Nitrobenzene Degradation Pathways In Horizontal Subsurface Flow Constructed Wetlands

The increase in wastewater discharged to the environment from rapid industrial development is putting high pressure on the proper functioning of the ecosystem. Re-evaluation of environmental impacts resulting from wastewater discharged is on at regional and international levels. Rural-based industries in developing countries have no proper wastewater treatment infrastructures due to economic constraints. Thus discharged untreated or poorly treated wastewater into the ecosystem.Discharge from industrial often contained inorganic and organic pollutants. The organic pollutants range from easily biodegradable to hard biodegradable compounds. The hard biodegradable compounds are used as raw material for products used for various applications. The main raw material for these products is nitrobenzene (NB). NB is a simple nitro-aromatic compound, which is a hazardous chemical. To remove it from the wastewater has been a challenge since the already establish techniques are expensive and not affordable to the developing economies. Biological methods appears to be an alternative affordable option to circumvent this challenge.Constructed wetlands (CWs) have gain popularity in the past decade due to being eco-frriendly and affordable. The use of CWs in wastewater treatment from industrial process have been documented. However, their use in nitrobenzene wastewater treatment is not advanced and is scarcely reported. The removal pathways of nitrobenzene in CWs is yet to be clearly understood. NB removal in CWs need to be studied to establish methods for optimising their complete removal.The main aim of the study was to investigate NB degradation pathways and interaction with N and S microbial mediated processes in horizontal subsurface CWs. Two-laboratory scale CWs planted with Juncus effusus were used. One was intermittently aerated and the other was not. The synthetic industrial wastewater containing NB, NH4-N and PO4-P were applied continuously for300-day study period. The NB influent concentration were35,140mg/L for phases1,2and280mg/L for phases3,4respectively. In phase4,290mg/L glucose was added to study the effect of co-metabolism on NB degradation. The instant increase in the wetland NB influent concentration was to investigate the impact of shock loads on unaerated and intermittently aerated CWs. The intermittently aerated wetland was able to withstand the shock loading as compared to unaerated wetland.The degradation performance for the two wetlands were not significantly different indicating that the presents of plants in constructed wetland may be able to supply sufficient oxygen required for NB biodegradation. The increase in NH4-N in phases2and3of20mg/L compared to the inlet in unaerated wetland is attributed to production of ammonia from NB hydrolysis degradation. In intermittently aerated wetland, most of NH4-N were converted to nitrates indicating high nitrification process, while nitrates were not denitrified completely. The detected nitrates averaged were15mg/L at the centre and outlet. Gradual change in pH trends in both wetlands was observed immediately after NB influent concentration was increased to280mg/L, which indicated changes in chemical reactions leading to the imbalance of protons and hydroxyl ions in the CWs. The sulphide concentration was averaged64mg/L in phase1and44mg/L in phase2, the reduction is attributed to increase in NB influent concentration indicating a negative impact on the sulphur reduction bacterial activities.Dominant NB removal pathway was partial reductive degradation due to production of aniline. The other pathways were mineralisation and volatilisation. The aerated CW had5.0mg/m2/d NB volatilised whereas the unaerated CW had2.0mg/m2/d. The average NB effluent concentration for phases1and2were less or equal to60μg/L accounting to99%removal. In phases3and4, the effluent concentration was less or equal to600μg/L accounting for between88to94%removal. The addition of glucose had slightly noticeable impact on the reduction of280mg/L NB. The wetlands performed relatively well in removal of low NB influent up to140mg/L. This indicates that the wetlands planted with Juncus effussus without aeration can comfortably remove NB from industrial wastewater. The use of intermittent aeration led to shock absorption in increase of NB, the impact due to volatilisation may discourage its application on high NB concentration industrial effluent.