Client – Innovative Water Efficient Building
The building described was constructed as a flagship example of “best in class” sustainable design with a focus on energy and water conservation. Water efficiency was based on principles of water efficient fittings, water recycling, and water reuse.
Central to the water reuse strategy in the building is a “blackwater” treatment plant. As well as treating both the blackwater (toilet) and greywater (shower and basin) waste produced by the building, the system also produces reusable water by treating sewerage ‘mined’ from the sewer in the adjacent city street. Sewerage is usually made up of 95 per cent water and the installed system was intended to publically demonstrate that sewers can be a source of useable water that, while not potable is ideal for flushing toilets, irrigation, and other non-potable uses.
The installed blackwater treatment plant was found to require significantly increased maintenance and was suffering from performance issues and the client desired to meet, and report on, the original system objectives.
The approach was to conduct a rigorous analysis of process streams, the expected and actual performance of installed technologies, as well as both theoretical and laboratory proof of proposed alternative solutions.
Areas of investigation included:
- Hardware modifications required to ensure continued availability of the system “as installed”, and taking account of variability of inflows and demand.
- Filtration and pre-treatment of incoming streams, and impact on system maintenance intervals, particularly membrane longevity and biofilm formation.
- Filter membrane selection, and impact on performance.
- Feasibility analysis of biological pre-treatment of “mined” sewer water.
- Laboratory bench proof of proposed concept.
Methodical investigation revealed that:
- Simple hardware changes (self-priming feed pump) were required to improve system performance.
- Chlorine pre-treatment of the “mined” sewage at the point of sampling was ineffective, and increased cost.
- Coarser inflow filtration extended maintenance intervals and resulted in no discernible reduction in water quality.
- Particle size homogenisation (Multicrusher processing) of incoming waste streams produced a highly emulsified waste solution that contributed to biological film formation on the filter membranes, significantly reducing the expected throughput of the units.
- Storage capacity between treatment steps was inadequate for buffering and processing needs.
- Membrane filtration as installed was undersized for the application.
- Systems installed to reduce the corrosion potential of reclaimed, RO filtered water lacked appropriate monitoring and, if undiscovered, posed significant risk to the building water reticulation system.
- Bench testing showed that alternative processing using a Membrane Bioreactor (MBR) would provide reliable performance, extending maintenance intervals, with no reduction in quality of water quality.
- Bench testing of each recommendation confirmed the efficacy of each proposed technology.
The following changes were recommended to the client to restore and improve system performance with reduced OPEX budget:
- Feed pumps to be replaced with appropriate self-priming designs at nominal cost.
- Ineffectual chlorine pre-treatment at the point of sampling to be eliminated, reducing consumable expenditure.
- Multicrusher to be eliminated to prevent emulsification of Oil and Grease (O&G) simplifying processing, reducing cost and extending the life of filter membranes.
- Incoming screening to be made coarser to reduce maintenance.
- A small capacity Dissolved Air Flotation unit to be installed as pre-treatment. Test results showed 97.6% efficiency in the removal of Total Suspended Solids (TSS) and 95.3% in removal of Oil and Grease (O&G).
- Installation of a small scale MBR unit to reduce organic load and nutrients, improving treated water quality and reducing operation and maintenance cost.
- Replacement of the existing RO filtration with a Nano Filtration unit to significantly reduce the corrosion risk to the building water distribution system, eliminating the need for corrosion inhibition systems and associated operational cost.