Increased population and industrialization generate a large number of organic pollutants that create problems on the planet earth. The level of freshwater is reducing which has pushed the society to reuse/recycle wastewater. Eco-friendly and economically sound treatment of industrial wastewater has attracted global attention and hence is a thrust area of research. Organic compounds rich wastewater can be used to generate bioenergy and value-added products from the resource recovery point of view. Wastewater treatment(s) can be used to trap energy from industrial effluents in form of biofuel, bioenergy and biogas. Recovered products can be used in various ways such as recovered nutrients for (bio)fertilizer production and algal biomass for bioplastic production. Microbial electrochemical technology is a promising approach for resource recovery. This review article aims to present and discuss trends and scientific developments about recovery of value-added products from dye industry effluent. It also provides state-of-art technical information about technologies for remediation of pollutants from dye industry effluent with emphasis on nanotechnological approaches and microbial electrochemical technologies (METs). It narrates literature on classification and properties of dyes, effects of dye pollutants on environment and human health and factors affecting degradation of dyes. Generation of bioenergy and recovery of valuables from dye industrial wastewater along with challenges and perspectives of this research area have been covered.

Trends in dye industry effluent treatment and recovery of value added products

Rapid urbanisation and industrialization have made significant increase in demand for water. Dyes are majorly organic compounds used by various industries as an identifier, colorant or for aesthetic purposes. Effluents discharged by such industries containing dyes may change the physical, chemical and biological properties of water in long run which causes adverse impacts on the ecosystem. While in humans and other aquatic animals, the dyes have toxic, carcinogenic and mutagenic effects. The treatment of dyes can be done by various methods like membrane filtration, ion exchange, photochemical oxidation, nanotechnological approaches and various other physicochemical methods. Plants and microorganisms like algae, fungi, and bacteria also play a vital role in remediation of dyes through enzymatic processes. Phytoremediation of dyes takes place under the influence of reductase enzyme which reduces the dye into simpler organic compounds. Algae are source of carbon sink in oceans and also help in remediation of pollutants present in water. Algal cells are useful feedstock for various sectors of our economy such as biofuels, microalgal biochar, nutraceuticals, microbial fuel cell for bioelectricity, phyconanoparticles for environmental and medical applications. This review article aims to discuss dyes, sustainability issues and ecotoxicity related to them. It will give an insight on various methods of dye removal emphasising phytoremediation and factors responsible in its removal. The opportunities associated with the recovery of valuable products from algae would suggest it a sustainable approach in treatment of dyes present in effluents.

Phycoremediation of effluents containing dyes and its prospects for value-added products: A review of opportunities

The role of extracellular polymeric substances (EPS) in the minimization of the excess sludge yield underlying hybrid activated sludge treatment (as MANODOX system) for tannery wastewater was experimentally elucidated. The operational strategies designed for the MANODOX system configuration and a parallel-run conventional aerobic activated sludge (p-CAS) treatment were tested throughout the study period of 320 days. In comparison with the p-CAS system, MANODOX experimentation received prominently lowered sludge yield (averaged around 0.36 gVSS/gCOD). The experimental observation further affirmed that the sludge generation found influenced by the variation in the sludge characteristics like protein to polysaccharides ratio (ranged around 0.62–0.73), food to microorganism ratio (0.41 ± 0.04), dissolved oxygen (2.23 ± 0.30 mg/L) and bioflocculation ability (71.8–73.8%). Here, MANODOX implementation exhibited three times lowered EPS accumulation and around 70% less sludge generation compared with the p-CAS system. However, the ascertained was positively attributed to the purposeful nourishment of cell-lysis in the MANODOX system. Importantly, the strict maintenance of desired sludge retention time of 23–26 days imposed a profound effect on the aforementioned floc characteristics and EPS degradation. Furthermore, the Fourier transform infrared spectroscopy and specific ultra-violet absorbance analysis reveals, the facultative microbiota had a prominent influence on organic functional groups that presumed to be originated from the disintegration of EPS in the sludge. The corroborated has further been supported by thermogravimetric analysis and batch anaerobic and aerobic digestion test of the volatile sludge pool performed at end of the mainstream operation.

Investigation of activated sludge characteristics and their influence on simultaneous sludge minimization and nitrogen removal from an advanced biological treatment for tannery wastewater

Characteristics of nitrogen removal and sludge reduction using a multi-redox environment coupled bioreactor

The presence of suspended contaminants in water and wastewater such as algae, colloids, fats and oil, necessitates the use of systems such as dissolved air flotation (DAF) for their removal. In the current study, a novel setup has been proposed for bubble enhancement. An industrial scale (pilot) DAF system was tested at saturator pressures of 3 to 7 atm and flow rates of 5 to 20 L/min in three different configurations, namely, empty, packed, and the innovative two-stage (TS) configuration. In the TS system, after the nucleation of micro bubbles, the water is returned to the saturator to undergo pressurization for a second time before it is passed through the nozzle once more and is released. The results show that the highest volume of released air as well as the smallest microbubbles are seen in the TS configuration, followed by packed mode, with the empty configuration showing the least favorable results. Moreover, the bubbles produced at the lowest residence time and pressure (3 atm) with the novel setup are better than the bubbles produced by the standard configuration even with pressures as high as 7 atm. Thus, the novel TS setup can allow for significantly lower energy requirements and lower capital costs. For real-world application of the TS system, the feed for the saturator would have to be extracted from within or near the contact zone, i.e., where the bubbles are released in the DAF tank.

A two-stage dissolved air flotation saturator configuration for significant microbubble improvement.

Two SBBR systems, designated FSCSBBR (floating spherical carrier) and SCSBBR (multi-faceted polyethylene suspension carrier), were constructed for in-situ sludge reduction. The effects of the operating temperature (T), hydraulic retention time (HRT) and dissolved oxygen (DO) concentration on in-situ sludge reduction in the two SBBR systems were investigated, and the optimum carrier and operating parameters were determined. The results showed that an FSC carrier exhibited a better in-situ sludge reduction efficiency and contaminant removal efficiency than an SC carrier. The optimal operating parameters were a temperature of 25°C, HRT of 12 h and DO concentration of 4–5 mg/L, and the observed sludge yield was 0.21 ± 0.10 g MLSS/g COD. The analysis of biofilm samples showed that FSCSBBR biofilms were richer in microbial species and exhibited higher diversity than SCSBBR biofilms. Biofilm sludge hydrolysis and cytolysis promoted in-situ sludge reduction.

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Comparison of in-situ sludge reduction in a sequencing batch biofilm reactor (SBBR) under different carriers: operation parameter optimizations

\n The countercurrent–cocurrent dissolved air flotation (CCDAF) process is a new type of air flotation process integrating countercurrent collision and cocurrent flow adhesion processes. The structural form of the CCDAF tank and its process parameters are the required conditions to achieve countercurrent collision and cocurrent adhesion. In this study, eight CCDAF tank process models were established with a flow rate of 0.5 m3/h. Flow field numerical simulation and process optimization of a CCDAF tank was conducted using Fluent software. The simulation results show that the optimal conditions for the CCDAF process are as follows: contact zone ascending velocity 10 mm/s, separation zone separation velocity 1.5 mm/s, dissolved gas pressure 0.45 MPa, and recirculating dissolved-gas distribution ratio R1/R2 1:1. Under these operating conditions, the flow state in the flotation tank is the most stable and the gas in the contact zone is evenly distributed. According to the simulation results, a 5 m3/h pilot plant was built. The structural dimensions were: B × L × H = 1,020 mm × 1,300 mm × 1,350 mm. The test results show that the CCDAF has a significant decontamination effect and is clearly superior to the cocurrent flow DAF process and countercurrent flow DAF process.

Zhou Anran

Papers

Comparison of in-situ sludge reduction in a sequencing batch biofilm reactor (SBBR) under different carriers: operation parameter optimizations

Research on the CFD numerical simulation and process optimization of countercurrent–cocurrent dissolved air flotation