## 6 Bioremediation: Feasibility and Limits
A bioreactor is another popular ex situ technique that can effectively reduce bioremediation time by monitoring process parameters including temperature, pH, agitation and substrate concentrations. Five operating modes of bioreactors include batch, fed-batch, sequencing batch, continuous and multistage. In fact, the use of bioreactors for remediation purpose in Antarctic research has not been reported thus far despite being proven to have great biodegradation efficiency for volatile compounds and recalcitrate hydrocarbon species in many studies [50; 51; 52]. Some recent data on statistical optimisation for bacterial growth conditions along with hydrocarbon degradation using Antarctic microbes in laboratory-based study were published [53; 54; 55]. The relationships of multiple rate-limiting factors were evaluated using response surface methodology to obtain optimised conditions. These established findings can be employed to the best advantage by adopting bioreactor system that provides excellent control of bioprocess parameters. Ex situ bioremediation techniques are favourable to treat low permeability and the heterogeneous nature of soil [56]. However, the excavation process is relatively laborious and demands higher cost compared to in situ bioremediation. In situ bioremediation is the on-site management of pollutants that requires no excavation, thus leading to little or zero disturbance to soil structure. Preliminary tests such as lab-scale and pilot-scale systems are essential in this technique to ensure safe and efficient remediation. According to the Madrid Protocol, remediation operations shall not result in any greater adverse environmental impacts than the \"do nothing\" approach. One of the complications in field study is the formation of a non-aqueous-phase liquid body when infiltration of hydrocarbons into the subsurface soils occurs after a large spill [10]. Compared to the heavy hydrocarbons present at the soil surface subjected to aerobic condition, the subsurficial hydrocarbons are less susceptible to degradation. The application of bioventing mitigates the issue by incorporating air injection. This remedial approach involves delivering oxygen to the unsaturated zone to stimulate airflow, which in turn increases microbial activity for biodegradation [40]. A study reported that microbiome using nine small injection rods (0.5 m apart) was more efficient in enhancing biodegradation than single-well bioventing as the distribution of oxygen is more uniform [57]. Aside from that, a shallow water table and thin soil cover also contributed to the inefficiency of single-well bioventing in hydrocarbon removal. The important roles of airflow rates and air intervals were described. Complex microbial interactions as well as the co-contamination of heavy metals and hydrocarbon have the potential to render in situ bioremediation unsuccessful. Hence, thorough prior studies of contaminated sites ex situ are significant including soil analysis, molecular screening and microbial isolation, as well as preliminary studies involving micro-cosms. Subsequent trials of mesococosms and macrocosms are recommended as laboratory-based study may not precisely reflect the observations in the field for the large-scale soil microbial interactions [58]. The freeze-thaw cycle is another pitfall that causes dissolved contaminants to be mobilised in surface and subsurface soils besides disrupting the nutrient distribution upon thawing. As bioremediation is more conventionally practiced and assessed for its efficiency, the goal to achieve a practical strategy should consider site-specific conditions and acceptable cost-time relationship. For instance, integrated remedial approaches (coupling solvent extraction and bioremediation), application of biosurfactant and algae-bacteria consortia are among the intriguing prospects [59; 60; 61].