Qualitative Assessment of Rain Water Harvested from Roof Top Catchments

 Case Study of Embakasi, Nairobi County

Author:Joanne N. Gakungu
Department of SMARTEC, Sustainable Materials Research & Technology Centre, JKUAT

ABSTRACT
Rain Water Harvesting (RWH), in its broadest sense, is a technology used for collecting and storing rainwater for human use from rooftops, land surfaces or rock catchments using simple techniques such as jars and pots as well as engineered techniques (Najmi 2008). A study was carried out with the aim of assessing the quality of harvested roof-top rainwater from three different roofing materials in Embakasi area in Nairobi County.

 The roofing materials are corrugated iron sheets, clay tiles and concrete tiles. Chemical analysis included testing for iron, copper, zinc, fluoride, aluminium, lead, zinc, manganese, sodium and potassium. Physical analysis of pH and turbidity was carried out as well as bacteriological analysis for Escherichia coli (E. Coli) and total coliform. All the rainwater samples results for the samples taken after first run-off, were within the guidelines for both chemical and microbiological parameters established by the World Health Organization (WHO, 2003).

On the contrary, turbidity levels were higher than the maximum allowable concentration for drinking purposes hence the need to allow the first run –off during any rainfall event. As revealed from the analysis, all the samples require some level of treatment e.g. chlorination in order to ensure they meet regulatory standards for drinking water. However all water samples are quite safe for all other domestic uses including laundry, toilet flushing, bathing and other general cleaning. The integrated management must consist of regular cleaning of the catchment areas and the storage tanks and the employment of automated mechanical systems for discarding the first portion of each rainfall. The use of clay roofing tiles is preferable but due to cost, corrugated iron sheets maybe adopted.

INTRODUCTION
Water quality degradation, soil erosion, deforestation and urbanization greatly compromise the quality and availability of surface and groundwater resources hence threatening sources of potable water supply .The problem of water scarcity is strongly connected to the problem of water quality. Surface and ground water resources in Kenya are increasingly becoming polluted from sources caused by the activities of agriculture, urbanization, industry, leachate from mining and garbage dumps, sediments, salts, eutrophication of lakes, infiltration of fertilizer and pesticide residues, all of which increase catchment degradation. Lack of effective pollution control compromises the quality of water, posing potential health hazards, increasing treatment and maintenance costs, and affecting inland, estuarine and coastal aquatic ecosystems.

Rainwater harvesting, in its broadest sense, is a technology used for collecting and storing rainwater for human use from rooftops, land surfaces or rock catchments using simple techniques such as jars and pots as well as engineered techniques (Najmi 2008). A typical rainwater harvesting model comprises of components for transporting rainwater through pipes or drains, filtration, and tanks for storage of harvested water.

Even though such a solution seems to be so attractive from an ecological point of view, potential health risks from ingestion of harvested rainwater related to microbiological and chemical contaminants should be taken into account. Contamination by chemical pollutants may arise from a variety of materials with which the rain water comes into contact starting from the atmosphere. Rain water could dissolve gases and wash off chemicals from contacting dust particles and roof materials. It could dissolve the metals or derived chemicals. Elution of chemicals from the walls is also possible from the storage tank.Heavy metals such as iron, aluminium and zinc have been detected in rooftop-harvested rainwater although several additional studies in other countries have examined the effect of roofing material on harvested rainwater quality, domestic studies of the effect of roofing material on harvested rainwater quality might be more useful because roofing materials, coatings, and building practices vary globally.

MATERIALS & METHODS

2.1    Description of the Study Area
Embakasi is considered part of Nairobi’s Eastlands area, lying 15km to the south-east of Nairobi province. Jomo Kenyatta International Airport, the main airport of Nairobi is located in Embakasi. Embakasi Division is divided into following locations: Dandora, Embakasi, Kariobangi South, Kayole, Mukuru kwa Njenga, Njiru, Ruai and Umoja.  As a residential estate it houses mostly middle to lower income citizens. Most of the residential houses are four to eight- storey flats.

2.2    Sampling
To compare rainwater contamination in different locations, rain water samples were collected in four sampling sites (see Figure 1) namely Fedha Estate, Tassia Estate, Nyayo Estate and Baraka Estate. These locations were chosen as they represent a sample of high density residential areas within Embakasi area. Harvested rain water samples were collected via roof-top run made of three selected roofing sheet materials namely corrugated iron sheets, clay tiles and concrete tiles. The samples were analyzed for physical, chemical and bacteriological content using standard methods for water examination at the NCWSC Kabete Laboratories. Samples were collected during the months of April and May, 2013 during the long season of rains.

RESULTS AND DISCUSSION

Chemical Analysis
Chemical analysis was conducted by the NCWSC according to the standard methods for the examination of water by the Kenya Standards for Drinking Water – KS459-2007, American Public Association, American Water Works Association and the Water Pollution Federation. The corrugated iron sheets samples had higher average total iron and zinc concentrations than the other two roofing materials.  The source of this iron and zinc may be the corrugated iron sheets itself and atmospheric deposition. The corrugated iron sheets samples also had higher average total aluminium concentrations than the other two roofing materials.  Given that corrugated iron sheets are composed of zinc and iron, it may be that the aluminium levels found in this study originated in atmospheric dust and dry deposition. None of the roofing materials were significantly different from each other when analysis of the fluoride and manganese levels was done. This suggests that roofing materials were unlikely to be the source of fluoride and manganese deposition. Potassium and sodium levels were lowest in clay tiles possibly as result of its porous nature, these metals were trapped in the clay tiles. Lead and copper were not detected in any of the roofing materials.

3.1    Bacteriological and Physical Analysis
Bacteriological and physical analysis was conducted by the NCWSC according to the same standard methods for the examination of water highlighted above. The samples were examined for the two widely used bacterial indicators, namely total coliforms and Escherichia coli (E coli). The average pH of the harvested rainwater from all of the three roof types was in the near-neutral range (pH 7.0 to 7.4). The pH of the samples taken from the concrete tile and clay tile roofs was higher than that from the corrugated iron sheets. The rainwater harvested from the concrete tile roof had the highest Ph level, with an average pH of 7.4, probably due to the reaction of pure rainwater (pH 4.8 to 5.9) to the alkaline components of the tiles. The WHO guideline for non- potable urban water reuse is that the turbidity should not exceed 5 mg/L and for drinking water, the turbidity should not exceed 1mg/L. The levels found in the harvested rainwater from all the three roofs in this study exceeded 1 mg/L. This shows that all harvested rain water needs some form of disinfection by chlorine before use. However, all water samples are quite safe for all other domestic uses including laundry, toilet flushing, bathing and other general cleaning.

1.    CONCLUSION
As revealed from the analysis, all the samples require some level of treatment e.g. chlorination in order to ensure their potability considering their regulatory standards. However all water samples are quite safe for all other domestic uses including laundry, toilet flushing, bathing and other general cleaning. Safety and health measures are paramount in storage of harvested rain water. The integrated management must consist of regular cleaning of the catchment areas and the storage tanks and the employment of automated mechanical systems for discarding the first portion of each rainfall. The quality of the rooftop-harvested rainwater generally increased with roof flushing as the rain event progressed, indicating the importance of an effective first-flush diverter. The use of clay roofing tiles is preferable but due to cost, corrugated iron sheets maybe adopted.

Kenya needs to make a major paradigm shift – from focus on the extraction and distribution of water to conservation and development of the water resources. This means that the institutions/departments dealing with water resources development need to be resourced accordingly, and policies that conserve water such as rain water harvesting, water re-use and catchments protection need to be put on the forefront of national development efforts. Furthermore, the local government should launch awareness campaigns highlighting the importance of rain water harvesting.