. METHODS: Cross-sectional study using parental questionnaire on asthma and respiratory symptoms combined with a 6-min free running test with peak expiratory flow rate (PEFR) measurement (n = 1051, response rate 89.3%). Results were compared with those of a similar study in the same area from 1990 to 1991. Main outcome measures were current physician-diagnosed asthma or BHR in children without physician-diagnosed asthma measured by either a decrease in lung function after standardized running test and/or variability in PEFR on home monitoring. RESULTS: The prevalence of current physician-diagnosed asthma was 4.0% [95% confidence interval (CI) 2.7-5.3%] in 1990-1991 and 3.6% (95% CI 2.4-4.8%) in 2001. The prevalence of BHR was 3.2% (95% CI 2.0-4.4%) in 1990-1991 and 2.0% (95% CI 1.1-2.9%) in 2001. The combined prevalence was 7.2% (95% CI 5.4-8.9%) in 1990-1991 and 5.6% (95% CI 4.2-7.1%) in 2001. CONCLUSION: The point prevalence of current physician-diagnosed asthma and BHR among unselected Danish schoolchildren aged 6-17 years was unchanged over 10 years between 1990-1991 and 2001.

Nitrification in bulk water and biofilms of algae wastewater stabilization ponds.

Nitrogen removal in wastewater stabilization ponds is poorly understood and effluent monitoring data show a wide range of differences in ammonium. For effluent discharge into the environment, low levels of nitrogen are recommended. Nitrification is limiting in facultative wastewater stabilization ponds. The reason why nitrification is considered to be limiting is attributed to low growth rate and wash out of the nitrifiers. Therefore to maintain a population, attached growth is required. The aim of this research is to study the relative contribution of bulk water and biofilms with respect to nitrification. The hypothesis is that nitrification can be enhanced in stabilization ponds by increasing the surface area for nitrifier attachment. In order to achieve this, transparent pond reactors representing water columns in algae WSP have been used. To discriminate between bulk and biofilm activity, 5-day batch activity tests were carried out with bulk water and biofilm sampled. The observed value for Rnitrbulk was 2.7 x 10(-1) mg-N L(-1) d(-1) and for Rbiofilm was 1,495 mg-N m(-2) d(-1). During the 5 days of experiment with the biofilm, ammonia reduction was rapid on the first day. Therefore, a short-term biofilm activity test was performed to confirm this rapid decrease. Results revealed a nitrification rate, Rbiofilm, of 2,125 mg-N m(-2) d(-1) for the first 5 hours of the test, which is higher than the 1,495 mg-N m(-2) d(-1), observed on the first day of the 7-day biofilm activity test. Rbiofilm and Rnitrbulk values obtained in the batch activity tests were used as parameters in a mass balance model equation. The model was calibrated by adjusting the fraction of the pond volume and biofilm area that is active (i.e. aerobic). When assuming a depth of 0.08 m active upper layer, the model could describe well the measured effluent values for the pond reactors. The calibrated model was validated by predicting effluent Kjeldahl nitrogen of algae ponds in Palestine and Colombia. The model equation predicted well the effluent concentrations of ponds in Palestine.

Quantification of nitrification and denitrification rates in algae and duckweed based wastewater treatment systems.

Nitrification and denitrification rates at three different depths (0.1, 0.45 and 0.9m from the water surface) in two series of four algae and duckweed based waste stabilisation ponds (ABPs and DBPs) were measured using nitrate reduction techniques in laboratory batch incubations. The effects of temperature and BOD5 loading were investigated. In situ measurements over the ponds' depths were also done for confirmation of laboratory results. Higher dissolved oxygen (DO) concentrations in ABPs, especially during the warm season, favoured higher nitrification in ABPs over DBPs. Organic surface loading also affected the rate of nitrification in the ponds. Nitrification rates did not increase along the treatment line despite the decrease in organic matter content. Adsorption of nitrifiers to available suspended particles and subsequent sedimentation was assumed to be the main reason for the similar nitrification rates in most ponds. In both systems, the presence of DO in the water column resulted in very low denitrification rates (5-45 mg-N m(-2)d(-1)). Higher denitrification rates (160-560 mg-N m(-2)d(-1)) were measured in the sediments when anoxic conditions prevailed in the overlaying water. The absence of nitrite or nitrate accumulation suggested sufficient nitrite and nitrate diffusion within the water column to allow full denitrification. The nitrification and denitrification rates in both systems were higher at high temperature. The range of nitrogen loss via denitrification in ABPs and DBPs corresponded to 15-25% of total influent nitrogen.

Nitrogen mass balance across pilot-scale algae and duckweed-based wastewater stabilisation ponds.

Nitrogen removal processes and nitrogen mass balances in algae-based ponds (ABPs) and duckweed (Lemna gibba)-based ponds (DBPs) were assessed during periods of 4 months, each under different operational conditions. During periods 1 and 2, the effect of cold and warm temperature was studied. During periods 2 and 3, the effect of low- and high-system organic loading (OL) was studied in warm seasons operation. The pilot-scale systems consisted of four similar ponds in series fed with domestic sewage with hydraulic retention time of 7 days in each pond. Overall nitrogen removal was higher during warm temperature in both ABPs and DBPs, but similar during periods 2 and 3. Nitrogen removal in DBPs was lower than in ABPs by 20%, 12% and 8% during cold temperature, warm temperature and high-OL periods, respectively. Depending on temperature and OL rate, ABPs showed higher nitrogen removal via sedimentation (46-245% higher) compared to DBPs. Also, ABPs also showed higher nitrogen removal via denitrification (7-37% higher) compared to DBPs. Ammonia volatilisation in both systems did not exceed 1.1% of influent total nitrogen during the entire experimental period. N uptake by duckweed corresponds to 30% of the influent nitrogen during warm/low OL period and decreased to 10% and 19% during the cold and warm/high OL period, respectively. Predictive models for nitrogen removal presented a good reflection of nitrogen fluxes on overall nitrogen balance under the prevailing experimental conditions.

Comparison of ammonia volatilisation rates in algae and duckweed-based waste stabilisation ponds treating domestic wastewater.

Quantification of ammonia volatilisation from wastewater stabilisation ponds is important in order to understand its significance for overall nitrogen removal in these widely applied low-cost treatment systems. Ammonia volatilisation rates were measured in pilot plant facilities consisting of one line of four algae-based ponds in series and a parallel line of four ponds with a floating mat of duckweed (Lemna gibba). Ammonia volatilisation was assessed during a period of one and a half years. The method applied is accurate, convenient and is proposed for analysis of a wide range of gasses emitted from stabilisation ponds and possibly other aquatic systems. The ammonia volatilisation rates in algae-based ponds (ABPs) were higher than in duckweed-based ponds (DBPs). This can be explained by the lower values of NH(3) in DBPs due to shading and lower pH values, since the volatilisation rate highly correlated with free ammonia concentration (NH(3)) in pond water. The duckweed cover appeared not to provide a physical barrier for volatilisation of unionised ammonia, because whenever NH(3) concentrations were equal in ABP and DBP also the volatilisation rates were equal. Volatilisation was in the range of 7.2-37.4 mg-Nm(-2)d(-1) and 6.4 -31.5 mg-Nm(-2)d(-1) in the ABPs and DBPs, respectively. Average influent and effluent ammonium nitrogen measurements showed that the ammonia volatilisation during the study period in any system did not exceed 1.5% of total ammonium nitrogen removal. Therefore this study confirmed results from simultaneous experimental work in our laboratory indicating that nitrification/denitrification, rather than ammonia volatilisation, is the most important mechanism for N removal in ABPs and DBPs.

Effect of duckweed cover on greenhouse gas emissions and odour release from waste stabilisation ponds.

Treatment of wastewater in stabilisation pond systems prevents the negative environmental impact of uncontrolled disposal of sewage. However, even a natural treatment system may generate secondary negative environmental impacts in terms of energy consumption, emission of greenhouse gases and emission of odorous compounds. Whereas natural systems have an advantage over electro-mechanical systems in that they use less hardware and less energy, it is not yet known whether secondary environmental effects in the form of greenhouse gas emissions are lower for these systems. This research intends to be a first step in the direction of answering this question by assessing gas emissions from two types of natural systems, namely algae-based and duckweed-based stabilisation ponds. The H2S volatilisation from laboratory scale pond-reactors has been determined by drawing the air above the water surface continuously through a solution of 1 M NaOH for absorption of sulphide. The amount of H2S that volatilised from the algae pond-reactor, and was trapped in the NaOH trap, was found to be 2.5-86 mg/m2/day. The H2S volatilisation from the duckweed pond-reactor was found to be negligible, even though the sulphide concentration was 9.7 mg/l S(2-). The duckweed cover was a physical barrier for volatilisation, since bubbles were trapped in the cover. In addition the duckweed layer was found to be afavourable environment for both aerobic sulphide oxidisers (Beggiatoa gigantae) as well as for photosynthetic purple sulphur bacteria belonging to the genus Chromatium. These may also have contributed to the prevention of H2S volatilisation. Results on methane emissions were not conclusive so far, but the same mechanisms that prevent H2S volatilisation may also prevent methane volatilisation. Therefore it was concluded that duckweed covers on stabilisation ponds may reduce the emission of both odorous and greenhouse gases.

Process performance assessment of algae-based and duckweed-based wastewater treatment systems.

A pilot plant experiment was carried out to assess differences in environmental conditions and treatment performance in two systems for wastewater treatment: algae-based ponds (ABP) and duckweed-based (Lemna gibba) ponds (DBP). Each system consisted of a sequence of 4 equal ponds in series and was fed with a constant flow rate of partially treated wastewater from Birzeit University. Physico-chemical parameters and the removal of organic matter, nutrients and faecal coliforms were monitored within each treatment system over a period of 12 months. The results show clear differences in the environmental conditions. In ABP significantly (P>0.05) higher pH and DO values were observed than in DBP. DBP were more efficient in removal of organic matter (BOD and TSS) than ABP. The faecal coliform reduction was higher in ABP. However, the quality of the effluent from the third and fourth duckweed pond (total retention time of 21 and 28 days) did not exceed the WHO-criteria for unrestricted irrigation during both the summer and winter period, respectively. During the summer period, the average total nitrogen was reduced more in ABP (80%) than in DBP (55%). Lower values were measured during the winter period. Seasonal nitrogen reductions of the two systems were significantly different (P>0.05). In DBP, 33% and 15% of the total nitrogen was recovered into plant biomass and removed from the system via duckweed harvesting during the summer and winter period, respectively. This study showed that there were differences in the environmental conditions and treatment efficiencies between the two systems.