Innovative Multistage Constructed Wetland for Municipal Wastewater Treatment and Reuse for Agriculture in Senegal

Abstract

This paper reports on the performance of using a multistage constructed wetland (CW) to treat municipal raw wastewater and an analysis of its suitability for agricultural irrigation. The pilot plant consists of two stages of vertical flow CWs plus one stage of a horizontal CW built in 2018 with different local materials at the Gaston Berger University Campus, Saint Louis (Senegal). Each CW stage is composed of several filters with different type of media (silex, granite, or river gravel), filtering media depths, and macrophytes (Phragmites and Typha). The physicochemical and microbiological indicators were monitored over six months at each bed inlet and outlet to evaluate the efficiency and achievements of the Senegalese, European, and WHO regulations/recommendations for disposal or reuse in irrigation. This study demonstrates the viability of this new multistage CW design to treat raw municipal wastewater and produce an effluent of good quality suitable for reuse in agriculture. The removal of organic matter, suspended solids, and nutrients was very high (>95% for SS, BOD5 and N-NH4+, >90% for COD and P-PO43−), as was the reduction of microbiological indicators (fecal coliform reduction >5 log units and helminth egg removal of 100%). First, trends related to the influence of design (the type of gravel, filter depth, and type of macrophyte), operational modes, and the CW treatment efficiency were determined. The use of non-crushed gravel and Typha spp. seemed to provide better removal rates. On the contrary, no differences were found between the use of silex or granite gravel. For the studied Senegalese conditions under dry and hot climates, the preliminary results indicate that no resting periods are necessary for vertical flow CWs (VFCWs), thus resulting in a reduction in construction and operation costs. The main outcome of our study is evidence that multistage CWs can provide robust, cost-effective treatments, as well as allow for safe water reuse, which is imperative in areas with severe water scarcity and endemic microbiological waterborne diseases.

Potential Use of Constructed Wetland Systems for Rural Sanitation and Wastewater Reuse in Agriculture in the Moroccan Context

“…This was due to the combination of different types of CWs, allowing various removal mechanisms to occur at various intensities (aerobic and anaerobic degradation, adsorption, filtration, volatilisation, photodegradation, etc.). The elimination of nitrogen can also be enhanced, since both nitrification and denitrification can take place [48,50,52].…”

“…The intensity of these different removal mechanisms varies according to the type of the CWs, water chemistry, retention time, seasonal variation, substrate, and vegetation [78,79]. Although several studies showed substantial elimination of microbial contamination reaching over 99% [36,39,50,52,80], other studies showed lower removal rates [51]. The removal of microbial contamination in CWs is still challenging, and the implementation of polishing step, such as disinfection using chemical agents or UV lights, is recommended if the treated wastewater is to be reused in agriculture [22,43].…”

“…Another factor that influences the treatment efficiency of CWs is temperature. Several studies demonstrated that seasonal variation has an impact on treatment performance, especially for the removal of organic matters, nutrients, and microbial contamination, since low temperatures directly impact the microbial activity, while higher removal rates were always observed in warm and hot seasons because higher temperatures promote plant growth and microbial activity [39,48,52]. For this reason, a new design was proposed to enhance the treatment efficiency of CW in cold regions using geothermal energy [49].…”

“…The bacteriological contamination also met the WHO guidelines for unrestricted irrigation (1148, 820, 520, 480, 7, and 3 cfu/100mL for TC, FC, FS, E. coli, P. aeruginosa, and S. aureus, respectively). Torrens et al [52] reported that a hybrid CW achieved a good removal efficiency, resulting in a quality that respects Senegalese discharge regulations and the WHO recommendations for reuse in unrestricted irrigation. On the other hand, a VSSF CW did not succeed in producing an effluent that respects the Egyptian guidelines for wastewater reuse in irrigation, with a COD concentration that exceeded the limits [47].…”

“…Torrens et al [52] evaluated the performance of a full-scale pilot constructed wetland. The system consisted of a hybrid CW comprising two stages of vertical flow French Reed Bed (FRB) and a horizontal subsurface flow CW.…”