Maleny sewage treatment plant

Two years in operation and more
R Kulkarni
Publication Date (Web): 3 November 2017

A hybrid of a new technology and a natural purification process
In 2014, Unitywater, a water distributor/retailer in South-East Queensland, embraced an immersed membrane bioreactor (iMBR) technology at Maleny Sewage Treatment Plant (STP). Increasing process capacity, in conjunction to higher effluent quality, and fostering a community-based-partnership and an ecosystem-based-approach for disposal of effluent to vegetated forest and treatment wetland were identified as the foremost drivers for successful implementation. 

Operational results under a range of operating conditions such as diurnal flows, wet weather events, and fluctuating membrane fluxes including varying mixed liquor feed quality were analysed. The iMBR process consisting of two ultrafiltration trains (UF1 and UF2) demonstrated a ‘stable performance’ in the first two years of its operation. Figure 2 summarises the iMBR performance and its indicators (MPIs) expressed as ‘plant health’.  Identical results were found for both trains indicating ‘higher-end’ of the membrane operating system under dynamic operating conditions (>75% versus the normal range of 50-80%).

Figure 2
Figure 2: iMBR plant performance and health in a) 2014 and in b) 2015

The iMBR surmounts its capability to produce a superior water quality effluent, especially with specific performance on parameters such as turbidity (0.11 and 0.13 <0.2 NTU (50th percentile), Escherichia coli (<1 versus <100 (median) CFU /100 mL) and Enterococci spp. (<1 versus <40 (median) and <150 (max) CFU /100 mL).

A clean-in-place (CIP) performed on one occasion found minimal impact on permeability recovery. No membrane damage, pinning, fibre-pull-out or fibre-rub-through were sighted due to ‘high’ agitation when in use.

Typical permeability recoveries achieved were between 62-67%. 

Getting a membrane recovery protocol ‘right’ depends upon choosing:
  • What condition-based ‘triggers’ and or ‘time-based’ conditions should be considered prior to CIP operation?

Excess chemical dependence and higher energy consumption (kW per kL of permeate produced) were the operational drawbacks requiring refinements moving forward. A validation in chemical cleaning protocol and recurrence interval for permeability recovery clean is ‘mandatory’ in order to achieve economics in plant operation.

An important parameter to watch is the mixed liquor concentration in the bioreactor and solids mass flux on a daily basis. A marginal drop in permeability was found when operating at >12 g up to 15 g MLSS/L. Operating within hydraulic capacity and with long sludge age resulted in additional thickening which is first of its kind in membrane operation in Australia.

Predicting useful life of in-situ membrane fibre(s) still remains an ongoing challenge.

Approximately 175 ML (annually) of reclaimed water discharged for night-time application to vegetated forest and an engineered treatment wetland has led to restoration of an old dairy pasture land. This essentially transformed a monoculture into a rich habitat land full of biodiversity, flora and fauna in the Maleny Community Precinct.

Performance of the treatment wetland was ‘competitive’ in removing nutrients. Nearly 94-97% removal was achieved in nitrogen especially with nitrate-nitrogen. Approximately 525 and 412 kg of nitrogen and 63 and 84.5 kg of phosphorous was reduced from being released into Obi Obi Creek – a major drinking water source to Sunshine Coast.

Lessons gained
A SWOT analysis was undertaken to improve operational excellence and extend useful service life of membranes, including areas that critique the selection of the membrane technology highlighted:
  • An iMBR is distinctly a decentralised process suitable for remote and or isolated communities such as Maleny.
  • Stable performance and a high quality of reuse water was produced.
  • Membrane life is a function of operating within MPIs. From an operational perspective, the iMBR carries additional costs associated with cleaning chemicals and energy cost for air scouring and permeate pumping.
  • Combined with irrigated forest and treatment wetland, the STP upgrade worked well in creating flora and fauna biodiversity in the Maleny Community Precinct - a recipe for a ‘win-win’ proposition.
  • Collaboration with the local community, Sunshine Coast Council, and other stakeholders yielded a range of benefits delivered back to the community and to the environment.

This paper was presented at the IWA World Water Congress and Exhibition, Brisbane, October 2016.


Click here to read the full paper