Redland Water is the department of the Redland City council that oversees the collection, treatment, and disposal of waste water in Redland City. Residential households and commercial premises such as shopping centers, cafes, commercial laundries, butchers, car-washing centers, and restaurants are the main sources of waste water in Redland City. Once collected, waste water is stored temporarily in abattoirs, before being released for treatment in one of the seven treatment plants located in Point Lookout, Dunwich, Mount Cotton, Victoria Point, Thorneside, Cleveland, and Capalaba (Redland City Council, 2012). The author paid the Capalaba Treatment Plant a visit, and found that the plant had just upgraded to a new waste water treatment technique referred to as the Vetiver Grass System, herein referred to as the VS. The Vetiver system, "a green and environmentally-friendly waste water technology as well as natural recycling method," had just been approved by the Queensland Department of Natural Resources and Mines, NRM (Ash and Truong, n.d.). Further necessitating an upgrade was the previous method's failure to comply with the new regulatory standards that had been established by the Environmental Protection Agency (EPA).
The plant's staff was just getting accustomed to the new treatment procedures and management found it necessary to compare the performance of the two systems so as to determine whether or not the VS was a perfect fit for the plant.
Capalaba Sewerage Plant
The traditional treatment scheme comprised of an Imhoff tank, three sewerage ponds, and a swamp area. Waste water from different quarters was collected in the Imhoff tank and left to settle. Large solids were removed through the process of sedimentation and the effluent first directed into the sewerage ponds before being left to overflow into the swamp area for disinfection. With the passage of the new EPA standards for pollution, the plant had to upgrade to a "system that would take up most of the water, as well as remove nutrients and heavy metals" (Ash and Truong, n.d., Para 2.1). Options such as rock filters and sand filters were considered, but these proved too costly and the city council eventually settled for the VS. Most of the land surrounding the plant belonged to the city council, and hence the plant would have ample space to develop the VS wetlands.
The nutrients passing through the sewerage ponds created a strongly alkaline environment, which produced high algae concentrations; a PH of between 7.7 and 9.4, against the 8.5 license limit (Truong, n.d.). The count of suspended solids, which also included algae counts and Fecal Coliform, stood at about 85 Mg/L, more than double the 30 Mg/L license limit (Truong, n.d.). Furthermore, the system recorded over 6000 organisms/100ml, almost six times the 1000/100ml license limit (Truong, n.d.).
The VS makes use of Vetiver grass (Vetiveria zizanioides L.), which "has a fast and very high capacity for absorption of nutrients, particularly nitrogen and phosphorus in waste water" (Ash and Truong, n.d., Para 3.2). It is also tolerant to heavy metals and high agrochemical levels (Truong & Hart, n.d.). Vetiver Grass has been proven to be more efficient than other commonly-used grass species in waste water treatment. For instance, 178 ha of land would be needed to adequately treat 2 ml of waste water consisting of 15Mg/L of phosphorus and 155Mg/L of Nitrogen if Kikuyu Grass is made use of, whereas approximately 86.5ha would be required in the case of Vetiver grass (Ash and Truong, n.d).
The VS Treatment Procedure
The VS treats effluent through a two-stage mechanism involving;
Stage 1: hydroponic treatment in ponds
Stage 2: Secondary treatment in Vetiver wetlands
Stage 1: Primary Treatment
Floating Vetiver pontoons absorb nitrogen and phosphorus as effluent flows through the three-pond system.
The Pontoon Structure: the floating pontoons are such that the Vetiver plants sit above the effluent, as the roots suspend in it (Ash and Truong, n.d; Truong & Hart, n.d). Each pontoon measures 2.4m by 2.4m, and carries approximately 300 Vetiver plants (Piemento, Falco and Basil, 2013).
Number of Pontoons: this depends on the size of the nutrient load (Piemento, Falco and Basil, 2013). Effluents with higher concentrations of phosphorus and nitrogen would require higher numbers of pontoons. It is advisable to increase the number regularly so as to ensure an effective treatment…