1. Introduction
The rapid economic development witnessed in recent years in China has led to vast increases in the production of human sewage and other waste effluents, most of which are rich in nitrogen (N) compounds. Nitrogen not only affects the water quality, but also contributes to water eutrophication [
1], especially in lakes and rivers [
2]. Therefore, N removal from wastewater has become a topic of growing concern, both in China and worldwide [
3,
4]. Wetlands play an increasingly important role in removing N from aquatic systems via various processes, and can achieve total nitrogen (TN) removals of between 40% and 55% [
5]. The two most common forms of N in wetlands are organic and inorganic. Organic N is made up of a variety of compounds, including amino acids, urea and uric acid, and purines and pyrimidines. Inorganic forms of N in wetlands include ammonia (NH
4+-N), nitrite (NO
2−-N), nitrate (NO
3−-N), nitrous oxide (N
2O), dissolved elemental N, or dinitrogen gas (N
2) [
6].
Constructed wetlands (CWs) are rapidly becoming a viable method for wastewater treatment worldwide [
7], because of their similarity to natural wetlands and the fact that they cost less to construct, implement, and maintain than other types of treatments [
8,
9]. Constructed wetlands can be used to prevent water eutrophication [
10]. For example, it was reported that, over a period of seven consecutive years, CWs were able to maintain a higher removal rate of N than traditional N removal processes [
11]. The HSSF-CWs have a purification function, in that the sewage water flows to the outlet end in a horizontal direction and transverses the sand medium and plant roots in turn. In HSSF-CWs, N removal is achieved by a combination of physical, chemical, and biological processes, which are affected by a range of factors, such as pH, dissolved oxygen (DO), and temperature [
12]. Mathematical models can be used to simulate N removal and the relationships between various parameters and the N removal rates in constructed wetlands to reflect the removal mechanisms in wetlands. This information can then be used to provide decision support and the scientific basis for improving ecosystem service functions of wetlands [
13]. Previous studies suggested that the first-order kinetics model could describe N removal in wetland ecosystems [
14,
15]. However, recent studies that have used the first-order kinetics model for N removal have mainly focused on the application of the area-based constant and volume-based rate constant in HSSF-CWs, and, to date, applications of the area-based constant to HSSF-CWs and assessments of the influence of water pollution loads, temperature, and DO on the area-based constant have been largely neglected [
16].
Using as its basis the wetland water balance principle, in which the potential water transfer mechanism inputs and outputs, including runoff, groundwater recharge, and lateral inflow or drainage processes, are balanced, the aim of this study was to examine the removal and seasonal variations of two N forms, namely NO
3−-N and NH
4+-N, in an HSSF-CW [
17,
18]. The first-order kinetic model, a commonly used hydrological model, has been frequently used to predict, for example, variations in water chemistry, sudden changes in influent organic loads, NO
3−-N removal, and pharmaceuticals removal [
19,
20,
21,
22]. We also used the first-order kinetic model to estimate the area-based constant and temperature coefficient of N removal in this study. Further, to improve the accuracy and consistency of the model parameters, we also examined the relationships between the area-based constant and factors that influenced it, such as temperature, pollution load, and DO.
4. Discussion
In this study, we examined the wetland hydrology of the HSSF-CW over a period of two years using the wetland water balance principal. The results showed that water injection was the main source of water to the HSSF-CW. The flows into and out of the wetland varied on a monthly basis. In this study, we combined information about the wetland hydrology and N removal, and the results suggest that, when the rainfall is higher, the hydraulic retention time is lower, which would then influence the N removal rate. The water balance model adopted in this study only included hydrological data, and did not consider factors such as soil, topography, and land use [
29]; this is a major limitation of the study.
There was obvious removal of both NO
3−-N and NH
4+-N in this HSSF-CW. The removal rates however varied on a seasonal basis, with higher NH
4+-N inflow concentrations in spring, because the swimming birds that live in the open water connected to the HSSF-CW were more active in April and May than in other months. The inflow NO
3−-N concentrations were higher in summer, reflecting the weaker denitrification caused by the abundant rainfall (
Figure 2) and shorter hydraulic retention time in summer. The NO
3−-N loads were strongly correlated with the removal rate (
R2 = 0.96,
p < 0.01), but the NH
4+-N load and removal rate (
R2 = 0.02,
p > 0.01) were not correlated. This may reflect the higher and lower inflow loads of NO
3−-N and NH
4+-N, respectively. Under certain pollutant loads, increased pollutant concentrations are conducive to biofilm formation on the substrate surface, which not only provides good conditions for microbial growth but can also adsorb large quantities of organic matter, thereby contributing to pollutant removal [
30].
We examined the relationship between the area-based constant and temperature, pollutant loads, and DO for both NO3−-N and NH4+-N, and also estimated the temperature coefficient θ. The area-based constant and the NO3−-N load were strongly correlated (R2 = 0.96, p < 0.01), but there was no correlation between temperature and DO. The area removal rate constant for NH4+-N was correlated with DO (R2 = 0.69, p < 0.01), but was not correlated with either the temperature or the NH4+-N load. The lower θ value estimated for the two N species illustrates that temperature had little or no impact on nitrification and denitrification.
While many factors influence the N removal efficiency in wetlands, the main controls are nitrification and denitrification [
4]. Nitrification needs aerobic conditions and denitrification requires anaerobic conditions. Both processes are affected by the DO concentration, meaning that N removal in constructed wetlands is strongly influenced by the DO concentration. There are three main sources of oxygen in constructed wetlands, including release from plants, atmospheric reaeration, and oxygen transported by water, of which release from plants and atmospheric reaeration dominate [
31]. Plants can also partly block the oxygen transfer process from water to the substrate [
32], so that the low oxygen concentration in the substrate becomes the main limit on NH
4+-N removal. In this study, the concentrations of DO in the HSSF-CW varied from 0.5 to 4.5 mg∙L
−1, and the DO% was as high as 87%. These DO concentrations are much lower than the water-saturated dissolved oxygen concentration [
33], indicating that the DO concentrations in this HSSF-CW were sufficiently low to satisfy the conditions for denitrification. The DO concentration did not limit NO
3−-N removal in this HSSF-CW. The NH
4+-N area-based constant and DO were correlated (
R2 = 0.69,
p < 0.01), suggesting that the low DO concentration became the limiting factor for NH
4+-N removal. Nitrification of NH
4+-N is slower than degradation of organics [
34], and the conditions for nitrifying bacteria are more stringent than those for fungus that consume organic matter. The oxygen was first used to remove organic matter when the concentrations of organics were high; this was followed by nitrification until the organic concentrations dropped to a low level. The controls on the N removal rate in this wetland met Liebig’s law of the minimum, therefore, the main way to improve the pollutant removal in constructed wetlands is to regulate and control the dissolved oxygen content [
35].
The flow regime and N migration and transformation processes in constructed wetlands involve many complex physical, chemical, and biological processes. The N removal models are more complicated than those for other conventional wastewater treatments [
36]. Also, the background concentrations of pollutants in the same type of wetland will vary [
37]. In this study, we considered only three controls on the area-based constants and did not consider the influence of background pollutant concentrations. These other factors will be explored in future studies.