Nitrogen Removal from Wastewater in Microalgal-Bacterial Treatment Ponds Otoyo Kurosu Abstract Much of agricultural and domestic wastewater contains high concentrations of nitrogen, which can affect public health and have harmful ecological impacts. Microalgal-Bacterial treatment ponds have generally been viewed as an effective method of removing nitrogen from wastewater. However, it is not well known under what physical and biological conditions nitrogen can most effectively be removed in the system. This study investigated how seasonal variables, namely water temperature and solar radiation influence the nitrogen removal in microalgal-bacterial treatment ponds. Additionally, the study aimed to elucidate what biological nitrogen removal mechanisms (ammonification, nitrification, denitrification, ammonia volatilization and algae assimilation) were mostly operating in a pond whose wastewater was municipal sewage and in a pond whose wastewater was agricultural drainage. Data of water temperature (°C) and solar radiation (w/m²) were collected from two Northern California wastewater sources, Panoche and Delhi. Concentrations of nitrogen (mg/L) in four different forms (organic nitrogen, ammonia, nitrite, and nitrate) in the water samples from both sites were calculated. Regression tests were used to predict the relationship between nitrogen removal and seasonal variables. The test showed that the total nitrogen removed from both sites was not significantly correlated with both water temperature and solar radiation. Ammonia volatilization and algae assimilation could be the two major removal mechanisms in ammonia-dominated Delhi, and denitrification was the most important in nitrate-dominated Panoche. This study gives a better understanding of microbial and biological processes in the microalgal-bacterial treatments ponds of Delhi and Panoche.Introduction Nitrogen is becoming increasingly important in wastewater management because nitrogen can have many effects on the environment (Halling-Sørensen and Jørgensen 1993). Nitrogen can exist in different forms because of various oxidation states, and it can readily change from one to another depending on the oxidation state present at the time. In the environment, living organisms can accomplish changes from one oxidation state to another. The principal forms of nitrogen are organic nitrogen, ammonia (NH₄⁺ or NH₃), nitrite (NO₂⁻), and nitrate (NO₃⁻) (Reed 1984). The presence of nitrogen in wastewater discharge can be undesirable because it has ecological impacts and can affect public health. Ammonia is extremely toxic to fish and many other aquatic organisms and it is also an oxygen-consuming compound, which can deplete the dissolved oxygen in water. The Depletion of dissolved oxygen in water is a problem in aquatic ecosystem since maintenance of a high oxygen concentration is crucial for survival of the higher life forms in aquatic ecosystem. Another ecological impact is eutrophication. All forms of nitrogen are taken up as a nutrient by photosynthetic blue-green bacterial and algae. The excessive growth of bacteria and algae due to the increase of the amount of nitrogen discharged into water, contributes to the reduction of the oxygen level in water. Although nitrate itself is not toxic, its conversion to nitrite is a concern to public health. Nitrite is a potential public health hazard in water consumed by infants (Sedlak 1991). In the body, nitrite can oxidize the iron (II) and form methemoglobin, which binds oxygen less effectively than normal hemoglobin. The resulting decrease in oxygen levels in young children leads to shortness of breath, diarrhea, vomiting, and in extreme cases even death (Kelter et al. 1997). These occurrences are generally associated with disposal of municipal sewage and fertilizer application to agricultural crops (Pinette 1993). The dangers that all these incidents have posed are a clear indication that municipal and agricultural wastewater must be treated before discharge. The treatment of wastewater in microalgal-bacterial treatment ponds exploits the physical and biochemical interactions that occur naturally in aquatic system to remove nitrogen (Hurse and Connor 1999). The system of microalgal-bacterial treatment ponds has been viewed as effective and low cost method of removing nitrogen from wastewater and are it is widely used all over the world. In north America alone, there are around 7000 microalgal-bacterial treatmentponds in use (Maynard et al. 1999). In theory, the nitrogen in wastewater will be converted to harmless nitrogen gas and will be lost to the atmosphere by going through three major biological transformations during removal of nitrogen in micoralgal-bacterial treatment ponds. The three biological transformations are ammonification, nitrification, and denitrificaion. In ammonification, microorganisms decompose the organic nitrogen and produce ammonia. In nitrification, microorganisms oxidize the ammonia compounds to nitrite and then to nitrate. The nitrate will be taken up by algae and either be converted to organic nitrogen in their cell tissues (algae assimilation) or will be reduced to elemental nitrogen (N₂) and lost as a gas by denitrification (Oswald 1996). In addition, many studies have been conducted and reported wide variations in the performance of microalgal-bacterial treatment ponds systems. However, there is some disagreement over the mechanisms responsible for the removal of nitrogen in treatment ponds. Hurse and Conner (1991) concluded that nitrification and denitrification were important nitrogen mechanisms in a wastewater treatment pond in Australia. On the other hand, several studies concluded that ammonia volatilization is the main nitrogen mechanisms from wastewater treatment ponds (Silva et al. 1995 and Soares et al. 1996). The differences from these two could be explained by Sørensen’s study that explains the advantages and disadvantages of various mechanisms presented for the removal of nitrogen from wastewater. Each mechanism has an effect on a specific form of nitrogen, which can be either organic nitrogen, ammonia or nitrate. Moreover, Gianelli (1971) stated that temperature and light energy were very important factors in microalgal-bacterial treatment ponds because temperature affects the growth rate of algae and algae require sufficient solar energy in order to release oxygen from waster by
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