K5/2579: Performance assessment of DEWATS constructed wetlands

Funded by: Water Research Commission

April 2016 to March 2019

Partner Organisations: eThekwini Water and Sanitation; BORDA; Trinity College, Dublin; Asian Institute of Technology

Project Description:

Researchers from the Pollution Research Group (PRG) at the University of Kwazulu-Natal, in partnership with eThekwini Water and Sanitation (EWS) and Bremen Overseas Research and Development Association (BORDA), have been investigating the use of Decentralized Wastewater Treatment Systems (DEWATS) in the South African context since 2010 (WRC project No 2002, Evaluation of the DEWATS process for decentralised wastewater treatment). The demonstration DEWATS plant at Newlands Mashu which treats wastewater generated by 83 existing middle-income households consists of a settler for solids separation followed by an anaerobic baffled reactor and anaerobic filter for primary treatment. Secondary treatment is facilitated in a constructed sub-surface flow vertical planted gravel filter (VPGF) which is fed by a mechanical siphon and a horizontal planted gravel filter (HPGF), both of which operate in series. It should be noted that these planted gravel filters (PGFs) are often referred to as constructed wetlands (CWs). Because a gravel media, rather than hydric soils, is used to support plant growth within the PGFs, they are not truly wetlands.

Due to design (excessive flow during storm events and no storm water overflow) and operational difficulties (occurrence of a thick scum layer in the first settling chamber), the constructed wetlands have been in operation for less than 12 months. These difficulties have been solved through the construction of an overflow and the institution of regular descumming operation (the treatment of the scum is the object of another project). Due to the lack of sufficient gradient across the site, the VPGF is only half of the recommended depth (750 mm vs 1.500 m). Furthermore, in November 2015, the HPGF was deemed non-functional due to cracks in the lining of the system and is still not in operation. Hence, final effluent from the VPGF flows back into the sewer.

Ongoing monitoring and system optimization has resulted in an effluent that meets general authorization discharge standards all of the time (provided the descumming takes place regularly). Although regular monitoring of the effluent at various points in the system has been conducted, little is known about the performance of the PGFs and how performance may be improved. In addition, since the VPGF is the only functional CW in the system, no evaluation has been done on the limitations of the depth, planted vegetation or hydraulic loading rate.

The purpose of this research is to get a more complete understanding of the hydraulics within the PGFs, as well as the removal rates of nutrients, pathogens, organics, and suspended solids. Further, this study will examine how changing certain variables (e.g. filter media, plant species, plant density, depth, rainfall, hydraulic retention time) affects removal rates. Maintenance requirements of the VPGFs will also be investigated (e.g. siphon and cropping of plants, etc.).

Aims:

1. Monitor the performance of the Newlands Mashu vertical flow constructed wetland/VPGF
2. Compare the performance with generally accepted design approaches
3. Highlight gaps or shortcomings in the design techniques
4. Provide guidance as to the design and operation of vertical flow CWs serving dense social housing or informal settlements

Publications and reports

Journal Papers

Posters

• Arumugam, P., Buckley, CA, Gill, L, Rodda, N and Trois C. Performance assessment of a full-scale vertical flow constructed wetland in a decentralized wastewater treatment system for instrumentation and modification. Presented at the International Conference on Natural and Constructed Wetlands, Galway, Ireland, 21st – 22nd June 2016

Conference Papers and Presentations

Reports and other

Deliverable 1: Performance of DEWATS constructed wetlands – Literature Review