Extracellular polymeric substances of bacteria and their potential environmental applications

Biopolymers are considered a potential alternative to conventional chemical polymers because of their ease of biodegradability, high efficiency, non-toxicity and non-secondary pollution. Recently, extracellular polymeric substances (EPS, biopolymers produced by the microorganisms) have been recognised by many researchers as a potential flocculent for their applications in various water, wastewater and sludge treatment processes. In this context, literature information on EPS is widely dispersed and is very scarce. Thus, this review marginalizes various studies conducted so far about EPS nature-production-recovery, properties, environmental applications and moreover, critically examines future research needs and advanced application prospective of the EPS. One of the most important aspect of chemical composition and structural details of different moieties of EPS in terms of carbohydrates, proteins, extracellular DNA, lipid and surfactants and humic substances are described. These chemical characteristics of EPS in relation to formation and properties of microbial aggregates as well as degradation of EPS in the matrix (biomass, flocs etc) are analyzed. The important engineering properties (based on structural characteristics) such as adsorption, biodegradability, hydrophilicity/hydrophobicity of EPS matrix are also discussed in details. Different aspects of EPS production process such as bacterial strain maintenance; inoculum and factors affecting EPS production were presented. The important factors affecting EPS production include growth phase, carbon and nitrogen sources and their ratio, role of other nutrients (phosphorus, micronutrients/trace elements, and vitamins), impact of pH, temperature, metals, aerobic versus anaerobic conditions and pure and mixed culture. The production of EPS in high concentration with high productivity is essential due to economic reasons. Therefore, the knowledge about all the aspects of EPS production (listed above) is highly essential to formulate a logical and scientific basis for the research and industrial activities. One of the very important issues in the production/application/biodegradation of EPS is how the EPS is extracted from the matrix or a culture broth. Moreover, EPS matrix available in different forms (crude, loosely bound, tightly bound, slime, capsular and purified) can be used as a bioflocculant material. Several chemical and physical methods for the extraction of EPS (crude form or purified form) from different sources have been analyzed and reported. There is ample information available in the literature about various EPS extraction methods. Flocculability, dewaterability and biosorption ability are the very attractive engineering properties of the EPS matrix. Recent information on important aspects of these properties qualitatively as well as quantitatively has been described. Recent information on the mechanism of flocculation mediated by EPS is presented. Potential role of EPS in sludge dewatering and biosorption phenomenon has been discussed in details. Different factors influencing the EPS ability to flocculate and dewaterability of different suspensions have been included. The factors considered for the discussion are cations, different forms of EPS, concentration of EPS, protein and carbohydrate content of EPS, molecular weight of EPS, pH of the suspension, temperature etc. These factors were selected for the study based upon their role in the flocculation and dewatering mechanism as well the most recent available literature findings on these factors. For example, only recently it has been demonstrated that there is an optimum EPS concentration for sludge flocculation/dewatering. High or low concentration of EPS can lead to destabilization of flocs. Role of EPS in environmental applications such as water treatment, wastewater flocculation and settling, colour removal from wastewater, sludge dewatering, metal removal and recovery, removal of toxic organic compounds, landfill leachate treatment, soil remediation and reclamation has been presented based on the most recent available information. However, data available on environmental application of EPS are very limited. Investigations are required for exploring the potential of field applications of EPS. Finally, the limitations in the knowledge gap are outlined and the research needs as well as future perspectives are highlighted.

Mixed culture of Kluyveromyces marxianus and Candida krusei for single-cell protein production and organic load removal from whey

The study was conducted to evaluate the potential of mixed culture of Kluyveromyces marxianus and Candida krusei to enhance COD removal efficiency, minimize contamination at extreme conditions (high temperature 40°C and low pH 3.5) during batch and continuous aerobic fermentation and to obtain improved quality single-cell protein (SCP) using whey as substrate. The batch fermentation of mono-culture and mixed culture result showed that the mixed culture resulted in 8.8% higher COD removal efficacy with 19% higher biomass yield and 33% increased productivity. The maximum COD removal 80.2% (including residual protein) was obtained at 24h HRT with biomass productivity of 0.17 g/L/h; however, maximum biomass productivity of 0.38 g/L/h and 34% COD removal were obtained at 6h HRT. The results showed that the mixed culture of acid resistance and thermo-tolerant yeasts was a potential way to produce SCP (animal feed) and simultaneous COD removal under extreme operating conditions.

Simultaneous single-cell protein production and COD removal with characterization of residual protein and intermediate metabolites during whey fermentation by K. marxianus

Cheese whey fermentation with Kluyveromyces marxianus was carried out at 40 °C and pH 3.5 to examine simultaneous single-cell protein production and chemical oxygen demand (COD) removal, determine the fate of soluble whey protein and characterize intermediate metabolites. After 36 h of batch fermentation, the biomass concentration increased from 2.0 to 6.0 g/L with 55 % COD reduction (including protein), whereas soluble whey protein concentration decreased from 5.6 to 4.1 g/L. It was confirmed through electrophoresis (SDS-PAGE) that the fermented whey protein was different from native whey protein. HPLC and GC-MS analysis revealed a change in composition of organic compounds post-fermentation. High inoculum concentration in batch fermentation resulted in an increase in biomass concentration from 10.3 to 15.9 g/L with 80 % COD reduction (including protein) within 36 h with residual protein concentration of 4.5 g/L. In third batch fermentation, the biomass concentration increased from 7.3 to 12.4 g/L with 71 % of COD removal and residual protein concentration of 4.3 g/L after 22 h. After 22 h, the batch process was shifted to a continuous process with cell recycle, and the steady state was achieved after another 60 h with biomass yield of 0.19 g biomass/g lactose and productivity of 0.26 g/L h. COD removal efficiency was 78-79 % with residual protein concentration of 3.8-4.2 g/L. The aerobic continuous fermentation process with cell recycle could be applied to single-cell protein production with substantial COD removal at low pH and high temperature from cheese whey.