Vetiver Grass System for Wastewater Treatment: Capalaba Case

Introduction

Redland Water is the department of Redland City Council that oversees the collection, treatment, and disposal of wastewater in Redland City. Residential households and commercial premises — such as shopping centres, cafes, commercial laundries, butchers, car-washing centres, and restaurants — are the main sources of wastewater in the city. Once collected, wastewater is stored temporarily before being released for treatment in one of seven treatment plants located at Point Lookout, Dunwich, Mount Cotton, Victoria Point, Thorneside, Cleveland, and Capalaba (Redland City Council, 2012).

The author visited the Capalaba Treatment Plant and found that it had recently upgraded to a new wastewater treatment technique referred to as the Vetiver Grass System, herein referred to as the VS. The Vetiver System — described as "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.). A further impetus for the upgrade was the previous method's failure to comply with new regulatory standards established by the Environmental Protection Agency (EPA).

The plant's staff were still accustoming themselves to the new treatment procedures, and management found it necessary to compare the performance of the two systems in order to determine whether the VS was a suitable fit for the plant.

Capalaba Sewerage Plant and License Limits

The traditional treatment scheme comprised an Imhoff tank, three sewerage ponds, and a swamp area. Wastewater from various sources was collected in the Imhoff tank and left to settle. Large solids were removed through sedimentation, and the effluent was directed first into the sewerage ponds before overflowing into the swamp area for disinfection. With the introduction of new EPA pollution standards, 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 ultimately settled on the VS. Because most of the surrounding land belonged to the city council, the plant had ample space to develop the VS wetlands.

The nutrients passing through the sewerage ponds created a strongly alkaline environment, producing high algae concentrations — a pH of between 7.7 and 9.4, against the 8.5 licence limit (Truong, n.d.). The count of suspended solids, which also included algae counts and fecal coliforms, stood at approximately 85 mg/L, more than double the 30 mg/L licence limit (Truong, n.d.). Furthermore, the system recorded over 6,000 organisms/100 ml, almost six times the 1,000/100 ml licence 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 of heavy metals and high agrochemical levels (Truong and Hart, n.d.). Vetiver grass has been proven to be more efficient than other commonly used grass species in wastewater treatment. For instance, 178 ha of land would be needed to adequately treat 2 ML of wastewater consisting of 15 mg/L of phosphorus and 155 mg/L of nitrogen if Kikuyu grass were used, whereas approximately 86.5 ha would be required with Vetiver grass (Ash and Truong, n.d.).

The Vetiver Grass System (VS)

The VS treats effluent through a two-stage mechanism:

Stage 1: Hydroponic treatment in ponds
Stage 2: Secondary treatment in Vetiver wetlands