Impacts of sedimentation on rainwater quality: case study at Ikorodu of Lagos, Nigeria

This study investigated the impact of sedimentation on rainwater storage system using a case study at theIkoroduareaofLagosstate,aruralareainNigeria.Inthisinvestigation,theproportionsof Escherichia coli ( E. coli ) that were settleable (due to sedimentation) and those that were at the free phase have been studied. Water samples were collected from different depths in the inspected rainwater storage tank at two different periods (i.e. rainy and dry periods) for 20 days. The samples collected from these periods have been analysed for physical and microbial measures before passing it through the serial ﬁ lters with poresizesof500 μ m,100 μ m,10 μ mand1.5 μ mtomeasuretheretainedparticlemass.Fromtheresults, it was observed that: (1) the water quality at the free-phase zone was better than that at the tank ’ s bottom; (2) the settleable bacteria rapidly sinked to bottom; (3) the correlation of turbidity, E. coli and total suspended solids(TSS) forall the rain eventsshoweda relatively highPearson ’ s coef ﬁ cientof0.9 to oneanother;and(4)over70%ofsettlingTSSoccurredwithin ﬁ rst36hours.Finally,ithasbeenfoundthat the physical sedimentation process can signi ﬁ cantly reduce the microbial measures.


INTRODUCTION
The availability of fresh-water is under threat from several factors including pollution and climate change. Hence, it is important to invest in research and technology that will safeguard the availability of fresh-water. The findings from occurrence and hazard from the physical, chemical and microbiological contaminants (Ahmed et al. , ).
Sedimentation is one of the processes used to improve the water quality, and it allows particles in water to settle under gravity. Studies have shown that sedimentation process can be aided by adding coagulants, such as aluminium sulfate (alum), that act to neutralise the negative electrical charge on particles and destabilise the forces to keep colloids apart (Zhou et al. ; Hussain et al. ).
This study aims to investigate the impact of sedimentation () reported that microbes (such as viruses, bacteria and protozoa) can be attached to solids in suspension that has unpredictable transport nature (Pu et al. ), thus the presence of suspended solids accommodates microbial contamination. In addition, elevated amounts of turbidity can shield microorganisms from the impacts of chlorination and stimulate the growth of bacteria, thus leading to a rise of chlorine demand in water treatment (Sharma & Bhattacharya ). In a water storage tank, the free phase bacteria can be killed by boiling or application of disinfectants since they are free floating in the water; while the sessile bacteria can be significantly reduced via the separation process of sedimentation (Olson et al. ; Characklis et al. ; Krometis et al. ). In this study, the level of settleable particles in the rainwater storage tank will be studied, and its correlation with the measured parameters (i.e. physico-chemical and microbial parameters) over varying depths and periods for different rain events will be investigated to draw necessary correlation between sedimentation process and water quality.

Fractionation of samples by serial filtration
Fractionation of samples, which is a process of separating different sizes of particulate matter into smaller quantities by using serial filters, is used in this study. This method prepares sub-samples of suspended particulate matter within a given size range for subsequent gravimetric and microbiological analysis. Its concept has been presented schematically by the flowchart at Figure 1(a). Based on the initial sample of 4 L, the volume of 2 L of raw-water was separated for further analysis after gently shaking to provide a homogeneous solution. Initially, the raw-water was analysed for E. coli and passed through the 1.5 μm pore size filter. The standard method was used to obtain the weight of the deposit and its enumerated bacterial count in the raw sample. The remaining 2 L of the water sample was then passed through the 500 μm pore size mesh filter. The weight of the 500 μm pore size filter was measured before and after the passing of water sample to determine the mass of retained deposit. Subsequently, 100 mL of the filtrate was separated for microbiological analysis. The remaining filtrate from the 500 μm was then passed through 100 μm. Finally, the water was passed through the 10 μm pore size filter, and the filtrate was then analysed for microbes. In the separate measure, the 1.5 μm filter was used to estimate TSS present in the raw sample while other filter sizes gave a range of the solid mass deposited.

Physico-chemical parameters
The measured physico-chemical parameters include the Total Suspended Solids (TSS), turbidity, conductivity and pH. TSS were determined using the vacuum filtration device which has been suggested in other studies (Mendez et al. ; Olowoyo ; Olaoye & Olaniyan ). The turbidity of the water samples was further determined using Hanna Turbidimeter. The pH of water samples was analysed using PHS-3D pH meter, while conductivity was measured using electrical conductivity meter. The procedures used to determine these parameters are as described by the standard methods in APHA AWWA and WEF ().

Sedimentation and microbial measurements
A gutter and storage vessel were fixed to the building to allow rainwater to be harvested from the roof during rain events in our field data collection. The gutter has a length of 2. All the storage vessel, gutter, PVC pipes and plastic bottles (that were used to collect the roof-harvested rainwater) were pre-washed with sterilised water to prevent external contamination. Furthermore, the sterilised water was analysed for microbes and the results showed that it was free of microbes (i.e. Total coliform and E. coli were not detected). The collected water samples were stored in a cooler of ice and taken to the laboratory for analysis within six hours of collection. After the analysis of each rain event, the remaining water in the vessel was discarded and the vessel was washed with sterilised water before use for the new harvested rainwater. Water samples taken at each level were subjected to analysis of suspended solids and microbes in fractionated and unfractionated samples. The sampling program is summarised in Table 1. The values of turbidity, pH and conductivity were measured from the time of cessation of a rain event till the 20th day. The unfractionated solids and coliform bacteria were analysed on the 0th, 72nd, 168th, 240th and 480th hour; while the fractionated solids and coliform bacteria were analysed for the 2nd, 8th and 36th hour after the end of a rain event (Table 1). Four rainfall events were harvested and analysed for our sedimentation experiment. Within these events, one was in the dry season and the remaining three were in the rainy season.
The E. coli and total coliform concentrations were enumerated using the standard Colilert-2000 ® procedure (IDEXX, Westbrook, Maine, USA). The Colilert technique was executed in accordance to the manufacturer's guidelines (i.e. APHA protocol number 9223 B. Enzyme substrate test).
All microbial testing in this paper was done in duplicates.
Other studies that have also employed this duplicate method

STUDY AREA
The chosen area for this study is the Ikorodu Local Government Area of Lagos, Nigeria. This area has been selected for

Physico-chemical parameters from sedimentation experiments
The characteristics of the four investigated rain events are shown in Table 2. From the results at Table 3, the quality of roof-harvested rainwater was poorer than the free-fall harvested rainwater (used as benchmarked comparison).
This is attributed to the roof as being one of the main sources of rainwater contamination. The quality of roof-harvested rainwater can be improved by regular cleaning of the roof before rain events especially after the long dry antecedent period. This study also showed that the water temperature at different levels of the storage vessel were similar. pH The World Health Organisation (WHO) recommends a guideline limit for pH between 6.5 and 8.   Note: FFRW and RHRW denotes Free-fall harvested rainwater and Roof-harvested rainwater respectively, while 1, 2, 3 and 4 represents the four different rain harvests as described at The acidity in the harvested rainwater in the studied area was caused by the reaction of rainwater with the atmospheric acid which was deposited from many activities, such as agricultural activities, industries, dusts from unpaved roads, un-vegetative areas, and exhaust fumes from vehicles.

Conductivity
The results from Figure  There was continuous increase in the conductivity

Fractionated total suspended solids (TSS)
The size range of suspended solids from four varying depths in the storage vessel has been investigated as shown on    , 8, 36, 72, 168, 240, and 480 (hours). The results evidenced that the minimum and peak particle settling velocities were obtained at level 4 and 1 respectively. The settling rate of particles in a storage tank is dependent on the depth of tank and the size of particles present. Conclusively from Figure 4, when water samples were considered unfractionated, it can be seen that over 70% settlement of TSS was observed within a period of 36 hours.

Microbial parameters from the sedimentation experiments
The enumerated E. coli obtained from the analysis of roofharvested rainwater, and the results from the serial filtration of the enumerated bacteria have been discussed here. The E.
coli analysis at Table 5 showed that the quality of the stored rainwater is generally poor for consumption as it exceeded the WHO permitted amount for drinking water (0MPN/ 100 mL).
Unfractionated E. coli bacteria for different periods Fractionation of E. coli    Table 5 from the fractionation of samples showed that the percentage of E.
coli (from the first rain event) retained in the 500 μm mesh filter during three different periods (2 hours, 8 hours and 36 hours) in the four levels ranged from 0% to 11.1%; while the percentage retained by 10 μm filter ranged from 4.9% to 19.3%. On the other hand, the same analysis for the second rain event showed the TSS deposited in the 500 μm mesh filter in the four levels ranged from 0% to 24.2%; and the percentage retained by 10 μm filter ranged from 17.2% to 39.6%. This trend was also observed for the third and fourth rain events. The analysis of Table 5 also evidenced that little or no E. coli was removed by the 500 μm mesh filter; while comparatively large amount of E. coli were removed by the 10 μm filter.
It can be concluded that the retention of solids by the filters during the serial filtration led to the reduction in the enumerated E. coli. The analysis of results from our experiments showed that not all the E. coli in the raw samples were collected after the serial filtration. This can be caused by two reasons: 1) some bacteria being in a free phase (i.e. they are not attached to any solids), and 2) some of the solids being smaller than the 10 μm filter. The results also showed that the process of particle sedimentation in water storage systems improved the quality of harvested rainwater over time.

Statistical analysis of the parameters
The Pearson's correlation was further conducted using the unfractionated sample data of turbidity, TSS and E. coli over 0, 72, 168, 240 and 480 hours.  The analysis also revealed that the quality of the free-fall rainwater was better than the roof-harvested rainwater, and that was resulted from the roof being a pathway for contaminants to enter into the storage tank especially after long dry antecedent period. The recorded quality of both free-fall and roof-harvested rainwater remained poor and did not meet the WHO guideline for drinking water, which present a caution to the store rainwater drinking practice that exists in this part of Africa.