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Effect of operational variables on nitrogen transformations in duckweed stabilization ponds

Effect of operational variables on nitrogen transformations in duckweed stabilization ponds


Department of Archaeology, University of Glasgow, Glasgow G12 8QQ, UNITED KINGDOM.


There is a diversity of conventional technologies available for removal of pollutants from wastewater. Most of these technologies are aerobic alternatives with high construction cost and high energy consumption and require skilled personal for operation and maintenance. As a consequence, only countries with a high gross national product (GNP) can afford these options. Where these technologies were introduced in developing countries, in most cases these could not be operated sustainably, leading to loss of investments and continued water resource contamination. Extensive investments in wastewater treatment plants world-wide during the last decades have greatly reduced the organic loading of receiving water bodies in high GNP countries. Only recently, many of these plants were appropriated to remove nitrogen and phosphorus. The increasing use of chemical fertilizer may cause high levels of eutrophication in water bodies, which may induce algae blooms resulting in strong fluctuations in oxygen concentration. Oxygen depletion causes fish kill as well as odor problems.

The situation in countries with a low GNP is worse than in the developed world. The unequal expansion of water supply coverage compared to the expansion in wastewater and sanitation services leads to increased contamination of surface and ground waters. The general trend is to use conventional WWT systems for big cities, but for medium and small sized cities non-conventional systems are often considered. Therefore, there is an urgent need to develop and improve low cost technologies for wastewater treatment that are within the economic and technological capabilities of developing countries. In countries like Colombia it is very common that the regulation controls mainly the removals of organic matter and suspended solids. Other parameters like nitrogen, phosphorus, pathogens, microcontaminants are also crucial and need to be addressed. This makes a response via conventional technologies very expensive, and for developing regions in fact unachievable. It would be ideal if new technologies can provide besides the removal of organic matter and solids, resource recovery like the generation of biogas (energy production) or high quality biomass (animal fodder). At the moment, no technological packages appear to be readily available.

Experience has shown that no single technology can offer an optimum treatment for the different components to be treated in wastewater or to recover them as valuable resources. Therefore an adequate combination of different technologies in an integrated system could convert a wastewater treatment into an attractive sustainable system. For example UASB reactor and duckweed ponds are relatively low cost technologies and their combination offers several advantages. Firstly, anaerobic treatment will reduce considerably the organic matter in the wastewater and convert it into methane, which can be used as a source of renewable energy. Secondly, the effluents of anaerobic treatment could be post-treated to meet discharge standards in duckweed ponds for nutrient recovery in the form of high quality biomass. At this point three valuable products can be listed: biogas for use as an energy source, biomass that can be used for aquaculture or animal feed and treated effluent that can be re-used in irrigation. A system that generates such by-products increases the feasibility and sustainability of pollution control programs. Furthermore, the products may help to address the increasing need for food production in the world.

The development of duckweed pond technology has been concentrated on the study of the processes occurring within the ponds, with respect to organic matter, nitrogen, phosphorus and pathogen removal and the corresponding mechanisms. Further research is needed in order to have a good control of effluent nitrogen levels. There are still important questions to be answer like how to maximize nitrogen recovery via duckweed production, how to get good effluent levels depending on effluent reuse. If the effluent is going to be used in crop irrigation, to reduce nitrogen effluent concentration to 15-20 mg l-1 will be enough. If the effluent is going to be discharge in surface waters the nitrogen level would have to be reduced as much as possible. Therefore it is important to study how the design and combination of technologies could generate the required nitrogen effluent levels. The present work was focus on the study of the effect of different operational variables, like the effect of anaerobic pre-treatment, the combination of algae and duckweed ponds, the effect of pond depth on nitrogen transformation and removals.

The effect of anaerobic pre-treatment on environmental and physicochemical characteristics of duckweed stabilization ponds was studied in Chapter 2. The environmental and physicochemical conditions affect both plant growth and microbiological treatment processes in the system. Two series of continuous-flow pilot plants, composed of seven ponds in series each, were operated side by side. One system received artificial sewage with anaerobic pre-treatment, while the other system received the same wastewater without anaerobic pretreatment. pH, temperature, dissolved oxygen, alkalinity, conductivity, biochemical oxygen demand, total and ammonium nitrogen, nitrites and nitrates, and phosphorus were monitored under steady state conditions. It was found that pH levels were very stable in both systems with and without anaerobic pretreatment. Vertical temperature gradients were present during daytime but not as strong as they may occur in conventional stabilization ponds. Oxygen levels were significantly higher in the duckweed system with anaerobic pretreatment, especially in the top layer, (up to 2 mg O2 l-1) than in the system without pretreatment (up to 1.2 mg O2 l-1). Nevertheless, aeration rates were low in both systems. Both systems were efficient in removing organic matter. The system without pretreatment obtained 98% of BOD5 removal in pond 4, so 12 days of retention time will be sufficient to reach high organic matter removal. The system with pretreatment obtained also 98% BOD5 removal (92% in UASB reactor). In this case the duckweed ponds will serve as a polishing step for remaining organic matter. Nutrient removals were 37-48% for nitrogen and 45-50 % for phosphorus in the lines with and without pretreatment respectively.

The main form of nitrogen in anaerobic effluent is ammonium. This is the preferred nitrogen source for duckweed, but at high levels it may become inhibitory to the plant. Renewal fed batch experiments at laboratory scale were performed (Chapter 3) to assess the effect of total ammonia (NH3 + NH4+) nitrogen and pH on the growth rate of the duckweed Spirodela polyrrhiza. The experiments were performed at different total ammonia nitrogen concentrations, different pH ranges and in three different growth media. The inhibition of duckweed growth by ammonium was found to be due to a combined effect of ammonium ions (NH4+) and ammonia (NH3), the relative importance of each one depending on pH.

The effect of anaerobic pre-treatment on the performance of a duckweed stabilization pond system was assessed in a pilot plant located in the Ginebra Research Station-Colombia (Chapter 4). The pilot plant consisted of two lines of seven duckweed ponds in series. One line received de-gritted domestic wastewater and the other received effluent of a 250 m3 Up-flow Anaerobic Sludge Blanket (UASB) reactor, treating the same wastewater. Both lines were operated at a total hydraulic retention time of 21 days. The systems were monitored for temperature, pH, oxygen, biochemical oxygen demand, chemical oxygen demand, total suspended solids, total phosphorus, biomass production, and different forms of nitrogen. No effect of anaerobic pretreatment was observed on pH and temperature in the two systems. Oxygen concentrations were higher in the system with UASB reactor. Although both systems complied with the Colombian regulation for BOD removal (> 85%), pretreatment with UASB reactor may contribute to the reduction of area requirement for the stabilization ponds. Effluent quality in terms of total suspended solids was excellent, i.e. 9 ± 2 and 4 ± 1 mg l-1 in the system with and without pre-treatment, respectively. Total nitrogen removals were 63 % and 68% and phosphorus removals were 24% and 29% in the system with and without pretreatment, respectively. The differences between the two systems were found not to be significant. Duckweed biomass production was in the range of 54-90 g m-2 -d-1 (fresh weight) in the system with pre-treatment and 36-84 g m-2 -d-1 in the system without pre-treatment. Total biomass productions were significantly different at 92% level of confidence. Protein content was 35.1% and 36.6% for the system with and without pre-treatment, respectively.

Nitrogen removal is nowadays one of the most important effluent treatment objectives because of the serious pollution problems it causes to the environment. How nitrogen is transformed and removed in duckweed ponds was studied and nitrogen balances were established (Chapter 5). The experimental system was the same as in the previous chapter. Ammonia volatilization was found to be not an important removal mechanism in duckweed ponds (less than 1%). Removal by sedimentation was also low at 2.1% and 4.7% for the systems with and without anaerobic pre-treatment, respectively. Instead, denitrification was found to be the most important removal mechanism (42% and 48%), followed by duckweed biomass up-take (15.6% and 15.1%). Average nitrogen biomass up-take rates were 199 mg N m-2 d-1 and 193 mg N m-2 d-1 for the system with and without pretreatment, respectively. Nitrification rates were in the range of 112-1190 mg N m-2 d-1 and 58-1123 mg N m-2 d-1 for the system with and without anaerobic pretreatment respectively. Denitrifícation rates were in the range of 112 - 937 mg N m-2 d-1 and 59 - 1039 mg N m-2 d-1 for the system with and without pre-treatment respectively. The configuration of the system, in particular the down and up flow pattetn seemed to have an important stimulating effect on denitrifícation rates, probably by causing alternative exposure of the pond water to aerobic and anoxic conditions.

Although the potential of duckweed ponds for removing carbonaceous and suspended material from wastewater has been demonstrated, the system could be further optimized for nitrogen removal. The effect of introducing algae-ponds (aerobic zones) into a series of duckweed stabilization ponds on nitrification and denitrifícation (Chapter 6) was studied in two consecutive phases. During the first phase, the seven ponds of the pilot plant were fully covered with duckweed (Spirodela polyrrhiza). Before the start of the second phase, the duckweed cover was removed from ponds 1 and 3, with a view to allow algae growth in the 'open' ponds. The feed of the duckweed pond system consisted of the effluent of a real scale UASB reactor, which treated domestic wastewater from Ginebra-Colombia. The system was operated with a continuous flow to produce a hydraulic retention time (HRT) of 3 days per pond and a total HRT of 21 days. Effluent total nitrogen was significantly different in the two phases, with 13.8± 2.9 mg TN l-1 (63 % removal) and 3.7±1.5 mg TN l-1 (90%) for first and second phase, respectively. Denitrifícation was the most important removal mechanism during both phases, and amounted to 43.5 % and 76.2 % of influent nitrogen, in first and second phase, respectively. Ammonia volatilization and sedimentation were insignificant processes for nitrogen removal in both phases. Nitrification played an important role in nitrogen transformations in the duckweed systems and it was favored by the introduction of aerobic zones in ponds 1 and 3. Denitrifícation also played a key role in nitrogen transformations and removal. Despite the presence of oxygen in the water column, denitrifícation occurred, probably due to the anaerobic microenvironment of system biofilms. Higher nitrogen removal might be obtained in duckweed pond systems through the introduction of aerobic zones in early stages of the system. Where effluents cannot be reused for crop irrigation, strict nitrogen effluent criteria can be met using hybrid duckweed-algal ponds at considerably shorter hydraulic retention time compared to fully duckweed covered systems.

The effect of pond depth on nitrogen removal in duckweed stabilization ponds was studied in Chapter 7. The pilot plant consisted of two Unes with seven duckweed ponds in series, with different depths and fed with effluent of a laboratory scale UASB reactor. Three experimental conditions were studied: DSP1 with pond depth 0.7 m and HRT= 21 days, DSP2 with pond depth 0.4 m and HRT = 12 days, and DSP3 with pond depth 0.4 m and HRT = 21 days. The systems were monitored for pH, temperature and oxygen profiles, organic matter removal (BOD5), nitrogen transformations, biomass production and biomass nitrogen content. Average total nitrogen removal rates were 598 mg N m"2 d"' for DSP 1, 589 mg N m-2 d-1 for DSP 2 and 482 mg N m-2 d-1 for DSP 3. In spite of the lower nitrogen removal rate in DSP 3, it had higher removal efficiency (44 %, 43 % and 62 % for DSP 1, 2 and 3 respectively) due to the lower surface loading rate in this system. This shows that using the percentage of removal as a parameter for comparison should be done with care and the operational parameters of the compared systems should be taken into account. Denitrification was the most important nitrogen removal mechanism for the three DSPs. Nitrogen removal via biomass production was the second most important removal mechanism for the three experiments. Pond depth does not seem to determine nitrification or denitrification. Nitrification seems to be related to surface organic loading rate, while denitrification was related to BOD availability. The comparison between two pond systems with different depth, but operated at the same hydraulic surface loading rate (DSP 1 and 2) showed similar nitrogen removals in the shallower system as in the deeper system. This suggests that duckweed pond system could be designed with shallow depth without affecting surface loading and nitrogen removal efficiency. Nitrogen removal appeared to be governed by surface loading rate rather than by hydraulic retention time.

Most of the research so far has been performed at laboratory or pilot scale. In the process of technology-development it is important to test findings at full scale. In Chapter 8, the performance of a full scale duckweed pond was compared with a full scale algae pond treating effluent of a UASB reactor operated under similar conditions of climate, configuration, wastewater composition and loading rate. The real scale experimental system was composed of two continuous flow channels. One operated as an algae pond and the other as a duckweed pond (Spirodela polyrrhiza and Lemna minor.). The volume of each channel was 225 m3 , an average surface area of 322 m2 , L/W ratio= 13.1 and depth of 0.7 m. The wastewater flow was 19.7 m3 d'1, for each system and the theoretical hydraulic retention time was 11.5 days. The ponds were monitored for the following parameters: Organic matter (BOD5), total suspended solids (TSS), ammonium nitrogen (NH4+-N), total Kjeldahl nitrogen (TKN), nitrite nitrogen (N02-N), nitrate nitrogen (NO3-N), total phosphorus (TP) and faecal coliform (FC). The duckweed pond developed different environmental conditions in terms of pH, temperature and oxygen, compared to the algae pond. The duckweed pond was more efficient in removing organic matter and the algae pond was more efficient in nitrogen removal. Denitrification accounted for most of the nitrogen removal in the algae and duckweed ponds. The second most important mechanism for nitrogen removal was ammonia volatilization for the algae pond and plant up-take for the duckweed pond. In the design of duckweed pond systems special attention should be paid to the reactor configuration and flow pattern in order to obtain good contact between water column and the duckweed cover and to reduce hydraulic problems.

Practical applications.
Wastewater treatment can be converted into an attractive, feasible and sustainable alternative by combining anaerobic pretreatment, duckweed ponds, and algae ponds. The integrated system UASB reactor, algae pond and duckweed pond offers the possibility to remove the various unwanted component in wastewater and to recover part of the valuable material present in the wastewater in the form of biomass or biogas The effluents may be suitable for discharge or for irrigation depending on the removal efficiencies of the system.

The design and operation of this integrated system may have two different approaches. Firstly, one could optimize nitrogen recovery by duckweed uptake and effluent irrigation. Secondly, one could maximize nitrogen removal in order to protect the receiving water resources.

If the objective of the treatment is recovery of nitrogen then the stimulation of duckweed incorporation and the reduction of effluent nitrogen to a suitable range for irrigation would be the best option. The configuration of an efficient anaerobic pretreatment followed by a series of ponds completely covered with duckweed would be recommendable. Influent ammonium nitrogen concentration below 50 mg l-1 and pH below 8 would be desirable to avoid biomass growth inhibition. The comparison between two pond Systems with different depths and the same hydraulic surface loading rate showed similar nitrogen removals in the shallower system as in the deeper system. This means that duckweed pond system could be designed with the shallower depth without affecting nitrogen removal efficiency. Shallow ponds are easier to build, to operate and to maintain and in the case of duckweed covered ponds, they can be regarded as a erop production system.

If the objective of the treatment is nitrogen removal due to disposai regulations, a strategy to enhance denitrification should be adopted. Higher nitrogen removals may be obtained in duckweed pond Systems through the introduction of aerobic zones in early stages of the system, which allows a considerable reduction of the hydraulic retention time. Strict nitrogen effluent criteria can therefore be met at relatively short hydraulic retention times. The configuration of the system, in particular the down and up flow pattern seems to have an important positive effect on denitrification rates.

Compartmentalization of the treatment system improves the pond performance. In the design of pond Systems special attention should be paid to the reactor configuration and hydraulic flow pattern, good contact water-biomass and to avoidance of short circuiting and dead zones.

In the process of technology development the following studies are envisaged and recommended for further research:

Future studies should be focused on shallow ponds with the views to enhance nitrogen removal via its recovery in the form of duckweed biomass. Shallow ponds will also reduce construction cost of the treatment Systems.
  • Alternative uses of treated effluent and produced biomass should be investigated. In the case of effluent reuse on irrigation, the reduction of nitrogen concentrations in the treatment system to 15-25 mg f' will be enough. The use of vegetable biomass as a food complement on the diet of fish and pork is an alternative that has been preliminary explored in the area of research. Further studies are necessary to determine its feasibility.
  • For safe discharge of effluent to open water bodies, effluent nitrogen concentration should be low. In this case nitrogen removal processes may be influence by affecting growth conditions of nitrifiers/dentrifiers like oxygen levels or availability of area for bacterial attachment. It is important to performed studies in order to find the best combination of duckweed and algae ponds for nitrogen removal. The introduction of baffles on the treatment Channels will increase the availability of area for biomass growth and will improved the hydraulic characteristics of the treatment Systems. The appropriated number and distribution of baffles should be investigated. Recycling of final aerobic effluent to the UASB reactor or to the entrance of the duckweed pond could be an interesting option to stimulate denitrification.
  • Pathogen removal will be affected by the use of low pond depths, the presence of aerobic zones and compartmentalization in the treatment system. These effects should be researched in order to optimize also the removal of pathogenic microorganisms.