Is monochloramine pre-oxidation a viable strategy for enhancing the treatment efficiency of algae-laden water with conventional drinking water treatment process?

Algal blooms worldwide pose many challenges to drinking water production. Pre-oxidation with NaClO, KMnO4, or ozone is commonly used to enhance algal removal in conventional drinking water treatment processes. However, these currently utilized oxidation methods often result in significant algal cell lysis or impede the operation of the subsequent units. Higher algal removal with pre-chlorination in algal solutions prepared with natural water, compared to those prepared with ultrapure water, has been observed. In the present studies, preliminary findings indicate that ammonium in natural water alters chlorine species to NH2Cl, leading to improved treatment efficiency. NH2Cl with 1.5-3.0 mg∙L-1 as Cl2 with an oxidation time of 3-7 h significantly enhancing algal removal by coagulation. The selective oxidation of surface-absorbed organic matter (S-AOM) by NH2Cl, followed by the subsequent peeling off of this material from the algal surface, leading to an increase in zeta potential from -20.2 mV to -3.8 mV, constitutes the primary mechanism of enhanced algal removal through coagulation. These peeled S-AOM retained their large molecular weight and acted as polymer aids. Compared with NaClO and KMnO4, NH2Cl displays the best performance in improving algal removal, avoiding cell lysis, and decreasing the potential for nitrogenous disinfection byproducts formation under the reaction conditions used in this study. Notably, in major Chinese cities, water purification plants commonly rely on suburban lakes or reservoirs as water sources, necessitating the transportation of raw water over long distances for times up to several hours. These conditions favor the implementation of NH2Cl pre-oxidation. The collective results indicate the potential of NH2Cl oxidation as a viable pretreatment strategy for algal contamination during water treatment processes.

A novel co-graft tannin-based flocculant for the mitigation of harmful algal blooms (HABs): The effect of charge density and molecular weight

In this study, for the first time, we developed a series of co-graft tannin-based flocculants, TA-g-P(AM-DMDAAC), with different charge densities (CDs) and molecular weights (MWs) and evaluated their algal-removal performances. The effects of TA-g-P(AM-DMDAAC) on the cell integrity of Microcystis aeruginosa and release of extracellular organic matter (EOM) and microcystin-leucine-arginine (MC-LR) in flocculation and floc storage were also studied. Results suggested that TA-g-P(AM-DMDAAC) could not only efficiently remove algal cells over a wide pH range (pH 3-11) but also EOM. CD and MW significantly affected flocculation performance and floc characteristics of TA-g-P(AM-DMDAAC). A higher CD helped achieve a higher removal efficiency of algal cells and EOM, whereas a higher MW resulted in the formation of larger and more compact flocs. Furthermore, the larger and denser flocs could better protect algal cells and reduce the release of EOM during floc storage. Notably, algal cells in the TA-g-P(AM-DMDAAC) flocs did not appear to show signs of massive rupture nor did they release EOM and MC-LR extensively for at least 20 days of storage. The abundance and easy availability of tannin resources effectively reduce the cost of preparing tannin-based flocculants. Therefore, TA-g-P(AM-DMDAAC) can have broad application prospects in the management of cyanobacteria bloom.

Diatomite-enhanced coagulation for algal removal in polluted raw water: performance optimization and pilot-scale study

Algae blooms have seriously threatened the health of aquatic ecosystems and the safety of drinking water. In this study, diatomite-enhanced coagulation technology was developed to improve the removal of algae and other pollutants. The dosage and ratio of diatomite and aluminum salts were optimized to 40mg/L and 1:1 which achieved algal removal efficiency of 98.8±0.65%. The effect of environmental factors was studied and it shows that cell density, pH, and temperature had a significant impact on algal removal. The mechanism of diatomite-enhanced coagulation was speculated to be adsorption bridging and sweep flocculation. Pilot-scale equipment was set up to verify the performance of diatomite-enhanced coagulation in engineering applications on algae polluted raw water. Results showed a better removal efficiency of algae, NH₄⁺-N, NO₂^--N, and COD_Mn and lower operation cost than the actual operation in the Waterwork Corporation were achieved with good application prospects and promotion value.

Removal of seven endocrine disrupting chemicals (EDCs) from municipal wastewater effluents by a freshwater green alga

The present endocrine disrupting chemicals (EDCs) in wastewater effluents due to incomplete removal during the treatment processes may cause potential ecological and human health risks. This study evaluated the removal and uptake of seven EDCs spiked in two types of wastewater effluent (i.e., ultrafiltration and ozonation) and effluent cultivated with the freshwater green alga Nannochloris sp. In ultrafiltration effluent cultivated with Nannochloris sp. for 7 days, the removal rate of 17β-estradiol (E2), 17α-ethinylestradiol (EE2), and salicylic acid (SAL) was 60%; but Nannochloris sp. did not promote the removal of other EDCs studied. The algal-mediated removal of E2, EE2, and SAL was attributed to photodegradation and biodegradation. Triclosan (TCS) underwent rapid photodegradation regardless of adding algae in the effluent with 63%-100% removal within 7 days. Triclosan was also found associated with algal cells immediately after adding algae, and thus the primary mechanisms involved were photodegradation and bioremoval (i.e., bioadsorption and bioaccumulation). After algal cultivation, TCS still has a bioaccumulation potential to pose high risks within the food web and the endocrine disrupting properties of the residual estrogens in the effluent are not eliminated. Algal cultivation can be exploited to treat wastewater effluents but the removal efficiencies of EDCs highly depend on chemical types.

UV-initiated synthesis of a novel chitosan-based flocculant with high flocculation efficiency for algal removal

In this study, maleyl chitosan-graft-polyacrylamide (MHCS-g-PAM), a novel chitosan-based flocculant, was prepared through UV irradiation, and maleyl chitosan (MHCS) was designed and prepared with maleic anhydride and acrylamide (AM) through maleyl acylation reaction. The effects of monomer concentration, MHCS-to-AM ratio, illumination time, initiator concentration, pH on viscosity, and grafting efficiency were investigated to optimize the synthesis of these substances. MHCS-g-PAM was characterized through Fourier transform infrared spectroscopy, nuclear magnetic resonance hydrogen spectroscopy, scanning electron microscopy, and thermal gravimetric analysis. Flocculation mechanisms in alga-containing wastewater at various pH levels and dosages were examined in detail on the basis of zeta potential measurements. Zeta potential experiments indicated that the adsorption-bridging and charge neutralization mechanisms played an important role in algal removal. Flocculation tests on algal removal demonstrated that the flocculation performance of MHCS-g-PAM was more effective than that of cationic polyacrylamide, polyferric sulfate, and polymeric aluminium. The optimal Chl-a and COD removal rate obtained by MHCS-g-PAM was 98.6% and 94.9% at pH7 and 4mg·L^-1, respectively.

Removal of Microcystis aeruginosa using hydrodynamic cavitation: performance and mechanisms

Algal blooms are a seasonal problem in eutrophic water bodies, and novel approaches to algal removal are required. The effect of hydrodynamic cavitation (HC) on the removal of Microcystis aeruginosa was investigated using a laboratory scale device. Samples treated by HC were subsequently grown under illuminated culture conditions. The results demonstrated that a short treatment with HC could effectively settle naturally growing M. aeruginosa without breaking cells. Algal cell density and chlorophyll-a of a sample treated for 10 min were significantly decreased by 88% andv 94%, respectively, after 3 days culture. Various HC operating parameters were investigated, showing that inhibition of M. aeruginosa growth mainly depended on treatment time and pump pressure. Electron microscopy confirmed that sedimentation of algae was attributable to the disruption of intracellular gas vesicles. Damage to the photosynthetic apparatus also contributed to the inhibition of algal growth. Free radicals produced by the cavitation process could be as an indirect indicator of the intensity of HC treatment, although they inflicted minimal damage on the algae. In conclusion, we suggest that HC represents a potentially highly effective and sustainable approach to the removal of algae from water systems.