Applicationof Duckweed in Treating Municipal Wastewater

Trialsat Inverell STW

ReportNo WSAA 151

April 1999




Thisreport presents the performance of the full scale duckweed demonstrationsystem, which was operated during the period December 1996 through to February1998.


Duckweedhas been utilised around the world for removing contaminants from wastewatersemanating from Sewerage Treatment Works (STW), intensive animal farmingindustries and food processing factories. Following successful duckweedresearch trials on the Northern Tablelands in New South Wales (NSW), a fullscale duckweed demonstration system was proposed for the Inverell SewageTreatment Works.


Theprocess employed at the Inverell STW is the intermittent decanting extendedaeration mode of the activated sludge process. At this plant the flow streamcould be easily divided into two parallel systems which meant that the duckweedsystem could be readily compared to a control system. Inverell Shire Councilcontinues to run the duckweed system as a functional component of their sewagetreatment plant.


Themain focus of the project was on the ability of duckweed to remove nutrientsand restrict the growth of algae in municipal wastewater treated by an extendedaeration sewage treatment process. The effect of duckweed coverage ondisinfection efficiency was also studied.


Theresults showed that duckweed is capable of taking up measurable quantities ofnutrients from municipal wastewater and restricting algae growth by blanketingthe surface. Disinfection efficiency was not found to be compromised with thearrangement used whereby 60% of the duckweed pond was covered with duckweed.


Thisproject 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 toprovide higher levels of nitrogen for improved duckweed growth, the effluentquality exiting the duckweed pond was found to be similar to that exiting thecontrol pond.


Theresults showed that the duckweed pond provided significantly higher removal ofseveral parameters when compared to the control pond:


        Various nitrogen compounds, particularly ammonia (30% reduction) andoxidised nitrogen (33% reduction);

        Total and reactive phosphorus (36 and 25% reductions, respectively);

        Faecal coliforms


Theduckweed was also found to suppress algae growth and thus reduce suspendedsolids (or NFR) exiting the duckweed section of the pond. However, algae growthdid occur in the section of the pond not covered in duckweed and this growthraised suspended solids concentrations exiting the duckweed pond.


Theproject demonstrated that the management of the Mixed Liquor Suspended Solids(MLSS) concentration and thus sludge age is important in an Extended AerationSystem using duckweed. With very low levels of Total Kjeldahl Nitrogen (TKN),the introduction of partially treated effluent will most likely encourage thegrowth of competing aquatic plants like azolla. Duckweed will not have thenutrient base to grow quickly enough to compete with the azolla. An analysis ofdried azolla revealed that it was not as efficient in uptaking phosphorus asthe duckweed.


Forextended aeration STWs, such as Inverell, where the level of TKN is low forduckweed cultivation, and the level of phosphorus is moderate, the duckweedcoverage is expected to be in the range of 3 to 4.5 kg/m2. If theduckweed mat is too thin, it encourages the growth of competing aquatic plants.


Theproject also demonstrated the development and operation of full scale duckweedmanagement facilities including the floating barrier system and the harvestingsystem. They are now ready for practical implementation.


Theseobservations have been made within the constraints of the operational dynamicsof the plant and the project boundary. The statistical relationships betweenthe observed performance, operation of the facility and the ambient environmentare not fully quantitatively defined. Further trials at STWs to ascertain theserelationships, are supported.


Aduckweed system is a simple, relatively low cost method of nutrient removal andalgae control compared to most mechanical and chemical systems. It provides apracticable, environmentally friendly option that required minimal power foroperation. Duckweed systems can be suitable for nutrient removal and algaecontrol for STWs with medium to high organic nitrogen discharge and for smallcommunities in remote areas.


Therecommendations from this project are that:


        For intermittently decanting extended aeration STWs, the system beoperated on the full aeration cycle to minimise the levels of nutrients. Inorder to obtain optimum growth of the duckweed, fertilising strategies shouldbe investigated that introduce additional sources of nitrogen to the duckweed.


        A small catch pond be provided to contain all the sludge, debris andfloatables that occasionally overflow into the pond especially after stormevents.


        Duckweed harvesting be carried out by using a boat harvester to pushthe duckweed out of the pond via a ramp for drying before removal.


        The floating baffles be shortened so the harvester can go around themfrom 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 effluentwith a low level of nitrogen, duck exclusion netting may be required tominimise the impact of ducks. However with vigorous duckweed growth this maynot be necessary as the ducks will have minimal impact.


        Duckweed systems be further trialed at STWs in NSW, especially thosethat discharge relatively high organic nitrogen concentrations.



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