Bioaugmentation: an effective commercial technology for the removal of phenols from wastewater

Phenol represents a huge problem in industrial wastewater effluents and needs to be removed due to its toxic and carcinogenic nature. The removal of phenol from the wastewater is often both expensive and time consuming; there is therefore a requirement for a more effective, sustainable solution for the removal of phenol from wastewaters. Bioaugmentation or the addition of phenol degrading microorganisms to contaminated effluents is one such sustainable approach being considered. Here, we describe how bioaugmentation has been applied for the biological treatment of phenol in industrial wastewaters.

Phenol and phenolic derivatives are often found in wastewater discharged from pharmaceutical treatment plants, oil refineries and are toxic and carcinogenic to both humans and animals. It has also been shown to inhibit photosynthesis¹. Phenol can also be released into the environment due to spillage or leaks from hazardous waste dumps. Phenol is resistant to degradation in the environment and considered a serious pollutant² and therefore it is included in the list of priority organic pollutants prepared by the USEPA³. Once released into the environment, due to its chemical properties phenol does not adhere to soil, and thus moves through the soil matrix and into groundwater⁴.

Bioaugmentation generally falls in two main strategies: (1) bioaugmentation by enrichment with indigenous microorganisms; and (2) bioaugmentation by enrichment with non-indigenous microorganisms. The reinoculation of an environment with previously adapted indigenous microorganisms directly isolated from the site is often termed indigenous bioaugmentation⁵. However, if the sites do not contain active, pollutant degrading microbes, addition of exogenous microbial strains could be a solution.

In comparison to other technologies used for reducing phenol content in contaminated water such as chemical oxidation, filtration and activated carbon, biological treatment has been shown to be cost effective and versatile resulting in the complete mineralisation of phenol^6,7. As such, industrial effluents containing phenol have often been treated using low cost biological treatment such as activated sludge systems. However, these systems often failed due to the high concentrations of phenol or fluctuations in phenol wastewater concentration. This has encouraged the development of more robust microbial systems able to accommodate large irregular fluctuations to meet compliance in a more consistent manner^8–10.

In nature there are some microbes that can use phenol as source of carbon and energy. The biodegradation of phenol using such phenol degraders has been studied extensively with many cultures including those from commonly occurring Gram negative bacteria e.g. Pseudomonas spp.^11,12 and Alcaligenes spp.⁹, Gram positive bacteria e.g. Bacillus spp.^13,14 and Nocardia spp.¹⁵, Gram variable bacteria e.g. Arthrobacteria¹⁶ and the yeast-like fungi Aureobasidium pullulans¹⁷. Phenol is normally degraded under aerobic condition where enzymes such as phenol monoxygenases (phenol 2-monooxygenase) are involved in its degradation¹⁸.

Reports on phenol degradation using single species of microorganisms are abundantly available^12–14,16, while reports on the application of mixed cultures of microorganisms are less prevalent but interest has increased in recent years^19–21. The reason for the interest in microbial consortia is the assumption that the application of mixed species consortia in the bioremediation of pollutants has greater stability and tolerance to changing environmental and physiological conditions together with increased metabolic capabilities.

Recently Poi et al.²² isolated 22 phenol degraders including Acinetobacter sp., Bacillus sp. and Pseudomonas sp. The screening results showed that all 22 isolates were able to degrade phenol in laboratory based studies. The bioaugmentation of these 22 isolates in a field study using a bioreactor (400 m³) (Figure 1) resulted in complete phenol degradation, with a phenol concentration reduced from 407 mg L⁻¹ to below detection limit (0.1 mg L⁻¹) over 104 days of incubation. An estimate for the treatment of wastewater from phenol using conventional technologies is around US$100 per tonne. However, through the use of bioremediation techniques such as the system described above, this cost can be reduced to less than US$30 per tonne. As a result, this environmental biotechnology is becoming an increasingly competitive commercial remediation technology.

Figure 1. (a) Seed Bioreactor (five drums, 750 L in total), (b) Treatment tank (400 m³) (on the left), (c) Bioballs inside each of the drums (placed under the ceramic porous rings), (d) Phenol-contaminated wastewater being added to the netted ceramic porous rings. The phenol contaminated water was added to the Seed bioreactor where the consortium was added and later moved to the treatment tank.

In conclusion, bioaugmentation represents a promising, sustainable and cost effective approach for the degradation of phenol in industrial wastewaters. This case study provides evidence of the scalability of the process to field studies and promotes its usage in similar contaminated sites.