Water and wastewater treatment plants (WWTPs) are going to have to work harder as Australia’s weather patterns turn drier with the arrival of the El Nino forecast, writes CST Wastewater Solutions MD Michael Bambridge.
One of their first lines of defence against process disruption, blockages and environmental spills will be WWTP headworks comprising mechanical screens, screening compactors, grit removal systems and grit washing systems installed in municipal and industrial plants.
The challenges for WWTP’s during El Nino may include widely variable flow patterns, including extended lower flow rates bringing with them greater concentrations of solids, debris, and grit carried in the reduced volumes of inlet water. Headworks will be asked to efficiently process throughputs without clogging and environmental spills – and continue to do it with less on-site maintenance, because many local authorities today can afford fewer specialist operations and engineering staff on call or on site.
Advantages of the Australian-engineered drum screen include mechanical simplicity, self-cleaning and high-efficiency screening for reduced maintenance and cheaper whole-of-life costs, compared with other types of screens, such as band and inclined drum screen designs, for example. In addition to the proven 2mm pre-MBR screen the new screen offers 5mm primary screening on flows up to 2000 l/sec.
Prevention better than downtime
Prevention is definitely better than expensive downtime. Damage or weakness is not always obvious. The first indication owners have that their wastewater plant screens need maintenance or replacement is when they fail.
If they fail, sewage, or untreated food waste, or organic and inorganic waste can seep through plant and neighbouring surroundings, including sensitive land and waterways, with potential production halts, OH&S clean up risks, pointed questions directed at those responsible for preventing spills and potentially serious environmental consequences for public and private entities.
Sometimes the build-ups to these failures will extend over many years. Grit build-up, for example, particularly in local authority WWTPs, may not show up until variable flows test the screens. For industrial plants, the problem is often a result of excess solids or hydraulic loads – or it could just be simple mechanical failure somewhere in the process.
Because screening headworks get the dirtiest job of removing all the lumps, chunks, strands, and unwanted foreign objects that can foul the often expensive, settling, clarification, treatment, filtration, and energy processes that follow downstream.
Why to check wastewater screens
The inlet screens can look pretty good to the untrained eye, so the temptation may always be there to check and forget, to save a few dollars if there is no obvious problem. This set-and-forget approach can become a costly lesson for several reasons, including:
Screening out microplastics with fine screening technologies is a step towards addressing this significant environmental threat. Such screening (top left) complements coarser screening (bottom) to prevent pollution entering wastewater systems and damaging them. Microfibres are rapidly becoming a significant source of pollution as they often enter marine systems and waterways and are subsequently consumed by aquatic and other wildlife.
1. Screens do deteriorate over time
They may have a design life of many years in normal service – but regular routine service and maintenance is needed to keep them in peak operating condition. This can be particularly important in sandy areas (noting that municipal councils’ WWTPs serve more the 80 per cent of Australia’s population living within 50 km of the coast. Many industrial processing plants fall within the same heavily populated areas).
2. Screen technology does evolve over time
What was a fine mesh screen 10 or 15 years ago, may no longer be up to the job of protecting newer processes downstream. Older, coarse screens used to remove large solids, rags, and debris from wastewater typically have openings of 6 mm or larger.
Such screens are no longer up to the higher standards and performance demanded today, including removal of nanoparticles such as plastics down to 1.5mm and finer. So, for environmental reasons, it could be be a matter of upgrading the screen to a finer mesh type now available or using newer technology, such as CST’s new horizontal rotary drum screen, which is specifically designed to protect downstream MBR (membrane bioreactor) plant.
3. Beware particularly of grit
Wastewater contains large solids and grit that can interfere with treatment processes or cause undue mechanical wear and increased maintenance on wastewater treatment equipment. To minimize potential problems, these materials require separate handling. Preliminary treatment removes these constituents from the influent wastewater. Fine screens are typically used to remove material that may create operation and maintenance problems in downstream processes, particularly in systems that lack primary treatment. Typical opening sizes for fine screens are 1.5 to 6 mm. Very fine screens – such as those proven in Australia by CST Wastewater Solutions – have openings of 1.5 down to 0.2mm.
CST’s Rotary Drum Screen, left, and Smith and Loveless PISTA 360™ grit removal technology, right are two examples of the broad range screening and grit removal technologies that CST services
4. Asset management
Inlet pre-treatment, including screens, is a robust but finite asset we all depend in the waste management business. The value of this asset should be maintained by regular servicing, just like a car. CST has some screens still operating after more than 25 years, because the companies involved have invested in cost-effective regular maintenance to extend product life and effectiveness.
5. Risk Management
Because screens are your front-line defence, they prevent or limit damage occurring in expensive processes downstream. These processes can be hard to get to if they do fail, then cause many more OH&S issues to rectify. Compounding the issue is the fact mentioned prior that many remote and smaller public and private waste treatment plants these days do not have the engineering manpower on hand to quickly rectify unexpected or complex and expensive events.
We know how important the risk management is because we engineer and locally manufacture dozens of waste management (and waste-to-energy) processes purpose-built to minimise maintenance. The advantage of local manufacture is that the product is built to local standards (some of the best in the world, recognised internationally) for arduous local conditions. And when they do ultimately need maintenance, having achieved top lifecycle value, local manufacture means local service is available without stretched supply lines.
Most companies these days are pretty risk-aware. They know production stoppages are costly, and environmental spills can affect their statutory licence to operate. But we still do see poorly maintained plants causing issues for the operator, reasonably often.
These should be a thing of the past, when small investment in yearly maintenance and check-ups can return good value and ROI throughout the long term.
