A PRELIMINARY STUDY ON FEASIBILITY OF USING TREATED WASTEWATER TO RECHARGE RECLAIMED LAND

Ahmad Sana, Shuy Eng Ban, Ngonidzashe Mzila and Clarinda Kem Mui Hoong
School of Civil and Structural Engineering, Nanyang Technological University, Singapore 639798
Tel. (65) 790-5326
Fax. (65) 791-0676
E-mail: cshuyeb@ntu.edu.sg

Abstract: An experimental study is being carried out to observe the transformation in water quality parameters when treated wastewater is recharged into a coarse sand medium placed in a laboratory flume. A significant improvement in the water quality is observed during the passage of treated wastewater through the sand. The results of this preliminary study would be useful in designing a full scale field recharge-and-recovery experiment on a reclaimed land in Singapore. 

Keywords: water reuse, water quality, groundwater recharge

1    INTRODUCTION

In the coming years, land reclamation works at the eastern coastal area of Singapore would yield a huge reclaimed land area of over 2500 hectares. The land is being reclaimed by filling up the original seabed with graded gravel sand to depths exceeding 10m. This piece of land may be considered as an unconfined coastal aquifer with the possibility to be used for groundwater storage. With a centralised wastewater treatment plant to be built on a part of the reclaimed land, the possibility of using the treated wastewater to recharge this aquifer is attractive. A rough estimate shows that the water thus recharged, and recovered subsequently, could be up to one third of the total current water demand of Singapore.
The major challenges facing such a scheme are saltwater intrusion due to excessive pumping from the aquifer and the quality issues of the treated wastewater to be injected and subsequently extracted from the aquifer. The former problem, which is physical in nature, can be resolved by carrying out a detailed analysis based on groundwater modelling and proposing an optimum injection-extraction well system to avoid saltwater intrusion into the aquifer. The later problem is however more complex in nature and needs considerable amount of research on the water quality transformation occurring through the aquifer, estimation of the rate and extent of clogging of the aquifer medium and the measures to rectify these problems. With the scarcity of literature on this topic, the job to resolve water quality issues has become more challenging. 
A significant large scale field experiment on solute transport has been reported by Mackay et al.(1986), where they have considered the transport of synthetic organic chemicals through the aquifer. Another study similar in nature to the former one has been reported by Ding et al.(1995) . 
In the present study, a laboratory scale model is being utilised to observe the changes occurring in the water quality as the treated wastewater passes through the sand brought from the reclaimed site. The treated wastewater is initially made to pass through a filter made from the reclaimed sand in order to observe the clogging phenomenon.

2    EXPERIMENTAL SET-UP

The sand was brought from the reclaimed site and filled in a flume as shown in Figure 1. The straight portion in the middle of the flume may be considered as the test section and the flow in this part may be reasonably assumed as two-dimensional. Two PVC pipes of 300mm diameter, with perforations and well screen on their bottom portions, are installed at both the ends of the flume as ‘recharge’ and ‘extraction’ pipes. Piezometers are installed along the centreline of the flume at 30cm c/c. Those are 5cm diameter PVC pipes with perforations and well screen wrapped on the bottom portion. A typical piezometer is shown in Figure 1.

Fig.1   Schematic diagram of the experimental set-up

A filter is placed at inlet in order to monitor the possible change in the permeability of the sand with the passage of time due to clogging by the wastewater constituents. The filter is fitted with two piezometers in order to observe the head difference between its inlet and outlet, to allow the permeability coefficient to be monitored (Figure 2).

Fig.2   The filter used at the inlet. The sand used in the filter is the same as in the flume

2.1  Physical properties of the sand

The source of the sand is ocean bottom near Malaysia and Indonesia. The chemical analysis would be done in order to ascertain the chemical composition of the filling material. 
The results of the sieve analysis of a sample taken from the reclaimed site are presented in Table 1 and the gradation curve is shown in Figure 3. The soil may be described as loose light brown gravelly sand. 
Table 1    The result of sieve analysis for the sand used in the experiment.

Fig.3  Gradation curve for the sand that was brought from the reclaimed site to be used in the experiment

2.2  Quality parameters of the wastewater used in the experiment

The treated wastewater to be used in the present study was brought in from a wastewater treatment plant nearby. It was tested at the Environmental Laboratory at Nanyang Technological University, Singapore and some of the important constituents would be shown later. 

3  METHODOLOGY

The present experiment is designed to gather information regarding the water quality transformation through the sand and the changes occurring in the sand due to the passage of treated wastewater. In order to carry out an experiment that matches the real phenomenon in the field, the whole flume was initially saturated with fresh water. Some preliminary tests were carried out to ascertain the hydraulic conductivity of the sand by measuring the discharge (volumetric method) and hydraulic gradient (from the piezometers) under steady conditions. The sand filter shown in Figure 2 was then installed at the inlet and the treated wastewater was then pumped through the filter to the flume at a constant discharge. The steady state was achieved by adjusting the valve opening at the bottom part of the outlet. The water levels in the piezometers were measured at different intervals after the start of experiment and water samples were taken in order to monitor the change in the water quality with respect to time. The residence time of the treated wastewater in the flume can be changed by changing the discharge, so the experimental data would help to find out an optimum residence time to be used for the design purposes in the field.

The detailed experiment is being carried out and those results would be produced later. In this paper only the preliminary results are included to show the promising nature of the present study.

In the beginning the fresh water was used to saturate the sand in the flume and a discharge of 0.018 l/s was maintained, a value obtained by trial and error to achieve steady state flow through the soil medium. The hydraulic gradient was measured from the piezometers in the flume and the hydraulic conductivity was calculated by using Darcy’s law, which had a range from 64 to 67 m/day for various readings taken during three days.

In the next step the treated wastewater was made to flow through the filter shown in Fig.2 and then allowed to enter in the flume at inlet. A significant improvement was observed in the water quality by virtue of most of the parameters tested in the samples. The results for the samples taken one day after the test started are shown in Table.2.

Table 2  Water quality parameters at various points along the flume (Distance X is in meters)

It may be observed from Table 2 that there is a significant (approximately 80%) reduction in Total Dissolved Solids (TDS) when the treated wastewater is allowed to flow through the sand. It may be also be seen that the filter (shown in Figure 2) does not affect TDS, it completely eliminates the Total Suspended Solids (TSS), though. A 76% reduction in the Conductivity, 85% reduction in Chemical Oxygen Demand and 90% improvement in colour are also obvious. Calcium, Sodium, pH, turbidity and alkalinity show random values, the reason for which is under investigation at present.

The main contribution of the filter is in the reduction of TSS, COD, colour and turbidity where a significant improvement is observed after the filter.

A preliminary laboratory study on the recharge of reclaimed land with treated wastewater has been presented. The initial results show that the experimental set-up used in the present study would produce promising data that would be helpful for the design of a large-scale storage and recovery experiment on the actual reclaimed land in the coastal area of Singapore.

Mackay, D.M., Freyberg, D.L. and Roberts, P.V., 1986, A natural gradient experiment on solute transport in a sand aquifer, 1. Approach and overview of plume movement, Water Resources Research, Vol. 22, No. 13, 2017-2029.