Application of Duckweed in Treating Municipal Wastewater – Trials at Inverell STW
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report no. UWRAA 151
This report presents the performance of the full scale duckweed demonstration system, which was operated during the period December 1996 through to February1998.
Duckweed has been utilised around the world for removing contaminants from wastewaters emanating from Sewerage Treatment Works (STW), intensive animal farming industries and food processing factories. Following successful duckweed research trials on the Northern Tablelands in New South Wales (NSW), a full scale duckweed demonstration system was proposed for the Inverell Sewage Treatment Works.
The process employed at the Inverell STW is the intermittent decanting extended aeration mode of the activated sludge process. At this plant the flow stream could be easily divided into two parallel systems which meant that the duckweed system could be readily compared to a control system. Inverell Shire Council continues to run the duckweed system as a functional component of their sewage treatment plant.
The main focus of the project was on the ability of duckweed to remove nutrients and restrict the growth of algae in municipal wastewater treated by an extended aeration sewage treatment process. The effect of duckweed coverage on disinfection efficiency was also studied.
The results showed that duckweed is capable of taking up measurable quantities of nutrients from municipal wastewater and restricting algae growth by blanketing the surface. Disinfection efficiency was not found to be compromised with the arrangement used whereby 60% of the duckweed pond was covered with duckweed.
This project demonstrated that even with low levels of nitrogen in the wastewater, it is possible to cultivate duckweed and remove detectable quantities ofphosphorus. However in adjusting the operation of the extended aeration STW to provide higher levels of nitrogen for improved duckweed growth, the effluentquality exiting the duckweed pond was found to be similar to that exiting the control pond.
The results showed that the duckweed pond provided significantly higher removal of several parameters when compared to the control pond:
· Various nitrogen compounds, particularly ammonia (30% reduction) and oxidised nitrogen (33% reduction);
· Total and reactive phosphorus (36 and 25% reductions, respectively);
· Faecal coliforms
The duckweed was also found to suppress algae growth and thus reduce suspended solids (or NFR) exiting the duckweed section of the pond. However, algae growth did occur in the section of the pond not covered in duckweed and this growth raised suspended solids concentrations exiting the duckweed pond.
The project demonstrated that the management of the Mixed Liquor Suspended Solids(MLSS) concentration and thus sludge age is important in an Extended Aeration System using duckweed. With very low levels of Total Kjeldahl Nitrogen (TKN),the introduction of partially treated effluent will most likely encourage the growth of competing aquatic plants like azolla. Duckweed will not have the nutrient base to grow quickly enough to compete with the azolla. An analysis of dried azolla revealed that it was not as efficient in uptaking phosphorus as the duckweed.
For extended aeration STWs, such as Inverell, where the level of TKN is low for duckweed cultivation, and the level of phosphorus is moderate, the duckweed coverage is expected to be in the range of 3 to 4.5 kg/m2. If the duckweed mat is too thin, it encourages the growth of competing aquatic plants.
The project also demonstrated the development and operation of full scale duckweed management facilities including the floating barrier system and the harvesting system. They are now ready for practical implementation.
These observations have been made within the constraints of the operational dynamics of the plant and the project boundary. The statistical relationships between the observed performance, operation of the facility and the ambient environment are not fully quantitatively defined. Further trials at STWs to ascertain these relationships, are supported.
A duckweed system is a simple, relatively low cost method of nutrient removal and algae control compared to most mechanical and chemical systems. It provides a practicable, environmentally friendly option that required minimal power for operation. Duckweed systems can be suitable for nutrient removal and algae control for STWs with medium to high organic nitrogen discharge and for small communities in remote areas.
The recommendations from this project are that:
· For intermittently decanting extended aeration STWs, the system be operated on the full aeration cycle to minimise the levels of nutrients. In order to obtain optimum growth of the duckweed, fertilising strategies should be investigated that introduce additional sources of nitrogen to the duckweed.
· A small catch pond be provided to contain all the sludge, debris and floatables that occasionally overflow into the pond especially after storm events.
· Duckweed harvesting be carried out by using a boat harvester to push the duckweed out of the pond via a ramp for drying before removal.
· The floating baffles be shortened so the harvester can go around them from one section of the pond to the next, without having to go over them.
· In areas where ducks are prevalent and the STW produces an effluent with a low level of nitrogen, duck exclusion netting may be required to minimise the impact of ducks. However with vigorous duckweed growth this may not be necessary as the ducks will have minimal impact.
· Duckweed systems be further trialled at STWs in NSW, especially those that discharge relatively high organic nitrogen concentrations.