High Rate Algal Ponds (HRAP)

Background

Wastewater treatment for CWMS typically consists of facultative lagoons or mechanical treatment plants (eg Sequencing Batch Reactors (SBR)). Treated water is stored over winter in storage lagoons or is disposed of via woodlot irrigation or evaporative lagoons. Both treatment options have their advantages and disadvantages and the CWMS Management Committee (CMC) identified a need for an alternative treatment system that was low-cost with a low carbon footprint and that requires minimal operator intervention while producing a large volume of recycled water available for beneficial re-use.

In 2008 the CWMS Management Committee (CMC) formed a research partnership with Professor Howard Fallowfield from Flinders University and funded the investigation of High Rate Algal Ponds (HRAP) as a sustainable, low energy, low cost alternative wastewater treatment process. HRAPs are typically used to treat high strength wastewater or to grow algal biomass for biofuel production and the focus of the research partnership was to investigate whether they are suitable to treat human wastewater to an acceptable standard.

In 2008 the CWMS program funded the construction of a HRAP test site at Kingston on Murray, a small regional STEDs scheme owned and operated by the District Council of Loxton Waikerie and this site has been used as a research site for Flinders University.

CWMS Kingston on Murray (KoM) HRAP

High Rate Algal Ponds (HRAP) are shallow raceway ponds that circulate wastewater via a low-power paddle wheel. HRAPs are designed to maximise the breakdown of organic waste via algal and bacterial growth and inactivate pathogens via sunlight exposure. The accelerated organic breakdown and pathogen reduction results in shorter retention times compared to standard lagoon systems. The residence time is between 4 and 10 days and the depth of ponds is typically between 300mm and 500mm.

Some of the reasons why a HRAP may be suitable for incorporation in a CWMS include:

  • HRAPs designed using the LGA HRAP Design Guidelines will achieve a minimum of 1 Log Reduction in pathogens and result in better public health outcomes.
  • Shallow depth between 300 to 500mm -construction cost reduced by 40 percent compared with conventional lagoon systems
  • Paddle wheel -low power consumption and carbon footprint
  • Shorter retention times of between 4 to 10 days for effective treatment resulting in smaller footprint and reduced evaporation by approximately 12 to 30 percent
  • Minimal operator intervention required when instrumentation and alarms are installed.

The table below summarises when each type of treatment process is most suitable for various site constraints and provides a useful comparison for when a HRAP may be suitable for installation.

Site constraints

Facultative lagoon

Mechanical treatment

HRAP

Limited power available

Y

N

Y

Limited land footprint

N

Y

Y

High evaporation rates due to climate

N

Y

Y

Limited access to experienced operators

Y

N

Y

Public Health Requirements - 1 LRV through wastewater treatment plant

N

N

Y

Holiday towns (peaking flows)

Y

N

Y

The Australian Guidelines for Water Recycling specify target log reductions of pathogens for different types of re-use. As the HRAP was a novel treatment process for the treatment of wastewater, SA Health required that the HRAP process underwent validation testing to demonstrate the indicative log reductions that can be achieved.

Validation testing of the Kingston on Murray HRAP was undertaken as a requirement of SA Health and resulted in the development of the HRAP Design Guideline which includes specifications for a validated HRAP design.

The High Rate Algal Pond (HRAP) Design Guideline (the Guideline) has been developed to provide guidance on designing a validated HRAP for wastewater treatment that will achieve a minimum of 1 log reduction of viral pathogens. The design criteria in this Guideline is based on a validated design that has been approved for use by Water Industry Entities (WIE) by the South Australian Department for Health and Wellbeing.

This is the first validated wastewater treatment design guideline in South Australia.

High Rate Algal Pond (HRAP) Design Guideline - June 2020

The first installation of a HRAP designed using the HRAP Design Guidelines was at Peterborough. The CWMS was designed as a gravity sewage collection system and accepts wastewater from the SAMEX abattoir. As part of Councils trade waste policy, the abattoir waste must be treated to a standard that is equivalent to human sewage.  The wastewater treatment plant can treat up to 470 kL of combined sewage and abattoir waste each day. It includes two anaerobic lagoons in parallel that provide primary treatment to the abattoir waste and the sewage. The Primary treated wastewater is dosed into the two HRAP lagoons in parallel which have a residence time of 10 days a total surface area of 4700m2. The Peterborough HRAP was designed using the “Serpentine” configuration and this can be seen in the photo below. Water gravity overflows from the HRAPs  into the two storage lagoons in series which store recycled water for irrigation of the Peterborough golf course and the town oval.

Peterborough HRAP

The Peterborough CWMS was recognised for its innovative design at the Australian Water Association (AWA) South Australian Water Awards in 2019 where the project won the Infrastructure Project Innovation Award. The project is a finalist in the 2020 AWA Australian Water Awards, with the winner to be announced at Ozwater in May.