Effectiveness of septage pre-treatment in vertical flow constructed wetlands

Abstract

Septage is wastewater stored temporarily in cesspools. A periodic supply of its significant quantities to small municipal wastewater treatment plants (WWTPs) may cause many operational problems. In the frame of the research, it has been proposed to utilize vertical flow constructed wetlands for pre-treatment of septage prior to its input to the biological stage of a WWTP. The aim of the work was to assess the effectiveness of pre-treatment in relation to factors such as: seasonality, hydraulic load, pollutants load of the VF bed and interactions between these factors. The results proved that application of a VF bed to septage pre-treatment can significantly reduce the concentration of pollutants (biochemical oxygen demand (BOD): 82%, chemical oxygen demand (COD): 82%, total suspended solids (TSS): 91%, total nitrogen (TN): 47%, ammonia nitrogen (NH-N): 70%), and thus decrease the loading of the biological stage of a WWTP. The mathematical models of mass removal process were created. They indicate that in case of all analysed parameters, removed load goes up with the increase of load in the influent. However, with the increase of hydraulic load, a decrease of the removed BOD, COD, TSS and total phosphorus, and in vegetation period an increase of TN, can be observed in terms of load. There are no statistically significant effects of seasonality.

Dewatering and Treatment of Septage Using Vertical Flow Constructed Wetlands

“…The mean concentration of COD and TN were also relatively low, which were 4549 ± 2477 mg L −1 and 192 ± 104 mg L −1 , respectively. The wide variation of septage quality was also reported in many studies [5,13,17]. Table 1.…”

“…In the VFCW, the granular substrate acts as a filter to physically trap the solids components. As for the system designed for septage treatment, the presence of a sand layer in the substrate profile [12,13] remains controversial as it increases the risk of substrate clogging. Therefore, it is crucial to determine whether the substrate built by coarse materials is adequate to deliver reliable treatment performance.…”

“…Therefore, the control of hydraulic load is a more practical feeding strategy from the operation perspective [12]. Karolinczak and Dabrowski [13] indicated that the increase of hydraulic load tended to reduce the treatment efficiency of TS and organic matter, but it is effective for the removal of total nitrogen (TN).…”

“…The retention of solids at the wetland surface forms a layer of sludge deposit, which has been recognised as an effective porous medium to enhance the treatment performance [14]. Significant amounts of TS and chemical oxygen demand (COD) are retained in the wetland bed (at least 80% in most studies [9,11,13,[15][16][17]) via physical filtration. However, the excessive accumulation of sludge deposit deteriorates the infiltration capacity of the wetland bed and extends the ponding period during operation, which eventually leads to the system failure, known as substrate clogging [18].…”

“…However, the excessive accumulation of sludge deposit deteriorates the infiltration capacity of the wetland bed and extends the ponding period during operation, which eventually leads to the system failure, known as substrate clogging [18]. Furthermore, the removal of nitrogen compounds is less effective [13,15]. This is due to the aerobic characteristic in the substrate of the VFCW, where nitrification is promoted while denitrification is prohibited.…”