It is difficult to manage sewage water in cities in developing countries as treatment is both costly and complex.
Manoj Kumar Pandey's doctoral thesis investigated the use of constructed wetlands for decentralised management of sewage in cities and urban areas in Nepal.
Constructed wetlands are natural systems for treating sewage water. Wetlands are ecosystems in which water and wet areas constitute a substantial part of the landscape’s character. The system can also be a potential method for the treatment of sewage water in cities. However, there are inadequate guidelines for dimensioning in subtropical areas.
To compare the purification efficiency of wetlands featuring horizontal flow (HF) below the surface with wetlands featuring vertical flow (VF) and the impact plants had on the treatment, Pandey constructed a pilot-scale wetland facility.
The facility comprised units with and without plants. A reduced amount of sewage water resulted in reduced purification and the treatment improved in both HF and VF facilities with plants in the wetland meadows. According to Nepalese emission requirements, a wetland with horizontal flow is sufficient, but in order to meet stricter requirements a combination of HF and VF is recommended.
A pilot study on the dewatering of sludge in planted drying beds was conducted to study the sludge loads and drying intervals that could be used in subtropical climates such as Kathmandu. The study found that the drying period can be substantially reduced if the facility is covered to divert precipitation.
Constructed wetlands result in greenhouse effect
The environmental impact from wetlands was compared to other purification alternatives in a life cycle analysis.
Together with a cost analysis, the life cycle analysis was used to examine the environmental impact and financial sustainability of three wetlands-based decentralised solutions and an existing central solution with secondary treatment.
The three decentralised options were: Constructed wetland, wetland combined with the separation of urine and sullage treatment and wetland combined with source separation of effluent.
The life cycle analysis was unable to determine the best option but did identify bottlenecks in the systems and provided the basis for decisions. Constructed wetlands release greenhouse gases but are also a drain on CO2 and the net greenhouse effect of wetlands is uncertain. The conventional system had the highest energy consumption, and transport contributes significantly to greenhouse gas emissions in the alternatives with separation of urine or effluent.
The treatment of effluent in planted drying beds could be a poor alternative due to the high greenhouse gas emissions. Wetlands combined with the separation of urine demonstrated a positive energy balance. The constructed wetlands had the lowest cost per year.
Wetlands can be improved
Pandey's work shows that large wetlands or a longer retention time for sewage water are necessary to achieve maximum purification.
If the flow conditions are improved so that a larger proportion of the wetlands actively participates in the purification processes, both the volume and area can be reduced.
The optimisation of flow conditions should therefore be prioritised in connection with further development of wetlands with horizontal flow in Nepal.
The numerous current septic tanks can be upgraded using shallow infiltration systems.
Shallow infiltration utilises the purification capacity in more of the soil and maximises the distance to ground water. Treatment in constructed wetlands with final disposal of sewage water through ground infiltration will result in excellent purification and resilient facilities.
There are several areas in Nepal where infiltration can be used but there are inadequate local guidelines for dimensioning and design. However, there is a substantial international knowledge base available that can facilitate the work on establishing dimensioning criteria for Nepal.