A comprehensive review on the use of algal-bacterial systems for wastewater treatment with emphasis on nutrient and micropollutant removal.

The scarcity of water resources and environmental pollution have highlighted the need for sustainable wastewater treatment. Existing conventional treatment systems are energy-intensive and not always able to meet stringent disposal standards. Recently, algal-bacterial systems have emerged as environmentally friendly sustainable processes for wastewater treatment and resource recovery. The algal-bacterial systems work on the principle of the symbiotic relationship between algae and bacteria. This paper comprehensively discusses the most recent studies on algal-bacterial systems for wastewater treatment, factors affecting the treatment, and aspects of resource recovery from the biomass. The algal-bacterial interaction includes cell-to-cell communication, substrate exchange, and horizontal gene transfer. The quorum sensing (QS) molecules and their effects on algal-bacterial interactions are briefly discussed. The effect of the factors such as pH, temperature, C/N/P ratio, light intensity, and external aeration on the algal-bacterial systems have been discussed. An overview of the modeling aspects of algal-bacterial systems has been provided. The algal-bacterial systems have the potential for removing micropollutants because of the diverse possible interactions between algae-bacteria. The removal mechanisms of micropollutants - sorption, biodegradation, and photodegradation, have been reviewed. The harvesting methods and resource recovery aspects have been presented. The major challenges associated with algal-bacterial systems for real scale implementation and future perspectives have been discussed. Integrating wastewater treatment with the algal biorefinery concept reduces the overall waste component in a wastewater treatment system by converting the biomass into a useful product, resulting in a sustainable system that contributes to the circular bioeconomy.

Start-up of a trickling photobioreactor for the treatment of domestic wastewater.

A stand-alone trickling photobioreactor (TPBR) was seeded with activated sludge and microalgae to treat domestic wastewater. The TPBR was started-up at 12-h hydraulic retention time at room temperature with 12:12 h light:dark cycle. The light was provided by blue LED strips. The reactor has a total volume of 30 L and is divided into six segments. Each segment is 30 cm long and has a diameter of 15 cm. Each segment was packed with polyurethane foam sponge cubes (2.5 × 2.5 × 2.5 cm3 ) with 40% occupancy. The chemical oxygen demand (COD), total organic carbon (TOC), total nitrogen (TN), and phosphorus (P) of domestic wastewater varied in the range of 164-256 mg/L, 84.4-133.8 mg/L, 34.2-55.6 mg/L, and 24.7-39.3 mg/L, respectively, during this period. The COD, TOC, TN, and P concentrations in the effluent after 45 days of operation were 30.24 ± 3.36 mg/L, 7.69 ± 0.09 mg/L, 16.67 ± 0.39 mg/L, and 17.48 ± 0.5 mg/L, respectively. The chlorophyll-to-biofilm biomass ratio increased during the experimental period. The above results indicate that the algal-bacterial symbiotic relationship is beneficial for carbon and nutrient removal from domestic wastewater. PRACTITIONER POINTS: Trickling photobioreactor works on natural ventilation and has low power requirements and a small footprint. The porous sponge media helped in immobilizing and subsequent harvesting of biomass. The reactor conditions favored the growth of diatoms (brown algae) over green algae.

Domestic wastewater treatment in a coupled sequential batch reactor-electrochemical reactor process.

The effectiveness of a sequenced biological-physicochemical reactor system for treating sewage was studied. The biological degradation was conducted in a Sequential Batch Reactor, which had innovative features for simplifying the operation and maintenance of the reactor. The reactor was operated at 4, 6, 8, and 12 hr cycle. Up to 82% removal of Chemical Oxygen Demand (COD), 50% removal of Dissolved Organic Carbon (DOC), 45% removal of Total Nitrogen (TN), and 45% removal of Total Phosphorus (TP) were achieved. The treated effluent was further polished in a continuous-flow bipolar-mode electrochemical reactor to remove additional recalcitrant organic matter from the wastewater. The process parameters were optimized using Response Surface Methodology. At the optimum condition (pH = 8.7; Current = 1.0; reaction time = 9.0), up to 90% removal of COD, 67% removal of DOC, 61% removal of TN, and 99.9% removal of TP were achieved in the coupled system. Micropollutants belonging to Pharmaceutically Active Compounds, pesticides, etc., were significantly removed. The coupled system completely removed Salmonella, Pseudomonas, and Staphylococcus. However, coliforms were detected at the outlet samples. A UV or ozone disinfection treatment is recommended for the safe reuse of the treated water for nonpotable purposes. PRACTIONER POINTS: Sequential sequential batch reactor-electrochemical reactor process (SBR-ECR) technology is effective for micropollutant removal from sewage. The coupled SBR-ECR system requires less footprint compared to conventional biological systems for wastewater treatment. Carbon material balance study revealed that more than 60% of carbon escapes from wastewater in the form of CO2.

Nutrient Removal from Wastewater Using Microalgae: A Kinetic Evaluation and Lipid Analysis.

The objective of this study was to examine the performance of mixed microalgal bioreactors in treating three different types of wastewaters-kitchen wastewater (KWW), palm oil mill effluent (POME), and pharmaceutical wastewater (PWW) in semi-continuous mode and to analyze the lipid content in the harvested algal biomass. The reactors were monitored for total nitrogen and phosphate removal at eight solid retention times (SRTs): 2, 4, 6, 8, 10, 12, 14, and 16 days. The nutrient uptake kinetic parameters were quantified using linearized Michaelis-Menten and Monod models at steady-state conditions. The nutrient removal efficiency and lipid production were found to be higher in KWW when compared with the other wastewaters. Saturated fatty acids (C16:0, C18:0, and C18:1) accounted for more than 60% of the algal fatty acids for all the wastewaters. The lipid is, therefore, considered suitable for synthesizing biodiesel.