Moss Vale Sewage Treatment Plant (EPL 1731) - Licence breach

Council has committed to constructing a new Sewage Treatment Plant (STP) to serve the community in Moss Vale. Whilst this is ongoing, the existing Moss Vale STP plant is being ran at its maximum capacity to try to meet the licence conditions. With this summers seasonal weather already being notably warmer and drier than recent years, the capacity of the process at the plant is proving particularly challenged in this current environment.

This has resulted in a licence breach in December 2024 downstream of the plant in relation to discharged ammonia and resultant decreased dissolved oxygen (DO) levels in Whites Creek.

In simple terms, the treatment plant is a biological process and requires oxygen to function. As the workload increases, the biomass requires more oxygen to succeed. The design of the plant means there is a limit to the amount of oxygen that can be supplied. This has been insufficient compared to the increased loading. The result has been that the biomass has been unable to meet the workload, resulting in the breach of licence.

Councils team has been working hard to react to the conditions that have emerged and has been open in its consultation and communication with both our Environment Protection Authority (EPA) and Department of Climate Change, Energy, the Environment and Water (DCCEEW) regulators in searching for an interim solution ahead of the new larger plant being completed.

In response to the breach in licence episode across December 2024 and the first week of January 2025,  the EPA have issued Council a Clean Up Notice which includes the directive to create this webpage to provide information about the incident, including up to date environmental monitoring data, to inform the community.

• Tuesday 26 November 2024: Council proactively reported a pre-notification to the EPA of an anticipated 90th percentile exceedance in those licence conditions with regard treated effluent discharge to Whites Creek. This was in respect elevated ammonia levels and resultant reduced dissolved oxygen levels at the discharge point and downstream monitoring point.

• Wednesday 27 November 2024: Council consulted with DCCEEW for their continued expert guidance, as the team continued to try to address the matter. We have a constructive working relationship with the regulator and the team regularly keep in contact. This enables the team to readily talk through ideas, options and gain their valued insight.

• Sunday 1 December 2024: Council received confirmation of a 90th percentile exceedance and notified the EPA as required. The EPA responded with a request that Council carry out daily sampling and lab analysis, with which we immediately complied.

• Friday 6 December 2024: Council participated in a meeting with both the EPA and DCCEEW, running through the options Council had investigated and applied to date in response at the site. These included:

• Reduced supernatant load into the plant and the Extended Aeration Tank’s (EAT) – to try to manage the workload for the biomass
• Reduced alum dosing to encourage biomass wellbeing
• Increased existing air transfer rate to maximum – to feed the biomass more air
• Re-seeded biomass with healthy MLSS from Bowral STP – to rejuvenate the biomass
• Replaced the diffuser heads to increase oxygen transfer – to feed the biomass more air

Council discussed the then future options to be investigated and implemented:

• Changing the compressor pulley sizing to change the ‘gearing’ and increase air transfer rate above design – to feed the biomass more air.
• - This has been completed on  Tuesday 10 December but has not solved the issue.
  
  
• Balancing flows between the EAT’s to try to optimise both treatment streams – to try to manage the workload for the biomass
• - This has been completed but has not solved the issue.
  
  
• Installing additional surface aeration to reinforce the existing submerged diffuse air system – to feed the biomass more air.
  - This has proven hard to source in practical timeframes over the holiday period, but the team has persisted and we have reviewed our options with DCCEEW.
  - It is an unknown outcome given surface aeration is not usually ran with existing submerged diffuse air systems.
  - A surface aeration system is going through the procurement process and should be installed from Monday 20 January 2025.

Council also discussed options suggested by DCCEEW and EPA in respect:

• Installing a temporary aeration system in the watercourse itself downstream of the discharge point to reinforce dissolved oxygen in the channel for waterway health.
  - This has been completed and has reinforced DO, but the seasonal reduction in DO did persist.

• Discharging to the storm pond and pumping overland to the UV system – to try to manage the workload on the plant.

• - This has proven to be unfeasible due to site layout.

• Discharging to the environment from the storm pond itself – effectively allowing the plant workload to overflow
  - Concerns around faecal coliform levels given the discharge is to WaterNSW downstream catchment – to be sampled to confirm anticipated levels.
  - Will dilute higher ammonia levels.
  
  
• Decant EAT1 to catchpond and/or EAT2 to catchpond – to try to manage the workload on the plant
  - Not feasible due to site pipework.
• Using new compressor planned for the new Bowral STP upgrade as an emergency compressor – to feed the biomass more air
  - Sized too small for the temporary task.

• Wednesday 11 December 2024:  Council hosted the EPA on site to inspect the STP

• Wednesday 18 December 2024: Council participated in an incident update meeting with the EPA team, discussing outcomes of the above points.

• Friday 20 December 2024: The EPA instructed Council to carry out a community notification, communicating the situation to the downstream residents. Council organised and distributed a letter drop to an EPA defined list of residents the same day.

• Thursday 9 January 2025: The EPA held a meeting with Council and informed us they were issuing the Clean Up Notice, instructing Council in respect Community Notification, Environmental assessment and expert Wastewater Treatment consultation to address the issues being experienced and to bring the plant back into licence. As at Thursday 9 January, monitoring indicates that the plant is within licence.

• Monday 13 January 2025: Council formally engaged the independent environmental consultant and independent wastewater specialists to complete the reporting requirements stipulated by the EPA in the Clean Up Notice.

• Monday 20 January 2025: Council is expecting to install a supplementary aeration system to bolster the existing plant treatment capacity. 

WSC Sample Data Moss Vale STP Ammonia and Dissolved Oxygen

December Sample Data(PDF, 57KB)

Council's team is working hard to maximise the treatment capability of the existing plant beyond its original design capacity whilst the new larger plant is constructed. We have also installed aeration into Whites Creek immediately downstream of the discharge point to mitigate the impact of the elevated ammonia and reinforce the dissolved oxygen content. The team are persisting with increasing the aeration capacity of the plant to enable the biological process to meet the workload. A supplementary surface aeration system will be installed from Monday 20 January 2025 and whilst this combination of systems is unproven, Council expects this to have a positive impact on the existing plant maintaining licence.

We have acted conscientiously to try to maintain licence and continue to investigate options to address the aeration issue which is fundamental to the success of the biological process given the level of loading on the existing plant. Each new option being enacted has taken time to implement and then assess over a few days to gauge its success. The team has been continuously working to try to address the challenge.

Council will continue to consult with the EPA and DCCEEW, enacting all practical solutions and communicate their outcome.

To reassure you, Council is committed to the health of the waterway and is acting to address the current incapacity in treatment capability compared to licence, whilst also significantly upgrading the sewer treatment plant capacity in parallel to meet the needs of the environment and the community in the future.

A discharge of elevated pollutant levels of ammonia and decreased levels of dissolved oxygen.

The EPA states - The discharge of elevated pollutant levels into Whites Creek may be harmful to the health of the waterway.

Specifically, increased levels of ammonia and decreased levels of dissolved oxygen reported at EPA Point 1 and downstream in Whites Creek can be toxic to aquatic life and can lead to an increase in algal activity in stagnant pools of water. There have been no reports of actual harm to aquatic life to date. 

Downstream of the STP discharge, along Whites Creek between the edge of Moss Vale and Medway Rivulet.

Along the affected section of Whites Creek (Whites Creek between the edge of Moss Vale and Medway Rivulet): landowners, residents and users such as stock and domestic users, holders of water irrigation licences or recreational water facilities.

• Council is constructing a new, larger capacity STP at Moss Vale to service the needs of the community and environment for decades to come.
• Council is formally engaging both Environmental and Wastewater Treatment specialist independent consultants to advise of any unexplored practical options to address the current challenges.
• Council will continue to consult with our EPA and DCCEEW regulatory bodies in regards their expert guidance on resolving the current challenges.
• Councils team will continue to work hard to operate the existing plant beyond its design capacity, enacting expert guidance to attain meeting licence through this challenging period.

The team have been operating the STP within licence up until December 2024, when the summer conditions and loading have proved beyond the capacity of the plant. Since the Summer of 2019/2020, the ‘La Nina’ wetter weather conditions have been pre-dominant over recent summers and it has been the change to warmer dryer environment in November 2024 that has impacted the treatment plant loading and processes negatively and proving beyond capacity.

Council is procuring an additional aeration system to reinforce the process, which is to be installed from Monday 20 January 2025. Whilst this is an unproven combination of technologies, Councils expects this to have a positive impact on the existing plant maintaining licence. DCCEEW are in support of this reinforcement methodology and are eager to assess the results. The new larger STP is programmed to be completed in 2026, though elements of the new process may be able to be brought online earlier, to avoid a recurrence of the licence breach next summer.

The new larger STP is programmed to be completed in 2026, though elements of the new process may be able to be brought online earlier, due to the current challenges being faced.