Development of biofiltration systems for treatment of greywater

The traditional paradigm of urban water management is shifting as a result of growing urban cities, increasing water demand, deteriorating aquatic ecosystem health and with climate change. Wastewater and stormwater recycling now form part of the new integrated and sustainable approach towards urban water management. The uptake and success of wastewater recycling schemes is heavily dependent on the development of economic and effective technologies capable of effectuating treatment at or close to origin.

This thesis assessed the performance and investigated the governing pollutant removal processes of greywater biofiltration systems in order to guide the design of an effective and reliable multi-functional greywater treatment system. This innovative technology could, for instance, be used to treat light greywater (wastewater discharges from washing basins, baths and showers) within tight urban areas in the form of living walls.

The study focused primarily on the removal of nitrogen and phosphorus, two critical environmental contaminants. With view to augment nitrogen removal via heterotrophic denitrification in biofiltration systems, the nitrate removal kinetics of eight different carbon substrates were evaluated in a batch scale laboratory experiment. Rates varied by two orders of magnitude between the most bioavailable (acetate) and the least biodegradable substrate (hardwood). In addition to rendering satisfactory rates, cotton and rice hulls produced lower levels of leachable nutrients and had the least variation in performance across the four tested water types (pure nutrient solution, light greywater, secondary-treated wastewater and tertiary-treated wastewater), and therefore can be recommended for further testing under continuous flow conditions. Light greywater on its own promoted adequate nitrate removal, implying that a treatment system operating under anoxic conditions may not require any additional electron donor for removal of nitrogen after its conversion to nitrate.

The hydraulic and pollutant treatment performance of 14 different biofilter designs, including 11 different plant species and two saturated zone configurations, were investigated in a large scale laboratory column experiment. Biofilter columns had an acceptable hydraulic performance after one year of operation; clogging was not a significant problem in systems operated at a reasonable loading rate (55 mm/d) and with rest periods between applications. Organics and suspended solids generally depicted high removal efficiencies (>90 % and >80 % respectively), allowing local standards for recycled water to be met for these two pollutants. High nitrogen removal efficiencies (>80%) and moderate to low phosphorus removal efficiencies were obtained, with plant type shown to exert significant influences on treatment performance. The application of an isotropic nitrogen tracer (15N-urea) revealed nitrogen removal in the biofilter columns to be dominantly occurring through adsorption and biological uptake processes as opposed to nitrification-denitrification. While rates of mineralisation and nitrification were effective in all configurations, denitrification rates were likely limited in the absence of an external carbon substrate. Consequently, species with poor nitrogen uptake experienced elevated levels of nitrate in their biofilter effluent.

A mass balance approach was employed to determine the fate of dissolved phosphorus in these systems. Storage in the plant biomass comprised a significant phosphorus removal pathway (60–70%) as opposed to accumulation onto the filter media (23-37%) in a 15-week laboratory column study. The use of a 32P radiotracer further demonstrated adsorption to be the primary phosphorus retention mechanism in the system followed by rapid plant uptake which liberated sorption sites for subsequent dosing events.

The presence of vegetation proved to be a key design parameter for overall system functioning. Ornamental species such as Canna lilies, Lonicera japonica, Grape vine (nitrogen only) and Pandorea jasminoides (nitrogen only) were extremely beneficial for nutrient removal. Proper specification of the filter media, including the incorporation of an adequate transition layer, was found to be important for reliable long-term performance. The inclusion of a permanently saturated zone at the bottom is recommended to maintain performance following dry periods.

The results of this study demonstrate that, with proper design and operation, biofiltration systems or greywater living walls coupled with a disinfection unit can be used as an advanced secondary greywater system within households and multi-dwelling residential, business and community for indoor recycling (toilet flushing), for watering gardens and lawns and for general sub-surface or surface irrigation. Future studies on mature systems will further improve designs for long-term reliable performance.