The Science of Recycling Nairobi's Sewage
In the heart of Nairobi County, a silent revolution is underway—one that transforms what we flush down our drains into a valuable resource for agricultural irrigation and economic activities. At the center of this transformation lies the Dandora Estate Sewage Treatment Works (DESTW) in Ruai, which processes approximately 80,000 cubic meters of wastewater daily 5 .
This treated wastewater represents both a significant challenge and an immense opportunity for a region grappling with water scarcity and pollution concerns.
Daily wastewater processed at Ruai treatment plant
The treatment plant releases its effluent into the nearby Nairobi River without any planned reuse scheme, despite the fact that some community members utilize it for their livelihoods, often unaware of its potential health risks 5 .
Wastewater is far from just "dirty water"—it's a complex chemical mixture containing organic matter, nutrients, pathogens, and various contaminants. The treatment process involves removing or reducing these components to levels that are safe for discharge or reuse.
Kenya primarily uses wastewater stabilization ponds for treatment, which are large, shallow bodies of water that utilize natural processes involving algae and bacteria to treat wastewater 1 . These systems are valued for their low operational costs and simplicity, making them particularly suitable for regions with limited infrastructure and technical expertise 1 .
In a comprehensive study conducted in 2020, researchers undertook a systematic assessment of the treated wastewater quality at DESTW in Ruai 5 . The experimental design was meticulous and multi-faceted:
Researchers collected wastewater samples from three different discharge points at the treatment plant to ensure representative data.
Samples were analyzed for heavy metals, macronutrients, and environmental characteristics including fecal coliforms, BOD, COD, and pH levels.
Raw data were analyzed using Statistical Package for Social Sciences (SPSS) software version 20 for both descriptive and inferential statistics 5 .
The analysis of Ruai's treated wastewater yielded fascinating results that painted a complex picture of its quality status:
The study found that while many parameters fell within acceptable limits, several concerning exceptions emerged:
| Parameter | Found in Ruai TWW | NEMA Standard | Meets Standard? |
|---|---|---|---|
| BOD | Exceeded limit | 30 mg/L | No |
| COD | Exceeded limit | 50 mg/L | No |
| Total Coliforms | Exceeded limit | 30 counts/100 mL | No |
| Lead (Pb) | Exceeded limit | 0.01 mg/L | No |
| Cadmium (Cd) | Below limit | 0.01 mg/L | Yes |
| Chromium (Cr) | Below limit | 0.05 mg/L | Yes |
Interestingly, the research revealed that most heavy metals in Ruai's treated wastewater were actually within safe limits for reuse. The content of cadmium, chromium, iron, manganese, phosphate, total solids, chlorides, and pH were all lower than their safe limits for reuse 5 .
Perhaps the most concerning finding was the microbial contamination. The study revealed that total coliform levels exceeded the recommended threshold limits 5 . This indicates the potential presence of disease-causing pathogens that could pose health risks.
Despite the quality issues, many farmers in the Ruai area already use the treated wastewater for irrigation purposes, often out of necessity rather than choice 5 . With freshwater resources becoming increasingly scarce and unreliable, this treated wastewater represents a valuable water source for agricultural activities that support livelihoods and food security.
The research concluded that the treated wastewater from DESTW is not entirely safe for use in its current quality state 5 . The exceedances in BOD, COD, lead, nitrates, and coliforms indicate a need for improved treatment processes to render the effluent suitable for reuse schemes.
Research indicates that constructed wetlands coupled with other treatment methods show higher pollutant removal compared to other methods . These nature-based solutions use plants and natural processes to further treat wastewater, often with lower costs and energy requirements than conventional mechanical systems.
| Treatment System | BOD Removal | COD Removal | TSS Removal |
|---|---|---|---|
| Ruai Treatment Plant | 82% | 71% | 72% |
| Kariobangi Treatment Plant | 48% | 51% | 68% |
| JKUAT WSP + HF-CW | 91% | 76% | 91% |
| Hybrid Wetland | 96.2% | 97.6% | 84.5% |
Conducting comprehensive wastewater analysis requires specialized equipment and reagents. Here are the key tools scientists use to assess water quality:
| Tool/Reagent | Function | Application in Wastewater Analysis |
|---|---|---|
| Statistical Package for Social Sciences (SPSS) | Statistical analysis software | Analyzing experimental data for significance and patterns |
| Culture Media | Microbial growth promotion | Cultivating and counting coliform bacteria |
| Chemical Test Kits | Parameter-specific testing | Measuring BOD, COD, nutrient levels, and heavy metals |
| pH Meter | Acidity/alkalinity measurement | Determining pH levels of water samples |
| Spectrophotometer | Light absorption measurement | Quantifying concentrations of specific chemicals |
The research at Ruai suggests that an improved optimal wastewater treatment configuration at DESTW is needed to render suitable effluent for reuse schemes 5 . This might involve:
Effective wastewater reuse requires robust regulatory frameworks that establish clear guidelines and monitoring protocols.
Perhaps most importantly, the communities potentially using treated wastewater need education and awareness about both the risks and proper safety measures. As the research noted, some people already use the treated wastewater from DESTW "oblivious of its quality status, putting at risk their own health, public health and the environment" 5 .
The research at Ruai's wastewater treatment plant reveals a complex picture of challenges and opportunities. While the current effluent quality doesn't consistently meet standards for safe reuse, the potential for improvement is significant.
With scientific insights guiding treatment enhancements, policy developments, and community engagement, Nairobi could transform its wastewater from a pollution problem into a valuable water resource. This is particularly crucial in the context of climate change and population growth, which are placing increasing pressure on Kenya's freshwater resources.