Multistage Horizontal Subsurface Flow vs. Hybrid Constructed Wetlands for the Treatment of Raw Urban Wastewater

José Alberto Herrera-Melián,Pilar García-Jiménez,Marina Carrasco-Acosta,Rayco Guedes-Alonso,Mónica Mendoza-Aguiar,Ezio Ranieri

doi:10.3390/su12125102

Abstract

In this study, pilot-scale hybrid constructed wetlands (CWs) and multistage horizontal subsurface flow CWs (HF CWs) have been studied and compared for the treatment of raw urban wastewater. In the hybrid CWs, the first stage was a mulch-based horizontal subsurface flow CW and the second stage was a vertical subsurface flow CW (VF CW). The VF CWs were used to determine if sand could improve the performance of the hybrid CW with respect to the mulch. In the multistage HFs, mulch, gravel and sand were used as substrates. The effect of water height (HF10: 10 cm vs. HF40: 40 cm) and surface loading rate (SLR: 12 vs. 24 g Chemical Oxygen Demand (COD)/m2d) has been studied. The results show that the use of sand in the vertical flow stage of the hybrid CW did not improve the average performance. Additionally, the sand became clogged, while the mulch did not. The effect of water height on average pollutant removal was not determined but HF10 performed better regarding compliance with legal regulations. With a SLR of 12 g COD/m2d, removals of HF10 were: 79% for COD, 75% for NH4+-N, 53% for dissolved molybdate-reactive phosphate-P (DRP), 99% for turbidity and 99.998% for E. coli and total coliforms. When SLR was doubled, removals decreased for NH4+-N: 49%, DRP: −20%, E coli and total coliforms: 99.5–99.9%, but not for COD (85%) and turbidity (99%). Considering the obtained results and the simplicity of the construction and operation of HFs, HF10 would be the most suitable choice for the treatment of raw urban wastewater without clogging problems.

Highlights

Constructed wetlands (CWs) are natural, low-cost reactors, adequate for wastewater treatment and reuse in small communities
Two important drawbacks of vertical subsurface flow CW (VF CW) when compared with Horizontal subsurface flow CWs (HF CWs) are achieving a homogeneous distribution of the influent over the bed, and the higher clogging risk because of the finer substrate material used
The influent can be considered as medium strength [25] for almost all the parameters, with the exception of Total N (TN) and NH4+-N, which were closer to high strength (Table 1)

Summary

Introduction

Constructed wetlands (CWs) are natural, low-cost reactors, adequate for wastewater treatment and reuse in small communities. Molinos-Senante et al [1] collected the opinions of 29 international experts from the academic, research and industrial fields about the most suitable wastewater treatment technology for small communities. Considering the water flow, CWs can be classified as horizontal flow or vertical flow and surface flow or subsurface flow CWs. Horizontal subsurface flow CWs (HF CWs) are designed and constructed. Horizontal subsurface flow CWs (HF CWs) are designed and constructed Their operation and maintenance is simple and inexpensive. Their efficacy regarding nitrification and total N removal is usually low. Two important drawbacks of VF CWs when compared with HF CWs are achieving a homogeneous distribution of the influent over the bed, and the higher clogging risk because of the finer substrate material used

Objectives

Results

Conclusion