Evaluation of nitrification rate and toxicity in urban wastewater treatment plants in terms of water recovery

The depletion of water resources worldwide, the increasing demand for water due to the growing population, and the impact of evolving industries make wastewater treatment and reuse imperative. The diminishing availability of clean water sources leads to considering recycled water as an alternative fresh water source. It is possible to meet the water demand in urban, industrial, agricultural, and industrial sectors through wastewater reclamation. In this thesis, the impact of the return (centrate) flow on the treatment efficiency, especially the critical nitrification rate, and its effect on the effluent toxicity in advanced biological wastewater treatment plants have been evaluated. Additionally, it has been determined whether the reclaimed water from the facility meets the quality parameters for irrigation water.As a result of this study, first and foremost, the characterization of large-scale advanced biological wastewater treatment plants was conducted, revealing an increase in plant flow rate and pollutant loads. The impact of return flows generated after anaerobic digestion and dewatering on the nitrification rate was examined in detail through laboratory-scale studies, and nitrification rates were found to be a maximum growth rate of 0.35 day-1 for 50% diluted centrate and 0.41 day-1 for synthetic wastewater. The maximum growth rate for 50% centrate was determined to cause approximately 15% reduction when compared to experiments with synthetic and wastewater. Even at maximum levels, the return centrate ratio to the main facility was estimated to be in the range of 0.5-1%, which is not expected to have a significant impact on the nitrification rate of the main facility. However, the effect of toxicity, an important factor influencing the nitrification rate, was examined, and the samples taken from different points and the studies classified the influent wastewater, primary settling tank effluent, active biomass sludge, and return sludge as toxic, while the content of the return flow (centrate) was classified as mildly toxic. Heavy metal analyses revealed the accumulation of heavy metals at high concentrations in the active biomass and return sludge. This situation is supported by toxicity experiments. Additionally, it was determined that toxic heavy metal ions pass into the return flow (centrate) water during anaerobic digestion and that continuous toxic effects increase with repeated inclusion in the main facility stream. At the same time, it was established that the toxicity and heavy metal content determined in the main and return flows do not cause any toxic effects in the reclaimed water facility's effluent. Thus, it was determined that the effluent meets Class A irrigation water quality parameters and Class I water classification based on chemical quality salinity values, and Class II water classification based on chloride values, according to regulations.