1. Introduction
Municipal wastewater discharge,
i.e., sewerage, is one of the most serious threats to the ecosystem. Therefore, the sewage needs to be treated appropriately before the wastewater can be released into the environment [
1]. A large number of technologies, such as oxidation ponds or activated sludge processes, have been applied for domestic wastewater treatment but most of these practices are expensive to erect and run [
2]. Accordingly, there is a need for a substitute system to overcome these drawbacks and achieve a high elimination rate of pollutants.
In recent years, the application of constructed wetlands (with rooted, emergent and free-floating aquatic plants), and facultative ponds treating domestic sewage have attracted considerable attention because they offer an environmentally sound approach [
3,
4,
5,
6]. The mechanisms of pollutant removal in constructed wetlands involve an interaction between the bacterial metabolism, plant uptake and accumulation [
7]. The impurities are removed in facultative ponds entirely by natural processes involving both algae and bacteria [
8]. In that order, vegetation is considered as a dominant feature of constructed wetlands, and acts as an important biotic factor in the treatment process [
9]. Among the free-floating species, the water hyacinth (
Eichhornia crassipes) appears to be a promising candidate for pollutant removal owing to its rapid growth rate and extensive root system [
10,
11,
12]. The water hyacinth lagoon functions as a horizontal trickling filter, where the submerged roots provide physical support for the bio-film bacterial to growth [
9]. Nevertheless, despite the efforts made worldwide, the construction of aquatic systems, particularly the water hyacinth treatment process, has not gained much popularity due to the requirement of a large land area and considerable capital investment [
5].
In any conventional water hyacinth system, the water column can be divided into three distinct zones, aerobic, facultative and anaerobic, depending on the oxygen transfer through the floating plants. An excess pond depth (typically ~50–100 cm depth) reduces the oxygen transfer efficiency through the roots and sustains high anaerobic microbial growth [
13,
14]. The oxygen concentration is likely to be high in the upper part of the lagoon, and begins to decrease further down the water column, approaching almost zero below a 200 mm depth. The higher pond depth can rise to the anaerobic zone; resulting a slow biodegradation process, and cause the emission of foul odors. To counter these disadvantages, a shallow pond water hyacinth system was reported [
15,
16].
The shallow pond system is an alternative to the conventional water hyacinth process because it has a low water depth (140–150 mm) based on the fully matured plant root submerged (80–130 mm) to avoid the anaerobic zone [
16]. This condition ensures the optimal interactions between the wastewater effluent and microbial biomass in the phytoremediation treatment practice. The shallow pond technique is an attempt to minimize these constraints due to the better oxygen diffusion efficiency through the roots and the accumulation of a larger aerobic bacterial population. This is a robust biological process that can be applied to the efficient and reliable elimination of pollutants at a lower hydraulic retention time (HRT) compared to the conventional water hyacinth system, even under environmental stress conditions [
15,
16]. Despite its adequate performance, further improvements can establish the shallow pond water hyacinth practice as an effective tool for purifying municipal wastewater effluent.
Application of phytoremediation with attached growth-based engineered procedure (using
Phragmites sp.) has been reported as a new relevant technology in constructed wetlands treating domestic sewage [
17,
18]. The concept of operation is void of the soil strata used in the root zone systems, and in lieu a support matrix (assembled by the number of vertical PVC pipes) is provided to enrich the microbial population in the form of biofilm within wetland unit. This approach overcomes the limitations of choking, clogging, slow mass transfer, poor root penetration into the multilayer soil column, high area requirement and capital investment owned by soil bed constructed wetlands. A further study also revealed the efficiency of this method by using
Thalia dealbata, Acorus calamus, Zizania latifolia and
Iris sibirica for river water treatment [
19,
20]. Marchand
et al. [
21] effectively applied this technology (combined with a homogeneous mix of gravels and perlite) for the removal of copper ion from synthetic Cu-contaminated wastewaters using
Phragmites australis, Juncus articulates and
Phalaris arundinacea. In addition, the vegetated system using
Typha sp. reported to be a promising solution in domestic wastewater treatment seriously stressed with total dissolved solids (TDS) and Cu metal salt [
22]. The integrated anaerobic baffled reactor and phytoremediation process with attached growth (using
Phragmites sp. and
Typha sp.), likewise, has been recommended as a novel approach for promoting a sustainable decentralization [
23]. As a result, it is recommended that using wetland systems upgraded with the phenomena of engineered attached growth matrix could be considered as a novel scientific advancement in domestic wastewater treatment.
Therefore, the present study aimed to enhance and examine the efficiency of the shallow pond water hyacinth system by adding a new feature to a treatment unit. In this approach, an attempt was made to incorporate the advantages of a shallow pond and attached growth microbial techniques by introducing Bio-hedge (mesh type structure), which is a support matrix to augment the indigenous microbial population. Although the performance of the water hyacinth treatment systems have been studied elsewhere before; until now, this innovation to our knowledge has not been previously documented. This new system identified to overcome the limitations associated with the traditional water hyacinth and facultative pond technologies, and permits treatment of effluent in the most cost effective method. Moreover, the role of plant, microbial biofilms and evapotranspiration in this phytoremediation system was also determined.
4. Conclusions
The study highlights the performance of an improved water hyacinth phytoremediation system coupled with attached microbial growth in a domestic wastewater treatment. The main features of the system are a low effective depth of 150 mm, highly dense plant population and the presence of plastic mesh type structures, called Bio-hedge. The Bio-hedges provide an efficient surface area for the microbial population to adhere and grow. The system also provides better oxygen transfer efficiency to sustain high levels of aerobic microflora for the degradation of many pollutants, particularly in tropical regions. The Bio-hedge technique has low space requirements, low capital investment and most importantly, a high degradation of organic pollutants, which was capable of achieving COD and BOD5 removal as high as 79% and 86%, respectively, at a HRT of 14 h. Regression analysis revealed a correlation between the inflow and outflow for the COD and BOD5 loadings. A high accumulation of nitrogen and phosphorus in the plant, complimented with high productivity and no anatomical changes, also indicated that water hyacinth could be a feasible macrophyte in the phytoremediation process. The system eliminates all the disadvantages of the conventional water hyacinth and facultative pond systems, i.e., odor and insect related problems, slow biodegradation rate, and so forth. Similarly, it can be a great deal of promise in overcoming the possible limitations associated with shallow pond water hyacinth technique. Therefore, a new Bio-hedge water hyacinth approach can be an ideal practice to promote low cost green technology for organic pollutant degradation and impurity elimination, but more detailed pilot scale and field studies will be needed.