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Corporate sustainability

Rottnest Island Wastewater Treatment Plant

Rottnest Island Wastewater Treatment Plant

Wastewater Innovations
Western Australia's Water Quality: Waste Water Treatment Projects
Environment Protection Group 1997

Summary of Project

The capability of Intermittent Activated Sludge Technology to treat sewage generated at remote, holiday locations in environmentally sensitive regions has been well demonstrated by the success of the Rottnest Island Wastewater Treatment Project. In this project, an intermittent biological nutrient removal process termed CASS (Cyclic Activated Sludge System) was installed on Rottnest Island by the Australian company ESI Ltd.

The plant treated the fluctuating flows and loads associated with the seasonal popularity of Rottnest Island to a high level, removing BOD (organic carbon), suspended solids, nitrogen and phosphorus to acceptably low levels. Moreover, the plant was shown to be robust and straightforward to operate, despite the variability of the incoming sewage flows - this can be attributed to a combination of particular design features of the CASS plant and the operating flexibility provided by the intermittent activated sludge treatment technology. The capital and operating costs of the Rottnest plant are consistent with those of other treatment plants of this capacity.

There is a growing market for small to medium sized plants to treat domestic wastewater both within Australia and also in Asia, with a move away from centralised plants to specialised plants such as this. This is particularly the case for situations such as Rottnest where the remote nature of the site and sensitive environment require plants which can produce a high quality effluent with minimal operator attendance and maximum operational flexibility. In the past, many such sites have used forms of wastewater disposal which are no longer environmentally acceptable. The successful operation of the CASS plant on Rottnest demonstrates that Intermittent Activated Sludge Technology has the potential to satisfy this demand.

The treatment plant

The treatment plant

The Rottnest Island plant is available as a reference site for this technology.

The Environmental Problem

Rottnest Island is located 18 km off the southern west coast of Western Australia. It is an "A" Class Reserve (No. 16713) for the purpose of Public Recreation with its management being vested in the Rottnest Island Authority. Much of the land held by the Commonwealth Government (Kingstown Barracks and Light Houses) forms part of the Reserve; the Reserve currently includes the waters off the coast of the Island.

Rottnest has particular and unique characteristics for wastewater treatment. It has a sensitive environment, and is subject to increasing and highly seasonal pressure from tourists. Rottnest has approximately 370,000 visitors per year, with the majority in summer (approximately 70,000 in January) and fewer in winter (approximately 10,000 in June). Access to the Island is by ferry and air, with normal supplies and services being relatively expensive to provide.

Rottnest Island is a popular holiday destination for WA residents; it has both overnight and day only visitors. Approximately 70% of visitors are day trippers. The demand generally peaks over weekends, long weekends and holiday periods. The Island has accommodation for up to 2850 visitors, while day only visitors can number up to 5000. In addition, there is a residential population of approximately 300 in winter and 400 in summer .

Sewage flows are generated principally from settlement areas at Thomson Bay and Geordie/Longreach Bays; the available accommodation consists mainly of villas, units and bungalows. Other sewage flows are generated from a small number of public buildings, including two churches, a community hall, museum, police station, medical centre, primary school, an airport and several food outlets. Prior to the construction of the wastewater treatment plant, the sewage was discharged directly to the ocean at Geordie and Longreach Bays. Sewage from Thomson Bay was treated in an Imhoff tank and effluent lagoons.

Treating sewage to produce an effluent low in BOD (organic carbon) and suspended solids as well as the nutrients nitrogen and phosphorus is traditionally carried out in large, centralised treatment plants. However, the need for equivalently high levels of treatment is now widespread even for small sewage flows. Intermittent activated sludge treatment technology represents an appropriate technology for such situations because of its ability to treat small, variable volume flows to a high level of purification and because of the simplicity of its operation. It is particularly suitable for situations such as Rottnest where the sensitive environment and the remote nature of the site require plants which can produce a high quality effluent with minimal operator attendance and maximum operational flexibility.

Outline of the Demonstrated Technology

Intermittent Activated Sludge technology is a form of the activated sludge process in which the liquid waste stream is treated in a single vessel by subjecting it to sequenced periods of filling, aeration, solids - liquid separation and decanting of the liquid. This compares with the conventional activated sludge process in which the liquid waste stream passes through different reactor compartments to achieve the same result. The basic principles of operation are identical, however, apart from the former being essentially a batch process and the latter a continuous process. Intermittent operation generally requires two reactor basins to be operated in parallel. All activated sludge processes use microorganisms which are trapped in the reactor to speed up the decomposition of the wastes.

The form of Intermittent Activated Sludge Technology used on Rottnest Island is a proprietary process termed CASS (Cyclic Activated Sludge System). CASS is a variable volume, intermittent activated sludge system which comprises (see Figure 1):

The fill/aerate sequence in progress

The fill/aerate sequence in progress

Figure 1

Figure 1

Rottnest Island Process Flow Diagram

Figure 2 - Rottnest Island Process Flow Diagram

The CASS plant on Rottnest Island was designed to treat a maximum of 176,000 kL of domestic wastewater per year (average of 482 kL/d), 27,700 kL in any one month (923 kL/d) and 3,800 kL in any 3 day consecutive period (1,270 kL/d). This flow was based on an equivalent population (E.P.) of 4,500. The plant was designed for continuous operation with in-built turn down capacity; however, no actual minimum flow was specified. Figure 2 provides an outline of the overall process.

The biological nutrient removal process begins by introducing wastewater into the first compartment of one of the CASS basins (termed the captive selector), where it is mixed with the microorganisms (return activated sludge). Operation of the plant involves one of the CASS basins to be filling whilst the other is operating ie. a sequential fill and draw mode. As the CASS basin is being filled, air is introduced to mix and aerate the biomass and thus begin the combined organic carbon removal, nitrification/denitrification and phosphorus removal processes. The air supply is then shutdown and the basin goes through a settling period where the activated sludge separates from the treated water.The treated effluent in the basin is then decanted by lowering the rotating surface decanters; floating scum barriers prevent foam from leaving in the effluent. The clear effluent is then directed towards effluent soakage lagoons. With disinfection the effluent would be suitable for irrigation. Cycle times for the process vary, depending upon the flowrate and conditions of the influent.Periodically, some of the sludge is wasted to the aerobic digester basin. The digested sludge is then directed to the sludge belt press for dewatering and removal. No chemicals are added to the activated sludge process and only polyelectrolyte is used for sludge dewatering.

A number of equipment and process patents related to the CASS process are held by ESI Ltd.

Results from Demonstration

The monitoring program showed that the plant was effective in removing Biological Oxygen Demand (BOD), Suspended Solids (SS), Total Nitrogen (TN) and Total Phosphorus (TP) from the wastewater, although the required concentrations of many of these parameters were occasionally exceeded in the effluent, particularly TP. The effluent quality is likely to have been affected by the low flows, as the plant was unable to operate as designed, and also by influent levels of the above parameters which frequently exceeded design levels.

The performance during 1995 is summarised in Table 1.

Table 1: Summary of Steady State Results

Target Performance
Average Actual Performance
Median Actual Performance
Inflow (kL/day)
482 (average)
BOD (mg/L) Influent


SS (mg/L) Influent


TKN (mg/L) Influent

TN (mg/L) Effluent

TP (mg/L) Influent



The plant was designed for continuous operation with in built turn down capacity for the low flow months. However, no actual minimum flow was specified. The average flows experienced during the monitoring period were only 250 kL/d, forcing the plant to operate on only one basin, rather than two as designed, for most of the monitoring program. As a result the plant operated effectively on a continuous inflow basis. This meant that influent was entering the basin during all stages of the operating cycle which is contrary to the intended design. Other influent parameters, notably nitrogen, were also often higher than expected. Despite these issues, the plant utilised operating cycles which are consistent with other plants of its type.

The plant has an alum dosing facility as a precautionary backup system for phosphorus removal from the effluent. This was not used throughout the monitoring program because the plant was considered to be operating well enough to avoid alum dosing (despite effluent phosphorus levels at times which were higher than required). The fact that the plant does not require alum dosing means that the volume of sludge produced is significantly less. The only chemical consumed in the plant was polymer which was added as an aid for sludge dewatering.

Filterpress for dewatering stabilised biosolids

Filterpress for dewatering stabilised biosolids

The capital cost of this plant, approximately $ 1.5M, is consistent with a large number of wastewater treatment plants across Australia when calculated on the basis of cost per kg BOD treated. Operating and maintenance costs have been estimated at $ 105,000/year which are also reasonable. The construction materials have been selected for low maintenance, and high maintenance costs are not expected.There is minimal odour from the plant. The odour is detectable only in close vicinity of the plant and was due to odour from the influent sewage.

What the Project Demonstrates

The project shows clearly that Intermittent Activated Sludge Technology is an effective means of treating variable, low volume sewage flows so that a high quality effluent results. The technology is particularly suitable for removing the nutrients, nitrogen and phosphorus, from sewage generated at resort locations, which are characterised by highly seasonal flows.

Two other features of the Intermittent Activated Sludge Technology, apart from its ability to generate high quality effluent under a range of load conditions, have been demonstrated by this project. They are its ease of operation and robustness. All operations can be controlled by a programmable time switch and operator training is straightforward.

This treatment plant will improve the environment of Rottnest Island and surrounding waters due to a much cleaner effluent being produced than in the past. There is a net reduction in nitrogen for the system and a low phosphorus level in the effluent. The majority of the phosphorus is taken up in the sludge and the sludge disposal can be controlled. The sludge can be used as an addition to soil for market gardens etc.

Water Corporation

Commonwealth of Australia