Abstract
Biological nutrient removal from wastewater to reach acceptable levels is needed to protect water resources and avoid eutrophication. The start-up of an anaerobic ammonium oxidation (anammox) process from scratch was investigated in a 20 L sequence batch reactor (SBR) inoculated with a mixture of aerobic and anaerobic sludge at 30 ± 0.5 °C with a hydraulic retention time (HRT) of 2–3 days. The use of NH4Cl, NaNO2, and reject water as nitrogen sources created different salinity periods, in which the anammox process performance was assessed: low (<0.2 g of Cl−/L), high (18.2 g of Cl−/L), or optimum salinity (0.5–2 g of Cl−/L). Reject water feeding gave the optimum salinity, with an average nitrogen removal efficiency of 80%, and a TNRR of 0.08 kg N/m3/d being achieved after 193 days. The main aim was to show the effect of a hydrazine addition on the specific anammox activity (SAA) and denitrification activity in the start-up process to boost the autotrophic nitrogen removal from scratch. The effect of the anammox intermediate hydrazine addition was tested to assess its concentration effect (range of 2–12.5 mg of N2H4/L) on diminishing denitrifier activity and accelerating anammox activity at the same time. Heterotrophic denitrifiers’ activity was diminished by all hydrazine additions compared to the control; 5 mg of N2H4/L added enhanced SAA compared to the control, achieving an SAA of 0.72 (±0.01) mg N/g MLSS/h, while the test with 7.5 mg of N2H4/L reached the highest overall SAA of 0.98 (±0.09) mg N g/MLSS/h. The addition of trace amounts of hydrazine for 6 h was also able to enhance SAA after inhibition by organic carbon source sodium acetate addition at a high C/N ratio of 10/1. The start-up of anammox bacteria from the aerobic–anaerobic suspended biomass was successful, with hydrazine significantly accelerating anammox activity and decreasing denitrifier activity, making the method applicable for side-stream as well as mainstream treatment.
Highlights
The removal of nutrients from wastewater to acceptable thresholds is essential for the protection of water bodies from algal blooms and from subsequent eutrophication, which may lead to reduced dissolved oxygen (DO) concentration and cause the death of life forms [1,2]
One of the major advancements in nutrient removal was the discovery of the anaerobic ammonium oxidation process, which occurs under anoxic conditions, and is characterized by the anoxic oxidation of ammonium to N2 gas, with nitrite serving as an electron acceptor [3]: NH4+ + 1.32NO2− + 0.066HCO3− + 0.13H+ → 1.02N2 + 0.26NO3− + 0.066CH2O0.5N0.15 + 2.03H2O
The anammox process shows a high autotrophic total nitrogen removal efficiency (TNRE), and low operational costs; it has no requirement for an addition of organic carbon, has low emission of greenhouse gases (CO2/N2O), and has low sludge production compared to nitrification– denitrification processes [4]
Summary
The removal of nutrients from wastewater to acceptable thresholds is essential for the protection of water bodies from algal blooms and from subsequent eutrophication, which may lead to reduced dissolved oxygen (DO) concentration and cause the death of life forms [1,2]. Water 2021, 13, 350 the process stability This hydrazine concentration range has stimulated anammox bacteria without harming AOBs, but has inhibited nitrite oxidizing bacteria (NOBs) [23]. Hydrazine addition contributes to a rapid recovery of the anammox biomass from nitrite inhibition, but its ameliorating effect in the case of detrimental effects resulting from excessive salinity is possible. A considerably higher hydrazine concentration (>15 mg of N2H4/L) was observed to slow down the nitrogen removal process (nitrite oxidation, denitrification, anammox), which is possibly attributable to the inhibitory effect [19,22,23]. The main objective is to evaluate the effect of hydrazine addition on the specific anammox activity (SAA) and denitrification activity during different stages of the start-up process in order to accelerate the overall start-up of autotrophic nitrogen removal from scratch. An additional objective is to evaluate the effect of salinity on the start-up process and overcome the detrimental effect of excess salinity with hydrazine
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