Carbon Limitation in Periphytic Algal Wastewater Treatment Systems

As global eutrophication poses an ever increasing threat to water quality new techniques must be implemented to improve the sustainability of natural resource consumption. Wastewater treatment facilities (WWTF) are designed to destroy pathogens, remove particulates, lower oxygen demanding substances, and reduce nutrients from influent waste water to avoid the degradation of receiving waters. WWTF are generally effective, however they are mostly inadequate at removing nutrients. When wastewater derived nutrients such as nitrate and phosphate are discharged to receiving waters, they stimulate algal blooms. Algal blooms can reduce dissolved oxygen, block sunlight for submerged aquatic plants, and render state waters unusable for recreational activities; these issues can negatively impact ecological systems and local economies. One experimental approach to reducing nitrogen and phosphorus in wastewater effluent uses algae as a tertiary treatment system. Algae are photosynthetic organisms that not only remove nutrients from wastewater, but they produce biomass for biofuels. Research is needed to increase the productivity of algae in tertiary treatment systems to make WWTF more ecologically sustainable and economically viable. Because algae in tertiary wastewater treatment systems are typically not limited by nutrients, the algae may become limited by dissolved inorganic carbon. To test this hypothesis, this study characterized the effects of adding carbon dioxide to lab scale, recirculating wastewater algal treatment floways. Eight 61 cm x 121 cm x 5.1 cm (height x length x diameter) recirculating floways, lined with unglazed clay tiles were administered gaseous carbon dioxide for 18 days while eight more were left untreated as controls. The floways, which were administered carbon dioxide, had a 41% greater algal dry mass, and 43% greater ash free dry mass compared to controls. By day 18 phosphate and total phosphorus concentrations in treated floways averaged 67% and 39% iower than controls respectively. Nitrate and total nitrogen concentrations in treated floways averaged 37% and 10% lower than controls respectively. The results demonstrated that carbon dioxide stimulates algae in tertiary wastewater treatment floways. Because floways administered carbon dioxide had a magnitude lower pH than control floways, a secondary experiment was designed to determine if lower pH stimulated algal production. Replicating the previous experiment, recirculating (8L, n=8) floways were treated with 2N hydrochloric acid daily to lower their target pH to 6.48. A pH 7 solution of HCI and sodium hydroxide (n=8) was administered to floways (n=8) and others were left unmanipulated (n=8). After 18 days the acidified floways had 51% less dry algal mass compared to controls. The only nutrient concentration to differ significantly was nitrate which was 17% higher in acidified floways compared to controls. A final experiment was conducted to determine if another carbon source could stimulate algal production. For the third experiment, floways (n=8) were spiked with 8 g/L of sodium bicarbonate (NaHC03l a carbon source) or left untreated for controls (n=8). After 18 days the floways administered NaHC03 showed no difference in nutrient removal compared to control floways. However, these floways did have a 50% increase in volatile solids, a measure of algal biomass, and 39% greater dry algal mass. The first experiment supported the hypothesis that algae may become carbon limited but confounding variables left assigning causality impossible. The second experiment directly assessed the pH effect on algae while the third experiment confirmed the carbon limitation hypothesis by manipulating dissolved inorganic carbon. By assessing each variable a stronger case can be made for the benefits of algal wastewater treatment flow ways such as reducing global eutrophication and improving water quality with a sustainable system