Peracetic acid (PAA) stands out as a safe and environmental-friendly oxidant and disinfectant which has been effectively used in wastewater treatment. Chemical oxygen demand (COD) is a very popular analysis in wastewater treatment; however, the interference of residual PAA on the COD measurement is still unknown. In this context, this study investigated the implications of applying the COD analysis in PAA-based treatment. Each 1 mg·L−1 of PAA increased the COD concentration around 13.5 mg O2·L−1. Residual PAA and hydrogen peroxide (H2O2) were efficiently neutralized by sodium metabisulfite (SMBS) at the optimal SMBS/PAA ratio of 10.2:1 in a wide pH range (5 to 9). The effect of PAA addition on the COD concentration was evaluated in different water matrices (potassium hydrogen phthalate and wastewater solutions). The COD results with the SMBS addition at optimal SMBS/PAA ratio were lower than the ones without it. It may happen due to the neutralization of residual H2O2/PAA and the complexity of the water matrices which can interfere in the COD results. This study discussed the impact of the residual H2O2/PAA neutralization before the COD analysis, and this investigation can be used as a practical guideline for the correct COD measurement in PAA-based treatment.

Nutrient availability plays a crucial role in microalgae growth in domestic wastewater. In this study, we investigated the impact of different nitrogen and phosphorus ratios (5:1, 10:1, and 20:1, m∙m⁻¹), and the addition of inorganic carbon on microalgae growth and nutrient uptake from domestic wastewater. Microalgae biomass achieved values ranging from 0.54 to 1.41 g·L⁻¹. The cultivation process had maximum removal efficiencies of 83.7% for soluble chemical oxygen demand (sCOD), 74.0% for total Kjeldahl nitrogen (TKN), and 100.0% for ammonia (NH₃) and orthophosphate (PO₄³⁻). All the NH₃ and PO₄³⁻ concentrations from domestic wastewater without supplementation were completely removed on the fourth day of cultivation. Moreover, no significant differences in microalgae growth, and NH₃ and PO₄³⁻ removals were observed between the conditions with and without nutrient supplementation on the fourth day of cultivation. This study has shown the feasibility of growing microalgae in domestic wastewater without any nutritional supplementation. Further investigations are required to check the long-term performance, energy requirements, and economic viability of this system for wastewater treatment and the production of nutrient-rich biomass for agricultural applications.

Biological reactors with immobilized biomass on free carriers have provided new perspectives for wastewater treatment, once they reduce the system size and increase the treatment capacity. In this study, the performance of three Moving Bed Biofilm Reactors (MBBRs) using different carriers (with and without protected surface area) were evaluated for domestic wastewater treatment in continuous flow. Each MBBR (i.e., R1, R2, and R3) was filled at a ratio of 50% with high-density polyethylene (HDPE) carriers with different characteristics: both R1-K1 and R2-Corrugated tube with protected surface and R3-HDPE flakes without protected surface. Chemical oxygen demand (COD) removal of 80 ± 5.0, 80 ± 3.5, and 78 ± 2.4% was achieved by R1, R2, and R3, respectively. The oxygen uptake by biofilm attached on the carriers was 0.0079 ± 0.0013, 0.0033 ± 0.0015, and 0.0031 ± 0.0026 μg DO·mm−2 for the K1, corrugated tube, and HDPE flakes, respectively. No significant differences were observed between the performance of the three MBBRs in terms of physico-chemical parameters (alkalinity, pH, and dissolved inorganic carbon) and COD removal. Results showed that the carrier type and its characteristics (total area and with/without protected area) did not affect the organic matter removal. Thus, the carrier without a protected surface in MBBR could be a promising low-cost option for domestic wastewater treatment.

Eutrophication in water reservoirs releases algal organic matter (AOM), a key precursor to the formation of disinfection by-products (DBPs) during the disinfection process. Typical drinking water treatment is not efficient for AOM removal, and advanced treatments are necessary for the removal of residual AOM before chlorination. UV-based technology with PS and TiO₂ is widely used as a pre-oxidation step in water treatment; however, no publications have focused on them for AOM degradation. In this context, this work investigated the effect of oxidant concentration (0.1 to 0.5 g∙L⁻¹) and pH (6 to 10) on AOM degradation with TiO₂/UV and persulfate (PS)/UV using response surface methodology. In general, PS/UV was more effective in removing protein, while TiO₂/UV was more effective in carbohydrate degradation. TiO₂/UV removals varied from 27 to 57% for protein and from 48 to 86% for carbohydrates. The optimal condition (57% for protein and 86% for carbohydrates) was obtained using 0.5 g∙L⁻¹ TiO₂ at pH 10. PS/UV removals varied from 33 to 81% for protein and from 24 to 53% for carbohydrates. The optimal condition (81% for protein and 53% for carbohydrates) was obtained using 0.5 g∙L⁻¹ PS concentration at pH 8. Degradation kinetics showed a good fit to the pseudo-first-order model (R² > 95%) for both processes. The DBP formation reductions observed with TiO₂/UV—trihalomethane (THM) (85 to 86%) and chloral hydrate (CH) (94 to 96%)—were similar to the efficiencies observed for PS/UV—THM (87 to 89%) and CH (83 to 88%). These results show the efficiency of UV-based technology for AOM degradation and the control of DBP formation.