Merimbula Sewage Treatment Plant Upgrade and Ocean Outfall Project

The Merimbula STP was built more than 30 years ago. It no longer meets today’s environmental standards and cannot support future population growth without significant improvements. The upgrade is needed to meet stricter treated water quality standards, support future growth in Merimbula and surrounding communities and ensure the system continues to operate safely and reliably.

The project will:

• Add tertiary treatment to produce higher-quality water
• Install solar generation and battery storage to reduce energy costs and emissions
• Phase out the use of dunal exfiltration ponds
• Continue supplying recycled water to farms and golf courses
• Construct a new deep ocean outfall pipeline and pump station.

The existing beach outfall at Merimbula is outdated and no longer acceptable from an environmental or public safety perspective. It discharges effluent close to shore, which poses risks to water quality, recreation and tourism. The new outfall will extend 3.5 km offshore, where natural ocean processes can safely disperse and dilute the effluent.

Following EPA direction in 2009, Council and a community focus group assessed options. In 2013, the deep ocean outfall was selected as the preferred option because it:

• Provides the greatest environmental benefit
• Minimises construction and operational impacts
• Protects culturally and environmentally sensitive areas
• Offers the most sustainable long-term solution.

The original ocean outfall was built in 1972, following the sewering of Merimbula, Pambula Beach and Pambula and along with construction of the Merimbula STP. It extended into the ocean and beyond the surf zone. In 1974 it was damaged by large seas and a temporary beach-face outfall was constructed. This temporary outfall remains in the same location to this day.

The current STP uses secondary treatment processes through a well-established intermittently decanted extended aeration activated sludge process. This method is reliable, cost-effective and produces good-quality effluent for recycled water use or disposal. The STP upgrades will incorporate modernised tertiary treatment processes, enhancing the quality of treated water. This will offer improved environmental protection and ensure the facility can continue to meet the demands of our growing community into the future.

The current estimate is $44 million for both the STP upgrade and constructing a modern deep ocean outfall.

The upgrade is funded through Council’s Sewer Fund, with additional support from the NSW Government’s Safe and Secure Water Program. Council is also seeking Federal Government funding to offset construction costs to minimise the impact on customers.

While some of the effluent is reused, during wet periods the excess must be disposed of. Currently, this happens via the beach outfall or the dunal ponds, neither of which is sustainable. The beach outfall no longer meets EPA or community expectations, while the ponds have limited capacity, threaten groundwater and lake water quality, and are located in endangered Bangalay Sand Forest on land of Aboriginal cultural significance.

Council is required to undertake development under the Environmental Planning and Assessment Amendment (Merimbula Sewage Treatment Plant and Ocean Outfall) Order 2016 under the Environmental Planning and Assessment Act 1979, section 115U (4).

As the works are considered NSW State Significant Infrastructure, they must meet the Secretary’s Environmental Assessment Requirements and will require approval from the NSW Minister for Planning and Public Places.

Community consultation included multiple community drop-in information sessions, briefing sessions for key stakeholders, an agency and stakeholder focus group in 2009 and an online Have Your Say. A Community Working Group was set up in December 2017 to represent the broader community. Further consultation occurred as part of the Environmental Impact Statement exhibition in 2021, followed by Agency and community group submissions to the Environmental Impact Statement and Council’s responses to the submissions.

Updates will be shared through Council’s website, media releases and public notices as the project reaches key milestones.

An Environmental Impact Statement (EIS) was prepared and publicly exhibited in 2021. It assessed potential social, economic and environmental impacts. Key investigations included:

Ongoing monitoring of STP effluent and Merimbula Bay water quality

• Ocean current and seabed studies to identify the best outfall location
• Groundwater and geotechnical investigations in the dunes
• Heritage assessments, including Aboriginal cultural heritage
• Dye dispersion testing in Merimbula Bay
• Flora and fauna surveys
• Noise, vibration, traffic and air quality studies
• Social, economic, sustainability and climate change risk assessments.

The EIS also identified the need for an ongoing monitoring program to track any potentially impacted environment.

With two treatment plants (Tathra and Bemboka) already utilising solar power, Council is well on its way to introducing solar across its most power-hungry water and sewerage assets, including the Merimbula STP.  

This combined $3 million investment over the next three years will see an $860,000 solar and battery system installed at Merimbula STP, offsetting about 25% of the plant’s annual power use and costs.  The system will pay for itself through savings within nine and a half years. Once paid for, the combined solar power output from Council’s treatment plants will introduce significant savings to annual energy costs and cut emissions equivalent to removing 123 cars from our roads every year. 

Council is committed to installing the largest solar and battery system possible within budget. However, expanding solar to offset the significant extra energy needed to pump recycled water to distant sites is not feasible. Limited space at the Merimbula STP and high pumping energy demands would cancel out any environmental gains.

The upgraded STP will produce higher-quality effluent, which will be discharged offshore at depth, where natural marine processes can disperse and dilute it effectively. Removing the dunal ponds will protect endangered ecosystems and culturally significant land. Recycled water schemes will continue, while biosolids will remain recycled for agriculture.

Sewage is collected from communities in Merimbula, Berrambool, Pambula, Pambula Beach, South Pambula and Millingandi. The largest proportion comes from residential and holiday dwelling via showers, baths, washing machines, sinks and toilets. On average, the plant receives about 700 million litres of inflow each year.

Effluent quality data for Merimbula STP (and other STPs) is available on Council’s website

The cost of upgrading all our sewer treatment plants to this level is prohibitive. Council does not have the available funds to implement this.

Council’s current NSW EPA license for the Merimbula STP specifies a 90th percentile limit of 10 mg/L for BOD5, a standard the plant has consistently met for several years. Normally, the BOD5 levels in the excess effluent discharge from Merimbula STP are less than 2 mg/L which is as sensitive as Council’s NATA-registered laboratory currently tests for.

BOD5 is used as a measure of ‘strength’ of sewage. At this stage of the concept design and EIS, the planned upgrade to the STP for effluent disposal will include:

• Removal of additional phosphorous
• Change in the disinfection process to include Ultra Violet light disinfection.

Neither of these proposed upgrades will materially affect the BOD5 removal performance.

However, now that EIS is finalised, the project team will be able to confirm any specific treatment upgrades that will be required (including for any further BOD5 removal).

Algal blooms are a naturally occurring phenomenon. The most common species in Merimbula Bay isHincksiasordida, which often moves closer to shorelines due to the summer easterly and north-easterly winds. A 2013 independent study of algal blooms in Merimbula Bay found that nutrient content from treated effluent was only one of many food sources this type of algae utilised for growth. The study also found numerous accounts from long-term Merimbula residents indicating the presence of regular algal blooms as far back as the 1950s, more than 20 years before the commissioning of the Merimbula STP.

Council has consulted with NSW DPI Fisheries regarding potential impacts on the artificial reef, as well as the natural reefs in Merimbula Bay. These were carefully assessed as part of the Environmental Impact Statement, which concluded there is minimal risk of impact from the operation of the proposed outfall.

This option has been considered but is not viable. Wetland environments are far more delicate than marine environments. For example, the protected saltmarsh at Panboola could be severely impacted by regular effluent inflows. Outflows would be via Merimbula and Pambula lakes, which could pose a risk to established seafood and recreational businesses. The NSW Coastal Management Act 2016 would not allow this option to happen.

Constructed wetlands are not a feasible option. They require large land areas and a discharge location for water flowing from them. There is no identified available land or suitable discharge location for a constructed wetland in the vicinity of Merimbula STP. Wetlands also have variable performance due to seasonal/climatic factors and fall outside NSW EPA licencing provisions.

Current concentration of phosphorus in the effluent is about 9 milligrams per litre (mg/L). This has been a limiting factor for new recycled water schemes.

Current concentration of nitrogen in the effluent is about 4 milligrams per litre (mg/L).

When sewage is treated, organic solids are separated from the effluent and matured in lagoons for around six months.

The process is like a compost pile. The result is nutrient rich organic material called biosolids.

Biosolids are stabilised and then supplied to local farms as a nutrient rich organic soil conditioner.

Biosolids from Cobargo, Wolumla, Candelo, Kalaru, Merimbula and Pambula are all processed at Merimbula STP.

Before 2019, all the 6,000 t/year of biosolids produced went to landfill. Council now targets 100% of that to be diverted to its contractor Arkwood to use on local farms. Whatever cannot be re-used is reprocessed and used on-site at Council’s sewage treatment plants. Almost all biosolids will now avoid landfill, and most will benefit local farmers.

Effluent is either reused as recycled water for irrigation or disposed of via effluent disposal schemes. Council supplies as much recycled water as possible for irrigation at Pambula Merimbula Golf Course and farmland at Oaklands. The remainder is disposed via the beach-face outfall or dunal exfiltration ponds.

AECOM developed the Merimbula Effluent Management Options Study between 2009 and2013. This study investigated strategic alternatives to the project including new and expanded recycled water schemes as well as STP upgrade and effluent disposal options. All schemes included the need for a disposal option. 

Council currently partners with community groups, farmers and golf clubs to operate 11 recycled water irrigation schemes between Bermagui in the north and Eden in the south. In an average rainfall year, about a quarter of the community’s treated sewage is recycled and used beneficially. Council is investigating other recycled water schemes through developing a Recycled Water Strategy.  

Recycled water schemes are limited by weather, land availability and environmental constraints. During wet periods, demand falls while inflows increase, meaning disposal is still required. For example, stormwater can reach the sewer system through groundwater infiltration or accidental inflow if gutters are connected incorrectly. This has a big effect on the volume of sewage that the STP must treat and dispose of. In a big storm the STP can receive an entire weeks’ worth of sewage in one day. Recycled water schemes will continue and expand where feasible, but a disposal option is always necessary.

Yes, there are currently 11 recycled water schemes operating across the shire. The total amount of recycled water supplied fluctuates according to the weather. In financial year 2019/20, Council supplied 40% of its recycled water for beneficial use, which is far more than the state average of 13.4%.Also, in thatsame year Council recycled a higher percentage of effluent than any other coastal council in NSW.

Oaklands Farm uses between 5% and 11% of the total volume of effluent produced at Merimbula STP each year. More is used in dry years, less in wet. Median use is around 8% per year.

Oaklands’ recycled water scheme has been operating since February 2013. Effluent is pumped through a 4km pipeline to a 20 million litre storage dam located on the Oaklands property.

The Merimbula landscape has unique constraints on where recycled water can be directed. Sensitive areas such as wetlands, endangered ecological communities, National Parks, oyster leases, and Aboriginal heritage sites must be protected.

Suitable land for new schemes must also meet strict requirements, including:

• Adequate buffer zones from waterways
• Slopes of less than 10 degrees
• Appropriate soil profiles
• Capacity for irrigation systems that apply water only when vegetation needs it and avoid overspray, drift, runoff, ponding or waterlogging.

Around 7 hectares of sports fields at the Pambula Sporting Complex could potentially benefit from recycled water. Council is preparing cost and reuse estimates and has committed to developing a Recycled Water Strategy for the entire shire.

When the existing outfall cannot be used — such as during the day or in summer —effluent is pumped into dunal exfiltration ponds behind Merimbula Beach. 

Their ongoing use is unacceptable as they are constructed on culturally sensitive land of great significance to the local Aboriginal community and on endangered bangalay sand forest environments.  

The new ocean outfall pipe will remove the need for exfiltration ponds, allowing the land to be handed back to the local Aboriginal community.