| Summary: | Despite the global acceptance for the application of vertical-flow constructed wetlands (VF CWs) as sustainable and cost-efficient technology in treating various types of wastewater, including urban wastewater, continuous loading of wetlands over time can lead to performance inefficiency and generate operational problems especially when high shock loads, such as petroleum hydrocarbon spills, are subjected to the system. Contamination with petroleum hydrocarbon compounds results in changing the structure, function and ecosystem service values of wetlands, which can eventually lead to clogging of the wetland substrate and affect the life time of the system. Sound knowledge of long-term performance in mature vertical-flow constructed wetlands linked with hydrocarbon treatment processes is needed to make guided judgments about the probable effects of a given suite of impacts and revise the management plans accordingly. A study was conducted to compare the impact of different design (aggregate size) and operational (contact time, rest time and chemical oxygen demand (COD) loading) variables on the long-term and seasonal performance of vertical-flow constructed wetland filters operated in tidal flow between June 2011 and March 2016. Ten different vertical-flow wetland systems were planted with Phragmites australis (Cav.) Trin. ex Steud. (Common Reed). Approximately 130 and 975 grams of diesel fuel (equivalent to 20 and 150 grams/litre, respectively) were each poured into four wetland filters on 26/09/2013 and 26/09/2014 respectively. Overall findings showed that the mature wetland system improved the water quality except for ortho-phosphate-phosphorus (PO₄-P), which reduced less over time. Findings also indicated that the wetland filter with the highest chemical oxygen demand (COD) loading but no diesel contamination performed the best in terms of COD and biochemical oxygen demand (BOD) removal. Filters contaminated by diesel performed worse in terms of COD and BOD but considerably better regarding nitrate-nitrogen removal. The removal efficiencies dropped for those filters impacted by the diesel spills. Seasonal analysis for water quality from different wetlands showed clear seasonal outflow concentration trends (low in summer) for COD, and nitrate-nitrogen (NO₃-N) while effluent BOD showed high treatment performance in winter. No clear seasonal trend for ammonia-nitrogen (NH₄-N), PO₄-P or suspended solids (SS) was noted. Serious clogging phenomena, impacting negatively on the treatment performance and the hydraulic conductivity, were not observed. The simulation model confirms the experimental findings that notable wetland clogging restricting the operation did not occur. Moreover, results showed that small aggregate diameter, low inflow COD load, and high contact and rest time were most efficient in reducing SS accumulation within the wetland filter bed. With regard to the treatment performance of the hydrocarbon contaminants, results indicated that all wetland systems had a relatively good performance in treating petroleum hydrocarbon compounds and the evaluation showed that all the hydrocarbon components were highly degraded and their concentrations were reduced in all treated effluents of wetland filters with time. This indicates that VF CW zones provide appropriate conditions for high treatment capacity of diesel compounds spilled with urban wastewater by a combination of processes taking place in the wetland filters, thus minimizing hydrocarbon compounds within the filter. A new experimental artificial ponds system, including: ponds with wastewater; ponds with wastewater and reeds; and ponds with wastewater, reeds and aeration, was operated in parallel with the mature experimental vertical-flow constructed wetland system, for the period between July 2015 and October 2015, to compare performance, design and operation variables between the two treatment technologies in the treatment of urban wastewater. Findings showed that highest COD and SS removals were observed for wetlands in comparison to ponds. Moreover, mature wetlands were better in removing NH₄-N and PO₄-P than ponds unless the ponds were aerated. Both systems were linked with medium to high levels of BOD removal. The aerated pond system demonstrated better treatment performance in terms of NH₄-N and PO₄-P. The NO₃-N concentration increased in the aerated ponds reflecting the high oxygen availability. Due to increasing water scarcity and droughts, which are key concerns worldwide, there is considerable interest in recycling various wastewater streams, such as treated urban wastewater, for irrigation in the agricultural sector. Recycling of effluents from various wetland filters (with/without diesel contamination) was assessed for the irrigation of chilli plants (De Cayenne; Capsicum annuum (Linnaeus) Longum Group ‘De Cayenne’) grown in a greenhouse environment. Concerning chilli fruit numbers, findings showed that the highest fruit yields for all wetland filters were associated with those that received inflow wastewater with a high loading rate, reflecting the high nutrient availability in treated wastewater, which is of obvious importance for yield production. Findings also indicated that wetlands without hydrocarbon contamination, with small aggregate size, low contact time, and low inflow loading rate provided high marketable yields (expressed in economic return). In comparison, chillies irrigated by filters with hydrocarbon contamination, small aggregate size, high contact time and high loading rate also resulted in high marketable yields of chillies, which pointed out the role of high contact time and high inflow load for better diesel degradation rates. The overall outcome of this research could considerably contribute to optimization of the design and development of long-term operation variables for constructed wetland technology particularly in petroleum industry applications. Statistically validated long-term data interpretation can particularly help the wetland modelling community and wetland managers to define, with insight into long-term and seasonal factors, removal processes for individual water quality parameters to maximize wetlands treatment performance.
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